Incorporation of an alpha-hydroxy substituent modulates activity of triazole bisphosphonate-based geranylgeranyl...

Geranylgeranyl diphosphate synthase (GGDPS), the enzyme responsible for producing the isoprenoid substrate used in protein geranylgeranylation reactions, represents a novel therapeutic target in cancer, including malignancies such as multiple myeloma and osteosarcoma. Our work has focused on the development of a series of isoprenoid triazole bisphosphonate-based GGDPS inhibitors. Previous work has revealed that our lead GGDPS inhibitors do achieve substantial systemic distribution, in contrast to clinically used nitrogenous bisphosphonates which accumulate primarily in the bone. We have determined that modifications to these GGDPS inhibitors impact not only potency at the level of the target enzyme, but also activity at cellular and in vivo levels. To explore further this structure-activity relationship, we have prepared a series of novel derivatives that contain an α-hydroxy substituent, but differ based on olefin stereochemistry (E vs Z), length of the isoprenoid side chain (C₁₀ vs C₁₁) as well as length of the linker between the triazole and bisphosphonate (0 vs 1 carbons). All new compounds were subjected to enzymatic and cellular assays in a panel of multiple myeloma and osteosarcoma cell lines. These studies revealed that olefin stereochemistry, isoprenoid chain length and triazole-to-bisphosphonate linker length all impacted inhibitor potency. In particular, decreasing the linker length from one to zero carbons resulted in a marked reduction in inhibitor activity. The most potent compound contained a neryl chain, had an IC₅₀ of 0.16 μM in the enzyme assay and demonstrated cellular activity at concentrations as low as 10-100 nM across the panel of tested cell lines. This work sets the stage for future studies which will explore the in vivo activity of the lead bisphosphonate, focusing on relative distribution to bone vs other organs, as well as determining anti-tumor efficacy in mouse models of multiple myeloma and osteosarcoma.