Metabolism of thymoxamine. III. Structure elucidation of the metabolites and interspecies comparison

Urine sulfated and nonsulfated bile acids as a diagnostic test for liver disease in cats

Urine bile acid (UBA) tests reflecting "average" serum bile acid (SBA) concentrations may have greater practical utility than paired SBA samples in cats. This study evaluated whether urine sulfated bile acids (USBAs), urine nousulfated bile acids (UNSBAs), or a combined approach had a clinical utility equivalent to SBAs. Routine serum biochemistry tests, SBA concentrations, and urine samples were collected from 54 cats with hepatobiliary disease, 17 cats with nonhepatic disorders, and 8 healthy cats. UBAs were measured by a quantitative enzymatic colorimetric method, and results were normalized with urine creatinine (UCr) concentrations. Significantly higher values occurred in cats with liver disease than in cats without liver disease for USBA : UCr, UNSBA:UCr, and (USBA and UNSBA) : UCr, P < .05 each. UBA tests with diagnostic performance (sensitivity [SS], specificity [SP], and positive and negative predictive values [PV+ and PV-]) equivalent to SBAs were the UNSBA : UCr and the combined test (SS: 87, 87 versus 85; SP: 88, 88 versus 88; PV+: 96, 96 versus 96; PV-: 68, 65 versus 68; UNSBA : UCr, [USBA, and UNSBA]: UCr versus SBA, respectively). Clinical applications of the UNSBA : UCr or the combined (USBA and UNSBA) : UCr test should be useful as convenient diagnostic tests for identifying cats with liver disease and high SEA concentrations.

Moxisylyte: a review of its pharmacodynamic and pharmacokinetic properties, and its therapeutic use in impotence

Moxisylyte is a competitive noradrenaline antagonist, acting preferentially on post-synaptic alpha-1 adrenoceptors. It was introduced more than thirty years ago for the treatment of cerebro-vascular disorders and shown more recently effective in the urological field due to its ability to modulate the urethral pressure. Renewal of interest in this drug has been observed in recent years since the demonstration of the possibilities of vasoactive drugs in evaluation and treatment of erectile dysfunctions. Moxisylyte is a prodrug, rapidly transformed into an active metabolite in plasma (Deacetylmoxisylyte or DAM). Elimination of the active metabolite occurs by N-demethylation, sulpho- and glucuroconjugation. The N-demethylated metabolite is sulphoconjugated only. Urine is the main route of excretion. The metabolites of moxisylyte can be determined in biological fluids by various methods using high-performance liquid chromatography. Their pharmacokinetics is dependent on the route of administration. By the oral route, the concentrations of the active metabolite are low, and the glucuroconide of DAM predominates over the sulphates. After intravenous and intracavernous injection, the active metabolite is proportionally higher, the two sulphates are equivalent and in larger amounts than the glucuronide. In the treatment of impotence, intracavernous injection of moxisylyte at 10, 20 or 30 mg can induce an erection adequate for intercourse in most of the patients. Compared to inducing agents such as papaverine and prostaglandin E1, moxisylyte must be considered as a facilitator of male erection, its interest lying in the low rate of adverse effects, either general or local.

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.

Multiple-dose pharmacokinetics of moxisylyte after oral administration to healthy volunteers

The pharmacokinetics of moxisylyte in plasma and urine was investigated after oral administration. Twelve subjects were treated orally, twice daily with 240 mg of the drug for 6 days; on day 7, the subjects received a last dose of 240 mg of moxisylyte. Moxisylyte was assayed in plasma and urine by a specific HPLC method with fluorimetric detection. Moxisylyte was absorbed rapidly and changed to its metabolites immediately after drug administration; unchanged moxisylyte was not found in plasma. Two metabolites were found in plasma and urine: conjugated desacetylmoxisylyte (DAM) and the conjugate of desmethylated DAM (MDAM). The pharmacokinetic parameters determined after the first oral administration were not modified on multiple dosing. The apparent elimination half-lives of conjugated DAM and MDAM were 2.3 and 3.5 h, respectively. Elimination of these two metabolites in urine averaged 50 and 10%, respectively.

Pharmacokinetics of moxisylyte in healthy volunteers after intravenous and intracavernous administration

The concentration-time profiles of metabolites of moxisylyte, an alpha-adrenergic receptor blocking agent, in the plasma of 12 healthy volunteers were investigated after intravenous (iv) and intracavernous (ic) administrations. The study was conducted in open, randomized, Latin Squares. Plasma levels of moxisylyte and its biotransformation products were assayed by a specific high-performance liquid chromatography method with fluorescence detection. Three metabolites, unconjugated desacetylmoxisylyte (DAM), conjugated DAM, and conjugated monodesmethylated DAM (MDAM), were found in plasma. After iv administration, unconjugated DAM appeared in plasma in < 5 min; the formation of this metabolite is slightly lower after ic administration (half-life, 6.08 +/- 2.33 min). Maximum plasma levels (57.2 +/- 29.4 ng/mL) and area under the curve of concentration versus time (43.3 +/- 11.4 micrograms.h/L) were significantly lower after ic administration than after iv administration (352.8 +/- 287.6 ng/mL and 152.6 +/- 0.247 micrograms.h/L, respectively). For conjugated DAM, the time to reach the maximum concentration is significantly increased after ic administration (0.9 h instead of 0.46 h) and the maximum concentration is significantly decreased (163.5 ng/mL instead of 203.4 ng/mL). The other pharmacokinetic parameters show no change between the two routes of administration. The pharmacokinetic parameters computed for MDAM are in the same range after iv and ic administrations, and there are no significant statistical differences.

Moxisylyte plasma kinetics in humans after intracavernous administration

Obtaining and sustaining an erection are common problems for the male spinal cord injury patient. Intracavernous injection of vasoactive substances offers a new treatment option but it must be approached with caution in this population. In this work, the use of an alpha-adrenergic blocking agent, moxisylyte, after intracavernous administration for complete paraplegic patients with erectile impotence is described. During this study, the pharmacokinetic profile of moxisylyte has been defined. Unchanged moxisylyte is not found in plasma, this drug is immediately metabolized after administration. Three metabolites were found in plasma: desacetylmoxisylyte (DAM), conjugated DAM, and conjugates of desmethylated DAM (MDAM). Maximum plasma levels of 72.3 ng ml-1, 301.4 ng ml-1, and 88.8 ng ml-1 are obtained 0.22 h, 0.9 h, and 2.08 h after drug administration for these three metabolites, respectively. The elimination half-lives are 0.89 h, 2.16 h, and 5.32 h and the MRT, 1.38 h, 3.23 h, and 8.45 h, respectively. No side-effects were noted, only one patient presented sleepiness. Successful erections (10 to 25 min) were obtained in all patients and no priapism was noted.

Pharmacokinetics of a prodrug thymoxamine: dose-dependence of the metabolite ratio in healthy subjects

Thymoxamine, a prodrug, is rapidly deacetylated in the plasma to give two phase I metabolites, DMAT and DAT, which are further sulpho- and glucuro-conjugated and then excreted mainly in the urine. In a cross-over study, the dose-dependence of the metabolite ratio was evaluated in nine healthy volunteers after three doses (120, 240, 480 mg) of thymoxamine-HCl. Regardless of the dose, DMAT and its glucuronide were not detected, while the amount of DMAT-sulphate was found to be proportional to the dose administered. Plasma levels of DAT were measurable in only four of the nine subjects after the 480 mg dose and showed great intersubject variability. The pharmacokinetics of both DAT-sulphate and DAT-glucuronide were dose-dependent. As the dose increased, the proportion of DAT undergoing sulphatation decreased; this saturation was compensated by glucuronidation.

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.

GLC method for the quantification of metabolites of thymoxamine in human plasma

A sensitive gas-chromatographic method for quantification of the pharmacologically active metabolites I-IV of thymoxamine in plasma is described. 4-(Hydroxythymyl)-(2'methylbutylaminoethyl)ether, a compound similar to metabolite I, is used as an internal standard. Metabolites I and II the internal standard are extracted with cyclohexane from alkalinized plasma followed by back-extraction into 0.1 N hydrochloric acid. After evaporating the hydrochloric acid solution, the sample is silylated with BSTFA and analyzed by gas-chromatography on a CRS 101/Carbowax 4000 column using a thermoionic detector. For subsequent determination of metabolites III and IV, the extracted plasma is hydrolyzed under conditions in which the phenol sulfates but not the glucuronide conjugates undergo cleavage. The resulting phenols (metabolite I and II) are analyzed as described above. The sensitivity threshold for all 4 compounds is approximately 5 ng/ml plasma based on a 2 ml plasma sample.

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. 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.