Patent Application
20170232110
The invention provides a peptide-polymer conjugate for use in treating malaria.
This application claims priority to South African provisional patent application number 2014/05884, which is incorporated by reference herein.
Malaria is endemic in 93 third world countries, with an estimated 198 million malaria cases being reported annually (http://www.who.int/malaria/media/world_malaria_report_2014/en/) and resulting in about 367,000-755,000 deaths per year. About 90% of deaths due to malaria occur in Sub-Saharan Africa, and children under the age of 5 years account for more than 70% of these deaths.
Chloroquine and its derivatives and the arternisinins are the main drugs for treating malaria, but global resistance to these drugs is on the increase. For example, chloroquine-resistant strains of Plasmodium falciparum (the malaria parasite responsible for most malaria cases) have spread to most malaria areas. There have also been reports of field strains of P. falciparum demonstrating in vitro resistance to the artemisinins.
There is therefore an urgent need for an alternative treatment for malaria, especially a last resort antimalarial drug for treating patients (mostly children) with advanced resistant malaria.
Cyclic decapeptides such as the tyrocidines and gramicidin S (i.e. peptides with 10 amino acids forming a ring) are known to have potent antiplasmodial activity. The closely-related tryptocidines and phenycidines, which are cyclic decapeptides with a similar structure, also fall within this group. However, the cyclic decapeptides are considered to be unsuitable for antimalarial treatment as they are haemolytic. Although having a 10-300 fold selectivity towards infected red blood cells, they also lyse healthy red blood cells and other cells, even at micromolar peptide concentrations. They are therefore toxic, especially in a systemic method of treatment.
According to a first embodiment of the invention, there is provided a peptide-polymer conjugate comprising a polymer to which an antiplasmodial peptide is covalently attached, wherein the peptide is a cyclic decapeptide having an amino acid sequence of cyclo(Val-X1-Leu-D-Phe-Pro-X2-X3-X4-X5-X6) (SEQ ID NO: 1), where:
The cyclic decapeptide may be a tyrocidine with the sequence cyclo(Val-X1-Leu-D-Phe-Pro-X7-X8-Asn-Gln-X9) (SEQ ID NO: 2) or an analogue or derivative thereof, where:
Alternatively, the cyclic decapeptide may be gramicidin S or a derivative or analogue thereof, with the amino acid sequence of cyclo(Val-X1-Leu-D-Phe-Pro-Val-X1-Leu-D-Phe-Pro) (SEQ ID NO: 3).
The cyclic decapeptide may be a derivative or analogue of a tyrocidine, tryptocidine, phenycidine or gramicidin S, with one or more of the following amino acid substitutions:
The cyclic decapeptide may have an amino acid sequence selected from any one of SEQ ID NOS: 6-177.
The peptide-polymer conjugate may be produced from a mixture of any two or more peptides of SEQ ID NOS: 6-177.
The polymer may be a water-soluble and biocompatible polymer, and may have a thiol end group. For example, the polymer is selected from the group consisting of poly(N-vinylpyrrolidone), poly(ethylene oxide), poly((ethylene oxide)-co-(propylene oxide)), poly(oligo(ethylene oxide)acrylate), poly(2-hydroxypropyl acrylamide) and poly(oligo(ethylene oxide)methacrylate). In a preferred embodiment, the polymer is poly(N-vinylpyrrolidone).
The peptide may be conjugated to the chain end of the polymer, such as by way of a linkage that is unstable at acidic pH. The linkage is formed via an acrylate ester or via an acrylamide.
The peptide-polymer conjugate may further include a hydrophilic targeting ligand. The targeting ligand may be a peptide which specifically binds to proteins or cell surface markers on the cell surface of red blood cells, and more specifically may be a peptide which specifically binds to proteins or cell surface markers on the cell surface of red blood cells infected with malaria parasites.
The targeting ligand may be selected from peptides having amino acid sequences of SEQ ID NOS: 178-185 and sequences which are at least 50%, at least 60%, at least 70%, at least 80% or at least 90% similar. In one embodiment, the targeting ligand is a peptide having an amino acid sequence which is at least 50% similar to SEQ ID NO: 178, and more particularly is a peptide of SEQ ID NO: 178.
According to a second embodiment of the invention, there is provided a pharmaceutical composition or formulation comprising the peptide-polymer conjugate as described above and a pharmaceutically acceptable carrier.
The peptide-polymer conjugate or pharmaceutical composition or formulation as described above may be for use in treating malaria.
According to a third embodiment of the invention, there is provided the use of a peptide-polymer conjugate as described above in a method of making a medicament for treating malaria.
According to a fourth embodiment of the invention, there is provided a method of treating malaria, the method comprising administering a therapeutically effective amount of a peptide-polymer conjugate or pharmaceutical composition or formulation as described above to a patient in need thereof.
The therapeutically effective amount of the peptide-polymer conjugate may comprise an amount of the peptide which is sub-therapeutic for treating malaria when the peptide is administered in an unconjugated or unmodified form.
According to a fifth embodiment of the invention, there is provided a process for preparing a
The acrylate or acrylamide functionalised peptide may be conjugated to the polymer by covalently linking an acrylate ester on a tyrosine residue or an acrylamide on a lysine or ornithine residue of the peptide to a terminal thiol on the polymer chain via Michael addition. Preferably, the resulting beta-thioether will cause the ester linkage or the amide linkage to be hydrolytically unstable at acidic pH.
The process may further include the step of conjugating a targeting ligand to the polymer.
The cyclic decapeptide, polymer and targeting ligand may be selected from those described above.
A peptide-polymer conjugate is described herein, wherein the conjugate is formed from a polymer to which an antiplasmodial peptide is covalently attached. The peptide is a cyclic decapeptide having an amino acid sequence cyc/o(Val-X1-Leu-D-Phe-Pro-X2-X3-X4-X5-X6) (SEQ ID NO: 1), where
Peptide-polymer conjugates of the invention have antiplasmodial activity against Plasmodium falciparum and analogous parasites (e.g. human malaria parasites including: P. vivax, P. ovale, P. malariae, P. knowlesi, P. brasilianum; P. cynomolgi, P. cynomolgi bastianellii, P. inui, P. rhodiani, P. schweitzi, P. semiovale and P. simium; primate malaria parasites other than those already listed above including : P. reichenowi, P. pitheci, P. silvaticum and P. fragile; non-primate malaria parasites including: P. atheruri ; P. berghei; P. bubalis; P. caprae, P. cephalophi; P. cyclopsi; P. odocoilei and P. voltaicum; avian malaria parasites including: P. relictum, P. anasum, P. hermani, P. gallinaceum, P. cathemerium and P. circumflexum; and others listed at https://en.wikipedia.org/wiki/Plasmodium), without the toxicity associated with unconjugated cyclic decapeptides, and are therefore useful for treating malaria in mammals.
Tyrocidines, tryptocidines, phenycidines and/or gramicidin S are β-sheet cyclic decapeptides, with the tyrothricin cyclodecapeptides (the tyrocidines, tryptocidines and phenycidines) being produced by B. aneurinolyticus (also known as Brevibacillus parabrevis) and gramicidin S being produced by A. migulanus (previously known as B. brevis). These peptides have high sequence identity, are highly conserved and adopt a similar backbone conformation/molecular topology. Tyrothricin (a tyrocidine/tryptophan/phenycidine-gramicidin complex, where the gramicidins are linear neutral 15-mer peptides not related to gramicidin S) was the first antibiotic to be used in clinical practices, but later fell into disrepute due to its haemolytic toxicity (Dubos and Cattaneo, 1939, J. Exp. Med. 70: 249; Hotchkiss and Dubos, 1941, J. Biol. Chem., 141: 155; Bradshaw, 2003, Biodrugs, 17: 233-240).
Tyrocidines and gramicidin S have a common sequence of Val-Orn-Leu-D-Phe-Pro (SEQ ID NO: 4), where the cationic residue can either be ornithine or lysine. The valine and leucine residues can also be substituted for leucine, isoleucine and valine for tyrocidines. The complete gramicidin S sequence is a repeat of the highly conserved sequence of cyclo(Val-Orn-Leu-D-Phe-Pro)2 (SEQ ID NO: 4). The complete tyrocidine sequence also contains this highly conserved sequence, but instead of a repeat thereof it is followed by a variable pentapeptide moiety, Phe-D-Phe-Asn-Gln-Tyr (SEQ ID NO: 5) or a derivative or analogue thereof. Any one or more of the three aromatic residues in the variable peptide moiety can be substituted with tyrosine, phenylalanine or tryptophan, giving rise to tyrocidines, phenycidines and tryptocidines. There are four tyrocidines (A-D), which may be used in this invention in a purified form or in a mixture of any two or more tyrocidines. Tyrocidine A has the sequence
The primary chemical structures of gramicidin S and tyrocidine A (one of the tyrocidines) are shown below:
The cyclic decapeptides of the present invention are known tyrocidines, tryptocidines, phenycidines or gramicidin S, or derivatives or analogues thereof, which have a highly conserved amino acid sequence comprising Val-X1-Leu-D-Phe-Pro-X2-X3-X4-X5-X6 (SEQ ID NO: 1), where X1 is ornithine or lysine, or a derivative or analogue thereof.
Suitable analogues of the tyrocidines, tryptocidines, phenycidines or gramicidin S may be those including any one or more of the following substitutions:
More preferably, the cyclic decapeptide derivatives may be one or more of the peptides selected from the group consisting of:
Tyrocidine Analogues:
Tryptocidine Analogues:
Phenycidine Analogues:
Gramicidin S Analogues:
In the sequences above, standard upper case abbreviations denote
References herein to “cyclic decapeptides” refer to the sequences stated above and analogues or derivatives thereof.
The cyclic decapeptides can be produced by their natural bacterial producers, by genetically modifying a suitable microorganism or by using an organic/semi-synthetic system. The amino acid residues in the derivatives or analogues can separately or in combination be replaced in the core cyclic decapeptide sequence (cyclo(Val-Orn-Leu-
In the present invention, the cyclic decapeptide can be covalently attached to the chain end of a polymer, which is preferably a hydrophilic and biocompatible polymer with a terminal thiol (although a person skilled in the art will understand that chemical linkers may alternatively be used if the polymer does not have a thiol end group). The covalent link can be formed by an acrylate ester on the tyrosine residue of a tyrocidine or by an acrylamide on the lysine or ornithine residue on tryptocidines/phenycidines/tyrocidines and gramicidin S, and by subsequent linking via Michael addition to a terminal thiol on the polymer chain. The beta-thioether causes the ester linkage or the amide linkage to be hydrolytically unstable at acidic pH.12 The peptides may be modified to have more than one acrylate ester or acrylamide.
For example, the acrylate can be introduced onto the tyrosine residue of tyrocidine with an acid chloride derivative of acrylate (e.g. acryloyl chloride) or an activated ester of acrylic acid via ester formation, using DMF or DMSO as a solvent and an organic base such as triethylamine or diisopropylethylamine (DIPEA) as a catalyst. The temperature of this reaction should be kept below 40° C. to avoid polymerisation of the acrylate, and more particularly should be kept at about 0° C. to prevent side reactions. The thiol-Michael reaction is a “click” reaction which can be performed under “green” conditions. In one method of carrying out this reaction, the polymer and cyclic decapeptides with an acrylate derivative or acrylamide derivative are reacted in a solvent such as water or water/DMF and a catalyst such as a nucleophilic phosphine (e.g. Tris(2-carboxyethyl)phosphine (TCEP) or tributyl phosphine) or an amine (e.g. triethylamine, hexylamine or ethylenediamine). The reaction can be performed at ambient temperature or a temperature in the range of from about 20° C. to about 40° C.
Suitable polymers include poly(N-vinylpyrrolidone), poly(ethylene oxide), poly((ethylene oxide)-co-(propylene oxide)), poly(oligo(ethylene oxide)acrylate), poly(oligo(ethylene oxide)methacrylate), poly(2-hydroxypropyl acrylamide) or any other suitably reactive water-soluble polymer. In one preferred embodiment, the polymer is poly(N-vinylpyrrolidone) (PVP).
The polymer preferably has a number average molecular weight between 1,000 and 20,000 g/mol, more preferably between 1,000 and 10,000 g/mol and most preferably between 1,000 and 6,000 g/mol.
The polymer chains can optionally be decorated with a peptide sequence that specifically targets an epitope on red blood cells, which may be only those red blood cells infected with a plasmodial parasite or normal red blood cells too (typically, about 1% of the polymer chains will be decorated with these targeting ligands). The targeting peptide (also referred to herein as the targeting ligand) is typically hydrophilic.
Examples of suitable targeting ligands include, but are not limited to, the peptides which Eda et al. 6 have previously described as targeting proteins displayed on the cell surface of malaria parasite-infected red blood cells (Gly-Ser-Arg-Ser-Lys-Gly-Thr (SEQ ID NO: 178), Leu-Val-Asp-Ala-Ala-Ala-Leu (SEQ ID NO: 179), Pro-Ile-Ala-Leu-Gly-Leu-Lys (SEQ ID NO: 180), Gly-Gly-Pro-Leu-Lys-Gly-Leu (SEQ ID NO: 181), Ile-Asn-Leu-Gly-Leu-Thr-Met (SEQ ID NO: 182), Phe-Ser-Leu-Gly-Leu-Ile-Lys (SEQ ID NO: 183), Pro-Ala-Tyr-Lys-Leu-Tyr-Ser (SEQ ID NO: 184) or Asn-Ser-Val-Gly-Gly-Arg-Ser (SEQ ID NO: 185)), or peptides which have amino acid sequences which are at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% similar thereto. In one embodiment, the targeting ligand has the amino acid sequence Gly-Ser-Arg-Ser-Lys-Gly-Thr (SEQ ID NO: 178) (
Due to the self-assembling nature of the more hydrophobic cyclic decapeptide and the hydrophilic nature of the polymer and targeting ligand, the conjugate is amphiphilic and self-assembles into micelles (measurements show that they are around 20 nm in diameter (
The micelles are efficiently taken up in red blood cells, with the peptide-polymer conjugate acting as a “Trojan-horse” drug carrier. In the case of parasite-infected erythrocytes, the micelles are subsequently taken up by the parasite and end up in their food vacuole where the pH is below 6 (about pH5.5). This is sufficient for the hydrolytically unstable ester or amide to be cleaved and for the cyclic decapeptide to be liberated inside the cell. After cleavage of the conjugate, the peptide reverts to its original form and is able to lyse the malarial/plasmodial parasite within the red blood cell. The polymer is excreted from the cell and body of the mammal.
Two versions of the conjugate of the invention were synthesized, using tyrocidine as the cyclic decapeptide and poly(N-vinylpyrrolidone) (PVP) of two different lengths as the polymer. These were tested in an anti-malaria assay (Malstat assay) and an haemolytic assay, and it was found that the concentration of a tyrocidine/tryptocidine mixture required to kill malaria in infected red blood cells dropped by a factor of close to 500 (it was reduced to the pg/mL range) when delivered via the conjugate compared to the administration of free tyrocidine. More importantly, at a concentration where all the malaria infected cells were killed within 24 hours, healthy red blood cells were unaffected.
Thus, the peptide-polymer conjugate of the invention was shown to be close to 500 times more potent than the tyrocidine/tryptocidine mixture on its own and is highly selective at therapeutically relevant concentrations. The therapeutically active drug loading is also exceptionally low (below 50 ng/mL), which could also have cost benefits or reduce potential side effects of the peptide-polymer conjugate or the cyclic decapeptides.
As the malaria assay was performed on a chloroquine-resistant malaria strain, the peptide-polymer conjugate could also be useful in treating drug-resistant malaria strains.
The invention will now be described in more detail with reference to the following non-limiting examples:
All chemicals and solvents were purchased from commercial sources and used without further purification, unless stated otherwise. 2,2′-azobis(isobutyronitrile) (AIBN) (Riedel-de Haën) was recrystallized from methanol and dried under vacuum at ambient temperature. All solvents and monomers were dried and distilled before use. Reactions were monitored using thin layer chromatography (TLC), utilising Machery-Nagel Silica gel 60 plates with a UV 254 fluorescent indicator. Dialysis tubing, MWCO 2000, was purchased from Sigma Aldrich. Moisture and oxygen sensitive reactions were carried out in an inert argon atmosphere. 1H-NMR and 13C-NMR spectra were recorded on a Varian VXR-Unity (400 MHz) spectrometer. Samples were prepared in deuterated solvents obtained from Cambridge Isotope labs. Chemicals shifts were reported in parts-per-million (ppm), referenced to the residual solvent protons. Size exclusion chromatography (SEC) was measured on a system that comprised a Shimadzu LC-10AT isocratic pump, a Waters 717+autosampler, a column system fitted with a PSS guard column (50×8 mm) in series with three PSS GRAM columns (300×8 mm, 10 μm, 2×3000 Å and 100 Å) kept at 40° C., a Waters 2487 dual wavelength UV detector and a Waters 2414 differential refractive index (DRI) detector. Dimethylacetamide (DMAc) was used as the eluent, stabilized with 0.05% BHT (w/v) and 0.03% LiCl (w/v), at a flow rate of 1 mL.min−1. The polymer samples were filtered through 0.45 μm GHP filters, to remove impurities, prior to analysis. Calibration was carried out using poly(methyl methacrylate) (PMMA) standard sets (Polymer Laboratories) ranging from 690 to 1.2×106 g/mol. Data acquisition was performed using Millennium32 software, version 4.
Fluorescence spectroscopy was performed on a Carl Zeiss Confocal LSM 780 Elyra S1, equipped with a LSM780 GaAsP detector, using an alpha Plan-Apochromat 100×/1.46 oil DIC objective. Samples were excited with a 488 nm laser with utilisation of a MBS 488/561 beam splitter. Images were acquired through z-stacking with an increment of 0.3 μm step width, and projected as maximum intensity projections using ZEN software (black edition, 2011).
Ultra performance liquid chromatography coupled to electrospray mass spectroscopy (UPLC-MS) was measured on a system that comprised a Waters Acquity Ultra Performance Liquid Chromatograph coupled to a Waters Q-Tof Ultima mass spectrometer fitted with a Z-spray electrospray ionisation source. The system was tailored with a Waters UPLC BEH C18 column (2.1×50 mm, 1.7 μm spherical particles, Millipore-Waters, La Jolla, USA). For direct injections and ESMS analysis, 3 μL of the sample solution (±50 ng peptide in 50% MeCN) were injected each time. For UPLC, 1-3 μL of the sample solution (±50 ng peptide in 50% MeCN) was injected each time. Separation was achieved using a 0.1% trifluoroacetic acid (A) to MeCN (B) gradient (100% A for 30 seconds, 0 to 30% B from 30 to 60 seconds, 30 to 60% B from 1 to 10 minutes, 60 to 80% B from 10 to 15 minutes at a flow rate of 300 μL/min), followed by re-equilibration of the column to initial conditions.
Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 20 TWIN with a Gatan Tridiem 863 energy filter, incorporating a built-in CCD camera microscope, operating with an accelerating voltage of 120 kV. The samples were prepared on a plasma-treated copper grid.
Light microscopy was performed on a Nikon Eclipse E600 fluorescence microscope fitted with a 100×Apochromat objective and images were captured with a Media Cybernetics CoolSNAP-Pro monochrome cooled CCD camera. Confocal fluorescence microscopy (CFM) was performed on a Carl Zeiss LSM 780. The images were taken using a 488 nm (100 mW) laser (green).
Human umbilical vein endothelial cells (HUVECs) passaged less than 6 times were used in the assessment of the toxicity of the conjugate. HUVECs (cc-2159; Lonza, Walkersville, Md.) were cultured in collagen-coated flasks using EBM™ Basal Medium with supplements added (cc-3121 & cc-4133; Lonza, Walkersville, Md.) and maintained in a humidified atmosphere at 37° C. in the presence of 5% CO2.
1. Synthesis of polyvinylpyrrolidone (PVP)
Polyvinylpyrrolidone (PVP) was synthesised by a RAFT polymerisation process. Akeroyd et al. showed that a 1,2,3 triazole-based RAFT agent offered good control over molar mass and dispersity for vinyl acetate, styrene, n-butyl acrylate and N-vinylpyrrolidone (NVP). It also allowed the introduction of a vast range of functionalities through copper click chemistry (scheme 1).1
A new RAFT agent (4) was designed in line with this strategy (Scheme 2). It contained both a xanthate and an acetal functionality.
But-3-yn-2-ol (10.0 g, 142 mmol), tosyl chloride (32.6 g, 171 mmol) and diethyl ether (100 mL) were introduced into a 250 mL round bottom flask and the mixture was cooled to 0° C. in a sodium chloride/ice bath. Potassium hydroxide (20.2 g, 360 mmol) was slowly added portion-wise over 20 minutes after which the suspension was stirred for 3 hours, warming to room temperature on its own accord. After that, the reaction mixture was filtered, washed with water (2×50 mL), dried over magnesium sulphate and concentrated, yielding the crude white crystalline product, which was used immediately in the next step. Potassium ethyl xanthate (20.6 g, 129 mmol) and THF (80 mL) were added to the crude activated alcohol (24.0 g, 107 mmol) in a 250 mL round bottom flask and allowed to run at room temperature overnight. The reaction mixture was filtered, concentrated and then purified via column chromatography (diethyl ether:pentante=8:2) to yield 1 as a pale yellow oil (16.41 g, 66% overall). 1H NMR (400 MHz, CDCl3) δ 4.66 (q, J=7.1 Hz, 2H), 4.47 (qd, J=7.1, 2.5 Hz, 1H), 2.33 (d, J=2.5 Hz, 1H), 1.62 (d, J=7.1 Hz, 3H), 1.43 (t, J=7.1 Hz, 3H). 13C NMR (400 MHz, CDCl3) δ 212.33, 83.08, 71.67, 70.33, 35.24, 21.42, 13.96.
This compound was prepared as described in Goddard-Borger and Stick (2007).2 1H NMR (400 MHz, D2O) δ 9.33 (t, J=1.3 Hz, 1H), 8.04-8.00 (m, 1H), 7.60 (dd, J=2.1, 1.2 Hz, 1H).
The synthesis of 3 was adapted from literature.21-Amino-3,3-diethoxypropane (5.00 g, 34.0 mmol), potassium carbonate (9.39 g, 68.0 mmol), copper (II) sulphate.5H2O (84.9 mg, 0.340 mmol) and MeOH (30 mL) were introduced into a 100 mL round bottom flask. 2 (8.50 g, 40.1 mmol) was dissolved in MeOH (20 mL) and added to the reaction mixture and the solution was stirred for 10 hours at room temperature. The mixture was diluted with water (30 mL), acidified to pH 6 using acetic acid and extracted into diethyl ether (4×50 mL). The combined organic layers were washed with water (2×50 mL) and brine (2×50 mL), dried over magnesium sulphate and concentrated affording a dark yellow oil 3 (4.52 g, 77%). 1H NMR (400 MHz, CDCl3) δ 4.59 (t, J=5.6 Hz, 1H), 3.74-3.58 (m, 2H), 3.55-3.45 (m, 2H), 3.36 (t, J=6.8 Hz, 2H), 1.87 (td, J=6.8, 5.7 Hz, 2H), 1.20 (t, J=7.1 Hz, 6H). 13C NMR (400 MHz, CDCl3) δ 103.56, 64.00, 49.85, 32.91, 15.45.
A 50 mL round bottom flask was charged with 1 (3.71 g, 21.4 mmol), 3 (3.71 g, 21.4 mmol), copper (II) sulphate.5H2O (0.53 g, 2.14 mmol), sodium ascorbate (1.27 g, 6.43 mmol), DMF (15 mL) and stirred overnight at room temperature. The product was concentrated and purified via column chromatography (diethyl ether:pentane=4:1) to afford a viscous yellow oil 4 (4.46 g, 60%). 1H NMR (400 MHz,CDCl3) δ 7.50 (s, 1H), 5.06 (q, J=7.2 Hz, 1H), 4.62 (q, J=7.1 Hz, 2H), 4.45 (t, J=5.4 Hz, 1H), 4.40 (t, J=7.1 Hz, 2H), 3.63 (dq, J=9.3, 7.1 Hz, 2H), 3.53-3.40 (m, 2H), 2.18 (td, J=7.0, 5.5 Hz, 2H), 1.79 (d, J=7.2 Hz, 3H), 1.38 (t, J=7.1 Hz, 3H), 1.18 (t, J=7.0 Hz, 6H). 13C NMR (400 MHz, CDCl3) δ 213.34, 148.28, 121.79, 100.48, 70.11, 62.40, 46.44, 41.02, 34.54, 20.44, 15.49, 13.97. MS (ESI): m/z=348.1 (calculated 348.1 for [M+H+]).
1H NMR confirmed that the RAFT agent 4 contained no impurities (
NVP was polymerised using RAFT agent 4. NVP (10.0 g, 90.0 mmol), AIBN (37.0 mg, 0.225 mmol), 4 (0.312 g, 0.900 mmol) and anisole (10 mL) were added to a 30 mL pear flask. The reaction flask was degassed with argon for 1 hour and immersed in a 60° C. oil bath. The polymerization was left to run for a predetermined time. When the polymerization was finished, the solution was precipitated into diethyl ether and centrifuged. The precipitate was re-dissolved in DCM, precipitated in diethyl ether and centrifuged again. This process was repeated twice. Finally, the polymer was dried under reduced pressure overnight.
Two molar mass poly(N-vinylpyrrolidone) (PVP) polymers were synthesised, DP=100 and DP=300, respectively. A conversion study was performed to determine whether RAFT agent 4 controlled the polymerisation of PVP. The solid data points represent the experimental data and the dotted lines represent the theoretical molar mass (
The data from the conversion study allowed molar masses to be targeted more accurately. Using this information, two PVP polymers were synthesised (Table 1).
aconversion
bMn,SEC based on PMMA standards
cMn,NMR determined by integrating the xanthate signal versus the polymer backbone signal
1H NMR was performed to determine whether the polymers 5 and 6 still contained end-group functionality. It can be seen in
2. Cleaving of the Protected Functionalities on PVP
Polymers 5 and 6 both retained their end-group functionality, an acetal and a xanthate. It is known that acetals can be deprotected to form aldehydes under acidic conditions,3 that xanthates can be cleaved to thiols under reducing conditions,4 and that thiols form disulfides under oxidising condtions.5 In the present study, the deprotection was performed in one step insitu (Scheme 3). The acetal and xanthate functionalities were cleaved in a one pot process by dissolving the PVP in acetone and adding a 3 times excess hexylamine. This was left to react for 4 hours, after which HCl in dioxane (4 M) was added until the overall concentration was 1 M HCl. This was left to react for an additional 4 hours. The solution was dialysed in 2000 molecular weight cutoff dialysis tubing for 2 days against a water/methanol (1:1) solution and for 1 day against water, after which it was freeze dried yielding polymers 7 and 8, respectively.
1H NMR spectroscopy confirmed the presence of the aldehydic proton i at 9.79 ppm, the loss of protons h at 4.48 ppm, ascribed to the acetal moiety, and the loss of protons f at 4.58 ppm, attributed to the xanthate moiety (
General One-Pot Deprotection of PVP End-Groups (7)
5 (2.50 g, 0.926 mmol aldehyde), hexylamine (0.281 g, 2.78 mmol) and acetone (13.9 mL) were introduced to a 50 mL round bottom flask and stirred for 4 hours at room temperature. HCl (13.9 mL, 4 M in dioxane) was added, to bring the overall HCl concentration to 1 M, and the reaction was stirred for an additional 4 hours at room temperature. The solution was purified via dialysis (2000 Da MWCO) against water/methanol (1:1) for 2 days and pure water for an additional day, after which the product was freeze-dried to obtain 7 as a white powder. End-group analysis was performed via 1H NMR spectroscopy, and molar mass and dispersity were determined via SEC.
3. Conjugation of Polymers to a Targeting Ligand
Polymers 7 and 8 were reacted with Gly-Ser-Arg-Ser-Lys-Gly-Thr (SEQ ID NO: 178) (2% mole equivalence) from GL Biochem, 6-aminofluoroscein (2% mole equivalence) and NaBH3CN (10×excess) in sodium borate buffer (pH 9.7) overnight. 200% propylamine was added the following day to render the remaining aldehyde groups inactive (Scheme 4). The solution was dialysed again in 2000 g/mol molecular weight cutoff dialysis tubing for three days against water and subsequently freeze dried to yield polymers 9 and 10, respectively. 1H NMR confirmed the functionalization (
In order to further clarify the presence of the targeting ligand GSRSKGT (SEQ ID NO: 178) within polymers 9 and 10, a qualitative Kaiser test was performed, which tests for the presence of primary amines. The only source of primary amines within 9 and 10 was as a result of the ε-amine within the lysine residue (K) of Gly-Ser-Arg-Ser-Lys-Gly-Thr (SEQ ID NO: 178)(FOG. 8). A positive Kaiser test result is a blue to green colour, while a negative result should remain yellow.
General Synthesis of Targeting Ligand-Terminal PVP (9)
NaOH (400 mg, 10.0 mmol) was dissolved in water and made up to 100 mL in a volumetric flask (0.1 M, solution A). In parallel, sodium borate (1.02 g, 5.00 mmol) was dissolved in water and made up to 100 mL in a volumetric flask (50.0 mM, solution B). Solution A (32.3 mL) was placed in a 100 mL volumetric flask and made up to 100 mL using solution B, affording a sodium borate buffer (pH=9.7). 6-Aminofluorescein (4.13 mg, 11.9 μmol) was dissolved in sodium borate buffer (1 mL, pH=9.7) to afford a 6-minofluorescein stock solution (11.9 mM). PVP 7 (0.321 g, 0.119 mmol), Gly-Ser-Arg-Ser-Lys-Gly-Thr (2.00 mg, 2.38 μmol), 6-aminofluoroscein stock solution (200 μL, 2.38 μmol), NaBH3CN (74.8 mg, 1.19 mmol) and sodium borate buffer (2 mL, pH=9.7) were introduced to a 10 mL round bottom flask and placed on a shaker overnight at room temperature. n-Propylamine (14.1 mg, 23.8 μmol) was added and the solution was shaken for a subsequent 4 hours, after which it was dialysed (2000 Da MWCO) against water for 2 days and freeze-dried to afford a white powder, 9.
4. Synthesis of acrylate functional ised-Tyrocidine
Synthesis of Acrylate-Modifed Tyrocidine
a) Acryloyl chloride (0.0278 g, 3.08×10−4 mol) and DMF (5 mL) were added to a 20 mL sample flask and cooled to 0° C. in an ice bath. Tyrocidine (0.020 g, 1.5×10−5 mol), Dimethylformamide (2 mL) and N,N-Diisopropylethylamine (0.038 g, 3.1×10−4 mol) were mixed in a 20 mL container and this mixture was then slowly added dropwise into the sample flask over 30 minutes. The reaction was left to mix over 48 hours on a shaker in a 4° C. room. The DMF was removed via rotorvap and the residue was dissolved in 500 μL ethanol. This was subsequently precipitated in diethyl ether/acetone (6:4, 20 mL). The precipitate was centrifuged and this was repeated twice to yield acrylate functionalised Tyrocidine (Scheme 5).
b) The above method was also repeated on a larger scale a follows: A 100 mL round bottom flask was charged with acryloyl chloride (78.3 μL, 0.969 mmol) and DMF (80 mL) and cooled to 0° C. in an ice bath. Tyrocidine (63.0 mg, 48.5 μmol) and DIPEA (169 μL, 0.969 mmol) were dissolved in DMF (10 mL) and added dropwise to the round bottom flask over 30 minutes. The round bottom flask was placed on a shaker in a 4° C. room and left to react for 2 days, after which the solution was concentrated. The remaining residue was redissolved in ethanol (500 μL) and precipitated in diethyl ether/acetone (3:2) and the mixture was subsequently centrifuged. The pellet was washed with diethyl ether/acetone (3:2) and centrifuged, and the process was repeated twice. The pellet was dissolved in 50% MeCN and freeze-dried (49.2 mg, 75%) (Scheme 5).
Mass spectrometry and 1H NMR were used to confirm the structure (
5. Conjugation of Polymers to Acrylate-Functionalised Tyrocidine
Polymers 9 and 10 were conjugated to the acrylate-functionalised Tyrocidine through Michael Addition (Scheme 6). Thiols react with acrylates 7 and acrylamides to form a β-thiopropionate and β-thiopropioamide linker respectively, which is acid-labile at pH 5.5 but stable within the blood (pH 7.4).8 This will allow the release of the Tyrocidine after administration to a patient and being taken up in the food vacuole of the parasite (pH 5.5).
General Synthesis of Conjugate (9/10)
a) In one experiment, PVP 9/10 and acrylate-functionalised Tyrocidine were dissolved in DMF/Water (1:1) and treated with an excess tris(2-carboxyethyl) phosphine (TCEP solution (0.5 M, pH 7.0), 10× excess) and a catalytic amount of Ethylene diamine (1%) and left to react for 3 days. Water was added slowly, via syringe pump, to induce self-assembly. The respective solutions were dialysed in 10 000 molecular weight cutoff dialysis tubing for 2 days to remove DMF, unreacted polymer and Tyrocidines.
b) In another experiment, ethylene diamine (10.0 mg, 166 μmol) was dissolved in water (20 mL) to afford an ethylene diamine stock solution (8.3 mM). PVP 9/10 (10.0 mg, 3.7 μmol), tyrocidine mixture (4.81 mg, 3.70 μmol), TCEP solution (7.5 μL, 37 μmol, 0.5 M, pH=7.0), ethylene diamine stock solution (4 μL, 37.0 pmol), water (500 μL) and DMF (500 μL) were introduced to a 50 mL round bottom flask and placed on a shaker at room temperature for 24 hours. Water (30 mL) was introduced via a syringe pump, over 1 hour, to facilitate self-assembly. The solution was purified by dialysis (2000 Da MWCO) against water for 2 days and further dialysis using a higher MWCO (10000 Da) for a further day, to afford a milky dispersion 11/12.
1H NMR confirmed functionalization (
6. Tyrocidine/Tryptocidine Release Studies
A qualitative study was performed on the release of the tyrocidines over time, via UPLC-MS, to confirm that the acid-labile linkage between the tyrocidines linker and PVP cleaved under acidic conditions over 24 hours. A 1 mL sample of 11 was freeze-dried and redissolved in a phosphate buffer (pH=5.5). Samples were taken initially and again after 3 hours, 6 hours, 12 hours and 14 hours, and were immediately frozen in liquid nitrogen and subsequently stored in the freezer until analysis. Normally, matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectroscopy (MS) is needed to visualise polymers above 3000 g.mol−1. However, as a result of the PVP being conjugated to the tyrocidines, they were found to ionise easily under normal MS conditions.
Three chromatograms were chosen for each time period. The average molar mass of conjugate 11 was 4000 g.mol−1 and multiple distributions were present. However, a penta-charged molecular ion was chosen to represent the conjugate (733.97 Da). TrcA was chosen to represent the released tyrocidines. In each case, the representative peaks were labelled with their retention time, mass-to-charge ratio (m/z) and molecular ion count. The spectra shown at 0 hours were not an absolute control, as time was taken for the sample to thaw prior to injection onto the UPLC-MS. This attributed to released tyrocidines and tryptocidines being seen in these spectra. It is clear that from inception to 14 hours, the count of tyrocidines released over time increased substantially, from 140 to 844, if TrcA release is taken as measure. In addition, visually, the signal intensity at the retention time where the conjugate eluted (2-4 minutes) decreased, and then dramatically increased where the tyrocidines eluted (7-11 minutes), over time.
The release study confirms that the tyrocidine/tryptocidine conjugates would act as a Trojan-horse drug carrier, releasing the tyrocidines/trptocidines after the conjugate is taken up into malaria-infected erythrocytes.
7. In Vitro Anti-Malaria Assay
In separate experiments, tyrocidine/tryptocidines conjugated to polymers 11 and 12, respectively, were administered at varying concentrations to blood samples containing cells infected with a chloroquine-resistant Plasmodium falciparum strain (D10 resistant mutant denoted D10R, IC50(chloroquine)>100 nM). For comparison purposes, the unconjugated/unmodified natural tyrocidine/tryptocidine mixture was also administered to blood infected with the same chloroquin-resistant malaria strain.
Malaria parasites primarily convert glucose to lactate in their energy metabolism and therefore the 48 hour growth assays will lead to accumulation of lactate/lactic acid in the media. As mentioned above, the cyclodecapetide-PVP conjugates used in these experiments contained primarily tyrocidines, but also small amounts of the tryptocidines. It is known that both the tryptocidines and tyrocidines cause haemolysis of red blood cells and because the haemolysis assay was only performed after 48 hours, it is possible that some or all of the haemolytic activity observed in the malaria assay was due to the presence of free tryptocidines and tyrocidines that were released from the conjugate because of the acidification of the media. If this is the case, the cyclodecapeptide-PVP conjugates may have even higher selectivity for the parasite.
Parasite Culturing
P. falciparum (D10 strain) was cultured and dose-response assays were performed as previously described.9 Briefly, parasites were cultured under an atmosphere of 3% CO2, 4 % O2 and 93% N2 in RPMI-1640 media, supplemented with 25 mM HEPES, 50 mM glucose, 0.65 mM hypoxanthine, 0.048 mg/mL gentamicin, 0.2% (w/v) NaHCO3, 0.5% (w/v) Albumax II and 2% fresh human O+ erythrocytes obtained from anonymous donors from the Western Cape Blood Blank. Fresh erythrocytes and media were mixed with the parasitised erythrocytes to yield a 2% haemotocrit and 2% parasitemia suspension, which were distributed in microtitre plates at 90 μL/well. A 10 μL aliquot of a serial double dilution range in media of conjugate 11 and 12 was added into 90 μL of the parasite culture. The plates were incubated under an atmosphere of 3% CO2, 4% O2 and 93% N2 at 37° C. for 48 hours. After incubation, the microtitre plates were centrifuged, to sediment the intact erythrocytes. A 10 μL aliquot from each well was diluted to 100 μL with phosphate buffered saline and analysed at 405 nm, on a plate reader, to determine the quantity of released haemoglobin and haemolytic activity of the conjugates. The remaining culture was then frozen overnight and the parasite survival in each well was determined from the residual lactate dehydrogenase activity, using the Malstat assay.9 Duplicate plates containing normal erythrocytes (2% haematocrit) were also incubated at 37° C. for 48 hours.
Dose-Response Data Analysis
Triplicate assays were performed for each conjugate against each target cell. The dose-response data was analysed using Graphpad Prism version 3.01 for Windows (GraphPad Software, San Diego, Calif. USA; www.graphpad.com). Percentage parasite viability (lactate dehydrogenase assay) and percentage haemolysis were determined from absorbance values. A non-linear regression (sigmoidal curve with a variable slope) was performed on the dose-response data. Total growth was determined from the control wells (no conjugate added) and total haemolysis, from the addition of 200 μM gramicidin. The 50 % P. falciparium inhibitory concentration (IC50) and the 50% haemolytic concentration (HC50) were calculated as described by Rautenbach et al.9 Apparent selectivity was calculated by the ratio of HC50 to IC50 and defined as the selectivity index.
8. Microscopy of Conjugate-Treated P. falciparum Cultures
Light microscopy was used to study the effect that the tyrocidine-PVP conjugates had on the P. falciparium infected erythrocytes. Smears of the conjugate-treated parasite cultures and growth controls were taken at 24 hours and subsequently stained with Giemsa, which stains the malaria parasite purple, to be viewed under the light microscope (
Similarly, smears of the conjugate-treated parasite cultures and growth controls were taken at 48 hours and prepared for confocal fluorescence microscopy (CFM). Conjugates 11 and 12 had been tagged with 1% fluorescein in order to visualise their distribution in the malaria-infected erythrocytes over the normal erythrocytes (
It is speculated that the conjugate was taken up into the erythrocytes, through the membrane. In addition, it is suggested that “vaccination” of the erythrocytes could be part of the mode of action, as the dormant/intact conjugate was present in all erythrocytes. The pH in erythrocytes is the same as blood pH, 7.4, and as a result conjugates would remain intact within the erythrocytes. Only when a merozoite entered and infected an erythrocyte would its food vacuole take up the conjugate, releasing the tyrothricin, and incapacitating the parasite. This is a possible mechanism explaining why no parasites with abnormal morphology or stunted growth, as well as no reinfection, were observed (as previously found for the tyrocidines).10
Microscopy
Blood smears were prepared at various incubation times for examination by light and fluorescence microscopy. Slides prepared for light microscopy were stained with Giemsa.
9. Conjugate Toxicity
Toxicity studies were conducted to assess whether the lower molecular mass PVP with conjugate Mr-2700 of the invention showed any toxicity towards human epithelial cells.
Cell Viability Assay:
Human umbilical vein endothelial cells (HUVECs) were seeded at a density of 80 000 cells per well in 48-well plates and maintained until 70-80% confluency was reached. The cell proliferation agent WST-1 (ab155902; Abcam, Cambridge, UK) was added to experimental wells, each well containing 1 ng/ml, 10 ng/ml or 1 μg/ml peptide-polymer conjugate, and were then incubated in a light-protected environment for 2 h. Viable cells have active mitochondrial dehydrogenases, which result in the production of formazan dye, measurable through spectrophotometry. Plates were placed on a shaker for 1 min to mix the well content, after which the absorbance was measured at 450 nm. Data are represented as a percentage of average control (Table 4).
No statistically significant difference was observed between control and 1 ng/ml or 10 ng/ml concentrations of the conjugate in terms of the cell viability depending on the MTT reductive capacity. However, a small but statistically significant increase in reductive capacity was observed between the control and the 1 μg/ml treatment group (Table 4). This increased reductive capacity may either indicate greater mitochondrial respiration and function, which could serve as protective mechanism, or it may be a result of a stress response, where reductive capacity is increased as a coping mechanism.
Fluorescence Microscopy
Cells were stained with Hoechst 33342 and tetramethyl rhodamine ethylester (TMRE, 500 nM) by adding pre-warmed media with both dyes (1:200) to wells for 3 min, followed by fluorescence microscopy. At least three images were captured for each treatment group at 100× magnification using 360 nm, 492 and 572 nm excitation wavelengths. An F-view-II cooled CCD camera (Soft Imaging Systems, Olympus Corporation, Tokyo, Japan) mounted on a wide field inverted microscope (Olympus IX81, Olympus Corporation, Tokyo, Japan) with a Xenon-Arc burner (Olympus Biosystems GMBH, Hamburg, Germany) as a light source was used. Emission was collected with a UBG triple-bandpass emission filter cube (Chroma). Images were captured and processed using Olympus Cell̂R (Hamburg, Germany) software.
The microscopy images showed that no nuclear condensation, as indicator for apoptotic cell death, is observed. The mitochondria display TMRE positive signal, and hence have a maintained polarisation, with a well-defined mitochondrial network morphology. The conjugate itself is rarely visible in the green channel, likely due to the very low fluorochrome concentration (
Statistical Analysis
One-way analysis of variance (ANOVA) with Bonferroni post hoc test was performed using Prism Graph Pad 5.0 to determine differences between groups. A p-value<0.05 was considered significant.
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| Number | Date | Country | Kind |
|---|---|---|---|
| 2014/05884 | Aug 2014 | ZA | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2015/056148 | 8/12/2015 | WO | 00 |
