Patent Application
20080152703
The present invention relates to a composition for interacting with a ligand, a method for producing such a composition and a method for producing a molecule based on the composition.
Protein receptors are known normally to bind to their target ligands via epitopes, which constitute a small proportion of the total protein molecule. For maximum binding or interaction, the structure of the epitope needs to be maintained in a rigid conformation in order to form a binding site containing all the necessary components of the epitope in close proximity. Attempts to produce an analogous peptide constructed solely of the amino acids comprising the binding site often fail because these peptides do not possess the same biological activity as the protein receptor. This is attributed to the peptide having a different conformation in free solution from that of the entire protein receptor. In addition, where the binding site of a protein is constructed of oligo-peptides from different, non-contiguous parts of a protein chain, mixing isolated oligopeptides in free solution does not result in reconstitution of the active binding site.
Being constrained to use such large proteins to present binding-site epitopes gives rise to several problems in development of new receptor-specific therapeutic strategies. One problem is that such large proteins can readily evoke an immune response. A second problem is that long peptide chains are susceptible to attack by endopeptidases, such as those in the lumen of the gut. Finally, these large proteins can be costly to manufacture, purify and maintain in stable form.
The present invention aims to overcome the disadvantages of the prior art.
In a first aspect, the invention provides a composition for interacting with a ligand, which composition comprises a non-covalent assembly of a plurality of distinct conjugates, each conjugate comprising a head group and a tail group, wherein the tail groups of the conjugates form a hydrophobic aggregation and the conjugates have freedom of motion with respect to each other within the assembly so that, in the presence of a ligand, at least two of the head groups (which are the same or different) are appropriately positioned to form an epitope capable of interacting with the ligand more strongly than each of head groups individually. The head groups are typically hydrophilic and the tail groups typically hydrophobic, eg lipophilic, composed of hydrocarbon chains, halophilic, constructed of fluorocarbon chains, or silane based.
By constructing conjugates with a head group and a tail group in accordance with the present invention, the tail groups can associate to form a hydrophobic aggregation which is typically a supramolecular assembly such as a micelle, a lamellar structure, a liposome or other lipid structure, in which the conjugate are oriented whereby the head groups are brought into close proximity when in an aqueous phase. Because the conjugates are movable within the assembly, the head groups are able to adopt a number of different positions within the assembly. The head groups, which are typically non-identical, are therefore free to move within the assembly and, surprisingly, to interact cooperatively to induce biological consequences which the head groups on their own are not capable of eliciting. A further unexpected finding is that assemblies composed of combinations of different headgroups are capable of eliciting biological responses or participating in binding with biological receptors while assemblies composed of single headgroups are not capable of acting in this way.
As indicated above, these supra-molecular assemblies are typically particulate or colloidal in nature, usually comprising many hundreds of sub-units (the conjugates) all oriented with the headgroups directed outwards from the centre of the particle as shown in
Each conjugate in the assembly may have a head group selected from one chemical or biological class or a number of different classes, such as an amino acid or peptide; a peptide analogue; a mono-, di- or poly-saccharide; a mono-, di- or poly-nucleotide; a sterol; an alkaloid; an isoprenoid; an inositol derivative; a single or fused aromatic nucleus; a water-soluble vitamin; a porphyrin or haem nucleus; a phthalocyanine; a metal ion chelate; a water-soluble drug; a hormone; or an enzyme substrate.
In one preferred embodiment, each head group comprises an amino acid or oligo-peptide, which may be the terminal portion of a peptide chain. It is desirable to keep the length of the peptide to a minimum so as to avoid eliciting an immune response where the composition is to be used in vivo. Accordingly, it is preferred that the peptide is no more than six amino acids long.
The amino acids employed can be any of the natural amino acids, substituted derivatives, analogues, and D-forms thereof.
The tail groups of the conjugates may be all the same or may be a mixture of different tail groups, each of which preferably comprises a hydrophobic group selected from a linear, branched, cyclic, polycyclic, saturated or unsaturated construct, with or without hetero-atoms included in the structure which can be substituted or unsubstituted, for example, a lipidic amino acid analogue; a prostaglandin; a leukotriene; a mono- or diglyceride; a sterol; a sphingosine or ceramide derivative; and a silicon or halogen-substituted derivative of such a hydrophobic group. The tail group preferably has from 6 to 24 carbon atoms and more preferably comprises from 10 to 14 carbon atoms. More than one tail group may be present in each conjugate. For example, one or more lipidic amino acids with hydrocarbon side chains may form part of each conjugate, linked to one or more amino acids in the head group.
Any chemical method may be used to link the head group to the tail group. For example, each conjugate may further comprise a spacer group linking the head group to the tail group so as to facilitate presentation of the head group on the surface of the non-covalent association. Such spacer groups are well known and include, for example, amino acids, hydroxy acids, sugars and polyethylene glycol.
In a further aspect, the present invention provides a composition as defined above, for use as a medicament, a prophylactic or a diagnostic.
An advantage of the invention is that strong specific binding interactions can be achieved with conjugates in which the head groups are small in comparison to conventional biological receptors. If the head group is an oligo-peptide, for example, then the length of the peptide chain would not normally exceed ten amino acids and would preferably be six or less. Accordingly, compositions according to the present invention can be made far less immunogenic than their protein counterparts.
In accordance with this aspect of the invention, not only can the composition of the present invention be formulated to interact with a ligand in vitro but also the composition can be used in vivo, optionally formulated with a suitable diluent, excipient or carrier in accordance with a suitable delivery route.
In a further aspect, the present invention provides use of a conjugate comprising a head group and tail group for the preparation of the composition as defined above.
There is further provided a method for producing a composition for interacting with a ligand, which method comprises:
The conjugates may be dispersed in aqueous phase by a variety of known methodologies for the preparation of lipid vesicles, including mechanical mixing, exposure to high shear forces, sonication, solvent dispersion or codissolution with detergents. Typically, the non-covalent supra-molecular assemblies formed thereby will be composed of several different conjugates mixed together. Additional lipidic materials may optionally be added to alter surface properties, to aid in the dispersion of the conjugates, to stabilise the non-covalently associated assembly of conjugates, to aid in the presentation of head groups of the conjugates, or to permit the construction of vehicles which can be targeted by the epitopes formed upon random movement of the conjugates and appropriate positioning of the head groups within the assembly.
An important aspect of the method according to the present invention involves the step of identifying the plurality of conjugates which has the desired biological activity. In a preferred aspect, this step comprises
Examples of assays for “sufficient interaction” may include binding assays such as those utilising the ELISA principle for detection of association between antibody and antigen. Other suitable in vitro assays include modification of fluorescence of environmentally-sensitive membrane-bound fluorescent probes, precipitation reactions, enhancement or inhibition of enzyme activity etc. Assays relying on the ability of materials to alter the behaviour of cells cultured in vitro may also be appropriate, such as assays for cell death, cell proliferation, apoptosis, inhibition or stimulation of cell-to-cell contact, secretion of cytokines or other soluble products, synthesis of specific m-RNA, intracellular vesicular transport, alteration of cell signalling processes etc. In vivo assays in whole animals or humans may also be carried out, for example incorporation of radiolabel into the supramolecular assemblies, followed by investigation of its subsequent distribution after administration by various routes.
According to this method a combinatorial approach is used in which a range of different supra-molecular assemblies (or “probes”) is prepared, each containing a different combination of conjugates selected from a pre-synthesised bank. Selection of the appropriate conjugates may be based on known properties of the target ligand or may simply involve the use of a very wide range of head groups to increase the probability that two or more of the head groups will form an epitope for the ligand. In this way, following the assay for sufficient interaction between the probe and the ligand as described above, the combination of conjugates found to be most effective may be modified by adding further head groups, removing some head groups, or both, and assaying the resultant probes once again for sufficient interaction. Eventually, the most favourable combination of head groups may be identified and selected for use in the composition.
The present invention therefore has a very clear advantage over traditional combinatorial chemistry. In combinatorial chemistry, the identification of the most favourable sequence for binding to a specific receptor must be carried out by synthesis of hundreds of possible combinations of different groups such as amino acids, in different orders, each one having to be tested for efficacy. This process is time-consuming, expensive and is limited by the nature of the chemistry which can be carried out in linking the different components together. In contrast, the present invention simply relies upon proximity of the head groups to provide association-derived epitopes. Once a set of conjugates has been synthesised, no further synthetic chemistry is required, only simple mixing of the conjugates to form the different probes by non-covalent association.
In a preferred simple embodiment, the present method uses conjugates having a single terminal amino acid linked via a spacer to a lipid tail group which can be combined simply by mixing in aqueous medium to form micelles in which different amino acid side chains would be presented together in a multiplicity of different configurations. Accordingly, the need to present amino acids in a specific order, or with a specific spacing or orientation, is circumvented. On statistical grounds, a proportion of the individual amino acid sub-units will always be associated in an ideal configuration.
In one arrangement, each of the conjugates would have the linear structure: X-spacer-spacer-lipid-lipid, where X represents a single amino acid different for each of the distinct conjugates employed.
When seeking to construct epitopes composed of natural amino acids it is possible to simplify further the number of head groups for selection. One can categorise the amino acid residues found in natural proteinaceous materials into six fundamental classes preferably using in any one class one amino acid rather than all members of that class because of the increased spatial flexibility of amino acids in the terminal position of the head group. This has the effect of reducing considerably the total number of amino acids required for constructing the pre-synthesised bank of conjugates and thereby the total number of head groups used. The main classes of amino acids are set out in Table 1 below.
A number of strategies are available for identifying active combinations of amino acid-containing conjugates.
In one embodiment, a restricted number of conjugates is employed to form a range of distinct probes where each probe is an aqueous suspension of supra-molecular assemblies, each assembly consisting of selected conjugates mixed together, and each differing from the other as a result of the inclusion of a different additional conjugate as shown below where each of the letters given represents a conjugate with a different terminal amino acid:
Each of the probes is tested separately in the biological assays for sufficient binding as outlined above.
In a second simple embodiment, an initial probe can be constructed which contains a large number of different conjugates from the bank, and its efficacy compared with probes each lacking a different conjugate in turn, to determine which headgroups in the bank are essential, and which are redundant for the biological interaction being investigated. This approach is illustrated below:
Combinations of the alternative approaches as outlined above can be made.
A knowledge of the target ligand may assist in designing a suitable starting array. For example, if the ligand is known to be basic, it would make sense to impart an acidic character to the conjugates by presenting them in the form where a free carboxyl group of the terminal amino acid is exposed. Introducing additional functionality by employing a particular amino acid as a spacer group adjacent to the terminal amino acid may also confer increased specificity. Where the involvement of, say, a short oligo-peptide sequence of known structure has already been implicated in binding to the target ligand, such a sequence may be incorporated into a conjugate to be included in the set of conjugates making up the composition.
In a final aspect, the present invention provides a method for producing a molecule for interacting with a ligand. The method comprises producing a composition according to one of the methods defined above; identifying the at least two head groups which form an epitope for the ligand in the composition; and producing a molecule incorporating the functional groups of the at least two head groups optionally spaced apart by one or more linker groups so that the molecule is capable of interacting with the ligand more strongly than each of the head groups individually.
Whilst the compositions of the present invention may themselves be useful in in vitro or in vivo systems perhaps to induce a biological response in a therapeutic, prophylactic or diagnostic method, in some circumstances a molecule may be produced based on the structure of the above compositions. By identifying the functional groups of the at least two head groups which form the epitope for the ligand a new molecule analogous to the composition may be produced containing the same or a similar epitope. The functional groups may, for example, be incorporated into a single linear oligo-peptide possibly with one or more linker groups to space the functional groups apart.
The invention will now be described in further detail, by way of example only, with reference to the following Examples and the attached drawings, in which:
Referring to
In the examples given below, the standard convention for representation of amino acids by single letters of the alphabet is employed, except that in all cases the letter refers to conjugates as described above in which that particular amino acid occupies the terminal position in the peptide chain. In the examples described here, the lipid comprises two amino acids linked via a peptide bond, in which both of the amino acids are glycine analogues, where in each case the alpha hydrogen has been replaced by a linear hydrocarbon chain containing either 12 or 14 carbons. Linkages between the headgroup and spacer and the spacer and lipid are all via peptide bonds. The headgroup bears a free amino group and the free end of the lipid bears a CONH2 group. The structure of each conjugate is thus: NH2-headgroup-spacer-amino acid (C14 side chain)-amino acid (C12 side chain)-CONH2.
It can be seen that some, but not all, of the combinations of different headgroups elicit strong biological responses, indicating that the response is specific to those particular combinations. The example illustrates the way in which the conjugates described can be employed in the combinatorial approach to identify efficacious combinations for the purpose of eliciting a desired biological response.
Comparison of Supra-molecular Assemblies Containing a Mixture of Conjugates, with a Mixture of Supra-molecular Assemblies Each Containing a Single Conjugate
Samples were prepared as described in Example b 1, with or without the inclusion of additional lipidic materials as described below. The combination of conjugates Y, S and L was chosen since this combination was a good performer in the experiment described in Example 1.
Probes containing phosphatidyl choline were prepared at a ratio of phospholipid to conjugate of 2:1 wt/wt.
Probes containing octyl glucoside were prepared at a ratio of glycolipid to conjugate of 1:1 wt/wt.
Results shown in the table below are optical densities at 450 nm of TNF ELISAs conducted on 18 hour culture supernatants. The concentration of conjugate in the wells was 10 μug/ml
This example shows that combinations of the conjugates can elicit biological responses either when presented alone, or when presented in conjunction with other lipids, such as phospholipids or lipid sugars. It also shows that for efficacy to be manifested, it is important for all of the conjugates to be presented in combination on the same supra-molecular assembly, and that activity is not observed if the same conjugates are presented together at the same time, but separated on different supra-molecular assemblies. This suggests that it is important to present the conjugates in close proximity to each other, in order to permit the formation of epitopes formed by non-covalent association of the conjugates, which can participate in specific binding with cell-surface receptors.
Percentage uptake was estimated on the basis of a 2 ml blood volume, of which 1 ml was assumed to be plasma.
Results are shown in the table below.
It can be seen that some, but not all, of the combinations of different headgroups enhance uptake of label via the oral route, indicating that the response is specific to those particular combinations. The example illustrates the way in which the conjugates described can be employed in the combinatorial approach to identify efficacious combinations capable of acting as targeting ligands.
It can be seen that maximal binding is achieved with samples 2, 3 and 4 (ie combinations YFL, YWL, and FWL).
It can be seen that some, but not all, of the combinations of different headgroups enter into strong binding interactions, indicating that the response is specific to those particular combinations. The example illustrates the way in which the conjugates described can be employed in the combinatorial approach to identify efficacious combinations for the purpose of eliciting a desired binding interaction.
| Number | Date | Country | Kind |
|---|---|---|---|
| 99150740 | Jun 1999 | GB | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10019052 | Apr 2002 | US |
| Child | 12041939 | US |
