Differential inhibition of farnesyl protein transferase and squalene synthase by isoprenoid phosphonic acids

Farnesyl protein transferase (FPTase) is an attractive target for anti-RAS therapies. We have synthesized a series of isoprenoid phosphonic acids to characterize the structural requirements for FPTase inhibition. Analogues were tested for mammalian FPTase and squalene synthase inhibitory activities in enzyme and intact cell assays. E,E-α-hydroxyfarnesylphosphonate inhibits FPTase with a 50% inhibitory concentration (IC₅₀) of 30 nM whereas the Z,E-isomer is inactive. In human-derived THP-1 myeloid and RPMI-8402 lymphoid leukemia cells 100 μM E,E-α-hydroxyfarnesylphosphonate induces accumulation of unmodified RAS proteins and the Z,E-isomer is inactive. In contrast, squalene synthase is only minimally inhibited by the active E,E-isomer with a reduction of squalene synthase activity to 80% of control occurring at 1 μM levels. Cholesterol synthesis in intact cells is not decreased with concentrations as high as 200 μM of the E,E-isomer. We conclude from these studies that there is differential inhibition of FPTase and squalene synthase by E,E-α-hydroxyfamesylphosphonate thus supporting the concept that isoprenoid phosphonic acids may have eventual clinical utility as anti-RAS therapies.