Selectivity and Safety Characterization of a Xanthine–Imidazothiazole Lead Structure: a Novel Tryptophan Hydroxylase Inhibitor of Peripheral Serotonin Synthesis

Serotonin (5-HT), a crucial neurotransmitter and peripheral
mediator,
regulates various physiological processes and is synthesized by tryptophan
hydroxylase 1 (TPH1), the rate-limiting enzyme responsible for its
production. 5-HT overproduction is implicated in multiple diseases,
making TPH1 a promising therapeutic target. However, selectivity remains
a challenge due to the structural similarity of TPH1 with other members
of the aromatic amino acid hydroxylase (AAAH) family, including TPH2,
phenylalanine hydroxylase (PAH), and tyrosine hydroxylase (TH). This
study aimed to evaluate the selectivity and inhibitory potential of
TPT-004, a novel TPH inhibitor, compared with Telotristat (LP778902)
and its prodrug (LX1606). We developed high-throughput fluorescence
assays to evaluate the inhibitory effects of the test compounds on
TPH1, TPH2, PAH, and TH enzymes. TPT-004 demonstrated high selectivity
for TPHs compared to LP778902 and LX1606. Structural analysis based
on a detailed sequence alignment within the AAAH enzyme family, combined
with cocrystal structures of TPH1 and TPH2 bound to different generations
of inhibitors, enhances our understanding of the molecular basis of
inhibitor binding and provides a framework for explaining TPT-004’s
selectivity for TPHs. Selectivity profiling against 97 targets confirmed
that TPT-004 showed minimal off-target interactions, underscoring
its specificity. A dose–range finding (DRF) study in rats assessed
the in vivo safety profile of TPT-004, showing no
adverse effects on survival and body weight at doses up to 400 mg/kg/day.
Hematology parameters remained normal, with only minor liver changes
observed. These results highlight TPT-004’s potential as a
selective and safe TPH inhibitor, offering a promising therapeutic
option for serotonin-related disorders.