MOLECULAR DECOYANTS AND METHODS OF USE THEREOF

Abstract: Molecular decoyants are chemical structures which functionally resemble an endogenous receptor. They may be used for the treatment of humans or other animals which have been subjected to a foreign agent which exerts an undesired effect only after first binding to that endogenous receptor. A decoyant is a fraction of an endogenous receptor which retains the essential elements of the binding site of the receptor for the foreign agent or is a synthetic or biosynthetic derivative thereof. The alpha -184-200 amino acid sequence of Torpedo californica acetylcholine receptor is an example of a decoyant which protects against curarimimetic neurotoxins. Another example is the fraction of the CD4 receptor which retains the essential elements of the receptor for protection against HIV.

Description: MOLECULAR DECOYANTS AND METHODS OF USE THEREOF Field of the Invention The present invention relates to substances useful for the treatment of animals, including humans, which have been subjected to a foreign agent, which agent exerts an undesired effect only after first binding to an endogenous receptor. More particularly, the present inven tion relates to such substances, and methods of use there¬ of, which are directly or indirectly derived from the natural endogenous receptor. Background of the Invention In order to exert their action in a living body, many pathogenic and toxic agents, such as viruses, bacteria and also toxins and poisons, become attached to specific binding sites, e.g., to cell surface receptors. Such binding phenomena may be necessary as a first step in viral infectivity or may be the essence of toxic inactivation of a critical physiological function. A receptor is a cellu¬ lar component that interacts with a specific ligand. Ligands classified as agonists, when bound to their recep¬ tors, activate an effector system and trigger a bio- response. Ligands classified as antagonists depress receptors or inhibit the action of the agonist. When, for example, cobra venom or curare is attached to a cholinergic receptor, the binding of acetylcholine is prevented. Prevention of such formation of certain ligand-receptor complexes should be beneficial in combatting the deleteri¬ ous effects of the pathogenic or toxic agent. Prevention of complex formation can be achieved via a number of basically different approaches. For exam¬ ple, the generation of anti-toxin antisera has proven to be a useful approach in the treatment of snake bites. Immuno- logical inactivation of viruses and bacteria is the essence of vaccination. In both of these cases the infiltrating substance is intercepted by highly stereospecifiσ immuno- globulins which in turn prevent the substance from reaching its target of action. Alternatively, a different approach has recently been suggested according to which an analogue of the for¬ eign material is used to preoccupy the host receptor bind¬ ing site, and thus viruses or bacteria are prevented from associating with the tissue they normally would infect. These known approaches suffer certain basic drawbacks. Immunological inactivation is "ligand- specific". Furthermore, many bacteria and viruses have the capability of periodically modifying their immunogenic epitopes by random mutations and recombination processes, thereby rendering the immunoglobulin ineffective. The use of ligand analogues is "receptor-specific". However, by definition, such analogues occupy the receptor preventing its functionability. The present invention employs a novel approach to the problem of prevention of formation of such ligand- receptor complexes which approach is "receptor-specific" yet does not prevent the functionability of the native receptor. Summary of the Invention It is an object of the present invention to obviate the drawbacks of the prior art. It is a further object of the present invention to provide a novel approach for the prevention of formation of ligand-receptor complexes which approach is "receptor- specific" yet does not prevent the functionability of the receptor sites. It is another object of the present invention to provide molecular decoyants which are not substantially larger than the molecular structure of the ligand binding site in the native receptor and which can be used to bind pathogenic or toxic agents .in vivo in a "target-specific" manner. It is yet another object of the present inventio to provide substances which are small enough as to be substantially non-immunogenic and which act as decoyants i vivo to compete with the natural binding sites and inter- cept the specific ligands and inactivate them. It is still a further object of the present invention to provide a method for the treatment of animals, including humans, which have been subjected to foreign agents which exert an undesired effect only after first binding to an endogenous receptor. It is still another object of the present inven¬ tion to provide such a method by administering a molecular decoyant which mimics the functional attributes of the binding site of the endogenous receptor. These and other objects of the present invention will be further understood after consideration of the following detailed description of preferred embodiments in conjunction with the attached drawings. The novel solution to the problem of prevention of formation of ligand-receptor complexes, when the ligand is a foreign agent which exerts an undesired effect by specifically associating with an endogenous receptor, involves the identification of the molecular structure of the ligand binding site in the native receptor and the production of mimic ligand binding sites. These sites can be used .in vivo to bind toxins or viruses or any other foreign agent in a "target-specific manner". Thus, the mimic ligand binding sites of the present invention compete in the animal's body with natural binding sites, thus acting as decoys. Such substances have been denominated "molecular decoyants" by the present inventor. It is believed that natural receptors are rather large structures, comprising some hundreds of amino acids, and can be as large as a molecular weight of about 250,000. The specific binding site, however, is much smaller and this opens up the possibility of preparing artificial, synthetic binding sites, which are effective in binding specific viruses, bacteria, toxins, etc., yet which com¬ prise a much smaller number of amino acids, preferably less than 100, and which have therefore a considerably lower molecular weight and thus a correspondingly lower im uno- genicity. It has been found possible to prepare such binding-site-mimicking molecular decoyants which are adap¬ ted to bind specific ligands with a size on the order of about 20 amino acids. Suc rather small peptide structures can be prepared by physically dividing the endogenous receptor or they can be prepared synthetically by the preparative procedures of peptide chemistry, such as Merri- field synthesis, or by genetic engineering. This opens up the possibility of large scale production of such specific polypeptide structures and their use as active materials in the treatment of animals subjected to pathogenic or toxic agents. Since molecular decoyants bind to the ligand at the very site required by the ligand to bind with the endogenous receptor required to exert its undesired effect, this site cannot be changed by the ligand without deacti¬ vating itself. Thus, decoyants are much more reliable than immunoglobulins and have a long term effect. The present invention is of a very wide applica¬ bility as molecular decoyants specific to a wide variety of ligands can be produced. The present invention includes prophylactic as well as therapeutic compositions which contain the active molecular decoyant structures in an adequate concentration and quantity. Brief Description of the Drawings Figure 1 shows the process steps for obtaining the 17 amino acid sequence WKHWVYYTCCPDTPYLD by recombinant DNA technology. Figure 2 shows the results of the separation on polyacrylamide gels of various samples of an R4137 clone cultured for induction of the trpE fusion protein. The cells were either solubilized in sample buffer (T) or sonicated in high salt buffer (500 mM) and centrifuged. The supernatant (S-, ) contained 40-60% of the fusion protein as did the pellet (Pi ) . The pellet was further extracted with water to generate a supernatant (S 2 ) which contained some 15% of the original fusion protein content and a pellet (P 2 ) . After polyacrylamide gel electrophoresis (PAGE) , the samples were either stained with Coomassie brilliant blue (top) or blotted and overlayed with 125 I- labeled α-bungarotoxin (BTX) followed by autoradiography (bottom) . The arrowheads indicate the position of the fusion protein. Numbers indicate relative molecular mass in kDa. Figure 3 is a graph showing the concentration of bound toxin at specific times (C t ) divided by that reached at equilibrium (C eq ) after incubation of R4137 with 12S _ labeled BTX for different periods of time as indicated. The concentrations were measured after applying aliquots to positively charged membrane filter discs. The time was measured in minutes in panel A or seconds in panel B. Figure 4 is a graph showing the Scatchard analy¬ sis of toxin binding to R4137. After incubation with different concentrations of 125 I-labeled BTX until equilib¬ rium was achieved (30 min) , net bound BTX was determined by adding a 1,000 fold excess of nonradioactive BTX and the bound versus the free toxin for each point was calculated. Figure 5 is a graph showing competition of BTX binding. The percent of 125 I-labeled BTX (2'10" 8 M) is plotted after mixing with ever increasing concentrations of: non-labeled BTX (•) , cobratoxin (Δ) , decamethonium (0) d-tuboσurarine (° ) , NaCl (O) , carbamylcholine (x) or glycine ( ) . The mixtures were incubated with equal amounts of R4137 for 30 min at 25°C and the net amount of bound radioactive toxin was determined. Figure 6A is a graph plotting the amount of bound 125 I-labeled BTX as a function of the total amount of 1 5 1- labeled BTX applied to a concanavalin-A column having AcChoR immobilized thereon. Figure 6B is a graph plotting the amount of bound 125 I-labeled BTX on a concanavalin-A column having AcChoR immobilized thereon as a function of the amount of R4137 applied to the column after different amounts of R4137 and constant amounts of 125 I-labeled BTX were applied thereto. Figure 7 is a graph showing the effect of R4137 on the survival rate of d-tubocurarine injected mice. Two groups of Balb/C mice (35 in each) were inj cted with either pATH2 or R4137 (approx. 3 nmole BTX binding sites/ mouse) intraperitoneally. Five minutes later the mice were given d-tubocurarine (approx. 15 nmole, 9 μg/mouse, subcu- taneously) . The number of survivors as a function of time after the injection of toxin is shown (the data are derived from experiments 2 and 3 of Table 1) . Detailed Description of Preferred Embodiments While the present invention is applicable to molecular decoyants which mimic the binding site of any endogenous receptor, it will be discussed in detail with respect to the cholinergic binding site. It is known that α-bungarotoxin (BTX) , which is a snake venom α-neurotoxin, exerts its toxic ef ect -by blocking the binding of cholin¬ ergic ligands to the nicotinic acetylcholine receptor (AcChoR) . The neuro-muscular junction is the site where nerves meet with muscle fibers. The point of contact is the synapse and is characterized by the fact that the nerve and muscle are not actually physically connected, but rather form a chemical junction. When the nerve pulse reaches the tip of the axon, acetylcholine, the neuro- transmitter, is secreted into the gap between the nerve and muscle, i.e., "the synaptic cleft". The acetylcholine is bound by its receptor which is situated on the extra¬ cellular membrane of the muscle, the post-synaptic side of the junction. The binding of two molecules of acetylcho- line to their receptor causes an ion-channel to open, the membrane to depolarize and eventually leads to muscle contraction. BTX is an antagonist which binds to AcChoR, thereby preventing acetylcholine from reaching its receptor and preventing muscle contraction. To make a molecular decoyant for the treatment of animals subjected to BTX, one must first identify the particular BTX binding site. The preferred method for doing this is by means of ligand overlay of protein blots. Once the binding site is identi- fied, the minimal sequence may be produced. Upon the administration of such a sequence, the decoy will mimic the binding site and bind with BTX, thereby blocking the unde¬ sired activity of the toxin. The specific binding site for BTX on the AcChoR is known to be situated on the α-subunit thereof. The minimal essential elements of the binding site which will still permit selective and specific binding of reasonable affinity to BTX may be further identified by means of protein blotting. The techniques of protein blotting are discussed in detail in Gershoni, "Protein Blotting: A Manual" in Methods of Biochemical Analysis, edited by David Glick, John Wiley and Sons, vol. 33, pp. 1-55, 1988, the entire contents of which are hereby incorporated by refer¬ ence. The process involves transferring resolved polypep- tides from chromatographiσ gels to immobilizing matrices. BTX is a polypeptide toxin (74 amino acid resi¬ dues) which can be iodinated and binds the receptor with an affinity of Q ^IO'^M. Purified AcChoR is subjected to polyacrylamide gel electrophoresis (PAGE) under mild denat- uration without boiling of the sample and use of lithium dodecyl sulfate, instead of sodium dodecyl sulfate (SDS) . Blots are then prepared and probed with 125 1-labelled BTX. Such experiments show that the α-subunit of the AcChoR is labelled (Gershoni et al, "Binding of α-Bungarotoxin to Isolated α-Subunit of the Acetylcholine Receptor of Torpedo californica; Quantitative Analysis with Protein Blots", Proc. Natl. Acad. Sci. (USA). 80: 493-4977 (1983)). The α-subunit is then proteolysed and then protein blots there¬ of probed with alkaline-phosphatase hydrazide, concana- valin-A, BTX and sequence specific antibodies which together have allowed the mapping of the toxic binding site to the region α-160-330 and more particularly α-160-210 and even more specifically to the region α-180-200 (see Neumann et al, "Mapping of the α-Bungarotoxin Binding Site with the α-Subunit of the Acetylcholine Receptor", Proc. Natl. Acad. Sci. (USA) . 83:3008-3011 (1986), the entire contents of which are hereby incorporated by reference) . In order to determine the cholinergic binding site more particularly, a number of synthetic peptides were prepared and their capacity to bind cholinergic ligands was assessed. Of the many peptides tested the only ones that were capable of BTX binding included the sequence α-185- 196. This peptide was found to have low affinity (10" 5 M) yet highly specific BTX binding ability. Higher affinity (10 * 7 M) was found for the sequence α-173-204. BTX-binding sequences can also be produced by recombinant DNA techniques, sub-clones of cDNA of α-sub- units of mouse or Torpedo californica were prepared using expression vectors. The trpE fusion vector pATH2 was used. Restriction fragments of the plasmid p42, a cDNA clone of the α-subunit of Torpedo californica AcChoR, were purified on 1% agarose gels. Preparative quantities of plasmids were obtained, and ligations in transformations of E_i_ coli strain HB101 were performed by the methods of Maniatis et al. , "Molecular Cloning: A Laboratory Manual", Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, 1982. By such means fusion proteins were prepared in E. coli trans- formants (Gershoni, "Expression of the α-Bungarotoxin Binding Site of the Niσotinic Acetylcholine Receptor by Escherichia coli Transformants", Proc. Natl. Acad. Sci. (USA) . 84, 4318-4321 (1987), the entire contents of which are hereby incorporated by reference) . These fusion pro¬ teins were shown to specifically bind BTX (affinity: 10" 7 M) . Thus, for example, bacterially expressed proteins containing α-166-200 bind toxin whereas those expressing α- 201-315 do not. Two oligonucleotides were prepared as described in Figure 1(1). They were designed using bacterially preferred codons to code for the amino acid sequence: Gly- Ile-Glu-Gly-Arg-Trp-Lys-His-Trp-Val-Tyr-Tyr-Thr-Cys-Cys- Pro-Asp-Thr-Pro-Tyr-Leu-Asp, which includes αl84-200 of Torpedo californica AcChoR as well as a pentapeptide intro- duced N-terminal to residue W184. The first glycine is the result of the trinucleotide dGGG which is necessary for the maintenance of a functional S al site. The following sequence IEGR is the specific cleavage site for the coagu¬ lation factor Xa (CFX) . Thus the expressed fusion protein becomes cleavable with this enzyme thereby enabling the eventual release of the site of interest, i.e., the 17 amino acid sequence: WKHWVYYTCCPDTPYLD. The two oligo¬ nucleotides were mixed at a ratio of 1:1 heated together and allowed to anneal forming a 9-base pair duplex. The complementary strands were then enzymatically "filled in" using Klenow polymerase (1 μl ; 5 units), as shown in Fig. 1(2), after which the DNA was phenol-extracted and precipi¬ tated in ethanol using glycogen as a carrier. The con¬ struct was then ligated with Smal-cut, purified pATH2 expression vector. The ligated vectors were used to trans¬ form E. coli strain HB101, all in accordance with the techniques described in Aronheim et al, "Characterization of the Binding of α-Bungarotoxin to Bacterially-Expressed Cholinergic Binding Sites", J. Biol. Chem.. 263, 20:9933- 9937 (1988) , the entire contents of which are hereby incor¬ porated by reference. The transformed bacterial clones which contained the insert in proper orientation were selected by 125 1- labeled BTX overlay of colony-blots. These transformants were found to produce an efficient toxin-binding fusion- protein (36kDa designated R4137, Figure 2). R4137 could be highly enriched by sonicating the transformed cells in phosphate buffer + 500mM salt. Centrifugation resulted in a pellet which contained 40-60% of the total R4137 and little more (Figure 2, P, ) . This pellet could be extracted with water to produce a soluble fraction which was predomi- nantly R4137 (Figure 2, S 2 ) . The R4137 in the S 2 fraction was biochemically characterized by measurement of toxin binding thereto in accordance with the techniques described in Gershoni et al, "Molecular Decoys: Ligand-Binding Recombinant Proteins Protect Mice from Curarimimetic Neurotoxins", Proc. Natl. Acad. Sci (USA) . 85, 4087-4089 (1988) , the entire contents of which are hereby incorporated by reference. In essence, aliquots of S 2 were incubated with 125 I-labelled BTX. Then the mixture was filtered through a charge modified membrane filter to separate the bound versus free toxin. The fil¬ ters were then counted for radioactivity. Using this technique it was found that BTX associates with R4137 in a pseudo first order kinetic which reaches 50% completion within 40 sec and about 90% completion after 6 min (Figure 3) . Measurements of toxin binding at equilibrium (30 min reaction) showed that toxin bound to R4137 with a K,j=1.2x10" 7 M (Figure 4). This binding could be competed with other cholinergic ligands with a progression of effi¬ ciencies which is similar to that known for the intact receptor (Figure 5) . Accordingly, R4137 demonstrates that the 17 amino acid sequence: α-184-Trp-Lys-His-Trp-Val-Tyr-Tyr-Thr-Cys- Cys-Pro-Asp-Thr-Pro-Tyr-Leu-Asp-200 is sufficient for BTX binding. Although this binding is appreciably less than that of the intact receptor, it does resemble the binding characteristics of the complete α-subunit (437 amino acids) of AcChoR. In order to establish whether R4137 could be used as a treatment for animals which have been subjected to a neurotoxin, an experiment was conducted to establish that R4137 can compete against intact AcChoR for a common and limiting pool of BTX. In order to do this, one must not only separate bound BTX from free, but one must also dis¬ tinguish between AcChoR-bound versus R4137-bound toxin. In order to do so, AcChoR was first bound to a concanavalin-A column. This was found to have little or no effect on toxin binding (Figure 6A) . Then such immobilized AcChoR was mixed with different concentrations of R4137 and con¬ stant amounts of 125 I-labelled-BTX. Afterwards, the col¬ umns were washed by repeated centrifugation/resuspension and the amount of radioactivity associated with the column was determined. As can be seen in (Figure 6B) , R4137 effectively competes for BTX against AcChoR. Finally, R4137 was tested for competition with AcChoR in. vivo. Male and female mice (approximately 5 weeks, 20-25g) , both inbred (Balb/C) and outbred (CD1) strains, were first injected intraperitoneally with R4137 or a placebo - a similar fraction derived from bacteria transformed with the unmodified vector pATH2 (these cells have no toxin-binding capacity) . Five minutes later all the mice were challenged with various amount of d- tubocurarine or α-cobratoxin (CTX) . The dose was gauged to cause 80% death of untreated mice. The toxins were admin¬ istered by subcutaneous injection to the nape of the necks of the animals. The animals were observed and the number of deaths during the course of two hours after the toxin injection was recorded. The results are shown in Table 1. As shown in Figure 7 the survival rate of the R4137 treated mice is markedly improved over the control and the absolute number of recoveries is at least 300% better than those that were given the placebo. TABLE 1 Bacterial Exp. protein Strain Sex Toxin Dose No. of Survivors (i.p.) (s.c.) g/kg mice pATH2 Balb/C M Tubo 0.40 R4137 pATH2 Balb/C M Tubo 0.36 R4137 pATH2 Balb/C M Tubo 0.36 R4137 pATH2 CD1 M Tubo 0.36 R4137 pATH2 CD1 Tubo 0.29 R4137 pATH2 CD1 M CTX 0.15 R4137 pATH2 CD1 M CTX 0.15 R4137 A further experiment was carried out where mice were first injected with a lethal dose of cobra toxin and one hour later given either a placebo or the cholinergic decoyant. The animals which received the placebo all died, whereas the decoy-treated animals were dramatically pro¬ tected (90% survival). The results are shown in Table 2. It should be understood that the same receptor site is involved with all of BTX, CTX and d-tubocurarine as well as decamethonium and rabies-virus. TABLE 2 Material injected Time after toxin Survivors (minutes) R4137 (decoyant) 30 6/10 60 9/10 90 6/7 pATH (placebo) 60 0/10 The fact that R4137 has been proven to be a decoyant against toxin is only a case in point for the general claim of molecular decoyants as therapeutic agents. In the specific case of the AcChoR, R4137, or improved versions of this molecular decoyant, can serve as an anti¬ dote against cobra-like snake bites by specifically inter¬ cepting the α-toxin constituent of these venoms. In addition, d-tubocurarine is routinely used in surgery as a neuro-muscular blocking agent and a decoyant based on R4137 could be extremely useful as its antidote. This utility is of particular importance as the use of ligand analogues as described in the prior art would be as detrimental as the blockers themselves. Moreover, it has recently been demon¬ strated that rabies-virus specifically binds AcChoR and that its binding can be competed against d-tubocurarine or BTX (Lentz et al. , Science, 215, 182-184 (1982)). Thus, a decoyant based on R4137 will also provide therapeutic affects in the treatment of rabies. It should be understood that R4137 is but an intermediate tool which, by its genetic manipulation or chemical processing, would allow those of ordinary skill in the art to design even more efficient cholinergic decoy- ants. Thus, the present invention is intended to include not only the specific 17 amino acid sequence of R4137, but variations and derivatives thereof which maintain, and preferably improve, its functional or pharmacological characteristics. For example, modified peptide sequences can be readily prepared and tested by routine techniques for preferred toxin-binding characteristics so as to more effectively compete against the native receptor. Such modification may involve substitution, deletion or inser¬ tion of amino acids or their chemical modification. For example, longer lived decoyants may be obtained in this manner. As enzymatic degradation of the decoyants in vivo may cause some decoyants to be relatively short-lived, one method of preventing such degradation would be by making synthetic peptides containing d-amino acids. Alternative¬ ly, based on the fusion-protein blueprint, organic mole- cules, i.e., not proteinaceous, can be designed so as to satisfy the physico-chemical requirements of a decoyant which must form a functional interface with the toxin. It should further be understood that the decoy¬ ants of the present invention can be modified by extending the polypeptide or by adding specific chemical moieties intended to aid in drug design or to permit the decoyants to be used for additional utilities. One such modification would be to extend the polypeptide by moieties intended to affect solubility, e.g. , by the addition of a hydrophilic residue, such as serine, or a charged residue, such as glutamic acid. Furthermore, the decoyant could be extended for the purpose of stabilization and preservation of a desired conformation, such as by adding cysteine residues for the formation of disulfide bridges. Another reason to modify the decoyants would be to make the decoyant detectable, even after administration. This might be done by radioiodination with a radioactive iodine isotope, directly, or by adding tyrosine for subse¬ quent radioiodination. Such detectable decoyants could be used to detect the presence and/or location of specified pathogenic agents or toxins. For example, detection of an accumulation of R4137 in the area of a dog bite would indicate the presence of rabies-virus. Thus, such detect¬ able decoyants could be used for the selective detection and mapping of given foreign agents or for diagnosis of the invasion of such an agent. A further reason for modifying decoyants would be for accelerated clearance of the conjugated foreign agent from the body. For example, a decoyant linked to an asialoglyco-moiety would be expected to be cleared by the liver. Thus, for example, a decoyant mimicking the recep¬ tor site of an anti-cancer chemotherapeutic agent and containing such an asialoglyco-moiety, or any other moiety which would aid in its clearance, could be used to inacti¬ vate and quickly remove excess chemotherapeutic agent after the therapy is completed in order to reduce side effects. Proof of the effectiveness of R4137 as a decoyant against toxin in vivo establishes the operability of the general concept of the present invention. Accordingly, it should be understood that the present invention comprehends not only decoyants based on the cholinergic binding site but decoyants based on any endogenous receptor for a for¬ eign agent which exerts an undesired effect only after binding to that endogenous receptor. The first requirement of a decoyant in accordance with the present invention is that it be a mimic of the endogenous receptor, i.e., it must functionally resemble the binding site, although it may differ physically. The term "functionally resemble" means that the decoyant binds to the foreign agent in question in a selective and specific manner and with rea- sonable affinity. For the purpose of this invention, a receptor can be any ligand-binding molecule as, for exam¬ ple, the ligand-binding site of a traditional cell-surface receptor, the substrate binding site of an enzyme, the ligand-binding site of gangliosides, etc. It must be understood, however, that a decoyant in accordance with the present invention cannot be an immunoglobulin nor can it be derived from immunoglobulins. While it is true that an immunoglobulin directed against the "binding domain" of a pathogen could generally function as a decoy, such is not intended to be included within the concept of the present invention. A decoyant in accordance with the present inven¬ tion should not be substantially immunogenic. Reduction of size is a means of diminishing the immunogenicity of a substance, but not all large molecules are as immunogenic as some small molecules. To be classified as a decoyant in accordance with the present invention, the substance must be substantially non-immunogenic in the system of the host, regardless of the size of the substance, although the smallest possible size is preferred. It is very important, however, that the decoyant not be sufficiently immunogenic to elicit an autoimmune response against the endogenous receptor when administered in vivo. In the case of the cholinergic receptor, such an autoimmune response might cause a case of myasthenia gravis. A decoyant in accordance with the present inven¬ tion must comprise the essential elements of the binding site of a receptor and not substantially more. For the purpose of this invention, the "essential elements" of a binding site are defined as those elements essential for the decoyant activity, i.e., ligand recognition and bind¬ ing. A receptor consists of many residues, only a few of which are involved in ligand recognition and binding. However, as discussed above, the decoyants of the present invention may be further modified for purposes of drug design. Thus, for example, the entire α-subunit of AcChoR would not qualify as a decoyant, being both immunogenic and also considerably longer than necessary. The α-subunit does, however, contain the potential information needed for the design and construction of a decoyant, i.e., sequence α-184-200. The fact that some additional peptide units may also be present, for example to improve the solubility of the essential required sequence, would not remove the structure from the category of decoyant as long as it is still substantially non-immunogenic and it is still selec- tive, specific and of reasonable affinity. Addition of sugar molecules could be a modification with the same effect. Thus, additions to the molecule for the purpose of drug design are not considered when determining whether the substance contains substantially more than the elements of the endogenous receptor which are required for binding to the foreign agent in question. As previously indicated, a decoyant must be selective, specific and of reasonable affinity with respect to the agent for which it is designed. Thus, for example, mannose, a simple sugar, may interfere with infection by bacteria that have type-I mannose specific pili; however, the selectivity of mannose is not sufficient and neither is its affinity. A decoyant is a drug designed to intercept an invading foreign agent having an undesired effect. Such foreign agents may include toxins, poisons, bacteria, viruses, including retroviruses, etc. As long as the foreign agent exerts its pathogenic or toxic effect (or any other effect which is desired to be eliminated) only after binding to a receptor site somewhere in the host, a decoy¬ ant can be designed in accordance with the present inven- tion to prevent such binding and thereby eliminate such undesired effect. Once the procedures of the present invention are known, as well as the fact that such receptor fractions containing ligand binding sites will still com¬ petitively bind the ligands in vivo, those of ordinary skill in the art will understand that decoyants derived from other receptors, designed to bind to other pathogenic agents and toxins, can be obtained using no more that routine experimentation. Among the pathogenic agent-receptor pairs for which decoyants can readily be obtained in accordance with the present invention is the T-cell surface glycoprotein CD4 (T4) , which is the cellular receptor for human immuno¬ deficiency virus, type 1 (HIV-1) , the first member of the family of viruses that cause acquired immunodeficiency syndrome (AIDS) . The infection of the HIV virus starts through the binding of its envelope protein (gpl20) to the T4 receptor (CD4) located on the T4' lymphocytes. It has recently been confirmed that soluble, secreted forms of CD4 can be used to competitively bind HIV-1 and thus neutralize the infectivity of HIV-1 (Smith et al, "Blocking of HIV-1 Infectivity by a Soluble, Secreted Form of the CD4 Anti- gen". Science. 238, 1704-1707 (1987)). Intact CD4 would not be a decoyant in accordance with the present invention in view of its size. It does, however, contain the essence for a decoyant. The minimal binding domain of CD4 can be identified using no more than routine experimentation by the means described herein for arriving at the minimal binding domain for the cholinergic receptor, e.g., by means of proteolysis and protein blotting followed by recombinant DNA procedures. Another ligand-receptor pair particularly suited for the preparation of decoyants in accordance with the present invention is organophosphate-acetylcholine ester- ase. Such a decoy would relieve some of the effects of nerve gas. Other examples are LSD and the serotonin recep¬ tor and strychnine and the glyσine receptor. Table 3 shows additional ligand-receptor pairs for which decoyants in accordance with the present inven¬ tion can be designed using no more than routine experimentation: TABLE 3 Ligand Receptor Calcium 28 kDa of bovine cerebellum and kidney Calmodulin Heparin apoE and apoB of human plasma Pili of Gonococcus 14 and 16 kDa proteins of CHO cells Virus Retrovirus type 3 67 kDa glycoprotein of rodent lymphoid and neuronal cells Sendai virus Human erythroσyte glycophorin Potato spindle tuber Nuclear proteins viroid The decoyants of the present invention may be administered to an animal, including a human patient, in order to ameliorate the undesired effects of the foreign agent for which it was designed. Such decoyants can be used not only for the treatment of humans, but also for the treatment of other animals, including mammals, poultry, fish, etc. Furthermore, decoyants in accordance with the present invention could be designed for the protection or treatment of plants. The specific effective dosages for the treatment of any given foreign agent can readily be empirically determined by those of ordinary skill in the art without undue experimentation. However, those skilled in the art will understand that the dosage of decoyant will depend to some extent on the amount of foreign agent in the system of the host. The ratio of decoyant to foreign agent molecules is preferably in the range of 1:1 to 1:10. Animal tests have shown that a large excess of decoyant is not necessary for effectiveness. Preferably, the amount of foreign agent in the bloodstream of the host will be moni¬ tored and the decoyant dosage adjusted accordingly during the course of treatment. Compositions within the scope of the present invention include compositions wherein the decoyant is present in an effective amount to achieve its intended purpose. Determination of the effective amounts is within the skill in the art. In addition to the decoyants of the present invention, the pharmaceutical compositions may contain suitable pharmaceutically acceptable carriers comprising excipients and auxiliaries which facilitate processing of the active compounds into preparations which can be used pharmaceutically. Preferably, the preparations, particu¬ larly those which can be administered by injection, con¬ tain from about 0.1 to 99 percent, and preferably from about 25 to 85 percent by weight, of the active ingredient, together with the excipient. Any conventional route of administration may be used for the decoyants of the present invention. Although the preferred mode of administration is by injection, e.g., intravenously, intradermally, mtraperitoneally, etc, they may also be administered orally, by suppository or by any other route. Other non-conventional means of administration can be envisioned which are also intended to be compre¬ hended within the scope of the present invention. For example, while the present system for the expression of the active ingredient by bacteria involves the bacterial ex¬ pression vector pATH2, other bacterial expression systems exist which actually secrete the expressed protein into the medium. It could be contemplated that such a secreting expression system could be used to generate the decoyant from within the host rather than producing it ex vivo and administering it to the host. Obviously, the secreting system must be compatible with the host. The term "admin¬ istration" as used in the present specification and claims is intended to include such in vivo secretion systems. The pharmaceutical preparations of the present invention are manufactured in a manner which is itself known, for example, by means of conventional mixing, dis¬ solving, or lyophilizing processes. Suitable formulations for parenteral administration include aqueous solutions of the active compounds in water-soluble form. In addition, suspensions of the active compounds as appropriate oily injection suspensions may be administered. Suitable lipo- philic solvents or vehicles include fatty oils such as sesame oil, or synthetic fatty acid esters such as ethyl oleate or triglycerides. Aqueous injection suspensions may contain substances which increase the viscosity of the suspension such as sodium carboxymethyl cellulose, sorbi- tol, and/or dextran. Optionally, the suspension may also contain stabilizers. The decoyants of the present inven¬ tion may also be administered in the form of liposomes, pharmaceutical compositions in which the active ingredient is contained either dispersed or variously present in corpuscles consisting of aqueous concentric layers adherent to lipidic layers. The active ingredient may be present both in the aqueous layer and in the lipidic layer, or, in any event, in the non-homogeneous system generally known as a liposomic suspension. The foregoing description of the specific embodi- ments will so fully reveal the general nature of the inven¬ tion that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and therefore such adaptations and modifications are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.

Claims: CLAIMS : 1. A molecular decoyant comprising a substance having a chemical structure which functionally resembles an endogenous receptor for a foreign agent which exerts an undesired effect only after first binding to the endogenous receptor, wherein said substance is neither an immuno¬ globulin nor a derivative of an immunoglobulin and wherein said substance is a substance selected from the group consisting of: (1) a substance derived from a natural endogenous receptor by physically dividing the endogenous receptor into a portion which is small enough so as not to elicit an autoimmune response against the endogenous receptor when administered jln vivo, or otherwise lacks substantial immunogenicity, yet retains the essential elements of the binding site of said receptor, (2) a chemical sequence derived from (1) synthe¬ sized by chemical and/or recombinant DNA techniques and having the required functional resemblance, and (3) a substance of (1) or (2) having a pharmaceu¬ tically acceptable accessory moiety attached thereto which does not effect said functional resemblance. 2. A molecular decoyant in accordance with claim 1, wherein said substance of (1) or (2) is not substantial- ly larger than that size needed to retain the essential elements of the binding site of the receptor. 3. A molecular decoyant in accordance with claim 1, wherein said endogenous receptor is a cholinergic recep¬ tor and said foreign agent is a curarimimetic neurotoxin. 4. A molecular decoyant in accordance with claim 1, wherein said endogenous receptor is the nicotinic acetylcholine receptor and said foreign agent is α- bungarotoxin, cobratoxin, d-tubocurarine, decamethonium, or rabies-virus. 5. A molecular decoyant in accordance with claim 1, wherein said endogenous receptor is the T4 cell antigen of lymphocytes and said foreign agent is an HIV virus. 6. A molecular decoyant in accordance with claim 1, wherein said substance is derived from a natural endoge¬ nous receptor by physically dividing the endogenous recep¬ tor into a portion which is small enough so as not to elicit an autoimmune response against the endogenous recep¬ tor when administered jLn vivo, or otherwise lacks substan- tial immunogenicity, yet retains the essential elements of the binding site of said receptor. 7. A molecular decoyant in accordance with claim 1, wherein said substance is a peptide chain expressed by cells genetically engineered by reco binant DNA techniques. 8. A molecular decoyant in accordance with claim 1, wherein said substance is a synthetic peptide. 9. A molecular decoyant in accordance with claim 3, including the amino acid sequence: Trp-Lys-His-Trp-Val-Tyr-Tyr-Thr-Cys-Cys-Pro-Asp-Thr-Pro- Tyr-Leu-Asp or a variation thereof maintaining binding characteristics with a curarimimetic neurotoxin which are at least as strong as those of said sequence with the same neurotoxin. 10. A method for the treatment of an animal, including a human, which has been subjected to a foreign agent which exerts an undesired effect only after first binding to an endogenous receptor, comprising administering to said animal a molecular decoyant in accordance with claim 1 which functionally resembles the endogenous recep¬ tor for the foreign agent to which the animal has been subjected, said molecular decoyant being administered in a quantity sufficient to ameliorate the effects of said foreign agent. 11. A method in accordance with claim 10, wherein said foreign agent is a snake venom toxin. 12. A method in accordance with claim 10, wherein said foreign agent is rabies virus. 13. A method in accordance with claim 10, wherein said foreign agent is HIV virus. 14. A pharmaceutical composition for the treat¬ ment of an animal, including a human, which has been sub¬ jected to a foreign agent which exerts an undesired effect only after first binding to an endogenous receptor, com¬ prising a molecular decoyant in accordance with claim 1 in an amount effective to ameliorate the effects of said agent, and a pharmaceutically acceptable excipient.

Inventors: GERSHONI JONATHAN M, GERSHONI, JONATHAN, M.
Applicants: GERSHONI JONATHAN M, GERSHONI, JONATHAN, M.
Application year: 1988
Main classification: A61K37
IPC codes: A61K38, A61K47, C07K7, C07K14