Chemical formula: C₂₀H₂₈O₂ Molecular mass: 300.435 g/mol PubChem compound: 5282379
Isotretinoin is a stereoisomer of all-trans retinoic acid (tretinoin). The exact mechanism of action of isotretinoin has not yet been elucidated in detail, but it has been established that the improvement observed in the clinical picture of severe acne is associated with suppression of sebaceous gland activity and a histologically demonstrated reduction in the size of the sebaceous glands. Furthermore, a dermal anti-inflammatory effect of isotretinoin has been established.
Isotretinoin is structurally and pharmacologically related to vitamin A which regulates epithelial cell growth and differentiation. It is thought that topically applied isotretinoin acts in a comparable way to its stereoisomer, tretinoin, and:
Isotretinoin has topical anti-inflammatory actions. Topically applied isotretinoin inhibits leukotriene-B4-induced migration of polymorphonuclear leukocytes, which accounts for topical isotretinoin’s anti-inflammatory action. A significant inhibition was produced by topically applied isotretinoin but only a weak inhibition by topical tretinoin. This may account for the reduced rebound effect seen with topical isotretinoin when compared with topical tretinoin.
Hypercornification of the epithelial lining of the pilosebaceous unit leads to shedding of corneocytes into the duct and blockage by keratin and excess sebum. This is followed by formation of a comedone and, eventually, inflammatory lesions. Isotretinoin inhibits proliferation of sebocytes and appears to act in acne by re-setting the orderly program of differentiation. Sebum is a major substrate for the growth of Propionibacterium acnes so that reduced sebum production inhibits bacterial colonisation of the duct.
The pharmacological action of isotretinoin remains to be fully elucidated.
Isotretinoin binds to the 3 retinoic acid receptors (RAR) alpha, beta and gamma with less affinity and is unable to bind to retinoid X receptors (RXR) and the retinoic acid cellular receptor (CRABP).
There are studies which show similar activity to systemic actions when administered topically. Inhibition of sebum production by topical isotretinoin has been demonstrated in the ears and flank organs of the Syrian hamster. Application of isotretinoin to the ear for 15 days led to a 50% reduction in sebaceous gland size, and application to the flank organ resulted in a 40% reduction. Topical application of isotretinoin has also been shown to have an effect on the epidermal differentiation of rhino mouse skin. Reduction in the size of the utriculi or superficial cysts leading to normal looking follicles was a predominant feature of isotretinoin treatment and has been used to quantify the antikeratinising effects of isotretinoin.
The absorption of isotretinoin from the gastro-intestinal tract is variable and dose-linear over the therapeutic range. The absolute bioavailability of isotretinoin has not been determined, since the compound is not available as an intravenous preparation for human use, but extrapolation from dog studies would suggest a fairly low and variable systemic bioavailability. When isotretinoin is taken with food, the bioavailability is doubled relative to fasting conditions.
Following isotretinoin 0.05% gel application to acne patients at a daily dose of 20g (equivalent to 10 mg of isotretinoin) to the face, chest and back for 30 days, plasma concentrations of isotretinoin and tretinoin were not measurable (<20 ng/mL).
Isotretinoin is extensively bound to plasma proteins, mainly albumin (99.9%).
The volume of distribution of isotretinoin in man has not been determined since isotretinoin is not available as an intravenous preparation for human use. In humans little information is available on the distribution of isotretinoin into tissue. Concentrations of isotretinoin in the epidermis are only half of those in serum. Plasma concentrations of isotretinoin are about 1.7 times those of whole blood due to poor penetration of isotretinoin into red blood cells.
After oral administration of isotretinoin, three major metabolites have been identified in plasma: 4-oxo-isotretinoin, tretinoin, (all-trans retinoic acid), and 4-oxo-tretinoin. These metabolites have shown biological activity in several in vitro tests. 4-oxo-isotretinoin has been shown in a clinical study to be a significant contributor to the activity of isotretinoin (reduction in sebum excretion rate despite no effect on plasma levels of isotretinoin and tretinoin). Other minor metabolites includes glucuronide conjugates. The major metabolite is 4-oxo-isotretinoin with plasma concentrations at steady state that are 2.5 times higher than those of the parent compound.
Isotretinoin and tretinoin (all-trans retinoic acid) are reversibly metabolised (interconverted), and the metabolism of tretinoin is therefore linked with that of isotretinoin. It has been estimated that 20-30% of an isotretinoin dose is metabolised by isomerisation.
Enterohepatic circulation may play a significant role in the pharmacokinetics of isotretinoin in man. In vitro metabolism studies have demonstrated that several CYP enzymes are involved in the metabolism of isotretinoin to 4-oxo-isotretinoin and tretinoin. No single isoform appears to have a predominant role. Isotretinoin and its metabolites do not significantly affect CYP activity.
After oral administration of radiolabelled isotretinoin approximately equal fractions of the dose were recovered in urine and faeces. Following oral administration of isotretinoin, the terminal elimination half-life of unchanged drug in patients with acne has a mean value of 19 hours. The terminal elimination half-life of 4-oxo-isotretinoin is longer, with a mean value of 29 hours.
Isotretinoin is a physiological retinoid and endogenous retinoid concentrations are reached within approximately two weeks following the end of isotretinoin therapy.
Since isotretinoin is contraindicated in patients with hepatic impairment, limited information on the kinetics of isotretinoin is available in this patient population.
Renal failure does not significantly reduce the plasma clearance of isotretinoin or 4-oxo-isotretinoin.
The acute oral toxicity of isotretinoin was determined in various animal species. LD50 is approximately 2000 mg/kg in rabbits, approximately 3000 mg/kg in mice, and over 4000 mg/kg in rats.
A long-term study in rats over 2 years (isotretinoin dosage 2, 8 and 32 mg/kg/d) produced evidence of partial hair loss and elevated plasma triglycerides in the higher dose groups. The side effect spectrum of isotretinoin in the rodent thus closely resembles that of vitamin A, but does not include the massive tissue and organ calcifications observed with vitamin A in the rat. The liver cell changes observed with vitamin A did not occur with isotretinoin.
All observed side effects of hypervitaminosis A syndrome were spontaneously reversible after withdrawal of isotretinoin. Even experimental animals in a poor general state had largely recovered within 1-2 weeks.
Like other vitamin A derivatives, isotretinoin has been shown in animal experiments to be teratogenic and embryotoxic.
Due to the teratogenic potential of isotretinoin there are therapeutic consequences for the administration to women of a childbearing age.
In a carcinogenicity study in Fischer 344 rats given oral isotretinoin up to 32 mg/kg/day, there was an increased incidence of phaeochromocytomas relative to controls in both sexes at 32 mg/kg/day and in males at 8 mg/kg/day. Given the high rate of spontaneous rate of occurrence of phaeochromoyctoma in Fischer 344 rats, the relevance of this tumour to humans is uncertain.
Studies in hairless mice suggest that concurrent dermal exposure to isotretinoin at dose levels up to 500 mg/kg may enhance the tumorigenic potential of UV irradiation. The significance of these studies to humans is not clear.
The mutagenic potential of isotretinoin was evaluated in the Ames assay with and without S9 metabolic activation and in the Chinese hamster lung cell for chromosome aberrations, both of which were negative.
In rats, no adverse effects on gonadal function, fertility, conception rate, gestation or parturition were observed at oral dose levels of isotretinoin up to 32 mg/kg/day.
In dogs, testicular atrophy was noted after approximately 30 weeks at isotretinoin dose levels of 20 or 60 mg/kg/day. However, in studies of men receiving oral isotretinoin, no significant effects have been seen on semen parameters.
Reproduction studies conducted in rabbits using isotretinoin gel applied topically at up to 60 times the human dose have revealed no harm to the foetus.
Topical application of high doses of tretinoin (an isomer of isotretinoin) induces maternal toxicity, which limits the maximum dose to a level potentially below that associated with embryofoetal alterations by other routes of administration.
In one study, topical doses of a 0.1% ethanol solution, given to Wistar rats through gestational days (GDs) 6 to 16, were not tolerated at 10 mg/kg/day, causing severe local and systemic maternal toxicity. Offspring of dams receiving 5 mg/kg weighed significantly less than those of controls. Maternal toxicity (reduced weight gain and food consumption) was also evident at doses of 2.5 mg/kg/day or more. A significant increase in the occurrence of supernumerary ribs was observed at this dose, a result thought to be nonspecific or maternally mediated.
Topical administration of tretinoin at a dose of 10.5 mg/kg/day for 3 days to intact skin of hamsters on GDs 7, 8, and 9 resulted in erythema and/or epidermal hyperplasia at the site of application, but did not cause a significant teratogenic response.
Topical administration of 5g 0.05% tretinoin ointment (corresponding to a dose of ~10 mg/kg) to the shaved backs of pregnant rats on GD 12 resulted in some retinoid-specific patterns of anomalies (humerus short 9%, radius bent 6%, ribs wavy 80%). This dose was ~100 fold that expected in humans.
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