Antipyrine pharmacokinetics in women receiving conjugated estrogens

Effect of a Triphasic Oral Contraceptive on Drug-Metabolizing Enzyme Activity as Measured by the Validated Cooperstown 5+1 Cocktail

The effects of a common oral contraceptive preparation on the activity of 7 drug-metabolizing enzymes were investigated using the validated Cooperstown 5+1 Cocktail. In a randomized crossover fashion, 10 premenopausal women received caffeine, dextromethorphan, omeprazole, intravenous midazolam, and warfarin + vitamin K with and without a triphasic oral contraceptive (ethinyl estradiol 35 microg) and varying doses of daily norgestimate (0.18, 0.215, and 0.25 mg). Bioequivalence testing showed nonequivalence in drug versus no-drug treatment on the activity of drug-metabolizing enzymes (as reflected by metabolite ratios following probe drug administration); the activity of CYP1A2, CYP2C19, and NAT-2 decreased following the oral contraceptive, whereas the activity of CYP2C9 and CYP2D6 increased. No effects on xanthine oxidase or hepatic CYP3A were seen. Application of a non-parametric statistical testing approach revealed a significant difference only for CYP1A2 and CYP2C19. This triphasic oral contraceptive may have a clinically significant effect on the activity of some drug-metabolizing enzymes.

Pharmacokinetic and pharmacologic variation between different estrogen products

Due to the complex nature of endogenous and exogenous hormone concentration, formation, and metabolism and assay complexity, the pharmacokinetics of estrogen are difficult to study. Oral estrogens have minimal systemic bioavailability (2% to 10%) due to gut and liver (first-pass) metabolism. High concentrations of estrone are achieved with oral administration, whereas higher concentrations of estradiol are generally achieved after percutaneous absorption. Although vaginal products (such as gel, rings, etc.) are administered locally, they achieve high serum concentrations. Estradiol and estrone concentrations and estradiol-to-estrone ratios vary with different estrogen therapies. Approximately 95% to 98% of estradiol is bound loosely to albumin or tightly to sex hormone binding globulin, the major binding protein. The terminal half lives for the different estrogen compounds (after oral or intravenous administration) vary from 1-12 hours. Some conversion rates have been calculated between estrogen and its metabolites. Smoking decreases achievable estrogen concentrations, and has a greater effect on oral products. Oral contraceptives have been found to decrease antipyrine clearance. In the one study evaluating conjugated estrogens, antipyrine clearance was not altered. Oral contraceptives have a variable effect on the elimination of medications. Acetaminophen clearance is increased, whereas clearance of some benzodiazepines, caffeine,and prednisolone is decreased. Phenytoin increases the metabolism of conjugated estrogens. The various estrogen products may produce different clinical effects based on composition. The metabolites (minor components) of conjugated estrogens have been found to have significant effects on lipid concentrations, uterine weight, liver generated compounds, and bone resorption. Because transdermal products bypass the first-pass effect, delayed or decreased effects on lipid profiles and liver generated compounds have been observed.

Menopause: pharmacodynamics and pharmacokinetics

Effects of menopause on pharmacokinetics and pharmacodynamics remain largely uninvestigated. We described decreased clearance of prednisolone in postmenopausal (POM) vs. premenopausal (PRM) women. Decreased clearance may be related to greater age in POM women, or to hormonal differences between PRM and POM women, independent of age. Estrogen replacement therapy (ERT) did not restore POM pharmacokinetic parameters to PRM values. However, in a single POM woman taking estrogen and progestin, clearance was similar to PRM values, suggesting that estrogen and progestin replacement may restore PRM prednisolone elimination. Lemmens and coworkers reported a significant negative correlation between age and alfentanil clearance in women, but not men, concluding that effects of age on alfentanil clearance were gender dependent. An alternative conclusion based on these same data was proposed by Rubio and Cox, who found that 67.5% of the variation in alfentanil clearance in women was explained when clearance values were divided into groups by menopausal status. Alfentanil clearance may exhibit hormonal dependence, independent of age. These data are consistent with an enzyme system that becomes less active after menopause. Gennari and coworkers examined calcium absorption before and after oophorectomy in women who received ERT or placebo therapy for 6 months after surgery. Placebo women showed significant decreases in vertebral bone density (VBD) and calcium absorption. ERT women maintained presurgical VBD and calcium absorption. The placebo group displayed intestinal resistance to normal 1,25-dihydroxyvitamin D3 stimulation of calcium absorption, whereas ERT preserved normal intestinal responsiveness.

Antipyrine kinetics in patients with primary biliary cirrhosis

Fourteen antimitochrodrial antibody-positive patients (13 women, 1 man) with biopsy-proven primary biliary cirrhosis, aged 40 to 71 years (mean, 57 years) weighing 43 to 102 kg (mean, 63 kg), along with 14 age- and sex-matched healthy controls, received a single 1.0- to 1.2-g dose of intravenous antipyrine. Plasma antipyrine levels were determined during a 12- to 24-hour period. Patients' mean serum chemistry values were: albumin, 3.9 g/dL (range, 3.1-4.4) and total bilirubin, 1.9 mg/dL (range, 0.3-10.9). Seven of the fourteen patients had cirrhosis. Mean kinetic variables for antipyrine in controls and primary biliary cirrhosis patients were: Vd, .54 versus .49 L/kg; half-life, 12.0 versus 15.1 hours (P < .07); clearance, .55 versus .41 mL/min/kg (P < .04). Within the primary biliary cirrhosis group, there was no correlation between total bilirubin and clearance (r = .09), nor did clearance vary significantly among histologic categories. Clearance of antipyrine in primary biliary cirrhosis patients is reduced by an average of 25%, but the clinical prognosticators of serum bilirubin levels and histologic grade do not correlate with or predict the degree of clearance impairment.