The '387 data I saw at ASCO looked compelling, even with the acknowledgement that crucial questions about the drug's efficacy and safety remain unanswered. And YM's valuation is definitely more attractive than what investors are paying to own Incyte ( INCY) today, even if the latter's competing myelofibrosis drug is a few years ahead. @trevor_McMullan tweets, "Can you look into the deaths for the Exelixis (EXEL) drug? Even cancer patients don't want a drug that could kill them." That's true, but remember, the biggest worry that these patients carry with them is that their cancer is going to kill them. When thinking about the risk-benefit of cancer drugs, don't forget that a higher level of safety risk is tolerated because cancer is a killer. Exelixis enrolled 490 patients in the phase II study of cabozantinib. Six patients, or 1%, died for reasons that investigators said were related to the drug and not their underlying cancer. Three of the six patients died due to perforations or bleeding in their gastrointestinal tract. This is a well-understood side effect for drugs like cabozantinib that work (in part) by blocking a protein known as VEGFR2. Roche's blockbuster cancer drug Avastin works similarly and patient deaths due to gastrointestinal perforations, while sad and unfortunate, are an accepted safety risk. The positive buzz over cabozantinib's ability to clear bone lesions in cancer patients is tempered by the drug's relatively high side-effect profile, which forces a majority of patients into dose reductions. With that said, the doctors presenting the cabozantinib data at this year's ASCO meeting said the drug's toxicities were manageable, and no worse than what's seen in other targeted cancer drugs. While I do think the concern over the patient deaths in the phase II cabozantinib study is overblown, I'm not naive enough to believe that this issue is going away any time soon. The next significant clinical update on cabozantinib will come next quarter when Exelixis announces top-line results from the phase III study in medullary thyroid cancer. Keep in mind, however, that patients in this thyroid cancer study were dosed with 175 mg of cabozantinib, which is higher than the 100 mg dose used in the phase II study presented at ASCO. Don't forget: The Biotech Stock Mailbag Live Chat, today at 12 p.m. ET. --Written by Adam Feuerstein in Boston. >To contact the writer of this article, click here: Adam Feuerstein. >To follow the writer on Twitter, go to http://twitter.com/adamfeuerstein. >To submit a news tip, send an email to: firstname.lastname@example.org.