pamidronate

Pamidronate belongs to the nitrogen-containing bisphosphonate class (along with alendronate, zoledronic acid, risedronate, ibandronate). Mechanism: bisphosphonates are pyrophosphate analogs that bind avidly to hydroxyapatite in bone, especially at sites of active remodeling. Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate synthase (FPPS) in the mevalonate pathway within osteoclasts, preventing prenylation of small GTPases (Ras, Rho, Rac) necessary for osteoclast cytoskeletal function, survival, and activity. This leads to osteoclast apoptosis and decreased bone resorption. Clinical uses: (1) Hypercalcemia of malignancy—often used for humoral hypercalcemia of malignancy (PTHrP-mediated) or due to osteolytic metastases; lowers serum calcium within days. (2) Osteolytic bone metastases from breast cancer, multiple myeloma—reduces skeletal-related events (fractures, pain, need for radiation). (3) Paget's disease of bone—reduces bone turnover. (4) Osteogenesis imperfecta in pediatric patients (off-label). Administration: given IV (unlike oral bisphosphonates like alendronate) due to poor oral bioavailability and GI irritation; typically infused slowly to reduce renal toxicity risk. Adverse effects: acute phase reaction (fever, myalgia, flu-like symptoms after first infusion), hypocalcemia, hypophosphatemia, hypomagnesemia, nephrotoxicity (especially with rapid infusion or renal impairment), osteonecrosis of the jaw (classically associated with IV bisphosphonates, particularly in cancer patients, especially after dental procedures), atypical femoral fractures (more with oral bisphosphonates, long-term use), ocular inflammation (uveitis, scleritis—rare). Contraindications/cautions: renal impairment (dose adjust/avoid in severe renal failure), hypocalcemia (correct before use), pregnancy. Monitoring: renal function, serum calcium, phosphate, magnesium before and during therapy. Distinguish from denosumab (RANKL inhibitor, monoclonal antibody) which has similar indications but different mechanism and is not renally cleared.