Discovery and biological evaluation of phthalazines as novel non-kinase TGFβ pathway inhibitors

TGFβ is crucial for the homeostasis of epithelial and neural tissues, wound repair, and regulating immune responses. Its dysregulation is associated with a vast number of diseases, of which modifying the tumor microenvironment is one of vital clinical interest. Despite various attempts, there is still no FDA-approved therapy to inhibit the TGFβ pathway. Major mainstream approaches involve impairment of the TGFβ pathway via inhibition of the TGFβRI kinase. With the purpose to identify non-receptor kinase-based inhibitors to impair TGFβ signaling, an in-house chemical library was enriched, through a computational study, to eliminate TGFβRI kinase activity. Selected compounds were screened against a cell line engineered with a firefly luciferase gene under TGFβ-Smad-dependent transcriptional control. Results indicated moderate potency for a molecule with phthalazine core against TGFβ-Smad signaling. A series of phthalazine compounds were synthesized and evaluated for potency. The most promising compound (10p) exhibited an IC₅₀ of 0.11 ± 0.02 μM and was confirmed to be non-cytotoxic up to 12 μM, with a selectivity index of approximately 112-fold. Simultaneously, 10p was confirmed to reduce the Smad phosphorylation using Western blot without exhibiting inhibition on the TGFβRI enzyme. This study identified a novel small-molecule scaffold that targets the TGFβ pathway via a non-receptor-kinase mechanism.