Calcium phosphates have been used in the pharmaceutical technology for many years. They have many physical and chemical properties that make them ideal candidates for the production of solid oral dosage forms. They are mainly used as fillers in order to bulk up formulations however, the function of calcium phosphates goes far beyond mere filler and the adequate utilisation of their full functionality can support achieving the intended formulation goals.
Calcium phosphates are inorganic substances of mineral origin and therefore are characterised by exceptional chemical stability. For the same reason, they are compatible with most of known drug substances. The few exceptions include indomethacin and tetracycline antibiotics which form with the calcium ions hardly absorbable complexes. Tribasic calcium phosphate (USP) shows incompatibility with the tocopheryl acetate which is related to the large number of hydroxyl groups on the surface of the substance.
Calcium salts of orthophosphoric acid do not interact with water and therefore can be successfully used in any technological operations involving water as a completely inert densifier. Due to the very high content of calcium and phosphorus they are very often used in dietary supplements. Especially hydroxyapatite finds wide spread use due to its calcium to phosphorous ratio which is identical to the one present in human bones.
Calcium phosphate based excipients possess many functional properties making them ideal candidates for direct compression process. Due to their favourable particle size and shape calcium phosphates exhibit excellent flowability. Furthermore, they enable to govern the flow pattern of poorly flowable powders, which allows for relatively easy preparation of tableting mixtures without the granulation. Very high density of calcium salts of orthophosphoric acid allows either using of larger amounts of excipients without increasing the size of the dosage form or decreasing it when the same quantity of excipient is used.