In This Issue

See article, p. 1334

MET amplification can confer resistance to EGFR tyrosine kinase inhibitors (TKI) in patients with EGFR-mutant tumors, suggesting the potential for dual targeting of EGFR and MET. Bahcall and colleagues treated a patient with recurrent non–small cell lung cancer (NSCLC) harboring an EGFR exon 19 deletion mutation who progressed on the EGFR inhibitor erlotinib. The recurrent tumor harbored high-level MET amplification in addition to the EGFR exon 19 deletion. The patient was enrolled in a phase I clinical trial of the MET TKI savolitinib plus the EGFR TKI osimertinib and initially responded, but eventually experienced disease progression, and the resistant tumor acquired a MET^D1228V mutation. Type I MET TKIs, including savolitinib, bind to the MET active conformation. Protein modeling revealed that the Asp1228 residue is key in maintaining the MET kinase activation loop, and MET^D1228V was predicted to disrupt savolitinib binding. In contrast, type II MET inhibitors such as cabozantinib bind the inactive MET conformation and thus would not be affected by MET^D1228V. As predicted, expression of MET^D1228V rendered cells resistant to type I, but not type II, MET inhibitors. Based on these findings, the patient was treated with erlotinib in combination with the type II MET inhibitor cabozantinib and achieved an ongoing response. Collectively, these findings provide support for targeting MET in NSCLC and indicate that acquired mutations can selectively induce resistance to type I MET inhibitors while maintaining type II inhibitor sensitivity. Thus, determination of resistance mechanisms may guide drug selection and improve MET inhibitor efficacy in patients.

See article, p. 1342

High-grade serous ovarian cancer (HGSOC) is a multifocal and heterogeneous disease thought to arise from in situ preneoplastic lesions in the fallopian tube known as serous tubal intraepithelial carcinomas (STIC), which progress to locally invasive fallopian tube lesions, primary ovarian tumors, and distant metastases in the peritoneal omentum. However, the spatiotemporal relationship between putative precursor STIC lesions and advanced metastatic HGSOC remains incompletely understood. Eckert and colleagues performed whole-exome sequencing of laser-capture microdissected tumor cells, including STIC and matched invasive fallopian tube, ovarian, and omental metastases, from eight patients with advanced sporadic HGSOC. Although patient-specific differences in mutation signatures and copy-number variations were observed, no differences in genomic alterations were detected across anatomic sites, which shared core events including TP53 mutations and deletion of DNA repair pathway genes, suggesting that genomic instability occurs early during HGSOC progression. Furthermore, phylogenetic clustering revealed distinct patterns of metastatic dissemination among patients, indicating that STIC was likely the precursor lesion in half of the patients, but that STIC arose from intraepithelial metastases to the fallopian tube in two patients. In support of this finding, ex vivo models demonstrated that HGSOC spheroids were capable of implanting in fallopian tube epithelium and expressed markers of STIC. These data provide further insight into the dynamics of HGSOC progression and suggest that STIC may represent either primary precursor lesions or secondary metastases in patients with HGSOC.

See article, p. 1352

Approximately 8% to 10% of patients with metastatic colorectal cancers harbor a BRAF^V600E mutation, which is associated with poor survival. Single-agent treatment with the BRAF inhibitor vemurafenib has a low response rate in these patients due to EGFR-mediated MAPK pathway reactivation, but preclinical data has suggested that addition of the BRAF^V600E inhibitor vemurafenib to the EGFR inhibitor cetuximab and the cytotoxic agent irinotecan, which are approved for RAS–wild-type metastatic colorectal cancer, may reduce MAPK signaling and enhance treatment efficacy. Hong, Morris, and colleagues performed a phase I dose escalation study to investigate the safety and activity of vemurafenib in combination with irinotecan and cetuximab in 18 patients with metastatic BRAF^V600E colorectal cancer and 1 patient with BRAF^V600E appendiceal cancer. This combination was generally well tolerated. Of the 17 evaluable patients, 6 (35%) achieved a response, and 15 exhibited disease control. The median progression-free survival was 7.7 months. Sequencing of cell-free DNA (cfDNA) from serial plasma samples from 12 patients revealed that reduced BRAF^V600E was correlated with radiographic response. Further, acquired mutations that reactivate MAPK signaling were observed at progression. The efficacy of the triplet regimen was confirmed in mouse xenograft models of BRAF^V600E metastatic colorectal cancer. Single agent irinotecan therapy and vemurafenib plus cetuximab reduced tumor progression, whereas triple combination therapy induced tumor regression. Overall, the results of this phase I trial suggest that vemurafenib in combination with cetuximab and irinotecan is well tolerated and may enhance antitumor activity in patients with metastatic BRAF^V600E colorectal cancer.

See article, p. 1366

In many tumor types, PD-1 (encoded by PDCD1) expression is elevated on tumor-infiltrating lymphocytes (TIL) and contributes to tumor immune evasion. However, the molecular mechanisms by which PD-1 is enriched on TILs are incompletely understood. Park and colleagues screened cytokines to identify regulators of T-cell PD-1 induction and found that transforming growth factor β1 (TGFβ1) promoted antigen-driven PD-1 upregulation. This effect required the kinase activity of the TGFβ1 receptor TGFβR1, which upon ligand binding heterodimerizes and phosphorylates SMAD2 and SMAD3. Inhibition of TGFβR signaling blocked the increase in PDCD1 expression, indicating that TGFβ1 regulates PDCD1 transcription. Consistent with these findings, TGFβ1 enhanced SMAD3 binding to the PDCD1 promoter, which harbors two SMAD-binding elements. Inhibition of SMAD3, but not SMAD2, prevented the TGFβ1-mediated induction of PDCD1, suggesting that SMAD3 is responsible for the induction of PD-1 by TGFβ1. In vivo, the growth of melanoma and colon cancer xenografts was reduced in Smad3-knockout mice, and these mice also lacked PD-1^hi CD8⁺ TILs. Further, Smad3-deficient CD8⁺ T cells displayed enhanced cytokine production, indicating increased T-cell function, and altogether demonstrating that TGFβ1 regulates PD-1 expression via SMAD3 in vivo. Treatment with an anti–PD-1 antibody had no effect in Smad3-deficient mice, confirming that TGFβ1/SMAD3 promotes immune evasion via PD-1 induction. The finding that TGFβ1 enhances PD-1 expression on TILs indicates an immunosuppressive role for TGFβ1 in promoting tumor growth and suggests that SMAD3 inhibitors may activate PD-1^hi TILs.

See article, p. 1382

Despite the success of immunotherapy in treating advanced cancers, it is not clear if immunotherapy will be more effective as a preoperative neoadjuvant therapy or postoperative adjuvant therapy in patients with resectable tumors. Liu and colleagues used two mouse models of metastatic triple-negative breast cancer to compare the effects of neoadjuvant and adjuvant immunotherapy on metastasis and survival. In these models, neoadjuvant depletion of regulatory T cells (Treg) to relieve tumor immunosuppression prior to tumor resection led to a greater increase in survival compared with postsurgery adjuvant Treg depletion, and long-term survivors remained free of residual metastatic disease, suggesting that neoadjuvant immunotherapy may be more effective than adjuvant immunotherapy. Similarly, neoadjuvant treatment with an antibody targeting PD-1 increased survival more than adjuvant therapy and was further enhanced by the addition of an anti-CD137 antibody. Neoadjuvant therapy depended on the presence of CD8⁺ and CD4⁺ T cells, natural killer cells, and IFNγ, and induced a strong increase in tumor-specific CD8⁺ T cells in the peripheral blood and organs, which remained elevated following resection of the primary tumor. High levels of tumor-specific CD8⁺ T cells in the blood following neoadjuvant immunotherapy were associated with improved survival, indicating that they may be a biomarker for outcome. Together, these findings indicate that neoadjuvant immunotherapy provides greater benefit than adjuvant immunotherapy in mouse models of metastatic triple-negative breast cancer, and suggest that further clinical investigation of neoadjuvant immunotherapy is warranted in patients with metastatic disease.

Note: In This Issue is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details.