Alendronate sodium is a bisphosphonate that acts as a specific inhibitor of osteoclast-mediated bone resorption. It inhibits bone resorption in vitro with an IC₅₀ value of 2 nM. Also acts as a farnesyl diphosphate synthase inhibitor (IC₅₀ = 460 nM) and CD45 protein tyrosine phosphatase inhibitor.