P.H. Rowe

An investigation of the pharmacokinetics of ethynylestsadiol in women using badioimmunoassay

Abstract A radioimmunoassay for ethynylestradiol (EE) which is applicable to plasma samples obtained from women, who have taken a combination type oral contraceptive, has been developed and fully validated. Plasma concentrations of EE rise to a peak of 128 pg/ml following the oral administration of 50 μg EE. Following the intravenous administration of the same dose of EE, plasma concentrations of the steroid declined biexponentially, the two half-lives being 0.83 and 6.75 hours. Comparison of the results of the intravenous and oral administration of the steroid suggested that its oral bioavailability is 42%. Although EE thus has a lower bioavailability than norethindrone, the pharmacokinetics of the two steroids, as reflected by half-lives, plasma clearance and volume of distribution, are very similar. The occurrence of a secondary peak in plasma at around 12 hours after dosing gave strong evidence that EE undergoes enterohepatic circulation in women; an event that may have considerable clinical significance.

The pharmacokinetics of levonorgestrel and ethynylestradiol in women: studies with Ovran and Ovranette.

A radioimmunoassay for levonorgestrel (Ng), which is applicable to plasma samples obtained from women who have taken a combination type oral contraceptive, has been developed and fully validated. Plasma concentrations of Ng rise to a peak of 3.6 ng/ml after an oral dose of 150 microgram and to 5.0 ng/ml after a 250 microgram dose. Following the intravenous administration of Ng, plasma concentrations of the steroid decline bi-exponentially with mean half-lives of 0.76 and 11.55 hours. Comparison of the results of the intravenous and oral administration of the steroid show a mean systemic bioavailability of 89% after the 150 microgram dose and 99% after a 250 microgram dose. This difference was not statistically significant. The values for volume of distribution were approximately half and the values for plasma clearance three times less than that previously reported for norethindrone. The plasma ethynylestradiol levels were also measured following administration of 30 microgram both intravenously and orally. The bioavailability and kinetics were similar to that previously reported for a 50 microgram dose. The peak values after oral dosing were 103 pg/ml and were reached by 1.0 hour in all subjects.

The effect of rifampicin oh the pharmacokinetics of ethynylestradiol in women

A single dose of Minovlar (50 microgram ethynylestradiol (EE) and 1 mg norethindrone acetate) was given to seven women during treatment with rifampicin (450-600 mg/day) and again one month after stopping rifampicin. Blood samples were taken at intervals over a 24-hour period. The area under the plasma EE versus time curve increased significantly on stopping rifampicin from 1014 +/- 317 pg/ml x h (mean +/- SE) to 1747 +/- 218 pg/ml x h (p less than 0.01). The terminal plasma half-life increased from 2.9 +/- 0.8 h to 6.3 +/- 1.4 h (p less than 0.005). The sex hormone binding globulin (S.H.B.G.) capacity was also reduced from 213.4 +/- 11.5 nmoles to 129.0 +/- 7.7 nmoles/1 after stopping rifampicin. In one patient starting rifampicin who was taking Minovlar as a long-term oral contraceptive, a fall in the plasma EE concentration was associated with a rise in the plasma follicle stimulating hormone concentration. These effects of rifampicin on EE pharmacokinetics are consistent with induction of the microsomal enzymes that metabolise EE.

The Effect of Rifampicin on Norethisterone Pharmacokinetics

SummaryThe pharmacokinetics of norethisterone have been studied in 8 women during and one month after treatment with rifampicin (450–600 mg/day). Rifampicin caused a significant reduction in the A. U. C. of a single dose of 1 mg norethisterone from 37.8±13.1 to 21.9±5.9 ng/ml X h (p<0.01). The plasma norethisterone half life (β-phase) was also reduced from 6.2±1.7 to 3.2±1.0 h (p<0.0025). In one additional woman on long term oral contraceptive therapy the 12 hour plasma norethisterone concentration was reduced by rifampicin from 12.3 ng/ml to 2.3 ng/ml. Rifampicin caused a significant increase in antipyrine clearance, 6β-hydroxycortisol excretion and plasma gamma-glutamyltranspeptidase activity but there were no significant correlations between changes in these indices of liver microsomal enzyme induction. There was a significant correlation between the percentage increase in antipyrine clearance and the percentage decrease in norethisterone A. U. C. during rifampicin. The changes in norethisterone pharmacokinetics during rifampicin therapy are compatible with the known enzyme inducing effect of rifampicin.

Kinetics of norethindrone in women. II. Single-dose kinetics.

In order to study the kinetics of norethindrone in women, we have administered single doses of 4 preparations of norethindrone (N) to 6 women (aged 21 to 23) in random order. The preparations were 1 mg N acetate and 50 µg ethinylestradiol EE2) (Minovlar), a dose of 1.05 mg N (Noriday), 3 mg N acetate and 50 µg EE2 (Gynovlar), and an intravenous preparation of 1 mg N with 50 µg EE2. Blood samples were taken at intervals up to 24 hr after dosing and plasma norethindrone concentrations were measured by radioimmunoassay. The plasma concentration decay slope fitted a two‐component open model with an initial rapid decay (half life 0.41 to 2.6 hr) followed by a slower beta phase with a half‐life of between 4.8 and 12.8 hr. The kinetics of norethindrone were not affected by the concomitant administration of EE2. The bioavailability of norethindrone after an oral dose of 1 mg was between 47% and 73% compared with an equivalent intravenous dose. The plasma clearance of norethindrone after 1 mg N acetate and 50 µg EE2 was 404.7 ± 31.7 ml/kg/hr and the apparent volume of distribution was 4.28 ± 0.56 L/kg. These values were not significantly different from the figures obtained after administration of 1.05 mg N. After an oral dose of 3 mg N acetate and 50 µg EE2, the bioavailability of norethindrone was significantly less (between 32% and 70%) than that after 1 mg N acetate and 50 µg EE2 explaining, at least in part, the lower than expected plasma concentrations of norethindrone in women on long‐term treatment with 3 mg N acetate and 50 µg EE2.

The effect of antibiotics on the enterohepatic circulation of ethinylestradiol and norethisterone in the rat.

Abstract Seventy-one percent of a given dose of [ 3 H]-ethinylestradiol ([ 3 H]-EE 2 ) and 76.0% of a given dose of [ 3 H]-norethisterone ([ 3 H]-N) were excreted in the bile of female rats in 4 h. The majority of radio-activity appeared in the glucuronide fraction. Characterization of the hydrolysed glucuronide fraction obtained after administration of [ 3 H]-EE 2 gave evidence that approximately 10.0% of the administered dose may be directly conjugated to form ethinylestradiol glucuronide. In contrast, there was no evidence of direct conjugation of norethisterone. In a linked rat preparation 15.4% of the given dose of [ 3 H]-EE 2 was excreted in the bile of control recipient rats in 7 h; this was reduced to 5.2% in neomycin pretreated and 6.0% in ampicillin pretreated animals. With [ 3 H]-N, 13.2% of the dose was excreted in the bile of control recipient rats in 7 hours and this was reduced to 3.6% in neomycin pretreated and 3.9% in ampicillin pretreated animals. These results show that antibiotic treatment reduces the enterohepatic circulation of synthetic steroids in the rat.

Reduction of the enterohepatic circulation of norethisterone by antibiotics in the rat: correlation with changes in the gut flora.

When bile containing tritiated conjugates of norethisterone, collected from donor rats, was infused either into the duodenum or caecum of recipient rats, 23.5% and 42.4% of the dose was excreted in the bile respectively. Pretreatment of recipient rats with ampicillin or neomycin (200 mg/kg/day for 4 days) or neomycin + lincomycin (100 + 100 mg/kg/day for 4 days) significantly reduced (P < 0.001) the amount of steroid appearing in the bile of recipient rats following intracaecal administration of conjugates. There was a correlation between the reduction in enterohepatic circulation (EHC) and suppression of the gut flora. Ampicillin treatment caused a reduction in the anaerobic element and partial suppression of aerobes. Neomycin pretreatment caused a great reduction in the aerobic flora, but only a small reduction in anaerobes. Following neomycin + lincomycin treatment both the aerobic and anaerobic flora were dramatically reduced. Rifampicin treatment (200 mg/kg/day for 4 days) did not cause a reduction in the EHC of norethisterone, or in the concentration of aerobic or anaerobic flora in the caecum. However, the aerobic flora were rifampicin resistant. To investigate the onset of this resistance recipient rats were treated with rifampicin for 1, 2 or 3 days and a graded effect both on the EHC of the steroid and the incidence of resistant organisms was evident. Following ampicillin administration (4 days) both the EHC and the gut flora were normalized 14 days after the final drug treatment.

Interindividual Variation and Drug Interactions with Hormonal Steroid Contraceptives

SummaryLarge inerindividual differences occur in the plasma concentration of contraceptive steroids. With the present tendency to decrease the dose of progestagen and oestrogen any factor which reduces the bioavailability of the lower dose preparations becomes very important. The possibility that other drugs and environmental chemicals may interact with contraceptive steroids is currently being investigated. Clinical reports suggest that the most important interacting drugs are the antituberculosis agent rifampicin, anticonvulsants and antibiotics. In the case of the first two, evidence is available suggesting that microsomal enzyme induction, either in the liver or the gut wall, is the operative mechanism. Antibiotics interfere with the pharmacokinetics of contraceptive steroids by interfering with their enterohepatic circulation in animal species; whether this is operative in man is still unclear.Other environmental and constitutional factors such as smoking, variations in diet, and concurrent disease may alter the disposition of contraceptive steroids and affect response accordingly. Additional studies are needed to estimate the significance of such interactions.

Kinetics of norethindrone in women; I. Radioimmunoassay and concentrations during multiple dosing

A radioimmunoassay for norethindrone has been developed with the use of an antiserum raised in rabbits after the injection of norethindrone‐3‐carboxymethyloxime linked to bovine serum albumin. The immunoassay is sensitive to 30 pg/ml norethindrone. With this assay plasma concentrations of norethindrone in 40 women receiving 1 mg norethindrone acetate with 50 µg ethinylestradiol were between 1.6 and 15.2 ng/ml 12 hr after dosing during multiple dosing. After a single dose of 1 mg of norethindrone acetate with 50 mg ethinylestradiol, the plasma half life (t½) of the terminal apparently monoexponential phase of norethindrone decay was between 6.1 and 12.3 hr in 6 patients. During long‐term therapy with contraceptive steroids the plasma t½ of norethindrone did not differ from the t½ after a single dose. In 4 women receiving contraceptive steroids for the first time. plasma concentrations of norethindrone rose progressively during the first cycle to concentrations higher than predicted from simple accumulation pharmacokinetics. The sex hormone‐binding globulin capacity also increased and there was a good correlation between the plasma concentration of norethindrone and the sex hormone‐binding capacity (r between 0.695 and 0.910 in 4 patients). In 3 women starting the contraceptive steroid 4 wk postpartum there was a reduction of plasma norethindrone concentration with time (r = 0.528 p < 0.01) and this correlated with a fall in the sex hormone‐binding capacity (p < 0.05). In women taking a preparation containing 3 mg norethindrone acetate. the mean plasma norethindrone concentration was only 1.4 times as high as that in women taking 1 mg norethindrone acetate.

The pharmacokinetics of a large (3 mg) oral dose of ethynylestradiol in women

Plasma levels of ethynylestradiol (EE), EE sulphate (EES), EE glucuronide (EEG) and prolactin were measured in women up to 72 h following the oral administration of 3 mg of EE. The decline in plasma EE levels showed a sharp discontinuity at 10 h which is assumed to be due to the beginning of enterohepatic circulation (EHC). After this event, an apparently terminal monoexponential decline was eventually established with a half-life of 13.1 h. As a result of EHC, the volume of distribution was increased by 60% to 6.0 1.Kg-1 but it does not appear that any significant accumulation of EE would occur during multiple dosing. Plasma levels of EES were, on average, 22.5 times greater than those of EE and this circulating metabolite may act as a reservoir of EE. No circulating EEG could be detected. Comparing these results with those previously obtained with 50 microgram EE, there was no evidence of dose dependency in the pharmacokinetics of this steroid. Plasma prolactin levels were markedly enhanced by this single dose of oestrogen.