Bone matrix hydroxyapatite (HAP) (HAP)

Bone matrix hydroxyapatite is the primary inorganic constituent of vertebrate bone, composed of crystalline calcium phosphate Ca10(PO4)6(OH)2 (PubChem, 2024). It provides structural rigidity to the skeleton and serves as a critical reservoir for calcium and phosphate ions, playing a central role in mineral homeostasis (StatPearls, 2023). In diseases like osteoporosis, Paget's disease, and bone metastases, the rate of bone remodeling is pathologically increased, exposing more hydroxyapatite surfaces (PubMed, 2021). This mineral serves as a pharmacological target for bone-seeking agents, including bisphosphonates and certain radiopharmaceuticals, which bind to the crystal lattice with high affinity (PubMed, 2002). By targeting hydroxyapatite at sites of high turnover, these drugs can effectively inhibit osteoclast-mediated resorption or deliver localized cytotoxic radiation to bone tumors (NIH, 2022). Consequently, hydroxyapatite acts as both a structural scaffold and a delivery platform for bone-targeted therapies (PubMed, 2020).

Other names

Calcium hydroxyapatiteHydroxylapatiteBone mineralCrystalline calcium phosphate

Drugs such as bisphosphonates possess a high affinity for hydroxyapatite crystals, allowing them to selectively bind to bone surfaces, particularly at sites of active remodeling. Once bound, they are internalized by osteoclasts during bone resorption, leading to osteoclast apoptosis and reduced bone turnover. Radiopharmaceuticals similarly use hydroxyapatite affinity to deliver targeted radiation to areas of high bone metabolism.