Geranylgeranyl diphosphate synthase inhibition by terpenoid bisphosphonates

Multiple myeloma is a cancer characterized by the large-scale production and export of monoclonal antibody proteins. The exocytosis of these proteins from the trans-Golgi apparatus is contingent upon and directed by the presence of Rab proteins on the vesicle membrane. Proper membrane localization of Rab is made possible by a post-translational modification called prenylation. In this modification, a geranylgeranyl chain is added to the Rab protein bound in a complex with another protein, Rep. The geranylgeranyl chain for this modification comes from geranylgeranyl diphosphate (GGPP). By depleting the cell’s GGPP concentration, Rab prenylation is diminished, and the unmodified protein does not properly localize to vesicle membranes. Consequently, exocytosis is inhibited, resulting in a buildup of intracellular monoclonal protein in the myeloma cells. Eventually, this buildup induces the unfolded protein response and leads to apoptotic cell death. Over the past decade, the research labs of Dr. David Wiemer and Dr. Sarah Holstein have explored a strategy to lower cellular GGPP concentrations in myeloma cells by inhibition of the enzyme responsible for its cellular synthesis - geranylgeranyl diphosphate synthase (GGDPS). The most active class of inhibitors developed to date consists of a bisphosphonate head group, a terpenoid tail, and a triazole linking the two halves of the molecule. The structure-activity relations (SAR) study through which this family has been optimized has focused primarily on modifications of the terpenoid tail. Far less work has been done on modifications at or around the bisphosphonate head, and even less work has been done looking at the linkage between the two halves of the molecule. This work concerns itself with furthering efforts to understand GGDPS inhibition. In the first section, the attempted synthesis of a number of oxygenated terpenoid triazole bisphosphonates is discussed. The successful syntheses of biologically active vinyl and cyclopropyl terpenoid triazole bisphosphonate also is presented. During the course of this work, a new Johnson-Corey-Chaykovsky strategy for preparing unsubstituted cyclopropyl bisphosphonates from vinyl bisphosphonates was developed. In the second part, a new, amide-based linkage is developed for these inhibitors in an effort to address challenges inherent to the triazole. These challenges include the practical difficulties of large-scale azide-based syntheses, observed hepatotoxicity of the triazoles in a mouse model, and the difficult synthesis of the most-active homoneryl triazoles. The family of terpenoid amides prepared is biologically active against GGDPS, and follows – broadly – the patterns of activity found in the triazoles. The synthesis of the most active bishomogeranyl amide was comparatively straightforward and avoided the use of azides. Furthermore, this synthesis does not contain fundamental issues of scalability. Finally, future work might well show that the replacement of a triazole linkage with an amide lowers the risk of hepatotoxicity in treating myeloma with these terpenoid bisphosphonate GGDPS inhibitors.