Zytiga works very fast in its pharmacological action of lowering testosterone working within 30 minutes of being taken. Following oral administration Zytiga is absorbed by the small intestine into the bloodstream and starts working within 30 minutes and its effects last longer than 24 hours. It has a half life of 12 hours. So after 12 hours the plasma concentrations have reduced by half which is still an active concentration. So Zytiga is rapidly absorbed and starts inhibiting the enzyme CYP17 which blocks all androgen production within a few hours. The testosterone levels then gradually reduce down to zero over the next few days and have reached their lowest within a week. This brings about total androgen deprivation (TAD) which prevents androgen receptor positive prostate cancer cells from growing and the absence of androgens promotes tumour cell death by apoptosis.
The effects of total androgen deprivation is different in each case. Some men respond quickly to TAD whilst others do not respond at all and their PSA continues to rise. In some cases a sharp rise in PSA is seen initially following Zytiga treatment but then reduces after 2 months. In order to evaluate the efficacy of Zytiga it is best to look at the PSA trend over 3 monthly tests. This gives a clearer idea of the response since any initial increase should have subsided after the second reading and declined further after 3 months.
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