Controlled release of 1-hydroxyethylidene diphosphonate: in vitro assessment and effects on bioprosthetic calcification in sheep tricuspid valve replacements

Abstract

Calcification (CALC) is the most frequent cause of the clinical failure of bioprosthetic valves (BHV's). Controlled-release (paravalvar) administration of the anticalcification agent ethanehydroxydiphosphonate (EHDP), as either Na2EHDP or in combination (1:1) with the less soluble CaEHDP, from a silicone rubber matrix (20% w/w EHDP) was studied both in vitro and in vivo for the prevention of BHV CALC. Seventeen sheep (6-7 months old, male, Suffolk) underwent tricuspid valve replacement using Hancock I, 25 mm porcine aortic bioprostheses. BHV explant evaluation after 16-20 weeks revealed that two of the 7 control BHV were calcified (139 +/- 20.8 [mu]gCa2+/mg of tissue), while none of the 9 BHV retrieved from animals receiving controlled release EHDP demonstrated CALC (4.41 +/- 1.09 [mu]g Ca2+/mg of tissue). No adverse effects of EHDP on bone or calcium metabolism were noted. The cumulative percent of EHDP released per electron microprobe analysis was 40.4% +/- 9.68 (Na, CaEHDP) to 79.0% +/- 4.82 (Na2EHDP) in vivo compared to 35.7% +/- 7.72 and 78.6 +/- 11.1 in vitro, respectively. Assessment of the Young's modulus (Y) using thermomechanical analysis (TMA) revealed a 1.5-fold (Silastic Q7-4840) to 9.5-fold (Silastic 382) increase in Y following drug loading. The Y for explanted, Silastic Q7-4840 polymer matrices ranged from 2.84 x 104 to 5.57 x 105 dyne/cm2. In vitro osmotic related matrix swelling of the Na2EHDP loaded, unsealed matrices (20% w/w) after 75 days was minimized to a 35.8% increase in weight due to coincorporation of CaEHDP with Na2EHDP in a 1:1 ratio and was further reduced (22.2% increase in weight) by sealing 76% of the releasing surface, compared to Na2EHDP matrices which demonstrated a 414% and 141% increase in weight, respectively.