They've still got a kid together -- potential kidney cancer treatment XL880 -- but GlaxoSmithKline (NYSE:GSK) and Exelixis (NASDAQ:EXEL) have decided to go their separate ways. Or, more specifically, Glaxo decided not to select any more drugs for development, and Exelixis didn't have much other choice.

This may be one of those rare instances in history where the one doing the breaking up says, "It's not you; it's me," and actually means it. Glaxo hasn't actually said anything about the breakup, but here's my interpretation of its actions.

Glaxo had to decide whether to take over development of XL184, Exelixis' most advanced drug, which recently began a phase 3 trial to treat a rare thyroid cancer. Exelixis submitted the data packet to Glaxo back in July, and Glaxo had 90 days to make a decision.

Under the terms of the deal, Glaxo could pick one additional compound out of the five remaining left in the basket. Glaxo could have passed on XL184 and waited to see the data for the remaining four compounds, but instead, it passed on all of them.

That leads me to think that Glaxo made an executive decision not to select an additional drug for business reasons, rather than science reasons. That's good news for Exelixis, since there are drugs that have made it to market after large pharmaceutical companies declined to develop them. For instance, Johnson & Johnson (NYSE:JNJ) passed development of high-blood-pressure medication Bystolic on to Mylan (NYSE:MYL), which in turn licensed the rights to sell the drug to Forest Laboratories (NYSE:FRX).

Where does that leave Exelixis? With a whole lot of drugs and not nearly enough money to develop them all. It gets even worse, because Exelixis owes Glaxo more than $100 million. Fortunately the Rule Breakers pick has a year before the first payment is due, which should be plenty of time to find a partner for one or more of its numerous compounds. Assuming I'm right about Glaxo's reasons, a new mate shouldn't be too hard to find.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Johnson & Johnson and Glaxo are Income Investor selections. The Fool owns shares of Exelixis. The Fool's disclosure policy prefers make-ups to break-ups.