Supplementation of 5-HTP has been shown to be more effective than tryptophan supplementation alone. This additional benefit of 5-HTP supplementation arises because 5-HTP bypasses the cell's L-tryptophan's own self-regulation on the IDO enzyme, in which it upregulates the activity of IDO (discussed in next section) to maintain body homeostasis of tryptophan[6] and it bypasses the tryptophan hydroxylase enzyme, which is the rate limiting step in serotonin biosynthesis.[7]

Tβ4 is a multifunctional regenerative small peptide containing 43-amino acids, and it is the major G-actin-sequestering molecule in eukaryotic cells.21 Tβ4 has pro-survival and pro-angiogenic properties, protects tissue against damage, and promotes tissue regeneration.22,23 It also plays a key role in corneal, epidermal and cardiac wound healing.21 Tβ4 participates in axonal path-finding, neurite formation, cell proliferation, and neuronal survival.24-26 Our previous studies show that Tβ4 reduces inflammation and stimulates remyelination and improves functional recovery in animal models of experimental autoimmune encephalomyelitis (EAE) and stroke.25,27 In summary, these pleiotropic properties make Tβ4 an ideal candidate for treatment of TBI.
The short half-life (<2h)[16] of 5-HTP may inherently limit the therapeutic potential of 5-HTP,[17] as the systemic 5-HTP exposure levels will fluctuate substantially, even with relatively frequent dosing. Such exposure fluctuations are usually associated with increased adverse event burden, resulting from Cmax drug spikes, and decreased clinical efficacy resulting from sub-therapeutic exposure for large parts of the day. It has been proposed that 5-HTP dosage forms achieving prolonged delivery would be more effective,[17] as is generally the situation with short-acting active pharmaceutical ingredients.[18]
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