Celldex shares dropped 8 percent in after-hours trading.

The drug, CDX-011, which links a tumor-targeting antibody to a cell-killing chemotherapy drug, was tested in 122 patients whose breast cancer had progressed despite several previous rounds of standard therapy.

In the study, released by the company on Wednesday, 19 percent of patients taking CDX-011 saw their tumors shrink, compared with 14 percent of those treated with standard chemotherapy.

CDX-011 is designed to target GPNMB, a protein produced on the surface of several types of malignant tumors. In trial patients with GPNMB in at least 25 percent of tumor cells, 32 percent responded to the drug, compared with 13 percent of chemotherapy patients.

"GPNMB helps the tumor cell to move around," said Dr. Linda Vahdat, director of the breast cancer research program at Weill Cornell Medical College and the study's lead investigator. "It almost seems like this drug puts the brakes on that process by taking away its legs."

For patients with high levels of GPNMB as well as "triple negative" breast cancer - meaning cancer that it's not sensitive to estrogen, progesterone or a protein known as HER2 - the response rate was 36 percent compared with no responses in the control group.

"I think that there is a lot of data and that the company has done a great study," said Wedbush Securities analyst Gregory Wade. "The results are very promising considering how heavily pre treated the patients are ... I think that if anything the reason the stock has moved around a little bit is there is a lot to interpret."

Celldex shares, which closed at $4.26, initially rose after the study results were announced before falling back to $3.90 in after hours trading

Side affects associated with CDX-011, also known as glembatumumab vedotin, included rash and peripheral neuropathy.

The company said it expects to announce updated results from the Phase II trial in the fourth quarter of this year.

"This patient population has a very real need," said Tom Davis, chief medical officer at Celldex. "It is reasonable to consider a single-arm study ... a randomized study is also a possibility."

In a randomized trial patients are allocated at random to receive one of several clinical interventions. In a single arm trial there is no comparison group.

About 15 percent of breast cancer patients are triple negative, while high expression of GPNMB occurs in about 27 percent, according to Celldex. Because there is some overlap, both groups account for an estimated 35 percent of the total breast cancer population.

About 227,000 U.S. women are diagnosed with invasive breast cancer each year, according to the American Cancer Society.

Chief Executive Officer Anthony Marucci said Celldex will consider partnerships to develop CDX-011, but plans to move forward with the drug regardless of any such deals.

(Reporting By Deena Beasley; Editing by Tim Dobbyn and Carol Bishopric)

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