A Ru(bpy)₃Cl₂ photocatalyst is applied to the rapid trans to cis isomerization of a range of alkene-containing pharmacological agents, including combretastatin A-4 (CA-4), a clinical candidate in oncology, and resveratrol derivatives, switching their configuration from inactive substances to potent cytotoxic agents. Selective in cellulo activation of the CA-4 analog Res-3M is demonstrated, along with its potent cytotoxicity and inhibition of microtubule dynamics.