Percentage of MDA-MB-435 human breast cancer cells remaining in vitro 72 hours after drug, prodrug, or hemiorthoester application versus the initial concentrations applied. Compounds were: (a) CPT, CPT-prodrugs 1−4, and hemiorthoesters I′-1 and I′-2 (inserts); (b) DB-67, DB-67 prodrugs 5 and 6, and hemiorthoesters I′-5 and I′-6 (inserts)