Pharmacokinetics of the retinoids isotretinoin and etretinate. A comparative review

The clinical pharmacokinetic profiles of two orally administered retinoids, isotretinoin and etretinate, are discussed and compared. The pharmacokinetic profile of isotretinoin is predictable and can be described using linear pharmacokinetic theory. The drug is rapidly absorbed following oral administration, is highly bound to plasma protein, and is metabolized to 4-oxo-isotretinoin. The apparent half-lives of elimination of isotretinoin and 4-oxo-isotretinoin following the oral administration of isotretinoin range from 10 to 20 hours and 24 to 29 hours, respectively. Steady-state pharmacokinetic profiles in patients are consistent with the single-dose pharmacokinetics in normal subjects. Following oral administration, etretinate undergoes significant first-pass biodegradation to its corresponding carboxylic acid; the acid appears rapidly in the circulation, often earlier than the parent drug, and its plasma concentration is usually comparable to, or greater than, that of the parent drug. The apparent elimination rates of drug and metabolite are similar (6-13 hours) following a single dose, suggesting that metabolite elimination may be formation-rate limited. During multiple dosing of etretinate, a very slow terminal elimination phase is observed which is not detected after single-dose administration. The prolonged half-life of this phase suggests accumulation in a deep tissue compartment. Differences between the two retinoids reflect their differing physicochemical properties.