While the Motley Fool believes investing should be a long term affair, investors should be aware when companies of interest make big moves higher or lower. Such big moves are usually due to important developments that could significantly influence investor's long term outlook.

Given that backdrop, let's take a look at why Endocyte Pharmaceuticals (NASDAQ:ECYT), Merrimack Pharmaceuticals (NASDAQ:MACK), and Pharmacyclics (UNKNOWN:PCYC.DL) are falling sharply today.

1. Endocyte Pharmaceuticals 

ECYT Chart

ECYT data by YCharts

Every trial has independent monitors that watch over it. If those monitors find that patients are doing really well on a drug, it can recommend that a company halts the trial early to allow other patients receiving placebo to receive the treatment.

In the case of Endocyte, those monitors came to the opposite conclusion, determining that it would be futile for Endocyte and Merck to continue their phase 3 study of Vynfinit as a treatment for platinum resistant ovarian cancer.

That trial had hoped to establish Vynfinit's ability to improve progression free survival in patients when dosed alongside Johnson & Johnson's Doxil versus Doxil alone.

Vynfinit's inability to meet its goal is especially disappointing given a key EU advisory panel had recommended EU regulators approve the drug for the indication in March, boosting investor's confidence.

Given that the EU panel's support was for conditional marketing authorization based on phase 2 trials, it's unclear if the phase 3 halt means EU support will be withdrawn.

If it is, patients will be particularly discouraged given that in cases where resistance develops to platinum therapy, Doxil would remain the only treatment option.

2. Merrimack Pharmaceuticals

MACK Chart

MACK data by YCharts

Shares in Merrimack surged 60% yesterday following news that its pancreatic cancer drug MM-398 outpaced chemotherapy in a late stage trials.

However, shares are giving back some of those gains today as investors digest the data and consider whether the share surge is justified.

Clearly, there's a significant need for new pancreatic cancer treatments. The disease is one of the toughest to treat and there are few approved drug alternatives.

Those that are most commonly used include generic Gemzar and Celgene's Abraxane, which won approval for use in pancreatic cancer patients last fall.

Despite the lack of alternatives, investors are right to wonder just how much of the market MM-398 may eventually capture. After all, MM-398 is being tested as a second-line treatment for patients who were previously treated with gemcitabine.

3. Pharmacyclics 

PCYC Chart

PCYC data by YCharts

Pharmacyclics shares have been tumbling as investors abandoned the recent rally in biotechnology stocks. Following Pharmacyclics' earnings release this morning, investors are taking the company's shares even lower.

In that earnings release. the company reported it had net product revenue of $56 million for Imbruvica during its first full quarter on the market. That led to non-GAAP earnings per share of $0.40, reversing a $0.40 loss a year ago.

Pharmacyclics' total revenue was $119 million thanks to $60 million in milestone payments from Johnson & Johnson, its partner on Imbruvica.

Imbruvica won its first approval in November as a second line treatment for mantle cell lymphoma, and a second approval in February as a second line therapy for chronic lymphocytic leukemia.

Thanks to the expanded label, Pharmacyclics expects second quarter net product sales will reach $80 million, and full year product sales of $295 million. That disappointed investors, who were likely hoping for an even bigger showing for this highly touted drug.

Regardless, given that Pharmacyclics has more than 11 ongoing phase 3 trials, and a total of 43 trials ongoing for Imbruvica overall, it's likely there's plenty of market moving news still to come from the company this year.

Editor's Note: A previous version of this article indicated that Merrimack's MM-398 could compete with gemcitabine and Abraxane if approved. This is incorrect as MM-398 is being tested as a second-line treatment. The Fool regrets the error.

Todd Campbell has no position in any stocks mentioned. Todd owns E.B. Capital Markets, LLC. E.B. Capital's clients may or may not have positions in the companies mentioned. Todd owns Gundalow Advisors, LLC. Gundalow's clients do not have positions in the companies mentioned. The Motley Fool owns and recommends shares of Johnson & Johnson, and recommends shares of Celgene. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.