With the observation in 1984 that the mechanism of action of CPT was through topoisomerase I inhibition [1], renewed interest in the drug led to the development of a variety of analogs, some of which had higher potency than the parent drug. Some derivatives are water soluble such as CamptosarÂ® (irinotecan, CPTÂ¬-11) and HycamptinÂ® (topotecan), which are currently approved for use in the USA for colon and ovarian cancers, respectively. Studies have shown that substitutions at the C-9 and C- 10 positions enhance activity, and may confer water solubility. In general, analogs that are water-soluble have reduced anti-cancer activity in preclinical models.