I think part of why you don't see prodrug-and-releaser combinations in one molecule very much is because you need a lot more of the monoamine precursor to have a useful effect than 1 mol equivalent. For e.g. MDMA, with a molecular weight of 193 g/mol and 5HTP m.w. 220 g/mol that means a ~100mg MDMA dose would only produce about 114mg of 5HTP). I was under the impression 5HTP doses needed to be at least twice that for pre-loading.
I believe that focusing instead on high-protien (make sure its balanced in all the amino acids you need, or even better use multiple sources of protien...) snacks and meals is probably more useful all-around than focusing on any single neurotransmitter precursors. A snack of dried fruit, mixed nuts, and some pepperoni is much better tasting than a fistful of gelatin capsules full of broad beans, anyway. As a bonus, you are likely to get a bunch of other food based nutrients along for the ride, which is always good for recovery - and health in general.
I also have suspicions that stapling a rather quite polar 5HTP amino acid onto otherwise active compounds will affect PK and distribution, maybe negatively. The unmasked phenolic OH is especially concerning because it will probably be ionized in the duodenum - where drugs would normally otherwise assume an uncharged form. A charged drug cannot diffuse throguh the fatty cell membranes and as a result, will not be absorbed into the hepatitic portal vein. This means oral BA can be expected to be quite low. (c.f. morphine, 5-OH-DMT)
L-DOPA is even worse in terms of pharmacokinetics due to polarity. The double aromatic hydroxide aka catechol arrangement is super duper water soluble. The only reason L-DOPA can make it into the brain is through active transport. Peripherally administered IV dopamine is actually used in cases of... uh, something to do with the heart, I'm not a cardiologist. But it's used, all right. However, dopamine itself won't cross the BBB so pure DA injecions outside the brain don't have a reinforcing effect.
On the topic of "why lysine in lisdexamphetamine" - I can hear the audience asking that question
I suspect that lisdexamphetamine was the most promising amphetamine prodrug selected after a quick screening effort; there are after all only 20-some amino acids that are common. Having some lab guy plug dexedrine on a Merrifield resin and whack it a few times with all the amino acids they have in their storeroom, and feeding the resulting compounds to rats to see how they behave, seems to me exactly the sort of primary research that gets your name on a patent, and puts smiles on the executives who sign your paycheck. (That's just a guess, though... it would be nice, however, to read a story straight from the farm - so to speak...)
