A new solid-phase route to prenylcysteine derivatives: Development of isoprenylcysteine carboxyl methyltransferase inhibitors

More than 120 human proteins terminate in a -CaaX sequence, targeting these proteins for a series of three post-translational modifications. These modifications are essential for the proper function of the -CaaX proteins. They consist of the sequential attachment of a 15-carbon farnesyl or 20-carbon geranylgeranyl group to the cysteine thiol of the -CaaX sequence, proteolysis of the -aaX residues, and methylation of the newly liberated cysteine carboxyl group. Several -CaaX proteins are involved in oncogenesis, making the enzymes that catalyze these three modifications targets for chemotherapeutic agents. The enzyme responsible for the C-terminal methylation of -CaaX proteins, and the target of this investigation, is isoprenylcysteine carboxyl methyltransferase (Icmt). A series of 24 amide-modified farnesylcysteine analogs (AMFCs) were synthesized based on the structure of the standard Icmt substrate N-acetyl-S-farnesylcysteine (AFC). Evaluation of these compounds against Icmt resulted in identification of several inhibitors. A solid-phase route to AMFCs was desired, but the farnesylcysteine thioether bond is very labile and requires mild conditions. The 2-chlorotrityl-chloride resin was developed as a solid support after significant optimization. A more comprehensive library of AMFCs was synthesized utilizing the new route and additional, more potent inhibitors of Icmt were identified. Given the significant differences in potency seen with amide-modified AFC analogs, it was hypothesized that the amino acid attached to the farnesylcysteine residue of -CaaX proteins may be a major binding determinate for substrate recognition. It is not currently known if all lipidated -CaaX proteins undergo proteolysis and methylation in vivo. Sequence analysis of -CaaX proteins revealed all 20 naturally occurring amino acids are found immediately upstream of the farnesylcysteine, though some appear much more frequently than others. The 2-chlorotrityl chloride resin solid-phase route was used to synthesize 26 Boc-protected farnesylcysteine dipeptides corresponding to all 20 naturally occurring and six unnatural amino acids. These dipeptides were evaluated as substrates for Icmt. While all 20 naturally occurring amino acid containing dipeptides were found to be substrates, as expected their ability to act as substrates varied greatly. These data suggests that while N-substitutions contribute to farnesylcysteine substrate ability, it is not the only determinant