The most desirable formulation for anti-cancer drugs requiring frequent administration would be a tablet, similar to the treatment of many other diseases. Oral treatment would not only reduce medical treatment costs but also allow cancer patients to spend less time in hospital and more time with their family and friends. In fact, cancer patients prefer oral treatment if it is as effective as parenteral treatment [1]. In addition, a tablet is most appropriate for the long-term frequent treatment of cancer cells. Although many cytotoxic drugs effectively inhibit tumor cells only in particular processes of the cell cycle, such as the S phase and M phase, not all tumor cells are present in the tissues during such processes. In order to be effective, cytotoxic drugs should be administered continuously over a period of time which is longer than one cell cycle. However, the daily administration of cytotoxic drugs is limited because of the accumulation of non-tumor-selective toxicity. When given orally, cytotoxic drugs generally lead to high concentrations of the drug, predominantly at the local sites, such as the intestine and liver, and this results in treatmentassociated adverse effects on these organs. On the other hand, oral doses of cytotoxic drugs are generally larger than parenteral doses, because drugs given orally are often subjected to first-pass metabolic effects in the intestinal tract and liver. Unfortunately, a reduction in dose to spare such local organ toxicity results in an insufficient efficacy compared with that of parenteral administration.In 1986, the present authors commenced studies into the design and synthesis of orally available cytotoxic compounds with high tumor selective activity and low myelotoxicity or intestinal toxicity, and which therefore would be safe for daily oral treatment over prolonged periods of time in the comfort of the home. The strategy employed, specifically identifying new cytotoxic compounds with high tumor-selective action, included the design and synthesis of a prodrug that, from oral administration, generates an active cytotoxic component from enzymes