Metabolism of 4-benzamido-1-[4-(indol-3-yl)-4-oxobutyl]piperidine in rats and monkeys

A review of the clinical pharmacokinetics and metabolism of the alpha 1-adrenoceptor antagonist indoramin

1. The alpha 1-adrenoceptor antagonist indoramin is rapidly and extensively absorbed after oral administration, but with only low to moderate bioavailability (8-24% median) from the tablet (Baratol). Although plasma protein binding is high (72-86%), the drug is widely distributed into tissues (with median Vz 6.3-7.7 l/kg after i.v. dosage). 2. Elimination of indoramin is rapid in most healthy volunteers, with median plasma clearances of 18-26 ml/min per kg, after i.v. dosage. Elimination occurs principally by metabolism, the major route being indole 6-hydroxylation, followed by sulphate conjugation of 6-hydroxyindoramin. The faecal route of excretion predominates (45-50% of dose), with a further 35-40% in the urine. 3. Extensive variation in single-dose oral pharmacokinetics of indoramin is due largely to the existence of a poor metabolizer phenotype which co-segregates with that of debrisoquine. 4. On repeated administration (37.5 mg twice daily) to healthy volunteers, plasma concentrations of indoramin accumulate 3-4-fold above those anticipated from single-dose kinetics. However, steady state is achieved within the first week of dosing. 5. The pharmacokinetics of indoramin are substantially altered in the elderly. The oral AUC for a 50 mg dose is increased approx. 5-fold and the t1/2 2.5-fold. 6. Cirrhotic liver disease enhances bioavailability and decreases clearance, approx. 2-fold in each case for single oral and i.v. doses of 50 mg and 0.15 mg/kg respectively. 7. After oral indoramin Cmax and AUC are both raised (58% and 25%, respectively, for a 50 mg dose) by co-ingested ethanol (0.5 g/kg). After i.v. indoramin, kinetics are unaffected by alcohol, but indoramin (0.175 mg/kg) slightly increases (26%) blood ethanol concentrations during the first hour after dosing. 8. The pharmacodynamics of indoramin appear to be related to the combined pharmacokinetics of the drug and its 6-hydroxylated metabolite, which contributes to the antihypertensive effect.

The pharmacokinetics of indoramin and 6-hydroxyindoramin in poor and extensive hydroxylators of debrisoquine

Five poor metabolisers (PM) and seven extensive metabolisers (EM), of debrisoquine, all healthy volunteers, received 50 mg indoramin orally following an overnight fast. Plasma concentrations of indoramin and 6-hydroxyindoramin were determined by HPLC with fluorimetric detection. In PM subjects, mean values of Cmax (158 ng/ml) and AUC(0-24) (2556 ng X h X m-1) for indoramin were substantially elevated and t 1/2 beta (18.5 h) prolonged by comparison with values in the EM subjects (21.6 ng/ml, 151 ng X h X ml-1 and 5.2 h respectively). For 6-hydroxyindoramin, on the other hand, Cmax (12.4 ng/ml) and AUC (0-8) (47.5 ng X h X ml-1) in PM subjects were significantly lower than in the EM subjects (28.2 ng/ml and 94.7 ng X h X ml-1). There was a tendency to a higher incidence of side-effects in the PM group. Although the difference did not achieve statistical significance (0.1 greater than p greater than 0.05), all the PM subjects experienced sedation compared to only two in the EM group. Differences in blood pressure and pulse rate between the two groups were small. It is concluded that the oxidative metabolism of indoramin is subject to genetic polymorphism, which is probably under the control of the same gene locus as that influencing debrisoquine oxidation. The clinical consequences are discussed.

Pharmacokinetics of oral indoramin in elderly and middle-aged female volunteers

Indoramin was administered as a single 50 mg oral tablet to 5 elderly (aged 68-71 years) and 6 middle-aged (aged 46-55 years) healthy female volunteers. The mean half life of indoramin in elderly subjects (14.7 +/- 4.8 h, mean +/- SEM) was statistically significantly longer than that seen in middle-aged subjects (5.1 +/- 1.0 h). The mean plasma concentrations of indoramin and mean area under the plasma concentration/time curve were also greater in elderly than in middle-aged subjects, although this did not achieve statistical significance. In many elderly patients, therefore, a reduction in dosage may be required due to the apparent reduction in clearance of indoramin. However, because of the wide inter-subject variability observed in the oral pharmacokinetics of indoramin, individual titration of indoramin dosage in all patients may be desirable.

Studies on the metabolism of the new anti-hypertensive agent, indoramin, in man

The absorption, metabolism and excretion of the new antihypertensive agent indoramin (Baratol) have been studied in male volunteers following oral administration of the drug labelled either with 14C or with tritium. Absorption of the drug proceeded at moderate rate, peak plasma radioactivity levels being seen by 3 h after dosing. Metabolism was extensive as shown by very little unchanged compound appearing in urine. Two major urinary metabolites accounting for some 35-40% of the renally excreted material were identified as acid labile conjugates of indoramin itself and indole 6-hydroxylated indoramin. The pattern of biotransformation appeared to be similar to that in the patas monkey, the species used in the long term safety evaluation of the drug. Excretion of the drug and metabolites occurred primarily via the faeces which accounted for 49.7 +/- 4.9% of the dose. A further 31.7 +/- 2.4% was recovered in the urine. Renal elimination of total radioactivity occurred in an apparently monoexponential manner with a half-life of 11.9 +/- 1.2 h.

Studies on the biotransformation of indoramin in the patas monkey

1. Studies on absorption, excretion and biotransformation of the antihypertensive agent indoramin (3-[2-(4-benzamidopiperidino)ethyl]indole hydrochloride) have been carried out in patas monkeys. 2. Absorption of the drug was extensive, as indicated by less than 8% of the dose appearing in faeces as the unchanged compound. 3. Approx. 35% of an orally administered dose of the 3H-labelled drug was recovered in urine while a similar amount was found in faeces. Urinary excretion of total 3H was mono-exponential with a half-life of 17 h. 4. Biotransformation was extensive, the unchanged drug constituting less than 4.5% of the urinary metabolites. The principal identified metabolites were a sulphate conjugate of 3-[2-(4-benzamidopiperidine)ethyl]indol-6-ol and an acid-labile conjugate of the parent drug. A minor metabolite resulting from N-dealkylation of the drug was identified as 4-benzamidopiperidine. Small amounts of 3-[2-(4-[4-hydroxybenzamido]piperidino)-ethyl]indole were found in a dichloromethane extract of acid-hydrolysed urine. 5. The faeces contained only negligible amounts of conjugated metabolites. The principal components were the indole-6-hydroxylated derivative of the parent drug with smaller amounts of the 5-hydroxylated compound and the unchanged compound.