Onconova's MDS Study Fails and I Explain What I Got Wrong.

Editor's note: On Wednesday night, Onconova Therapeutics (ONTX) announced negative results from a phase III study of its experimental drug rigosertib in patients with high risk myelodysplastic syndrome (MDS). Rigosertib plus best supportive care failed to prolong survival in these MDS patients compared to best supportive care alone. Onconova shares are down 39% to $8.40 in Thursday pre-market trading.

I have been writing about Onconova Therapeutics for quite some time and owned the stock. While acknowledging the outcome of the rigosertib high-risk MDS study would be a close call, I believed it would squeak by as a success. In biotech investing, you analyze the data and decide what you think is most likely to occur. Sometimes (a lot of the times) your best guess ends up being wrong. Unfortunately, that's what happened Wednesday night with Onconova.

Why did the rigosertib high-risk MDS trial fail? 

In the first article I wrote on Onconova, I discussed the assumption used to predict survival for patients in the best supportive care arm. Cancer drug studies often fail not because the experimental therapy under-performs, but because the control arm out-performs expectations. For rigosertib to be successful, Onconova needed to make accurate predictions about how long patients with high-risk MDS would live if treated with best supportive care alone. For this study, the company assumed best supportive care patients (the control arm) would have a median overall survival of approximately 17 weeks. In reality, as we learned Wednesday night, these control arm patients reported a median overall survival of about 24 weeks. They lived longer than Onconova predicted.

As reported by Onconova Wednesday, high-risk MDS patients treated with rigosertib plus best supportive care showed a median overall survival of 8.2 months compared to 5.8 months in the best supportive care alone arm. Overall, rigosertib added to best supportive care reduced the risk of death by only 14% compared to best supportive care alone. This survival benefit favoring rigosertib was not large enough to be statistically significant.

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