The retinoblastoma tumor suppressor protein, Rb, is a fundamental regulator of the mammalian cell cycle, and most human tumor cells present inactivation or interference of the Rb-regulatory pathway. By specifically binding and phosphorylating Rb, the serine-threonine kinase Raf-1 initiates a cascade of events that eventually leads to its and inactivation, which in turn leads to cell growth. Our search for small molecule inhibitors of Rb-Raf-1 binding has identified two benzylisothiouroniumsalts [NSC-35400 (1), IC50 100 nM; NSC-35950 (2), IC50 1 µM] from a screen of a 1,981 compound library. Various analogues of the hits were then synthesized and evaluated for their ability to disrupt Rb-Raf-1 binding. We have identified a series of benzylisothiourea, arylisothiourea, benzylguadinium and benzylthioimidazole derivatives as potent disruptors of the Rb-Raf-1 binding with IC50 values ranging from 80 to 500nM. One of these 2,4-dichlorobenzylisothiourea, 3a, suppresses human tumor growth in nude mice when administered orally at a dose of 150 mg/kg daily. The benzylisothioureas and analogues are the basis for a new strategy for anticancer drug development, and provides proof of principle that small molecules can be used to halt tumor growth by disrupting the Rb-Raf-1 interaction.

AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics-- Oct 22-26, 2007; San Francisco, CA