Investors are punishing Celldex Therapeutics (CLDX - Get Report) today because of significant changes made to a phase III breast cancer study. 

Last December, Celldex started a phase III study of glembatumumab (CDX-011) in patients with triple-negative breast cancer. Tuesday night, the company acknowledged enrollment into the study -- dubbed "METRIC" -- was slower than expected, in part because doctors believe the eligibility criteria are too limiting. To fix the problem, Celldex will now enroll a broader population of triple-negative breast cancer patients. The downside is enrollment into the study will extend into 2016, longer than investors expected. 

Celldex also changed the primary endpoint of the CDX-011 study, keeping progression-free survival but eliminating overall response rate. The company discussed these study changes with European regulators, not the FDA. 

All in, investors are selling Celldex today because of concerns the changes made to the METRIC study of CDX-011 introduce more risk. Celldex shares are down 20% to $13.21 in Wednesday trading.

CDX-011 is a monoclonal antibody drug conjugate. The antibody portion targets cancer cells that express the GPNMB protein, which has been shown to correlate with poorer outcomes in breast cancer patients. When the CDX-011 antibody attaches to GPNMB-expressing tumor cells, it releases a toxic chemotherapy payload. This "drug conjugate" was licensed from  Seattle Genetics (SGEN)  and is the same one used in the newly approved lymphoma drug Adcetris.

Celldex announced results from a phase II study of CDX-011 in May 2012.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.