A suspicion from retrospective data has now been confirmed by a prospective clinical trial: Adding panitumumab (Vectibix) to standard chemotherapy in left-sided RAS wild-type metastatic colorectal cancer (mCRC) is more effective than adding bevacizumab (Avastin).
Patients treated with panitumumab alongside chemotherapy saw a 16% improvement in overall survival versus those given bevacizumab after a median follow-up of over 5 years.
The overall survival benefit rose to 18% in those with left-sided tumors.
However, there was no difference in overall survival between the two treatment groups in the small subgroup of patients with right-sided primary tumors.
These findings come from the PARADIGM trial conducted in Japan.
The results were presented during a plenary session at the annual meeting of the American Society of Clinical Oncology.
“If gene testing shows that a tumor is RAS wild-type, the choice of initial treatment with panitumumab plus mFOLFOX6 chemotherapy is superior ... for those people with left-sided tumors,” said lead researcher Takayuki Yoshino, MD, PhD, department of gastrointestinal oncology, National Cancer Center Hospital East, Chiba, Japan, in an ASCO press release.
“It has long been believed that the sequence of metastatic colorectal cancer treatment does not matter as long as patients had access to the drugs at some point, which has now been disproven,” he noted.
Dr. Yoshino added in a press conference about the trial that the results establish “a standard first-line combination regimen for patients with RAS wild-type, left-sided mCRC.”
commented Cathy Eng, MD, ASCO Expert in gastrointestinal cancers.
The findings “emphasize the importance of taking into account sidedness, as well as including comprehensive biomarker testing,” she said.
Dr. Eng underlined that this is especially the case for RAS gene status testing, “which is critical for all colorectal cancer patients at the time of diagnosis of metastatic disease.”
These results are of particular relevance in the United States, where the choice between an anti-EGFR or anti-VEGF antibody for the treatment of mCRC has been an area of “controversy” because of the lack of supporting data.
Panitumumab is a human monoclonal antibody that targets EGFR. It was approved in 2006 for use in mCRC by the U.S. Food and Drug Administration and also approved in 2014 for use in combination with FOLFOX for the first-line treatment of patients with wild-type KRAS (exon 2 in codons 12 or 13) mCRC, having previously been shown to be equally effective as cetuximab (another EGFR inhibitor) in this population.
In contrast, bevacizumab is a monoclonal antibody that targets the VEGF receptor. It was approved by the FDA for use in mCRC in 2004 in combination with intravenous 5-fluorouracil–based chemotherapy.
Dr. Yoshino explained that around 36% of patients with CRC have metastatic tumors at diagnosis and that adding an anti-EGFR or anti-VEGF antibody to chemotherapy improves overall survival in these patients by up to 30 months.
There has been “accumulating” evidence from retrospective studies suggesting that patients with RAS wild-type mCRC whose primary tumor is on the left side, which accounts for approximately 35% of mCRC cases, have a longer survival benefit with an anti-EGFR antibody, he commented.
Despite this, both antibody types continue to be used in these patients, he added.
PARADIGM was the first prospective trial to compare the two antibody types. Patients were randomized to receive either panitumumab or bevacizumab plus the combination chemotherapy regimen modified FOLFOX6 (mFOLFOX6).
The trial involved 823 Japanese patients with previously untreated wild-type mCRC with unresectable disease. Most patients had left-sided primary tumors (312 of 400 patients in the panitumumab group, and 292 of 402 patients in the bevacizumab group).
After a median follow-up of 61 months, panitumumab was associated with a significant improvement in overall survival in the overall study population, at a hazard ratio of 0.84 (P = .030, with the boundary of significance set at P < .05).
In addition, panitumumab was associated with a significant improvement in overall survival in the large subgroup of patients with left-sided primary tumors, at 37.9 versus 34.3 months, or a hazard ratio of 0.82 (P = .031).
However, there was no significant difference in overall survival between the two treatment groups in the smaller subgroup of patients with right-sided tumors, at a hazard ratio of 1.09.
Median progression-free survival was no different between the panitumumab and bevacizumab groups, at 13.7 versus 13.2 months in patients with a left-sided tumor and 12.9 versus 12.0 months in the overall cohort.
There was, however, a difference in response rates in left-sided patients between those receiving the two antibodies, at 80.2% with panitumumab versus 68.6% with bevacizumab, and in curative resection rates, at 18.3% and 11.6%, respectively.
These results demonstrate the “superiority of first-line panitumumab versus bevacizumab in combination with mFOLFOX6 in the left-sided and overall populations,” Dr. Yoshino concluded.
He also highlighted that the team has undertaken a large-scale biomarker analysis of pre- and posttreatment plasma and tissue samples from patients in the PARADIGM study to identify potential biomarkers of treatment response.
At the plenary session, discussant for this abstract Chiara Cremolini, MD, PhD, professor of medical oncology, Pisa (Italy) University Hospital, commented that “location matters” when it comes to mCRC tumors.