Amgen's proposed takeover of Onyx Pharmaceuticals is all but agreed, but is being held up by a dispute over access to data from a trial of multiple myeloma treatment Kyprolis, according to Bloomberg.

Citing sources familiar with the matter, the news service claims that financial agreement on a $130-a-share offer for Onyx has been reached, valuing the cancer drug specialist at around $9.5 billion.

Amgen's original $120-per-share bid was turned down in June, with Onyx claiming it was in negotiations with other suitors and AstraZeneca and Bayer both mentioned in dispatches from the rumour mill.

Amgen is reportedly asking for data from a study of Kyprolis (carfilzomib) designed to support registration of the drug in Europe as it tries to value Onyx. At the moment it is unclear what type of data Amgen hopes to see, although it is likely to be interim data that would allow comparisons between groups but remain blinded.

Meanwhile, Onyx is resisting because it is concerned about disrupting the EU regulatory process. If the issue can resolved it is expected that the deal will however move ahead quickly, says Bloomberg.

Kyprolis was launched in the US in July 2012 as a second- and third-line therapy for multiple myeloma, and pulled in $125 million in the first half of 2012. It has been tipped to reach peak sales of $2 billion a year or more, despite the fact that the multiple myeloma market is looking increasingly crowded.

Europe is the largest market for the drug outside the US, according to Onyx, so it is understandable that the company is anxious not to rock the boat there. Onyx is planning to file for approval of Kyprolis in the EU in the second half of 2014, and is in the process of establishing sales affiliates in Germany, France, Italy, Spain and the UK in anticipation of a swift launch.

Onyx has studies trials supporting EU registration, namely the FOCUS trial looking at Kyprolis monotherapy in relapsed and refractory multiple myeloma patients, backed by data from the ASPIRE trial of Kyprolis plus Celgene's Revlimid (lenalidomide) and low-dose dexamethasone in relapsed patients who have received one to three prior therapies.