A method of measuring the efficacy of an anticancer agent in generating abasic (AP) sites in DNA of cancer cells of a subject includes administering to the subject an anticancer agent that generates AP sites in at least one cancer cell and an AP endonuclease inhibitor probe. The AP endonuclease inhibitor probe includes a detection moiety for detecting the probe in the subject. The amount of probe bound to cancer cells of the subject is then measured. The amount of probe bound to cancer cells of the subject is indicative of efficacy of the anticancer agent in generating AP sites in cancer cells of the subject.