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, 6-trimethylphenylsulfonyl-L-aspartyl-D-4-amidinophenylalanyl- piperidide) is a competitive peptide-based trombin inhibitor with high affinity to human alpha-thrombin (K-i 2.5 nM). The amphiphilic compound exhibits virtually no systemic bioavailability despite proteolytic stability and proven enteral absorption. After intravenous application (V. jejunalis) in rats CRC 220 is almost completely excreted into bile. Simultaneous administration of bile acids considerably decreases this first-pass elimination. CRC 220 is extensively taken up in isolated 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 this transport is energy dependent. At temperatures above 20 degrees C, the uptake is accelerated exponentially. The activation energy amounts to 82 kJ/mol. A minor portion of CRC 220 uptake occurs by physical diffusion 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 addition, a negative membrane potential facilitates the uptake. CRC 220 transport is only observed in hepatocytes: it is absent in BHK, FAO, HepG2, HPCT 1E3, and HPCT 1E3-TC cells. In the presence of 4-amidinophenylalanine derivatives, CRC 220 uptake is considerably decreased. Inhibition also occurs with bile acids and bromosulfophthalein, but less with bumetanide. Because CRC 220 inhibits bile acid uptake into hepatocytes and vice versa, the results suggest that the first pass elimination of this amphiphilic thrombin inhibitor is due to an active carrier-mediated transport process in the basolateral plasma membrane of rat hepatocytes, and that this transport occurs via a bile acid transport system.