High-performance liquid chromatographic determination of the conjugate metabolites of moxisylyte in human plasma and urine.

Sensitive and specific high-performance liquid chromatographic methods with fluorescence detection are described for the determination of the metabolites of moxisylyte (4-(2-dimethylaminoethoxy)-5-isopropyl-2-methylphenyl acetate) in human plasma and urine. Deacetylmoxisylyte glucuroconjugate (DAM-G) was hydrolysed enzymatically using 1-glucuronidase and quantified as the difference between the DAM concentrations determined after and before hydrolysis. The two sulphate derivatives (deacetylmoxisylyte sulphoconjugate, DAM-S and monomethyldeacetylmoxisylyte sulphoconjugate, MDAM-S), were analysed without prior hydrolysis. Their extraction from plasma and urine, as well as that of DAM from plasma, involved the use of C18 cartridges adapted on a Benchmate workstation. DAM in urine was quantified after liquid-liquid extraction. The two methods were validated for specificity, linearity, intra- and inter-day precision and accuracy. Precision was generally < or = 15% and accuracy < or = 12%. In plasma, the limits of quantification were 2.5 ng/ml for DAM and 2.8 ng/ml for the two sulphates, in urine, they were 40 ng/ml for DAM and 200 ng/ml for the sulphates. These methods were used for pharmacokinetic studies in healthy subjects.

Pharmacokinetics of moxisylyte in healthy volunteers after intravenous infusion and intracavernous administration with and without a penile tourniquet.

The concentration-time profiles of metabolites of moxisylyte (or thymoxamine), an alpha-blocking agent, were investigated in 18 healthy volunteers after intravenous (i.v.) and intracavernous (i.c.) administrations with and without a tourniquet. Four metabolites, unconjugated desacetylmoxisylyte (DAM), DAM glucuronide, and DAM and monodesmethylated DAM (MDAM) sulfates, were found in plasma and urine. For all metabolites, tmax was significantly increased after i.c. administrations and Cmax was significantly decreased. Maximum plasma level of unconjugated DAM was lower after i.c. administration with (1.81-fold) and without (1.26-fold) a tourniquet than after i.v. administration (43.6 +/- 19.6 ng/ml). The elimination half-life of each metabolite showed no change between the three treatments. The difference of 19 min between the mean residence times of unconjugated DAM after i.c. administration with and without a tourniquet may be compared with the difference between the mean duration of the intumescence, that is, 19 min (73 and 54 min with and without a tourniquet, respectively). Total percentages of metabolites recovered in urine were 66.2 +/- 20.9, 61.4 +/- 12.2, and 58.7 +/- 9.1% after i.v. and i.c. administrations with and without a tourniquet, respectively. In conclusion, tourniquet placed before i.c. administration increased the mean residence time of unconjugated DAM of approximately 25% and seemed to increase the efficacy of the drug in healthy volunteers.

Pharmacokinetics of moxisylyte in healthy volunteers after intracavernous injection of increasing doses.

OBJECTIVE: The concentration-time profiles of specific metabolites of moxisylyte, an alpha-adrenoceptor blocking agent, in the plasma and urine from 18 healthy volunteers were investigated after intracavernous (IC) administrations at three dose levels (10, 20 and 30 mg). RESULTS: Four metabolites, unconjugated desacetyl-moxisylyte (DAM), DAM glucuronide, and DAM and monodesmethylated DAM (MDAM) sulphates were found in plasma and urine. For all metabolites, t1/2 elimination was independent of the administered dose (1.19 h for unconjugated DAM; 1.51 h for DAM glucuronide; 1.51 h for DAM sulphate; and 2.17 h for MDAM sulphate). Cmax and AUC increased in direct proportion to dose, except for the inactive DAM glucuronide. Any the differences detected were small and equivalence of the three doses can be accepted. CONCLUSION: The pharmacokinetics of moxisylyte in humans following intracavernous administration were linear in the dose range 10 to 30 mg.

Metabolism of 14C-thymoxamine in rat and man.

1. After oral administration of 14C-thymoxamine to rat and man the total 14C excreted in urine and faeces was determined. 2. Six metabolites were isolated from the excret of man and rat by chemical extraction and identified by g.l.c.-mass spectral analyses. 3. Two other metabolites, highly polar and resistant to enzymic hydrolysis, were isolated by extraction on XAD2 resin and h.p.l.c. analysis. These two metabolites were identified by n.m.r. and by mass spectrometry in the fast atomic bombardment mode. 4. These two major metabolites of thymoxamine in man and rat have been identified and characterized as the sulphate conjugates of desacetyl-thymoxamine and N-monodesmethyl-desacetyl-thymoxamine.

Metabolism of thymoxamine. II. Studies with 14C-thymoxamine in man.

Thymoxamine is rapidly and completely absorbed in man. Rapid biotransformation is observed after intravenous and oral administration of 40 mg 14C-thymoxamine HCl. No unchanged compound is found in the body. More than 90% of plasma and urine radioactivity could be ascribed to six metabolites: the desacetyl compound (metabolite I), the monodemethylated metabolite I (metabolite II), the sulfate conjugates of I and II (metabolites III and IV) and the glucuronides of I and II (metabolites V and VI). The unconjugated metabolites are observed in plasma only after intravenous administration. Similar patterns for polar metabolites are found in plasma and urine for both routes of administration. The sulfate fraction amounts to about 50-60% and the glucuronide fraction to about 30-40% of the radioactivity, the conjugates of metabolite I being more abundant than those of metabolite II. The elimination of the metabolites is rapid, the half-life of radioactivity elimination being 1.5 h during the first 12 hours and 12 h thereafter. 80% of the radioactivity dose is recovered in the urine within 4 hours. Recovery after four days amounts to 99.8% (i.v.) and 97.7% (oral). The results are discussed with regard to the application of the drug in man, taking into account that not only the unconjugated metabolites but also the sulfate conjugates are pharmacologically active.

Metabolism of thymoxamine. I. Studies with 14C-thymoxamine in rats.

Thymoxamine is rapidly and completely absorbed in rats. It is a prodrug which does not enter the systemic circulation in its unchanged form. After either oral or intravenous administration it undergoes rapid and intense metabolism involving four biotransformation reactions: Enzymatic hydrolysis to the corresponding phenol (metabolite I), Monodemethylation to metabolite II, Sulfate conjugation of I and II (metabolites III and IV) and Conjugation of I and II with glucuronic acid (metabolites V and VI). With these 6 metabolites identified approximately 95% of the radioactivity can be accounted for in plasma, urine and bile. Whereas the systemic availability of I and II is low, III and IV show high bioavailability. Metabolites I to IV are pharmacologically active, while III and IV are less potent than I and II. The radioactivity distribution in tissues is different after oral and intravenous administration consistent with the higher portion of unconjugated metabolites in the body after administration by parenteral route. Although 60% of the labelled compounds is eliminated via bile, the radioactive compounds are almost completely excreted in the urine after both routes of administration. This demonstrates complete reabsorption of the biliary metabolites. Secondary peaks of radioactivity in plasma and organs at 4 hours are explained by the participation of the metabolites in the enterohepatic circulation.

Metabolism of thymoxamine: identification of metabolites in rat.

Thymoxamine hydrochloride administered by mouth to rats at 25 or 100 mg kg-1 was excreted in the urine as the deacetyl and N-demethyl-deacetyl metabolites. These were completely sulpho- and glucuronoconjugated at 25 mg kg-1 but only partially so at the higher dose. Thymoxamine deacetylation in vitro is catalysed by plasma and hepatic cytosol esterases and the deacetyl metabolite undergoes N-demethylation catalysed by the cytochrome P 450 hepatic microsome mixed function monooxygenase system. Because of the rapidity of the deacetylation it is concluded that thymoxamine is a prodrug leading in vivo to the active deacetyl thymoxamine.