Radioimmunological determination of fenoterol. Part II: Antiserum and tracer for the determination of fenoterol.

A radioimmunological method for the determination of the concentration of fenoterol in biological samples is described. The mixture of antibodies against the enantiomers of fenoterol obtained with the selected hapten shows a high affinity for racemic fenoterol and for the monoiodo125-fenoterol used as a tracer, which is also present as a racemate. The limit of detection of the radioimmunoassay for fenoterol (racemate) in biological samples (plasma, urine) is 10-20 pg/ml. The precision and accuracy of the radioimmunoassay, in the presence of racemic fenoterol, are sufficient for an analysis and meaningful interpretation of samples from human pharmacokinetic studies. A relationship between the cross-reactivity of the antibodies against fenoterol and a preferred conformation of the fenoterol molecule in aqueous solution is discussed.

Stereoselective formation of fenoterol-para-glucuronide and fenoterol-meta-glucuronide in rat hepatocytes and enterocytes.

The glucuronidation of fenoterol (Berotec, Partusisten) in isolated rat hepatocytes and enterocytes was investigated. Two different glucuronides, fenoterol para-glucuronide and fenoterol meta-glucuronide, were formed in proportions, that were constant over the concentration range investigated (0-1 mM). The fraction of para-glucuronide formed was 0.40 +/- 0.01 for hepatocytes and 0.54 +/- 0.01 for enterocytes. Fenoterol consists of a racemic mixture of SS'(+)fenoterol and RR'(-)fenoterol. The maximum glucuronidation rate of the (-)enantiomer (Vmax = 3.6 +/- 0.3 nmol/min/mg in hepatic microsomes and 3.4 +/- 0.1 nmol/min/mg in intestinal microsomes) is significantly lower than the same values of the (+)isomer (Vmax = 6.7 +/- 0.8 nmol/min/mg in hepatic microsomes and 5.8 +/- 0.4 nmol/min/mg in intestinal microsomes). Kmapp-values for the (-)enantiomer were lower than for the (+)enantiomer. Similar, but less pronounced, differences in Vmax were observed in isolated cells: Vmax = 148 +/- 13 and 372 +/- 50 pmol/min/mg [(-)fenoterol in hepatocytes and enterocytes], Vmax = 173 +/- 12 and 444 +/- 57 pmol/min/mg [(+)fenoterol in hepatocytes and enterocytes]. Calculation of intrinsic metabolic clearance (Clint = Vmax/Kmapp) from the cellular data suggests that the (+)enantiomer may be more efficiently eliminated by liver metabolism in vivo than the (-)enantiomer. This can result in stereoselective first-pass metabolism of the fenoterol enantiomers.