Let the bidding war begin. At least, that’s what ImClone Systems (NASDAQ:IMCL) is hoping will occur, after it announced yesterday that an unnamed "large pharmaceutical company" had made a proposal to potentially acquire it for $70 per share in cash. Names being bandied about for the unknown bidder include Merck KGaA, Pfizer (NYSE:PFE), and GlaxoSmithKline (NYSE:GSK).

In late July, ImClone’s longtime partner Bristol-Myers Squibb (NYSE:BMY) offered to acquire it for $60 a share. In conjunction with yesterday’s announcement about the potential $10-a-share larger deal, ImClone’s board of directors also rejected Bristol-Myers’ bid as “inadequate.” The delay in deciding raised eyebrows at Bristol-Myers.

Bristol-Myers has been ImClone’s North America marketing partner for its colorectal cancer drug Erbitux since its FDA approval in 2004. For several reasons, getting ImClone under its wings makes sense for Bristol-Myers. It already owns approximately 17% of ImClone’s shares, and acquiring the whole company would give Bristol-Myers control over how to market and promote Erbitux. Just as importantly, getting ImClone would significantly enhance Bristol-Myers’ biotech operations at a time when biopharmaceutical assets are hot commodities.

For these same reasons, having little control over ImClone’s most valuable asset makes ImClone less desirable to other drugmakers. Therefore, it doesn’t make much sense for an acquirer other than Bristol-Myers or ImClone’s European marketing partner Merck KGaA to make a bid for ImClone.

ImClone also has one other issue that makes acquiring it less attractive to any other drugmaker besides Bristol-Myers. It has a second-generation follow-on compound in phase 2 testing that is similar to Erbitux; the new drug could become a major competitor to Erbitux if approved and marketed by a drugmaker other than Bristol-Myers.

Recalling some of the partnership tangles that made a potential Biogen-Idec (NASDAQ:BIIB) buyout so difficult last year, ImClone claims that Bristol-Myers has no rights to this compound, dubbed IMC-11F8. Bristol-Myers claims the opposite. What drugmaker would want to wade into this potential legal mess?

There are many important differences between activist investor Carl Icahn’s attempts to foster a sale of ImClone and his experience trying to get Biogen sold last year. Investors should recall that with Biogen, Icahn once even purportedly offered $23 billion to acquire the company himself. Ultimately, nothing came of his “expressions of interest.”

Bristol-Myers has said it won't up its offer, since it would be competing with itself. Until a firm offer is publicly on the table and accepted, ImClone’s announcement that a $70-per-share offer has been made shouldn’t be taken as anything close to a done deal.

With ImClone’s shares already trading near this potential $70-per-share “proposal,” there's little upside left in ImClone's shares, even if a deal is made in this price range.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has an A+ disclosure policy.