(5Z,2E)-CU-3 is a potent and selective inhibitor of DGKα isozyme with an IC50 value of 0.6 μM and inhibition of DGKα by competing ATP with a Km value of 0.48 mM. (5Z,2E)-CU-3 is selective, targeting the catalytic region of DGKα, but does not act on the regulatory region. (5Z,2E)-CU-3 has antitumor and immunogenic effects, promoting apoptosis of cancer cells and activation of T cells.