RGB-286638 is a novel CDK inhibitor with IC50s of 1 nM/2 nM/3 nM/4 nM/5 nM for cyclin T1-CDK9/cyclin B1-CDK1/cyclin E-CDK2/cyclin D1-CDK4/cyclin E-CDK3/p35-CDK5 respectively; less potent against cyclin H-CDK7 and cyclin D3-CDK6.
RGB-286638 inhibited several tyrosine and serine/threonine non-CDK enzymes, i.e. GSK-3, TAK1, AMPK, Jak2, MEK1. RGB-286638 demonstrated equimolar (50nM) activity in freshly isolated tumor cells from MM patients, while manifesting less cytotoxicity in healthy donor PBMCs. Treatment with RGB-286638 resulted in rapid downregulation of p-RNAPII S2/S5 across all tested MM cell lines. RGB-286638 (50nM) reduced the phosphorylation of Rb at S807/811 in MM.1S and MM.1R cells, but had no effects on p-Rb S780.