May 28 (Reuters) - Aspen Investment Fund, a shareholder of obesity drug maker Vivus Inc, said it was planning to offer to buy the company for $640 million.
Aspen, which reported a 9.65 percent stake in Vivus on Wednesday, said it expects to submit its offer to the company by June 13.
Vivus shares were up 6 percent at $4.95 in morning trading.
The company, which was embroiled in a long-drawn proxy war with another shareholder First Manhattan Co last year, has disappointed investors with weak sales of its diet pill Qsymia that was once touted as a potential blockbuster. (Reporting by Esha Dey in Bangalore; Editing by Saumyadeb Chakrabarty)