DESIGN AND SYNTHESIS OF NOVEL IMIDAZOLE-SUBSTITUTED DIPEPTIDE AMIDES AS POTENT AND SELECTIVE INHIBITORS OF CANDIDA-ALBICANS MYRISTOYLCOA-PROTEIN N-MYRISTOYLTRANSFERASE AND IDENTIFICATION OF RELATED TRIPEPTIDE INHIBITORS WITH MECHANISM-BASED ANTIFUNGAL ACTIVITY

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 ne eded to maintain the viability of Candida albicans and Cryptococcus ne oformans, the two principal causes of systemic fungal infections in im munocompromised humans. Beginning with a weak octapeptide inhibitor AL YASKLS-NH2 (2, K-i = 15.3 +/- 6.4 mu M), a series of imidazole-substit uted Ser-Lys dipeptide amides have been designed and synthesized as po tent and selective inhibitors of Candida albicans NMT. The strategy th at led to these inhibitors evolved from the identification of those fu nctional groups in the high-affinity octapeptide substrate GLYASKLS-NH 2 la necessary for tight binding, truncation of the C-terminus, replac ement 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-methylimid azole moiety. Initial structure-activity studies led to the identifica tion of 31 as a potent and selective peptidomimetic inhibitor with an IC50 Of 56 nM and 250-fold selectivity versus human NMT. 2-Methylimida zole as the N-terminal amine replacement in combination with a 4-subst ituted phenacetyl moiety imparts remarkable potency and selectivity to this novel class of inhibitors. The (S,S) stereochemistry of serine a nd lysine residues is critical for the inhibitory activity, since the (R,R) enantiomer 40 is 10(3)-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(a) of the imidazole substituent as illustrated b y the benzimidazole analog 35 which is about 10-fold less potent than 31. The measured pK(a) (7.1 +/- 0.5) of 2-methylimidazole in 31 is com parable with the estimated pK(a) (similar to 8.0) of the glycyl residu e in the high-affinity substrate la. Groups bulkier than methyl, such as ethyl, isopropyl, or iodo, at the imidazole 2-position have a detri mental effect on potency. Further refinement of 31 by grafting an a-me thyl group at the benzylic position adjacent to the serine residue led to 61 with an IC50 Of 40 nM. Subsequent chiral chromatography of 61 c ulminated in the discovery of the most potent Candida NMT inhibitor 61 a reported to date with an IC50 Of 20 nM and 400-fold selectivity vers us the human enzyme. Both 31 and 61a are competitive inhibitors of Can dida NMT with respect to the octapeptide substrate GNAASARR-NH2 with K -i(app) = 30 and 27 nM, respectively. The potency and selectivity disp layed by these inhibitors are dependent upon the size and orientation of the cc-substituent. An or-methyl group with the R configuration cor responding to the (S)-methyl-4-alanine in 2 confers maximum potency an d selectivity. Structural modification of 31 and 61 by appending an (S )-carboxyl group beta to the cyclohexyl moiety provided the less poten t tripeptide inhibitors 73a and 73b with an IC50 Of 1.45 +/- 0.08 and 0.38 +/- 0.03 mu M, respectively. However, these tripeptides (73a and 73b) exhibited a pronounced selectivity of 560- and 2200-fold versus t he human NMT. More importantly 73a displayed fungistatic activity agai nst C. albicans with an EC50 Of 51 +/- 17 mu M in cell culture. Compou nd 73b also exhibited a similar antifungal activity. An Arf protein ge l mobility shift assay for monitoring intracellular myristoylation rev ealed that a single dose of 200 mu M of 73a or 73b produced <50% reduc tion in Arf N-myristoylation, after 24 and 48 h, consistent with their fungistatic rather than fungicidal activity. In contrast, the enantio mer 73d which had an IC50 > 1000 mu M against C. albicans NMT did not exhibit antifungal activity and produced no detectable reduction in Ar f N-myristoylation in cultures of C. albicans. These studies confirm t hat the observed antifungal activity of 73a and 73b is due to the atte nuation of NMT activity and that NMT represents an attractive target f or the development of novel antifungal agents.