Inhibitors of Ras farnesyl-protein transferase are described. These are reduced pseudopeptides related to the C-terminal tetrapeptide of the Ras protein that signals farnesylation. Deletion of the carbonyl groups between the first two residues of the tetrapeptides either preserves or improves activity, depending on the peptide sequence. The most potent in vitro enzyme inhibitor described (IC50 = 5 nM) is Cys [psi CH2NH]Ile[psi CH2NH]Phe-Met (3). To obtain compounds able to suppress Ras farnesylation in cell culture, further structural modification to include a homoserine lactone prodrug was required. Compound 18 (Cys[psi CH2NH]Ile[psi CH2NH]Ile-homoserine lactone) reduced the extent of Ras farnesylation by 50% in NIH3T3 fibroblasts in culture at a concentration of 50 microM. Structure-activity studies also led to 12 (Cys[psi CH2NH]Val-Ile-Leu), a potent and selective inhibitor of a related enzyme, the type-I geranylgeranyl protein transferase.

Derivatives of benzofuran- and indole-2-sulfonamide were prepared for evaluation as topically active ocular hypotensive agents. These compounds were found to be excellent inhibitors of carbonic anhydrase and to lower intraocular pressure in a rabbit model of ocular hypertension. However, the development of these compounds for clinical use was precluded by the observation that they cause dermal sensitization in guinea pigs. A correlation between electrophilicity, as assessed by in vitro reactivity with reduced glutathione, and dermal sensitization potential was further documented.

A series of pseudodipeptide amides are described that inhibit Ras protein farnesyltransferase (PFTase). These inhibitors are truncated versions of the C-terminal tetrapeptide (CAAX motif) of Ras that serves as the signal sequence for PFTase-catalyzed protein farnesylation. In contrast to CAAX peptidomimetics previously reported, these inhibitors do not have a C-terminal carboxyl moiety, yet they inhibit farnesylation in vitro at < 100 nM. Despite the absence of the X residue in the CAAX motif, which normally directs prenylation specificity, these pseudodipeptides are greater than 100-fold selective for PFTase over type 1 protein geranylgeranyltransferase.

We have identified a novel fungal metabolite that is an inhibitor of human farnesyl-protein transferase (FPTase) by randomly screening natural product extracts using a high-throughput biochemical assay. Clavaric acid [24, 25-dihydroxy-2-(3-hydroxy-3-methylglutaryl)lanostan-3-one] was isolated from Clavariadelphus truncatus; it specifically inhibits human FPTase (IC50 = 1.3 microM) and does not inhibit geranylgeranyl-protein transferase-I (GGPTase-I) or squalene synthase activity. It is competitive with respect to Ras and is a reversible inhibitor of FPTase. An alkaline hydrolysis product of clavaric acid, clavarinone [2,24,25-trihydroxylanostan-3-one], lacking the 3-hydroxy-3-methylglutaric acid side chain is less active as a FPTase inhibitor. Similarly, a methyl ester derivative of clavaric acid is also inactive. In Rat1 ras-transformed cells clavaric acid and lovastatin inhibited Ras processing without being overtly cytotoxic. Excess mevalonate reversed the effects of lovastatin but not of clavaric acid suggesting that the block on Ras processing by clavaric acid was due to inhibition of FPTase and not due to inhibition of HMG-CoA reductase. Despite these results, the possibility existed that clavaric acid inhibited Ras processing by directly inhibiting HMG-CoA reductase. To directly examine the effects of clavaric acid and clavarinone on HMG-CoA reductase, cholesterol synthesis was measured in HepG2 cells. No inhibition of HMG-CoA reductase was observed indicating that the inhibition of Ras processing by this class of compounds is due to inhibition of FPTase. To date, clavaric acid is the second reported nitrogen-free compound that competes with Ras to inhibit FPTase activity. A series of related compounds derived from computer-based similarity searches and subsequent rational chemical synthetic design provided compounds that exhibited a range of activity (0.04 --> 100 microM) against FPTase. Modest changes in the structures of these inhibitors dramatically change the inhibitory activity of these inhibitors.