I dont see why it shouldn't be orally bioavailable. It looks like foods as far as the body is concerned (amino acids). It would get actively transported but could it go across BBB to get to the brain and act on GABArgics, I don't know. Unlike digestive tract, for BBB there are more discriminating amino acids transport system there. It won't just transport any AA (it would be disaster if it does: as you could imagine: getting high on foods-like amino acids everytime one eats!) Aromatic AAs have one active transport system, aliphatic AA have their own ..etc etc). Maybe it can get across as "stealth" Proline. Tiagabine may not have such problem. On the other hand, as you said Tiagabine could get metabolized to Nipecotic acid and diarylallkyledene acetic acid. Whether that could take place in the brain (eg by MAO-B metabolism)..who knows?
