Bioequivalence Studies of Tibolone in Premenopausal Women and Effects on Expression of the Tibolone‐Metabolizing Enzyme AKR1C (Aldo‐Keto Reductase) Family Caused by Estradiol

Abstract

This study aimed to investigate the bioequivalence of a test formulation of tibolone with the marketed reference formulation in 24 young healthy female volunteers. Tibolone is a synthetic steroid hormone for menopausal women. Volunteers were treated with the 2 formulations of tibolone (total dose of active ingredient 2.5 mg) according to a 2 x 2 crossover design with a 1-week washout period. Plasma concentrations of 3alpha- and 3beta-hydroxytibolone, which are major metabolites of tibolone, were assayed in timed samples over a 24-hour period with a validated gas chromatography/mass spectrometry (GC/MS) method that had a lower limit of quantification of 0.5 ng/mL. The reference and test formulations gave a mean 3alpha-hydroxytibolone C(max) of 5.0 and 5.2 ng/mL, respectively, and a mean 3beta-hydroxytibolone C(max) of 16.4 and 16.5 ng/mL, respectively. The mean AUC(t) of 3alpha-hydroxytibolone was 24.7 and 24.3 ng h/mL, whereas the mean AUC(t) of 3beta-hydroxytibolone was 57.6 and 54.8 ng h/mL for the test and reference formulations, respectively. The authors did not find significant differences in pharmacokinetic parameters between the 2 formulations, but metabolite formation was different from reports in postmenopausal women. The authors therefore measured the effects of estradiol on the expression of the tibolone-metabolizing enzymes, from the aldo-keto reductase (AKR1C) family, using HepG2 cell (human hepatoma cells) and MCF-7 cell (human breast cancer cells). Estradiol increased mRNA levels of AKR1C1, AKR1C2, and AKR1C3 and protein levels of total AKR1C in HepG2 cells. Estradiol selectively enhanced levels of AKR1C2 mRNA in MCF-7 cells. Thus, changes in the major metabolites of tibolone might result from changes in AKR1C family expression by patient estrogen status.

Modulated Expression of Genes Encoding Estrogen Metabolizing Enzymes by G1-Phase Cyclin-Dependent Kinases 6 and 4 in Human Breast Cancer Cells

“…The dashed black line indicates the interaction of the androgen receptor with cdk6, which can stimulate androgen receptor-directed gene transcription, independently of cyclin D1 [42]. Steroid hormones bound to cognate steroid receptors regulate transcription of many genes that can affect tumor cell growth and function, including those encoding proteins that directly affect the cell cycle, such as c-myc and cyclin D1 [38], [39] and perhaps SME genes themselves [54].…”

Appendix: Drug Metabolizing Enzymes and Biotransformation Reactions

“…EHs are a class of proteins that catalyze the hydration of chemically reactive epoxides to their corresponding 1,2 -diol products. The hydride transfer from NADPH to the substrate is generally stereospecifi c. Human AKRs are involved in metabolism of synthetic hormones (Kang and Kim, 2008 ), chemotherapeutic agents (Jin and Penning, 2007 ;Novotna et al, 2008 ), and CNS drugs (Jin and Penning, 2007 ). Other human AKRs include the human homologues of afl atoxin aldehyde reductases (AKR7A2 and AKR7A3) (Jin and Penning, 2007 ;Barski et al, 2008 ).…”

“…A majority of the human enzymes involved in drug metabolism such as the CYPs, UGTs, MAOs, FMOs, (Kang and Kim, 2008 ).…”

Application of UPLC–MS/MS for separation and quantification of 3α-Hydroxy Tibolone and comparative bioavailability of two Tibolone formulations in healthy volunteers

“…Timmer et al [5] applied gas chromatography–mass spectrometry (GC–MS) method described by Verhoeven et al [8] for quantification of Tibolone and its metabolites in pharmacokinetic study of Tibolone in human plasma demonstrating LLOQ 0.100 ng/mL without any method validation details. Kang and Kim [9] developed solid phase extraction (SPE) GC–MS method for quantification of 3α-Hydroxy Tibolone and 3β-Hydroxy Tibolone in plasma with LLOQ 0.500 ng/mL. Verheul et al [10] reported GC–MS method for quantification of Tibolone and its metabolites from monkey plasma (LLOQ 0.100 ng/mL) and urine (LLOQ 0.500 ng/mL) which involved a solid phase extraction followed by derivatisation of samples.…”