NEEDHAM, Mass. ( TheStreet) -- A targeted cancer drug, CDX-011, from Celldex Therapeutics ( CLDX) delayed tumor growth and prolonged survival in patients with an advanced and aggressive form of breast cancer compared to single-agent chemotherapy, according to final results from a mid-stage study presented Saturday. Dr. Denise Yardley of the Sarah Canon Research Institute said the CDX-011 data showed a "clear signal of activity" because the breast cancer patients enrolled in the study typically respond poorly to currently approved medicines. However, the data presented Saturday was derived from a small number of patients, meaning Celldex will have to run a larger, confirmatory study to more definitively vet CDX-011's benefit, she cautioned. Celldex plans to meet soon with the U.S. Food and Drug Administration to get sign-off on a plan for a confirmatory study of CDX-011, which if positive, could lead to an accelerated approval, said CEO Anthony Marucci. The CDX-011 phase II results were released at the San Antonio Breast Cancer Symposium. Interim results from the same study were announced by Celldex last May. CDX-011 is a monoclonal antibody drug conjugate. The antibody portion latches on to cancer cells that contain a protein known as GPNMB, 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 with tumors containing high levels of the GPNMB protein. The 120 patients enrolled in the study were screened to make sure they all had at least some tumor cells that contained GPNMB but were then separated into high-expression and low-expression groups. 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. In patients with triple-negative breast cancer that also contained high levels of GPNMB -- a group of patients most difficult to treat -- treatment with CDX-011 led to a 33% response rate compared to no responses in patients treated with "investigator's choice" chemotherapy. CDX-011 doubled the time before tumors started to grow again to a median of 3 months compared to 1.5 months for control arm patients.