Isis Pharmaceuticals ( ISIS) rocketed Monday in postclose trading after Genzyme ( GENZ) announced a licensing partnership worth $325 million upfront for mipomersen, a drug in late-stage development for high-risk cardiovascular patients.

Isis, which before the announcement was up 44% in the last year and some 177% since the beginning of 2006, surged $8.02, or 55%, to $22.60 after hours. The news was a clear surprise for some who questioned the company's ability to partner the drug -- there were some 20 million shares short.

But Isis CEO Stanley Crooke, who has been very outspoken about the company's plans to partner on mipomersen, told TheStreet.com's senior columnist Adam Feuerstein that the company had multiple offers for the drug, some higher, but that they went with Genzyme because of its ability to see orphan drugs through.

At first glance in a note to investors, Bear Stearns analyst Mark Schoenebaum said that in addition to being a good fit for Genzyme's emerging cardiovascular franchise, the transaction "brings Genzyme a late stage asset that could launch in the 'critical period' of 2009/2010 -- a period during which many investors are worried that GENZ will struggle to grow."

The company will pay Isis $325 million upfront, with $150 million of that in a purchase of 5 million Isis shares at $30 a share, to codevelop and commercialize Isis' drug. Carlsbad, Calif.-based Isis could also receive an additional $825 million in milestones tied to development, and $750 in commercial milestone payments.

Per the agreement, Genzyme will record 70% of the profits until the drug reaches sales of $2 billion, and thereafter the companies will split sales 50/50.

The companies anticipate a regulatory filing for mipomersen in 2009 with an initial indication for patients with familial hypercholesterolemia (FH), a severe form of high cholesterol. There are approximately 1.5 million people in the U.S. and Europe with FH, according to the companies. The second indication the companies plan to pursue for the drug is the much larger population of other patients with high cholesterol, at high risk of cardiovascular events.

Conservatively assuming 20% penetration and $7 pricing per day, Schoenebaum preliminarily pegs peak sales at around $750 million.

Crooke would not comment on pricing, but did say that if the company expanded into broader patient populations, such as those who can't control their cholesterol with statins, pricing would potentially be lowered.

Investors have speculated that the company may not be able to price mipomersen highly enough to make it worth treating the relatively small number of familial hypercholesterolemia patients if it also wants to develop the drug for the much broader population of "ordinary" high-cholesterol patients. Pricing will have to be more constrained to get reimbursement for that patient population.

Genzyme, which also will have preferred access to future Isis drugs for CNS and certain rare diseases, reconfirmed its 2008 and five-year earnings guidance. It said the impact will fit into its goal of 20% compound annual growth in adjusted earnings through 2011. It was trading down 60 cents, or 0.8%, at 76.50 in after-market trading.