The Difference Between Agonists, Antiandrogens, and Antagonist in ADT

First line Hormone Therapy (ADT) involves the use of different drugs. These drugs include three different classes of medications with three different modes of action.  They include the GnRH agonists, such as leuprolide, antagonists like Firmagon and the antiandrogens like Casodex. 

The underlying mode of action of the GnRH agonists (e.g., Lupron and Zoladex) is to block the brain from sending signals to the gonads to make testosterone where most testosterone is produced.  The GnRH agonist act by interrupting this signal, ultimately leading to the cessation of testosterone production.   

However, when the GnRH agonists first disrupt the signal the gonads go into a state of overdrive and produce even larger qualities of testosterone (which support the growth of prostate cancer causing a PSA flare).  Over a period the feedback loop between the brain and the gonads breaks down entirely and the production of testicular testosterone halts.

The antiandrogens have a different mode of action.  They work by blocking the hormone receptors in the cells by binding to them and clogging the receptor.  Since antagonists do not disrupt the feedback loop between the brain and the gonads they do not flood the body with testosterone, so there isn’t a PSA flare. 

Actually, antiandrogens can block the adverse effects of the initial flood of testosterone caused by the GnRH agonists.   Antiandrogens should be used for two weeks before the start of any GnRH agonists, to prevent the surge of testosterone from supporting the cancer when the feedback loop between the brain and gonads is disrupted as described earlier in this post. 

In some countries, antiandrogens are used as a monotherapy, or without using any GnRH agonist.  Usually, when antiandrogens are used alone, their dosage is higher than when they are used along with a GnRH agonist.

A much newer ADT drug is now available. This drug, degarelix (Firmagon), which is a GnRH antagonist (not to be confused with an agonist), works very differently than either the agonists or the anti-androgens as it bypasses the time where that testosterone surge occurs. In clinical trials, within one day of receiving degarelix, 90% of the testosterone production was shut down, leaving most men castrated within one or two days.  There is no surge [in testosterone], and castration takes place within 24 to 48 hours.  Becoming castrated takes considerably longer with a GnRH agonist as opposed to a GnRH antagonist.

If you need to quickly drop the levels of testosterone to gain better control of the cancer you should use an antagonist (no antiandrogen will be needed). Antagonists work much faster in getting that testosterone level down. There is no evidence that one is superior to the other, the primary difference is the need to use an antiandrogen and the speed at which castration can be achieved.