The water-insoluble native camptothecin (CPT) caused diarrhea, which proved to be the dose limiting toxicity. Measurements demonstrated very little closed lactone ring CPT in the plasma of subjects receiving this compound [4]. This was later explained by the demonstration that CPT binds to human albumin, an action which promotes opening of the lactone ring [5]. Mouse albumin is much less efficient in this activity, hence explaining the greater antitumor activity observed in mice models.