As one biotech executive said, "oncology is a data driven market." For this reason, the data that oncology-driven drugmakers announce from clinical trials are incredibly important to their future sales and financial prospects.

This weekend marks the most important time of the year for drugmakers working on cancer-fighting compounds. The annual American Society of Clinical Oncology (ASCO) medical conference, at which drugmakers present the most promising data from their lead compounds, begins on Saturday.

Compared to other indications like sleep disorders, where big pharmas are willing to pay hundreds of millions of dollars in direct-to-consumer advertising, the drugs used to treat cancer indications are determined more by the compound's efficacy and safety in clinical trials. Many a drug's market share has been sunk when a competitor announces better trial results or when it fails to show much benefit in a pivotal label-expanding study.

ASCO gives these competing biopharmaceutical firms a chance to duel over whose compound is best. Let's preview some of the data duels that will take place over the weekend.

Millennium versus Celgene
The duel between Rule Breakers pick Millennium Pharmaceuticals (NASDAQ:MLNM) and Celgene (NASDAQ:CELG) has not yet produced a clear winner in their competition over compounds to treat multiple myeloma, and potentially other cancers.

Celgene's Revlimid has gotten off to a better start with sales outpacing Millennium's Velcade, but Millennium isn't standing by watching Celgene's fast start. The company is running clinical trials that could potentially expand the Velcade label and get the drug indicated as a first-line treatment for multiple myeloma.

At ASCO, both Millennium and Celgene plan on presenting massive amounts of data on Velcade and Revlimid, plus Celgene's other myeloma compound Thalomid. Most interesting will be the numerous abstracts that Celgene will present on Revlimid's use in lymphomas and early studies in solid tumors, which could open the door to on-label usage of the drug in many new patient groups.

ImClone versus Amgen
The battle over ImClone System's (NASDAQ:IMCL) Erbitux and Amgen's (NASDAQ:AMGN) Vectibix is clearly being won by Erbitux so far this year. ImClone has already announced that Erbitux has succeeded in a clinical study using the drug in an earlier setting for colorectal cancer. However, Amgen had to halt a study of Vectibix since patients taking it in a front-line setting with other cancer compounds were doing worse than patients not on the drug.

At ASCO, over 60 Erbitux-related abstracts will be presented, including the positive front-line metastatic colorectal cancer study data and a failed pancreatic cancer trial. Amgen will present seven pieces of clinical data for Vectibix. With the preponderance of the data favoring Erbitux and much more data on Erbitux in different settings, Amgen needs to produce some miraculous data to swing the sales battle back in its favor. Don't look for an upset in this targeted therapy war.

Onyx versus Pfizer
Another battle over competing drugs where sales levels don't tell the whole story is Onyx's (NASDAQ:ONXX) Nexavar and Pfizer's (NYSE:PFE) Sutent. Both drugs have some of the same cancer cell targets, are small molecule compounds, and are indicated as treatments for kidney cancer. Since the drugs' profiles are very similar, the label-expanding clinical trials and data will be very important in determining which one becomes the long-term standard of care for many cancers.

Onyx desperately needs to release more positive Nexavar data at ASCO. Sutent was only approved in January of last year, but sales of the drug have nearly doubled Nexavar's totals -- and halted Nexavar's U.S. sales growth -- as a result of Sutent showing better efficacy versus some other compounds in treating kidney cancer.

On the other hand, Nexavar produced such robust results as a treatment for liver cancer that Onyx and partner Bayer (NYSE:BAY) had to halt a study earlier in the year. If the clinical results from the liver cancer study are really that good, then Onyx should get Nexavar approved in this indication as soon as the fourth quarter of this year.

At ASCO, Onyx plans on releasing the data from this liver cancer study and its failed phase 3 melanoma study that was announced in November. Investors should take a close look at the safety data from these studies and other Nexavar clinical trials and compare them with the Sutent results to see which compound has the better adverse event profile, since this will affect sales of the drugs in kidney cancer as well.

Final thoughts
While pouring over the data and statistics from abstracts and journal articles may be boring for some investors, this sort of research is critical in determining which oncology compounds will become the standard of care and which will be eventual busts in the marketplace. Even if you don't have access to the abstracts or study data on the drugs that will be presented at ASCO, most of the companies will be having conference calls in conjunction with their data presentations, and investors can listen in on these. I highly recommend that every investor with shares of these oncology-targeting drugmakers at least listen to these post-ASCO conference calls and those of their competitors.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy. Pfizer is an Inside Value recommendation.