Two Five Star Quality Care shareholders will have to put on hold their plans to substantially increase their stake in the company until after March 31, the company said in a letter Thursday.

Katherine E. Potter, the Newton, MA-based company’s senior vice president, general counsel and assistant secretary, told William Thomas that because the company already had granted a conditional waiver to allow ABP Acquisition the right to acquire up to 18 million shares of Five Star common stock, the company must wait until that waiver expires on March 31 before considering a request made by Thomas and his brother, Robert.

The Thomases, founders of Tulsa, OK-based Senior Star Management Co., earlier this month had asked Five Star to grant it the same waiver as it had granted ABP, but Potter said that ownership limits in Five Star’s bylaws are designed to protect the value of certain tax attributes and that those attributes would be lost if it extended an additional waiver.

Also, she said, the company’s charter restricts ownership to no more than 9.8% of shares without the approval of Senior Housing Properties Trust, or SNH, from which Five Star was spun off in 2001.

“Five Star currently operates 246 senior living communities (approximately 28,444 residence units) under long-term leases and management agreements with SNH,” she said. “SNH has retained the right to terminate these contracts in the event any individual or group acquires more than 9.8% of Five Star shares without SNH’s prior approval. Five Star’s ability to preserve its relationship and the benefit of its contracts with SNH is critical to its evaluation of your request.”

Senior Star sought to purchase up to 10 million shares of Five Star common stock, which would represent approximately 20% of outstanding shares, by offering stock owners $3.45 per share. The waiver request followed a $3-per-share offer that ABP Acquisition LLC announced as part of a plan to buy the same number of shares. (ABP upped its tender offer to 18 million shares on Thursday.)

Together, the Thomases already own approximately 3.36 million shares of Five Star, or 6.8% of outstanding shares.

Read about the history between Five Star and Senior Star in the Related Articles below.