Pharmacokinetics, hydrolysis and aromatisation of norethisterone-3-oxime in female cynomolgus monkey

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Abstract
Norethisterone-3-oxime (NETO) was administered to 3 female cynomolgus monkeys intragastrically and, after a wash-out period of 2–5 weeks, intravenously at a dose of 1 mg/kg. The radioactive dose of tritiated NETO was 20 μCi/kg for both treatments. For i.v. injection, a 30% propylene glycol/water solution and for i.g. administration an aqueous microcrystalline suspension was used. Excretion of radiolabel in urine and feces was followed for 5 days and plasma samples were obtained up to 2 days after administrations. In all samples (urine, feces and plasma) radioactivity was determined. Extracts from plasma samples were subjected to HPLC separation of drug and metabolites, as well as NETO and NET (metabolite of NETO after hydrolysis of the oxime group) levels were determined. In addition, EE2 (ethinylestradiol, A-ring aromatised metabolite of NET) levels were estimated using a specifically designed HPLC system for separation. Quantification of EE2 was achieved by radioimmunoassay (RIA) of specific eluate fractions. The results demonstrate that [3H]-NETO was absorbed completely at a dose level of 1 mg/kg, and excreted predominantly via the kidneys. A urinary to fecal excretion ratio of 1.5 (i.v.) or 1.0 (i.g.) was found. Renal excretion of total radiolabel proceeded with a half-life of about 0.8 (i.v.) or 1.1 (i.g.) days. Balances were incomplete, probably due to technical reasons. Orally administered NETO was highly bioavailable (84.0 ± 16.9% of dose) but rapidly cleared from plasma (total clearance corresponded to 97% of plasma liver flow). The clearance from plasma is equivalent to the metabolic clearance because almost no unchanged NETO is excreted. Extensive metabolism of the parent drug was observed leading to at least two pharmacologically active metabolites (NET, EE2). The main progestogenic metabolite was NET reaching similar high plasma levels as NETO. EE2 turned out to be a metabolite of NETO and a conversion rate of below 0.5% of dose was estimated. However, due to its high estrogenic potency EE2 might contribute to the overall pharmacological pattern of NETO in the cynomolgus monkey.