NEW YORK (CNNMoney.com) - Amgen and Genentech, the long-ruling kings of the cancer drug market, could find the lucrative business growing more crowded as potential new competitors come to market.

The world's biggest biotechs, Amgen (up $0.45 to $68.14, Research) and Genentech (down $1.82 to $78.21, Research), are expected to unveil a slew of studies on existing drugs at the annual conference of the American Society of Clinical Oncology, to be held in Atlanta from June 2-6.

But GlaxoSmithKline (down $0.67 to $55.77, Research) could steal the show with data on an experimental drug, tykerb, that could eventually rival Genentech's Herceptin, the $750 million breast cancer drug.

"It's my sense that this ASCO is going to be more of a competitive meeting [against] Genentech," said Christopher Raymond, analyst for Robert W. Baird.

GlaxoSmithKline's tykerb, a potential treatment for women with breast cancer that has spread to the brain, is in late-stage testing. Analysts consider the drug a potential billion-dollar blockbuster.

Like Herceptin, tykerb targets the aggressive HER2 form of breast cancer, which is present in about half of all cases. But tykerb is a pill, while Herceptin is an injected drug, which could give the Glaxo drug a competitive edge if it gets approved by the Food and Drug Administration.

"There's obviously an advantage to having an oral therapy," said Raymond of Robert W. Baird, who said that taking tykerb as a pill would be more convenient that injecting Herceptin.

Bristol-Myers Squibb (down $0.18 to $23.99, Research) is also presenting data on an experimental new drug: dasatinib for the treatment of chronic myeloid leukemia. Though the drug has not yet entered late-stage testing, the FDA is conducting accelerated review of dasatinib and is expected to reach a decision on June 28.

Amgen and Genentech will dominate ASCO with presentations of new studies and data. Amgen is expected to show studies on experimental drug denosumab for metastatic bone cancer and panitumubab for colorectal cancer, but these studies are not as advanced as the data for tykerb.

"We've done quite well in this area, but now we are looking to expand it into the wider cancer paradigm," Amgen spokeswoman Christine Regan said, noting her firm is researching "pathways" in the human body to try and discover new treatments, some of them on the market and others in the experimental stages.

Sales of drugs for cancer, America's second-biggest killer behind heart disease, are expected to more than double over five years, from $24 billion in 2004 in $55 billion in 2009, according to IMS Health (down $0.08 to $26.98, Research).

And while new data could help the biotechs get their drugs approved for additional uses and bring in additional sales in the rapidly-growing market, studies for new drugs are comparatively scarce.

Genentech will present data mostly on drugs that are already on the market. Genentech spokeswoman Kristina Becker said that Avastin, the biotech's $1.1 billion drug for colorectal cancer, is the subject of no less than 80 studies involving 20 different types of cancer.

This includes a study that combines Avastin with Tarceva, the $275 million drug for non-small cell lung cancer, to create a single treatment that can be used without chemotherapy. Any one of these studies could lead to FDA approval of additional uses for the drug, which means additional sales.

But Geoff Porges, analyst for Sanford C. Bernstein, said he is concerned about a lack of early-stage products in Genentech's pipeline, and said the biotech could have problems following up on the glories of its past.

"Genentech is a great company and has done remarkable things in the last few years, but that's fully priced in [the stock]," said Porges. "When investors start thinking about competition, they might start to question their long-term projections for Genentech, and that's where the vulnerability is."

The analysts interview for this story do not own stock in the companies mentioned here.

To read about Merck's dance with the religious right with its new cancer vaccine, click here.