Patent Application
20220378956
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Radiopharmaceuticals typically contain a radioisotope attached to a targeting moiety or carrier. The radioisotope is carried to the target by the carrier where it decays. The mode of isotope decay determines the type of radiopharmaceutical.
Typically, gamma emitting isotopes are used to detect the fate of the construct and are used for diagnostic purposes. Constructs with particle emitters are preferred for therapy. Although beta-emitting radionuclides were used previously, alpha-emitting radionuclides have shown excellent efficacy in recent years.
Alpha-emitting radionuclides are effective at killing cells in part due to the short particle range and high linear energy transfer (LET). Poty et al. (J Nucl Med. 2018 June: 59(60):878-884) describe the use of alpha emitters for therapeutic radiopharmaceuticals.
The relatively long half-life of the alpha-emitting radionuclide actinium-225 (Ac-225) compared to other alpha emitters is one of the reasons that it has become popular as a therapeutic radioisotope for the treatment of cancer. Clinical trials with constructs using the isotope have shown excellent results. The about 10-day half-life is a good match for the in vivo biological half-life of monoclonal antibodies, and the four alpha emissions produced by Ac-225 and its daughters were responsible for a high rate of tumor cell kill. However, the chemistry necessary to attach Ac-225 to a targeting moiety was lacking.
Ac-225 ions exhibit a valence of +3, with a documented ionic radius of 112 pm. Due to its lack of polarizability, Ac+3 is classified as a “hard” Lewis acid according to the Hard and Soft Acids and Bases (HSAB) [Pearson, J Am Chem Soc 1963, 85:3533-3539] theory and is therefore likewise predicted to prefer “hard,” nonpolarizable, electronegative Lewis bases such as anionic oxygen donors. The hard/soft acid-base properties of a specific ion can be quantified using the concept of absolute (h) chemical hardness. The absolute chemical hardness (h) of an ion is given by the equation (h)=(I−A)/2, where I is the ionization energy and A is the electron affinity of the species of interest. [Parr and Pearson, J Am Chem Soc, 1983; 105:7512-7516; and Pearson, Inorg Chem 1988; 27:734-740.]
Absolute chemical hardness of Ac+3 and La+3 so calculated are 14.4 eV and 15.4 eV, respectively. Soft ions such as Au+, Ag+ and Cu+ exhibit absolute chemical hardness values that's range from 5.7 to 6.3 eV, whereas conventional hard ions, like Sc+3 and Al+3 are characterized by absolute chemical hardness values of greater than 24 eV. Thiele et al., Cancer Biother Radio, 2018 33(8):336-348.
The large ionic size of Ac+3 is suited to large polydentate chelators of high denticities, because most commonly used chelates for Ac(III) range between 8-12 coordinate. Actinium is similar to other actinides and rare earth elements, and can undergo hydrolysis in solution in the absence of a chelating agent to form [Ac(OH)3-x]x-; the sub-picomolar concentrations of Ac-225 cause the hydroxide species in turn to form radiocolloids that bind to surfaces such as reaction vessels.
Emission of multiple alpha-particles in the Ac-225 decay chain makes Ac-225 a particularly effective isotope to kill cancer cells, yet also makes the directed delivery of the nuclide and its decay daughters a challenge. Due to the conservation of momentum, the emission of an energetic alpha particle imparts a recoil energy to the daughter nucleus often >100 keV, 1000 times larger than the binding energy for any chemical bond. This results in release of the daughter nuclide from the chelator of the original delivery vector. The subsequent redistribution of the alpha-emitting daughter nuclides in vivo can cause substantial harm to untargeted healthy tissues and reduce the therapeutic effect.
Davis et al., Nuc Med Biol 1999, 26(5):581-589 reported that limited information exists regarding the behavior of Ac-225 in vivo. Preliminary studies have evaluated Ac-225 complexed to citrate with respect to tissue uptake, biodistribution, and tumor tropism in animal models. Previous studies using Ac-225 complexed to either of the polyaminocarboxylate chelators, ethylenediamine tetraacetic acid (EDTA), or cyclohexyl diethylenetriaminepentaacetic acid (CHX-DTPA) showed varied tissue tropism and elevated blood clearance compared with uncomplexed Ac-225.
Ac-225-CHX-DTPA-monoclonal antibody (Mab) complexes used to determine biokinetic behavior on tumor-bearing nude mice showed successful in vitro complexing but poor stability in vivo. Thus, whereas Ac-225 may prove useful in radiotherapeutic models, information regarding potentially effective chelators and the relative stability of such Ac-225 complexes in vivo is lacking.
A recent review article on Ac-225 radiopharmaceuticals, Robertson et al., Curr Radiopharm, 2018, 11(3):156-172, noted that the discovery of a chelating agent that binds Ac(III) with sufficient stability and that also controls the release of its daughter nuclides remains a challenge. Moreover, limited Ac-225 global availability of and the absence of a stable surrogate nuclide has limited the study of this isotope to a handful of institutions around the world that have secured a reliable Ac-225 supply.
The above review authors included the Davis et al. article, above, and noted that biodistribution profiles over the course of 8 days for each of the purified Ac-225-complexes were assessed by injecting 92 kBq (2.5 mCi) of each complex and compared to the Ac-225-acetate biodistribution as a control.
Because uncomplexed Ac-225 accumulates predominantly in the liver with small amounts in the bone, kidney, and heart, high Ac-225 liver uptake of a chelate indicates an unstable complex in vivo. Cyclohexyldiethylenetriamine-pentaacetic acid “a” isomer (CHX-A″-DTPA), and 1,4,7,10,13-pentaazacyclo-pentadecane-N,N′,N″,N′″,N″″-pentaacetic acid (PEPA) reduced liver uptake Ac-225 of the complex by more than 5.5 times compared to Ac-225 acetate, and although the Davis et al. data suggested —CHX-A″-DTPA to be the most effective tested chelator complex with regard to its in vivo stability, the Robertson et al., review authors wrote that “improvements can still be made to further reduce non-target tissue accumulation.” [Robertson et al., at page 164.]
As such, CHX-A″-DTPA provides inadequate chelation of Ac(III). Another important finding of the initial in vivo study on which Robertson et al. commented was that the maximum tolerated dose of Ac-225-CHX-A″-DTPA was less than 185 kBq (5 mCi), because at doses of 185 kBq (5 mCi) or higher, severe tissue damage was observed as early as 1 hour post-injection (p.i.), which ultimately led to study animal death, causing 100% mortality by day 8 p.i.
Attachment of actinium to a targeting molecule was accomplished by Sheinberg's research group (Sheinberg, Science 2001 Nov. 16; 294(5546):1537-1540. doi: 10.1126/science.1064126). The chelator of choice was a bifunctional molecule based on DOTA. However, in the Sheinberg group's report, a two-step method was used to obtain enough Ac-225 on the targeting moiety. In addition, yields based on Ac-225 starting material were very low, less than 10% of the isotope was incorporated into the targeting moiety. More than 90% of the isotope was wasted. Specific activities with this process ranged from about 50 to 70 μCi per mg of antibody. Clearly, a one-step process with higher yields would be preferred.
Further studies of possible chelators by the Scheinberg research group [McDevitt et al., App. Radiat. Isot., 2002, 57(6):841-847] found that of six possible chelators studied, showed that only DOTA and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra-propionic acid (DOTMP) showed any complexation of Ac-225 after 2 hours at 37° C. with radiochemical yields (RCYs) of >99 and 78%, respectively. However, subsequent in vitro stability assays in serum suggested that the Ac-225 DOTA complex was robust, remaining >90% intact after 10 days, whereas the Ac-225-DOMTMP complex rapidly dissociated.
A two-step labeling process was again employed that required radiolabeling of the bifunctional DOTA-NCS ligand first, followed by mAb conjugation (pH 8.7, 37° C. for 52 minutes). Despite low overall radiochemical yields of only 9.8±4.5%, reasonable specific activity (4.1±2.6 GBq/g, or 0.11±0.07 Ci/g) was achieved that permitted preclinical therapeutic studies. Low yields were attributed to the first Ac-225 labeling step of DOTA-NCS that required heating and, consequently, degradation of the isothiocyanate linker resulting in poor mAb conjugation in the following step.
The Scheinberg group and co-workers [McGuire et al., J. Nucl. Med., 2014, 55(9):1492-1498] later reported a one-step process for preparation of Ac-225-DOTA-antibody constructs. That process proceeded in 2 M tetramethyl ammonium acetate buffer (pH 7.5) with the addition of L-ascorbic acid as radioprotectant to the addition of DOTA-antibody construct and Ac-225+3 with a typical final reaction pH value of 5.8. Heating to 37° C. for 2 hours allowed a 10-fold increase in radiochemical yield (80%) compared to previous 2-step methods (6-12%), and resulted in the preparation of bioconjugates with up to 30-fold higher specific activities (120 GBq/g compared to 3.7-14.8 GBq/g). The highest specific activity achieved was equivalent to 1 actinium for every 25 antibodies.
US 2004/0067924 A1 (Frank) teaches the use of 12-membered macrocyclic amine-based polyacetate and polyphosphonate chelating agents for complexing Ac-225. DOTA-based chelating agents were found useful for chelating Ac-225.
Paragraph [0082] of that patent publication noted that the nitrobenzyl group of one depicted DOTA chelant can be reduced to an aniline, whose amine can be subsequently converted to an isothiocyanate to form a bifunctional compound for linking to a targeting peptide antibody or other entity. A bifunctional analog of PCTA (below) was said could be prepared by attaching a linking group to one of the acetate carbons.
Chelating agents based 3,6,9,15-tetraazabicyclo-[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-acetic acid (PCTA) were mentioned in the text of the Frank application and binding data with actinium were shown. The PCTA compound shown and utilized was not adapted for linkage to a targeting molecule such as a peptide or antibody other than by the possible use of one of the chelating carboxyl groups. No disclosure of a targeted construct using PCTA was disclosed.
Yapp et al., Mol Imaging June 2013 12(4):263-272 reported on the use of PCTA, DPTA and 1-oxa-4,7,10-triazacyclododecane-4,7,10-triacetic acid (Oxo-DO3A) for the chelation of Cu-64 [Cu (II)-64] for use in PET scan studies of tumor vasculature. The chelates were bonded to the cyclic tetrapeptide cyclic-(RGDyK) via benzylisothiocyanate linkages to the added lysine of the cyclic peptide.
Another study, reported by Bryan et al., Cancer Biol Ther Jun. 15, 2011 11:12, 1001-1007, discussed the radioimmunotherapy and PET scan results of Cu-64 linked to an internalizing mAb and to a non-internalizing mAb, using 1,4,7,10-tetraazacyclo-dodecane-1,4,7,10-tetraacetic acid (DOTA) as the chelating agent in treating colon cancer tumors in xenografted mice. Measuring the tumors daily, the results showed that the PET scans were useful and that use of an internalizing antibody did not improve the outcome of Cu-64 radioimmunotherapy.
An earlier one-step process was disclosed in Simón, WO 2011/011592 A1. This patent application teaches the preparation of a protein conjugated with chelators as a first step. After removal of excess chelating agent, the protein-chelated conjugate was reacted with the isotope. Again, a DOTA-based chelator was used for the work showing that the still current thinking in the art was that DOTA-based chelators would be the best for Ac-225.
The method in the Simon disclosure required the use of high concentrations of acetate ion and a high chelator to antibody ratio (CAR). Starting reactions were conducted using a molar reactant ratio of 100 chelators per antibody to yield a CAR number of 10-12.
It is desirable to produce high specific activity Ac-225 constructs with conjugates that have a lower CAR number. This is because as the CAR number increases, the biological targeting of the antibody decreases. Thus, even though the one-step process is taught with Ac-225 and DOTA type chelators, the CAR numbers required were high using DOTA-type chelating agents. Clearly there is a need for better chelating agents for preparing Ac-225 constructs.
The difficulties in using DOTA as a chelating agent for Ac-225 as discussed above notwithstanding, Thiele et al., Cancer Biother Radio, 2018 33(8):336-348, as recently as 2018 used the phrase “DOTA: the current gold standard” (at 340) for a section of their review. The last sentence of those authors' DOTA section reads: “Collectively, these shortcomings indicate that DOTA is not ideal for use in 225Ac-TAT [225Ac targeted alpha therapy]applications, highlighting the need for more suitable chelating scaffolds for 225Ac.”
The chemistry associated with attaching Ac-225 to targeting moieties has been challenging for the users and writers. It is apparent that better methods of attaching Ac-225 to molecules are needed. The present invention helps address that need. Surprisingly, we have found that PCTA-based chelating agents form stable chelates with Ac-225 under mild conditions and at lower CAR numbers than were previously reported when using DOTA, the prior “GOLD standard”.
Bi-213 is a radioactive decay product of Ac-225, whereas Bi-212 produced by the radioactive decay of lead-212 (Pb-212) after step-wise decay of uranium-234 (U-234). The short half-life of Bi-212 and Bi-213 can limit the application of these radionuclides in radionuclide therapy.
Bismuth isotopes, Bi-212 and Bi-213, are also candidates for use in radioimmunotherapy. Several preclinical studies have been published utilizing one, the other or both isotopes. Both have valences of +3, and as such tend to remain complexed after actinium has decayed.
Bi-212 has a half-life of about one hour and emits both alpha and beta particles in an almost 1:2 ratio. Bi-213 has a half-life of about 45 minutes and decays almost completely by beta emission to polonium-213, which then emits an alpha particle to form lead-209 as is shown in
Illustratively, Park et al., Blood, 2020 116(20):4231-4239, reported a preclinical study in mice having xenografts of Ramos lymphoma that were treated with anti-CD20 antibody fused to streptavidin followed by [213Bi]DOTA-biotin. The treated mice with tumors exhibited marked growth delays and mean survival times about four-times longer than untreated controls. A review by Yong et al., AIMS Med Sci, 2021, 2(3):228-245, discussed recent work using Pb-212/Bi-212 in targeted a-particle therapy (TAT), such as work that utilized the chelator 2-(4-isothio-cyanatobenzyl-1,4,7,10-tetraaza-1,4,7,10-tetra-(2-carbamonylmetyl)cyclododecane (TCMC) linked to a mAb, trastuzumab, that binds to HER2. A review by Mulford et al., J Nucl Med 2005, 46(1 Suppl):199S-204S discusses several TAT therapies that utilize one or the other of the above bismuth isotopes.
The labeling of biomolecules with precursor Pb-212 instead of Bi-212 or Bi-213, as discussed in Yong et al., above, has the advantage of obtaining a conjugate with a half-life of 10.6 hours, compared with of 60 minutes for Bi-212 or 46 minutes for Bi-213. Previous attempts to prepare a potential in vivo generator with Pb-212 complexed by the DOTA chelator failed, because about 36% of Bi was reported to escape as a result of the Pb-212 decaying via a beta particles to form Bi-212, which were not held by DOTA. It can be important that Bi-212 formed in the decay of Pb-212 remain bound to the carrier because free bismuth ions localize in the kidneys. Bartos et al., J Radioanal Nucl Chem, 2013 295:205-209.
Zirconium-89 is another useful radioisotope in that zirconium has a valence and the Zr-89 emits a gamma ray (909 keV) and also a positron at about 397 keV, both emissions being useful in diagnostics. The half-life of Zr-89 is 3.3 days, which is similar to the circulation half-lives of many monoclonal antibodies used in medicine. Those isotopes have been used in radiolabelling and evaluation of mAbs in positron emission tomography (Immuno-PET). [Saleem et al., Sci World J 2014, Article ID 269605, 9 pages.] The final decay product of Zr-89 is yttrium-89, a stable non-radioactive isotope.
A further useful isotope in the present invention is indium-111. Indium also has a valence of +3, and In-111 has a half-life of about 2.8 days. Indium-111 decay provides gamma rays of 0.171 MeV and 0.245 MeV, that can be used in diagnostic scans such as single photon emission computed tomography (SPECT) imaging. In-111 decays to cadmium-111, which is non-radioactive and stable.
The invention disclosed below teaches the using various targeting species and a single chelating agent for both therapeutic and diagnostic (theranostic) uses, providing a single chelator-linked targeting system for both uses. Such a theranostic has significant benefits in development and manufacturing as the targeting species and chelation manufacturing steps can be common with the labeling of the radioisotope being distinct. This provides some time and cost advantages in development, toxicity studies with the unlabeled targeted-chelator, common stability and bulk drug substance.
This invention relates to the use of a chelating agent containing a 12-membered macrocyclic amine with a pyridine ring imbedded in the structure surprisingly that easily makes stable metal ligand complexes with trivalent radioactive isotope ions such as Ac-225, Bi-212, Bi-213, Zr-89 and In-111, and also with a targeting species molecule to form a radiotherapeutic agent or a radiodiagnostic agent (or generically, a radiopharmaceutical agent). These radiopharmaceuticals can also be referred to as radiotheranostic agents.
In one embodiment of the invention, the chelating agent is bonded to a targeting species molecule, permitting the chelation of the Q+3 ion to that part of the molecule. The chelating agent is bonded to a part of the targeting molecule that does not interfere with the ability of the targeting molecule to reach its target. The targeting species binds the radiopharmaceutical agent to cells that are to be killed or one or more of whose presence, location, size and shape are to be determined.
More specifically, a targeting species is chemically-bonded to a PCTA chelator with its chelated trivalent radioactive isotope ion, Q+3, to form the theranostic radiopharmaceutical that has the general structural formula shown below in Formula I and which, depending on the radioactive isotope that is chelated can be used therapeutically to kill targeted cells or to bind to targeted cells to signal the one or more of the presence, location, size or shape of the bound cells
In that chelator, the chelated Q+3 ion is a radioactive isotope having a valence of +3, six of R1, R2, R3, R4, R5, R6, and R7 are H, and the seventh contains a reacted functionality, Z, that forms the chemical bond with the targeting species, T. X1, X2, and X3, are the same or different substituent groups that can coordinate to that ion and/or help neutralize the ionic charge of the chelated Q+3 ion such as trivalent Ac-225, Bi-213, Bi-212, Zr-89 or In-111. “g” is a number whose average value is 1 to about 12 that indicates the average number of chelated PCTA-chelated trivalent radioactive ions, Q+3, per each molecule of targeting species, T. An optional anion, Y−, can be present in an amount needed to balance the ionic charge.
The chelation reaction with the Q+3 ion can be performed first followed by attachment to the targeting species molecule, T. This is referred to as a two-step process because the isotope is handled twice. Alternatively, the conjugation reaction (attaching the chelating agent to the targeting species) can be accomplished first followed by insertion of Q+3 ion. This is called a one-step process as the isotope is only handled once, and is preferred. 225Ac+3 is a preferred Q+3 ion.
A pharmaceutical composition is contemplated that comprises a theranostic effective amount of a targeted radiopharmaceutical of Formula I dissolved or dispersed in a pharmaceutically acceptable diluent. Preferably, the pharmaceutically acceptable diluent is an aqueous liquid at ambient temperature and is adapted for parenteral administration.
In one embodiment, that pharmaceutical composition is used in a method for treating a mammalian host having a disease, disorder or condition characterized by undesired angiogenesis, tumor growth and/or tumor metastasis comprising administering to the host a targeted cell-killing (therapeutic) effective amount of the targeted radiopharmaceutical.
In further embodiments, a contemplated targeted radiopharmaceutical is used as a diagnostic agent. As such, the invention contemplates a method for assaying a mammalian host thought or known to have a disease, disorder or condition characterized by undesired angiogenesis, tumor growth and/or tumor metastasis by administering to the host a target cell-binding effective amount of the targeted radiopharmaceutical followed by scanning the host to detect and locate the radiation emitted by the bound targeted radiopharmaceutical.
In the drawing forming a portion of this disclosure,
This invention relates to a targeted radiopharmaceutical that comprises PCTA-chelated Q+3 ion chemically-bonded to a targeting species. A contemplated targeted radiopharmaceutical of this invention has the general structural formula shown below in Formula I
In that targeted radiopharmaceutical, Q+3 is a trivalent radioactive isotope ion; six of R1, R2, R3, R4, R5, R6, and R7 are H and the seventh contains a reacted linking functionality, Z, that forms a chemical bond with the targeting species, T. The X1, X2, and X3 groups are the same or different substituent groups that can coordinate to the Q+3 ion such as 225Ac+3, 212Bi+3, 213Bi+3, 89Zr+3 or 111In+3 and help neutralize the ionic charge of the chelated Q+3 ion. “g” is a number whose average value is about 1 to about 12 that indicates the average number of chelated PCTA-chelated trivalent radioactive ions, Q+3, per each molecule of targeting species, T. An optional anion, Y−, can be present in an amount needed to balance the ionic charge.
Illustrative targeted radiopharmaceutical chelates are illustrated in Formulas Ia, Ib, Ic and Id below without depicting a specific targeting species, the number of chelates bonded to each target species, or a reacted linking functionality, Z. The two bismuth isotopes (212Bi+3 and 213Bi+3) are written together as 212/213Bi+3 for convenience.
Actinium-225 is a preferred radiopharmaceutical isotope because it has a half-life of almost ten days and as shown in
Bismuth-212 is a decay product of lead-212. Once obtained, the bismuth-212 can be separated from the lead-212 and complexed with an appropriately linked PCTA to form a chelated Q+3 ion chemically-bonded to a targeting species. Bi-212 ultimately decays to lead-208, which is not radioactive and is stable.
Some versions of the chelating agent are referred to as pyridine-based 12-membered tetraaza-macrocyclic ligands or PCTA (First published: 2 Apr. 2019 chemistry-europe.onlinelibrary-.wiley.com/-doi/abs/10.1002/ejoc.201900280.
In one embodiment, the reacted functionality, Z, is selected from the group consisting of one or more of a reacted Michael reaction acceptor, a reacted isocyanato group, a reacted carboxyl group, and a 1,4-disubstituted-1,2,3-triazine formed by the reaction of an azide and an alkyne. A reacted isothiocyanate [—NH—C(═S)—NH—; a thiourea] is one preferred reacted functionality.
The number of chelators bonded per antibody molecule is an average number because some antibody molecules in a given composition do not react whereas others do react. Average numbers of chelators bonded per antibody molecule are 1 to about 12, preferably about 3 to about 12, and more preferably about 8 to about 10 when an isothiocyanate group is being bonded to an intact antibody. Where a paratope-containing portion (or antigen-binding fragment) thereof is the targeting species, the number of PCTA chelators per targeting species molecule tend to be fewer such as about 1 to about 5 as there are fewer lysine amino groups with which the isothiocyanate group can react when the two pairs of CH2 and CH3 portions of the heavy chain are absent.
An illustrative chelator reacted functionality, Z, prior to reaction can be a Michael reaction acceptor such as maleimide can link up to about 8 chelators to a reduced intact antibody thiol groups. Of course, with smaller targeting species such as a the peptidomimetic cyclic-(RGDyK) discussed hereinafter has only one amine that can bond to the chelator, thereby limiting the number of radiopharmaceutical chelates to which it can be linked.
A Michael reaction acceptor contains an a,b-unsaturated carbonyl group that can react with a nucleophile such as an amine or a mercaptan. Illustrative Michael reaction acceptor groups include acryloyl, methacryloyl and maleimido groups.
Precursors for the formation of the 1,4-disubstituted-1,2,3-triazine, an azide and an alkyne can be present, one each, on either the pre-reacted functionality of the chelator or on the targeting species. The coupling reaction can be catalyzed by a copper(II) ion or by irradiation with UV light.
The targeting species, T, is selected from the group consisting of one or more of a chemically-bonded antibody or paratope-containing portion of an antibody, a chemically-bonded hormone, a chemically bonded non-antibody protein, a chemically-bonded cytokine, a chemically bonded aptamer, a chemically bonded oligonucleotide, a chemically-bonded cytokine, a straight chain or cyclic oligopeptide or peptide mimetic, and a straight chain or branched chain oligosaccharide. A monoclonal antibody (mAb) or a paratope-containing portion thereof is a preferred targeting group, and a humanized monoclonal antibody or a paratope-containing portion thereof is particularly preferred.
The X1, X2, and X3 groups are the same or different substituent that is a functional group useful for chelation that can coordinate to the Q+3 ion and/or help neutralize the ionic charge of the targeted radiopharmaceutical. Exemplary X substituents include a —(CH2)nCO2M group, a phosphonic acid (—PO3M2) group and half-esters thereof, as well as carboxamides —(CH2)nCONH2, and —(CH2)nCH2NR10R11 primary, secondary or tertiary amines where R10 and R11 are the same or different H or C1-C4 alkyl. In such a substituent, M is a proton (H+), an ammonium ion or an alkali metal ion. It is preferred that each of the X1, X2, and X3 groups is the same, and more preferably each is a —COOM group. “n” is zero or 1, preferably zero so that an X group is —CO2M. It is to be understood that once in an aqueous composition such as a buffer, the cationic M is likely exchanged for another cation present in the aqueous composition.
A preferred chelator is referred to in the art as PCTA. The chemical formula for a particularly preferred form of PCTA is the (4-isothiocyanato-phenyl)methyl derivative that enables the chelator to be bifunctional, and is shown in Formula II, below, where M is as before described.
The chelator of Formula II is commercially available from Macrocyclics Inc. (Dallas, Tex.) under the designation p-SCN-Bn-PCTA.
Antibodies are one group of preferred targeting species molecules in one aspect of the invention because many bind to cell surface antigens of undesirable cells in the body such as cancer cells. Once bound, the antibody and its bonded chelated Q+3 ion can be taken into the unwanted cell (cell to be treated) at which time the Q+3 ion such as Ac-225 or one of its daughter atoms such as 213Bi+3 can decompose to release its cytotoxic alpha particle within the unwanted cell. The particular combination of a Q+3 ion with a PCTA chelator forms particularly stable chelation products as compared to those formed using DOTA as the chelator with a Q+3 ion. As a consequence, there is a greater concentration of radioisotope at the target cell and a lower concentration of radioisotope elsewhere in the recipient body than when a chelator such as DOTA is utilized.
An illustrative list of monoclonal antibodies for use as a targeting species is provided in the table below. Most on the list are human or humanized and are approved for being used in treating a human, whereas others are mouse antibodies. It is to be understood that this list is only illustrative with approximately 80-90 possibly useful monoclonal antibodies awaiting approval by the U.S. FDA for use in humans.
B. anthrasis
The mAb used illustratively herein designated mAb MNBR-101 is a humanized version of mouse mAb ATN-658, whose hybridoma has ATCC Accession Number BTA-8191, disclosed and claimed in U.S. Pat. No. 8,101,726. Mouse mAb ATN-615 that is also disclosed and claimed in U.S. Pat. No. 8,101,726, is secreted by a hybridoma that has ATCC Accession Number BTA-8192.
These mAbs specifically bind to (immunoreact with) the binary complex referred to as uPA-uPAR; i.e., urokinase plasminogen activator (uPA) and its cell surface receptor, uPAR, as well as to uPAR at a locus that does not interfere with formation of the binary complex. U.S. Pat. No. 8,101,726 notes that expression of uPA and uPAR has been demonstrated in numerous tumor types.
The mAb MNPR-101 paratopic amino acid residue sequence (CDR; complementarity determining region; variable region) binds to its uPA-uPAR antigen very similarly to the binding of mAb ATN-658 Table 3, hereinafter). The heavy chain constant regions (CH1, CH2 and CH3) are those of a human IgG1 antibody.
Humanization of ATN-658 to prepare MNPR-101 utilized the Xoma HE™ synthesis platform that utilizes the human antibody amino acid residue sequences reported in Wu and Kabat, 1992 Mol. Immunol., 29(9):1141-1146 (hereinafter Kabat) combined with the sequences of the variable regions of the antibody to be humanized to form one or more consensus sequences. There are several steps in this process:
(1) Human Engineer™ (HE™) the ATN-658 Light and Heavy chains using the XOMA Corp. (Emeryville, Calif.) proprietary HE™ method to generate the low risk and low plus moderate risk HE™ variants;
(2) HE™ Variable (V) region sequences codon optimization, energy minimization and gene synthesis;
(3) Clone the 4 HE™ V regions into XOMA's proprietary transient expression vectors which contain human gamma-1 and kappa constant region modules;
(4) Transiently express the HE™ variants;
(5) Purify the humanized antibodies and characterize them for purity and endotoxin; and
(6) Verify the affinity of the 4 HE™ variants.
The phrase “low risk” discussed above and hereinafter relates to whether a mouse-to-human amino acid residue change results in a major reduction in therapeutic immunogenicity with little chance of affecting binding affinity. The second phrase “high risk” relates to modifying positions at which a mouse-to-human amino acid residue change results in a degradation or abolition of binding activity with little or no actual reduction in therapeutic immunogenicity.
Humanization of ATN-658 to create mAb MNPR-101 using the Xoma HE™ platform was performed pursuant to the “low risk”, “moderate risk” and “high risk” substitutions suggested in the following publications, patents and application: 1) WO 93/11794 “Methods and materials for preparation of modified antibody variable domains and therapeutic uses thereof”; 2) U.S. Pat. No. 5,766,886 “Modified antibody variable domains”; 3) U.S. Pat. No. 5,770,196 “Modified antibody variable domains and therapeutic uses thereof”; 4) U.S. Pat. No. 5,821,123 “Modified antibody variable domains”; 5) U.S. Pat. No. 5,869,619 Modified antibody variable domains, and 6) Studnicka et al. 1994 Protein Eng 7:805-814, all of whose disclosures are incorporated by reference.
Further details related to the preparation of mAb MNPR-101 are discussed in the Examples hereinafter.
The term “antibody” is meant to include both intact immunoglobulin (Ig) molecules as well as fragments and derivative thereof, that can be produced by proteolytic cleavage of Ig molecules or engineered genetically or chemically. Paratope-containing portions or fragments include, for example, Fab, Fab′, F(ab′)2 and Fv, each of which is capable of binding antigen. These fragments lack the Fc fragment of intact antibody (Ab) and have an additional advantage, if used therapeutically, of clearing more rapidly from the circulation and undergoing less non-specific tissue binding than intact antibodies. Papain treatment of Ig's produces Fab fragments; pepsin treatment produces F(ab′)2 fragments. These fragments can also be produced by genetic or protein engineering using methods well known in the art.
A Fab fragment or portion is a multimeric protein consisting of the portion of an Ig molecule containing the immunologically active portions of an Ig heavy (H) chain and an Ig light (L) chain covalently coupled together and capable of specifically combining with antigen. Fab fragments are typically prepared by proteolytic digestion of substantially intact Ig molecules with papain using methods that are well known in the art. However, a Fab fragment can also be prepared by expressing in a suitable host cell the desired portions of Ig H chain and L chain using methods well known in the art. A F(ab′)2 fragment is a tetramer that includes a fragment of two H and two L chains.
The Fv fragment is a multimeric protein consisting of the immunologically active portions of an Ig H chain variable (V) region (VH) and an Ig L chain V region (VL) covalently coupled together and capable of specifically combining with antigen. Fv fragments are typically prepared by expressing in suitable host cell the desired portions of Ig VH region and VL region using methods well known in the art.
Single-chain antigen-binding protein or single chain Ab, also referred to as “scFv,” is a polypeptide composed of an Ig VL amino acid residue sequence tethered to an Ig VH amino acid residue sequence by a peptide that links the C-terminus of the VL sequence to the N-terminus of the VH sequence. In a preferred embodiment, the Ab is a mouse monoclonal antibody (mAb) designated ATN-615 (Creative Biolabs, Inc., Shirley, N.Y.) or ATN-658 (hybridoma B cell:ATCC PTA-8191; Manassas, Va.), both of which are IgG1 antibodies.
An Ab of the present invention can be produced as a single chain Ab or scFv instead of the normal multimeric structure. Single chain Abs include the hypervariable regions from an Ig of interest and recreate the antigen binding site of the native Ig while being a fraction of the size of the intact Ig (Skerra et al., Science, 1988 240:1038-1041; Pluckthun et al. Methods Enzymol 1989 178:497-515; Winter et al., Nature 1989 349:293-299); Bird et al., Science 1988 242:423-426; Huston et al. Proc. Natl. Acad. Sci. USA 1988 85:5879-5883; Jost et al., J Biol Chem. 1994 269:26267-26273; U.S. Pat. Nos. 4,704,692, 4,853,871, 4,946,778, 5,260,203, and 5,455,030).
DNA sequences encoding the V regions of the H chain and the L chain are ligated to a linker that encodes a sequence of at least about 4 amino acid residues (typically small neutral amino acids). The protein encoded by this fusion permits assembly of a functional variable region that retains the specificity and affinity of the original Ab.
A different type of single chain Ab is an antibody induced in a camelid such as a dromedary, llama, alpaca, of vicuna. These animals produce single heavy-chain-only antibodies (HcAbs) that have a variable region and a constant region, with many unique properties such as small size, excellent solubility, superior stability, quick clearance from blood, and deep tissue penetration. The nomenclature of “nanobody” originally adopted by the Belgian company Ablynx® due to its nanometric size and molecular weight of less than about 15 kDa. Sanofi's affiliate Ablynx N.V. is the holder worldwide of the NANOBODY® trademark.
The antigen-binding capacity of nanobodies, however, remains similar to that of conventional antibodies for the following reasons. First, the complementarity-determining region 3 (CDR3) of nanobodies is similar or even longer than that of human VH domain (variable domain of heavy immunoglobulin chain). The former consists of 3 to 28 amino acids (AAs), whereas the latter only 8 to 15 AAs. As therapeutic agents, nanobodies enable a targeted therapy by lesion-specific delivery of drugs and effector domains, thereby improving the specificity and efficacy of the therapy. [Bao et al., EJNMMI Res (2021) 11:6.] Humanized versions of monoclonal nanobodies are typically prepared in a manner similar to that used for the preparation of double-chained mAbs, except that they typically require fewer steps.
Human genes that encode the constant (C) regions of the chimeric antibodies of the present invention can be derived from a human fetal liver library or from any human cell including those which express and produce human Igs. The human CH region can be derived from any of the known classes or isotypes of human H chains, including g, m, a, d or e, and subtypes thereof, such as G1, G2, G3 and G4.
Because the H chain isotype is responsible for the various effector functions of an Ab, the choice of CH region will be guided by the desired effector functions, such as complement fixation, or activity in Ab-dependent cellular cytotoxicity (ADCC). Preferably, the CH region is derived from g1 (IgG1), g3 (IgG3), g4 (IgG4), or m (IgM).
The human CL region can be derived from either human L chain isotype, k or 1.
Genes encoding human Ig C regions are obtained from human cells by standard cloning techniques [Sambrook, J. et al., Molecular Cloning: A Laboratory Manual, 2nd Edition, Cold Spring Harbor Press, Cold Spring Harbor, N.Y. (1989)]. Human C region genes are readily available from known clones containing genes representing the two classes of L chains, the five classes of H chains and subclasses thereof.
Generally, the chimeric antibodies of the present invention are produced by cloning DNA segments encoding the H and L chain antigen-binding regions of a specific Ab of the invention, preferably non-human, and joining these DNA segments to DNA segments encoding human CH and CL regions, respectively, to produce chimeric Ig-encoding genes.
Thus, in a preferred embodiment, a fused gene is created that comprises a first DNA segment that encodes at least the antigen-binding region of non-human origin, such as CDR1, CDR2 and CDR3 of the V region with joining (J) segment, linked to a second DNA segment encoding at least a part of a human C region.
Chimeric Ab fragments, such as F(ab′)2 and Fab, can be prepared by designing a chimeric H chain gene that is appropriately truncated. For example, a chimeric gene encoding an H chain portion of an F(ab′)2 fragment would include DNA sequences encoding the CH1 domain and hinge region of the H chain, followed by a translational stop codon to yield the truncated molecule.
One common feature of all Ig H and L chain genes and their encoded mRNAs is the J region. H and L chain J regions have different sequences, but a high degree of sequence homology exists (greater than 80%) among each group, especially near the C region. This homology is exploited in this method and consensus sequences of H and L chain J regions can be used to design oligonucleotides for use as primers for introducing useful restriction sites into the J region for subsequent linkage of V region segments to human C region segments.
C region cDNA vectors prepared from human cells can be modified by site-directed mutagenesis to place a restriction site at the analogous position in the human sequence. For example, one can clone the complete human k chain C (Ck) region and the complete human g-1 C region (Cg-1). In this case, the alternative method based upon genomic C region clones as the source for C region vectors would not permit these genes to be expressed in bacterial systems where enzymes needed to remove intervening sequences are absent. Cloned V region segments are excised and ligated to L or H chain C region vectors. Alternatively, the human Cg-1 region can be modified by introducing a termination codon thereby generating a gene sequence that encodes the H chain portion of an Fab molecule. The coding sequences with linked V and C regions are then transferred into appropriate expression vehicles for expression in appropriate hosts, prokaryotic or eukaryotic.
In another aspect of the invention, the targeting molecule is a relatively small molecule such as a straight chain or cyclic oligopeptide or peptidomimetic having a molecular weight of about 400 to about 1000 amu. One illustrative cyclic oligopeptide is the cyclic tetrapeptide referred to as cyclic-(RGDyK) that binds to the avb3 receptor expressed on some tumors and on the endothelial cells of tumor neovasculature [Yapp et al., Mol Imaging June 2013 12(4):263-272].
A peptidomimetic is a compound whose essential elements (pharmacophore) mimic a natural peptide or protein in 3D space and retain the ability to interact with the biological target and produce the same biological effect as the natural peptide or protein [Vagner et al., Curr Opin Chem Biol June 2008 12(3):292-296]. An illustrative peptidomimetic of interest here is an inhibitor of prostate-specific membrane antigen (PSMA).
PSMA is a surface type 2 integral membrane glycoprotein with folate hydrolase and carboxypeptidase, and internalization activities [Cimadamore et al., Front Oncol Dec. 21, 2018 8:article 653]. PSMA is highly expressed in prostate cancer tumor cells as well as vessels of various non-prostatic solid tumors.
Monoclonal antibody J591 and the three other anti-PSMA monoclonals are each mouse monoclonals. One or more of those mAbs is the subject disclosed and/or claimed in the following U.S. Pat. Nos. 6,107,090; 6,136,311; 6,649,163; 6,770,450; 7,045,605; 7,112,412; 7,163,680; 7,192,586; 7,514,078; 7,666,414; 7,666,425; and 8,951,737.
Structural and functional homology between N-acetylaspartylglutamate peptidase (N-acetylated-alpha-linked acid dipeptidase;NAAALDASE) has been identified as have several inhibitors for NAAALDASE. One of the most advanced peptidomimetic inhibitors are urea-based PSMA ligands, which usually consist of 3 components: a binding motif (glutamate-urea-lysine [Glu-urea-Lys]), a linker and a radiolabel-bearing moiety (chelator molecule for radiolabeling). A particularly useful such molecule is illustrated below without the chelator.
Illustrative of branched oligosaccharide targeting species are the sialyl-Lewis a (sLea) sialyl-Lewis x (sLex) antigens that bind to selectin receptors on endothelial cells and others that can lead to metastisis. The anti-CD33 mAb, lintuzumab, binds to the sialoadhesin receptor CD33.
Folic acid and derivatives can be used as a targeting species for cancerous kidney cells that overexpress the folic acid receptor. The folate receptor is also overexpressed in cancers of the brain, kidney, lung, ovary, and breast relative to lower levels in normal cells (see, e.g., Sudimack et al., Adv Drug Deliv Rev 2000 41:147-162). Figliola et al., RSC Adv 2019 9:14078-14092, provides a synthetic route for preparation of folate targeting species with the drug prodigiosene using several α,ω-amino linking groups such as ethylene diamine, an ethylene oxide, cystamine, and a diamino oligo oxytheylene.
Immune cells have an affinity for mannose, and several RGD-containing targeting peptides that contain 4 to 30 amino acid residues are known and many are described by Beer et al., Methods Mol. Biol. 2011 680:183-200; Beer et al., Theranostics 2011 1:48-57; Morrison et al., Theranostics 2011 1:149-153; Zhou et al., Theranostics 2011 1:58-82; and by Auzzas et al., Curr. Med. Chem. 2010 17:1255-1299, and Goonewardena et al., U.S. Pat. No. 9,931,412.
A pharmaceutical composition containing a theranostic effective amount of a contemplated targeted radiopharmaceutical dissolved or dispersed in a pharmaceutically acceptable diluent is utilized in a contemplated method. In one embodiment, a theranostic effective amount is a targeted cell-killing effective amount as the treatment is therapeutic. Such a composition is administered in vivo into in a mammalian host animal to bind to and kill unwanted targeted cells such as cancer cells and aberrant immune cells.
Illustrative unwanted targeted cells include cells associated with undesired cell migration, invasion, proliferation, immune response or angiogenesis. Illustrative of such cells are aberrant immune cells and, cancer cells such as those of lung cancer, ovarian cancer, prostate cancer, brain cancer, bladder cancer, head and neck cancer, pancreatic cancer and colon cancer. Treatment of blood cancers such as acute myeloid leukemia that express the CD33 marker, and breast cancers that express the HER2 marker is also contemplated.
A theranostic amount of targeted radiopharmaceutical Q+3 ion administered therapeutically to provide a targeted cell-killing effective amount usually varies with the patient and the severity of the disease such a tumor load in cancer situations that the patient has. However, two to about four cycles of about 80 to about 120 kBq/kg body weight every other month (bimonthly; at about 60-day intervals) typically shows positive results. The use of three cycles of about 100 kBq/kg body weight with the same administration regimen was reported to provide positive results using 225AC-PSMA-617 that utilizes a DOTA-based chelating agent in prostate cancer patients leading to complete remissions in some patients. See, Kratochwil et al., J Nucl Med 2016 57(12):1941-1944; Langbein et al., J Nucl Med 2019 60:13S-19S; and Eder et al., Pharmaceuticals 2022 15:267. Such dosages can be used to provide a basis for dosages for therapeutic treating of other conditions.
For diagnostic purposes, the host is administered a theranostic amount that is a target cell-binding (diagnostic) effective amount of the targeted radiopharmaceutical. The host is thereafter maintained for a time period of about 1 hour to several days, more usually about 1 to about 4 hours, for the radiopharmaceutical to bind to the targeted cells. The maintenance times can depend on several factors such as the decay rate of the trivalent isotope used and the clearance rate of the targeted radiopharmaceutical. The maintained host mammal is thereafter scanned as by a PET scan for positron emissions (PET scan) or by a gamma ray detector (e.g., SPECT scan) to detect and locate the radiation emitted by the target cell-bound targeted radiopharmaceutical, and thereby identify one or more of the following 1) that targeted cells were present in the host, 2) the location in the host body of the targeted cells, 3) the size and possibly 4) the shape of the mass of cells bound by the targeting species.
The diagnostically-effective amount of targeted radiopharmaceutical administered is typically enough radioisotope to provide about 0.5 to about 6 mCi for an adult, and appropriately less for a child. In-111 is typically used at about 111 MBq (3 mCi) to about 222 MBq (6 mCi) for intravenous administration to an average adult (70 kg). Patients can receive Zr-89 at about 0.5 to about 2 mCi by intravenous administration for a whole-body PET scan.
Because a contemplated targeted radiopharmaceutical pharmaceutical composition is intended for parenteral administration as by injection, such a composition should contain an electrolyte, and preferably have approximately physiological osmolality and pH value of the mammalian species intended as the recipient. A preferred concentration of singly charged electrolyte ions in a targeted radiopharmaceutical pharmaceutical composition is about 0.5 to about 1.5% (w/v), more preferably at about 0.8 to about 1.2% (w/v), and most preferably at a concentration of about 0.9% (w/v). The about 0.9% (w/v) concentration is particularly preferred because it corresponds to an approximately isotonic solution for a human. In a further preferred embodiment, the electrolyte in a chemoablative pharmaceutical composition is sodium chloride.
Electrolytes at such levels increase the osmolality of the targeted radiopharmaceutical pharmaceutical composition. Thus, as an alternative to specifying a range of electrolyte concentrations, osmolality can be used to characterize, in part, the electrolyte level of the composition. It is preferred that the osmolality of a composition be greater than about 100 mOsm/kg and less that about 520 mOsm/kg, more preferably that the osmolality of the composition be greater than about 250 mOsm/kg, and most preferably that it be about 300 to about 500 mOsm/kg.
It is preferred that the pH value of the targeted radiopharmaceutical composition be about 4 to about 9, to yield maximum solubility of the targeted radiopharmaceutical in an aqueous vehicle and assure compatibility with biological tissue. A particularly preferred pH value is about 5 to about 8, and more preferably between about 6 to about 7.5.
The pH value of the targeted radiopharmaceutical pharmaceutical composition can be regulated or adjusted by any suitable means known to those of skill in the art. The composition can be buffered or the pH value adjusted by addition of acid or base or the like.
Because a contemplated targeted radiopharmaceutical pharmaceutical composition is intended for parenteral administration route, it is further preferred that it be sterile, such as required for conformance to U.S. Pharmacopeia (USP) <71>, and further that it contains negligible levels of pyrogenic material, such that it conforms to USP <85> (limulus amebocyte lysate assay) or to USP <151> (rabbit pyrogen test), or to substantially equivalent requirements, at a pyrogen or endotoxin level equivalent to not more than (NMT) 10 endotoxin units (EU) per mL. Moreover, the pharmaceutical composition should conform to requirements limiting content of particulate matter as defined in USP <788> (i.e., NMT 3000 particulates greater than 10 microns in size, and NMT 300 particulates greater than 25 microns in size, per container) or substantially equivalent requirements. Each of these references from the USP is incorporated herein by reference.
Illustrative mammalian animal hosts to which a contemplated targeted radiopharmaceutical composition can be administered include a primate such as a human, an ape such as a chimpanzee or gorilla, a monkey such as a cynomolgus monkey or a macaque, a laboratory animal such as a rat, mouse or rabbit, a companion animal such as a dog, cat, horse, or a food animal such as a cow or steer, sheep, lamb, pig, goat, llama or the like.
A contemplated pharmaceutical composition is usually administered a plurality of times to a mammalian host over a period of weeks, or months. As noted, a usual administration regimen is carried out every other month. Screenings of the host between administrations provides updates from which an attending physician can make determinations concerning further treatments. As noted before, a series of three bimonthly (about 60-days apart) administrations of a composition of a different Ac-225-containing targeted radiopharmaceutical pharmaceutical at 100 kBq/kg each produced complete remissions in some prostate cancer patients.
Formulation of parenteral compositions is discussed in, for example, Hoover, John E., Remington's Pharmaceutical Sciences, Mack Publishing Co., Easton, Pa.; 1975 and Liberman, H. A. and Lachman, L., Eds., Pharmaceutical Dosage Forms, Marcel Decker, New York, N.Y., 1980.
For injectable preparations, for example, sterile injectable aqueous suspensions can be formulated according to the known art using a suitable dispersing or wetting compound and suspending materials. The sterile injectable preparation can also be a sterile injectable solution or suspension in a nontoxic parenterally acceptable diluent or solvent, for example, as a solution in 1,3-butanediol. Among the acceptable vehicles and solvents that can be employed are aqueous liquids at ambient temperature such as water, Ringer's solution, and isotonic sodium chloride solution, phosphate-buffered saline. Liquid pharmaceutical compositions include, for example, solutions suitable for parenteral administration. Sterile water solutions of targeted radiopharmaceutical or sterile solution of the targeted radiopharmaceutical in solvents comprising water, ethanol, DMSO or propylene glycol are examples of liquid compositions suitable for parenteral administration.
Sterile solutions can be prepared by dissolving the targeted radiopharmaceutical component in the desired solvent system, and then passing the resulting solution through a membrane filter to sterilize it or, alternatively, by dissolving the sterile compound in a previously sterilized solvent under sterile conditions.
Two bifunctional chelators were purchased from Macrocyclics, Dallas, Tex. The structure of the two are shown below. The Macrocyclics catalog names them as: S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid and 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-4-S-(4-isothiocyanatobenzyl)-3,6,9-triacetic acid. They are also named p-SCN-Bn-DOTA and p-SCN-Bn-PCTA, respectively, as unreacted precursors. Once reacted as with a targeting species, they are more simply referred to as DOTA and PCTA, and are so named herein.
Conjugation reactions with a monoclonal antibody (MNPR-101) were performed in metal-free vials and glassware was acid washed to remove potential metal contamination. Reactions were performed with 2 mg of antibody and increasing molar reactant ratios of bifunctional chelating agents.
Monoclonal antibody (mAb) MNPR-101 is a humanized version of mouse IgG1, k, mAb ATN-658 having ATCC Accession Number PTA-8191 that is disclosed and claimed in U.S. Pat. No. 8,101,726. The mAb MNPR-101 paratope amino acid residue sequence (CDR; complementarity determining region) is the same as that of ATN-658, whereas the framework portions of the variable region are humanized and the Fc portion is that of a human IgG1 antibody.
For PCTA chelators, the molar reaction ratios were 1, 3, 5, 10 and 20. For the DOTA chelators the molar reaction ratios were 3, 10, 25, 50 and 100. The pH of the solutions was adjusted to 9.2 with 1 M Na2CO3. Reactions were run at 37° C. for 1.5 hours.
Bio-Rad 10DG gravity-fed columns with a 6,000 Dalton molecular weight cut-off were used purify the conjugates. The columns were rinsed with 15 mL of 0.1 M HEPES buffer in 0.1M NaCl. The pH of the buffer was 7.3. The total contents of the reaction vials were introduced to the top of the column and collected in 2 mL tubes. Multiple 0.5 mL elutions with the same buffer were also captured in separate tubes. The UV absorbance at 280 nm of each fraction was measured to determine the fractions containing protein. Typically, protein eluted in 4 fractions that were combined. The protein content of the combined fractions was measured using a Pierce BCA assay kit. The concentrations of the protein conjugates produced was about 1 mg/mL.
Analysis of each conjugate was performed with size exclusion HPLC. The column was from IGM Tosoh (TSKgelG3000SWx1; Tosoh Bioscience LLC, King of Prussia, Pa.). The mobile phase was phosphate-buffered saline and the flow rate was 1 mL/minute. A UV detector at 280 nm was used. HPLC results showed an early-eluting peak with about an 8-minute retention time consistent with high purity conjugates. The retention time of the conjugates decreased slightly with higher ratios of bifunctional chelators consistent with the addition of chelants to the antibody.
Conjugates were prepared by the method of Example 1 but with a chelant to protein molar reaction ratios of 12 and 25. Typical reaction yields are about 30%. Thus, average CAR numbers of about 4 and 8 are expected for the reactions.
Ac-225 was obtained from ORNL. The reaction vial contained solid Ac-225 which was dissolved using 0.2 M HCl. The same 4 conjugates as described in the previous section were used to prepare Ac-225 chelates. A ratio of 50 μCi of Ac-225 to 50 μg MNPR-101-PCTA chelant conjugate was used such that if there were a 100% yield, the specific activity would be 1 mCi/mg. Reactions were run in 100 μL volumes. That volume included about 4 μL Ac-225 in 0.2M HCl, 60 μL 0.1 M ammonium acetate buffer, and 36 μL MNPR-101-PCTA or -DOTA conjugate. Reactions were incubated at pH 5.8 and 37° C. for 60 minutes.
The radiochemical yield of the reactions was determined by diluting a 50 μL aliquot of reaction to 3 mL in buffer and passing through a 30 kDa Amicon® filter. Small, non-chelated Ac-225 ions pass through the filter, whereas the conjugate is retained by the filter. Samples were counted on a Ge detector after 45 minutes using the first daughter of Ac-225 (Fr-221). In addition, samples were counted using a dose calibrator after overnight (about 18 hours) equilibration of Ac with its daughters. The results of the chelation are shown below.
The table above shows quantitative yields for both the PCTA conjugates whereas the DOTA conjugates have much lower yields and there is a significant difference between the 12:1 and 25:1 conjugates. Surprisingly, even at low CAR numbers, the PCTA conjugates exhibit high yields.
The specific activity of the chelates formed from PCTA was 1,000 μCi/g, whereas the specific activity for the chelates from conjugates prepared from DOTA ranged from about 216 to 284 mCi/g. This head-to-head comparison between DOTA and PCTA shows the superiority of the PCTA chelators compared to DOTA chelators for chelating Ac.
High Performance Liquid Chromatography (HPLC) using a size exclusion column with phosphate-buffered saline as a mobile phase was used to determine the purity of the above samples. The HPLC data gave practically the same results as the filtration method.
The MNPR-PCTA conjugate of Example 2 with a chelant to antibody starting reaction ratio of 12:1 was chelated with Ac-225. This would produce specific activity of 1 mCi/mg if the reaction were quantitative. The same chelation reaction was performed with the DOTA conjugate of MNPR-101 using a 25:1 chelant to antibody molar reaction ratio. In addition, bovine serum albumin with no chelants added was used as a negative control.
The total volume of each of the reactions was 150 μL. Each of the reactions was measured for yield using the filtration method of Example 2. The percent of the activity in the retentate was used as the yield of the reaction.
In parallel studies, the above reactions were carried out further containing 35 μL of 0.1M diethylenetriamine pentaacetate (DTPA) and the reactions stood at room temperature for one hour. At this time, the filtration method using counts as above was again used to determine the yield or purity. The results of both studies are shown in the Table below.
MNPR-PCTA(12) had an initial yield of 99.1%. After DTPA challenge, the chelate lost only about 1% of the activity. In contrast, the MNPR-DOTA (25) only had an initial yield of 8.9% and that decreased to 5.6% after the DTPA challenge. In addition, the control BSA only showed 3.4% of the activity associated with the protein (non-specific binding) decreasing to 1.3% after DTPA wash. The data are consistent with PCTA outperforming DOTA in the ability to chelate Ac-225 even at a lower CAR ration. In addition, the lack of binding with naked BSA shows that non-specific binding is not an issue.
The conjugate between MNPR-101 (MNPR) and PCTA has been shown to efficiently chelate Ac-225. In a head-to-head comparison, Ac-225 chelated much more efficiently to the PCTA conjugate than with the DOTA conjugate.
Ac-225 was obtained from ORNL. The conjugates used for these reactions were previously prepared and described in Examples 2 and 3, above. Bovine serum albumin (BSA) was used as a negative control protein without any chelators attached to it. MNPR-PCTA(12) refers to the MNPR-101 conjugate made with PCTA with a starting molar reaction ratio of 12:1 p-SCN-Bn-PCTA (PCTA) to antibody. MNPR-DOTA(25) refers to the conjugate of MNPR-101 with a starting molar reaction ratio of 25 p-SCN-Bn-DOTA (DOTA) chelators to antibody.
Reactions were targeted to produce 1 mCi/mg assuming 100% incorporation of the Ac into the antibody. The reactions were run in 150 μL volumes and incubated at pH 5.8 and 37° C. for 60 minutes. Following the reaction, 35 μL aliquots of each reaction were mixed with 35 μL of 1M diethylenetriamine pentaacetate (DTPA) and allowed to stand at room temperature for 1 hour.
The solutions were tested for the percent Ac-225 associated with the protein by filtration (counts) as described above as a function of time (1, 24 and 72 hours). The results of the initial study are shown in the table below as the percent Ac-225 associated with the protein as a function of time.
The percentage designates the relative amount of activity in the filter compared to the total (filter+filtrate). MNPR-PCTA(12) gave the best results with 99% and 98% attached to the antibody (on the filter) after 1 and 24 hour incubation with DTPA. The purity dropped to 73% after 72 hours. Note that there is no radioprotectant added and Ac-225 gives a high radiation dose to the solution. The fact that the isotope remained associated with the protein shows a high degree of stability, and that there was little loss of the bismuth decay product to the solution during that time periods examined.
Both the control (BSA) and MNPR-DOTA(25) have significantly lower percentages of the activity associated with the protein. High resolution gamma spectroscopy analysis of the solutions was consistent with the filtration results in Table 1.
The antibody MNPR-101 conjugated with PCTA with a starting chelator to antibody molar reaction ratio of 12:1 was shown to reproducibly chelate Ac-225 in high yield and high specific activity (1,000 μCi/mg). Incubation of the material in excess DTPA showed a high degree of stability even when the formulation did not contain any radioprotectant. A head-to-head comparison with the same antibody conjugated with DOTA with a starting ratio of 25:1 ligand to protein molar ratio gave much lower yields showing the advantage of PCTA over DOTA for chelating Ac-225. Naked BSA was used as a control showing low amounts of non-specific binding.
A targeted radiopharmaceutical containing Ac-225 chelated by PCTA bonded to mAb MNPR-101 as illustrated by Formula I, where Q+3, is 125Ac+3, was prepared as described earlier. The starting molar ratio of chelator to antibody was 12 to 1. Fifty μCi of Ac-225 was combined with 50 μg of the MNPR-PCTA conjugate and the pH value adjusted to 5.8 with ammonium acetate for 60 minutes at 37 C. The total volume of the reaction was 100 μL.
A volume of 25 μL of the reaction mixture was analyzed on high performance liquid chromatography using a size exclusion column. The mobile phase was 0.1 M phosphate buffer at pH=7.4 and the flow rate was 1 mL/minute. Detection was by UV absorption at 280 nm and also by radiometric detector.
Evaluation of the UV and radiometric detector showed the radioactivity co-eluting with the protein. A size exclusion column separates chemicals based on size. Because most of the radioactivity from a solution of Ac-225 comes from its radioactive daughters, we would expect radioactive metals that are not attached to the protein to elute at a later time. There was no radioactive signal with retention times consistent with small molecules.
This result is consistent with the MNPR-101-PCTA conjugate chelating radioactive Ac-225 daughters such as Bi-213. Not to be bound by theory but the excellent binding properties of the PCTA conjugate are believed to be a result of the chelator binding not only Ac-225 but daughters such as Bi-213 being trivalent and/or not radioactive.
Bismuth ions can form very insoluble compounds that could precipitate carrying both bismuth and actinium ions. Prevention of bismuth compounds precipitation by the mAb-linked-PCTA chelating functionality provides another benefit of this invention.
Similar size exclusion column studies using DOTA as the chelating agent linked to mAb MNPR-101 show different results. Thus, when DOTA is used, the on-line radiation detector shows very little signal associated with the protein and most of the activity in later-eluting peaks that are indicative of radioactive metals that are not attached to the protein.
PCTA conjugates were prepared with humanized mAb MNPR-101 in parallel with two other illustrative mouse monoclonal antibodies: mAb ATN-616 and mAb ATN-292. The chelator to protein molar ratios of 12 and 75 were used to optimize subsequent chelation of Ac-225.
MNPR-101 and ATN-616 were conjugated with PCTA at the molar reaction ratio of 12:1, whereas ATN-292 was conjugated at a 75:1 excess. The pH values of the solutions were adjusted to 9.2 with 1 M NaH2CO3 and 0.2 M HCl. Reactions were run at 37° C. for 1.5 hours.
The conjugates so formed were purified using Bio-Rad 10DG gravity-fed columns (6,000 Dalton (Da) molecular weight cut-off) in which each conjugate was eluted with 0.1 M ammonium acetate buffer, pH 5.77. Eluted fractions (0.5 mL) were collected in 1.5 mL metal-free tubes and were measured at UV absorbance 280 nm. 3 or 4 fractions were combined, depending on concentration of protein in the eluant, and re-concentrated using Amicon® concentrators (30 kDa). Combined fractions were analyzed using a Pierce™ BCA Assay Kit (Thermofisher; Final protein concentrations were about 2-3 mg/mL).
Size-exclusion high performance liquid chromatography was utilized to analyze conjugate purity, as previously described, with phosphate-buffered saline solvent and flow rate of 1 mL/minute. HPLC results revealed the expected about 8-minute peaks observed from the naked antibodies and decreased retention time (Rt) of the conjugates consistent with addition of the bifunctional chelator PCTA.
Results suggest that the increase in retention time (ΔRt) observed between the conjugates and respective naked mAb(s) is related to the conjugates' subsequent ability to chelate Ac-225, in that a greater ΔRt correlates to a higher number of chelating agents bonded to the antibody. Differences in retention times from the three conjugates and naked antibodies are shown below.
A reaction vial containing solid Ac-225 was obtained from ORNL and was dissolved using 0.2 M HCl. The three conjugates from Example 6 were used to prepare Ac-225 chelates. A ratio of 100 μCi of Ac-225 to 100 μg mAb-PCTA chelant conjugate for all reactions was used such that if there were a 100% yield, the specific activity would be 1 mCi/mg. Reactions were run in about 110 μL volumes including approximately 10 μL Ac-225 in 0.2 M HCl, 60 μL 0.1 M ammonium acetate buffer, and 40 μL mAb-PCTA conjugate, dependent upon and normalized against each protein concentration. Reactions were incubated at pH 5.7 and 37° C. for 60 minutes.
25 μL aliquots of each chelation reaction were purified by eluting 0.5 mL fractions on Bio-Rad 10DG gravity-fed columns with 0.1 M ammonium acetate buffer. The Ac-labeled conjugate is expected to elute in 3-4 “peak fractions”, which are summed against the activity remaining on the column to determine radiochemical yield. After a minimum of 5 hours (to permit Ac equilibration with its daughters), the fractions and respective columns were measured on the dose calibrator (Capintec, setting #086). Results from each chelations' gravity-fed fractions measured on the dose calibrator are shown in the following tables.
Peak fractions from each reaction were measured at time points 24 hours and 48 hours post-purification to determine the chelants' ability to retain Ac-225 daughter isotopes. Peak activity increasing as a function of time provides evidence that the chelants did not effectively control the daughter isotopes. However, if activity decreased at a rate consistent with Ac-225 degradation, evidence suggests that the chelants were able to retain Ac-225 and its daughters.
Results observed in the tables below provide evidence of the three chelating systems effectively controlling Ac-225 daughters, as each peak activity does not exhibit a radioactivity increase as a function of time.
20 μL samples of each chelation reaction were analyzed via HPLC using an isocratic method (1×PBS solvent, pH 7.4) with detection method via UV absorption at 280 nm and radiometric detector. 1 mL fractions were collected per minute and permitted to equilibrate (>5 hours) then were measured on a NaI detector with a wide window.
As observed in Example 5, HPLC results showed the radioactivity co-eluting with the proteins from the three reactions. There was no radioactive signal with a retention time consistent with a small molecule, further supporting the inference that the PCTA chelator binds Ac-225 and its daughters. The results of each reaction yield are shown in the table below:
Although the Peak Yields of the three reactions when analyzed by the dose calibrator show 80.6%, 77.0%, and 75.5%, respectively, as described in Example 7, these same reactions show Reaction Yields of 96.2%, 92.7%, and 97.8%, respectively, when analyzed using HPLC purification and NaI detection. This variance is understood to stem from a lack of the ability to measure activity remaining on the size-exclusion column, thus observing the more conservative yields from the Bio-Rad gravity-fed columns and dose calibrator values.
The methods and results described suggest the bifunctional chelator in question, PCTA, shows remarkable ability to bind Ac-225 not only with humanized mAb MNPR-101, but also when linked to other antibodies as well, such as the two mouse monoclonal antibodies mAb ATN-616 and mAb ATN-292.
Humanization of Variable (V) Region Amino Acid Residue Sequence of Mouse mAb ATN-658 The consensus amino acid sequence (single-letter code) of the light chain variable region (VL) and heavy chain variable region (VH) polypeptides of mAb ATN-658 are set out in U.S. Pat. No. 8,191,726 to Parry and Mazar, will not be repeated here and are incorporated by reference. cDNA was prepared from total RNA extracted from the hybridoma expressing ATN-658 and the variable regions were cloned, amplified and sequenced using standard techniques.
Following the course set out by Studnicka et al., above, human V Kappa light chain subgroup 2 (VK2) and human heavy chain subgroup 1 (VH1) consensus sequences were utilized. The cognate mouse signal sequences were retained.
Two sequences for each of the light chain and the heavy chain variable regions were prepared. One sequence for each chain contained only low risk changes and the other sequence that contained both the low risk and the moderate risk changes were prepared for the VK2 and VH1 regions, providing a total of four sequences. Ten low risk and 1 moderate risk changes were introduced into the light chain framework sequences and 11 low risk and 5 moderate risk changes were introduced into the heavy chain framework sequences. Low risk residue position changes, those exposed to solvent but not contributing to antigen binding or antibody structure, are likely to decrease immunogenicity with little or no effect on binding affinity.
The amino acid residue sequences were sent to Blue Heron Biotech LLP, (Bothell, Wash.) for codon (Chinese Hamster Ovary cells) and expression optimization. The optimized DNA sequences were received and sent back to Blue Heron for gene synthesis.
Transient Expression Vector Construction
Codon- and expression-optimized low risk and low plus moderate risk Human Engineered™ light chains and heavy chains were cloned in-frame into XOMA's proprietary transient antibody expression vectors that contain human Kappa and Gamma-1 constant region modules. The DNA sequences were verified (at ELIM Biopharmaceuticals, Inc., Hayward, Calif.) prior to initiating expression.
Production of Human Engineered™
ATN-658 Antibodies
The four HE™ ATN-658 variants (referred to as HE™ ATN-1, HE™ ATN-2, HE™ ATN-3 and HE™ ATN-4) were produced by transient transfection in HEK293E cells. XOMA's transient transfection approach is described in detail in a poster presented at the 2005 ASCB Annual Meeting.
Briefly, the light and heavy chains were co-transfected into XOMA's suspension-adapted HEK293E cells grown in IS293 medium (Irvine Scientific, Irvine, Calif.) using 2 liter shake flasks. After 24 hours in shake flasks, 200 ml of transfected cells were centrifuged, resuspended in 40 ml of fresh medium and transferred to Integra flasks (Wilson Wolf Manufacturing, Inc., New Brighton, Minn.) for production. After incubation for seven days, the cell suspensions were removed from the Integra flasks, centrifuged and the culture supernatants retained. Antibodies in the culture supernatants were purified on protein A spin columns (Pro-Chem), dialyzed against PBS, concentrated and sterile filtered.
The variable region constituent sequences of those four antibodies are illustrated in Table 1, below.
Concentration was determined by A280 using an extinction coefficient of 1.52. The proteins were analyzed for purity by SDS-PAGE (4-20%) and for endotoxin using an LAL assay. Purification results demonstrate that all of the antibody preparations had concentrations ≥1 mg/ml, were >90% pure and had low levels of endotoxin (<1 EU/mg).
Evaluation of Affinity of Human Engineered™
ATN-658 Antibodies by Biacore Assay Method
Kinetics analysis of mouse monoclonal antibody ATN-658 and Human Engineered™ ATN-658 variant antibodies was conducted on a Biacore 2000® surface plasmon resonance instrument analyzer (Uppsala, Sweden) to produce sensograms based on the antibody-surface interactions. Kinetic determinations were performed using a capture method.
Mouse parental mAb ATN-658 was diluted in PBS to 2 mg/mL and injected over a rabbit anti-mouse capture surface. The HE™ variants were diluted to 1 mg/mL and injected over a protein A/G surface. Antibody injections were optimized to produce antibody densities of 100-200 RU.
Six serial 3-fold dilutions of soluble UPAR (suPAR) were prepared in running buffer (PBS), and each dilution was injected in triplicate in random order at 25° C. Buffer injections were evenly distributed throughout the run. The sample injections were double-referenced against the blank flow cells and buffer injections to correct for any bulk shift or non-specific binding. Data were analyzed with BiaEvaluation software from Biacore®. Sensorgrams were fit utilizing a 1:1 Langmuir model.
Humanized mAb MNPR-101
As compared to mAb ATN-658, one residue was changed in one CDR of each of the VK2 and VH1 regions in mAb MNPR-101 as compared to the CDR sequences of mAb ATN-658 (CDR L1 and CDR H2) in arriving at the six CDRs of mAb MNPR-101. The complementarity-determining regions (CDRs) for each variable region that are present in paratropic regions of mAb MNPR-101 and are set out in Table 2, below.
Sequences
Sequences for the VL and VH as well as the CL and CH regions of the Fab portion of mAb MNPR-101, and also the low risk sequences of the variable regions of both chains (HE™ ATN-1) are shown below.
A SalI restriction site was placed in frame and up-stream of the encoded N-terminus of each of the heavy and light chains and a XhoI site was inserted in frame and down-stream from the encoded C-terminus of each chain for insertion of coding nucleic acids into their expression vectors.
The heavy and light chain of the monoclonal antibody candidate were packaged in a pUC19 plasmid. cDNA inserts encoding the monoclonal antibodies were cloned out and heavy and light chains were inserted into expression vectors.
After confirmation of the sequences, the DHFR-deficient CHO cell line DUX B11 was transfected with light chain and heavy chain containing vectors and a cationic liposome mixture (Lipofectamine® 2000; Invitrogen Corp., Carlsbad, Calif.). Forty-eight hours after transfection, cells were subcloned in 96 well dishes using a purine-free growth medium in the presence of geneticin (G418) and 20 nM methotrexate (MTX).
After selection, all subclones were screened using a hIgG Bethyl ELISA kit. Three vials were frozen down for each of the 12 best subclones. The top 6 best producing subclones were then transferred to a medium supplemented with increasing amounts of methotrexate (MTX), an inhibitor of DHFR. MTX concentrations were sequentially increased from 20 to 1,000 nM during the selection process and then to 1,500 nM MTX. The MTX-resistant clones that grew out were screened by ELISA. After a first series of amplification, the two highest expressing population subclones were obtained in medium containing 1,000 nM MTX. These two clones were amplified up to 1,500 nM MTX before being subcloned at 1,000 nM and 1,500 nM MTX. These subclones are currently being expanded to 6 well plates and will be screened by ELISA in the next few days. The top 2-3 best subclones will be then expanded for the production of a Research Cell Bank after adaptation to serum free medium.
Results
The ligand-binding kinetics of mouse mAb ATN-658 and the above discussed Human Engineered™ ATN-658 antibodies were measured once. The sensorgram results of individual assays indicated that all of the transiently-expressed antibodies displayed a similar affinity with mAb ATN-658 as well as among themselves. Results for the four combinations of two VL and two VH chains are shown below in Table 3.
Antibody HE™ ATN-4 was renamed MNPR-101.
An initial study of the chelation characteristics and stability of In-111 using a contemplated PCTA-MNPR-101 chelator-targeting species. Thus, PCTA-MNPR-101 (produced at 12:1) freshly prepared in an aqueous solution at a pH value of 9.2 (1M NaHCO3 and HCl) that contained 4.0 mg/mL by protein analysis was incubated for 1.5 hours at 37° C. The conjugate (220.0 mL MNPR-101-PCTA) was purified by passage through a PD10 column with elution using 0.1M ammonium acetate. Samples containing the conjugate were collected and concentrated using a 30 kDa Amicon® concentrator (4000 rpm for 20 minutes).
Three aqueous chelation reactions were set up, each with activity of about 200 μCi for a target specific activity of 10 mCi/mg. Each was mixed with In-111 chloride obtained from BWXT Medical, Ottawa, ON, Canada. All reactions were stored at 4° C. and assayed for stability after 24, 48 and 72 hours.
Stability in this context is the maintenance of radioactive ion chelation over time. Stability was determined by gravity fed SEC column (PD10 6,000 Dalton cut off), HPLC and TLC for comparison.
Three aqueous chelation reactions were set up, each with activity of about 200 μCi for a target specific activity of 10 mCi/mg. These were as follows:
1) Incubated at 37° C. for 30 minutes. Stored at 4° C. for 72 hours.
2) Incubated at room temperature for 30 minutes. Stored at 4° C. for 24 hours.
3) Incubated at room temperature for 1.5 hours. Stored at 4° C. for 48 hours.
The results of this initial study are shown in the Table below.
The results of this initial study showed that relatively high yields of chelation were obtained at 10 mCi/mg targeted specific activity. Conditions could likely be optimized to increase yields. Each of the three different analytical methods showed that a chelate was formed. Given that the half-life of Indium-111 is about 2.8 days, reasonable chelated In-111 stability was observed.
Each of the patents, patent applications and articles cited herein is incorporated by reference. The use of the article “a” or “an” is intended to include one or more.
The foregoing description and the examples are intended as illustrative and are not to be taken as limiting. Still other variations within the spirit and scope of this invention are possible and will readily present themselves to those skilled in the art.
This application claims priority to U.S. application Ser. No. 63/191,499 and Ser. No. 63/191,506, both filed on May 21, 2021, whose disclosures are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63191499 | May 2021 | US | |
| 63191506 | May 2021 | US |
