The metabolism of dexamethasone in the rat--effect of phenytoin

Roles of human cytochrome P450 3A4/5 in dexamethasone 6β-hydroxylation mediated by liver microsomes and humanized liver in chimeric mice metabolically suppressed with azamulin

The urinary metabolic ratio of 6β-hydroxydexamethasone to dexamethasone reportedly acts as a noninvasive marker for human cytochrome P450 (P450) 3A4/5, which is induced by rifampicin in humanized-liver mice. In the current study, the pharmacokinetics of dexamethasone in humanized-liver mice after intravenous administration (10 mg/kg) were investigated using azamulin (a time-dependent P450 3A4/5 inhibitor). After intravenous dexamethasone administration, significant differences were observed in the time-dependent plasma and 24-h urinary concentrations of 6β-hydroxydexamethasone between untreated humanized-liver mice and humanized-liver mice treated with azamulin (daily oral doses of 15 mg/kg for 3 days). The mean ratios of 6β-hydroxydexamethasone to dexamethasone for the maximum concentrations, the areas under the plasma concentration-versus-time curves, and urinary concentrations were significantly lower in the azamulin-treated group (59%, 58%, and 41% of the untreated values, respectively). 6β-Hydroxydexamethasone formation was suppressed by 93% by replacing control human liver microsomes with P450 3A4/5-inactivated liver microsomes. These results suggest that the oxidation of dexamethasone in humans is mediated mainly by P450 3A4/5 (which is suppressed by azamulin), and that humanized-liver mice orally treated with azamulin may constitute an in vivo model for metabolically inactivated P450 3A4/5 in human hepatocytes transplanted into chimeric mice.

Modeling Combined Anti-Inflammatory Effects of Dexamethasone and Tofacitinib in Arthritic Rats

Tofacitinib (TOF), a Janus kinase (JAK) inhibitor, which was approved in 2012, has been recommended for the treatment of clinically active rheumatoid arthritis (RA). Dexamethasone (DEX), a potent corticosteroid, is also used in RA therapy but with limited usefulness due to dose- and time-dependent adverse effects. This pilot study examines the single and combined effects of DEX and TOF in order to explore the steroid-sparing potential of TOF. Collagen-induced arthritic (CIA) rats were subcutaneously (SC) dosed with vehicle, 1.5 mg/kg TOF, 5 mg/kg TOF, 0.225 mg/kg DEX, or a combination of 1.5 mg/kg TOF and 0.225 mg/kg DEX. Paw sizes were measured as an index of disease and drug efficacy and dynamically depicted using a logistic function for natural paw growth, a turnover model for disease progression, an indirect response model for inhibitory effects of TOF and DEX and a non-competitive interaction model for the combined effect of DEX and TOF. TOF alone exerted only a slight inhibitory effect on RA paw edema compared to DEX, which reduced edema by 40%. In combination, TOF and DEX had additive effects with an interaction factor of 0.76. Using model simulations, a single SC dose of TOF does not have a visible steroid-sparing potential, although BID oral dosing has such potential. The current study suggests an additive effect of TOF and DEX and simulations indicate that further exploration of TOF and DEX administration timing may produce desirable drug efficacy with lower DEX doses.

Preliminary studies of 6 beta-hydroxydexamethasone and its importance in the DST

The role of the metabolites of dexamethasone (DEX) in the dexamethasone suppression test (DST) has never been fully elucidated. We report here our preliminary studies of 6 beta-hydroxydexamethasone (6 OH-Dex), a known metabolite of DEX, on the hypothalamic-pituitary-adrenal (HPA) axis of the rat; its activity in the most commonly used radioimmunoassay for plasma DEX; and its plasma concentrations in a normal human subject during the standard 1.0 mg DST. Six OH-Dex administered subcutaneously to rats at a dose of 1 mg/kg was able to completely suppress corticosterone production for at least 3 hr. In the IgG Corp. radioimmunoassay for plasma DEX, 6 OH-Dex was moderately cross-reactive yielding a 50% cross-reactivity of 10%. Gas chromatographic coupled mass spectroscopic analysis of human plasma samples, obtained 12 to 20 hr after the oral ingestion of 1.0 mg DEX, demonstrated similar plasma concentrations for both the parent compound and the 6-hydroxyl metabolite. The relevance of these findings, particularly to pharmacokinetic studies of the DST, is discussed.

Influence of sex and oestrogen replacement on the disposition of dexamethasone in rats

The disposition of dexamethasone (DXM, 2 mg/kg, iv) was studied in ovariectomized female rats treated with oestrogen (0.1 mg and 1 mg of oestradiol benzoate) and in male rats. Oestradiol replacement had no effect on body or liver weights or on the DXM pharmacokinetic parameters (CL, Vdss, AUC, MRT and t1/2) of the female groups. If the Vdss seemed slightly greater in male than in female rats, this difference disappeared after normalization based on body weight. In contrast, CL was greater in the male rats even after normalization. For all the animals, significant correlations were observed between body weight and Vdss (r = 0.731, P less than 0.001) or CL (r = 0.639, P less than 0.001). Terminal half life and MRT were negatively correlated with CL (r = -0.481, P less than 0.01 and r = -0.575, P less than 0.01, respectively) but not with Vdss. Although oestrogen replacement did not seem to affect the pharmacokinetics of DXM, the increase in the CL in male rats should be the main determinant observed between the sexes. These results are consistent with a slower metabolism found for various drugs metabolized by the cytochrome P-450 in female rats.

Two distinct mechanisms for regulation of gamma-glutamyl transpeptidase in cultured rat hepatocytes by glucocorticoid-like steroids

Adult rat hepatocytes maintained in primary monolayer culture with defined medium were used to characterise two effects of glucocorticoid-like steroids in regulating gamma-glutamyltranspeptidase (GGT). Low concentrations of glucocorticoids alone had little effect on GGT but synergistically enhanced induction of the enzyme by liver tumor-promoting xenobiotics such as 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane (DDT) and hexachlorocyclohexane. The enhancing effect appears to be mediated by the classical glucocorticoid hormone receptor since structural requirements and concentration-dependence for enhancement were similar to those for induction of tyrosine aminotransferase in parallel cultures. Higher concentrations (1-100 microM) of various glucocorticoids alone increased GGT activity. Most glucocorticoids induced GGT but their order of potency did not parallel that for induction of tyrosine aminotransferase under similar culture conditions. Among the most potent glucocorticoids, triamcinolone was a weak GGT inducer and cortivazol appeared to act as an antagonist of GGT induction by steroids. Some non-glucocorticoids including pregnenolone 16 alpha-carbonitrile, and some progestins, also induced but required addition of 30 nM dexamethasone for maximal effect. Some specific steroid structural features were identified which increased (presence of a 16 alpha methyl group) or impaired GGT-inducing activity. Although interpretation is complicated by differential metabolism of individual steroids in culture, the results suggest that GGT induction by pharmacological levels of steroids may be mediated, directly or indirectly, by one or more relatively specific receptors distinct from the classical glucocorticoid receptor.

Dexamethasone bioavailability: implications for DST research

The bioavailability of dexamethasone (DEX) has recently been demonstrated to be a critical factor in determining Dexamethasone Suppression Test (DST) status in psychiatric patients. This brief review focuses on several aspects of DEX bioavailability as they relate to the use of the DST in neuroendocrine research. Several methodologies, including radioimmunoassay, high-performance liquid chromatography, and gas chromatography-mass spectrometry are available for quantification of DEX in biological fluids, although few detailed comparisons between methods have been reported. Surprisingly, little systematic research on the metabolism of DEX has been reported, but it appears that hepatic rather than renal mechanisms are the major source of DEX elimination. The marked variability in serum DEX levels following oral administration in psychiatric patients is also observed in normal controls and patients with Cushing's syndrome. A variety of drugs can modify serum DEX levels and thereby after the effectiveness of DEX in suppressing serum cortisol levels. Simultaneous measurement of serum DEX and cortisol levels appears to be necessary for the appropriate evaluation of DST results. This procedure may help explain many of the inconsistencies in recent DST research.

The biotransformation and urinary excretion of dexamethasone in equine male castrates

The pro-drugs of dexamethasone, a potent glucocorticoid, are frequently used as anti-inflammatory steroids in equine veterinary practice. In the present study the biotransformation and urinary excretion of tritium labelled dexamethasone were investigated in cross-bred castrated male horses after therapeutic doses. Between 40-50% of the administered radioactivity was excreted in the urine within 24 h; a further 10% being excreted over the next 3 days. The urinary radioactivity was largely excreted in the unconjugated steroid fraction. In the first 24 h urine sample, 26-36% of the total dose was recovered in the unconjugated fraction, 8-13% in the conjugated fraction and about 5% was unextractable from the urine. The metabolites identified by microchemical transformations and thin-layer chromatography were unchanged dexamethasone, 17-oxodexamethasone, 11-dehydrodexamethasone, 20-dihydrodexamethasone, 6-hydroxydexamethasone and 6-hydroxy-17-oxodexamethasone together accounting for approx 60% of the urinary activity. About 25% of the urinary radioactivity associated with polar metabolites still remains unidentified.

Identification and quantification of 6 beta-hydroxydexamethasone as a major urinary metabolite of dexamethasone in man

Identification of 6 beta-hydroxydexamethasone as a major urinary metabolite of dexamethasone in man has been accomplished by nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry. Mass fragmentographic measurements revealed that more than 30% of the intravenously or orally administered dexamethasone dose was excreted in the 24-h urine as 6 beta-hydroxydexamethasone, while only a small fraction of the dose was excreted as unchanged dexamethasone and its glucuronic acid conjugate.

Influence of phenylbutazone, mofebutazone and aspirin on the pharmacokinetics of dexamethasone in the rat

Phenylbutazone suppresses the C-6 hydroxylation, absorption rate, bioavailability, and renal and plasma clearanceè rates of dexamethasone administered orally to normal and oedemateous rats. It increases the half life and the volume of distribution. Aspirin exerts an effect which is less pronounced and involves the enhancement of the C-6 hydroxylation. Aspirin suppresses the half life and renal clearance of dexamethasone and enhances its hepatic clearance. Mofebutazone does not exert any pronounced influence. Also, unlike phenylbutazone, it does not interfere with the gastrointestinal absorption of dexamethasone. More rapid onset of absorption, decrease of half life and increase of the contribution of renal clearance to total plasma clearance of dexamethasone, are characteristics of the oedematous condition in the rat. The contribution of renal clearance to the elimination of dexamethasone is much greater in the rat than in human subjects. The presence of a third unconjugated metabolite of dexamethaone in the urine of rat has been demonstrated.