(Reuters) - Swiss-based drugmaker Actelion Ltd had offered to buy U.S. biotech company ZS Pharma Inc last month in a deal valued at $2.5 billion, Bloomberg reported, citing people with knowledge of the matter said.

It was unclear how ZS Pharma responded to the offer or if the two companies had held discussions, Bloomberg reported. (http://bloom.bg/1XQC5AR)

Actelion is assessing a number of potential acquisitions and is not wedded to any particular deal, Bloomberg reported.

ZS Pharma's stock shot up nearly 46 percent to touch a record high of $84.85 on Thursday.

ZS Pharma spokeswoman Denise Powell declined to comment on the report. Actelion was not immediately available for comment.

Coppell, Texas-based ZS Pharma had a market capitalization of $1.5 billion as of Wednesday's close.

ZS Pharma's lead experimental drug, ZS-9, is aimed at treating hyperalkemia, a build up of potassium in the body that could lead to heart failure.

The company had filed a marketing application for ZS-9 with the U.S. Food and Drug Administration in May. The health regulator is set to rule on the application in May next year.

ZS Pharma shares were up 26 percent at $73.20 in late afternoon trading. They had gained about 40 percent so far this year through Wednesday's close.