The pharmacokinetics and metabolism of idazoxan in the rat

Abstract

1. [2′-¹⁴C]Idazoxan was rapidly and completely absorbed after its oral administration to rats.

2. After administration of either [2′-¹⁴C] or [6,7-³H] idazoxan, radioactivity was taken up by a wide range of tissues and became localized, especially in the organs of metabolism and excretion. Quantitative distribution patterns were route-dependent such that oral dosing resulted in lower radioactivity concentrations in all tissues apart from liver.

3. Clearance of idazoxan (94-144 ml/min per kg) was due mostly to metabolism and was independent of dose. Oral bioavailability in male rats at low oral doses of idazoxan (10 mg/kg) was about 1%, but increased with increasing dose to 23% at 100 mg/kg. Oral bioavailability in female rats was considerably higher than in male rats, at all doses studied. Brain idazoxan levels were in equilibrium with those in plasma, but ten-fold higher.

4. Elimination of radioactivty after administration of ¹⁴C-idazoxan was via the urine and the faeces (about 75% and 20% of dose respectively) and occurred essentially in the 24 h period immediately after dosing. By 96 h after dosing, elimination was virtually complete, with less than 0˙5% dose remaining in the carcasses.

5. Biotransformation was by hydroxylation at positions 6 and 7 to form phenolic metabolites, which were excreted as glucuronide and sulphate metabolites in urine, but unconjugated in faeces. Other minor metabilic routes were 5-hydroxylation or oxidative degradation of the imidazoline ring, but these pathways were of quantitatively minor importance in the rat.