MADRID (Reuters) - U.S.-based investment fund Mason Capital said on Monday it had sent a letter to Spain's stock market regulator CNMV demanding more transparency from the embattled pharmaceutical company Grifols.

The activist fund said in a statement it had identified potential conflicts of interest linked to related party transactions by current board members, failure of internal controls and a "rumoured clause" in a recent bond issuance that benefits Canadian fund Brookfield at shareholders' expense.

"We urge the CNMV to consider the benefits to all Grifols shareholders from improved disclosure," Mason Capital said.

The CNMV declined to comment about the letter.

Grifols shares were down around 3% in late afternoon trading following the statement by Mason Capital, which owns a stake of about 2.5% in Grifols.

In early January 2024, Grifols lost about a third of its value in a few hours after short seller fund Gotham City Research questioned its accounting and the size of its debt. The share price has not fully recovered.

Barcelona-based Grifols denied any wrongdoing and sued Gotham City. It also made a series of governance changes, appointing a new CEO following the incident, and revised the value of its debt.

In November, Brookfield dropped a plan to take over the human-plasma drug developer, together with the founding Grifols family, that valued it at 6.45 billion euros. The deal fell through due to a disagreement over the valuation.