Roche's Gazyva cancer drug did not show a significant overall survival benefit and raised greater safety concerns than its predecessor Rituxan, a study showed, raising doubts over the Swiss pharmaceuticals group's bid to replace a key blockbuster.

Gazyva is used with the chemotherapy drug bend amustine to treat follicular lymphoma (FL) and Roche is counting on it to help mitigate the impact of biosimilars on Rituxan, whose patents expired this year.

Roche failed to find a follow-up drug to its Avastin blockbuster this year and faces competition from rivals including Novartis whose Sandoz generics unit is hot on Rituxan's heels.

An analysis released at the weekend meeting of the American Society of Hematology found that for patients with previously untreated FL, those getting Gazyva and bendamustine showed a 94 percent overall survival rate at three years.

That compares with 92 percent overall survival among those getting Rituxan and chemotherapy, raising concerns among analysts that Roche will have trouble differentiating Gazyva.

"A few physicians we spoke to were unimpressed which could make it more difficult to replace Rituxan with Gazyva," Jeffrey Holford at Jefferies said in a note to investors.

Roche said the trial had shown Gazyva to be superior to the current standard of care and the improvement was "meaningful".

"People with this disease need better initial treatment options that delay their cancer getting worse, since the disease is incurable and becomes more difficult to treat with each relapse," a Roche spokesman said in an emailed response.

Roche shares gained 1 percent, in line with Swiss stocks.

Analysts on average expect Gazyva to reap $1.7 billion in 2020, according to consensus forecasts compiled by Thomson Reuters, and in May, Roche raised hopes for the drug by releasing initial findings showing it met its main goal of superior progression-free survival compared to Rituxan, which in some markets is known as Mabthera.

In the more-detailed analysis released at the U.S. meeting this weekend, Roche said Gazyva improved progression-free survival by 34 percent among 1,202 patients in the study.

But analysts said three-year-old Gazyva's safety profile in combination with bendamustine was a concern, with the rate of severe, life-threatening and fatal adverse events 74.6 percent with Gazyva, versus 67.8 percent with Rituxan.

"The data presented was critically perceived by haematologists, as the favored combination with bendamustine appears relatively too toxic," Bank Vontobel analyst Stefan Schneider said. "It will take a strong effort from Roche to achieve a broad switching."