Absorption:
Norethindrone acetate is completely and rapidly deacetylated to norethindrone (NET) after oral administration, and the disposition of norethindrone acetate is indistinguishable from that of orally administered norethindrone. Norethindrone acetate is rapidly absorbed from norethindrone acetate tablets, with maximum plasma concentration of norethindrone generally occurring at about 2 hours post-dose. The pharmacokinetic parameters of norethindrone following single oral administration of norethindrone acetate tablets in 29 healthy female volunteers are summarized in Table 1.
Table 1 Pharmacokinetic Parameters after a Single Dose of norethindrone acetate tablets in Healthy Women |
| Norethindrone acetate tablets (n=29) Arithmetic Mean ± SD |
| Norethindrone (NET) |
| AUC(0-inf)(ng/mL*h) | 166.90±56.28 |
| Cmax(ng/mL) | 26.19±6.19 |
| tmax(h) | 1.83±0.58 |
| t1/2(h) | 8.51±2.19 |
AUC= area under the curve Cmax= maximum plasma concentration tmax= time at maximum plasma concentration t1/2= half-life SD = standard deviation |
Figure 1. Mean Plasma Concentration Profile after a Single Dose of 5 mg Administered to 29 Healthy Female Volunteers under Fasting Conditions
Effect of Food:
The effect of food administration on the pharmacokinetics of norethindrone acetate tablets has not been studied.
Distribution:
Norethindrone is 36% bound to sex hormone-binding globulin (SHBG) and 61% bound to albumin. Volume of distribution of norethindrone is about 4 L/kg.
Metabolism:
Norethindrone undergoes extensive biotransformation, primarily via reduction, followed by sulfate and glucuronide conjugation. The majority of metabolites in the circulation are sulfates, with glucuronides accounting for most of the urinary metabolites.
Excretion:
Plasma clearance value for norethindrone is approximately 0.4 L/hr/kg. Norethindrone is excreted in both urine and feces, primarily as metabolites. The mean terminal elimination half-life of norethindrone following a single dose administration of norethindrone acetate tablets is approximately 9 hours.