In vitro data suggested that topoisomerase I inhibitors could have activity in MDS, and a prolonged exposure appeared to be particularly effective. Topotecan was the first agent of this class explored for this purpose. A phase I study established the maximum tolerated dose (MTD) at 2 mg/m2/day as a continuous infusion for 5 days. Using this dose, a phase II study in patients with MDS and CMML, a complete remission rate of 31% was achieved. These agents are now considered the most effective available to date in the treatment of MDS and constitute standard therapy. Because of the different times when the studies were conducted and the different eligibility criteria used (e.g., patients with secondary MDS not eligible for decitabine, only patients with high-risk MDS eligible for topotecan), comparisons are somehow limited. However, results with topotecan compare favorably to those obtained with both 5-azacytidine and decitabine. The CR rate is the highest achieved with any of these agents. Response duration is shorter than that reported for AZA, but the remission duration for AZA includes hematologic improvements which constitute most of the responses with AZA. In addition, the AZA study continued therapy until the response was lost, while the topotecan study (as well as the decitabine study) administered by design only 4-6 cycles of therapy. Survival was shorter for patients treated with topotecan, but this is not unexpected considering that this population included 50% of patients with CMML and 32% with secondary MDS, both important adverse prognostic characteristics in MDS. The AZA study had the best survival, but it included 37% patients with RA or RAES and only 20% with intermediate-2 or high IPSS. Because the eligibility included patients with greater than 10% blasts, all MDS patients would have had a score of at least 1.5 and thus be classified in the intermediate-2 risk group.