Comparability of in vitro and in vivo methods for the determination of alterations in drug metabolism.

The above series of experiments taken in toto suggest the usefulness of lindane as a model substrate for studying the effects of a variety of compounds on drug metabolism in vivo. Excellent correlations were observed in comparison with the in vitro measurements both qualitatively and quantitatively. Unlike some of the other compounds discussed lindane offers some distinct advantages. One is that because the metabolites can be monitored in the urine, it is non-invasive in nature. A second is that a number of mixed function oxidase pathways (phase I reactions) can be determined at the same time. This would be of great importance if the effect of a compound is rather selective and does not alter the single pathway measured in the metabolism of other substrates which have been suggested as model compounds. However, the tradeoff is obviously the need for more analytical work. A third advantage is the ability of the system to detect changes in conjugative or phase II reactions at the same time. Further studies will be necessary with all of these model substrates to detect their usefulness and their limitations.

Effect of 7,8-benzoflavone pretreatment on diethylstilbestrol metabolism, drug-metabolising enzymes and the aromatic hydrocarbon (Ah) receptor in male hamster liver.

Pretreatment of male Syrian golden hamsters with 7,8-benzoflavone (7,8-BF) leads to a marked increase of cytochrome P450 and cytochrome b5 levels in the liver, whereas phenobarbital (PB) and 3-methylcholanthrene (MC) induce cytochrome P450 but not cytochrome b5 7,8-BF pretreatment has only minor effects on the activities of aryl hydrocarbon hydroxylase and 7-ethoxycoumarin-O-deethylase, but 7-ethoxyresorufin-O-deethylase is increased 3-fold. In contrast to PB, pretreatment with 7,8-BF or MC reduces the oxidative metabolism of diethylstilbestrol (DES) by hepatic microsomes in vitro. The cytosolic level of the aromatic hydrocarbon (Ah) receptor in hamster liver is decreased by 7,8-BF and slightly enhanced by MC pretreatment. PB increases the receptor level 1.5-fold. The affinity of 7,8-BF to the Ah receptor in vitro is of the same order of magnitude as that of the known ligands 5,6-benzoflavone and 2,3,7,8-tetrachlorodibenzofurane. PB and DES show no binding to the receptor protein.

Possible hemodynamic basis to urethane anesthesia-induced reductions in renal clearance.

The hemodynamic basis of reported urethane anesthesia-induced reductions in the clearance of some renally eliminated compounds has been investigated in the laboratory rat. The use of urethane as an anesthetic in pharmacokinetic studies still persists, particularly in experiments of long duration. Using a microsphere technique, the per cent distribution of cardiac output to the kidneys, in both ip urethane- and ip Hypnorm/Hypnovel-anesthetized animals was significantly (p less than 0.01) lower than that observed in ip pentobarbital-anesthetized animals. However, the cardiac index in the Hypnorm/Hypnovel-anesthetized animals was 42% greater (p less than 0.01) than in the pentobarbital-treated animals and 86% greater (p less than 0.01) than in the urethane-treated group. Additionally, the cardiac index in the pentobarbital-anesthetized animals was significantly greater (p less than 0.01) than in the urethane-anesthetized animals. The lower cardiac index and reduced cardiac distribution to the kidneys in the urethane-treated group resulted in significant (p less than 0.01) reductions of approximately 40% in renal blood flow and glomerular filtration rate compared with the animals anesthetized with pentobarbital or Hypnorm/Hypnovel. The transport capacity of the basolateral organic anion secretory mechanism was not compromised by an ip injection of urethane, as demonstrated by p-aminohippuric acid transport studies conducted in isolated renal tubule fragments prepared from anesthetized animals. In conclusion, urethane anesthesia results in significant alterations of renal hemodynamics compared with pentobarbital and Hypnorm/Hypnovel anesthesia in the rat. For primarily renally eliminated compounds, such alterations are likely to influence pharmacokinetic data generated using this anesthetic agent.