Amgen (NASDAQ:AMGN) investors can finally breathe a little sigh of relief after the biopharma giant announced on Friday that the Food and Drug Administration had finally approved its blood disorder drug Nplate for marketing.

After having its Nplate marketing application decision delayed twice by the FDA, first in April and a second time in July, Amgen finally got the go-ahead to sell Nplate as a treatment for a rare chronic blood platelet disorder called chronic immune thrombocytopenic purpura (ITP).

Amgen estimates that there are 60,000 patients in the U.S. being treated with steroids and other drugs for thrombocytopenia and 140,000 total, counting Europe. None of these treatments are very effective, and Amgen will have a good opportunity to carve out a large market for itself, depending on how another potential thrombocytopenia treatment, Promacta, from GlaxoSmithKline (NYSE:GSK) and Ligand Pharmaceuticals (NASDAQ:LGND), fares with the FDA.

Amgen has been tight-lipped about how it will price Nplate. It is already approved for marketing in Australia, but taking a look at Australia's drug pricing regulatory agency shows that Nplate reimbursement hasn't been finalized over there. Nplate's final pricing will likely depend on whether Glaxo and Ligand's Promacta is approved for marketing when it comes up for FDA review around Sept. 19 (assuming no more FDA review delays), and Ligand has presented charts in the past saying that the annual drug costs of "blood-related treatments" are approximately $15,000 for currently used medications.

Whether Promacta gets approved or not, Nplate currently has the advantage in that it is approved as a long-term treatment for ITP whereas Promacta is only up for approval as a short-term treatment. Glaxo and Ligand hope to get Promacta approved in the long-term setting as well, but that will come in a future marketing application that the drugmakers expect to file later this year.

Amgen has been on a bit of a roll lately with positive phase 3 results for its potential osteoporosis treatment denosumab and now Nplate's approval. Denosumab still has a while to go until approval and some strong competition from other drugmakers like Merck (NYSE:MRK) and Eli Lilly (NYSE:LLY) up against it, but Amgen's stock price has responded to the recent successes with denosumab and Nplate, up about one-third in the past six months. Now investors get to find out how big the market for Nplate will be and whether it will be able to pick up the slack for some of Amgen's other top compounds.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. Eli Lilly and GlaxoSmithKline are active Income Investor picks. Try any of our Foolish newsletters today, free for 30 days. The Fool has an A+ disclosure policy.