Influence of oral isotretinoin on hepatic and cutaneous P-450-dependent isozyme activities

A pilot study on the effect of isotretinoin on serum etonogestrel concentrations in contraceptive implant users

OBJECTIVE

To explore the pharmacokinetic interaction between isotretinoin, a cytochrome P-450 (CYP) inducer and potent teratogen, and the etonogestrel contraceptive implant.

STUDY DESIGN

We enrolled healthy reproductive-age women initiating isotretinoin and using an etonogestrel implant. We compared serum etonogestrel concentrations at baseline and after four and nine weeks of isotretinoin co-administration using a validated assay.

RESULTS

Among eight implant users, all serum etonogestrel concentrations remained >90 pg/mL during isotretinoin co-administration with no significant changes from baseline (p = 0.25, Friedman's test).

CONCLUSION

In this exploratory study, we found that isotretinoin did not cause serum etonogestrel concentrations to fall below the threshold for ovulatory suppression (<90 pg/mL) among implant users.

IMPLICATIONS

Reproductive-age women treated with isotretinoin require reliable contraception to prevent pregnancies impacted by teratogenic-effects. This small study demonstrates that contraceptive implant users maintained serum etonogestrel concentrations above the threshold for consistent ovulatory suppression during isotretinoin co-administration. The contraceptive implant remains an appropriate option for patients considering isotretinoin therapy.

Retinoids induce cytochrome P450 3A4 through RXR/VDR-mediated pathway

A panel of retinoids and carotenoids was screened as potential inducers of CYP3A4 through the RXR/VDR-mediated signaling pathway. Transient transfection assays revealed that 3 out of 12 retinoids screened transactivated RXRalpha/VDR and induced CYP3A4 reporter activity. These three retinoids are the active metabolites of retinoids, 9-cis-retinal, 9-cis-retinoic acid (9-cis-RA), and all-trans-retinoic acid (all-trans-RA). 9-cis-RA and all-trans-RA preferentially transactivated the RXR/VDR heterodimers and RXR homodimers. Retinoids and VDR agonist 1alpha, 25-dihydroxyvitamin D(3), but not PXR or CAR activator, could induce Cyp3a11 mRNA level in hepatocytes derived from PXR/CAR-double null mouse. Moreover, retinoids induced CYP3A4 enzyme activity in HepG2 human hepatoma and Caco-2 human colorectal adenocarcinoma cells. A direct role of retinoid-mediated CYP3A4 induction through RXRalpha/VDR was proved by the results that 9-cis-retinal, 9-cis-RA, and all-trans-RA recruited RXRalpha and VDR to CYP3A4 regulatory region pER6 (proximal everted repeat with a 6-nucleotide spacer) and dXREM (distal xenobiotic-responsive enhancer module). Thus, using various approaches, we have unequivocally demonstrated that retinoids transactivate RXR/VDR heterodimers and RXR homodimers and induce CYP3A expression at mRNA as well as enzyme activity levels in both liver and intestinal cells. It is possible that retinoids might alter endobiotic metabolism through CYP3A4 induction in vivo.

Drug-metabolizing enzymes in the skin of man, rat, and pig

The mammalian skin has long been considered to be poor in drug metabolism. However, many reports clearly show that most drug metabolizing enzymes also occur in the mammalian skin albeit at relatively low specific activities. This review summarizes the current state of knowledge on drug metabolizing enzymes in the skin of human, rat, and pig, the latter, because it is often taken as a model for human skin on grounds of anatomical similarities. However only little is known about drug metabolizing enzymes in pig skin. Interestingly, some cytochromes P450 (CYP) have been observed in the rat skin which are not expressed in the rat liver, such as CYP 2B12 and CYP2D4. As far as investigated most drug metabolizing enzymes occur in the suprabasal (i.e. differentiating) layers of the epidermis, but the rat CYP1A1 rather in the basal layer and human UDP-glucuronosyltransferase rather in the stratum corneum. The pattern of drug metabolizing enzymes and their localization will impact not only the beneficial as well as detrimental properties of drugs for the skin but also dictate whether a drug reaches the blood flow unchanged or as activated or inactivated metabolite(s).

Retinoids activate the RXR/SXR-mediated pathway and induce the endogenous CYP3A4 activity in Huh7 human hepatoma cells

Steroid and xenobiotic receptor (SXR) or human pregnane X receptor (hPXR) dimerizes with retinoid X receptor (RXR) and regulates the transcription of genes encoding xenobiotic-metabolizing enzymes such as CYP3A4. Rifampin, the classical activator of CYP3A4, binds to SXR directly. It is unclear whether various natural and synthetic retinoids can regulate the expression of CYP3A4. To evaluate the effects of retinoids on the RXR/SXR-mediated pathway, transient transfection assays were performed on both CV-1 and human hepatoma Huh7 cells using a reporter construct containing multiple RXR/SXR consensus binding elements (an everted repeat with a 6-nucleotide spacer, ER-6). The results revealed that eight out of 13 retinoids screened significantly induced the RXR/SXR-mediated pathway in Huh7 cells. At an equal molar concentration, the acid forms (9-cis-RA, 13-cis-RA, and all-trans-RA) or aldehyde, the direct precursor of acid (9-cis-retinal and 13-cis-retinal), exhibited a greater or similar potency than rifampin. Depending on the ligands, RXR may serve as a silent or an active partner of SXR. Additionally, retinoids can increase CYP3A4 enzyme activity in Huh7 cells. To further evaluate the potential drug-drug interactions, which may be caused by retinoids, Huh7 cells were pretreated with 9-cis-RA and followed by acetaminophen. We showed that 9-cis-RA enhanced the covalent binding of N-acetyl-p-quinoneimine, a toxic intermediate of acetaminophen produced by phase I enzymes oxidation. This result suggested that drug-drug interaction might occur between 9-cis-RA and acetaminophen in human liver cells. Taken together, retinoids activate the RXR/SXR-mediated pathway and regulate the expression of CYP3A4. Thus, retinoids potentially can cause drug-drug interactions when they are administered with other CYP3A4 substrates.

Safety issues in isotretinoin therapy

Isotretinoin (Accutane, Roche Laboratories Inc, Nutley, NJ) is an important drug, not only for the treatment of severe acne, but also for other diagnoses and in chemoprevention settings. Because the use of isotretinoin is increasing, it is important for physicians to be aware of the adverse events, toxicities, and management issues related to its use. The most important issue is that of congenital defects, which has resulted in new pregnancy prevention policies and programs implemented by the manufacturer. A relatively new concern is that of depression associated with isotretinoin use, also resulting in new policies placed by the manufacturer and the FDA. The most common adverse effects observed during treatment are mucocutaneous and ocular in nature, but laboratory abnormalities and effects in the nervous, musculoskeletal, gastrointestinal, pulmonary, hematologic, and other systems are also described. Additionally, potential drug interactions, follow-up, and toxicity prevention measures are discussed.

Ah receptor-dependent CYP1A induction by two carotenoids, canthaxanthin and beta-apo-8'-carotenal, with no affinity for the TCDD binding site

The assays of several phase I and phase II xenobiotic-metabolizing enzyme activities, as well as CYP1A immunoblot analysis, were performed in liver microsomes and cytosol of male C57BL/6 mice (Ah receptor-responsive), of male DBA/2 mice (Ah receptor-low responsive) and of female Ah receptor gene knockout mice that were fed diets containing 300 mg/kg of a nonprovitamin A carotenoid, canthaxanthin, or a provitamin A carotenoid, beta-apo-8'-carotenal for 14 days, or which were injected i.p. with 3-methylcholanthrene. Previous studies have shown that some carotenoids, such as canthaxanthin and beta-apo-8'-carotenal, are strong inducers of liver CYP1A1 and 1A2 when given to rats. In this work, only canthaxanthin induced both CYP1A1 and 1A2 in C57BL/6 mice, whereas beta-apo-8'-carotenal induced only CYP1A2 in this strain. Neither of the two carotenoids modified CYP1A1/2 protein contents or enzyme activities in Ah receptor-low responsive DBA/2 or in Ah receptor gene knockout mice. Cytosol prepared from C57BL/6 mice liver tissue was incubated with [3H] 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the presence of canthaxanthin or beta-apo-8'-carotenal and analyzed by sucrose density gradient sedimentation: neither of the carotenoids, even when present in large excess, competed with TCDD for the TCDD binding site of the cytosolic Ah receptor of C57BL/6 mice. In brief, the carotenoids canthaxanthin or beta-apo-8'-carotenal induced Cyp1a genes in mice through an Ah receptor-dependent pathway, but did not bind to the Ah receptor.