Hydroxyapatite of bone matrix (HA (rarely, HAP))

Hydroxyapatite is a naturally occurring crystalline calcium phosphate (Ca$_{10}$(PO$_{4}$)$_{6}$(OH)$_{2}$), forming the principal inorganic fraction of the bone matrix[2][4][5][6]. By volume, it constitutes approximately 40–45% of bone matrix, with type I collagen composing the remainder[1][2][5]. Hydroxyapatite crystals are plate-shaped (20–50 nm × 15 nm × 2–5 nm) and contribute to bone’s strength and rigidity[1][2][4]. Bone hydroxyapatite contains physiological impurities (carbonate, magnesium, strontium, fluoride, etc.), which affect crystal properties such as solubility and mechanical strength[2][4]. The deposition of hydroxyapatite onto collagen during bone formation (osteogenesis) is regulated by osteoblasts and supports mineral homeostasis, skeletal structure, and calcium/phosphate balance[2][4][6]. Disorders of hydroxyapatite metabolism or structure (e.g., osteoporosis, osteomalacia) impact bone quality and fracture risk[5][6]. Although not a traditional therapeutic receptor or target, drugs affecting bone metabolism often interact directly or indirectly with hydroxyapatite[6].\n\nHydroxyapatite is not a protein nor a drug receptor—it is the inorganic phase of bone tissue, targeted for imaging or modulation in bone disease therapy, but not as a classic molecular drug target[2][3][5][6].

Other names

HydroxyapatiteHAPBone mineralCalcium phosphate crystalline complexBioapatite

Bisphosphonates: bind to hydroxyapatite surfaces and inhibit osteoclast-mediated bone resorption; Strontium ranelate: substitutes for calcium in the hydroxyapatite lattice; Denosumab: reduces bone resorption, indirectly affecting hydroxyapatite content