Summaries of more than 4,000 cancer studies - including several notable ones - will be released Thursday evening, two weeks before a key, closely watched medical conference begins.

While the biggest news regarding cancer research likely will be saved for the annual meeting of the American Society of Clinical Oncology, beginning May 29 in Orlando, Fla., Thursday's release includes much-anticipated updates from companies like Roche Holding AG's (RHHBY) Genentech unit, OSI Pharmaceuticals Inc. (OSIP) and Exelixis Inc. (EXEL)

This is the second year that ASCO has released such data two weeks before the actual meeting in order to avoid the previously famous "ASCO Effect," in which leaked information from embargoed studies caused stocks to move before the actual meeting.

The data coming Thursday are likely to be incremental and could be subject to misinterpretation because the information will be in abstract form, or a condensed summary, which could be incomplete until the full data is presented at the meeting.

Nonetheless, Wall Street will scrutinize the findings, especially those on Tarceva's use in lung cancer as a maintenance therapy - when it is given after chemotherapy, but prior to new tumor growth

Observers will be looking for Tarceva to show progression-free survival exceeding the two months already seen in Eli Lilly & Co.'s (LLY) Alimta.

If its effectiveness is more robust, OSI's stock could jump, notes Barclays Capital analyst Jim Birchenough. He warns that any stock gains could be short-lived as investors will quickly shift attention to overall survival data from the study coming later this year.

"If you are just delaying the progression of cancer but patients aren't living any longer, it doesn't really become an impetus for changing physician behavior," Birchenough said.

Tarceva, sold by OSI Pharmaceuticals and Genentech, is currently approved for use as later treatment in advanced lung cancer patients who have progressed after at least one prior chemotherapy regimen. Another study examining Tarceva use with Avastin in advanced lung cancer will also be presented at the meeting.

Some insight into Tarceva's potential could come at the meeting, when overall survival data on Lilly's Alimta as a maintenance therapy in lung cancer is presented.

As usual, Genentech/Roche will be presenting a plethora of data at the meeting, with more than 500 studies covering 20 different kind of tumors. The most closely watched of those, Avastin's use in preventing a relapse following surgery in colorectal cancer, won't be available Thursday but will come at the meeting.

Among the flood of data expected Thursday is the so-called Phase III RIBBON-1 trial of Avastin's use in advanced breast cancer. The study was presented to the Food and Drug Administration to support full approval in the indication after the agency granted accelerated approval last year.

Other notables possibly coming Thursday include early Phase II data on Exelixis Inc.'s (EXEL) XL184 in treating progressive or recurrent glioblastoma multiforme, a form of brain cancer, in the first or second relapse. Bristol-Myers Squibb Co. (BMY) signed a co-development pact for the drug late last year.

Cougar Biotechnology Inc. (CGRB) may offer mid-stage data on Abiraterone in treating prostate cancer. While the data are much anticipated, Rodman & Renshaw analyst Simos Simeonidis doesn't expect the stock to move more than 10% in either direction unless there is a major negative surprise

Also on the earlier side of development, Arqule Inc. (ARQL) may have interim data from a mid-stage trial of ARQ-197 in Microphthalmia Transcription Factor tumors - a group of cancers that tend to effect young adults. Last year, Arqule partnered with Japan's Daiichi Sankyo Co. (4568.TO) on the drug.

-Thomas Gryta; Dow Jones Newswires; 201-938-2053; thomas.gryta@dowjones.com