In vivo evaluation of isoprenoid triazole bisphosphonate inhibitors of geranylgeranyl diphosphate synthase: impact of olefin stereochemistry on toxicity and biodistribution.

The enzyme geranylgeranyl diphosphate synthase (GGDPS) synthesizes the 20-carbon isoprenoid geranylgeranyl diphosphate (GGPP) which is used in geranylgeranylation reactions. In multiple myeloma (MM) cells, we have demonstrated that GGDPS inhibitors disrupt Rab geranylgeranylation, leading to inhibition of monoclonal protein trafficking, induction of the unfolded protein response pathway (UPR) and apoptosis. We have previously reported preclinical studies with the GGDPS inhibitor VSW1198, which is a mixture of homogeranyl/homoneryl triazole bisphosphonates. Additional structure-function efforts have led to the development of the α-methylated derivatives RAM2093 (homogeranyl) and RAM2061 (homoneryl). As little is known regarding the impact of olefin stereochemistry on drug properties in vivo, we pursued additional preclinical evaluation of RAM2093 and RAM2061. In MM cell lines, both isomers induce activation of UPR/apoptotic markers in a concentration-dependent manner and with similar potency. Single dose testing in CD-1 mice identified a maximum tolerated dose of 0.5 mg/kg IV for RAM2061 and 0.3 mg/kg for RAM2093. Liver toxicity was the primary barrier to dose escalation for both compounds. Disruption of geranylgeranylation in vivo was confirmed following multi-dose administration of either compound. Pharmacokinetic studies revealed plasma terminal half-lives of 29.2 ± 6 h (RAM2061) and 22.1 ± 4 h (RAM2093). Relative to RAM2061, RAM2093 levels were significantly higher in liver tissue but not in other tissues. Using MM.1S flank xenografts we observed a significant reduction in tumor growth in mice treated with RAM2061 relative to controls. Collectively, these studies reveal olefin stereochemistry-dependent effects on GGDPS inhibitor biodistribution and confirm the in vivo efficacy of this novel therapeutic approach. SIGNIFICANCE STATEMENT: These studies reveal olefin stereochemistry-dependent effects on the in vivo properties of two novel triazole bisphosphonate inhibitors of geranylgeranyl diphosphate synthase and demonstrate the therapeutic potential of this class of inhibitors for the treatment of multiple myeloma.