Alendronate Is a Specific, Nanomolar Inhibitor of Farnesyl Diphosphate Synthase

Abstract

Alendronate, a nitrogen-containing bisphosphonate, is a potent inhibitor of bone resorption used for the treatment and prevention of osteoporosis. Recent findings suggest that alendronate and other N-containing bisphosphonates inhibit the isoprenoid biosynthesis pathway and interfere with protein prenylation, as a result of reduced geranylgeranyl diphosphate levels. This study identified farnesyl disphosphate synthase as the mevalonate pathway enzyme inhibited by bisphosphonates. HPLC analysis of products from a liver cytosolic extract narrowed the potential targets for alendronate inhibition (IC₅₀ = 1700 nM) to isopentenyl diphosphate isomerase and farnesyl diphosphate synthase. Recombinant human farnesyl diphosphate synthase was inhibited by alendronate with an IC₅₀ of 460 nM (following 15 min preincubation). Alendronate did not inhibit isopentenyl diphosphate isomerase or GGPP synthase, partially purified from liver cytosol. Recombinant farnesyl diphosphate synthase was also inhibited by pamidronate (IC₅₀ = 500 nM) and risedronate (IC₅₀ = 3.9 nM), negligibly by etidronate (IC50 = 80 μM), and not at all by clodronate. In osteoclasts, alendronate inhibited the incorporation of [³H]mevalonolactone into proteins of 18–25 kDa and into nonsaponifiable lipids, including sterols. These findings (i) identify farnesyl diphosphate synthase as the selective target of alendronate in the mevalonate pathway, (ii) show that this enzyme is inhibited by other N-containing bisphosphonates, such as risendronate, but not by clodronate, supporting a different mechanism of action for different bisphosphonates, and (iii) document in purified osteoclasts alendronate inhibition of prenylation and sterol biosynthesis.