Design and synthesis of novel imidazole-substituted dipeptide amides as potent and selective inhibitos of Candida albicans myristoylCoA:protein N- myristoyltransferase and identification of related tripeptide inhibitors with mechanism-based antifungal activity

Balekudru Devadas, Sandra K. Freeman, Mark E. Zupec, Hwang Fun Lu, Srinivasan R. Nagarajan, Nandini S. Kishore, Jennifer Lodge, David W. Kuneman, Charles A. McWherter, Dutt V. Vinjamoori, Daniel P. Getman, Jeffrey Gordon, James A. Sikorski

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Abstract

A new class of antifungal agents has been discovered which exert their activity by blockade of myristoylCoA:protein N-myristoyltransferase (NMT; EC 2.1.3.97). Genetic experiments have established that NMT is needed to maintain the viability of Candida albicans and Cryptococcus neoformans, the two principal causes of systemic fungal infections in immunocompromised humans. Beginning with a weak octapeptide inhibitor ALYASKLS-NH2 (2, k(i) = 15.3 ± 6.4 μM), a series of imidazole-substituted Ser-Lys dipeptide amides have been designed and synthesized as potent and selective inhibitors of Candida albicans NMT. The strategy that led to these inhibitors evolved from the identification of those functional groups in the high-affinity octapeptide substrate GLYASKLS-NH3 1a necessary for tight binding truncation of the C-terminus, replacement of the four amino acids at the N-terminus by a spacer group, and substitution of the glycine amino group with an N-linked 2- methylimidazole moiety. Initial structure-activity studies led to the identification of 31 as a potent and selective peptidomimetic inhibitor with an IC50 of 56 nM and 250-fold selectivity versus human NMT. 2- Methylimidazole as the N-terminal amine replacement in combination with a 4- substituted phenacetyl moiety imparts remarkable potency and selectivity to this novel class of inhibitors. The (S:S) stereochemistry of serine and lysine residues is critical for the inhibitory activity, since the (R,R) enantiomer 40 is 102-fold less active than the (S,S) isomer 31. The inhibitory profile exhibited by this new class of NMT ligands is a function of the pK(o) of the imidazole substituent as illustrated by the benzimidazole analog 35 which is about 10-fold less potent than 31. The measured pK(o) (7.1 ± 0.5) of 2-methylimidazole in 31 is comparable with the estimated pK(o) (~8.0) of the glycyl residue in the high-affinity substrate 1a. Groups bulkier than methyl, such as ethyl, isopropyl, or iodo, at the imidazole 2- position have a detrimental effect on potency. Further refinement of 31 by grafting an α-methyl group at the benzytic position adjacent to the serine residue led to 61 with an IC50 of 40 nM. Subsequent chiral chromatography of 61 culminated in the discovery of the most potent Candida NMT inhibitor 61a reported to date with an IC50 of 20 nM and 400-fold selectivity versus the human enzyme. Both 31 and 61a are competitive inhibitors of Candida NMT with respect to the octapeptide substrate GNAASARE-NH2 with K(i)(app) = 30 and 27 nM, respectively. The potency and selectivity displayed by these inhibitors are dependent upon the size and orientation of the o-substituent. An α-methyl group with the R configuration corresponding to the (S)-methyl- 4-alanine in 2 confers maximum potency and selectivity. Structural modification of 31 and 61 by appending an (S)-carboyl group β to the cyclohexyl moiety provided the less potent tripeptice inhibitors 73a and 73b with an IC50 of 1.45 ± 0.08 and 0.38 ± 0.03 μM, respectively. However, these tripeptides (73a and 73b) exhibited a pronounced selectivity of 560- and 2200-fold versus the human NMT. More importantly 73a displayed fungistatic activity against C. albicans with an EC50 ± 17 μM in cell culture. Compound 73b also exhibited a similar antifungal activity. An Arf protein gel mobility shift assay for monitoring intracellular myristoylation revealed that a single dose of 200 μM of 73a or 73b produced <50% reduction in Arf N-myristoylation, after 24 and 48 h, consistent with their fungistatic rather than fungicidal activity. In contrast, the enantiomer 73d which had an IC50 >1000 μM against C. albicans NMT did not exhibit antifungal activity and produced no detectable reduction in Arf N-myristoylation in cultures of C. albicans. These studies confirm that the observed antifungal activity of 73a adn 73b is due to the attenuation of NMT activity and that NMT represents an attractive target for the development of novel antifungal agents.

Original languageEnglish
Pages (from-to)2609-2625
Number of pages17
JournalJournal of Medicinal Chemistry
Volume40
Issue number16
DOIs
StatePublished - Aug 1 1997

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