Cancer drugs have been taking center stage in recent months. Some of the drugs making the biggest headlines come from the field of immuno-oncology, which are drugs that manipulate the body's immune system in an effort to fight cancer.

Three companies are vying to be one of the first to get their drug to market.

The early leader was Bristol
Bristol-Myers Squibb (NYSE:BMY) was the early leader in the immuno-oncology market thanks to its drug Yervoy. This drug has been approved by the FDA to treat melanoma and is expected to generate $1 billion in sales this year and potentially almost $2 billion a year by 2020 if used in combination with other drugs.

Bristol was unquestionably the early leader in the PD-1 field with its nivolumab, but was bypassed by Merck's (NYSE:MRK) MK-3475 when Merck began the rolling submission process at the beginning of the year and received priority review from the FDA last month.

Bristol did not start the rolling submission process with nivolumab until April. Nonetheless at the American Society of Clinical Oncology meeting this week the company presented very solid survival data when nivolumab is used in combination with Yervoy. The survival rate of advanced melanoma patients was 79%.

This is welcome news for Bristol after seeing its stock dip in mid-May after reports were published that showed nivolumab plus Yervoy performed rather poorly with patients suffering from squamous non-small-cell lung cancer (NSCLC). The studies indicated a rather average 22% response rate, with a high adverse event profile, and a 6.5% drug-related death rate.

Needed good news for Merck as MK-3475 is its most important drug
MK-3475 is Merck's most important drug at the moment. The drug now covers 30 tumor types and the company expects 24 clinical trials by the end of the year involving roughly 6,000 patients. Perhaps more importantly, the FDA has a deadline of October 28 to make the marketing decision on the drug. Right now Merck is clearly in the lead to be at market at first.

At ASCO this week MK-3475 produced an overall survival rate of 69% in a study of 411 melanoma patients. Also at ASCO the company presented data showing that MK-3475 produced an overall response rate of 47% in the treatment of patients in previously-untreated advanced NSCLC.

AstraZeneca's aggressive timeline
AstraZeneca (NYSE:AZN) also reported positive results with its immuno-oncological drug MEDI4736. In its trial of 27 patients with advanced tumors 19% saw tumor shrinkage and 39% had their cancer stabilize. The drug also appears to have a low adverse event rate and relatively low toxicity.

AstraZeneca appears conscious of Merck's timeline advantage and is being rather aggressive in pushing its lung cancer timeline. The company is skipping phase 2 trials and is beginning phase 3 trials for Medi4736. 

High stakes battle
All three companies appear to have some reason to feel optimistic about immuno-oncology. Bristol's presentation at ASCO should partially erase some of the negative sentiments about nivolumab that appeared last month. Merck has taken the lead with MK-3475 and should be the first to market (if the drug is approved by regulatory agencies). And Astra is moving quickly as well by pushing into phase 3. The stakes are high as this is undoubtedly a market worth billions annually. Investors need to keep a close eye.

Editor's Note: A previous version of this article incorrectly referred to Merck's MK-3475 as MK-4375. The Fool regrets the error.