FIRST-PASS ELIMINATION OF A PEPTIDOMIMETIC THROMBIN INHIBITOR IS DUE TO CARRIER-MEDIATED UPTAKE BY THE LIVER - INTRODUCTION WITH BILE-ACID TRANSPORT-SYSTEMS

CRC 220 (4-methoxy-2, 3, phenylsulfonyl-L-aspartyl-D-4-amidinophenylal anyl- piperidide) is a competitive peptide-based trombin inhibitor wit h high affinity to human alpha-thrombin (K-i 2.5 nM). The amphiphilic compound exhibits virtually no systemic bioavailability despite proteo lytic stability and proven enteral absorption. After intravenous appli cation (V. jejunalis) in rats CRC 220 is almost completely excreted in to bile. Simultaneous administration of bile acids considerably decrea ses this first-pass elimination. CRC 220 is extensively taken up in is olated rat hepatocytes by a saturable carrier-mediated transport with K-m 23.7 mu M and V-max 775 pmol x mg(-1) x min(-1). A large part of t his transport is energy dependent. At temperatures above 20 degrees C, the uptake is accelerated exponentially. The activation energy amount s to 82 kJ/mol. A minor portion of CRC 220 uptake occurs by physical d iffusion with a permeability coefficient of 7.83 x 10(-7) cm/sec at 12 degrees C. Sodium ions energize CRC 220 uptake. Replacement of sodium by choline or lithium decreases the transport rate of 23-40%. In addi tion, a negative membrane potential facilitates the uptake. CRC 220 tr ansport is only observed in hepatocytes: it is absent in BHK, FAO, Hep G2, HPCT 1E3, and HPCT 1E3-TC cells. In the presence of 4-amidinopheny lalanine derivatives, CRC 220 uptake is considerably decreased. Inhibi tion also occurs with bile acids and bromosulfophthalein, but less wit h bumetanide. Because CRC 220 inhibits bile acid uptake into hepatocyt es and vice versa, the results suggest that the first pass elimination of this amphiphilic thrombin inhibitor is due to an active carrier-me diated transport process in the basolateral plasma membrane of rat hep atocytes, and that this transport occurs via a bile acid transport sys tem.