5-Aminothiazoles Reveal a New Ligand-Binding Site on Prolyl Oligopeptidase Which is Important for Modulation of Its Protein-Protein Interaction-Derived Functions

A series of novel 5-aminothiazole-based ligands for prolyl oligopeptidase (PREP) comprise selective, potent modulators of the protein-protein interaction (PPI)-mediated functions of PREP, although they are only weak inhibitors of the proteolytic activity of PREP. The disconnected structure-activity relationships are significantly more pronounced for the 5-aminothiazole-based ligands than for the earlier published 5-aminooxazole-based ligands. Furthermore, the stability of the 5-aminothiazole scaffold allowed exploration of wider substitution patterns than that was possible with the 5-aminooxazole scaffold. The intriguing structure-activity relationships for the modulation of the proteolytic activity and PPI-derived functions of PREP were elaborated by presenting a new binding site for PPI modulating PREP ligands, which was initially discovered using molecular modeling and later confirmed through point mutation studies. Our results suggest that this new binding site on PREP is clearly more important than the active site of PREP for the modulation of its PPI-mediated functions.

Nonpeptidic Oxazole-Based Prolyl Oligopeptidase Ligands with Disease-Modifying Effects on α-Synuclein Mouse Models of Parkinson's Disease

Prolyl oligopeptidase (PREP) is a widely distributed serine protease in the human body cleaving proline-containing peptides; however, recent studies suggest that its effects on pathogenic processes underlying neurodegeneration are derived from direct protein-protein interactions (PPIs) and not from its regulation of certain neuropeptide levels. We discovered novel nonpeptidic oxazole-based PREP inhibitors, which deviate from the known structure-activity relationship for PREP inhibitors. These new compounds are effective modulators of the PPIs of PREP, reducing α-synuclein (αSyn) dimerization and enhancing protein phosphatase 2A activity in a concentration-response manner, as well as reducing reactive oxygen species production. From the best performing oxazoles, HUP-55 was selected for in vivo studies. Its brain penetration was evaluated, and it was tested in αSyn virus vector-based and αSyn transgenic mouse models of Parkinson's disease, where it restored motor impairment and reduced levels of oligomerized αSyn in the striatum and substantia nigra.

Azulene as a biphenyl mimetic in orexin/hypocretin receptor agonists

Azulene is a rare ring structure in drugs, and we investigated whether it could be used as a biphenyl mimetic in known orexin receptor agonist Nag 26, which is binding to both orexin receptors OX₁ and OX₂ with preference towards OX₂. The most potent azulene-based compound was identified as an OX₁ orexin receptor agonist (pEC₅₀ = 5.79 ± 0.07, maximum response = 81 ± 8% (s.e.m. of five independent experiments) of the maximum response to orexin-A in Ca²⁺ elevation assay). However, the azulene ring and the biphenyl scaffold are not identical in their spatial shape and electron distribution, and their derivatives may adopt different binding modes in the binding site.

2-Imidazole as a Substitute for the Electrophilic Group Gives Highly Potent Prolyl Oligopeptidase Inhibitors

Different five-membered nitrogen-containing heteroaromatics in the position of the typical electrophilic group in prolyl oligopeptidase (PREP) inhibitors were investigated and compared to tetrazole. The 2-imidazoles were highly potent inhibitors of the proteolytic activity. The binding mode for the basic imidazole was studied by molecular docking as it was expected to differ from the acidic tetrazole. A new putative noncovalent binding mode with an interaction to His680 was found for the 2-imidazoles. Inhibition of the proteolytic activity did not correlate with the modulating effect on protein–protein-interaction-derived functions of PREP (i.e., dimerization of alpha-synuclein and autophagy). Among the highly potent PREP inhibiting 2-imidazoles, only one was also a potent modulator of PREP-catalyzed alpha-synuclein dimerization, indicating that the linker length on the opposite side of the molecule from the five-membered heteroaromatic is critical for the disconnected structure–activity relationships.

A scaffold replacement approach towards new sirtuin 2 inhibitors

Sirtuins (SIRT1-SIRT7) are an evolutionary conserved family of NAD⁺-dependent protein deacylases regulating the acylation state of ε-N-lysine residues of proteins thereby controlling key biological processes. Numerous studies have found association of the aberrant enzymatic activity of SIRTs with various diseases like diabetes, cancer and neurodegenerative disorders. Previously, we have shown that substituted 2-alkyl-chroman-4-one/chromone derivatives can serve as selective inhibitors of SIRT2 possessing an antiproliferative effect in two human cancer cell lines. In this study, we have explored the bioisosteric replacement of the chroman-4-one/chromone core structure with different less lipophilic bicyclic scaffolds to overcome problems associated to poor physiochemical properties due to a highly lipophilic substitution pattern required for achieve a good inhibitory effect. Various new derivatives based on the quinolin-4(1H)-one scaffold, bicyclic secondary sulfonamides or saccharins were synthesized and evaluated for their SIRT inhibitory effect. Among the evaluated scaffolds, the benzothiadiazine-1,1-dioxide-based compounds showed the highest SIRT2 inhibitory activity. Molecular modeling studies gave insight into the binding mode of the new scaffold-replacement analogues.

Tetrazole as a Replacement of the Electrophilic Group in Characteristic Prolyl Oligopeptidase Inhibitors

4-Phenylbutanoyl-aminoacyl-2(S)-tetrazolylpyrrolidines were studied as prolyl oligopeptidase inhibitors. The compounds were more potent than expected from the assumption that the tetrazole would also here be a bioisostere of the carboxylic acid group and the corresponding carboxylic acids are at their best only weak inhibitors. The aminoacyl groups l-prolyl and l-alanyl gave potent inhibitors with IC₅₀ values of 12 and 129 nM, respectively. This was in line with typical prolyl oligopeptidase inhibitors; however, we did observe a difference with N-methyl-l-alanyl, which gave potent inhibitors in typical prolyl oligopeptidase inhibitors but not in our novel compound series. Furthermore, all studied 4-phenylbutanoyl-aminoacyl-2(S)-tetrazolylpyrrolidines decreased α-synuclein dimerization at the concentration of 10 μM, also when they were only weak inhibitors of the proteolytic activity of the enzyme with an IC₅₀ value of 205 μM. Molecular docking studies revealed that the compounds are likely to bind differently to the enzyme compared to typical prolyl oligopeptidase inhibitors represented in this study by 4-phenylbutanoyl-aminoacyl-2(S)-cyanopyrrolidines.

Structure-Activity Relationships of 1-Benzoylazulenes at the OX1 and OX2 Orexin Receptors

We previously demonstrated the potential of di- or trisubstituted azulenes as ligands (potentiators, weak agonists, and antagonists) of the orexin receptors. In this study we investigated 27 1-benzoylazulene derivatives, uncovering seven potentiators of the orexin response on OX₁ and two weak dual orexin receptor agonists. For potentiators, replacement of the azulene scaffold by indole retained the activity of four out of six compounds. The structure-activity relationships for agonism and potentiation can be summarized into a bicyclic aromatic ring system substituted with two hydrogen-bond acceptors (1-position, benzoyl; 6-position, carboxyl/ester) within 7-8 Å of each other; a third acceptor at the 3-position is also well tolerated. The same pharmacophoric signature is found in the preferred conformations of the orexin receptor agonist Nag26 from molecular dynamics simulations. Subtle changes switch the activity between weak agonism and potentiation, suggesting overlapping binding sites.

Stapled truncated orexin peptides as orexin receptor agonists

The peptides orexin-A and -B, the endogenous agonists of the orexin receptors, have similar 19-amino-acid C-termini which retain full maximum response as truncated peptides with only marginally reduced potency, while further N-terminal truncations successively reduce the activity. The peptides have been suggested to bind in an α-helical conformation, and truncation beyond a certain critical length is likely to disrupt the overall helical structure. In this study, we set out to stabilize the α-helical conformation of orexin-A_15-33 via peptide stapling at four different sites. At a suggested hinge region, we varied the length of the cross-linker as well as replaced the staple with two α-aminoisobutyric acid residues. Modifications close to the peptide C-terminus, which is crucial for activity, were not allowed. However, central and N-terminal modifications yielded bioactive peptides, albeit with decreased potencies. This provides evidence that the orexin receptors can accommodate and be activated by α-helical peptides. The decrease in potency is likely linked to a stabilization of suboptimal peptide conformation or blocking of peptide backbone-receptor interactions at the hinge region by the helical stabilization or the modified amino acids.

Replacement of the Disulfide Bridge in a KLK3-Stimulating Peptide Using Orthogonally Protected Building Blocks

Peptide "B-2", which is one of the most potent kallikrein-related peptidase 3 (KLK3)-stimulating compounds, consists of 12 amino acids and is cyclized by a disulfide bridge between the N- and C-terminal cysteines. Orthogonally protected building blocks were used in the peptide synthesis to introduce a disulfide bridge mimetic consisting of four carbon atoms. The resulting pseudopeptides with alkane and E-alkene linkers doubled the proteolytic activity of KLK3 at a concentration of 14 μM. They were almost as potent as the parent "B-2" peptide, which gives a 3.6-fold increase in the proteolytic activity of KLK3 at the same concentration.

Pseudopeptides with a centrally positioned alkene-based disulphide bridge mimetic stimulate kallikrein-related peptidase 3 activity

First successful pseudopeptides of the KLK3-activating bicyclic peptide “C-4” are reported.

N(epsilon)-thioacetyl-lysine-containing tri-, tetra-, and pentapeptides as SIRT1 and SIRT2 inhibitors

N()-Thioacetyl-lysine-containing tri-, tetra-, and pentapeptides, based on the alpha-tubulin and p53 protein sequences, were studied as SIRT1 and SIRT2 inhibitors. The potency of the pentapeptides depended on the selection of the side chains. The removal of N- and C-terminal residues of the pentapeptides yielded tripeptides with retained SIRT1 inhibitory activity but decreased SIRT2 inhibitory activity. The most potent SIRT1 inhibitors were equipotent with the reference compound (6-chloro-2,3,4,9-tetrahydro-1H-carbazole-1-carboxamide) with the IC(50) values of 180-330 nM.

Characterization of the binding properties of SIRT2 inhibitors with a N-(3-phenylpropenoyl)-glycine tryptamide backbone

SIRT2 inhibitors with a N-(3-phenylpropenoyl)-glycine tryptamide backbone were studied. This backbone has been developed in our group, and it is derived from a compound originally found by virtual screening. In addition, compounds with a smaller 3-phenylpropenoic acid tryptamide backbone were also included in the study. Binding modes for the new compounds and the previously reported compounds were analyzed with molecular modelling methods. The approach, which included a combination of molecular dynamics, molecular docking and cluster analysis, showed that certain docking poses were favourable despite the conformational variation in the target protein. The N-(3-phenylpropenoyl)-glycine tryptamide backbone is also a good backbone for SIRT2 inhibitors, and the series of compounds includes several potent SIRT2 inhibitors.

Oxadiazole-carbonylaminothioureas as SIRT1 and SIRT2 inhibitors

A new inhibitor for human sirtuin type proteins 1 and 2 (SIRT1 and SIRT2) was discovered through virtual database screening in search of new scaffolds. A series of compounds was synthesized based on the hit compound (3-[[3-(4-tert-butylphenyl)1,2,4-oxadiazole-5-carbonyl]amino]-1-[3-(trifluoromethyl)phenyl]thiourea). The most potent compound in the series was nearly as potent as the reference compound (6-chloro-2,3,4,9-tetrahydro-1H-carbazole-1-carboxamide).

Synthesis of 2,3,6,8-tetrasubstituted chromone scaffolds

A useful and efficient synthetic strategy to 2,3,6,8-tetrasubstituted chromone derivatives has been developed. 2-Aryl/styryl-8-bromo-6-chloro-3-hydroxychromone derivatives were synthesized and used as scaffolds by introducing a variety of substituents in the 3-, 6-, and 8-positions using palladium-mediated reactions. Excellent regioselectivity in all positions could be obtained by performing reactions in the 8-position first, in which Stille, Heck, Suzuki, and Sonogashira reactions gave good to excellent yields of product (63-98%). Stille and Heck reactions in the 6-position also gave the desired products in good yields (64-86%). The hydroxy group in the 3-position was activated as a triflate and used in productive Stille reactions (63-94%). This hydroxyl group was also used in O-alkylation reactions with different functionalized alkyl bromides (57-88%). The flavonoids, which are based on the chromone structure, and other related ring systems, have several interesting biological activities. The chromones are also interesting structural scaffolds, and they have for example been designed to be used as mimetics of short peptides. The versatile applicability of chromone derivatives and especially their potential use in drug discovery implicates the importance of access to efficient synthetic routes to such compounds.

N-(3-(4-Hydroxyphenyl)-propenoyl)-amino acid tryptamides as SIRT2 inhibitors

A series of N-(3-(4-hydroxyphenyl)-propenoyl)-amino acid tryptamides was based on a previously reported new SIRT2 inhibitor from our group, and it was designed to study if the molecular size of the compound could be reduced. The most potent compounds, N-(3-(4-hydroxyphenyl)-propenoyl)-2-aminoisobutyric acid tryptamide and N-(3-(4-hydroxyphenyl)-propenoyl)-L-alanine tryptamide, were equipotent, 30% smaller in molecular weight, and slightly more selective (SIRT2/SIRT1) than the parent compound.

Synthesis of 3-aminomethyl-2-aryl- 8-bromo-6-chlorochromones

[reaction: see text] An efficient synthetic route to Cbz-protected 3-aminomethyl-2-aryl-8-bromo-6-chlorochromones has been developed. 3-Aryl-1-(3-bromo-5-chloro-2-hydroxyphenyl)-2-propen-1-one or 2-aryl-8-bromo-6-chlorochroman-4-one could be reacted under Mannich conditions yielding 2-aryl-8-bromo-6-chloro-3-methylenechroman-4-one, which was further converted to the target compound via an aza-Michael reaction followed by an SeO(2) oxidation. This procedure represents a new method to introduce a primary aminomethyl group at the 3-position of a 2-arylchromone scaffold. The Cbz-protected 3-aminomethyl-2-aryl-8-bromo-6-chlorochromones can, e.g., be used in the synthesis of chromone-based beta-turn peptidomimetics.

2(S)-(Cycloalk-1-enecarbonyl)-1-(4-phenyl-butanoyl)pyrrolidines and 2(S)-(aroyl)-1-(4-phenylbutanoyl)pyrrolidines as prolyl oligopeptidase inhibitors

In order to replace the P2-P1 amide group, different 1-cycloalkenyls and 2-aryls were studied in the place of the P1 pyrrolidine group of a 4-phenylbutanoyl-L-Pro-pyrrolidine structure, which is a well-known prolyl oligopeptidase inhibitor SUAM-1221. The 1-cyclopentenyl and the 2-thienyl groups gave novel compounds, which were equipotent with the corresponding pyrrolidine-analog SUAM-1221. It was shown that the P2-P1 amide group of POP inhibitors can be replaced by an alpha,beta-unsaturated carbonyl group or the aryl conjugated carbonyl group.

Beneficial Effect of Prolyl Oligopeptidase Inhibition on Spatial Memory in Young but Not in Old Scopolamine-Treated Rats

The effects of a novel prolyl oligopeptidase (POP) inhibitor KYP-2047 on spatial memory of young (3-month-old) and old (8- to 9-month-old) scopolamine-treated rats (0.4 mg/kg intraperitoneally) was investigated in the Morris water maze. In addition, the concentrations of promnesic neuropeptide substrates of POP, substance P and neurotensin in various brain areas after acute and chronic POP inhibition were measured in young rats. In addition, inositol-1,4,5-trisphosphate (IP(3)) levels were assayed in rat cortex and hippocampus after effective 2.5-day POP inhibition. KYP-2047 (1 or 5 mg/kg 30 min. before daily testing) dose-dependently improved the escape performance (i.e. latency to find the hidden platform and swimming path length) of the young but not the old rats in the water maze. POP inhibition had no consistent effect on substance P levels in cortex, hippocampus or hypothalamus, and only a modest increase in neurotensin concentration was observed in the hypothalamus after a single dose of KYP-2047. Moreover, IP(3) concentrations remained unaffected in cortex and hippocampus after POP inhibition. In conclusion, the behavioural data support the earlier findings of the promnesic action of POP inhibitors, but the mechanism of the memory-enhancing action remains unclear.

N,N‘-Bisbenzylidenebenzene-1,4-diamines andN,N‘-Bisbenzylidenenaphthalene-1,4-diamines as Sirtuin Type 2 (SIRT2) Inhibitors

A series of N,N'-bisbenzylidenebenzene-1,4-diamine and N,N'-bisbenzylidenenaphthalene-1,4-diamine derivatives were synthesized as inhibitors for human sirtuin type 2 (SIRT2). The design of the new compounds was based on two earlier reported hits from molecular modeling and virtual screening. The most potent compound was N,N'-bis(2-hydroxybenzylidene)benzene-1,4-diamine, which was equipotent with the most potent hit compound and well-known SIRT2 inhibitor sirtinol.

An introduction of a pyridine group into the structure of prolyl oligopeptidase inhibitors

A series of ionizable prolyl oligopeptidase inhibitors were developed through the introduction of a pyridyl group to the P3 position of the prolyl oligopeptidase inhibitor structure. The study was performed on previously developed prolyl oligopeptidase inhibitors with proline mimetics at the P2 position. The 3-pyridyl group resulted in equipotent compounds as compared to the parent compounds. It was shown that the pyridyl group improves water solubility and, in combination with a 5(R)-tert-butyl-l-prolyl group at the P2 position, good lipophilicity can be achieved.

Binding kinetics and duration of in vivo action of novel prolyl oligopeptidase inhibitors

Prolyl oligopeptidase (POP) is a serine protease that specifically hydrolyses small peptides at the carboxyl end of the proline residue. POP has gained pharmaceutical interest, since its inhibitors have been shown to have antiamnesic properties in rat. We examined the effect of the 2(S)-substituents CN and COCH(2)OH at the P1 site of the parent inhibitors isophthalic acid 2(S)-(cyclopentanecarbonyl)pyrrolidine-l-prolyl-pyrrolidine amide and 4-phenylbutanoyl-l-prolyl-pyrrolidine and bulky 5-t-butyl group at the P2 site l-prolyl residue of the parent inhibitor 4-phenylbutanoyl-l-prolyl-pyrrolidine on the binding kinetics to the enzyme. In addition, we studied the duration of POP inhibition in the rat tissues in vivo after i.p. administration. CN and COCH(2)OH substituents at the P1 site pyrrolidine group were found to greatly increase the affinity of the inhibitor and the enzyme-inhibitor complex half-life. In addition, 5-t-butyl group at the P2 site l-prolyl residue increased the dissociation half-life of the enzyme-inhibitor complex, without much affecting the inhibitory potency. The duration of the inhibition in the rat tissues followed the inhibition kinetic properties in that the compounds with fast dissociation produced shorter inhibition in the rat tissues than the compounds with slow dissociation. The duration of POP inhibition of compounds was evidently not governed by their serum clearance. The fact that the in vivo pharmacodynamic behaviour of POP inhibitors can be predicted by their in vitro-properties may be of importance when designing therapeutically useful POP inhibitors.

Synthesis and Characterization of the Novel Fluorescent Prolyl Oligopeptidase Inhibitor 4-Fluoresceinthiocarbamoyl- 6-aminocaproyl-l-prolyl-2(S)-(Hydroxy- acetyl)pyrrolidine

The synthesis and characterization of the first fluorescent prolyl oligopeptidase inhibitor 4-fluoresceinthiocarbamoyl-6-aminocaproyl-L-prolyl-2(S)-(hydroxyacetyl)pyrrolidine is described. This compound has an IC50 value of 0.83 nM and a dissociation half-life of 160 min, and its fluorescence signal is detectable using standard filters for fluorescein. These properties make this compound a suitable probe for visualizing prolyl oligopeptidase in various applications.

Dicarboxylic Acid Azacycle l-Prolyl-pyrrolidine Amides as Prolyl Oligopeptidase Inhibitors and Three-Dimensional Quantitative Structure−Activity Relationship of the Enzyme−Inhibitor Interactions

A series of dicarboxylic acid azacycle l-prolyl-pyrrolidine amides was synthesized, and their inhibitory activity against prolyl oligopeptidase (POP) from porcine brain was tested. Three different azacycles were tested at the position beyond P3 and six different dicarboxylic acids at the P3 position. l-Prolyl-pyrrolidine and l-prolyl-2(S)-cyanopyrrolidine were used at the P2-P1 positions. The IC(50) values ranged from 0.39 to 19000 nM. The most potent inhibitor was the 3,3-dimethylglutaric acid azepane l-prolyl-2(S)-cyanopyrrolidine amide. Molecular docking (GOLD) was used to analyze binding interactions between different POP inhibitors of this type and the POP enzyme. The data set consisted of the novel inhibitors, inhibitors published previously by our group, and well-known reference compounds. The alignments were further analyzed using comparative molecular similarity indices analysis. The binding of the inhibitors was consistent at the P1-P3 positions. Beyond the P3 position, two different binding modes were found, one that favors lipophilic structures and one that favors nonhydrophobic structures.

Slow-binding inhibitors of prolyl oligopeptidase with different functional groups at the P1 site

POP (prolyl oligopeptidase) specifically hydrolyses a number of small proline-containing peptides at the carboxy end of the proline residue and POP inhibitors have been shown to have cognition-enhancing properties. It has been noted that certain functional groups at the P1 site of the inhibitor, which correspond to the substrate residue on the N-terminal side of the bond to be cleaved, increase the inhibitory potency. However, detailed mechanistic and kinetic analysis of the inhibition has not been studied. In the present study, we examined the effect of different functional groups at the P1 site of the parent inhibitor isophthalic acid bis-(L-prolylpyrrolidine) amide on the binding kinetics to POP. Addition of CHO, CN or COCH(2)OH groups to the P1 site increased the inhibitory potency by two orders of magnitude (K(i)=11.8-0.1 nM) and caused a clear slow-binding inhibition. The inhibitor containing a CHO group had the lowest association rate constant, k(on)=(2.43+/-0.12) x 10(5) M(-1) x s(-1), whereas the inhibitor with a CN group exhibited the fastest binding, k(on)=(12.0+/-0.08)x10(5) M(-1) x s(-1). In addition, the dissociation rate was found to be crucially dependent on the type of the functional group. Compounds with COCH(2)OH and CHO groups had much longer half-lives of dissociation (over 5 h) compared with the compound with the CN group (25 min), although the K(i) values of the compounds were relatively similar. A possibility to optimize the duration of inhibition by changing the functional group at the P1 site is important when planning therapeutically useful POP inhibitors.

A cyclopent-2-enecarbonyl group mimics proline at the P2 position of prolyl oligopeptidase inhibitors

With the aim to replace the natural amino acid proline by a proline mimetic structure, a cyclopent-2-enecarbonyl moiety was studied at the P2 position of prolyl oligopeptidase (POP) inhibitors. The cyclopent-2-enecarbonyl moiety proved to be an excellent proline mimetic at the P2 position of POP inhibitors. The replacement is particularly useful when increased lipophilicity is needed.

New prolyl oligopeptidase inhibitors developed from dicarboxylic acid bis(l-prolyl-pyrrolidine) amides

Isophthalic acid bis(l-prolyl-pyrrolidine) amide is a very potent prolyl oligopeptidase inhibitor, but it has a log P value of -0.2, which is very low for a compound targeted to the brain. Therefore, these types of compounds were further modified to improve the structure-activity relationships, with the focus on increasing the log P value. The inhibitory activity against prolyl oligopeptidase from pig brain was tested in vitro. The most promising compounds resulted from replacing the pyrrolidinyl group at the P5 site by cycloalkyl groups, such as cyclopentyl and cyclohexyl groups, and by a phenyl group. These compounds are slightly more potent, and they have a significantly higher log P value. The potency of these compounds was further increased by replacing the pyrrolidinyl group at the P1 site by 2(S)-cyanopyrrolidinyl and 2(S)-(hydroxyacetyl)pyrrolidinyl groups.

Conformationally rigid N-acyl-5-alkyl-L-prolyl-pyrrolidines as prolyl oligopeptidase inhibitors

In the N-acyl-L-prolyl-pyrrolidine type of prolyl oligopeptidase inhibitors the L-prolyl group was replaced by different 5-alkyl-L-prolyl groups, resulting in a series of N-acyl-5-alkyl-L-prolyl-pyrrolidines. Since N-amides of 5-alkyl-L-prolines are conformationally more rigid than those of L-proline, the main objective was to make more rigid prolyl oligopeptidase inhibitors. In the series of compounds where the N-acyl group was a Boc group, the 5(R)-tert-butyl group increased the potency strongly. A similar effect was not observed for the 5(S)-tert-butyl group. In the series of compounds where the N-acyl group was a 4-phenylbutanoyl group, the 5(R)-tert-butyl, 5(R)-methyl and 5(S)-methyl groups did not have an effect on the potency [the 5(S)-tert-butyl group was not tested in this series]. As an additional effect, the 5-tert-butyl groups increased the log P of the compounds 1.5 log units, which might be beneficial when targeting the compounds to the brain.

Dicarboxylic acid bis(L-prolyl-pyrrolidine) amides as prolyl oligopeptidase inhibitors

New dicarboxylic acid bis(L-prolyl-pyrrolidine) amides were synthesized, and their inhibitory activity against prolyl oligopeptidase from pig brain was tested in vitro. As compared with earlier described prolyl oligopeptidase inhibitors, these new compounds have in common an L-prolyl-pyrrolidine moiety, but the typical lipophilic acyl end group is replaced by another L-prolyl-pyrrolidine moiety connected symmetrically with a short dicarboxylic acid linker. These compounds are a new type of peptidomimetic prolyl oligopeptidase inhibitor.

4-Phenylbutanoyl-2(S)-acylpyrrolidines and 4-phenylbutanoyl-L-prolyl-2(S)-acylpyrrolidines as prolyl oligopeptidase inhibitors

New 4-phenylbutanoyl-2(S)-acylpyrrolidines and 4-phenylbutanoyl-L-prolyl-2(S)-acylpyrrolidines were synthesized. Their inhibitory activity against prolyl oligopeptidase from pig brain was tested in vitro. In the series of 4-phenylbutanoyl-2(S)-acylpyrrolidines, the cyclopentanecarbonyl and benzoyl derivatives were the best inhibitors having IC(50) values of 30 and 23 nM, respectively. This series of compounds shows that the P1 pyrrolidine ring, which is common in most POP inhibitors, can be replaced by either a cyclopentyl ring or a phenyl ring, causing only a slight decrease in the inhibitory activity. In the series of 4-phenylbutanoyl-L-prolyl-2(S)-acylpyrrolidines the cyclopentanecarbonyl and benzoyl derivatives were not as active as in the series of 4-phenylbutanoyl-2(S)-acylpyrrolidines. The hydroxyacetyl derivative did however show high inhibitory activity. This compound is structurally similar to JTP-4819, which is one of the most potent prolyl oligopeptidase inhibitors. The acyl group in the two series of new compounds seems to bind to different sites of the enzyme, since the second series of new compounds did not show the same cyclopentanecarbonyl or benzoyl specificity as the first series.