IN-VIVO DISPOSITION OF DERMORPHIN ANALOG (DALDA) IN NONPREGNANT AND PREGNANT SHEEP

Although synthetic opioid peptide analogs have been used extensively t o study the functional roles of opioid receptors, little is known abou t their in vivo disposition. Our goal was to develop novel opioid drug s with limited transfer across the placenta. DALDA (Tyr-D-Arg-Phe-Lys- NH2) is a potent and highly selective mu agonist that is quite polar b ecause of its 3+ charge at physiological pH. It can therefore be expec ted that the distribution of DALDA across the placenta would be highly restricted. In this study, we determined the pharmacokinetics and pla cental transfer of DALDA after systemic administration in sheep. DALDA was infused intravenously to four nonpregnant and four pregnant sheep at a dose of 0.6 mg/kg/hr for 4 hr. Steady state plasma levels of DAL DA were 5436 +/- 464 ng/ml in nonpregnant sheep and 5214 +/- 661 ng/ml in pregnant sheep. A one-compartment open model provided an excellent fit for nonpregnant and pregnant plasma data. The apparent volume of distribution was estimated to be 45.6 +/- 4.4 and 59.2 +/- 7.9 ml/kg i n nonpregnant and pregnant animals, respectively. There was no differe nce in the elimination half-life of DALDA in nonpregnant (1.4 +/- 0.1 hr) and pregnant (1.7 +/- 0.2 hr) animals, and clearance was also simi lar in nonpregnant (23.1 +/- 1.7 ml/kg/hr) and pregnant (23.7 +/- 1.3 ml/kg/hr) animals. These data suggest that the distribution of DALDA i s restricted to plasma volume and that its disposition is not altered in pregnancy. DALDA was not detected in any of the fetal plasma sample s (<50 ng/ml), indicating that fetal plasma concentration is <1% of ma ternal concentration. The highly restricted placental distribution of DALDA suggests that it may be a promising opioid drug for obstetrical use.