The use of human hepatocytes to select compounds based on their expected hepatic extraction ratios in humans.

PURPOSE: The present investigation retrospectively evaluates the use of human hepatocytes to classify compounds into low, intermediate or high hepatic extraction ratio in man. METHODS: A simple approach was used to correlate the in vivo hepatic extraction ratio of a number of compounds in man (literature and in-house data) with the corresponding in vitro clearance which was determined in human hepatocytes. The present approach assumes that, for compounds eliminated mainly through liver metabolism, intrinsic clearance is the major determinant for their in vivo hepatic extraction ratio and subsequently their bioavailability in man. The test compounds were selected to represent a broad range of extraction ratios and a variety of metabolic pathways. RESULTS: The present data show that in vitro clearances in human hepatocytes are predictive for the hepatic extraction ratios in vivo in man. Most of the test compounds (n = 19) were successfully classified based upon human hepatocyte data into low, intermediate or high hepatic extraction compounds, i.e. compounds with potential for high, intermediate or low bioavailabilities in humans. CONCLUSIONS: The present approach, validated so far with 19 test compounds, appears to be a valuable tool to screen for compounds with respect to liver first-pass metabolism at an early phase of drug discovery.

Bosentan, a new endothelin receptor antagonist: prediction of the systemic plasma clearance in man from combined in vivo and in vitro data.

1. Accurately predicting the kinetics in man greatly improves the design of the phase I clinical studies. This was particularly crucial in the case of bosentan, a new endothelin receptor antagonist, as very large interspecies differences in systemic clearance were observed in the animal species investigated, namely from 1.5ml/min/kg in the dog up to 70 ml/min/kg for the rabbit. 2. Bosentan was shown to be metabolized by the hepatic cytochrome P450, therefore the rate of metabolism was investigated in vitro in liver microsomes and hepatocytes, across the species which had been tested in vivo. The same rank-order of metabolism was found for the laboratory animals both in vitro and in vivo, and hepatocytes appeared to be more representative of the in vivo situation than liver microsomes. The in vitro clearance in human hepatocytes was very close to that observed in dog hepatocytes. 3. A plasma clearance for bosentan in man of 1-2 ml/min/kg was predicted by combining the in vivo and in vitro data from a few animal species with the in vitro data in man. This expectation was subsequently found to agree reasonably well with the plasma clearance observed in healthy volunteers: ca 2 ml/min/kg. Integrating this prediction into the design of the first clinical protocols substantially improved the quality of the human pharmacokinetic data obtained.

Carnitine supplementation and fat emulsion clearance and utilization.

The aim of the present work is to learn if intravenous administration of L-carnitine accelerates the clearance of a lipid emulsion. Intravenous fat tolerance tests have been done on rats (0.4 g triacylglycerols.kg-1 B.W.). Measurement of the light scattering index of the plasma permitted determination of the exogenous lipid concentration and thus allowed to represent the clearance curve of the infused emulsion. It was found that prior administration of L-carnitine (110, 160 or 560 mg.kg-1 B.W.) does not modify the clearance rate either of a long chain triglyceride emulsion or of a medium chain triglyceride based emulsion. The observed clearance in L-carnitine deficient animals, resulting either from intraperitoneal injections of D-carnitine (3 g.kg-1 B.W. for four days) or from a 6-months long diet of suboptimal amounts of precursor amino acids and vitamins for carnitine biosynthesis, was not reduced relative to the clearance in corresponding control animals. Also, the in vitro activities of the two enzymes involved in the clearance of the infused lipids, lipoprotein lipase (diaphragm and adipose tissue) and hepatic endothelial lipase, were unmodified by the L-carnitine.