Disposition and metabolism of Ro 24-4736 in the rat.

Ro 24-4736, a new platelet activating factor antagonist, is currently under preclinical and clinical development. The tissue distribution of the 14C-label in male rats following a single intravenous dose of 1.0 mg/kg of 14C-Ro 24-4736 indicated appreciable uptake by the liver, kidney, heart and gastrointestinal tract. Peak plasma and tissue concentrations were seen at 5 minutes after dosing except for the small intestine (4 hrs) and abdominal fat, stomach and large intestine (4 hrs). Thereafter, the 14C-label rapidly declined in all tissues. At 48 hours, only 3.5% of the dose was present in the tissues, and 6.1% in the lumen of the gastrointestinal tracts. The excretion of 14C was essentially completed; 94% of the administered 14C was excreted in the feces and 4.0% in the urine. Overall recoveries of the administered 14C label ranged from 96 to 116%. The purified major 14C-labelled component in the fecal extracts yielded essentially the same NMR spectrum as authentic Ro 24-4736 which accounted for 11% of the dose. In vitro incubations of Ro 24-4736 with rat liver 9S supernatant in an NADPH generating system produced two metabolites. NMR spectra indicated that one metabolite was hydroxylated at carbon-1 while the other one contained a hydroxyl at carbon-10 of the parent molecule. Interestingly, the sites of hydroxylation were at carbons C1, and C10 bearing the protons guarding the bay area of the phenanthrenoid ring, rather than carbons of the phenyl-methyl-thienotriazolodiazepine moiety.

The metabolism of [14C]rimantadine hydrochloride in rats and dogs.

Metabolism and route of excretion of [14C]rimantadine hydrochloride was studied in male rats after single po (60 mg/kg) and iv doses (15 mg/kg) and in male dogs (5 or 10 mg/kg po and 5 mg/kg iv). Total 14C excretion in urine (po and iv) in both species reached 81-87% of the dose in 96 hr. Rimantadine was excreted in rats free (1.0% po, 1.7% iv) and conjugated (0.8% of the dose, po and iv, both in 24 hr) and in dogs, free (2.6% po, 3.0% iv) and conjugated (6.4% po, 7.7% iv, both in 48 hr). In both species, rimantadine metabolism is essentially independent of the route of administration. In rats and dogs, m-hydroxyrimantadine (mostly unconjugated) was the major metabolite, 22% (po) and 24% (iv), and 27% (po) and 21% (iv), respectively. Rats, but not dogs, excreted trans-p-hydroxyrimantadine (23.5% and 25.2%, po and iv, free plus conjugated). An oxidative pathway in dogs produced the m- and p-hydroxylated analogs with a hydroxyl in place of the amino group (3.7% and 5.7% of the dose, both conjugated). A p-hydroxylated compound with a nitro group in place of the amino group may have originated from an N-hydroxy metabolite by spontaneous oxidation during isolation. Comparison of total 14C excretion, in rats (81%, po; 82%, iv) and dogs (81%, po; 84%, iv) after po and iv administration after 96 hr indicates good absorption of rimantadine.

Metabolism of the anti-obesity agent, 4-amino-5-ethyl-3-thiophenecarboxylic acid methyl ester hydrochloride, in rats and dogs.

1. In 24 h, male rats excreted in urine 1% of an intra-gastric 100 mg/kg dose of 4-amino-5-ethyl-3-[4-14C]thiophenecarboxylic acid methyl ester hydrochloride (I) as unchanged I and 59% as 4-amino-5-ethyl-3-thiophenecarboxylic acid (II), mostly conjugated. 2. In rats dosed intra-duodenally with I (50 mg/kg), little I was found in the systemic circulation (less than 2 micrograms/ml) but high concentrations (26 micrograms/ml) were present at five minutes in portal plasma. At five minutes, II was found at 89 and 93 micrograms/ml in systemic and portal plasma, respectively. First-pass ester hydrolysis by the duodenum and liver may explain the near absence of I and the high concentrations of II in systemic plasma. 3. Dogs which received 30 mg/kg 14C-I intra-gastrically, excreted 0.3% I, 30.8% II and 6.8% as 5-ethyl-4-(methylamino)-3-thiophenecarboxylic acid (III), the N-methyl derivative of II. 4. Dogs which received approximately equivalent intra-venous or intra-gastric doses of non-radioactive I and II had high plasma concentrations of II but only small concentrations of I. Plasma concentrations of II after intra-gastric doses of non-radioactive I or II were similar, indicating that both compounds are pharmacokinetically equivalent. I may be a prodrug of II.

The metabolism of 14C-cibenzoline in dogs and rats.

The disposition of the new antiarrhythmic agent cibenzoline (CBZ) (racemic 4,5-dihydro-2-(2,2-diphenylcyclopropyl)-1H-imidazole) in three male dogs was investigated after oral administration of 13.8 mg/kg of 14C-CBZ base. Within 6 days, 60.5 +/- 6.0% of the dose was excreted in urine and 19.2 +/- 4.6% in feces. In 0-24-hr urine, unchanged drug was excreted (41.6% of the dose) as well as the unconjugated 4,5-dehydro metabolite (DHCBZ, 3.7%), conjugated p-hydroxybenzophenone (0.8%, only in one dog), and a phenolic metabolite, p-hydroxycibenzoline (HCBZ) in a rearranged form (RHCBZ) at 5.2% of the dose (free plus conjugated). Studies with synthetic HCBZ indicated that unrearranged HCBZ was excreted and that rearrangement occurred during purification. CBZ from dog urine displayed slight optical activity, based on ORD/CD data, corresponding to an optical purity of 15% of the S-(-)-CBZ, indicating a limited extent of stereoselective metabolism of CBZ in dogs. After an oral 50-mg/kg dose of 14C-CBZ succinate, male rats excreted in 3 days 27.0 +/- 2.8% in urine and 41.5 +/- 2.6% of the dose in feces, and in a repeated experiment 32.1 +/- 1.9% in urine and 54.5 +/- 0.7% in feces. CBZ (7.6%) and DHCBZ (0.2%) were determined in 0-24-hr urine, and CBZ (4.2%) and RHCBZ (4.2% of the dose) were determined in 0-24-hr feces. RHCBZ (3.1%), m-methoxy p-hydroxycibenzoline (8.3%), and p-hydroxybenzophenone (5.3% of the dose) were identified as glucuronide/sulfate conjugates in bile from rats. Evidence that p-hydroxybenzophenone arose from an unstable unidentified metabolite is discussed.

The disposition and metabolic fate of 14C-cibenzoline in man.

The disposition and metabolic fate of cibenzoline (CBZ) following single oral 153-mg doses of 14C-CBZ succinate were studied in five healthy adult males. The mean maximum plasma radioactivity of 386 ng eq/ml occurred at 2.4 hr after administration. The mean half-life, determined from the 14C plasma concentration and urinary excretion rate data, was 13.1 and 14.8 hr, respectively. The mean maximum CBZ concentration was 196 ng/ml at 1.2 hr post-dose. The mean half-life, determined from the plasma concentration and urinary excretion rate data, was 7.2 and 7.3 hr, respectively. The mean total clearance of radioactivity and CBZ was 300 ml/min and 1224 ml/min, respectively, due to elimination via both renal and nonrenal pathways. The only unconjugated metabolite in the plasma was 4,5-dehydrocibenzoline which, together with other unidentified metabolites, is presumed responsible for the longer observed half-life for total radioactivity. Approximately 75% of the dose was recovered in the urine in the first 24 hr after dosing, 80% of which was present at CBZ and known metabolites. After 6 days, a mean of 85.7% of the dose was excreted in urine and 13.2% in feces. The predominant excreted compound was CBZ (55.7% of the dose) in the 0-72 hr urine. Although several metabolites were identified in urine samples, none were found in substantial amounts relative to the parent drug. Two of these substances showed slight antiarrhythmic activity, whereas the 4,5-dehydro metabolite, representing approximately 4% of radioactivity in urine, was inactive.

Biotransformation of cibenzoline to 2-(2,2-diphenylcyclopropyl)-1H-imidazole.

A microsomal metabolite of cibenzoline, 4,5-dihydro-2-(2,2-diphenylcyclopropyl)-1H-imidazole butanedioate, was identified by n.m.r. as the 4,5-dehydro analogue, 2-(2,2-diphenylcyclopropyl)-1H-imidazole. Three dogs dosed orally with 13.8 mg/kg 14C-cibenzoline base excreted 1.8-3.5% of the dose as this metabolite in the urine. Mean plasma concentrations of cibenzoline reached a peak of 1.5 micrograms/ml at 2 h while mean concentrations of the metabolite of 0.4-0.5 micrograms/ml were found between 2 and 7 h. The metabolite was synthesized and found to decrease the frequency of ventricular premature depolarizations in conscious dogs having a two-stage occlusion of the left anterior descending coronary artery performed 48 h before. It did not inhibit ventricular arrhythmia in rats induced by i.v. infusion of aconitine. Thus, in contrast to cibenzoline, the metabolite does not appear to be a true antiarrhythmic agent.

Pharmacodynamic studies with (-)-3-phenoxy-N-methylmorphinan in rats.

Analgesia and brain and plasma concentrations of (-)-3-phenoxy-N-methylmorphinan (PMM) and its metabolites were determined in rats administered 50 mg/kg of 3H-labeled PMM p.o., an approximate ED50. Unchanged PMM and two active metabolites, levorphanol and a different phenol, p-hydroxylated on the 3-phenoxy group (pOH-PMM), were present in brain at concentrations greater than in plasma. Analgesia was observed from 1 to 6 hr and was associated with brain concentrations of 400-1400 ng/g of PMM, 190-300 ng/g of pOH-PMM, and 16-27 ng/g of levorphanol. The presence of 58% of the administered dose as unchanged PMM in the gastrointestinal tract at 6 hr may reflect slow absorption and explain the persisting brain concentrations of PMM and its metabolites as well as the prolonged analgesia. Analgesia may have been due to the presence in brain of only PMM, pOH-PMM or levorphanol, or to the combined activity of two or three of these substances. Administration of the approximate ED50 of 3H-labeled levorphanol (0.1 mg/kg, s.c., or 6 mg/kg, p.o.) resulted in brain levorphanol concentrations (11-18 ng/g) close to those observed when PMM was administered p.o. at 50 mg/kg. After administration of an approximate subcutaneous ED50 of [3H]pOH-PMM of 24 mg/kg, the brains contained pOH-PMM (1500-4100 ng/g) and levorphanol (60-100 ng/g); these levorphanol concentrations were higher than those found after administration of the approximate ED50 of PMM or levorphanol. The findings indicate that brain levorphanol concentrations resulting from administration of PMM or pOH-PMM to rats may account for the analgesic activity observed, i.e. that PMM and pOH-PMM may act as prodrugs for levorphanol