Resistance Mechanisms to Anti–Epidermal Growth Factor Receptor Therapy in RAS/RAF Wild-Type Colorectal Cancer Vary by Regimen and Line of Therapy

Abstract

PURPOSE Acquired resistance to anti-EGFR therapy (EGFRi) in CRC has previously been explained by the model of acquiring new mutations in KRAS/NRAS/EGFR, among other MAPK-pathway members. However, this was primarily based on single-agent EGFRi trials and little is known about the resistance mechanisms of EGFRi combined with effective cytotoxic chemotherapy in previously untreated patients. METHODS We analyzed paired plasma samples from RAS/BRAF/EGFR wild-type mCRC patients enrolled in three large randomized trials evaluating EGFRi in the first-line in combination with chemotherapy and as a single-agent in third-line. The mutational signature of the alterations acquired with therapy was evaluated. CRC cell lines with resistance to cetuximab, FOLFOX, and SN38 were developed, and transcriptional changes profiled. RESULTS Patients whose tumors were treated with and responded to EGFRi alone were more likely to develop acquired mutations (46%) compared to those treated in combination with cytotoxic chemotherapy (9%). Further, contrary to the generally accepted hypothesis of the clonal evolution of acquired resistance, we demonstrate that baseline resistant subclonal mutations rarely expanded to become clonal at progression, and most remained subclonal or disappeared. Consistent with this clinical finding, preclinical models with acquired resistance to either cetuximab or chemotherapy were cross-resistant to the alternate agents, with transcriptomic profiles consistent with epithelial-to-mesenchymal transition (EMT). In contrast, commonly acquired resistance alterations in the MAPK pathway do not impact sensitivity to cytotoxic chemotherapy. CONCLUSION These findings support a model of resistance whereby transcriptomic mechanisms of resistance predominate in the presence of active cytotoxic chemotherapy combined with EGFRi, with a greater predominance of acquired MAPK mutations after single-agent EGFRi. The proposed model has implications for prospective studies evaluating EGFRi rechallenge strategies guided by acquired MAPK mutations, and highlights the need to address transcriptional mechanisms of resistance.

Reply to Y. Yu et al

“…2,4 Patterns similar to our current study were also seen for other acquired mutations in this study. 2,4 Furthermore, the study, using preclinical models, also revealed that extra-genomic transcriptomic mechanisms, specifically epithelial-to-mesenchymal transition, may explain cross-resistance between anti-EGFR-Ab and chemotherapy. 4 In summary, we maintain that these results indicate differing mechanisms of resistance to anti-EGFR-Ab used in combination with first-line chemotherapy and in later lines.…”

“…Additionally, and perhaps more importantly, these results are not isolated observation and are supported by similar findings from one of our other studies (Parseghian et al 4 ), which used data from large randomized trials comparing first-line and third-line anti–EGFR-Ab therapy. The rate of Acq-GAs seen was 9% versus 46% (our study: 7% v 62%) for the up-front use of anti-EGFR chemotherapy versus anti–EGFR-Ab therapy alone in studies with later lines.…”

“…2,4 Furthermore, the study, using preclinical models, also revealed that extra-genomic transcriptomic mechanisms, specifically epithelial-to-mesenchymal transition, may explain cross-resistance between anti-EGFR-Ab and chemotherapy. 4 In summary, we maintain that these results indicate differing mechanisms of resistance to anti-EGFR-Ab used in combination with first-line chemotherapy and in later lines. These estimates and the potential biology behind progression on therapy should be kept in mind while employing anti-EGFR-Ab rechallenge in clinics and on clinical trials.…”

Line by Line: Distinct Patterns of Anti-EGFR Antibody Resistance by Line of Therapy

“…The studies by Topham et al, 14 Raghav et al, 16 and Parseghian et al 17 reveal strikingly different molecular patterns of acquired resistance to anti-EGFR antibodies in colorectal cancer, with frequent heterogeneous genomic resistance mechanisms observed after late-line therapy but rarely observed after first-line therapy. These observations raise interesting questions about potential mechanistic differences in the emergence of acquired resistance between different lines of targeted therapy.…”

“…Notably, even well-known anti-EGFR resistance mutations in KRAS and even EGFR-ECD-the latter not previously observed in the absence of prior anti-EGFR exposure-were observed at similar frequency between the cetuximab and bevacizumab arms after first-line therapy. This suggests a much different pattern of Finally, in a third study, Parseghian et al 17 directly compared paired pretreatment and postprogression plasma samples between patients receiving anti-EGFR antibody therapy in one first-line trial versus two third-line trials. Similarly, they observed the frequency of acquired genomic resistance alterations was substantially higher in the thirdline setting (46%) compared to the first line (9%).…”