An orally bioavailable inhibitor of cyclin-dependent kinases 8 and 19 (CDK8/19), with potential antineoplastic and chemoprotective activities. Upon oral administration, CDK8/19 inhibitor SEL 120 targets, binds to and inhibits the activity of CDK8/19, which prevents activation of CDK8/19-mediated oncogenic signaling pathways, blocks selective transcription of various tumor-promoting genes, and inhibits proliferation of CDK8/19-overexpressing tumor cells. CDK8/19, serine/threonine kinases involved in the regulation of the cell cycle, are overexpressed in certain cancer cell types and play key roles in tumor cell proliferation.