It isn't. Sixteen percent and 14% of the pixantrone and comparator arm patients, respectively, underwent prior stem-cell transplants, according to the EXTEND study data made available by Cell Therapeutics. 6. Where in the world did pixantrone actually work? The EXTEND study was an international study but most of the patients were enrolled outside the U.S. Geographic disparity of response to pixantrone could become a troublesome regulatory issue, if for instance, U.S. patients are completely missing from those who recorded complete responses or unconfirmed complete responses. 7. Is pixantrone less cardio-toxic? Pixantrone belongs to the anthracycline class of chemotherapy drugs, which are well known to cause heart failure at high doses and/or prolonged exposure. Pixantrone is designed to be less toxic to the heart, allowing it to be used in patients that have been treated previously with other anthracyclines, according to Cell Therapeutics. This claim will likely be a review issue on Feb. 10, with the FDA and the panel experts going over pixantrone's safety data closely. Almost 9% of pixantrone patients experienced a serious cardiac event in the EXTEND study compared to 4.5% for patients in the comparator arm, according to the data presented to date by Cell Therapeutics. If FDA and panel experts reach the conclusion that pixantrone is not more heart safe than other anthracyclines, Cell Therapeutics loses one of the most important data points it needs to market the drug effectively (if approved). The last FDA advisory panel for blood cancer drugs in September was very rough, with Genzyme ( GENZ) and Vion Pharmaceuticals both receiving negative votes. Allos Therapeutics ( ALTH) emerged victorious, but even there, the debate and review were no holds barred. Cell Therapeutics best be prepared for a brawl if it's going to walk away with a vote for approval. -- Reported by Adam Feuerstein in Boston. Follow Adam Feuerstein on Twitter.