What is the primary mechanism of action of bisphosphonates in osteoporosis?

• A. Stimulate osteoblast activity
• B. Increase calcium absorption in the gut
• C. Mimic PTH to build bone
• D. Inhibit osteoclast-mediated bone resorption

Answer: (D)

Osteoporosis treatment with bisphosphonates works by dampening osteoclast activity to slow bone resorption. These drugs bind strongly to bone mineral and are taken up by osteoclasts during resorption. Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate synthase in the mevalonate pathway, disrupting the prenylation of small signaling proteins essential for osteoclast function and survival. The result is impaired osteoclast activity and increased osteoclast apoptosis, which lowers bone turnover and helps preserve or increase bone density, thereby reducing fracture risk.

This mechanism is distinct from stimulating osteoblasts, increasing gut calcium absorption, or mimicking parathyroid hormone, which would involve anabolic or hormonal pathways rather than directly suppressing bone resorption.