Potent Imidazothiazole-based Inhibitor of BRAF V600E Overcomes Acquired Resistance via Inhibition of RAF Dimerization in PLX4032-resistant Melanoma.

ABSTRACT

BACKGROUND/AIM: The B-raf proto-oncogene, serine/threonine kinase (BRAF) V600E mutation is frequent in patients with advanced melanoma. PLX4032, an inhibitor of BRAFV600E kinase, is effective for the treatment of melanoma in BRAF V600E-positive patients; however, resistance eventually develops due to paradoxical activation of the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinases (ERK) pathway resulting from RAF dimerization. In this study, we investigated the inhibitory effects of a novel imidazothiazole-based compound, KS28, on RAF dimerization and resistance to PLX4032 in melanoma. MATERIALS AND METHODS: The effects of KS28 were examined by immunoblotting, cell viability, terminal deoxynucleotidyl transferase dUTP nick-end labeling, reporter-gene, and soft-agar assays. RESULTS: KS28 treatment inhibited RAF dimerization in PLX4032-resistant A375 (A375R) cells, leading to suppression of the MEK/ERK pathway. In addition, KS28 reduced activator protein 1 transactivation in A375R cells, reduced cell viability, and increased DNA fragmentation. Moreover, treatment with KS28 suppressed anchorage-independent growth of A375R cells. Similarly, in an orthotopic tumor xenograft model, KS28 treatment suppressed the growth of tumors formed by A375R cells in BALB/c mice. CONCLUSION: KS28 plays a vital role in overcoming PLX4032 resistance in melanoma by down-regulating the MEK/ERK pathway.