Sixty-seven percent of GA101-treated patients experience a serious side effect, including 34% reporting significant drops in white blood cell counts known as neutropenia. By comparison, the rate of serious adverse events associated with chemotherapy was 41 percent, including 15% occurrence of neutropenia.

These are the data Roche filed with FDA seeking GA101's approval but investors are more interested in how GA101 stacks up against Rituxan. The second stage of the phase III study is not ready for a final analysis, but preliminary data released Wednesday shows GA101 might be a better drug.

CLL patients treated with Rituxan reported a median PFS of 15.7 months, which is numerically inferior to the 23-month median PFS for GA101. The Rituxan data will change, however, as the study matures. Roche expects to have more definitive results from the second stage of the study later this year.

New CLL therapies are also being developed by Pharmacyclics ( PCYC) (ibrutinib) and Gilead Sciences ( GILD) (idelalisib.)

-- Reported by Adam Feuerstein in Boston.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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