Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

Let's take a look at three stocks -- Horizon Pharma (NASDAQ:HZNP), Prothena (NASDAQ:PRTA), and Portola Pharmaceuticals (NASDAQ:PTLA) -- all of which could loom large in biotech headlines this morning.

Horizon buys Vidara for $660 million
Horizon Pharma is up 22% as of this writing after it announced that it was acquiring privately held specialty pharmaceutical company Vidara Therapeutics via a reverse merger for approximately $660 million in stock and cash. About 26% of the combined company would be owned by Vidara shareholders.

Horizon Pharma markets three key drugs -- Duexis, Rayos/Lodotra, and Vimovo -- which target unmet therapeutic needs in arthritis, pain, and inflammatory diseases. Net sales of Duexis, which accounted for 80% of Horizon's top line, rose 473% year-over-year in fiscal 2013. Horizon notably acquired the U.S. rights for Vimovo from AstraZeneca last November.

Acquiring Vidara adds Actimmune, an approved interferon treatment for reducing serious infections caused by Chronic Granulomatous Disease, to Horizon's portfolio as its fourth marketed product. Vidara bought the rights to Actimmune from InterMune (NASDAQ:ITMN.DL) in May 2012 for $55 million and a two-year royalty stream.

Horizon Pharma finished fiscal 2013 with $74 million in net sales. Shares of Horizon have risen nearly 500% over the past 12 months on optimism regarding its pipeline.

Prothena jumps on two positive developments
Meanwhile, shares of Prothena, a clinical-stage biotech specializing in monoclonal antibodies, are up 22% as of this writing on two positive headlines.

First, the company announced that interim data from a phase 1 study of NEOD001 -- a treatment for immunoglobulin light chain (AL) amyloidosis and persistent organ dysfunction --will be presented at the XIV International Symposium on Amyloidosis (ISA) in Indianapolis at the end of April. NEOD001 was granted an orphan drug designation for the treatment of AA (secondary) and AL amyloidosis by the FDA in 2012. The European Medicines Agency also designated the drug as an orphan drug for the treatment of AL amyloidosis in 2013.

NEOD001 is the most advanced of Prothena's three primary drug candidates, which also include PRX002, a treatment for Parkinson's Disease, and PRX003, a treatment for inflammatory disease and late-stage cancer. PRX002 and PRX003 are currently in preclinical studies.

Second, Prothena shares were boosted by a big analyst upgrade. RBC Capital just raised its price target from $38 to $52 for the stock, based on the belief that NEOD001 could hit peak sales of $500 million. Shares of Prothena have notably soared nearly 500% over the past 12 months, despite having no marketed or late-stage pipeline products.

Portola initiates a phase 3 study of andexanet alfa
Last but not least, Portola Pharmaceuticals just announced that it has initiated a phase 3 study of andexanet alfa, the company's investigational Factor Xa inhibitor reversal agent. Factor Xa is a clotting factor which is blocked by blood thinners to keep blood clots from worsening.

Andexanet alfa is an FDA-designated breakthrough therapy which could potentially reverse the anticoagulation activity of Factor Xa inhibitor-treated patients who are suffering major bleeding or require immediate surgery. Portola's new phase 3 study will evaluate the safety and efficacy of andexanet alfa with Bristol-Myers Squibb's and Pfizer's Factor Xa inhibitor Eliquis.

Portola doesn't have any marketed products, but has three other products in its clinical pipeline -- Betrixaban (anticoagulant), Cerdulatinib (genetically defined blood cancers), and PRT2607 (allergic asthma). Betrixiaban, which is already in phase 3 trials, is Portola's most advanced pipeline product.

Andexanet alfa has attracted the most attention, with five clinical collaborators -- Pfizer, Bayer, Bristol-Myers Squibb, Johnson & Johnson, and Daiichi Sankyo. Biogen Idec currently collaborates with Portola in the development of PRT2607.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.