Histone Deacetylase Inhibitors Show Selective Inhibition of Aromatic Amino Acid Hydroxylases

Aromatic amino acid hydroxylases (AAAHs) like phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH), and tryptophan hydroxylases (TPH) 1 and 2 are enzymes that require an iron atom and tetrahydrobiopterin to function. They play key roles in producing serotonin, dopamine, noradrenaline, and adrenaline, and are involved in various disorders. 

Many TPH inhibitors have been developed and tested in clinical trials. One of the earliest, 4-chloro-l-phenylalanine (L-PCPA), has been widely used since the 1950s in research. TPH inhibitors vary in their chemical classes and modes of action, with some existing drugs showing potential as AAAH inhibitors. Panobinostat, a nonselective histone deacetylase inhibitor, also inhibits PAH and TH, suggesting new therapeutic uses for histone deacetylase inhibitors (HDIs).

To test whether AAAHs are targeted by other HDIs and explore their binding selectivity, we screened an in-house library of 307 HDIs and structural analogs against human TPH1. Compounds that showed stabilizing effects and had more than 50% enzyme inhibition were subjected to dose–response testing against PAH, TH, and TPH2. We show that panobinostat and structurally related compounds such as TB57 and EG22, which are also cinnamoyl hydroxamates, bind to human TH, PAH, and TPH and inhibit these enzymes at low micromolar concentrations. Binding of these inhibitors to PAH in intact cells was validated by using a cellular thermal shift assay. Our results highlight panobinostat and structurally related compounds as promising new AAAH inhibitors worthy of further development.

Aromatic Amino Acid Hydroxylases as Off-Targets of Histone Deacetylase Inhibitors