* Tecentriq drug cocktail cuts risk of disease progressing
* Drugmaker to file for expanded approval with latest data
* Analysts see drug sales topping $4.6 bln by 2023 - poll (Adds analyst comment)
By John Miller
ZURICH, Dec 7 (Reuters) - Adding Roche's immunotherapy Tecentriq to older drugs doubled the percentage of lung cancer patients who survived a year without their disease advancing, an outcome some experts on Thursday labelled unprecedented.
Its shares retreated, however, as analysts said uncertainty lingers over whether the result is sufficient to give the Swiss drugmaker a jump on rivals Merck & Co and Bristol-Myers Squibb, whose immunotherapy sales dwarf Roche's own.
Thirty-seven percent of patients in Roche's closely watched clinical trial who got Tecentriq, Avastin and chemotherapy reached the one-year mark without their cancer progressing (PFS), according to data released at the European Society for Medical Oncology meeting in Geneva.
For patients getting just Avastin and chemotherapy in the Impower 150 study, that fell to 18 percent.
Roche is counting on Tecentriq to help replace revenue from its $20 billion-per-year trio of Avastin, Herceptin and Rituxan whose patents have expired or will shortly, exposing them to cheaper competition.
Chief Executive Severin Schwan hopes Roche can leap-frog ahead of Merck and Bristol-Myers Squibb, both of which are still awaiting key lung cancer combination trial results of their own.
"This is very, very promising," Dr. Solange Peters, the head of Medical Oncology at the Centre Hospitalier Universitaire Vaudois in Lausanne, Switzerland, said of the latest results.
"Doubling PFS (progression-free survival) at one year is something we have not seen with any targeted therapy in unselected patients to date."
Tecentriq, already approved in bladder cancer treatment and second-line lung cancer treatment, is seen posting annual sales of $4.6 billion by 2023, according to the average forecast of analysts polled by Reuters.
It costs $150,000 per patient a year.
Roche, which said it would submit Thursday's results to regulatory authorities, had said in November that the Impower 150 trial had broadly succeeded in first-line lung cancer patients, helping spur a one-day, $12 billion rally in the Basel-based drugmaker's shares.
With the release of the specific numbers, however, the Swiss company's shares had fallen 2.2 percent by 1330 GMT, giving up a quarter of their gain since mid-November, while Bristol-Myers and Merck shares both rose.
Roche investors feared Thursday's results might not be good enough, analysts said.
For instance, patients getting the Swiss company's immunotherapy survived an average of 8.3 months without their disease getting worse, compared with the PFS of 6.8 months for those getting chemotherapy and Avastin, according to the study.
That trailed the benefit of Merck's Keytruda plus chemotherapy in a trial that led to U.S. approval for treatment of first-line lung cancer in May.
"The market's view is that the 1.5 months difference versus the Avastin plus chemo arm is insufficient, as Merck's Keytruda has shown a 4.3 months difference," Morgan Stanley analyst Vincent Meunier said.
Still, several analysts said such comparisons are difficult and potentially misleading given vast differences in how trials are organised.
For instance, Merck's trial of its Keytruda cocktail was earlier stage and involved just 123 patients, tiny compared to Roche's 1,202-person study of Tecentriq. It also did not include a targeted therapy like Avastin.
"Impower 150 is one important piece of a complex, still largely incomplete, puzzle," wrote Bernstein's Tim Anderson.
Martin Reck, a chief oncology physician at the Lung Clinic in Grosshansdorf, Germany, and lead author of the Tecentriq study, said that a larger PFS benefit would have been better.
Still, Reck said the combination's 38 percent reduction in risk of disease progression or death, combined with a durable survival benefit that improved at the six- and 12-month treatment marks, should carry more weight.
And while the trial's overall survival data will not be mature until the first half of 2018, Reck said, a preliminary reading of 19.2 months versus 14.5 months among patients not getting Tecentriq suggests an "encouraging trend".
"In the end, I'm not really disappointed ... because this is something we've seen in other trials with immunotherapies," he told a news conference.
Some analysts were also optimistic, saying Thursday's results will clearly be enough to help Roche win expanded U.S. approval in first-line lung cancer treatment by next year.
"We see the highly significant 38 percent reduction in risk of disease progression or death securing a swift adoption of Tecentriq on top of Avastin," wrote Bruno Bulic, a Baader Helvea analyst with a "buy" rating on Roche shares.
Roche said serious adverse events were seen in 25.4 percent of patients getting the Tecentriq combination, compared with 19.3 percent in the Avastin-chemo group. (Reporting by John Miller; Editing by Ben Hirschler, Mark Potter and Susan Fenton)
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