Circulating free DNA as a surrogate for tumor material for EGFR and KRAS analysis

The analysis of a patient9s tumor mutation status has become increasingly important. Pharmaceuticals such as gefitinib require that a mutation test is performed prior to treatment to help predict a positive outcome to the patient before the drug is administered. However, obtaining a suitable tumor sample for analysis is not always possible and therefore some patients that may benefit will not receive the drug. The use of easily obtainable samples such as plasma or serum would be ideal if tumor derived material could be readily extracted and contained the same predictive mutations as the source tumor. We have evaluated circulating free (cf) DNA from plasma, and to a lesser extent serum, from lung cancer patients for EGFR and KRAS mutations using allele specific PCR tests (ARMSTM). The majority of the samples had no matched tumor sample available so the mutation detection rate was compared to the expected frequency of mutations in this tumor and population type. EGFR mutations were detected in 21 of 237 plasma cf DNA samples (8.9%) (expected: 10-15%). KRAS mutations were detected in 11 of 168 plasma cf DNA samples (6.6%) (expected: 17%). Where analysis was performed on the matched cf DNA extracted from serum the mutation detection rate was lower than in plasma. Interestingly the EGFR mutation frequency was higher than KRAS in the plasma samples, although the KRAS mutation frequency was expected to be higher than EGFR mutation frequency in the tumors. This could be due to the frequent amplification of mutant EGFR in lung tumors resulting in more target copies in the cf DNA, the EGFR assays could be more sensitive or perhaps the nature of EGFR mutation containing tumors is different making them shed more DNA or survive longer in the blood. In conclusion, cf DNA derived from plasma is a useful surrogate for mutation detection when no tumor sample is available, but the mutation detection rate is not equal for different mutations in different genes.