Ring-A reduction of medroxyprogesterone acetate [17 alpha-acetoxy-6 alpha-methyl-4-pregnene-3,20-dione (MPA)] in biological systems

McCoubrey L, Freire C, W. Basit A, Conlan RS, Gonzalez D. Progestogens Are Metabolized by the Gut Microbiota: Implications for Colonic Drug Delivery

Following oral administration, the bioavailability of progestogens is very low and highly variable, in part due to metabolism by cytochrome P450 enzymes found in the mucosa of the small intestine. Conversely, the mucosa in the colon contains much lower levels of cytochrome P450 enzymes, thus, colonic delivery of progestogens may be beneficial. Microbiota in the colon are known to metabolize a great number of drugs, therefore, it is important to understand the stability of these hormones in the presence of colonic flora before developing formulations. The aim of this study was to investigate the stability of three progestogens: progesterone, and its two synthetic analogues, medroxyprogesterone acetate (MPA) and levonorgestrel (LNG), in the presence of human colonic microbiota. Progesterone, MPA, and LNG were incubated in mixed fecal inoculum (simulated human colonic fluid) under anerobic conditions. Progesterone was completely degraded after 2 h, whereas levels of MPA and LNG were still detectable after 24 h. The half-lives of progesterone, MPA, and LNG in fecal inoculum were 28, 644, and 240 min, respectively. This study describes the kinetics of colonic microbial metabolism of these hormones for the first time. MPA and LNG show promise for delivery to the colon, potentially improving pharmacokinetics over current oral delivery methods.

A Comparative Pharmacokinetic Study of Micronized Estradiol Valerate Administered Alone and in Combination with Medroxyprogesterone Acetate in Postmenopausal Women

The objective of this study was to evaluate a possible pharmacokinetic interaction between 17beta-estradiol (E2) and medroxyprogesterone (MP) when administered together in a combined tablet because both hormones have common metabolic routes of biotransformation. The study assessed the mean pharmacokinetics parameters of E2 found after 1-dose administration of 2 different tablets containing E2, 1 containing 2 mg of micronized 17beta-estradiol valerate (E2V) and the other, administered after 2 weeks, 2 mg of E2V in combination with 5 mg of medroxyprogesterone acetate (MPA). The subjects were 15 healthy postmenopausal women with normal laboratory and clinic tests. The study was randomized, double blind, crossover, with 2 periods and 2 sequences. The blood samples were obtained at 0, 1, 2, 3, 4, 6, 8, 10, 12, and 24 hours after each administration. The E2 serum concentrations were determined by electrochemoluminiscence assay. From these data, the following pharmacokinetic parameters were calculated for E2 alone and E2 in combination with MPA (E2V/MPA): Cmax = 104.89 +/- 26.96, 103.27 +/- 44.40; AUC0-24 =1900.30 +/- 392.23, 1783.70 +/- 756.39; AUC0-infinity = 5576.06 +/- 4065.87, 5317.89 +/- 3702.54; ka = 1.06 +/- 0.31, 1.09 +/- 0.13; t1/2 = 35.65 +/- 20.62, 36.12 +/- 18.04; MRT = 16.29 +/- 8.77, 16.27 +/- 4.88; V/F = 16.29 +/- 8.76, 16.27 +/- 4.88. No significant differences between the pharmacokinetic parameters of E2 and E2/MPA were found, which led us to conclude that there is no pharmacokinetic interaction.

Modulation of [3H]flunitrazepam binding by natural and synthetic progestational agents

Progesterone is metabolized by ring-A reduction with subsequent oxidoreduction to 3 alpha-hydroxy-5 alpha-dihydroprogesterone (3 alpha-OH-5 alpha-DHP), a naturally occurring metabolite that has been shown to enhance [3H]flunitrazepam ([3H]FNZ) binding. Medroxyprogesterone acetate (MPA), a commonly prescribed progestational agent, is a synthetic progesterone derivative that has a metabolic profile similar to that of progesterone. In this study, the effects of MPA and its ring-A reduced metabolites DHMPA and THMPA on [3H]FNZ binding were investigated. While known modulators of specific [3H]FNZ binding demonstrated expected effects in frozen and fresh rat cortical tissue, 3 alpha-OH-5 alpha-DHP enhanced [3H]FNZ binding only in fresh, not frozen, tissue. Neither DHMPA nor THMPA affected binding, while MPA partially inhibited [3H]FNZ binding by 40%. In addition, five test drugs were used to assess the effect of gender and hormonal status on [3H]FNZ binding. Neither gender nor hormonal status influenced binding. Thus, ring-A reduced metabolites of progesterone but not of MPA enhance [3H]FNZ binding. The clinical implications of these in vitro results are currently under investigation.

Mass spectrometric and high-performance liquid chromatographic studies of medroxyprogesterone acetate metabolites in human plasma

Medroxyprogesterone acetate (MPA) treatment has been shown to exert several beneficial effects in cancer patients. It has been suggested that such effects are due in part to the metabolites derived from MPA in vivo. The first results are reported on the identification of 2 alpha-hydroxy- and 21-hydroxy-MPA, 20-dihydro-MPA, 17 alpha-acetoxy-2 alpha,3 beta-dihydroxy-6 alpha-methylpregn-1,4-dien-20-one and two X,21-dihydroxy-MPAs, one of them presumably being 6 alpha-hydroxymethyl-21-hydroxy-MPA, in patient's plasma by high-performance liquid chromatographic (HPLC), gas chromatographic-mass spectrometric and NMR methods. Additionally, the presence of other metabolites such as di- and tetrahydro-MPAs and 6,21-dihydroxy-MPA, found in urine and other samples, was demonstrated in plasma. For routine clinical examinations an HPLC method is described for determination of, e.g., the unreduced MPA metabolite group in Sep-Pak-ODS column extracts of patients' plasma.

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.

Comparison of mass spectrometry and radioimmunoassay to measure medroxyprogesterone acetate in patients with endometrial cancer

Serum medroxyprogesterone acetate (MPA) was measured by radioimmunoassay (RIA) and gas chromatography-mass spectrometry (GC-MS) in patients with endometrial cancer. Samples were obtained 3, 6 and 24 h after the oral administration of 100 or 200 mg MPA once a day. The levels obtained by GC-MS were lower (median 16-29%) than those obtained by RIA, which is probably attributable to the presence of metabolites interfering with the RIA. Two commercial MPA formulations gave different MPA serum levels by both RIA and GC-MS. The levels obtained by GC-MS were so low that frequently only partial saturation of the endometrial progesterone receptor may be achieved which may explain why high oral doses are needed to produce optimum therapeutic response.

Medroxyprogesterone acetate pharmacokinetics following oral high-dose administration in humans: a bioavailability evaluation of a new MPA tablet formulation

The pharmacokinetics of medroxyprogesterone acetate (MPA) in healthy female volunteers have been investigated following oral administration of single doses of six different high-dose MPA tablet formulations. Blood samples were obtained over 96 hrs following administration. The plasma was separated and analyzed in duplicate for MPA by radioimmunoassay (RIA) after extraction with petroleum ether. A two compartment open model with first order absorption was computer-fitted to the plasma concentration of MPA. Following oral administration MPA is rapidly transferred from the gastrointestinal tract to the blood circulation with a half-life of the absorption process of 15-30 min. The peak plasma concentration is reached 1-3 hrs after administration, and the biological half-life of MPA is 40-60 hrs. Following administration of 1000 mg MPA the areas under the plasma concentration-time curves (AUC 0-infinity) were calculated to (mean and S.E.): 3357 (438) nmol/l and 2403 (245) nmol/l for Leo formulation A and Farlutal, respectively (P less than 0.02). Following administration of 500 mg the areas were: 2325 (389) nmol/l, 1793 (312) nmol/l, 1778 (239) nmol/l, 1178 (209) nmol/l, and 556 (89) nmol/l for Gestapuran, Leo formulation A (P = n.s.), Leo formulation B (P = n.s.), Provera (P less than 0.001), and Lutopolar (P less than 0.001), respectively. The in vitro dissolution rates of MPA from the tablet formulations were determined and compared with the results of the bioavailability studies, indicating that a rapid dissolution rate as well as the particle size of MPA are two important factors to ensure optimal absorption of MPA from the gastrointestinal tract.

Treatment of metastatic breast cancer patients with different dosages of megestrol acetate; dose relations, metabolic and endocrine effects

Megestrol acetate (MA) is of therapeutic value in breast cancer patients. This study was designed to evaluate the effects of different dosages of MA on endocrine events potentially influenced by the drug in relation to plasma level of MA and clinical effects in patients with advanced breast cancer. Eighteen postmenopausal patients were randomly distributed over six groups to receive daily 90, 180 or 270 mg of MA (niagestin) orally in a cross-over study consisting of 3 periods of 6 weeks. Complete remission was observed in 1 patient, partial remission in 9, no change in 4 and failure in 4 patients. During the 18 weeks of treatment plasma levels of MA gradually increased, irrespective of the dose administered. Significant rises of the basal and TRH-stimulated plasma PRL and basal insulin levels were observed, whereas LH and FSH, estradiol, SHBG and the pituitary-adrenal axis were suppressed. None of these metabolic effects showed a correlation with the clinical response. We concluded that treatment of metastatic breast cancer with 180 mg MA/day is effective and causes minimal adverse effects.

Studies on the role of intestinal bacteria in metabolism of synthetic and natural steroid hormones

Administration of antimicrobial agents to subjects taking oral contraceptives has been reported to lead to contraceptive failure and subsequent pregnancy. In women taking oral contraceptives antimicrobial agents could have an effect on both endogenous hormone levels and on the metabolism of the exogenously administered steroids. To investigate these possibilities, antimicrobial agents were administered for short periods to normal women taking various steroid drugs: Megestrol acetate (MA), medroxyprogesterone acetate (MPA), norethisterone (NET), a combination of NET and ethinylestradiol (EE) or a combination of lynestrenol and EE. During ampicillin administration the 24-h morning plasma concentrations of MA, MPA and NET were increased compared to the control values. In the MA and MPA experiments the afternoon values were determined and also found to be increased. In the subjects taking oral contraceptives plasma EE concentration showed a tendency to decrease during ampicillin administration on the third, fourth or fifth morning of ampicillin administration, but was never lower than the pretreatment values. In other experiments plasma estrone (E1) and estradiol (E2), urinary total E1, E2 and estriol (E3) and fecal unconjugated and conjugated E1, E2 or E3 were determined by RIA before, during and after administration of oxytetracycline (2 X 500 mg/day for 5 days) to 5 young male subjects. Furthermore urinary and fecal estrogens were determined in 1 male subject after administration of erythromycin for 6 days and in 2 normally menstruating women after tetracycline and trimethoprim administration, respectively. During treatment with antimicrobial drugs an increase in the excretion of fecal conjugated and, with the exception of the oxytetracycline experiments, also of unconjugated estrogens paralleled a decrease in urinary estrogen excretion, especially for E2 and E3. In both urine and feces the E1/E2 and E1 + E2/E3 ratios increased due to diminished reductive metabolism of estrogens in the gut. No significant effects on plasma unconjugated estrogen concentrations were observed. The results suggest that the intestinal bacterial flora plays a significant role in estrogen metabolism. However, further studies are necessary, because our results do not explain why administration of antibiotics may cause contraceptive failure.

Inactivation of contraceptive steroid hormones by human intestinal clostridia

Steroid hormones reduced in ring-A are devoid of hormonal activity. In metabolic experiments we found that human fecal flora reduced the delta 4-3-keto structure of natural progestins to 3 alpha-hydroxy, 5 beta-steroid metabolites (3 alpha,5 beta) and of synthetic progestins to a mixture of 3 alpha,5 beta and 3 beta,5 beta compounds. 3 alpha,5 beta-Reductase was synthesized by Clostridium paraputrificum and had a strong affinity for natural progestins such as progesterone. 3 beta,5 beta-Reductase was synthesized by Clostridium innoculin and had a stronger affinity for synthetic progestins. A third enzyme, 3 beta,5 alpha-reductase, was synthesized by St. Luke's strain 209 (Clostridium species "J-1") but was only observed when pure cultures were used. Ring-A reduction of synthetic progestins was 3 to 10 times slower than that of natural progestins, thus explaining the pharmacological superiority of synthetic progestins over naturally occurring analogs.

Plasma concentrations of megestrol acetate and medroxyprogesterone acetate after single oral administration to healthy subjects

Plasma concentrations of megestrol acetate (MA) were measured by radioimmunoassay (RIA) after a single oral dose of 60 mg either in the form of one tablet, or four 15 mg tablets, to 10 women 21-40 years old using a cross-over design. No statistically significant difference between the two preparations was observed with respect to plasma concentrations, the area under the curve from 0 to 24 h or the maximum concentration (c(max)). For comparison, data are presented on the plasma level of medroxyprogesterone acetate (MPA) following a single oral dose of 100 mg given using a cross-over design in two different tablet forms to 10 healthy men, when no significant difference was observed for these parameters. The mean c(max) for MA after 2.6 h was 43.9 ng/ml (range 21.7-87.7 ng/ml), whereas that for MPA at 3.1 h was 13.1 ng/ml (range 4.4-29.5 ng/ml) despite the higher dose. After 24 h immunoreactive MA and MPA ranged from 9.6 to 29.0 ng/ml and from 0.2 to 4.0 ng/ml respectively. Moreover, it was found that petroleum ether extraction gives the most specific result by RIA, although considerable amounts of metabolites are still co-estimated. By comparison with selected ion monitoring using GC-MS, metabolite interference in RIA increases with time after administration of the steroids and is considerably greater for MPA than for MA. It is concluded that after oral administration the relative bioavailability of MA is significantly better than that of MPA.