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.

What: Shares of Nektar Therapeutics (NASDAQ:NKTR) fell by more than 11% today on heavy volume, reflecting the company's announcement that its experimental breast cancer drug, NKRT-102, failed to significantly improve overall survival in a pivotal late-stage study. The drug failed to achieve its secondary endpoints of overall response rate and progression-free survival as well. 

Digging into the details, NKTR-102's anti-cancer activity was compared against patients receiving a single chemotherapy that was chosen by the attending physician. All patients in the study were heavily pre-treated with roughly three prior therapies for metastatic disease.

On the bright side, the drug did significantly improve median overall survival in select sub-groups of patients afflicted with brain and liver metastases. Specifically, patients with brain metastases showed an improvement in overall survival of 5.2 months (10 months for patients taking NKTR-102, relative to only 4.8 months for patients on chemotherapy). Patients with liver metastases also exhibited a moderately higher median overall survival of 2.6 months when taking NKTR-102, compared to chemotherapy. 

So what: While Nektar does sport a broad clinical pipeline, it doesn't have many experimental drugs that can be considered true blockbuster candidates. NKTR-102 was one of the few exceptions, given that a fair chunk of FDA-approved breast cancer drugs do go on to to reach blockbuster status. 

What now: Nektar is reportedly discussing these data with regulatory agencies in the U.S. and Europe today, meaning that they might push for a limited approval based on sub-group analysis. Indeed, findings for these certain patient populations were both significant and clinically meaningful. 

So the drug shouldn't be completely written off by investors yet. But it almost certainly won't be the huge revenue generator that investors were hoping for at the start of the study.