SITE-SPECIFIC DELIVERY OF ETHANEHYDROXY DIPHOSPHONATE FROM REFILLABLEPOLYURETHANE RESERVOIRS TO INHIBIT BIOPROSTHETIC TISSUE CALCIFICATION

Calcification (CALC) is the most frequent cause for the failure of bio prosthetic heart valves fabricated ted from glutaraldehyde-pretreated porcine aortic valve, and contributes to the failure of glutaraldehyde pretreated bovine pericardial (BHV) bioprosthetic heart valves as wel l. Although systemic therapy in rats using ethanehydroxy diphosphonate (EHDP) has proven successful in inhibiting CALC, adverse effects on s erum calcium, bone development, and overall somatic growth have been n oted. The present study was designed to evaluate the potential of site -specific delivery of EHDP to arrest CALC of glutaraldehyde-pretreated bovine pericardium when implanted subdermally in rats using a refilla ble reservoir drug delivery device. The refillable reservoir devices e valuated in these studies exhibited constant (zero-order) release of E HDP in vitro and replenishment of the drug supply when implanted subde rmally in rats was achieved in a noninvasive fashion using an exterior ized entrance and exit cannula. The refillable reservoirs evaluated we re fabricated from a commercially available polyurethane (Biomer(TM). Glutaraldehyde-pretreated bovine pericardium was implanted subdermally in 21-day-old rat either alone (control) or with refill able Biomer(T M) reservoirs with (treatment) or without (sham) a 2 M solution of Na2 EHDP. Implanted reservoirs which initially contained a 2 M solution of Na2EHDP were refilled with a fresh 2 M solution of Na2EHDP on days 7 and 14 post-initial surgery using a syringe and the exteriorized entra nce and exit cannulas. Pericardium retrieved following 21 days and ass ayed for calcium showed significant (P<0.001 ) inhibition in CALC for tissue implanted adjacent to refillable Biomer(TM) reservoirs containi ng EHDP (6.9 +/- 2.1 mug/mg) compared to control (179.0 +/- 13.5 mug/m g) and sham-implanted ( 1 52.0 +/- 10.2 mug/mg) rats. Unimplanted peri cardium had a mean tissue calcium concentration of 3.0 +/- 0.5 mug/mg. Based on the in vitro release studies of EHDP from refillable Biomer( TM) reservoirs, the estimated dose delivered when implanted subdermall y in rats in the present study was 5.5 +/- 0.7 mg/kg per day. For rats implanted with EHDP-containing refillable reservoirs, histological ex amination of retrieved pericardium and femurs from rats in each group confirmed both complete inhibition of CALC of the glutaraldehyde cross linked pericardium and no untoward effects on bone development, respec tively. In addition, blood samples obtained at sacrifice showed no cha nge in serum Ca2+ concentrations in EHDP-treated animals compared to c ontrols. Thus, the site-specific delivery of EHDP using refillable Bio mer(TM) reservoirs was successful for inhibiting BHV CALC in a rat sub dermal model with no untoward effects on bone development, serum Ca2concentrations, or overall growth. The advantages of the refillable re servoir system are its constant (zero-order) rate of EHDP release and its potential for replenishment of EHDP by noninvasive means when the EHDP solution inside the reservoir has been depleted.