Inhibition of ectopic calcification of glutaraldehyde crosslinked collagen and collagenous tissues by a covalently bound diphosphonate (APD)

Calcification of collagen-derived prosthesis, such as glutaraldehyde crosslinked porcine heart valves or heart valves assembled out of bovine pericardium, presents a major clinical problem. Their subcutaneous implantation into young rats provides us with a reproducible method of assessing this form of ectopic calcification. Long-term implantation is essential, since some materials which do not calcify within the first month frequently exhibit a delayed calcific response. Crosslinked pericardium is much more likely to calcify than crosslinked tendon or reconstituted crosslinked pepsin extracted bovine type I collagen. The covalent binding of a diphosphonate to collagen and collagen-rich tissues can prevent calcification. The binding of this diphosphonate and its ability to inhibit calcification can be enhanced by increasing the number of amino groups on the collagen molecule. The degree of calcification is inversely related to the number of diphosphonate molecules covalently bound to collagen. Under standard conditions, chemical modifications appear to occur primarily on the surface of the collagen fibrils, as evidenced by the relationship between the number of molecules of APD bound and fibril diameter. The bound diphosphonate seems to interfere with crystal growth and prevent the formation of highly insoluble hydroxyapatite on the surface and interstices of the collagen fibrils.