Measurement of penbutolol and 4-hydroxypenbutolol in plasma or serum by HPLC.

A simple HPLC method for penbutolol and 4-hydroxypenbutolol assay has been developed. Plasma or serum (200 microliters) is vortex-mixed (30 s) with Tris solution (2 M, pH 10.6) containing an internal standard (50 microliters) and methyl t-butyl ether (200 microliters). After centrifugation, the extract (100 microliters) is analysed using an unmodified silica column (250 x 5 mm ID) and iso-octane-methanol-methyl t-butyl ether (55:25:20) containing ammonium perchlorate (10 mM, pH 5.7) as eluent and with fluorescence detection. No interference has been encountered and the limit of accurate measurement for both compounds is 5 micrograms/l.

Liquid-chromatographic determination of penbutolol and its principal metabolites in plasma and urine.

We describe a sensitive, specific liquid-chromatographic determination of penbutolol and its 4-hydroxy metabolite in plasma and urine. The method involves a simple organic extraction, evaporation of the solvent, reconstitution in methanol/water, and injection into the chromatograph. Penbutolol, its metabolites, and the internal standard, propranolol, are resolved on a CN reversed-phase column and detected fluorometrically. Conjugates of penbutolol and its 4-hydroxy metabolite may be determined after a 2-h enzymic hydrolysis. Detection limits are in the range of 3 to 12 micrograms/L of plasma. The assay is reproducible and nearly free of interferences. Representative concentrations in blood and urine of normal volunteers are reported.

Direct assay of conjugates of penbutolol and its metabolites in urine.

The metabolites of 1-tert.-butylamino-3-(2-cyclopentylphenoxy)propan-2-ol (penbutolol, Betapressin), penbutolol 2-glucuronide, 4'-OH-penbutolol 2-glucuronide, 4'-OH-penbutolol 4-sulfate and 1"-dehydropenbutolol 2-glucuronide were determined in urine by high-performance liquid chromatography (HPLC). The compounds were determined after direct injection, that is to say without prior cleavage of the conjugates to the corresponding aglycones. In the case of the glucuronides, the urine was injected into the HPLC system without further sample preparation. The sulfate was determined after ion-pair extraction. Fluorimetric detection was employed. Depending on the compound, the detection limits lay between 0.07 and 0.3 micrograms/ml. This method was used to determine the cumulative urinary excretion of a subject.

Pharmacokinetics of penbutolol and its metabolites in renal insufficiency.

The pharmacokinetics of penbutolol, its 4-hydroxylated metabolite and of their conjugates was studied in hypertensive patients with various degrees of renal impairment. A single oral dose of penbutolol 40 mg, was rapidly absorbed after a lag-time of 0.34 h. Its plasma concentration reached a maximum after 0.84 h and then declined bi-exponentially, with an apparent elimination half-life of 21.8 h. The hydroxylation of penbutolol was negligible and conjugation was of major importance for its elimination. Consequently, the kinetics of unchanged penbutolol were not altered by renal impairment. The 48 h-urinary excretion of penbutolol and its metabolites reached 13-14% of the administered dose, which is consistent with extensive metabolism of the drug. After treatment for 30 days with penbutolol 40 mg/d there was no accumulation of the parent drug but the concentration of its conjugates was increased. It is concluded that the dose of penbutolol need not be changed in patients with mild renal insufficiency, 4-hydroxypenbutolol is unlikely to participate in the anti-hypertensive effect of the drug, due to its low concentrations, and biotransformation of penbutolol may be enhanced during chronic treatment.

Penbutolol Pharmacokinetics: the influence of concomitant administration of cimetidine.

A possible interaction of penbutolol and cimetidine was investigated in healthy volunteers treated orally for 7 days. The plasma levels of unmetabolized penbutolol showed a slight but non-significant increase. The biphasic elimination kinetics of penbutolol (half-lives 0.8 and 17 h) was not affected by coadministration of cimetidine. Plasma levels of penbutolol were not significantly altered by chronic treatment with cimetidine, whereas the levels of 4-hydroxypenbutolol and 4-hydroxypenbutolol glucuronide were significantly reduced.

Penbutolol: beta-adrenoceptor interaction and the time course of plasma concentrations explain its prolonged duration of action in man.

Beta-adrenoceptor binding of (-) penbutolol and its active metabolite 4-hydroxy-penbutolol to rat reticulocyte membranes was shown in the presence of native human plasma. Due to the high plasma protein binding (approximately 99%) the apparent Ki-values of penbutolol were shifted 100-fold to the right after inclusion of plasma in the assay; the Ki was approximately 40-70 ng/ml. That value is comparable to the IC50-values calculated from clinical studies. The interaction of 4-hydroxy-penbutolol with beta-adrenoceptors was not affected to the same extent by inclusion of plasma protein binding approximately 80%, apparent Ki-value approximately 7 ng/ml. Thus, the active metabolite of penbutolol displays higher potency at beta-adrenoceptors in vitro due to its lesser degree of plasma protein binding. A prediction procedure for antagonist activity after penbutolol administration using beta-adrenoceptor interaction and plasma concentration kinetics suggests that, in addition to a rapid elimination process from human plasma, a slow elimination phase of penbutolol (or an active metabolite) is necessary to explain the long duration of action observed in clinical studies after a single oral dose. Inhibition in vitro of beta-adrenoceptor binding by plasma samples obtained after oral administration of 40 mg penbutolol to 3 healthy volunteers indicated a biphasic concentration-time profile of the antagonist in plasma and was in accordance with the time course of the reported reduction in exercise tachycardia. Finally, plasma concentrations of penbutolol equivalents derived from the receptor assay were in the range of penbutolol concentrations detected by physico-chemical methods.(ABSTRACT TRUNCATED AT 250 WORDS)