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A study investigating long term, or "maintenance" use of the cancer drug Rituxan in patients with low-grade, or indolent non-Hodgkin's lymphoma was halted early due to positive efficacy results,

TheStreet.com

has learned.

The news is a win for Rituxan's co-marketers

Biogen Idec

(BIIB) - Get Report

and

Genentech

(DNA)

because the study has the potential to re-energize sales at a time when Rituxan sales growth was slowing. Rituxan sales should exceed $1.5 billion this year, making it the most successful biotech-derived cancer drug ever, but market share already approaches 70%.

Biogen and

Idec Pharmaceuticals

closed their merger Wednesday night.

Wall Street has been

fixated on a new strategy to reinvigorate Rituxan's growth -- using the drug as "maintenance" therapy in cancer patients. The idea is pretty simple: Non-Hodgkin's patients respond well to Rituxan therapy. If their cancer comes back, they get Rituxan again and respond well again. Therefore, why wait for patients to relapse? Instead, why not administer Rituxan on a regular basis to keep non-Hodgkin's patients in remission permanently?

The study halted today was being conducted by the Eastern Cooperative Oncology Group, and is known as ECOG 1496. The study is investigating Rituxan maintenance therapy in patients with indolent, or low-grade non-Hodgkin's (the smaller of the two major non-Hodgkin's categories).

Wednesday, ECOG sent an email and fax to investigators in the 1496 study, informing them that the study was being halted after a planned interim analysis. "The interim results met the protocol criteria for improvement in time to failure with maintenance

Rituxan," according to the ECOG letter, which was obtained by

TheStreet.com

.

Executives with Biogen Idec and Genentech were not aware that the study had been halted, so neither company had an immediate comment.

Last week, Genentech announced that a different study of Rituxan maintenance therapy, this one in patients with aggressive, or high-grade NHL, failed to boost survival. Final results from this study will be presented at the upcoming American Society of Hematology meeting in December.

But low-grade non-Hodgkin's patients live longer and take longer to relapse, compared to patients with high-grade non-Hodgkin's, so proving that maintenance Rituxan therapy works with these patients (long-term, chronic use of the drug) will have a greater, positive impact on Rituxan sales.

The ECOG 1496 study was not scheduled for presentation at the upcoming ASH meeting, but given these positive results, there is a chance it will be shoe-horned into the meeting.

Other studies, also investigating Rituxan maintenance therapy in both low-grade and aggressive non-Hodgkin's patients, are already scheduled for presentation at the ASH meeting. The totality of these studies, and not just the findings from a single one like ECOG 1496, likely will be the final determination of whether doctors expand their Rituxan use significantly by using it as maintenance therapy in non-Hodgkin's patients.

Idec shares closed up 2 cents to $33.42 in Wednesday trading, but shares rose 2% to $34.30 in the after-hours market. The stock starts trading under its new symbol, BIIB, on Thursday. Genentech shares were up 2% to $83.19 in the regular session and up just 4 cents after hours.

Adam Feuerstein writes regularly for RealMoney.com. In keeping with TSC's editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet.com. He also doesn't invest in hedge funds or other private investment partnerships. He invites you to send

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