* Co to apply for U.S. label expansion within 4 mths
* Shares surge more than 50 pct
* Rival Tesaro's shares slip 3 pct (Adds analyst comment; updates shares)
By Natalie Grover
June 19 (Reuters) - Clovis Oncology Inc said on Monday late-stage data on its already-approved ovarian cancer drug showed that the treatment could benefit four times as many patients, sending shares of the U.S. biotech soaring 50 percent.
Clovis's late-stage trial was designed to move its drug, Rubraca, up to a second-line treatment and later, a maintenance treatment. Maintenance therapy immediately follows initial treatment to keep patients cancer-free if they go into remission.
Rubraca, like Tesaro Inc's Zejula and AstraZeneca Plc's Lynparza, belongs to a closely watched class of new medicines called PARP inhibitors, which blocks enzymes that repair damaged DNA, helping kill cancer cells in the process.
Rubraca was granted accelerated approval in December by the U.S. Food and Drug Administration (FDA) to treat patients whose cancer tested positive for defective BRCA genes, and whose disease had advanced despite two or more rounds of chemotherapy.
BRCA gene mutations are known to raise the risk of breast and ovarian cancers.
Clovis's latest study included 564 patients and tested Rubraca against a placebo in patients with various gene mutations who had undergone initial platinum-based chemotherapy.
When given Rubraca, women with recurrent ovarian cancer lived a median 10.8 months without their disease worsening, compared with 5.4 months for women on a placebo, Clovis said.
Overall, data showed Rubraca slowed recurrence in patients who were in remission, and shrunk tumors in those who entered the study with residual disease, in some cases even eliminating tumors.
Shares of Tesaro, whose Zejula is already approved as a maintenance treatment for ovarian cancer, were off 3.6 percent at $139.12 in morning trading.
"Rubraca and Zejula appear to be more similar than different, leading to a narrowing of the valuation gap between Clovis and Tesaro," Morgan Stanley analysts said.
Clovis plans to submit an application to the U.S. FDA within the next four months to expand Rubraca's label.
If approved, the drug could be used as a maintenance therapy, or to treat the cancer if the patient prefers to wait for real evidence that the tumor has recurred, Clovis CEO Patrick Mahaffy said.
More than 22,400 American women will be diagnosed with ovarian cancer this year, according to the American Cancer Society.
About 25 percent of those diagnosed will carry BRCA mutations, Mahaffy said, adding that with an expanded label, Rubraca's addressable population could effectively quadruple.
Shares of Boulder, Colorado-based Clovis were up 48 percent at $88.76 in morning trading. (Reporting by Natalie Grover in Bengaluru; Editing by Amrutha Gayathri and Sai Sachin Ravikumar)