Pharmacokinetic model for the successive demethylation and biliary secretion of methyl orange in the rat.

1. A one-compartment pharmacokinetic model was developed in which a drug underwent two successive metabolical reactions (for example, metabolism followed by conjugation) and free drug and both metabolites were excreted.2. Techniques were described whereby graphical estimates of the first-order rate constants may be derived from cumulative excretion data on the drug and its metabolites. Computer simulation techniques were used to show that the experimental data permit reasonably accurate estimation of the rate constants of the model by graphical and computer methods.3. Tritiated methyl orange (2 mg) was administered to five groups of six rats with biliary cannulation. The bile produced by each animal was collected at hourly intervals for 6 h and the amounts of methyl orange and its metabolites, 4'sulpho-4-methylaminoazobenzene and 4'sulpho-4-aminoazobenzene, determined by thin layer chromatography and radioactive counting techniques.4. The data were analysed graphically and with an iterative digital computer programme to yield the first-order rate constants for the successive demethylation steps in the metabolism of methyl orange. The removal of the first methyl group had a rate constant of 0.684+/-0.142 h(-1) (+/-S.D.) and the second methyl group 1.00+/-0.302 h(-1). The rate constant for biliary excretion of the free methyl orange was 0.164+/-0.042 h(-1), for the monomethyl derivative 0.672+/-0.461 h(-1), and for the demethylated metabolite 6.413+/-3.222 h(-1).