Search

Small-molecule inhibitors of the CDK4/6 cell-cycle kinases have shown clinical efficacy in estrogen receptor (ER)-positive metastatic breast cancer, although their cytostatic effects are limited by primary and acquired resistance. Here we report that ER-positive breast cancer cells can adapt quickly to CDK4/6 inhibition and evade cytostasis, in part, via noncanonical cyclin D1-CDK2-mediated S-phase entry. This adaptation was prevented by cotreatment with hormone therapies or PI3K inhibitors, which reduced the levels of cyclin D1 (CCND1) and other G1-S cyclins, abolished pRb phosphorylation, and inhibited activation of S-phase transcriptional programs. Combined targeting of both CDK4/6 and PI3K triggered cancer cell apoptosis in vitro and in patient-derived tumor xenograft (PDX) models, resulting in tumor regression and improved disease control. Furthermore, a triple combination of endocrine therapy, CDK4/6, and PI3K inhibition was more effective than paired combinations, provoking rapid tumor regressions in a PDX model. Mechanistic investigations showed that acquired resistance to CDK4/6 inhibition resulted from bypass of cyclin D1-CDK4/6 dependency through selection of CCNE1 amplification or RB1 loss. Notably, although PI3K inhibitors could prevent resistance to CDK4/6 inhibitors, they failed to resensitize cells once resistance had been acquired. However, we found that cells acquiring resistance to CDK4/6 inhibitors due to CCNE1 amplification could be resensitized by targeting CDK2. Overall, our results illustrate convergent mechanisms of early adaptation and acquired resistance to CDK4/6 inhibitors that enable alternate means of S-phase entry, highlighting strategies to prevent the acquisition of therapeutic resistance to these agents. [ABSTRACT FROM AUTHOR]

Fulvestrant is used to treat patients with hormone receptor-positive advanced breast cancer, but acquired resistance is poorly understood. PlasmaMATCH Cohort A (NCT03182634) investigated the activity of fulvestrant in patients with activating ESR1 mutations in circulating tumor DNA (ctDNA). Baseline ESR1 mutations Y537S are associated with poor outcomes and Y537C with good outcomes. Sequencing of baseline and EOT ctDNA samples (n = 69) revealed 3/69 (4%) patients acquired novel ESR1 F404 mutations (F404L, F404I, and F404V), in cis with activating mutations. In silico modeling revealed that ESR1 F404 contributes to fulvestrant binding to estrogen receptor-alpha (ERα) through a pi-stacking bond, with mutations disrupting this bond. In vitro analysis demonstrated that single F404L, E380Q, and D538G models were less sensitive to fulvestrant, whereas compound mutations D538G + F404L and E380Q + F404L were resistant. Several oral ERα degraders were active against compound mutant models. We have identified a resistance mechanism specific to fulvestrant that can be targeted by treatments in clinical development., Significance: Novel F404 ESR1 mutations may be acquired to cause overt resistance to fulvestrant when combined with preexisting activating ESR1 mutations. Novel combinations of mutations in the ER ligand binding domain may cause drug-specific resistance, emphasizing the potential of similar drug-specific mutations to impact the efficacy of oral ER degraders in development. This article is featured in Selected Articles from This Issue, p. 201., (©2023 The Authors; Published by the American Association for Cancer Research.)

Cyclin-dependent kinase 4/6 (CDK4/6) and PI3K inhibitors synergize in PIK3CA -mutant ER-positive HER2-negative breast cancer models. We conducted a phase Ib trial investigating the safety and efficacy of doublet CDK4/6 inhibitor palbociclib plus selective PI3K inhibitor taselisib in advanced solid tumors, and triplet palbociclib plus taselisib plus fulvestrant in 25 patients with PIK3CA -mutant, ER-positive HER2-negative advanced breast cancer. The triplet therapy response rate in PIK3CA- mutant, ER-positive HER2-negative cancer was 37.5% [95% confidence interval (CI), 18.8-59.4]. Durable disease control was observed in PIK3CA -mutant ER-negative breast cancer and other solid tumors with doublet therapy. Both combinations were well tolerated at pharmacodynamically active doses. In the triplet group, high baseline cyclin E1 expression associated with shorter progression-free survival (PFS; HR = 4.2; 95% CI, 1.3-13.1; P = 0.02). Early circulating tumor DNA (ctDNA) dynamics demonstrated high on-treatment ctDNA association with shorter PFS (HR = 5.2; 95% CI, 1.4-19.4; P = 0.04). Longitudinal plasma ctDNA sequencing provided genomic evolution evidence during triplet therapy. SIGNIFICANCE: The triplet of palbociclib, taselisib, and fulvestrant has promising efficacy in patients with heavily pretreated PIK3CA -mutant ER-positive HER2-negative advanced breast cancer. A subset of patients with PIK3CA -mutant triple-negative breast cancer derived clinical benefit from palbociclib and taselisib doublet, suggesting a potential nonchemotherapy targeted approach for this population. This article is highlighted in the In This Issue feature, p. 1 ., (©2020 American Association for Cancer Research.)

Unlabelled: FGFR1 and FGFR2 are amplified in many tumor types, yet what determines response to FGFR inhibition in amplified cancers is unknown. In a translational clinical trial, we show that gastric cancers with high-level clonal FGFR2 amplification have a high response rate to the selective FGFR inhibitor AZD4547, whereas cancers with subclonal or low-level amplification did not respond. Using cell lines and patient-derived xenograft models, we show that high-level FGFR2 amplification initiates a distinct oncogene addiction phenotype, characterized by FGFR2-mediated transactivation of alternative receptor kinases, bringing PI3K/mTOR signaling under FGFR control. Signaling in low-level FGFR1-amplified cancers is more restricted to MAPK signaling, limiting sensitivity to FGFR inhibition. Finally, we show that circulating tumor DNA screening can identify high-level clonally amplified cancers. Our data provide a mechanistic understanding of the distinct pattern of oncogene addiction seen in highly amplified cancers and demonstrate the importance of clonality in predicting response to targeted therapy., Significance: Robust single-agent response to FGFR inhibition is seen only in high-level FGFR-amplified cancers, with copy-number level dictating response to FGFR inhibition in vitro, in vivo, and in the clinic. High-level amplification of FGFR2 is relatively rare in gastric and breast cancers, and we show that screening for amplification in circulating tumor DNA may present a viable strategy to screen patients. Cancer Discov; 6(8); 838-51. ©2016 AACR.This article is highlighted in the In This Issue feature, p. 803., (©2016 American Association for Cancer Research.)

Unlabelled: Activation of fibroblast growth factor receptors (FGFR) is a common oncogenic event. Little is known about the determinants of sensitivity to FGFR inhibition and how these may vary between different oncogenic FGFRs. Using parallel RNA interference (RNAi) genetic screens, we show that the EGF receptor (EGFR) limits sensitivity to FGFR inhibition in FGFR3-mutant and -translocated cell lines, but not in other FGFR-driven cell lines. We also identify two distinct mechanisms through which EGFR limits sensitivity. In partially FGFR3-dependent lines, inhibition of FGFR3 results in transient downregulation of mitogen-activated protein kinase signaling that is rescued by rapid upregulation of EGFR signaling. In cell lines that are intrinsically resistant to FGFR inhibition, EGFR dominates signaling via repression of FGFR3, with EGFR inhibition rescued by delayed upregulation of FGFR3 expression. Importantly, combinations of FGFR and EGFR inhibitors overcome these resistance mechanisms in vitro and in vivo. Our results illustrate the power of parallel RNAi screens in identifying common resistance mechanisms to targeted therapies., Significance: Our data identify a novel therapeutic approach to the treatment of FGFR3-mutant cancer, emphasizing the potential of combination approaches targeting both FGFR3 and EGFR. Our data extend the role of EGFR in mediating resistance to inhibitors targeting a mutant oncogene, showing that EGFR signaling can repress mutant FGFR3 to induce intrinsic resistance to FGFR targeting., (©2013 AACR.)