Ectopic hydroxyapatite deposit (BCP)

Ectopic hydroxyapatite and calcium phosphate deposits are pathological mineral accumulations that occur in soft tissues, such as blood vessels, tendons, and skin, rather than in the skeletal system (StatPearls, 2023). These deposits primarily consist of basic calcium phosphate (BCP) crystals, which can induce significant inflammation and mechanical tissue damage (Nature Reviews Nephrology, 2020). They are a major complication in chronic kidney disease, where vascular calcification increases cardiovascular mortality, and in conditions like calcific tendinitis or fibrodysplasia ossificans progressiva (FOP) (Journal of the American Society of Nephrology, 2018). Therapeutic strategies target these deposits either by preventing their formation or by promoting their dissolution. For example, the drug SNF472 binds directly to the growth sites of hydroxyapatite crystals to inhibit their expansion, while sodium thiosulfate is used clinically to increase the solubility of these deposits (Circulation, 2019). Additionally, emerging therapies target enzymes like tissue-nonspecific alkaline phosphatase (TNAP) to preserve levels of inorganic pyrophosphate, a natural inhibitor of crystal formation (Arteriosclerosis, Thrombosis, and Vascular Biology, 2021). Bisphosphonates like etidronate also interact with these deposits by inhibiting crystal growth and osteoclast-like activity in soft tissues.

Other names

Ectopic and non-osseous hydroxyapatite/calcium phosphate depositsEctopic calcificationExtra-skeletal calcificationHydroxyapatite deposition disease (HADD)Basic calcium phosphate (BCP) depositsCalcinosis cutisDystrophic calcificationMetastatic calcification

Inhibition of hydroxyapatite crystal nucleation, growth, and aggregation through direct binding to the crystal surface; chelation of calcium ions; and inhibition of tissue-nonspecific alkaline phosphatase (TNAP) to increase levels of the endogenous mineralization inhibitor pyrophosphate.