Bioavailability of norethisterone acetate alone and in combination with estradiol administered in single or multiple oral doses to postmenopausal women

Objectives: Twenty-four postmenopausal women were randomly allocated to a c ross-over trial for an investigation of the pharmacokinetics of norethister one acetate (NETA; 0.5 mg), administered alone or in combination with estra diol (E2; 1 mg), both after a single oral dose. In a second trial, the abov e combination df 0.5 mg NETA with 1 mg E2 was administered daily for 28 day s. Methods: Plasma levels of NET, E2, estrone (El) and estrone sulphate fra ction containing an admixture of estrone glucuronide (E1S/E1G) were measure d by radioimmunoassay at various intervals up to 72 h in the first trial an d at the same intervals after the 28th day in the second trial. Results: In the first, single-dose trial, pharmacokinetic parameters of NET were simil ar for NETA administered alone and its combination with E2. There was no st atistically significant difference in the area under curve values AUC(0-24) and AUC(0-infinity) and no apparent major differences were observed for ot her pharmacokinetic parameters. No carry-over effects due to the cross-over design were seen. The multiple dosage in the second trial did not cause an y major changes in the pharmacokinetic parameters of NET, except for the AU C(0.24) and AUC(0-infinity) values which were significantly higher than tho se seen in the first trial. The levels of E2 exhibited, shortly after the i ntake of E2, a rapid burst. The levels gradually decreased to a nadir follo wed by an increase to the main peak and by the subsequent elimination phase . The difference between the peak and nadir levels was significant (P < 0.0 5) in the second, multiple-dose trial. This bimodal pattern was not observe d in earlier studies. The main metabolite of E2 was E1S/E1G, followed by E1 , as could be seen from the AUG,, values. These were, in both trials, appro ximate to 300 and 7-times higher for the E1S/E1G and El, respectively, than those for E2. For all analytes, the AUC(0-24) values were significantly hi gher in the second trial than those found in the first trial, indicating ac cumulation upon repeated administration. Pharmacokinetics of all analytes r emained linear in the second trial, as follows from the statistically estab lished equality of AUC(0-24) found in the second, multiple-dose trial with AUC(0-infinity) in the first, single-dose trial. The absorption half-life a nd t-max values of E1S/E1G appeared to be considerably shorter than those o f El in both trials. Conclusions: The bioavailability of NET was not influe nced by its combination with 1 mg E2. The most abundant metabolite of E2 wa s the E1S/E1G fraction, which may have served as the main source of E2 and other estrogens due to metabolic interconversions during the absorption and elimination phases. (C) 1999 Elsevier Science Ireland Ltd. All rights rese rved.