JOURNAL OF MEDICINAL CHEM ISTRY Q Copyright 1993 by the American Chemical Society

Volume 36, Number 21 October 15,1993

Pe rspe c t iu e

Concepts and Progress in the Development of Mimetics

Gary L. Olson,. David R. Bolin, Mary Pat Bonner, Michael BOs, Charles M. Cook, David C. Fry, Bradford J. Graves, Marcos Hatada, David E. Hill, Michael Kahn, Vincent S. Madison, Victoria K. Rusiecki, Ramakanth Sarabu, Jerry Sepinwall, George P. Vincent, and Matthew E. Voss Roche Research Center, Hoffmann-La Roche Inc., Nutley, New Jersey 07110

Received May 19, 1993

Medicinal chemistry research has been dramatically ology is applicable to inhibition as well. The broad transformed by biotechnology. Previously, synthetic use of the term “peptide mimetics” is unavoidable, but chemistry and natural products screening dominated drug advocates of rational design do not favor its use to describe research, but now molecular biology has become a driving compounds found by screening. Nevertheless, leads found force behind screening and the establishment of macro- by screening contribute to the understanding of phar- molecular targets. Most of the research in biotech macophores that recognize peptide binding sites. Simi- companies has been directed to peptide and protein larly, natural product opiate alkaloids are frequently cited therapeutics in spite of problems associated with their as examples of peptidomimetics2J because they validate low bioavailability, rapid metabolism, and lack of oral many of the concepts invoked in rational design. In fact, activity. Because of these limitations, research groups they also make a case for how structurally different continue to rely upon chemical synthesis of nonpeptide nonpeptides may be from their peptide parents (lacking substances for drug discovery, recognizing that small flexibility, amide bonds, and obvious pharmacophore molecules are likely to remain the most viable avenue for similarity) and how their modification can lead to highly the identification and optimization of potential drugs. The selective ligands for subtypes of receptors in both peptide challenge is to devise strategies that take advantage of the and nonpeptide c~mpounds.~ strengths of biotechnology to define and characterize Another class of peptide mimetic research that lies molecular targets and of medicinal chemistry to develop between the screening approach and de novo mimetics compounds. A promising approach integrating these design is centered on the replacement of individual peptide disciplines is the field of peptide mimetics, a conceptual bonds by a nonpeptidic These peptide surrogates approach which considers and proteins not as are complemented by the synthesis of peptides incorpo- potential therapeutics but rather as leads for the discovery rating unnatural amino acids, conformational constraints, of other classes of compounds. and larger subunits, for example, dipeptide mimetics. The terms “peptide mimetics” and “peptidomimetics” These compounds bridge the gap between simple peptide have been utilized interchangeably to describe compounds analogs and the completely nonpeptide structures. In this discovered through a variety of research strategies.l respect, they represent a step along the path toward the Indeed, even compounds identified by random screening rationally designed nonpeptide. In many cases, peptide and subsequently optimized through structural modifi- surrogates are likely to be an essential stage in the cation have been termed peptidomimetics if the initial development of a pharmacophoric hypothesis for confor- lead was found in an assay in which the natural ligand is mationally flexible peptides.6 In addition, new technol- a peptide or protein.2 The field of enzyme inhibitors uses ogies for the rapid screening of peptide libraries7 will peptide mimetics terminology for replacements of seg- generate many highly flexible lead structures with modest ments of peptide-based substrates and inhibitors. In this binding affinities. Optimization and eventual translation Perspective, the emphasis will be on mimetics of peptide of a peptide lead to a small molecule drug will probably and protein ligands for receptors, although the method- require isosteric replacements, cyclic peptide derivatives, 0022-2623/93/1836-3039$04.00100 1993 American Chemical Society 3040 Journal of Medicinal Chemistry, 1993, Vol. 36, No. 21 Perspective

H

Bnoy:o@

En0 +-O(CH,),NHR

6 Figure 2. mimetic 6 (R = H)based on a glucose 0 H-f ternplate.l8 analogous to those of the peptide. Even in protein ligand structures where backbone conformations are known from X-ray crystallography, the side chains involved in inter- action with a receptor may move considerably from their orientation on the crystal surface they dock into 3 4 as recognition sites on the receptor. If a lead activity is seen for a flexible mimetic, adding conformational constraints to side chain groups is then a rational approach to enhance potency and selectivity. 4. Select appropriate targets based on availability of a pharmacophore hypothesis, or develop the information as a first step. Attempting to carry out a peptide mimetic H 0 project on a system for which there is no idea of structure- 5 activity relationships and no three-dimensional hypothesis of the bioactive conformation is irrational. Such a program Figure 1. @-Turnmimetics (1,l12,12 3,13 4,14 P). is likely to generate many inactive compounds and little useful guidance for further work. For systems where the bond surrogates, and conformational constraints as logical biological rationale or medical need is sufficient to justify steps in the process. a peptide mimetics approach, efforts to provide a phar- The rational design of nonpeptide compounds thus is macophore hypothesis as a first step are justified and not feasible without the information obtained from the critical to the outcome of the peptide mimetics programs. study of the structure-activity relationships and confor- Nonpeptide leads designed by this strategy might be mational properties of peptide structures. When such expected to behave more like the structures derived from information is available, and the indication is that the screening synthetic compound collections or natural peptide backbone is not a critical element in receptor products in terms of their bioavailability, metabolic binding,* the design of a nonpeptide becomes feasible. stability, and transport. With an appropriate and suc- The transition to a mimetic can then be made rational by cessful design and subsequent modification of an initial adopting some general principles, adapted from the treatise lead structure, the likelihood that these peptide mimetics of Farmer.g will become drug candidates is high. Progress toward this goal is just beginning to emerge. In this Perspective, we Design Criteria for Peptide Mimetics will describe some of our initial efforts to design peptide 1. Replace as much of the peptide backbone as possible mimetics to constrain secondary structural elements of by a nonpeptide framework. If bond surrogates have been peptides and will review the progress in our first cases of shown to retain activity, or peptide bonds are not exposed de novo design following the basic principles outlined in the presumed bioactive conformation, then a structural above. template may be designed to eliminate amide bonds. 2. Maintain peptide side chain pharmacophoric groups Mimicking Architectural Elements of Protein as in the peptide. Rather than completely dispensing with Structure a relationship between peptide and nonpeptide, initially &Turn Mimetics. Major effortslo have been devoted to designed mimetics may retain groups as found on the the development of templates that mimic or stabilize peptide, as these are most likely to be recognized by a several common architectural elements of peptide and receptor. For example, if a lysine residue is known to be protein structure, Le., &turns, helices, and @-sheets. The required for activity in the peptide, then the first- majority of the @-turn mimetics have been dipeptide generation mimetic should have a primary amino group mimetic replacements for the i + 1 and i + 2 residues at at the end of a methylene chain. As the design progresses the corners of the turn. These have been designed to retain through other generations, amine variants, different chain the intramolecular hydrogen bonding network of an lengths, conformational constraints, and substitutions on appended peptide chain (e.g., Figure 1, compounds 1-4). the nitrogen may be fruitful ways to enhance binding However, these examples lackan appendage for the corner affinity. residue side chains. Since these are the residues that are 3. Retain some conformational flexibility in first- most exposed to the surface of a protein, and most likely generation mimetics. The probability that a pharma- to be involved in intermolecular interactions with a cophore hypothesis will be definitive for flexible peptide receptor, their elimination limits the scope of the mimetic side chains is low, making it necessary that the first designs to that of a group that enforces a conformational constraint. leave some of the side chain pharmacophoric groups While these structures are valuable for answering unconstrained so that they may adopt conformations fundamental questions about protein folding, we felt it Perspective Journal of Medicinal Chemistry, 1993, Vol. 36, No. 21 3041 Scheme I

FH3 y3 PW13 H?NOH*HCI

N-methyllonnanilide odichlorobenzene CHO ? 81% "GO 1. LAH, THF, reflux - Y Br Br 2. (~%uoco)~o, - dioxane, 5% NaOH NKo-'BuCH2CI2, -10 'C 0

CO, PdC12(PPh3)2 7 m Br o-'Bu PPh3,llO0C "2 MeOH, Bu~N K0 88% 62%

7 0 83%

14

Chart I m Ac-Cys-(-N-Me)Arg-Gly-Asp-Pen-", Gly-Pen-Gly-ArgGIy-Asp-Pro-Cys-Ala 7 8 r-- I Ac-Cys-Asn-Dtc-Amf-Gly-Asp-Cys-OH

0

NH

O*COOH 11 10 NH

NH

12 (Pen= Penicillamine, Dtc= 5,5-dimethylthiazolidine-4-carboxylicacid, and Amf= p- aminometh ylp henylalanine) 3042 Journul of Medicinal Chemistry, 1993, Vol. 36, No. 21 Perspectiue

Figure 3. VIP sequence and critical residues (shadowed).

Figure 4. VIP structureby NMR in solution (left, 25 9% MeOH in H20;center, 50% MeOH in HnO;right, superposition showing closely aligned central core). was important to devise a more highly functionalized Another system that has shown promise in mimicking a system that could theoretically carry side chain groups in p-tum ina biologically activecompoundisthesomatostatin positions that corresponded to those of the corner residues mimetic 6 (Figure 2) based on a glucose template described of the tum.16 Two possibilities emerge for such com- by Hir~chmann.'~This compound, designed on the basis pounds. In one case, the functionalized turn could be of the cyclic hexapeptide developed by the Merck group," incorporated into a peptide sequence to generate the is an early example of a mimetics approach following required turn and simultaneously present the comer "Farmer's rules." The fact that 6 is relatively weak in residues to a potential receptor. Another hypothetical comparison to the cyclic peptide as a somatostatin case would allow the corner residues and turn geometry mimetic21 probably reflects the greater conformational to participate in a binding interaction in the absence of mobility of the side chains appended to the glucose the rest of the protein molecule. In a case in which the template. tumislocatedatacriticalrecognitionsite,suchcompounds have the greatest potential to antagonize a biological RGD Mimetic Inhibitors of GpIIbIIIa-Fibrinogen response. The 9-membered ring lactam system 5 was Receptor Binding. The basicchemicalstudiesand model designed with the idea of replacing the intrachain hydrogen systems for 8-turn mimetics have prepared the way for bond between residues i and i + 3 with a methylene group the development of specific compounds with defined and replacing all of the amide bonds except for the exposed biological targets. Some of the most promising examples corner with carbon-carbon bonds. to date are the diverse structures that have emerged from In addition to our synthetic and structural studies with work on inhibitors of GpIIbIIIa binding to fibrinogen modelcompounds related to5, othergroupa haveextended receptors based on the ubiquitous, inhibitory RGDW this approach to specific target peptide turns and have (Arg-Gly-Asp-(Ser)) peptide sequences. This sequence described initial characterization of these systems.'6*17 has been constrained into cyclic peptideszz(e.g., 7-9), and Unfortunately, progress toward structures of this type that a number of nonpeptide mimetics have been described could he prepared by short sequence~"b~~has been slow. (e.g., 1&13). Benzene ring~,2~.~~steroids,z6 and even Perspective Journul of Medicinal Chemistry. 1993, Vol. 26, No. 21 304s 7 “-f ...... - ...... ( ...... hl....”“ 0 n H

Spacer D$tance Torsbn Acgk

-132.5 ‘

0 ...... 04 A4, NIH

VIP 13-20 [50% MeOH] gsso

Standard a-helix i. k7 -102.9 @ = -50*,T- -41‘ Figure 5. Parameters for an ci-helix mimetic in VIP.

t

I- -$

Figure 6. VIP hybrid model6 (left, hybrid 14 and VIP; right, space-filling models of VIP hybrid 14). benzodiazepines26 have been employed as templates and its shape. If recognition of a particular helix involved a represent excellent examples of the application of ‘Farm- large number of residues, or a series of adjacent side chains, er’s rules.” mimicry would be impractical because of the highly a-Helix Mimetics-Replacement of a Bridging complex templates that would be required to present side Helix in Vasoactive Intestinal Peptide. Among the chains correctly. However, there are two cases in which best characterized structural features of proteins is the mimicry of an a-helical structure can be envisioned. (1) a-helix. Unlike a @-turn,the a-helix incorporates a larger The residuesresponsihle for biological recognitionlie along number of residues (e.g., 3.4/turn) to achieve one face of the helix:? so that a small bridging template 3044 Journul of Medicinul Chemistry, 1993, Vol. 36. No. 21 Perspective structure of VIP in 25% and 50% aqueous methanol --e-.VIP EC3 = 10 nM I ...*' solution was highly helical, particularly so in the middle 'O01 0- I of the sequence where the Ala substitutions had no effect on potency (Figure 4). These two results suggested strongly that the region between Tyr'O and TyP was performing as a helical spacer to hold the important residues at an appropriate distance, but without having a direct side-chain interaction with the VIP receptor. Our approach to mimetics of a @-turn where we developed a specific replacement for the turn structure * 204 could be adapted to the VIP problem by designing an amino acid that could replace a portion of the structural whelii segment by a nonpeptide helix mimetic. This spacer molemle would obviously have to span the reguired x 0 distance between the residues at either end of the excised helix. Inaddition, the peptide bondsemergingfrom either Concantration (nM) end of the spacer should be consistent with the path of the Figure 7. Relaxation of airway smooth muscle from Guinea pig peptide helix. After some initial measurements, the trachea by VIP and VIP-helix mimetic 8. segment between residues 13 and 20 was selected for can be designed analogously to the @-turntemplates. (2) replacement, giving distances and angles to be spanned by the spacer shown in Figure 5. Thehelixperformaafunctionasaspacerwithnoindividual as amino acid side chains specifically required for activity Types of structures that could have the appropriate within a segment of the helix. In such a case, a spacer distance between the excision points and an extended, template of modest complexity could be envisioned. relatively rigid shape that could fit within the diameter An example of this second case emerged from Bolin's of the helix axis were considered. After modeling some studies" of vasoactive intestinal peptide (VIP)(Figure 3). linear compounds, we realized that bent tricyclic ring VIP is a 28 amino acid peptide of interest as a potential systems, such as phenothiazine or dibewepine, would therapeutic agent for the treatment of asthma In have a similar distance between the connecting points on asthmatics, VIP is present at very low levels in the lung the helix. Importantly, the bend in the middle of the ring compared to normals. VIP is synthesized in the lung, system positioned the vectors of the amide bonds at the whereitactaasabronchcdilator,byrelaxingairwaysmooth proper angles to follow the continuation of the peptide muscle and by increasing mucus secretion. Extensive helix. The template selected for Synthesis was the research has led to the discovery of highly potent, phenothiazine amino acid 7. This structure was synthe- metabolically stable VIP peptide anal05 of potential sized as outlined in Scheme I and was incorporated into interest as therapeutics. a VIP sequence by solid-phase synthesis giving 14. As an approach to elucidate the important residues of Models of the VIP hybrid structure incorporating the VIP required for biological activity, Bolin performed an phenothiazine spacer are illustrated in Figure 6. The 'alanine scann of the VIP sequence, synthesizing VIP spacer fits the helical segment well and does not distort analogs with Ala substitution at each individual residue. the remaining peptide hackhone. In addition, the tricyclic The study revealed that a significant segment of the ring system lies within the volume of the helical backbone structure between the important residues Tyr'O and TyP and side chains, presenting the important Tyr residues in was insensitive to the substitution by Ala. At the same their native orientation. Whatthe modeldoes not address, time, NMR studies by Fry et al." indicated that the however, is the disruption of the hydrogen bonding lattice

ib-

1 s-

Figure 8. u-Carbon backbone of interleukin-la showing exposed surface loops. Perspective Journal of Medicinnl Chemistry, 1993, Vol. 36, No. 21 304s

2

Figure 9. Left: SI-loop in IL-la sequence 41-48. Right n-loop mimetic 16 incorporating naphthalene spacer. systems will be needed to prepare more potent variants as well as soluble hybrids whose structures can be studied directly. %Loop Mimetic in Interleukin-la. The approach devised for the design of the a-helix mimetic for VIP was to replace a segment of the peptide with a spacer that would maintain the distance and torsion angles of the native helii. A similar strategy can be applied to mimic Dismcc bcfween Leu 41 CU and Val 48 NH ...... 10.944 A an R-loop structure" if a spacer is designed to constrain Torsion Angle beween the highlighted bonds .__._...._,.,..,_125.9' the ends of a loop sequence. We have applied this ideas1 to mimic a prominent loop of the immunomodulatory Figure 10. Structural parameters for n-loop spacers. cytokine, IL-la, whose crystal structure was determined that is present in the normal helix and missing in the by Graves and Hatada.s2 In IL-la, the 12-stranded beta hybrid molecule. The tricyclic ring system does not have barrel is interconnected by a series of loops and turns hydrogen bond donors or acceptors to initiate a helical (Figure 8). Among these is a large, prominent R-loop turn following the spacer and thus might mimic the between residues 41 and 48 of the sequence. biologidy irrelevant portion well while failiig to preserve The ends of this loop are too far apart (ca. 11 A) to be the conformation needed for activity at those regions constrained by formation of a cyclic peptide or by disulfide outside of the spacer. bridges. However, since the side chains of residues Leu" The results of testing the VIP hybrid 14 in a model of and Val'* project into the center of the loop, they could airway smooth muscle relaxation are shown in Figure 7. be replaced by a nonpeptide spacer with the proper In this test, guinea pig tracheal rings are treated with test distances and torsional constraints (Figures 9 and 10). compounds, and their relaxation is compared to the The system which was chosen for this purpose is the maximal relaxation elicited by the @-agonist,isoproterenol. l-(Z-aminoethyl)naphthalene-Z-propionicacid, 15. This The hybrid 14 was remarkably potent, exhibiting full amino acid was capable of fitting into the space occupied agonist activity at about 10% of the potency of native by the side chains of Leu4I and Val,@ and modeled low- VIP. Unfortunately, the low solubility of 14 precluded an energy conformations of the loop with the spacer included NMR analysis of the structure to determine the extent of wereconsistent with the loop conformation from theX-ray helicity of the hybrid. Nevertheless, the activity of 14 as structure. Thus, the hybrid cyclic peptide 16 could be a full agonist supports the data from the Ala scan that the expected to mimic the role of the remainder of the IL-la coreofthehelixinVIPmaybecharacterizedasastructural protein in presenting the loop in a conformation that could scaffolding. Properly designed mimetics, like the phe- potentially be recognized by a receptor. nothiazine system, have the potential to replace the The synthesis of amino acid 15 and the cyclic peptide polypeptide spacer helix and to position the N- and hybrid 16 have been de~cribed.~~The structure of 16 was C-terminal pharmacophores. Further research on these studied by 2D-NMR (COSY and NOESY) experiments in

Fqure 11. Overlay of 90 lowestenergy conformations of 0-loop hybrid 10 consistent with NMR data 3046 JOUrMl of MediciMl Chemistry, 1993, Vol. 33, No. 21 Perspective

Figure 12. X-ray (open bonds) and propcaed bioactive wnformations (solid bonds) of TRH.

=@J& pqR

TRH MIMETICS Figure 13. TRH phmawphore and design of cyclohexane template for TRH mimetics.

15

(" w

16 DMSO-de at room temperature. NOES were quantified and converted into distances which were then employed as constrainta for molecular dynamics simulations. For the cyclic peptide hybrid 16, optimization yielded 30 low- energy conformations consistent with the NMR data. A superposition of the backbones of these 30 structures is mimetics and volume overlap. illustrated in Figure 11. The figure illustrates how the to a concentration of 2 mM. This observation suggesta naphthalene spacer is able to constrain the backbone C, that the constrained 0-loop mimetic is not recognized by to similar Loop structures for all the conformations of the the receptor, a conclusion that is supported by recent peptide loop, even though the spacer itself is capable of findings that the IL-1 binding epitope consists of 7-9 adopting a number of low-energy conformations. residues which are clustered in space, but none of which The cyclic peptide hybrid 16 was assayed for its ability are found in the 41-47 sequence of the 0-loop." Current to compete with 1261-labeledIL-lor in binding to type I work is directed toward mimetics that could present IL-1 receptors on mouse EL-4 cell rnernbrane~.~~These importantresiduesfromthe bindingepitopeonnonpeptide assays indicated that 16 did not inhibit IL-1 binding up scaffolding. Perspective Journal of Medicinal Chemistry, 1993, Vol. 36, No. 21 3047

Fd 17

H H

H H Figure 15. Orientation of side chains in cyclohexane TRH mimetic diastereoisomers. Mimetics of a Target Peptide-TRH Mimetics In designing our mimetic, we wanted to maintain the Based on a Cyclohexane Framework. The concepts spatial orientation of the pharmacophore using a scaf- outlined above, and the @-turn, a-helix, and Q-loop folding which would not extend beyond the boundaries of examples described, illustrate the validity of our approach the peptide. For this purpose, we chose the 1,3,5-cis- to design mimetics of defined structural elements of trisubstituted cyclohexane framework to replace the peptide and protein architecture. In the case of smaller, peptide backbone (Figure 13). The mimetic retains the conformationally mobile peptides, the design of a mimetic pyFoglutamy1 amide, prolineamide, and imidazole phar- requires an appreciation of the bioactive conformation of macophoric groups hypothesized to be required for activity the peptide. Except for systems in which enzyme- or and permits orientations consistent with the proposed receptor-bound peptides have been observed directly by bioactive conformation (Figure 14). The cyclohexane ring X-ray or NMR, this information can be obtained only system should contribute only nonspecific, hydrophobic through the study of constrained peptide analogs and character to the mimetic while permitting conformational peptides containing rigid bond surrogates. One system mobility at the side chains. The design, synthesis, and for which much of this information is available is the simple biological activity of our first TRH mimetics have recently , thyrotropin-releasing (TRH, pGlu- been reported46 following our initial disclosures of the His-ProNH2). results.& TRH is a hypothalamic peptide which functions as a The most potent of the mimetics in the Morris water neuroendocrine hormone by increasing thyrotropin-stim- maze test147 a behavioral model of cognitive impairment, ulating hormone (TSH) leading to an elevation of was found to be the N-benzyl compound 17 (Ro 24-9975). hormone levels. TRH binds to high- and low-affinity In this test, TRH reduced mean latencies over a dose range receptors labeled by 3H-TRH and 3H-3-MeHis2-TRHin from 0.003 to 3 mg/kg ip but was inactive by the oral route. discrete regions.36 TRH has pronounced CNS Compound 17 was active from 0.0003 to 3.0 mg/kg ip and effects36 and is able to enhance performance in cognitive from 0.0003 to 3.0 mg/kg PO in this test. behavioral models in animals3' suggesting its potential to Because of our route of synthesis, it was also possible treat cognitive disorders, including those associated with to determine the effects of diastereoisomers in the series. Alzheimer's disease. When the ring center bearing the imidazolylmethyl group Crystal38139 and solution structuresrn of TRH and TRH was inverted, the compounds were ca. 10-fold less active. peptide analogs are known, and models have been proposed By contrast, the compound in which all three of the ring for the pharmacophore and bioactive conformation(s) of asymmetric centers is reversed (18) was virtually equi- TRH and TRH analogs. Moore and Marshallq1proposed potent to 17. This result is significant from the perspective the lactam moiety of the pyroglutamyl residue, the of the ability of the cyclohexane ring to serve as a neutral histidine imidazole ring, and the carboxamide of the scaffoldingfor the pendant groups, since the ring inversion terminal prolineamide as pharmacophoric groups based at all three centers, as in 18, also orients the pharma- on activities in TSH release and high affinity receptor cophoric groups in a 1,3,5-cis relationship as indicated in binding. Figure 15. For the purposes of our mimetics design, we chose a The conclusion that the cyclohexane compounds are starting conformation in which the peptide backbone mimicking TRH is also supported by the activity of other approximates the Y-shaped X-ray structure of TRHq2 compoundsa having substituents chosen from among those except that, in our model, the imidazole ring is not locked known to lead to active TRH peptide analogs. Thus, in the hydrogen-bonded interaction with the terminal analogs of 17 having phenyl, 2-imidazolyl-, and l-methyl- carboxamide as in the crystal but is oriented in a 4-imidazolyl groups in place of the 4-imidazolyl group gave conformation consistent with the observations from so- structureactivity results which paralleled those obtained lution NMR studies (Figure 12). This model is analogous from corresponding TRH peptide analogs. Although 17 to that proposed by Marshall and colleagues.&*M does not compete with SH-3-MeHis2-TRHin the standard 3048 Journal of Medicinal Chemistry, 1993, Vol. 36, No. 21 Perspec tiue

TRH binding assay, direct binding studies with 3H-R~ (5) (a) Spatola, A. F. Peptide Backbone Modifications: A Structure- Activity Analysis of Peptides Containing Amide Bond Surrogates, 24-9975 have recently identified a lower affinity binding ConformationalConstrainta,.and Related Backbone Replacements. site in rat brain, and work on biochemical characterization In Chemistry and Biochemutry of Amino Acids, Peptides, and of this receptor and its regional localization by autorad- Proteins; Weinstein, B., Ed.; Marcel Dekker: New York, 1983; Vol. VII, pp 267-57. (b) Rich, D. H. Peptidase Inhibitors. In iography is being pursued.49 Further studies will be Comprehensive Medicinal Chemistry;Hanach, C., Sammes, P. G., required to establish whether the cyclohexane mimetics Taylor, J. B., Eds.;Pergamon Press: Oxford, 1989; Vol. 2, pp 391- are exhibiting cognitive enhancement activities by a 441. (6) Tilley, J. W.; Danho, W.; Shiuey, S.-J.; Kulesha, I.; Sarabu, R.; mechanism that is associated with this receptor or through Swistok, J.; Makofeke, R.; Olson, G. L.; Chiang, E.; Rusiecki, V. K.; some other process. Wagner, R.; Michaelewski, J.; Triicari, J.; Nelson, D.; Chiruzzo, F. The peptide mimetics projects described in this Per- W.; Weatherford, S. Structure Activity of C-Terminal Modified Analogs of Ac-CCK-7. Znt. J. Pept. Rotein Rea. 1992,39, 322- spective are representative of efforts being made in many 336. laboratories to translate peptide and protein structures (7) (a) Houghten, R. H. General Method for the Rapid Solid-Phase into small molecular weight, nonpeptide compounds that Synthesis of Large Numbers of Peptides: Specificity of Antigen- Antibody Interaction at the Level of Individual Amino Acids. Roc. could mimic their biological functions. The research is Natl. Acad. Sci. U.S.A. 1986,82,5131-6136. (b) Geysen, H. M.; truly interdisciplinary and dependent upon molecular Meleon, R. H.; Barbling, S. J. Use of Peptide Synthesis to Probe biology and peptide chemistry to provide targets and Viral Antigens for Epitopes to a Reeolution of a Single Amino Acid. Proc.Natl. Acad. Sci. U.S.A. 1984,81,3898-4002. (c) Lam, K. S.; structure-activity data. These results, coupled with Salmon, S. E.; Hersh, E. M.; Hruby, V. J.; Kazmierski, W. M.; structural insights from crystallography, NMR, and mod- Knapp, R. J. A New Type of Synthetic Peptide Library for eling provide the basis for design, while the implementation Identifying Ligand-Binding Activity. Nature 1991,354,82-84. (8) Backbonerecognitionmayplay an importentrolein peptide binding and practicality of the ideas is ultimately dependent on in specific cases, e.g., MHC molecules and . See: (a) organic synthesis. With the progress being made now, it Bjorkman, P. J.; Snper, M. A.; Samroui, B.; Bennet, W. S.; is reasonable to predict that some general principles of de Strominger, J. L.; Wiley, D. C. Structure of the Human ChI Histocompatibility Antigen,HLA-A2. Nature 1987,326,606-512. nouo peptide mimetics design will evolve from today’s (b) Brown, J. H.; Jardetsky, T.; Saper, M. A.; Samraoui, B.; efforts and that important therapeutic agents will emerge Bjorkman, P. J.; Wdey, D. C. A Hypothetical Model of the Foreign as a result of this collaboration between biotechnology Antigen Binding Site of ChI1 Hietocompatibility Molecules. Nature 1988, 332, 845-860. (c) Smith, A. B.; Keenan, T. P.; and medicinal chemistry. The future will present many Holcomb, R. C.; Sprengeler, P. A,; Guzman, M. C.; Wood, J. L.; opportunities for these two sciences to work together and Caroll, P. J.; Hirschmann, R. S. Design, Synthesis, and Cry&al indeed to become partners in the search for new treatments Structure of a Pyrrolhone-Baaed Peptidomimetic Pomeaing the Conformation of a &Strand Potential Application to the Design for disease. of Novel Inhibitors of Proteolytic Enzymes. J. Am. Chem. SOC. 1992,114,10672-10674. Acknowledgment. The coauthors whose research is (9) Farmer, P. S. Bridging the Gap between Bioactive Peptides and described in detail in this paper and many other individuals Nonpeptides: Some Perspectives in Design. In Drug Design; Ari&ns,E. J.; Ed.; Academic Prem: New York, 1980, Vol. X, pp have contributed their efforts to the projects reviewed 119-143. here. Their enthusiasm and dedication to idea of the (10) For recent comprehensive reviews we: (a) Hblzmann, G. Peptide peptide mimetics has been exceptional. We are indebted conformation Mimetics (Part 1). Kontakte (Darmutadt) 1991, 3-12 and references cited therein. (b) Hblzmann, G. Peptide to them for their hard work and intellectual stimulation. Conformation Mimetice (Part 2). Kontakte (Darmutadt) 1991, In particular, the TRH mimetics project could not have . 56-63 and references cited therein. been completed without the experimental work of Mr. (11) Nagai,U.; Sato,K. SyntheeisofaBicyclicDipeptidewiththeShape of a &Turn Central Part. Tetrahedron Lett. 1985,26, 647650. Ho-Chuen Cheung and Mr. Elliot Chiang. Dr. Thomas (12) Feigel, M. 2,&Dimethyl-~(carbox~ethyl)-6-~inomethyl)phe- Zebovitz also contributed through supplying large quan- noxathiin &Dioxide: An Organic Sub~titutefor the @-Turnin tities of key intermediates. Ms. Dawn Johnson was Peptides? J. Am. Chem. SOC.1986,108,181-182. (13) Dlaz, H.; Espina, J. R.; Kelly, J. W. A Dibenzofuran-Bd Amino responsible for testing of the TRH mimetics in mice, and Acid Designed to Nucleate Antiparallel j3-Sheet Structure: Evi- the compounds are being studied biochemically by Dr. dence for Intramolecular Hydrogen-BondFormation. J.Am. Chem. Andrew Winokur and colleagues at the University of SOC.1992,114,8316-8318. (14) Genin, M. J.; Johneon, R. L. Design, Synthesis,and Conformational Pennsylvania. In the IL-1 project, Drs. Bradford Graves Analysis of a Novel Spiro-Bicyclic System EE a Type II &Turn and Marcos Hatada determined the crystal structures of Peptidomimetic. J. Am. Chem. SOC.1992,114,877&8783. the IL-1 proteins that formed the starting point for (15) Olson, G. L.; Vose, M.; Hill, D.; Kahn, M.; Madion, V. Design and Synthesis of a Protein &Turn Mimetic. J. Am. Chem. SOC.1990, modeling studies. Members of the Physical Chemistry 112,323-333. 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