Pharmacokinetics Absorption

The bioavailability of isotretinoin (13-cis-retinoic acid) is approximately 25% after oral administration, and it can be increased by food 1.5- to 2-fold. After 30 min, the drug is detectable in the blood, and peak serum levels are reached 1-4 h after oral ingestion. As a result of enterohepatic circulation, secondary and tertiary blood concentration maxima may occur. About 6 h after oral administration, the main metabolite, 4-oxo-isotretinoin, is present in a 2- to 4-fold higher concentration. Isotretinoin and 4-oxo-isotretinoin reach steady-state concentrations within 10 days. The mean elimination half-life of isotretinoin is 19 h and of 4-oxo-isotretinoin is 29 h.

Similar to isotretinoin, the oral absorption of acitretin, the major metabolite of etretinate, is variable. Given with food, the bioavailibility is approximately 60% (range, 36%-95%). Peak serum levels are reached 2 h after a single dose is taken. Plasma concentrations of acitretin and its major metabolite, cis-acitretin, reach steady-state within 2-3 weeks. Mean peak concentrations of cis-acitretin are considerably higher at steady-state and result from the longer half-life and lower clearance of cis-acitretin compared with acitretin. The half-life after multiple doses of acitretin ranges from 3-96 h and of cis-acitretin from 35-148 h (Orfanos et al. 1987,1997, Wie-gand and Chou 1998b).

Retinol

Retinol

2nd generation retinoids

3rd generation retinoids

1st generation retinoids

Arotinoids

Tretinoin (all-frans-retinoic acid)

Etretinate

3rd generation retinoids

Arotinoids

1st generation retinoids

Tretinoin (all-frans-retinoic acid)

2nd generation retinoids

Etretinate

Isotretinoin (13-c/s-retinoic acid)

Fig. 27.3. Structural modification of the retinol molecule resulting in three generations of retinoids

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