Onyx Pharmaceuticals has given potential buyers, including Amgen, access to trial data on its new cancer drug, removing a key hurdle that was holding up deal talks, according to three people familiar with the matter.

The company's attempt to sell itself for around $9.5 billion hit an impasse in recent weeks after lead bidder Amgen sought trial data about blood cancer drug Kyprolis — something that Onyx was reluctant to share, Reuters reported last week.

Onyx made more information available this week to Amgen and other parties involved in the sale process, and is now waiting for a firm offer, the people said on Wednesday. Discussions could still fall apart and there is no guarantee a deal will happen, they cautioned.

All the people were not authorized to speak with the media and asked not to be identified. Representatives for Onyx and Amgen declined to comment.

Amgen, the drugmaker in advanced talks to buy Onyx for $130 per share, or about $9.5 billion based on shares outstanding, had asked to see data from an ongoing trial on Kyprolis in order to better evaluate the treatment, people familiar with the matter said previously.

Onyx had been reluctant to meet the request because the study is continuing. Details of the trial data that Onyx made available to potential buyers could not be learned, but one of the people said bidders have all the data that Onyx has access to.

South San Francisco, California-based Onyx put itself up for sale in late June after spurning a $120-per-share offer from Amgen as significantly undervaluing the company. While Onyx has been in talks with other parties, including AstraZeneca, it remains unclear if a rival bid will emerge.

Access to the trial data had proved to be a sticking point in the negotiations since the sales potential of Kyprolis - a drug with the generic name of carfilzomib that treats multiple myeloma, a type of blood cancer - hinges on the outcome of the clinical studies.

The drug was approved last year by the U.S. Food and Drug Administration based on robust mid-stage trial results, but full approval is contingent on the outcome of Phase 3 trials.

Kyprolis, given intravenously, is currently used for patients whose myeloma has worsened despite at least two prior therapies. Successful trials would allow Onyx to expand that to patients who have failed just one treatment - a larger market currently dominated by Takeda Pharmaceutical's Velcade.

Velcade, sold by Takeda in the United States and Johnson & Johnson in Europe and the rest of the world, generated sales near $2.2 billion in fiscal 2012. Wall Street analysts, on average, have forecast that Kyprolis will outstrip that, reaching $2.23 billion by 2018, according to Thomson Pharma.

Both Kyprolis and Velcade are members of a class known as proteasome inhibitors. Bullishness over the Onyx drug stems largely from the fact that it causes fewer central nervous system side effects such as neuropathy, or nerve pain.

However, the Kyprolis label does list several warnings, including a risk of heart problems. The label notes that cardiac failure was reported in 7% of mid-stage trial patients.

"There have been concerns raised about carfilzomib possibly having more cardiac effects than Velcade and possibly more kidney issues," said Dr Robert Orlowski, professor in the department of lymphoma/myeloma at the University of Texas MD Anderson Cancer Center in Houston. "We need to have the randomized study results."

The Onyx drug irreversibly binds to and blocks activity of a proteasome, responsible for digesting proteins inside cells that the cells no longer need. Velcade also targets the proteasome, but is known as a "slowly reversible" inhibitor.

"There may be longer inhibition with carfilzomib," said Dr Orlowski. "Every cell in the body, including heart cells, have proteasomes in them."