[The RAS paradox of the EGFR-targeted therapy in colorectal cancer].

During the carcinogenesis of colorectal cancer in about half of the cases K-RAS, while much less frequently B-RAF mutation occur in early adenomas. While K-RAS mutant tumors are more likely present in male patients, B-RAF mutant tumors develop more frequently in females and are independent of the microsatellite status. Colorectal cancers are characterized by EGFR expression; the gene is not mutated, rarely amplified and increased copy number is due to chromosomal polysomy. This phenotype/genotype of colorectal cancer lent support to the introduction of anti-EGFR antibody therapies. For a while positive EGFR expression status of the tumor was the basis of patient selection for these targeted therapies in colorectal cancer. Monotherapies with the two available anti-EGFR antibodies of chemoresistant colorectal cancers resulted in appr. 10% objective response rate, which was independent of the level of EGFR expression. In case of panitumumab it was discovered that the efficacy of this targeted therapy depends on the K-RAS mutant status of the tumors. Furthermore, preliminary data suggest that cetuximab combined with chemotherapy is effective also exclusively in K-RAS wild-type tumors. Based on these data it is safe to say that K-RAS mutant status of colorectal cancer is a negative predictor for EGFR-targeted therapies of colorectal cancer. Accordingly, it is necessary to determine the K-RAS status of colorectal cancer before making therapeutic decisions.