Alkylating nucleosides. 2. Synthesis and cytostatic activity of bromomethylpyrazole and pyrazole nitrogen mustard nucleosides.

Glycosylation of ethyl 3(5)-(bromomethyl)pyrazole-5(3)-carboxylate (3) and 3(5)-(bromomethyl)pyrazole-5(3)-carboxamide (4) with poly-O-acetylated sugars via an acid-catalyzed fusion method afforded the corresponding ethyl 3-(bromomethyl)pyrazole-5-carboxylate and 3-(bromomethyl)pyrazole-5-carboxamide substituted nucleosides 5 and 7, respectively. In some cases, the positional isomers 6 and 8 were also obtained. Treatment of 5 and 7 with methanolic ammonia gave the deprotected 3-(aminomethyl)pyrazole-5-carboxamide nucleosides 9. Reaction of 3--5 and 7 with bis(2-chloroethyl)amine led to the corresponding pyrazole nitrogen mustards 10--13. All the bromomethylpyrazole nucleosides described showed significant cytostatic activity against HeLa cell cultures.

Analgesic dipeptide derivatives. 4. Linear and cyclic analogues of the analgesic compounds arginyl-2-[(o-nitrophenyl)sulfenyl]tryptophan and lysyl-2-[(o-nitrophenyl)sulfenyl]tryptophan.

The syntheses of Trp(Nps)-Arg-OMe.HCl (15) [Trp(Nps) = 2-[(o-nitrophenyl)sulfenyl]tryptophan], its three stereoisomers, and their corresponding cyclic analogues are reported. The preparation of Trp(Nps)-Lys-OMe (19) and its cyclic analogue is also described. All these compounds have been designed as analogues of the analgesic dipeptide derivatives X-Trp(Nps)-OMe (1b, X = Arg; 2b, X = Lys). In the case of dipeptides containing Arg or D-Arg, the coupling reactions were achieved via the isobutyl chloroformate and N-methylmorpholine mediated mixed anhydride procedure, while in the case of the Lys analogue, the N,N-dicyclohexylcarbodiimide method was employed. Sulfenylation reactions were carried out with Nps-Cl in acidic media. Cyclization to the diketopiperazines was achieved by using acetic acid as catalyst. The antinociceptive effects of all these new Trp(Nps)-containing dipeptides were evaluated after icv administration in mice, and the effects were compared with those of 1b, 2b, Tyr-Arg (Kyotorphin), and Tyr-D-Arg. The most active compounds, 15 and 19, were found to exhibit a naloxone-reversible antinociceptive effect similar to those of 1b and 2b and approximately 50 and 12.5 times higher than those of Kyotorphin and its D isomer, respectively. Trp(Nps)-D-Arg-OMe.HC1, D-Trp(Nps)-Arg-OMe.HC1, and cyclo[Trp(Nps)-Arg].HC1 were also more effective than Kyotorphin (5, 10, and 10 times, respectively). In view of the structure-activity relationships obtained, several similarities between this series of Trp(Nps)-containing dipeptides and that of Kyotorphin analogues have emerged.

Analgesic dipeptide derivatives. 3. Synthesis and structure-activity relationships of o-nitrophenyl-modified analogues of the analgesic compound H-Lys-Trp(NPS)-OMe.

A series of analogues of the analgesic dipeptide derivative H-Lys-Trp(NPS)-OMe has been designed to determine the influence of the (2-nitrophenyl)sulfenyl (NPS) moiety on the activity. The syntheses and antinociceptive effects of these analogues of general formula H-Lys-Trp(R)-OMe [R = phenylsulfenyl (PS) (9); R = (2-carbomethyoxyphenyl)sulfenyl (CmPS) (10); R = (4-nitrophenyl)sulfenyl (pNPS) (11); R = (2,4-dinitrophenyl)sulfenyl (DNPS) (12); R = [2-(acetylamino)-2-carbomethoxyethyl]sulfenyl (AacCmES) (13); R = [2-(acetylamino)phenyl]sulfenyl (AacPS) (17); R = tert-butylsulfenyl (t-BuS) (23); R = (2-carbomethoxyethyl)sulfenyl (CmES) (24)] are described. Reaction of Z-Lys(Z)-Trp-OMe (3) with PS-, CmPS-, pNPS-, DNPS-, and AacCmES-Cl afforded the corresponding 2-(sulfenyl)tryptophan derivatives, which on treatment with boron-tris(trifluoroacetate)/trifluoroacetic acid or trimethylsilyl iodide in acetonitrile (Me3SiI/CH3CN) provided 9-13, respectively. Sulfenylation of 3 with NPS-Cl gave Z-Lys(Z)-Trp(NPS)-OMe, which, on catalytic hydrogenation of the nitro group using 10% Pd/C followed by acetylation of the resulting amino function and removal of the protecting Z groups, gave 17. Condensation of 2-(tert-butylsulfenyl)- and 2-[(2-carbomethoxyethyl)sulfenyl]tryptophan methyl ester, obtained by reaction of methyl 3a-hydroxy-1,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole-2-carboxyla te with the corresponding thiol, with Z-Lys(Z)-OSu afforded Z-Lys(Z)-Trp(t-BuS)-OMe and Z-Lys(Z)-Trp(CmES)-OMe, which on treatment with Me3SiI/CH3CN provided 23 and 24, respectively. Intracerebroventricular administration of 10 elicited a naloxone-reversible antinociceptive effect in mice similar to that of H-Lys-Trp(NPS)-OMe. No analgesia was however found with the phenylsulfenyl or acyclic sulfenyl substituted dipeptides 9, 11, and 17 or 13, 23, and 24. The Trp(DNPS)-containing analogue was neurotoxic. Structure-activity studies indicate that the role of the NPS and CmPS moieties could be related to the adoption of a preferential active conformation.

beta-Turned dipeptoids as potent and selective CCK(1) receptor antagonists.

To improve our knowledge of the bioactive conformation of CCK(1) antagonists, we previously described that replacement of the alpha-MeTrp residue of dipeptoids with the (2S,5S, 11bR)-2-amino-3-oxohexahydroindolizino[8,7-b]indole-5-carbox ylate (IBTM) skeleton, a probed type II' beta-turn mimetic, led to restricted analogues (2S,5S,11bR,1'S)- and (2S,5S,11bR, 1'R)-2-(benzyloxycarbonyl)amino-5-[1'-benzyl-2'-(carboxy)ethyl]carbam oyl-3-oxo-2,3,5,6,11,11b-hexahydro-1H-indolizino[8,7-b]indole, 1a,b, showing high binding affinity and selectivity for CCK(1) receptors. In this report, we describe the synthesis and binding profile of new analogues of compounds 1 designed to explore the importance of the C-terminal residue and of the type of beta-turn on the receptor binding affinity and selectivity. Structure-affinity relationship studies show that a C-terminal free carboxylic acid and an S configuration of the Phe and betaHph residues are favorable for CCK(1) receptor recognition. Moreover, selectivity for this receptor subtype is critically affected by the beta-turn type. Thus, while compounds 15a and 16a, containing the (2S,5S,11bR)- and (2R,5R, 11bS)-IBTM frameworks, respectively, are both endowed with nanomolar affinity for CCK(1) receptors, restricted dipeptoid derivative 15a, incorporating the type II' IBTM mimetic, shows approximately 6-fold higher CCK(1) selectivity than analogue 16a, with the type II mimetic. From these results, we propose that the presence of a beta-turn-like conformation within the peptide backbone of dipeptoids could contribute to their bioactive conformation at the CCK(1) receptor subtype. Concerning functional activity, compounds 15a and 16a behave as CCK(1) receptor antagonists.

Analgesic dipeptide derivatives. 7. 3,7-Diamino-2-hydroxyheptanoic acid (DAHHA) containing dipeptide analogues of the analgesic compound H-Lys-Trp(Nps)-OMe.

A series of diastereomeric dipeptides, analogues of the analgesic compound H-Lys-Trp(Nps)-OMe (2), containing 3,7-diamino-2-hydroxyheptanoic acid (DAHHA) and 2-[(o-nitrophenyl)sulfenyl]tryptophan [Trp(Nps)] has been synthesized. These compounds were tested as enkephalin-degrading aminopeptidases (APs), AP-M and AP-B inhibitors, and analgesics. The inhibitory potencies and the antinociceptive effects depended on the stereochemistry of the compounds. (2S,3R)-DAHHA-L-Trp(Nps)-OMe (26d) was a highly potent and selective enkephalin-degrading APs inhibitor, with an IC50 value in the 10(-8) M range. Although this derivative was about 10(3)-fold more potent than 2 against these enzymes, their antinociceptive effects were completely similar. These results indicate that the inhibitory capacity of this series of Trp(Nps)-containing dipeptides against enkephalin-degrading enzymes is not an important factor for their antinociceptive effects.

5-(Tryptophyl)amino-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-based potent and selective CCK(1) receptor antagonists: structure-activity relationship studies on the central 1,3-dioxoperhydropyrido[1,2-c]pyrimidine scaffold.

To further define the pharmacophore of the potent and selective 5-(tryptophyl)amino-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-based CCK(1) receptor antagonists the electronic and topographic properties of the central 1,3-dioxoperhydro-pyrido[1,2-c]pyrimidine scaffold have been modified. With this aim, the 1- and 3-oxo groups have been replaced by the thioxo- and deoxi-analogues, and the fused piperidine ring has been contracted to the corresponding pyrrolidine moiety. The results of the evaluation of the new analogues as CCK receptor ligands, in rat pancreas and cerebral cortex preparations, showed that, whereas replacement of oxygen with sulfur is allowed, reduction of the 1- or 3-oxo groups or the contraction of the fused piperidine ring lead to the complete loss of binding affinity at CCK(1) receptors. The thioxo-analogues 5a, 8a, 12a, and 12b showed functional CCK(1) antagonist activity, inhibiting the CCK-8-stimulated amylase release from pancreatic acinar cells. The 1-thioxo analogue 5a, with subnanomolar affinity (IC(50) = 0.09 x 10(-9) M), was found to be the most potent and selective compound within the family of 5-(tryptophyl)amino-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-based CCK(1) antagonists.

Synthesis and antiviral evaluation of nucleosides of 5-methylimidazole-4-carboxamide.

Due to the antiviral activity of certain 5-substituted imidazole nucleosides related to ribavirin, 5-methylimidazole-4-carboxamide nucleosides having beta-D-ribofuranosyl, 2-deoxy-beta- and -alpha-D-ribofuranosyl, and (2-hydroxyethoxy)methyl moieties have been prepared and tested as antiviral agents. 1-beta-D-Ribofuranosyl-5-methylimidazole-4-carboxamide was obtained by deacetylation of the corresponding tri-O-acetyl nucleoside 11 or by deacetylation and ammonolysis of the blocked ethyl 5-methylimidazole-4-carboxylate nucleoside 10, which was prepared from the stannic chloride catalyzed condensation of the trimethylsilyl derivative of ethyl 4(5)-methylimidazole-5(4)-carboxylate. Glycosylation of 4(5)-methylimidazole-5(4)-carboxamide with 3,5-di-O-p-toluoyl-2-deoxy-D-erythro-pentofuranosyl chloride via mercuric cyanide method provided an anomeric mixture of the blocked 5-methylimidazole-4-carboxamide deoxynucleoside 14 along with an anomeric mixture of the 4-methyl 5-carboxamide isomer 15. Separation of compound 14 into the corresponding beta and alpha anomers was achieved by conversion to the 3',5'-di-O-acetyl derivatives 17 and 18, which after chromatographic separation were deacetylated to give 1-(2-deoxy-beta-D-erythro-pentofuranosyl)-5-methylimidazole-4-carboxa mid e and its alpha anomer 20. 1-[(2-Hydroxyethoxy)methyl]-5-methylimidazole-4-carboxamide was prepared by alkylation of the imidazole 13 with (2-acetoxyethoxy)methyl bromide followed by treatment with methanolic ammonia. All these imidazole nucleosides were tested in HeLa cell cultures against type 1 herpes simplex and vesicular stomatitis viruses. The ribofuranosyl derivative 12 showed a significant activity against type 1 herpes simplex virus.

Uridine 5'-diphosphate glucose analogues. Inhibitors of protein glycosylation that show antiviral activity.

A series of analogues of uridine 5'-diphosphate glucose and uridine 5'-diphosphate glucosamine have been synthesized by reaction of 2,3,4,6-tetra-O-benzyl-, 2,3,4,6-tetra-O-benzoyl-, 2,3,4,6-tetra-O-acetyl-, and 2,3,4,6-tetra-O-palmitoyl-alpha-D-glucopyranose and 2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-alpha-D-glucopyranose with chlorosulfonyl isocyanate and 2',3'-O-isopropylideneuridine. Isopropylidene and acetyl groups of the resulting 5'-O-[[[[(alpha-D-glucopyranosyl)oxy]carbonyl]amino]sulfonyl] -2',3'-O-isopropylideneuridine derivatives were removed by reaction with a TFA/water (5:1) mixture and methanolic ammonia, respectively. The 5'-O-[[[[(2",3",4",6"-tetra-O-benzyl-and 2",3",4",6"-tetra-O-benzoyl-alpha-D-glucopyranosyl)oxy]carbonyl] amino]sulfonyl]-2',3'-O-isopropylideneuridine (13 and 19) and the corresponding deisopropylidenated derivatives showed antiviral activity as determined by the inhibition of the cytopathic effect induced by HSV-1 replication and by the plaque assay method. Compound 13 inhibited glycosylation of proteins in HSV-1 infected HeLa cells.

Alkylating nucleosides. 4. Synthesis and cytostatic activity of chloro- and iodomethylpyrazole nucleosides.

The synthesis and cytostatic activity of several chloromethyl- and iodomethylpyrazole nucleosides are described. Glycosylation of ethyl 3(5)-(chloromethyl)pyrazole-5(3)-carboxylate (3) and 3(5)-(chloromethyl)pyrazole-5(3)-carboxamide (4) with poly(O-acetylated) sugars via an acid-catalyzed fusion method gave the corresponding 3-(chloromethyl)-5-carboxylate and 3-(chloromethyl)-5-carboxamide substituted nucleosides 7 and 9, respectively. From the reaction of 4 with tetra-O-acetyl-beta-D-ribofuranose, the 5-(chloromethyl)-3-carboxamide-substituted derivative 11 was also obtained. Reaction of 7, 9, and 11 with sodium iodide in acetone provided the related iodomethylpyrazole nucleosides 8, 10, and 12. In general, chloromethyl-substituted nucleotides showed moderate activities against HeLa cells, while all the corresponding iodomethyl derivatives exhibited high activities. Some of these latter compounds increased the life span of mice bearing ECA tumor.

5-(Tryptophyl)amino-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-based potent and selective CCK(1)receptor antagonists: structure-activity relationship studies on the substituent at N2-position.

To establish structure-activity relationships a new series of analogues of the highly potent and selective CCK(1) receptor antagonist (4aS,5R)-2-benzyl-5-(N-Boc-tryptophyl)amino-1,3-dioxoperhydropyrido[1,2-c]-pyrimidine (1a) modified at N2-position of the central scaffold has been prepared and evaluated as CCK receptor ligands. With this aim the N2-benzyl group has been replaced by methyl, cyclohexyl, aromatic groups, 1-phenylethyl, and 1-carboxy-2-phenylethyl group. Then, substituents with different electronic and steric properties were introduced into different positions of the phenyl group of analogues 19a and 19b. The results of the CCK receptor binding and in vitro functional activity evaluation suggest the importance of the lipophilic character and an appropriate spatial orientation of the moiety linked at the N2-position of the 1,3-dioxoperhydropyrido[1,2-c]pyrimidine template for potent and selective binding and antagonist activity at CCK(1) receptor subtype. The 2-cyclohexyl and (2S)-1-naphthyl derivatives 18a and (2S)-20a have emerged as more potent and selective CCK(1) receptor antagonists than the lead compound 1a. Additionally, the results confirm the (4aS,5R)-stereochemistry at the central bicyclic skeleton as an essential structural requirement for potent binding to this receptor subtype.

Synthesis and stereochemical structure-activity relationships of 1,3-dioxoperhydropyrido[1,2-c]pyrimidine derivatives: potent and selective cholecystokinin-A receptor antagonists.

The synthesis and stereochemical structure--activity relationships of a new class of potent and selective non-peptide cholecystokinin-A (CCK-A) receptor antagonists based on the 1,3-dioxoperhydropyrido[1,2-c]pyrimidine skeleton are described. The most potent member of this series of eight diastereoisomers, (4aS,5R)-2-benzyl-5-[N-[(tert-butoxycarbonyl)-L-tryptophyl]-amino] - 1,3-dioxoperhydropyrido[1,2-c]pyrimidine, displays nanomolar CCK-A receptor affinity and higher than 8000-fold potency at the CCK-A than at the CCK-B receptor. As CCK-A antagonist, this compound inhibits the CCK-8-evoked amylase release from pancreatic acinar cells at a low concentration, similar to that of the typical antagonist Devazepide. Highly strict stereochemical requirements for CCK-A receptor binding and selectivity have been found. The L-Trp and the 4a,5-trans disposition of the bicyclic perhydropyrido[1,2-c]pyrimidine are essential for binding potency and selectivity.

Ketomethylene and (cyanomethylene)amino pseudopeptide analogues of the C-terminal hexapeptide of neurotensin.

A series of pseudopeptide analogues of the C-terminal hexapeptide of neurotensin (NT8-13), namely [Tyr11 psi[COCH2]Phe12]-, [Ile12 psi[COCH2]Phe13]-, and [Tyr11 psi[CH(CN)NH]Ile12]NT8-13 with different stereochemistries, has been synthesized and evaluated for its potency in displacing labeled NT from rat cortex membranes. Ketomethylene pseudohexapeptides were prepared from the corresponding Boc-protected ketomethylene dipeptide derivatives, previously formed, using different solid phase synthesis (SPS) conditions, while (cyanomethylene)amino analogues were directly prepared by SPS using Fmoc strategy. H-Arg-Arg-Pro-Tyr psi[COCH2]-Phe-Leu-OH was nearly as potent as NT8-13 and [Phe12]NT8-13 in binding to the receptor. Comparison of the affinities for the pseudohexapeptides, here reported, with those of the psi-[CH2NH] analogues indicates the importance of the CO group in the amide or surrogate linkage at 11-12 and 12-13 positions in the receptor binding process.

5-(Tryptophyl)amino-1,3-dioxoperhydropyrido[1,2-c]pyrimidine-based potent and selective CCK(1) receptor antagonists: structural modifications at the tryptophan domain.

Analogues of the previously reported potent and highly selective CCK(1) receptor antagonist (4aS, 5R)-2-benzyl-5-(N-Boc-tryptophyl)amino-1,3-dioxoperhydropyrido-[1, 2-c]pyrimidine (2a) were prepared to explore the structural requirements at the Boc-tryptophan domain for CCK(1) receptor affinity. Structural modifications of 2a involved the Trp side chain, its conformational freedom, the Boc group, and the carboxamide bond. Results of the CCK binding and in vitro functional activity evaluation showed three highly strict structural requirements: the type and orientation of the Trp side chain, the H-bonding acceptor carbonyl group of the carboxamide bond, and the presence of the Trp amino protection Boc. Replacement of this acid-labile group with 3, 3-dimethylbutyryl or tert-butylaminocarbonyl conferred acid stability to analogues 14a and 15a, which retained a high potency and selectivity in binding to CCK(1) receptors, as well as an in vivo antagonist activity against the acute pancreatitis induced by caerulein in rats. Oral administration of compounds 14a and 15a also produced a lasting antagonism to the hypomotility induced by CCK-8 in mice, suggesting a good bioavailability and metabolic stability.

Pharmacological evaluation of IQM-95,333, a highly selective CCKA receptor antagonist with anxiolytic-like activity in animal models.

1. The pyridopyrimidine derivative IQM-95,333 ((4aS,5R)-2-benzyl-5-[N alpha-tert-butoxicarbonyl)L-tryptophyl] amino-1,3dioxoperhydropyrido[1,2-c]pyrimidine), a new non-peptide antagonist of cholecystokinin type A (CCKA) receptors, has been evaluated in vitro and in vivo in comparison with typical CCKA and CCKB receptor antagonists, such as devazepide, lorglumide, L-365,260 and PD-135,158. 2. IQM-95,333 displaced [3H]-CCK-8S binding to CCKA receptors from rat pancreas with a high potency in the nanomolar range. Conversely, the affinity of this new compound at brain CCKB receptors was negligible (IC50 > 10 microM). IQM-95,333 was a more selective CCKA receptor ligand than devazepide and other CCKA receptor antagonists. 3. Like devazepide, IQM-95,333 was a more potent antagonist of CCK-8S- than of CCK-4-induced contraction of the longitudinal muscle from guinea-pig ileum, suggesting selective antagonism at CCKA receptors. 4. IQM-95,333 and devazepide were also potent inhibitors of CCK-8S-stimulated amylase release from isolated pancreatic acini, a CCKA receptor-mediated effect. The drug concentrations required (IC50s around 20 nM) were higher than in binding studies to pancreas homogenates. 5. Low doses (50-100 micrograms kg-1, i.p.) of IQM-95,333 and devazepide, without any intrinsic effect on food intake or locomotion, blocked the hypophagia and the hypolocomotion induced by systemic administration of CCK-8S, two effects associated with stimulation of peripheral CCKA receptors. 6. IQM-95,333 showed an anxiolytic-like profile in the light/dark exploration test in mice over a wide dose range (10-5,000 micrograms kg-1). Typical CCKA and CCKB antagonists, devazepide and L-365,260 respectively, were only effective within a more limited dose range. 7. In a classical conflict paradigm for the study of anxiolytic drugs, the punished-drinking test, IQM-95,333, devazepide and L-365,260 were effective within a narrow dose range. The dose-response curve for the three drugs was biphasic, suggesting that other mechanisms are operative at higher doses. 8. In conclusion, IQM-95,333 is a potent and selective CCKA receptor antagonist both in vitro and in vivo with an anxiolytic-like activity in two different animal models, which can only be attributed to blockade of this CCK receptor subtype.

Ketomethylene and methyleneamino pseudopeptide analogues of insect allatostatins inhibit juvenile hormone and vitellogenin production in the cockroach Blattella germanica.

Metabolic studies on insect allatostatins have suggested that the dipeptide Leu-Tyr may be a target for endopeptidases. In order to increase resistance to degradation, methyleneamino psi [CH2NH] and ketomethylene psi [COCH2] peptide bond surrogates have been introduced at the position Leu3-Tyr4 of the allatostatin Asp-Arg-Leu-Tyr-Ser-Phe-Gly-Leu-amide (BLAST-2), and Leu3-Phe4 of [Phe4]BLAST-2, respectively. Assays of inhibition of juvenile hormone (JH) synthesis in vitro by corpora allata from the cockroach Blattella germanica showed that both analogues were similarly active to the respective model peptides. The methyleneamino analogue was further tested in vivo as an inhibitor of JH synthesis, and in vivo and in vitro as an inhibitor of vitellogenin production by the fat body of B. germanica. The analogue was less active than BLAST-2 when tested in vitro, but more active than it when tested in vivo.

Antinociceptive effects in rodents of the dipeptide Lys-Trp (Nps) and related compounds.

Intracerebroventricular administration of the synthetic dipeptide derivative Lys-Trp (Nps) (LTN) elicits a potent and naloxone-sensitive antinociceptive effect in mice and in rats using heat and electrical current respectively as the noxious stimuli. LTN does not induce analgesia by directly acting on opioid receptors but the peptidase inhibiting activity of the new compound may account in part for the behavioral effect. LTN produces also a marked decrease in the met-enkephalin content of the periaqueductal gray suggesting a possible enkephalin releasing property. Structure-activity studies with different analogs of LTN indicate that replacement of Lys by other basic amino acids results also in compounds with a potent antinociceptive effect whereas replacement by neutral or acidic amino acids leads to a complete loss of activity.

Prolonged antinociceptive activity of pseudodipeptide analogues of Lys-Trp(Nps) and Trp(Nps)-Lys.

Peptide bond substitution in the molecules of Lys-Trp(Nps) (LTN) and Trp(Nps)-Lys (TNL) by an aminomethylene and ketomethylene bond, respectively, afforded pseudodipeptides with analgesic activity. The new compounds Lys psi(CH2NH)-Trp(Nps)-OMe (LTNAM) and Trp(Nps)psi(COCH2)(R,S)-Lys (TNLKM) induced a dose-dependent and naloxone-reversible analgesia following intracerebroventricular (ICV) administration to mice. The antinociceptive effects were longer lasting compared to those induced by the parent compounds. The pseudodipeptides protected Met-enkephalin degradation by rat striatal slices and, combined with an ineffective dose of the opioid peptide, induced analgesia. LTNAM and TNLKM were as potent as LTN to inhibit brain aminopeptidase in vitro and ex vivo. An increased resistance to proteolysis of the pseudodipeptides may explain their prolonged analgesic activity.

2,5-Diketopiperidine derivatives as non-peptide ligands for cholecystokinin receptors.

Easily accessible 2,5-diketopiperidines have been used as templates for the construction of Trp-Phe and Trp-Asp-Phe-NH2 mimics. The cycle [L-Trp psi[COCH2]-L-Phe] analogue 1a has shown to possess significant and selective affinity for CCKA receptors.