Phosphonocarboxylate inhibitors of Rab geranylgeranyl transferase disrupt the prenylation and membrane localization of Rab proteins in osteoclasts in vitro and in vivo Fraser P. Coxon a, * , Frank H. Ebetino b , Emilie H. Mules c , Miguel C. Seabra c , Charles E. McKenna d , Michael J. Rogers a a Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen, AB25 2ZD, UK b Procter & Gamble Pharmaceuticals, Cincinnati, OH 45250-0859, USA c Cell and Molecular Biology, Division of Biomedical Sciences, Imperial College School of Medicine, London SW7 2AZ, UK d Department of Chemistry, University of Southern California, Los Angeles, CA 90007, USA Received 6 December 2004; revised 4 March 2005; accepted 22 April 2005 Available online 11 July 2005 Abstract Nitrogen-containing bisphosphonate drugs such as risedronate act by inhibiting farnesyl diphosphate synthase, thereby disrupting protein prenylation in osteoclasts. We recently found that an anti-resorptive phosphonocarboxylate analogue of risedronate, 3-PEHPC (previously referred to as NE10790), selectively prevents prenylation of Rab GTPases in vitro by specifically inhibiting Rab geranylgeranyl transferase. In this study, we demonstrate that unprenylated Rab6 could be detected in J774 cells after treatment with 3-PEHPC or risedronate for as little as 4 h, and reached 50% after 24 h. Furthermore, treatment of J774 cells or osteoclasts with either 3-PEHPC or risedronate disrupted membrane association of several Rab family proteins. Like risedronate, the effects of 3-PEHPC are likely to be restricted to osteoclasts in vivo, since both risedronate and 3-PEHPC inhibited Rab prenylation in osteoclasts, but not in general bone marrow cells, when administered to rabbits in vivo. Analysis of two new phosphonocarboxylate analogues of 3-PEHPC (3-PEPC and 2-PEPC) revealed that, first, the geminal hydroxyl group is not essential for inhibition of Rab prenylation by phosphonocarboxylates, but does contribute to their anti-resorptive potency, most likely by enhancing their affinity for bone mineral. Second, the position of the nitrogen in the side chain of phosphonocarboxylates is crucial for their ability to inhibit Rab prenylation and hence to inhibit bone resorption. In addition, there is a good correlation between the ability of the phosphonocarboxylates to inhibit Rab prenylation and to inhibit bone resorption in vitro, indicating that these compounds are a new class of pharmacological agents that inhibit bone resorption by specifically preventing prenylation of Rab proteins. Furthermore, although phosphonocarboxylates are analogues of bisphosphonates, the structure – activity relationships of phosphonocarboxylates for inhibiting Rab geranylgeranyltransferase appear to differ from the structure–activity relationships of bisphosphonates for inhibiting farnesyl diphosphate synthase. D 2005 Elsevier Inc. All rights reserved. Keywords: Bisphosphonate; Phosphonocarboxylate; Osteoclast; Protein prenylation; Rab GGTase Introduction Protein prenylation is a post-translational modification involving the transfer of either a farnesyl or geranylgeranyl isoprenoid lipid to the c-termini of specific target proteins, chiefly small GTPases of the Ras, Rho, and Rab families. This modification is critical for the function of these proteins, since it is required for membrane association and for specific protein – protein interactions [1]. The process of prenylation is carried out by one of three distinct protein:- prenyl transferase enzymes, the specificity being determined by the prenylation motif in the protein substrate. These 8756-3282/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.bone.2005.04.021 * Corresponding author. Fax: +44 1224 559533. E-mail address: f.p.coxon@abdn.ac.uk (F.P. Coxon). Bone 37 (2005) 349 – 358 www.elsevier.com/locate/bone