The molecular evolution of acquired resistance to targeted EGFR blockade in colorectal cancers

Abstract

Colorectal tumours that are wild type for KRAS are often sensitive to EGFR blockade, but almost always develop resistance within several months of initiating therapy. The mechanisms underlying this acquired resistance to anti-EGFR antibodies are largely unknown. This situation is in marked contrast to that of small-molecule targeted agents, such as inhibitors of ABL, EGFR, BRAF and MEK, in which mutations in the genes encoding the protein targets render the tumours resistant to the effects of the drugs. The simplest hypothesis to account for the development of resistance to EGFR blockade is that rare cells with KRAS mutations pre-exist at low levels in tumours with ostensibly wild-type KRAS genes. Although this hypothesis would seem readily testable, there is no evidence in pre-clinical models to support it, nor is there data from patients. To test this hypothesis, we determined whether mutant KRAS DNA could be detected in the circulation of 28 patients receiving monotherapy with panitumumab, a therapeutic anti-EGFR antibody. We found that 9 out of 24 (38%) patients whose tumours were initially KRAS wild type developed detectable mutations in KRAS in their sera, three of which developed multiple different KRAS mutations. The appearance of these mutations was very consistent, generally occurring between 5 and 6 months following treatment. Mathematical modelling indicated that the mutations were present in expanded subclones before the initiation of panitumumab treatment. These results suggest that the emergence of KRAS mutations is a mediator of acquired resistance to EGFR blockade and that these mutations can be detected in a non-invasive manner. They explain why solid tumours develop resistance to targeted therapies in a highly reproducible fashion.

Fig. 1. Emergence of circulating mutant KRAS.

The time course of circulating mutant KRAS alleles, CEA and tumor burden are depicted in two patients where fragments of circulating DNA containing mutant KRAS were detected. (A) Demonstrates the emergence of four different mutant KRAS alleles in codon 12 (cDNA nt 35T, 34T, 35C and 34C) in the serum of Patient #1 and (B) demonstrates the emergence of two different mutant KRAS alleles in codon 12 (cDNA nt 34T and 35C) in the serum of Patient #12. Tumor burden refers to the aggregate cross-sectional diameter of the index lesions.

Fig. 2

Predicted probability distribution of times from when treatment starts until resistance mutations become observable in circulating DNA. Overlaid is the observed time to detection of mutant KRAS fragments (23 ± 5 weeks, mean ± 1 standard deviation) and time to clinical progression (25 ± 10 weeks, mean ± 1 standard deviation) in the studied patients. Predictions were based on the Lea-Coulson model with death, as introduced by Dewanji et al., or equivalently, the branching process model of Iwasa et al^,.