CZ48 is an analog of the topoisomerase I inhibitor Camptothecin (CPT). CPT is an alkaloid extracted from the Chinese tree, Camptotheca acuminata. Only the S isomer (the natural form) is biologically active. The intact E lactone ring, which tends to be preserved in an acid environment, is essential for anti-tumor activity. Moreover, the open lactone ring moiety tends to promote toxicity [1-3]. In phase I and II trials conducted from 1970-1972 with the sodium salt of CPT (later shown to be largely inactive) toxicities included myelosuppression, diarrhea and cystitis [2, 3].