With its secondary hyperparathyroidism treatment, Sensipar, set to face generic competition in a few years, Amgen (Nasdaq: AMGN) made the most obvious move to solve the problem. It bought a biotech developing a compound for the disease.

Amgen is paying $315 million for KAI Pharmaceuticals -- entirely, it seems, for KAI-4169, which has completed phase 2 development. KAI had a partnership with Bristol-Myers Squibb (NYSE: BMY) to develop a heart drug, but that appears to have died in development.

Amgen wants to push KAI-4169 into phase 3 trials quickly, so it's lending KAI some money so the company can press on with trial planning before the deal closes. The fact that KAI needs a loan probably signals that it was a little desperate for a sale and that Amgen probably got a good deal.

There's no doubt the KAI acquisition is a good fit. There's the obvious experience in selling Sensipar, which brought in more than $800 million last year. But secondary hyperparathyroidism is a hormone imbalance that is often caused by kidney failure, so it fits well with Amgen's anemia drugs.

Follow-on compounds often struggle after the first-generation drug goes generic -- think Pfizer's (NYSE: PFE) Pristiq and Johnson & Johnson's (NYSE: JNJ) Invega competing against Effexor and Risperdal, respectively -- but KAI-4169 has one advantage because it'll be administered intravenously at the same time a patient is undergoing dialysis. Sensipar and Abbott Labs' (NYSE: ABT) Zemplar, which is also approved for secondary hyperparathyroidism, are taken orally. Patients usually prefer popping pills to other methods, but not if you're already hooked up to a machine.

The true sales potential for KAI-4169 will ultimately depend on the phase 3 efficacy data, so it'll be a while before we know whether the KAI acquisition was a good buy. But for now, it looks like a good move to combat the inevitable loss of Sensipar.

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