Structural factors influencing the ability of compounds to inhibit hydroxyapatite formation.

For a compound to inhibit potently the transformation of amorphous calcium phosphate into hydroxyapatite, it is suggested that the minimum structural requirement is a phosphate group and, at some other position, either another phosphate group preferably or a carboxylic moiety. Primary amino groups abolish inhibitor potential. Inhibitor potency is modified by various secondary factors, including the number and proximity of active groups, their stereochemistry, steric factors, the lability of the molecule, and in special instances its lipophilicity. Parameters used to monitor the transformation indicate that inhibitors can be grouped into two classes, and it is suggested that this is because one class acts as a hydroxyapatite crystal growth inhibitor. The close proximity of two phosphate groups or of a phosphate and multiple carboxylic groups is proposed to determine in part whether or not a compound acts as a crystal growth inhibitor. Further, bulky side groups render a molecule inactive as a crystal growth poison, although it will still inhibit by other mechanisms.