Aurora kinase A drives the evolution of resistance to third-generation EGFR inhibitors in lung cancer.

Although targeted therapies often elicit profound initial patient responses, these effects are transient due to residual disease leading to acquired resistance. How tumors transition between drug responsiveness, tolerance and resistance, especially in the absence of preexisting subclones, remains unclear. In epidermal growth factor receptor (EGFR)-mutant lung adenocarcinoma cells, we demonstrate that residual disease and acquired resistance in response to EGFR inhibitors requires Aurora kinase A (AURKA) activity. Nongenetic resistance through the activation of AURKA by its coactivator TPX2 emerges in response to chronic EGFR inhibition where it mitigates drug-induced apoptosis. Aurora kinase inhibitors suppress this adaptive survival program, increasing the magnitude and duration of EGFR inhibitor response in preclinical models. Treatment-induced activation of AURKA is associated with resistance to EGFR inhibitors in vitro, in vivo and in most individuals with EGFR-mutant lung adenocarcinoma. These findings delineate a molecular path whereby drug resistance emerges from drug-tolerant cells and unveils a synthetic lethal strategy for enhancing responses to EGFR inhibitors by suppressing AURKA-driven residual disease and acquired resistance.

Inactivation of Capicua drives cancer metastasis.

Metastasis is the leading cause of death in people with lung cancer, yet the molecular effectors underlying tumor dissemination remain poorly defined. Through the development of an in vivo spontaneous lung cancer metastasis model, we show that the developmentally regulated transcriptional repressor Capicua (CIC) suppresses invasion and metastasis. Inactivation of CIC relieves repression of its effector ETV4, driving ETV4-mediated upregulation of MMP24, which is necessary and sufficient for metastasis. Loss of CIC, or an increase in levels of its effectors ETV4 and MMP24, is a biomarker of tumor progression and worse outcomes in people with lung and/or gastric cancer. Our findings reveal CIC as a conserved metastasis suppressor, highlighting new anti-metastatic strategies that could potentially improve patient outcomes.

Circulating tumour DNA profiling reveals heterogeneity of EGFR inhibitor resistance mechanisms in lung cancer patients.

Circulating tumour DNA (ctDNA) analysis facilitates studies of tumour heterogeneity. Here we employ CAPP-Seq ctDNA analysis to study resistance mechanisms in 43 non-small cell lung cancer (NSCLC) patients treated with the third-generation epidermal growth factor receptor (EGFR) inhibitor rociletinib. We observe multiple resistance mechanisms in 46% of patients after treatment with first-line inhibitors, indicating frequent intra-patient heterogeneity. Rociletinib resistance recurrently involves MET, EGFR, PIK3CA, ERRB2, KRAS and RB1. We describe a novel EGFR L798I mutation and find that EGFR C797S, which arises in ∼33% of patients after osimertinib treatment, occurs in <3% after rociletinib. Increased MET copy number is the most frequent rociletinib resistance mechanism in this cohort and patients with multiple pre-existing mechanisms (T790M and MET) experience inferior responses. Similarly, rociletinib-resistant xenografts develop MET amplification that can be overcome with the MET inhibitor crizotinib. These results underscore the importance of tumour heterogeneity in NSCLC and the utility of ctDNA-based resistance mechanism assessment.

Discovery of a mutant-selective covalent inhibitor of EGFR that overcomes T790M-mediated resistance in NSCLC.

UNLABELLED: Patients with non-small cell lung cancer (NSCLC) with activating EGF receptor (EGFR) mutations initially respond to first-generation reversible EGFR tyrosine kinase inhibitors. However, clinical efficacy is limited by acquired resistance, frequently driven by the EGFR(T790M) mutation. CO-1686 is a novel, irreversible, and orally delivered kinase inhibitor that specifically targets the mutant forms of EGFR, including T790M, while exhibiting minimal activity toward the wild-type (WT) receptor. Oral administration of CO-1686 as single agent induces tumor regression in EGFR-mutated NSCLC tumor xenograft and transgenic models. Minimal activity of CO-1686 against the WT EGFR receptor was observed. In NSCLC cells with acquired resistance to CO-1686 in vitro, there was no evidence of additional mutations or amplification of the EGFR gene, but resistant cells exhibited signs of epithelial-mesenchymal transition and demonstrated increased sensitivity to AKT inhibitors. These results suggest that CO-1686 may offer a novel therapeutic option for patients with mutant EGFR NSCLC. SIGNIFICANCE: We report the preclinical development of a novel covalent inhibitor, CO-1686, that irreversibly and selectively inhibits mutant EGFR, in particular the T790M drug-resistance mutation, in NSCLC models. CO-1686 is the fi rst drug of its class in clinical development for the treatment of T790M-positive NSCLC, potentially offering potent inhibition of mutant EGFR while avoiding the on-target toxicity observed with inhibition of the WT EGFR.

Adenosine modification may be preferred for reducing siRNA immune stimulation.

The immune stimulation induced by short interfering RNAs (siRNAs) has been reported to be quieted or abrogated by methoxy or fluoro modifications of the 2' position of the ribose sugar. However, variables such as the type of modification, nucleotide preference, and strand bias have not been systematically evaluated. Here, we report the results of a screen of several modified siRNAs via a human peripheral blood monocyte cytokine induction assay. Unlike corresponding modifications of guanosine, cytidine, or uridine, 2'-fluoro modification of adenosine significantly reduced cytokine induction while retaining siRNA knockdown activity. The results of this study suggest adenosine as an optimal target for modification.

In vivo activity and duration of short interfering RNAs containing a synthetic 5'-phosphate.

Endogenous and exogenous short interfering RNAs (siRNAs) require a 5'-phosphate for loading into Ago2 and cleavage of the target mRNA. We applied a synthetic 5'-phosphate to siRNA guide strands to evaluate if phosphorylation in vivo is rate limiting for maximal siRNA knockdown and duration. We report, for the first time, an in vivo evaluation of siRNAs with a synthetic 5'-phosphate compared to their unphosphorylated versions. siRNAs that contained a 5'-phosphate had the same activity in vivo compared with unphosphorylated siRNAs, indicating phosphorylation of an siRNA is not a rate limiting step in vivo.

mRNA knockdown by single strand RNA is improved by chemical modifications.

While RNAi has traditionally relied on RNA duplexes, early evaluation of siRNAs demonstrated activity of the guide strand in the absence of the passenger strand. However, these single strands lacked the activity of duplex RNAs. Here, we report the systematic use of chemical modifications to optimize single-strand RNA (ssRNA)-mediated mRNA knockdown. We identify that 2'F ribose modifications coupled with 5'-end phosphorylation vastly improves ssRNA activity both in vitro and in vivo. The impact of specific chemical modifications on ssRNA activity implies an Ago-mediated mechanism but the hallmark mRNA cleavage sites were not observed which suggests ssRNA may operate through a mechanism beyond conventional Ago2 slicer activity. While currently less potent than duplex siRNAs, with additional chemical optimization and alternative routes of delivery, chemically modified ssRNAs could represent a powerful RNAi platform.

Nucleobase and ribose modifications control immunostimulation by a microRNA-122-mimetic RNA.

Immune stimulation is a significant hurdle in the development of effective and safe RNA interference therapeutics. Here, we address this problem in the context of a mimic of microRNA-122 by employing novel nucleobase and known 2'-ribose modifications. The nucleobase modifications are analogues of adenosine and guanosine that contain cyclopentyl and propyl minor-groove projections. Via a site-by-site chemical modification analysis, we identify several immunostimulatory 'hot spots' within the miRNA guide strand at which single base modifications significantly reduce immune stimulation. A duplex containing one base modification on each strand proved to be most effective in preventing immune stimulation.

Analysis of acyclic nucleoside modifications in siRNAs finds sensitivity at position 1 that is restored by 5'-terminal phosphorylation both in vitro and in vivo.

Small interfering RNAs (siRNAs) are short, double-stranded RNAs that use the endogenous RNAi pathway to mediate gene silencing. Phosphorylation facilitates loading of a siRNA into the Ago2 complex and subsequent cleavage of the target mRNA. In this study, 2', 3' seco nucleoside modifications, which contain an acylic ribose ring and are commonly called unlocked nucleic acids (UNAs), were evaluated at all positions along the guide strand of a siRNA targeting apolipoprotein B (ApoB). UNA modifications at positions 1, 2 and 3 were detrimental to siRNA activity. UNAs at positions 1 and 2 prevented phosphorylation by Clp1 kinase, abrogated binding to Ago2, and impaired Ago2-mediated cleavage of the mRNA target. The addition of a 5'-terminal phosphate to siRNA containing a position 1 UNA restored ApoB mRNA silencing, Ago2 binding, and Ago2 mediated cleavage activity. Position 1 UNA modified siRNA containing a 5'-terminal phosphate exhibited a partial restoration of siRNA silencing activity in vivo. These data reveal the complexity of interpreting the effects of chemical modification on siRNA activity, and exemplify the importance of using multiple biochemical, cell-based and in vivo assays to rationally design chemically modified siRNA destined for therapeutic use.

Hsp90 inhibition suppresses mutant EGFR-T790M signaling and overcomes kinase inhibitor resistance.

The epidermal growth factor receptor (EGFR) secondary kinase domain T790M non-small cell lung cancer (NSCLC) mutation enhances receptor catalytic activity and confers resistance to the reversible tyrosine kinase inhibitors gefitinib and erlotinib. Currently, irreversible inhibitors represent the primary approach in clinical use to circumvent resistance. We show that higher concentrations of the irreversible EGFR inhibitor CL-387,785 are required to inhibit EGFR phosphorylation in T790M-expressing cells compared with EGFR mutant NSCLC cells without T790M. Additionally, CL-387,785 does not fully suppress phosphorylation of other activated receptor tyrosine kinases (RTK) in T790M-expressing cells. These deficiencies result in residual Akt and mammalian target of rapamycin (mTOR) activities. Full suppression of EGFR-mediated signaling in T790M-expressing cells requires the combination of CL-387,785 and rapamycin. In contrast, Hsp90 inhibition overcomes these limitations in vitro and depletes cells of EGFR, other RTKs, and phospho-Akt and inhibits mTOR signaling whether or not T790M is present. EGFR-T790M-expressing cells rendered resistant to CL-387,785 by a kinase switch mechanism retain sensitivity to Hsp90 inhibition. Finally, Hsp90 inhibition causes regression in murine lung adenocarcinomas driven by mutant EGFR (L858R) with or without T790M. However, efficacy in the L858R-T790M model requires a more intense treatment schedule and responses were transient. Nonetheless, these findings suggest that Hsp90 inhibitors may be effective in T790M-expressing cells and offer an alternative therapeutic strategy for this subset of lung cancers.

Bronchial and peripheral murine lung carcinomas induced by T790M-L858R mutant EGFR respond to HKI-272 and rapamycin combination therapy.

The EGFR T790M mutation has been identified in tumors from lung cancer patients that eventually develop resistance to erlotinib. In this study, we generated a mouse model with doxycycline-inducible expression of a mutant EGFR containing both L858R, an erlotinib-sensitizing mutation, and the T790M resistance mutation (EGFR TL). Expression of EGFR TL led to development of peripheral adenocarcinomas with bronchioloalveolar features in alveoli as well as papillary adenocarcinomas in bronchioles. Treatment with an irreversible EGFR tyrosine kinase inhibitor (TKI), HKI-272, shrunk only peripheral tumors but not bronchial tumors. However, the combination of HKI-272 and rapamycin resulted in significant regression of both types of lung tumors. This combination therapy may potentially benefit lung cancer patients with the EGFR T790M mutation.