Source: Pfizer.

Last week, Portola Pharmaceuticals (NASDAQ:PTLA) reported interim results from a phase 3 trial of its drug andexanet alfa at the American Heart Association's Scientific Sessions 2014 conference -- results that could have big implications on Bristol-Myers Squibb (NYSE:BMY) and Pfizer(NYSE:PFE).

Why exactly does this matter for Bristol and Pfizer? Their blood thinner Eliquis needs an antidote -- something that can be used to reverse its effects -- and as of right now, no such antidote exists. Andexanet alfa is being developed by Portola to do just that, allowing more patients to potentially use Eliquis -- and so far, results are promising. An eventual approval of andexanet alfa has potentially meaningful impact on revenue growth for Pfizer and Bristol. Let's take a closer look.

Reinventing a proven market
Doctors have used a blood thinning drug called warfarin to help reduce the risk of blood clots in heart disease and post-operative patients for decades. However, warfarin's risk of brain hemorrhage, difficulty in adjusting doses, and regular monitoring make it a less-than-ideal medicine.

As a result, a slate of drugmakers have launched warfarin alternatives, including Johnson & Johnson's Xarelto and Bristol-Myers and Pfizer's Eliquis. These next-generation anti-coagulants offer an arguably better safety profile, more flexible dosing, and reduced monitoring. As a result, sales of these drugs have been building quickly.

In the first nine months of this year, Johnson & Johnson reported that sales of Xarelto had climbed 84.5% to $1.09 billion, and Bristol-Myers reported that global sales of Eliquis surged from $75 million to $493 million. While sales are growing, revenue could be even higher if not for one big problem: the lack of an approved antidote. Without one, hospital ER's have few options when treating a patient that's been taking these drugs.

Relying on an upstart
Since there isn't an antidote available to reverse the affects of these drugs, but there is an antidote available for warfarin, some doctors are shying away from using Xarelto and Eliquis, especially in older or sicker patients that are more likely to suffer a bleeding event. 

In a bid to accelerate the development of such an antidote, Johnson & Johnson, Bristol-Myers, and Pfizer have agreed to help fund clinical trials of andexanet alfa. Although none of these companies got commercial rights in exchange for their help, results announced so far suggest that it's been money well spent toward growing sales of their anticoagulants.

In the first part of Portola's phase 3 trial studying andexanet alfa in Eliquis patients, 24 of the 33 volunteers participating were given Eliquis twice daily for four days and then given andexanet alfa. Within two to five minutes of being given andexanet alfa, about 94% of the anticoagulant activity of Eliquis was reversed. Across all 24 participants, andexanet alfa reversed between 90% and 96% of Eliquis activity. Importantly, there were no adverse events reported other than a mild infusion reaction in three patients.

Timing its impact
Since the absence of an antidote to these anticoagulant drugs could prove life-threatening, the FDA awarded andexanet alfa breakthrough status last fall. That means that Portola will enjoy an accelerated pathway to eventual approval. Since Portola announced in October that it hopes to file its application for FDA approval by the end of 2015,  andexanet alfa could reach the market, and indirectly begin benefiting Bristol-Myers and Pfizer's Eliquis, in 2016. Since Portola is itself a speculative company without any revenue, it's best suited for risk-taking investors. However, less risk-tolerant investors interested in Bristol-Myers and Pfizer may want to keep an eye on andexanet alfa given its potential to boost sales of Eliquis.