WASHINGTON — Twice-weekly doses of the investigational monoclonal antibody panitumumab reduced short-term disease progression by 46% in previously treated patients with metastatic colorectal cancer, Dr. Marc Peeters reported at the annual meeting of the American Association for Cancer Research.
After 8 weeks of twice-weekly 6-mg/kg doses of panitumumab plus the best standard of care, 49% of 231 patients were alive and had no disease progression, compared with 30% of 232 patients who received the best standard of care without panitumumab, said Dr. Peeters of Ghent (Belgium) University Hospital.
Disease progression continued to be slower in patients treated with panitumumab, compared with the standard-care group until about 20 weeks of treatment, and more panitumumab-treated patients remained alive after 32 weeks of treatment, compared with the standard therapy group.
Panitumumab, which is being developed by Amgen Inc., targets the epidermal growth factor receptor.
The randomized study included 463 patients aged 27–82 years of age, with a median age of 62 years. Overall, 67% of the patients had colon cancer, and 33% had rectal cancer, and all but one patient had undergone at least two chemotherapy regimens. Other demographic and clinical characteristics were similar between the groups.
Skin rash, the most common adverse event, was reported in about 90% of the panitumumab patients and in 9% of the standard group. Other side effects that were more common among panitumumab patients included fatigue, abdominal pain, nausea, and diarrhea. No treatment-related deaths were reported.
“This is the first randomized controlled phase III study comparing a monoclonal antibody with best supportive care in chemoresistant patients,” Dr. Peeters noted. “These data support further investigations with panitumumab, and additional studies are ongoing,” he said.
Dr. James Abbruzzese, of the University of Texas M.D. Anderson Cancer Center in Houston, was the study's discussant at the meeting. The results were unsurprising, he said. “The objective response rate was very consistent with prior phase II studies.”
Dr. Abbruzzese, who was not financially associated with the study, reviewed the data and noted that the hazard ratio for panitumumab was significantly lower during the first weeks of care, stabilized to 0.5 at about 5 weeks, and then tapered off after about 20 weeks.
Although the long-term survival rates were no longer significantly different between the two groups, “the impact of panitumumab during the early weeks of treatment was substantial, and prevented patients from progressing or dying during that period of time,” he said. Panitumumab's impact on disease progression after 20 weeks remains uncertain, and may suggest some emergent resistance over time, he noted.
Panitumumab is being incorporated into investigational front-line cancer therapy regimens, and future research will examine its long-term effects.