The pharmacokinetic profile of different dosage (e.g. 300 mg vs. 600 mg) of progesterone administered into the vagina is non-linear. Systemic progesterone concentrations are the same with different dosages, because of local pharmacokinetic processes, such as direct passive diffusion or transport through the local blood circulation or lymph circulation, due to which progesterone will be transported from the vagina to the womb.
Absorption
Micronised progesterone administered into the vagina will be absorbed quickly and stable concentrations in plasma (4 – 12 mg/ml depending on the daily dosage) and a average Cmax at around the 8 hour mark is achieved with less individual fluctuation compared to orally taken medicine.
With a 600 mg daily dose of progesterone administered into the vagina the progesterone concentration in plasma were stable throughout administration times so that the highest average concentration was 11.63 ng/ml.
Distribution
Micronised progesterone administered into the vagina undergoes the first metabolic cycle in the womb, when progesterone distributes primarily or selectively into the womb, causing higher hormone levels in the womb and nearby tissues.
Progesterone is transported via the lymph and blood vessels and approximately 96%-99% is bound to serum proteins, primarily to serum albumin (50%-54%) and transcortin (43%-48%).
Elimination
By administering progesterone into the vagina, the first pass metabolism in the liver can be avoided, which enables concentrations in plasma to remain higher for longer.
95% of progesterone is eliminated from the urine as glycurone conjugated metabolites, mainly as 3 α , 5 β – pregnanediol (pregnandiol).
Biotransformation
Progesterone is metabolised by the liver.
Oral progesterone is excreted via the gallbladder and kidneys, with a half-life of 5 - 95 minutes. It is detectable in urine after 24 hours, and a small amount (8 - 17%) is secreted in the faeces.
After vaginal administration, observable levels of pregnanolone and 5α - dihydroprogesterone are very low, due to the lack of first-pass metabolism.