Interaction with the pharmacokinetics of ethinylestradiol and progestogens contained in oral contraceptives

Pharmacokinetics and pharmacodynamics of oral contraceptive steroids: factors influencing steroid metabolism

The time-dependent alterations in the serum concentrations of ethinyl estradiol, gestodene, and 3-keto-desogestrel during treatment with 30 micrograms of ethinyl estradiol + 75 micrograms of gestodene or 30 micrograms of ethinyl estradiol + 150 micrograms of desogestrel were investigated during 12 months. The levels of gestodene and 3-keto-desogestrel increased between days 1 and 21 of each cycle, reaching maximal levels during the third and sixth cycles. The serum concentrations of gestodene were fourfold to fivefold higher than those of 3-keto-desogestrel. The ethinyl estradiol levels increased significantly between days 1 and 10 during each cycle and were significantly higher by 70% during intake of ethinyl estradiol/gestodene compared with ethinyl estradiol/desogestrel, although the dose was identical. Intake of gestodene, in addition to 35 micrograms of ethinyl estradiol + 2 mg of cyproterone acetate, caused a rise in ethinyl estradiol levels. During treatment with ethinyl estradiol/gestodene and an additional 150 micrograms of levonorgestrel, there was a continuous increase in gestodene levels, although sex hormone-binding globulin level did not change. During treatment with 30 or 35 micrograms of ethinyl estradiol and 75 micrograms of gestodene, 150 micrograms of desogestrel, or 2 mg of cyproterone acetate, there were large intraindividual and interindividual variations in the steroid levels and ratios of estrogen: progestogen levels. There was no correlation with the occurrence of intermenstrual bleedings. It is concluded that ethinyl estradiol and nortestosterone derivatives may inhibit steroid-metabolizing enzymes in the liver, which results in a rise in the serum levels of contraceptive steroids. The cause of the large intraindividual variations is as yet unknown, but it is probably from changes in steroid metabolism.

Inhibition of oral contraceptive steroid-metabolizing enzymes by steroids and drugs

The major 17 alpha-ethinyl estradiol 2-hydroxylase is humans is the hepatic enzyme cytochrome P-450 IIIA4 (P-450NF), which is known to be inducible by rifampicin or barbiturates. The literature indicates that 17 beta-estradiol, progesterone, and norgestrel are competitive inhibitors and that primaquine and tolbutamide are rather weak noncompetitive inhibitors. Recent experiments in this laboratory indicate that gestodene is a relatively potent mechanism-based inactivator of cytochrome P-450 IIIA4 in vitro. Inhibition requires incubation with the reduced form of nicotinamide adenine dinucleotide phosphate, is time and concentration dependent, and can be partially blocked by the presence of noninhibitory cytochrome P-450 IIIA4 substrates. The in vitro activation by gestodene provides a possible explanation for the increase in plasma estrogen levels reported in women administered gestodene along with 17 alpha-ethinyl estradiol.

Contraception with a vaginal ring releasing 3-keto desogestrel and ethinylestradiol

Eleven healthy women used a contraceptive vaginal ring for three cycles. The rings released 15 micrograms of ethinylestradiol and 150 micrograms of 3-keto desogestrel per 24 hours. The rings were used for 21 days and then removed, rinsed in clean water and stored. After a 7-day ring-free period, the rings were reinserted. Totally, each ring was used for 3 cycles. The rings were well tolerated. The bleeding pattern was excellent. No woman showed any signs of ovulation when using the ring. Estradiol levels were suppressed, but showed an increase during the ring-free period as can be found during the pill-free period of an oral contraceptive regimen. When the ring was reinserted, the estradiol levels were rapidly suppressed again. SHBG capacity increased during use of the ring.

A pharmacodynamic and pharmacokinetic study of the Chinese No. 1 pill

A pharmacodynamic and pharmacokinetic study of the Chinese No. 1 pill, a combined oral contraceptive containing 35 micrograms ethynyloestradiol (EE) and 600 micrograms norethisterone (NET), was performed in 29 women over a period of six months. Blood samples for analysis were taken during a pretreatment cycle, the first and 6th treatment cycles and post-treatment. Minor changes in carbohydrate metabolism occurred and these were particularly noticeable when the incremental areas under the serum concentration-time curves for both glucose and insulin in response to a glucose tolerance test were calculated. No changes occurred in the serum glycosylated haemoglobin levels. The serum concentrations of all the lipids measured (total cholesterol, triglycerides, LDL-C, HDL-C and apolipoproteins AI, AII and B) were significantly increased on treatment as were levels of Factor X, SHBG and caeruloplasmin whereas antithrombin III decreased. In 38 of the 40 treatment cycles, ovulation was suppressed. In one cycle serum oestradiol and progesterone levels showed a typical ovulatory pattern and in another there was evidence of follicular activity without ovulation. Serum EE concentrations showed a similar pattern in both treatment cycles showing that co-administration of NET did not affect EE metabolism. Serum NET levels were higher in the 6th than in the first treatment cycles. On comparing pharmacodynamic and pharmacokinetic parameters, the only statistically significant correlations were between the percentage change in triglycerides and SHBG and serum NET, but not EE concentrations, and between apolipoproteins AI and serum EE.

Intra- and interindividual variations in contraceptive steroid levels during 12 treatment cycles: no relation to irregular bleedings

During one year of treatment with oral contraceptives containing 30 micrograms ethinylestradiol and 150 micrograms desogestrel (EE/DG) or 30 micrograms EE and 75 micrograms gestodene (EE/GSD), the serum concentrations of EE, 3-keto-desogestrel (KDG) and GSD were determined on day 1, 10 and 21 of the 1st, 3rd, 6th and 12th cycle. The areas under the time-versus-concentration curves were calculated from the levels before and 0.5, 1, 1.5, 2, 3, 4 and 24 hours after intake of a tablet. There were large intra- and interindividual variations both revealing coefficients of variation (C.V.) between 25% and 80% (EE),, 30% and 50% (KDG) and 30% and 65% (GSD). During each cycle, the EE levels increased significantly between day 1 and 10 by 70% on average reaching a steady-state, while the progestogen concentrations rose by 100% (KDG) and 150% (GSD) up to a steady-state between day 10 and 21. After reaching the steady-state, the C.V. were generally lower. The ratios between the levels of EE and the progestogens showed still higher variations indicating different influences on the estrogen and progestogen component. There was no correlation between the steroid levels and weight, height or age. In spite of the large intraindividual variations, most of the women showed a distinct pattern of the levels of EE and the progestogens throughout the year of treatment indicating a genetic or acquired predisposition. The difference in the average AUC of EE, KDG and GSD between the women was 300% at most. During the first cycle of treatment with EE/DG and EE/GSD, about half of the women recorded intermenstrual bleedings which decreased thereafter. There was no relation between the occurrence of irregular bleedings and the average serum levels of EE and the progestogens of the individual women, neither during the first cycle nor during the whole treatment period of 12 cycles. It is concluded that spottings or breakthrough bleedings during treatment with oral contraceptives are not dependent on a distinct pattern of the serum levels of EE and the progestogen.

Pharmacokinetics of oestrogens and progestogens

There are large inter- and intra-individual variations in the serum concentrations of natural and synthetic sex steroids irrespective of the route of administration. Oral ingestion of steroids has a stronger effect on hepatic metabolism than parenteral administration, as the local concentration in liver sinusoids are 4-5 times higher during the first liver passage. Oestradiol and oestrone are interconvertible, dependent on the local concentrations in liver and target organs, and oestrone sulphate serves as a large reservoir. The oestrone/oestradiol ratio has no physiological significance, as oestrone is only a weak oestrogen. Oestrone is both a precursor and a metabolite of oestradiol. Oestriol is extensively conjugated after oral administration. Therefore, the oestriol serum levels are similar after oral intake of 10 mg and after vaginal application of 0.5 mg oestriol resulting in similar systemic effectiveness. Conjugated oestrogens can easily enter the hepatocytes but are hormonally active only after hydrolyzation into the parent steroids. Ethinylestradiol which exerts strong effects on hepatic metabolism and inhibits metabolizing enzymes, should not be used for hormone replacement therapy. Among the progestogens, the progesterone derivatives have less effects on liver metabolism than the norethisterone derivatives (13-methyl-gonanes and 13-ethyl-gonanes). The highly potent 13-ethyl-gonanes are effective at very low doses, because of a slow inactivation and elimination rate due to the ethinyl group.

Pharmacokinetic drug interactions with oral contraceptives

Oral contraceptive steroids are used by an estimated 60 to 70 million women world-wide. Over the past 20 years there have been both case reports and clinical studies on the topic of drug interactions with these agents. Some of the interactions are of definite therapeutic relevance, whereas others can be discounted as being of no clinical significance. Pharmacological interactions between oral contraceptive steroids and other compounds may be of 2 kinds: (a) drugs may impair the efficacy of oral contraceptive steroids, leading to breakthrough bleeding and pregnancy (in a few cases, the activity of the contraceptive is enhanced); (b) oral contraceptive steroids may interfere with the metabolism of other drugs. A number of anticonvulsants (phenobarbital, phenytoin, carbamazepine) are enzyme-inducing agents and thereby increase the clearance of the oral contraceptive steroids. Valproic acid has no enzyme-inducing properties, and thus women on this anticonvulsant can rely on their low dose oral contraceptive steroids for contraceptive protection. Researchers are now beginning to unravel the molecular basis of this interaction, with evidence of specific forms of cytochrome P450 (P450IIC and IIIA gene families) being induced by phenobarbital. Rifampicin, the antituberculous drug, also induces a cytochrome P450 which is a product of the P450IIIA gene subfamily. This isozyme is one of the major forms involved in 2-hydroxylation of ethinylestradiol. Broad spectrum antibiotics have been implicated in causing pill failure; case reports document the interaction, and general practitioners are convinced that it is real. The problem remains that there is still no firm clinical pharmacokinetic evidence which indicates that blood concentrations of oral contraceptive steroids are altered by antibiotics. However, perhaps this should not be a surprise, given that the incidence of the interaction may be very low. It is suggested that an individual at risk will have a low bioavailability of ethinylestradiol, a large enterohepatic recirculation and gut flora particularly susceptible to the antibiotic being used. Two drugs, ascorbic acid (vitamin C) and paracetamol (acetaminophen), give rise to increased blood concentrations of ethinylestradiol due to competition for sulphation. The interactions could have some significance to women on oral contraceptive steroids who regularly take high doses of either drug. Although on theoretical grounds adsorbents (e.g. magnesium trisilicate, aLuminium hydroxide, activated charcoal and kaolin) could be expected to interfere with oral contraceptive efficacy, there is no firm evidence that this is the case. Similarly, there is no evidence that smoking alters the pharmacokinetics of oral contraceptive steroids. These agents are now well documented as being able to alter the pharmacokinetics of other concomitantly administered drugs.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of 17 alpha-ethynylestradiol in humans

17 alpha-Ethynylestradiol is extensively sulfated but the sulfate is thought to primarily be a storage form of this estrogen. 2-Hydroxylation is clearly the major oxidative reaction, and the 2-hydroxy derivative is further transformed by methylation and glucuronidation prior to urinary and fecal excretion. Alterations in the rate of 2-hydroxylation can have major effects on the pharmacokinetics and effectiveness of 17 alpha-ethynylestradiol as a contraceptive. The major human catalyst of the 2-hydroxylation reaction is liver microsomal cytochrome P-450 IIIA4. Lesser amounts of this enzyme are found in other tissues such as the intestine and may contribute to overall clearance of the orally administered contraceptive. In individuals with very low amounts of this enzyme other forms of cytochrome P-450 may make some contribution. Levels of cytochrome P-450 IIIA4 vary widely among individuals and can explain the variation in rates of 17 alpha-ethynylestradiol 2-hydroxylation. The known inducibility of the enzyme by barbiturates and rifampicin explains their effects in enhancing 17 alpha-ethynylestradiol clearance and reducing the effectiveness of the drug. Mechanism-based inactivation of cytochrome P-450 IIIA4 can be seen with 17 alpha-ethynylestradiol and other 17 alpha-acetylenic steroids, and the progestogen gestodene appears to be unusually active in this regard. Other unknown factors may also modulate levels of cytochrome P-450 IIIA4 and its ability to catalyze 17 alpha-ethynylestradiol 2-hydroxylation.