Among patients with metastatic colorectal cancer that does not have a mutation in the KRAS gene, initial treatment with a combination of chemotherapy and Vectibix® (panitumumab) delays cancer progression by 1.6 months compared with chemotherapy alone. These results were presented at the 2010 ASCO Gastrointestinal Cancers Symposium.

Colorectal cancer remains the second leading cause of cancer-related death in the United States. Metastatic colorectal cancer refers to cancer that has spread from the colon to distant sites in the body.

Targeted therapies are anticancer drugs that interfere with specific pathways involved in cancer cell growth or survival. Some targeted therapies block growth signals from reaching cancer cells; others reduce the blood supply to cancer cells; and still others stimulate the immune system to recognize and attack the cancer cell. Depending on the specific “target,” targeted therapies may slow cancer cell growth or increase cancer cell death.

Vectibix inhibits cancer cell growth and survival by targeting a protein known as the epidermal growth factor receptor (EGFR). Vectibix has been approved for the treatment of EGFR-expressing, metastatic colorectal cancer that has progressed on or following fluoropyrimidine-, oxaliplatin-, and irinotecan-containing chemotherapy regimens. Vectibix appears to benefit only those patients whose cancers do not contain a mutation in a gene known as KRAS. KRAS mutations occur in an estimated 40-50% of metastatic colorectal cancers and can be identified by testing a sample of tumor tissue.

To evaluate the effectiveness of Vectibix in the initial (first-line) treatment of metastatic colorectal cancer, researchers conducted a Phase III clinical trial known as PRIME (Panitumumab Randomized trial In combination with chemotherapy for Metastatic colorectal cancer to determine Efficacy). The study enrolled 1,183 patients. Study participants were assigned to receive treatment with FOLFOX4 chemotherapy alone or FOLFOX4 plus Vectibix.

• Among patients without KRAS mutations, the addition of Vectibix delayed cancer progression. Progression-free survival was 9.6 months among patients treated with chemotherapy plus Vectibix compared with 8.0 months among patients treated with chemotherapy alone.
• Among patients with KRAS mutations, the addition of Vectibix worsened outcomes. Progression-free survival was 7.3 months among patients treated with chemotherapy plus Vectibix compared with 8.8 months among patients treated with chemotherapy alone.
• Side effects of Vectibix included skin rash, low magnesium levels, and diarrhea.

The results of this study suggest that the addition of the targeted therapy Vectibix to first-line chemotherapy modestly improves progression-free survival among patients with metastatic colorectal cancer. The benefit of Vectibix only applies to patients whose tumors do not contain KRAS mutations.

Reference: Siena S, Cassidy J, Tabernero J et al. Randomized phase 3 study of panitumumab (pmab) with FOLFOX4 compared to FOLFOX4 alone as first-line treatment (tx) for metastatic colorectal cancer (mCRC): PRIME trial. Presented at 2010 ASCO Gastrointestinal Cancers Symposium. Orlando, FL, January 22-24, 2010. Abstract 283.

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