NEEDHAM, Mass. ( TheStreet) -- Celldex Therapeutics ( CLDX) announced results Wednesday from a mid-stage study of its targeted cancer drug CDX-011 demonstrating superior response rates in patients with advanced metastatic breast cancer compared to single-agent chemotherapy. As hoped, CDX-011 worked best in a subset of patients with tumors containing high levels of a protein linked to more aggressive disease. Treatment with CDX-011 partially shrank tumors in 19% of all patients enrolled in the phase II study compared to 14% of patients treated with "investigator's choice" chemotherapy. In patients with breast cancer tumors that contained high levels of the protein GPNMB, the CDX-011 response rate was 32% compared to 13% for patients treated with single-agent chemotherapy. CDX-011 also demonstrated strong response rates in a subgroup of patients with so-called triple negative breast cancer, regardless of GPNMB levels -- 21% of these patients responded to CDX-011 against none of the patients treated with chemotherapy. Based on these data, Celldex said it plans to meet with the U.S. Food and Drug Administration to design registration studies aimed at getting CDX-011 approved as a treatment for advanced breast cancer patients who overexpress GPNMB and with triple-negative disease. 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 designed the phase II study to test the theory that CDX-011 would work better in breast cancer patients whose tumors over-express the GPNMB protein. The 122 patients enrolled in the study had breast cancer in which at least 5% of the cells screened positive for GPNMB. All the patients had advanced breast cancer that was no longer responding to a median of 5-6 prior therapies. Two-thirds of patients were treated with injections of CDX-011 while the remaining third of patients were treated with chemotherapy of the doctor's choice. Dr. Linda Vahdat of Weill Cornell Medical College and the lead investigator in the phase II study said the results were "better than I hoped for," especially in the subset of patients with tumors that overexpress GPNMB and in triple-negative disease because this confirmed the mechanism by which CDX-011 was designed to work. These are also the breast cancer patients that have the worst prognosis and the shortest life expectancy, Vahdat added. To put the CDX-011 results in some perspective, Celldex executives point out that Eisai's Halaven and Bristol-Myer Squibb's ( BMY) Ixempra were approved with breast cancer response rates of 12-13% and 11-18%, respectively, in similar patients. About half of the patients in the CDX-011 study were treated with either Halaven or Ixempra. Neither drug was able to shrink tumors in patients who over-expressed GPNMB, said Celldex. The CDX-011 study is still ongoing so it's not known whether the drug will prolong survival in these breast cancer patients. Celldex declined to provide specific data on progression-free survival but showed slides indicating CDX-011 was delaying tumor re-growth compared to single-agent chemotherapy in some patients. In the small subset of patients with triple negative disease and high GPNMB expression, the progression-free survival benefit was statistically significant in favor of CDX-011. If higher tumor responses do not lead to significant delays in tumor re-growth or prolonged survival, Celldex may face a tougher hurdle getting CDX-011 approved by U.S. and European regulators. Cancer drugs can be approved on the basis of tumor shrinkage alone but generally only in patients who have run out of all other treatment options. In recent years, FDA regulators have made it more difficult for cancer drugs to be approved without some evidence of a survival benefit or delay in tumor progresssion. CDX-011 was generally well tolerated by patients in the phase II study, with rash the common adverse event reported by patients treated with the drug. Celldex shares were up 8 cents, or 2%, to $4.25 in late Wednesday trading. --Written by Adam Feuerstein in Boston. >To contact the writer of this article, click here: Adam Feuerstein. >To follow the writer on Twitter, go to http://twitter.com/adamfeuerstein. >To submit a news tip, send an email to: email@example.com. Follow TheStreet on Twitter and become a fan on Facebook.