Patent cliffs be damned, Pfizer (NYSE: PFE) is selling rather than buying drugs for its pipeline. Yesterday, AstraZeneca (NYSE: AZN) announced that it had licensed an oncology compound, tremelimumab, from Pfizer for an undisclosed amount of money.

Why is the pharma giant out-licensing drugs while it's facing the loss of Lipitor to generics next month? Tremelimumab has a checkered past, and it seems that Pfizer has decided to give up on it. Mostly.

Tremelimumab works in much the same way as Bristol-Myers Squibb's (NYSE: BMY) Yervoy by activating the patient's immune system to fight melanoma. Unfortunately in 2008, Pfizer stopped a phase 3 trial testing tremelimumab in melanoma patients because the drug wasn't offering any benefit over standard chemotherapy.

Further analysis revealed a biomarker that might predict whether patients would respond to the therapy, and Pfizer licensed the drug to Debiopharm, which was responsible for running a phase 3 trial enrolling patients that would respond. That strategy worked well for Roche's Herceptin and GlaxoSmithKline's (NYSE: GSK) Tykerb and might have resulted in an easier approval for Eli Lilly (NYSE: LLY) and Bristol-Myers Squibb's (NYSE: BMY) Erbitux and Amgen's (Nasdaq: AMGN) Vectibix had they discovered the genetic connection earlier.

Fast-forward nearly two years, and it appears Pfizer has regained the rights to tremelimumab; Debiopharm doesn't list the drug on its pipeline page. What's happened during that time isn't clear, but it is clear Pfizer has decided that tremelimumab would be better off in the hands of another company. AstraZeneca bought MedImmune a few years ago so it has plenty of experience with biologics; perhaps it can jump-start the program.

Pfizer isn't completely giving up on tremelimumab though. It's retained the rights to use the drug in combination with other drugs. Bristol and Roche are taking the same tactic with Yervoy and Roche's recently-approved Zelboraf. The theory is that one drug can do the heavy lifting killing the tumor and then the immune system can mop up the remaining cells.

Given Pfizer's revenue predicament, retaining the full rights to tremelimumab would be ideal if the drug works. But given the drug's history, perhaps it's best to just let this one go and focus resources on other potential replacements for Lipitor sales.

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