Eli Lilly’s $6.5 billion proposed acquisition of ImClone BioSystems has upset a group of the latter’s shareholders which have filed a lawsuit against the firms.

The Massachusetts Public Pension Forum and several individual shareholders are looking to block the deal and the lawsuit alleges that ImClone's board breached fiduciary duties to investors by agreeing to the takeover. It also claims that the board neglected to provide investors with material information so they could make an informed decision about whether to tender their shares.

The suit is seeking class action status on behalf of all ImClone stockholders and an injunction preventing the acquisition from proceeding until investors are given more information.

The lawsuit was disclosed in regulatory filings from both Lilly and ImClone which noted that a court hearing is scheduled for tomorrow. The drugmakers stated that the complaint was without merit and they are ready to vigorously defend themselves.

Lilly recently outbid Bristol-Myers Squibb to acquire ImClone.