Prolidase, a Potential Enzyme Target for Melanoma: Design of Proline-Containing Dipeptide-like Prodrugs

Bioinformatics tools such as Perl, Visual Basic, Cluster, and TreeView were used to analyze public gene expression databases in order to identify potential enzyme targets for prodrug strategies. The analyses indicated that prolidase might be a desirable enzyme target based on its differential expression in melanoma cancer cell lines and its high substrate specificity for dipeptides containing proline at the carboxy terminus. RT-PCR expression of prolidase and hydrolytic activity against N-glycyl-l-proline (GLY-PRO), a standard substrate of prolidase, determined in tumor cell lines, exhibited a high correlation (r(2) = 0.95). These results suggest the possibility of targeting prolidase with prodrugs of anticancer agents for enhanced selectivity. The feasibility of such a scenario was tested by (a) synthesizing prodrugs of melphalan that comprised linkage of the carboxy terminus of the l-phenylalanine moiety of melphalan to the N-terminus of l and d stereoisomers of proline and (b) determining their bioconversion and antiproliferative activities in SK-MEL-5 cells, a melanoma cancer cell line with high expression levels of prolidase. The results of hydrolysis studies of the l- and d-proline prodrugs of melphalan, designated as prophalan-l and prophalan-d, respectively, indicated a approximately 7-fold higher rate of activation of prophalan-l compared to prophalan-d in SK-MEL-5 cell homogenates. Prophalan-l exhibited cytotoxicity (GI(50) = 74.8 microM) comparable to that of melphalan (GI(50) = 57.0 microM) in SK-MEL-5 cells while prophalan-d was ineffective, suggesting that prolidase-specific activation to the parent drug may be essential for cytotoxic action. Thus, melphalan prodrugs such as prophalan-l that are cleavable by prolidase offer the potential for enhanced selectivity by facilitating cytotoxic activity only in cells overexpressing prolidase.

Synthesis of ?uorescent label, DBD-?-proline, and the resolution ef?ciency for chiral amines by reversed-phase chromatography

DBD-d(and l)-beta-proline, new fluorescent chiral derivatization reagents, were synthesized from the reaction of 4-(N,N-dimethylaminosulfonyl)-7- fl uoro-2,1,3-benzoxadiazole (DBD-F) with beta-proline. The racemic mixture synthesized was separated by a chiral stationary phase (CSP) column, Chiralpak AD-H, with n-hexane-EtOH-TFA-diethylamine (70:30:0.1:0.1) as the mobile phase. The dl-forms were decided according to the results obtained from a circular dichroism (CD) detector after separation by the CSP column. The fractionated enantiomers reacted with chiral amine to produce a couple of diastereomers. The labeling proceeded in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and pyridine as the activation reagents. The reaction conditions were mild and no racemization occurred during the diastereomer formation. The resulting diastereomers fluoresced at around 570 nm (excitation at around 460 nm). Good linearity of the calibration curves was obtained in the range 1-75 pmol and the detection limits on chromatogram were less than 1 pmol. The separability of the diastereomers was compared with the diastereomers derived from DBD-d(or l)-proline. The resolution values (Rs) obtained from the diastereomers of three chiral amines with DBD-d(or l)-beta-proline were higher than those derived from DBD-d(or l)-proline, e.g. dl-phenylalanine methylester (dl-PAME), 2.23 vs 1.37; (R)(S)-1-phenylethylamine [(R)(S)-PEA], 2.09 vs 1.13; and (R)(S)-1-(1-naphthyl)ethylamines [(R)(S)-NEA], 5.19 vs 1.23. The results suggest that the position of COOH group on pyrrolidine moiety in the structures is one of the important factors for the efficient separation of a couple of the diastereomers.

Asymmetric aza-[2,3]-Wittig sigmatropic rearrangements: chiral auxiliary control and formal asymmetric synthesis of (2S, 3R, 4R)-4-hydroxy-3-methylproline and (–)-kainic acid

A survey of 16 different chiral auxiliaries and a variety of strategies found that an (-)-8-phenylmenthol ester of a glycine derived migrating group can control the absolute stereochemistry of aza-[2,3]-Wittig sigmatropic rearrangements with diastereoselectivities of ca. 3 : 1 with respect to the auxiliary. In two specific examples, ca. 50% yields of enantiomerically pure products were obtained after chromatographic purification. These were synthetically manipulated with no erosion of stereochemistry into intermediates that completed formal asymmetric syntheses of (+)-HyMePro and (-)-kainic acid.

Endothelin-Converting Enzyme-1 Inhibition and Growth of Human Glioblastoma Cells

Endothelin-1 (ET-1) is mitogenic and/or antiapoptotic in human cancers, and antagonists to ET-1 receptors are under evaluation for cancer treatment. Inhibition of ET-1 activation by the endothelin-converting enzymes 1(a)(-)(d) (ECE-1(a)(-)(d); EC 3.4.24.71) represents another approach to block the ET-1 effect in cancer. To evaluate this potential, we synthesized and characterized a series of low nanomolar nonpeptidic thiol-containing ECE-1 inhibitors, and evaluated their effect, as well as the effect of inhibitors for the related metalloproteases neprilysin (NEP; EC 3.4.24.11) and angiotensin-converting enzyme (ACE; EC 3.4.15.1), on human glioblastoma cell growth. Only ECE-1 inhibitors inhibited DNA synthesis by human glioblastoma cells. Exogenous addition of ET-1 or bigET-1 to glioblastoma cells did not counterbalance the growth inhibition elicited by ECE-1 inhibitors, suggesting that ECE-1 inhibitors block the proliferation of human glioblastoma cells most likely via a mechanism not involving extracellular production of ET-1. This class of molecules may thus represent novel therapeutic agents for the potential treatment of human cancer.

Synthesis and α4β2 nicotinic affinity of 2-pyrrolidinylmethoxyimines and prolinal oxime ethers

Homochiral E and Z isomers of N-methylprolinal O-isopropyloxime and (1-methyl-2-pyrrolidinyl)methoxyimines were synthesized as candidate bioisosteres of nicotine and its isoxazolic analogue ABT 418. Two of them, namely (S)-2-isopropylideneaminooxymethyl- and (Z)-(S)-2-ethylideneaminooxymethyl-1-methylpyrrolidine, proved to bind at alpha4beta2 nicotinic acetylcholine receptor with submicromolar affinity and remarkable selectivity over alpha7 and muscarinic receptors thus supporting the hypothesized bioisosteric relationship between their methyloxyimino group and the aromatic heterocycles of the reference ligands.

Inhibitors of hepatitis C virus NS3.4A protease. Part 3: P2 proline variants

We recently described the identification of an optimized alpha-ketoamide warhead for our series of HCV NS3.4A inhibitors. We report herein a series of HCV protease inhibitors incorporating 3-alkyl-substituted prolines in P(2). These compounds show exceptional enzymatic and cellular potency given their relatively small size. The marked enhancement of activity of these 3-substituted proline derivatives relative to previously reported 4-hydroxyproline derivatives constitutes additional evidence for the importance of the S(2) binding pocket as the defining pharmacophore for inhibition of the NS3.4A enzyme.

Asymmetric Synthesis of 3-Substituted Proline Chimeras Bearing Polar Side Chains of Proteinogenic Amino Acids

The amino-zinc-ene-enolate cyclization reaction is a straightforward route to the synthesis of 3-substituted prolines. Herein we report the application of this reaction to the syntheses of proline chimeras of lysine, glutamic acid, glutamine, arginine, and serine. All these compounds were obtained in enantiomerically pure form and suitably protected for peptide synthesis.

Potent and selective proline derived dipeptidyl peptidase IV inhibitors

In-house screening of the Merck sample collection identified proline derived homophenylalanine 3 as a DPP-IV inhibitor with modest potency (DPP-IV IC50=1.9 microM). Optimization of 3 led to compound 37, which is among the most potent and selective DPP-IV inhibitors discovered to date.

Diastereoselective [2+2]-Cycloaddition Reactions of Unsymmetrical Cyclic Ketenes with Imines: Synthesis of Modified Prolines and Theoretical Study of the Reaction Mechanism

The synthesis of enantiomerically pure modified proline derivatives was achieved by using spiro beta-lactams as starting material that were prepared in turn by the [2+2]-cycloaddition of unsymmetrical cyclic ketenes with optically active imines. A theoretical study of the [2+2]-cycloaddition reaction, using density-functional methods, gave insights on the origin of the observed stereoselectivity of the Staudinger reaction. The spiro beta-lactams were transformed in the N-Boc derivatives and subjected to nucleophilic ring opening, affording the corresponding enantiomerically pure modified proline derivatives, isolated as orthogonally protected compounds.

Substituted 2-Azabicyclo[2.1.1]hexanes as Constrained Proline Analogues: Implications for Collagen Stability

Among the proteinogenic amino acids, only proline is a secondary amine and only proline has a saturated ring. Electronegative substituents on C-4 (that is, C(gamma)) have a substantial effect on the trans/cis ratio of the prolyl peptide bond and the pucker of the pyrrolidine ring. 2-Azabicyclo[2.1.1]hexane is, in essence, a proline analogue with two C(gamma) atoms, one in each of the two prevalent ring puckers of proline. Here, 2-azabicyclo[2.1.1]hexane analogues of 2S-proline, (2S,4S)-4-hydroxyproline, and (2S,4S)-4-fluoroproline residues were synthesized, and their trans/cis ratios were shown to be invariant in a particular solvent. Thus, the substitution of a proline residue on C-4 affects the trans/cis ratio by altering the pucker of its pyrrolidine ring. This finding has implications for the conformation of collagen, which has an abundance of 2S-proline and (2S,4R)-4-hydroxyproline residues, and can be stabilized by (2S,4R)-4-fluoroproline and (2S,4S)-4-fluoroproline residues.

Asymmetric Synthesis of α-Amino 1,3-Dithioketals from Sulfinimines (N-Sulfinyl Imines). Synthesis of (2S,3R)-(−)-3-Hydroxy-3-methylproline

[reaction: see text] N-Sulfinyl alpha-amino 1,3-dithioketals are prepared in high de and good yield by treating sulfinimines with lithio-1,3-dithianes. Selective removal of the N-sulfinyl or the thioketal groups affords stable alpha-amino 1,3-dithioketals and N-sulfinyl alpha-amino ketones, respectively. This new sulfinimine-derived chiral building block is employed in the asymmetric synthesis of polyoxypeptin amino acid (2S,3R)-(-)-3-hydroxy-3-methylproline.

Synthesis of 310-Helix-Inducing Constrained Analogues of l-Proline

A bicyclic indolizidinone carboxylic acid and a tricyclic constrained analogue of l-proline were synthesized and evaluated for their ability to induce helix formation as l-Ala tetrapeptides. Variable-temperature NMR, DMSO titration, CD spectra, and X-ray structure analyses, in conjunction with molecular modeling, confirmed the existence of 3(10)-helical motifs with di- and tetrapeptides of l-Ala.

Virtually Complete Control of Simple and Face Diastereoselectivity in the Michael Addition Reactions between Achiral Equivalents of a Nucleophilic Glycine and (S)- or (R)-3-(E-Enoyl)-4-phenyl-1,3-oxazolidin-2-ones: Practical Method for Preparation of β-Substituted Pyroglutamic Acids and Prolines

This study demonstrates a new strategy for controlling the stereochemical outcome of the Michael addition reactions between nucleophilic glycine equivalents and alpha,beta-unsaturated carboxylic acid derivatives: The addition reactions between achiral Ni(II)-complex of the Schiff base of glycine with o-[N-alpha-pycolylamino]acetophenone and (S)- or (R)-3-(E-enoyl)-4-phenyl-1,3-oxazolidin-2-ones were shown to occur at room temperature in the presence of nonchelating organic bases and, most notably, with very high stereoselectivity at both newly formed stereogenic centers. Thus, the chiral 4-phenyl-1,3-oxazolidin-2-one moiety was found to control efficiently both face diastereoselectivities of the glycine derived enolate and the C,C double bond of the Michael acceptor. The new strategy developed in this work is methodologically superior to previous methods, most notably in terms of generality and synthetic efficiency. Excellent chemical yields and diastereoselectivities, combined with the simplicity of the experimental procedures, render the present method of immediate use for preparing various 3-substituted pyroglutamic acids and related amino acids (glutamic acids, glutamines, prolines, etc.) available via conventional transformations of the former.

Catalytic Route to the Synthesis of Optically Active β,β-Difluoroglutamic Acid and β,β-Difluoroproline Derivatives

Beta,beta-difluorinated amino acid derivatives were synthesized via Mg(0)-promoted defluorination of alpha-trifluoromethyl iminoester. Bromination of the difluoroenamine afforded the bromodifluoromethyl iminoester in good yield. Pd-catalyzed asymmetric hydrogenation of the bromodifluoromethyl iminoester and the subsequent transformations provided optically active beta,beta-difluoroglutamic acid and beta,beta-difluoroproline derivatives.

Synthesis and evaluation of chiral bicyclic proline FKBP12 ligands

As part of our ongoing program to explore novel structural classes of FKBP12 ligands, we herein wish to report a new class of FKBP12 ligands containing chiral bicyclic proline analogues. Details of the synthetic routes, together with preliminary biological activity, will be presented.

Synthesis of Fused Heteroarylprolines and Pyrrolopyrroles

Fused heteroarylprolines were prepared starting from 4-oxo-N-(PhF)proline benzyl ester (6, PhF = 9-(9-phenylfluorenyl)) following two approaches. First, allylation of oxoproline 6 followed by Wacker oxidation gave 1,4-dione 8 that was selectively converted to pyrroloproline 10b, pyrrolopyrrole 12, and pyridazinoproline 9. Second, aldol condensation of oxoproline 6 with a series of N-(Boc)-alpha-amino aldehydes 15a-e and acid-catalyzed cyclization gave pyrroloprolines 17a-e possessing a variety of pyrrole 5-position substituents. Conditions for the selective deprotection and alkylation of the pyrrole nitrogen of pyrroloprolines 17 were developed to expand the diversity of the heteoaryl systems. Finally, hydrogenolytic cleavage of the PhF and benzyl groups followed by subsequent protection with Boc, Fmoc, and Moz groups was performed to obtain analogues suitable for peptide synthesis. The enantiomeric purity of N-(Boc)pyrrolo-proline 21a was ascertained by coupling to l- and d-phenylalanine methyl ester and examination of the diastereotopic pyrrole protons, which demonstrated the dipeptides to be of >99% diastereomeric purity. These approaches have thus delivered the first series of enantiopure fused arylprolines for application as arylglycine-proline chimeras in structure-activity studies of biologically active compounds.

Synthesis of Novel Potent Dipeptidyl Peptidase IV Inhibitors with Enhanced Chemical Stability: Interplay between the N-Terminal Amino Acid Alkyl Side Chain and the Cyclopropyl Group of α-Aminoacyl-l-cis-4,5-methanoprolinenitrile-Based Inhibitors

A series of methanoprolinenitrile-containing dipeptide mimetics were synthesized and assayed as inhibitors of the N-terminal sequence-specific serine protease dipeptidyl peptidase IV (DPP-IV). The catalytic action of DPP-IV is the principle means of degradation of glucagon-like peptide-1, a key mediator of glucose-stimulated insulin secretion, and DPP-IV inhibition shows clinical benefit as a novel mechanism for treatment of type 2 diabetes. However, many of the reversible inhibitors to date suffer from chemical instability stemming from an amine to nitrile intramolecular cyclization. Installation of a cyclopropyl moiety at either the 3,4- or 4,5-position of traditional 2-cyanopyrrolidide proline mimetics led to compounds with potent inhibitory activity against the enzyme. Additionally, cis-4,5-methanoprolinenitriles with beta-branching in the N-terminal amino acid provided enhanced chemical stability and high inhibitory potency. This class of inhibitors also exhibited the ability to suppress prandial glucose elevations after an oral glucose challenge in male Zucker rats.

Peptide deformylase inhibitors as antibacterial agents: identification of VRC3375, a proline-3-alkylsuccinyl hydroxamate derivative, by using an integrated combinatorial and medicinal chemistry approach

Peptide deformylase (PDF), a metallohydrolase essential for bacterial growth, is an attractive target for use in the discovery of novel antibiotics. Focused chelator-based chemical libraries were constructed and screened for inhibition of enzymatic activity, inhibition of Staphylococcus aureus growth, and cytotoxicity. Positive compounds were selected based on the results of all three assays. VRC3375 [N-hydroxy-3-R-butyl-3-(2-S-(tert-butoxycarbonyl)-pyrrolidin-1-ylcarbonyl)propionamide] was identified as having the most favorable properties through an integrated combinatorial and medicinal chemistry effort. This compound is a potent PDF inhibitor with a K(i) of 0.24 nM against the Escherichia coli Ni(2+) enzyme, possesses activity against gram-positive and gram-negative bacterial pathogens, and has a low cytotoxicity. Mechanistic experiments demonstrate that the compound inhibits bacterial growth through PDF inhibition. Pharmacokinetic studies of this drug in mice indicate that VRC3375 is orally bioavailable and rapidly distributed among various tissues. VRC3375 has in vivo activity against S. aureus in a murine septicemia model, with 50% effective doses of 32, 17, and 21 mg/kg of body weight after dosing by intravenous (i.v.), subcutaneous (s.c.), and oral (p.o.) administration, respectively. In murine single-dose toxicity studies, no adverse effects were observed after dosing with more than 400 mg of VRC3375 per kg by i.v., p.o., or s.c. administration. The in vivo efficacy and low toxicity of VRC3375 suggest the potential for developing this class of compounds to be used in future antibacterial drugs.

N-Tritylprolinal: An Efficient Building Block for the Stereoselective Synthesis of Proline-Derived Amino Alcohols

N-Tritylprolinal (prepared in four steps from l-proline) shows a very high Felkin diastereoselectivity in its reaction with various nucleophiles, leading to a straightforward and highly stereoselective access to syn-proline-derived amino alcohols.

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.

Synthesis, molecular modeling and biological evaluation of aza-proline and aza-pipecolic derivatives as FKBP12 ligands and their in vivo neuroprotective effects

Nonimmunosuppressant ligands, exemplified by GPI 1046 (1), for the peptidyl-prolyl isomerase FKBP12 have been found to unexpectedly possess powerful neuroprotective and neuroregenerative effects in vitro and in vivo. We have extensively explored the therapeutic utility of FKBP12 ligands based on analogues of proline and pipecolic acid. As part of our ongoing program to explore novel structural classes of FKBP12 ligands, we herein wish to report a new class of FKBP12 ligands containing aza-proline and aza-pipecolic acid analogues. Details of the synthetic studies, together with biological activity will be presented.

Asymmetric Synthesis of Unsaturated, Fused Bicyclic Proline Analogues through Amino Alkylation of Cyclic Bis(allylsulfoximine)titanium Complexes and Migratory Cyclization of δ-Amino Alkenyl Aminosulfoxonium Salts

Described is an asymmetric synthesis of new Delta(3a,4)-unsaturated, fused bicyclic proline analogues from cyclic bis(allylsulfoximine)titanium complexes and N-tert-butylsulfonyl imino ethyl ester. Treatment of the enantiomerically pure five-, six-, seven-, and eight-membered cyclic bis(allylsulfoximine)titanium complexes with the imino ester gave mixtures of the corresponding (E,syn)- and (Z,syn)-configured, delta-sulfoximine substituted, cyclic gamma,delta-unsaturated alpha-amino acid esters with high regio- and diastereoselectivities in good yields. Activation of the N-methyl sulfoximine group of these amino acid derivatives through methylation with Me(3)OBF(4) afforded in nearly quantitative yields the corresponding (dimethylamino)sulfoxonium salts. A novel migratory cyclization of these salts with DBU gave via an isomerization to the corresponding allylic (dimethylamino)sulfoxonium salts and an intramolecular substitution of the (dimethylamino)sulfoxonium group the enantio- and diastereomerically pure, bicyclic, N-tert-butylsulfonyl protected proline analogues having a six- and eight-membered unsaturated carbocyclic ring. Cyclization of the alkenyl (dimethylamino)sulfoxonium salts was independent of the configuration of the double bond. N,N-Dimethylphenylsulfinamide of > or =99% ee was obtained in good yield as a further reaction product. Conversion of the sulfinamide to N,S-dimethyl-S-phenylsulfoximine of > or =99% ee, the starting material for the synthesis of the allylic sulfoximines, had been accomplished previously. Finally, cleavage of the tert-butylsulfonyl protecting group with anhydrous acid furnished the fused bicyclic proline analogue containing an unsaturated six-membered ring in high yield.

Studies on the synthesis of (2S,3R)-3-hydroxy-3-methylproline via C2-N bond formation

A new efficient synthesis of (2S,3R)-3-hydroxy-3-methylproline (3) is reported. During the course of a recent study on the Lewis acid promoted intramolecular opening of an epoxide by a carbamate NH, a highly concerted rearrangement was unexpectedly observed. Further investigations of substrate generality show that delta-carbamate-alpha,beta-epoxide esters commonly underwent similar rearrangements with the aid of Lewis acids. Retrosynthetic analysis of such a C(2)-N disconnection can lead to an efficient synthesis of (2S,3R)-3-hydroxy-3-methylproline (3) in high enantio purity. Stereochemistries were established by a Sharpless asymmetric dihydroxylation and a diastereoselective reductive amination.

Synthesis of cyclic proline-containing peptides via ring-closing metathesis

[reaction: see text] Several dienes embedded in di- and tripeptides which incorporate proline have been prepared and subjected to ring-closing metathesis. Bicyclic peptides of well-defined amide geometry and of varying ring sizes were prepared. Several limitations of the cyclization step were revealed.