Factors affecting the duration of action of the injectable contraceptive norethisterone enanthate
Fotherby, K.
Contraceptive Delivery Systems 2(3): 249-257
1981
ISSN/ISBN: 0143-6112 PMID: 12278912 Document Number: 385651
Experimentation was conducted with 15 human female volunteers to assess the pharmacokinetics of injectable NET-EN (norethisterone enanthate) in determining intersubject variations. There was a large variation among the women as to the concentration of NET-EN found in the blood at varying times following injection. This variation in blood levels resulted from variations both in the rate at which the norethisterone entered the circulation from the injection site and in the rate of metabolism of the drug. Graphed results illustrate this point. There was no correlation between the rate at which the NET-EN was metabolized and the height, weight, or ponderal index of the women. There were also variations in the time necessary for the serum levels of NET-EN to become undetectable and for ovulation to return. The study results showed that the bioavailability of NET-EN is not complete and there is a wide variation among women as to the amount metabolized. Factors which influenced the uptake of the drug after injection are: 1) volume of the drug injected and concentration of the formulation; 2) the relative surface area of the depot and the drug; 3) the viscosity and nature of the drug; and 4) the rate of dissolution. Some of the difficulties encountered with intramuscular injection of solutions and suspensions may be overcome by using biodegradable polymer-steroid complexes.