Transdermal application leads for a slow, steady release into the system; therefore you're body will have more stable blood concentration at all times.
Oral use, leads to huge spikes in your blood concentration. Bioperine would be useful in oral application because it enhances your intestine's ability to 'absorb' nutrients and the like. Bioperine and Grapefruit may inhibit some of the enzymes that degrade formestane in the liver, which lead to a longer half-life.
It's important to remember that the active will degrade from your system based on how concetrated it is: in half-lives. Half of a little bit is still just a little bit. Half of a lot is a lot less than before.
Since oral formestane is poorly absorbed and causes for large spikes in serum concentrations, it will degrade a lot more quickly than transdermal use. Oral half-life is approximately 3 hours. Transdermal half-life (depending on the carrier you use) can be several times longer.