Cell experiment:

Human liver microsomes (0.2 mg/mL) are incubated with 4-OH-atRA (500 nM) and NADPH, NADP+ or NAD+ (each at 2 mM) in 100 mM KPi buffer pH 7.4. In addition, 4-OH-atRA is incubated with human liver microsomes in the presence and absence of Talarozole (1 μM), a CYP26A1 specific inhibitor, and Ketoconazole (10 μM) a pan-P450 inhibitor and with NADPH as a cofactor. Following a 5 min pre-incubation, the reactions are initiated with the addition of cofactor and incubated for 30 minutes. At 30 min the reactions are quenched with equal volume of Acetonitrile and centrifuged at 3,000 g for 15 min. The supernatants are collected and 4-oxo-atRA formation is analyzed by LC-MS/MS. All incubations are normalized to a no cofactor control[2].

Animal experiment:

Mice[3]Talarozole is administered to mice as a single dose (2.5 mg/kg) or as multiple doses for three days. Serum Talarozole concentrations and serum, liver and testes atRA concentrations are measured by LC-MS/MS. Inhibition of CYP26 and changes inatRA concentrations in each tissue are predicted based on CYP26 activity in vitro and Talarozole disposition. Markers of fatty acid oxidation in the liver and spermatogonial differentiation in the testes are measured following Talarozole treatment.

Talarozole is a selective inhibitor of cytochrome P450 with an IC50 at 4 nM [1].

Cytochrome P450 is concerned with oxidative metabolism of many organic chemicals of diverse structure,both in exogenous and endogenous environment. Cytochrome P450 are notable both for the diversity of reactions that they catalyze and the range of chemically dissimilar substrates upon which they act.

Talarozole is a potent and selective inhibitor of cytochrome P450 26-mediated breakdown of endogenous all-trans-retinoic acid for the treatment of psoriasis and acne. Talarozole treatment increased the mRNA expression of CRABP2, KRT4, CYP26A1 and CYP26B1 dose, meanwhile, compared with vehicle-treated skin, decreased the expression of KRT2 and IL-1α. There was no mRNA change in retinol-metabolizing enzymes. No induction of epidermal thickness or overt skin inflammation in talarozole-treated skin. Immunofluorescence analysis substantiated an upregulation of KRT4 protein, however, there were no upregulation of CYP26B1 and CYP26A1 expression was found.[1, 2]

There were 0.1% of the dose found in the skin itself after 12-24 h. The results of distribution of talarozole within the skin show that 80% was located in the epidermis, meanwhile, the remaining 20% was detected in the dermis. [3]

References:
[1] Pavez Loriè E, Cools M, Borgers M, Topical treatment with CYP26 inhibitor talarozole (R115866) dose dependently alters the expression of retinoid-regulated genes in normal human epidermis. The British Journal of Dermatology,2009,160(1):
26-36.
[2] Geria AN, Scheinfeld NS. Talarozole, a selective inhibitor of P450-mediated all-trans retinoic acid for the treatment of psoriasis and acne. Current Opinion In Investigational Drugs,2008 ,9(11):1228-1237.
[3] Baert B, De Spiegeleer B. Local skin pharmacokinetics of talarozole, a new retinoic acid metabolism-blocking agent. Skin Pharmacol Physiol,2011,24(3):151-159