Thousand Oaks biotech giant Amgen Inc. could decide to pay less than it offered earlier this month for Onyx Pharmaceuticals Inc., after Onyx reportedly provided less-than-complete clinical trial data on a blood-cancer drug.

Amgen’s $130-a-share offer would value Onyx at about $9.5 billion. Bloomberg News and Reuters reported that San Francisco-based Onyx provided study data on Kyprolis to Amgen and some other potential suitors, including British drugmaker AstraZeneca Plc. But sources told Bloomberg on Thursday that Amgen is holding out for details that would enable it to better estimate the eventual market value of the drug, which is the only approved product that Onyx markets on its own.

The data is from a European study that hasn’t been completed, the sources said. If successful it could lead to a larger market for Kyprolis, which now is approved only for patients who haven’t seen improvement after two other therapies have been tried.

Amgen initially offered $120 a share, which Onyx’s board rejected as too low. Onyx has two other approved cancer drugs, but any purchaser of the company would have to share profits of those with German drugmaker Bayer AG, Onyx’s current marketing partner.

Amgen shares closed up $1.64, or 1.6 percent, to $106.29 on the Nasdaq.