Cyclopropane-derived peptidomimetics. Design, synthesis, and evaluation ofnovel Ras farnesyltransferase inhibitors

Trisubstituted cyclopropanes have previously been established as rigid repl acements of dipeptide arrays in several biological systems. Toward further evaluating the utility of these dipeptide mimics in the design of novel CA( 1)A(2)X-based inhibitors of Ras farnesyltransferase (FTase), the conformati onally constrained, diastereomeric pseudopeptides CAbu Psi [COcpCO]FM 7-9, the flexible analogue CAbu Psi [CHOHCH2]FM (10), and the tetrapeptide CAbuF M (6) were prepared. The orientations of the two peptide backbone substitue nts and the phenyl group on the cyclopropane rings in 7-9 were specifically designed to probe selected topological features of the hydrophobic binding pocket of the A(2) subsite of FTase. The syntheses of the requisite trisub stituted cyclopropane carboxylic acid 22 and the diastereomeric cyclopropyl lactones 32a,b featured diastereoselective intramolecular cyclopropanation s of chiral allylic diazoacetates and a new method for introducing side cha ins onto the C-terminal amino acid of cyclopropane-derived dipeptide replac ements via the opening of an N-Boc-aziridine with an organocuprate. These c yclopropane intermediates were then converted into the targeted FTase inhib itors 7-9 by standard peptide coupling techniques. The pseudopeptides 7-9 w ere found to be competitive inhibitors of Ras FTase with IC(50)s of 1055 nM for 7, 760 nM for 8, and 7200 nM for 9. The flexible analogue 10 of these constrained inhibitors exhibited a IC50 of 320 nM and hence was slightly mo re potent than 7 and 8. All of these pseudopeptides were less potent than t he tetrapeptide parent CAbuFM (6), which had an IC50 of 38 nM. Because 7 an d 8 are approximately equipotent, it appears that the orientation of the pe ptide backbone substituents on the cyclopropane rings in 7 and 8 do not hav e any significant effect on binding affinity and that multiple binding mode s are possible without significant changes in affinity. On the other hand, this flexibility does not extend to the orientation of the side chain of th e Az residue as 7 and 8 were both nearly 1 order of magnitude more potent t han 9. Comparison of the relative potencies of 6 and 10 suggests that the a mide linkage between the-(A)1 and the A(2) residues of CA(1)A(2)X-derived F Tase inhibitors is important.