Primary analysis from the phase 2 study of continuous talazoparib (TALA) plus intermittent low-dose temozolomide (TMZ) in patients with relapsed or refractory extensive-stage small cell lung cancer (ES-SCLC)

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Background: TALA exhibits cytotoxic effects by inhibiting poly (ADP-ribose) polymerase (PARP) proteins 1 and 2 in addition to “trapping” PARP on DNA. TMZ has been shown to increase antitumor response when combined with TALA in SCLC models (Wainberg AACR 2016). TALA plus TMZ as second-line therapy for ES-SCLC may improve disease-related outcomes. Methods: This is a phase 2, open-label, single-arm study of the safety and efficacy of TALA plus TMZ in patients with ES-SCLC, relapsed or refractory to a first-line platinum-based regimen. Participants receive TALA 0.75 mg (or 0.5 mg if creatinine clearance < 60 mL/min) po daily on 28-day cycles with TMZ 37.5 mg/m2 po on days 1-5. The primary endpoint is objective response rate (ORR) based on RECIST 1.1 criteria, versus a historical control of 15% ORR in second-line topotecan, with the null hypothesis rejected for 8 or more confirmed responses among 28 evaluable subjects (29% ORR). Secondary endpoints include progression-free survival, overall survival, duration of response, and time to response. Exploratory endpoints include biomarker studies such as status of DNA damage response genes (DDR) and patient reported outcomes. A Simon two-stage design was utilized to reach a total accrual of 28 evaluable patients. Results: Thirty-one subjects were enrolled, of which 3 were non-evaluable due to ineligibility (1) or early withdrawal of consent prior to first disease assessment (2). Eleven of 28 evaluable subjects (39.3%) achieved a confirmed partial response. The ORR was similar among platinum-refractory (3/6), -resistant (4/9), and -sensitive subgroups (4/13). The median time to response was 1.8 months (m), duration of response 5.8 m, progression free survival 4.5 m, and overall survival 11.9 m. Adverse events (AEs) were manageable, with grade ≥ 3 AEs being thrombocytopenia (61.3%), anemia (54.8%), neutropenia (41.9%), and atypical pneumonia (3.2%), which responded well to dose-hold or dose-reduction and transfusion or growth factor support as needed. Cell free DNA and tissue analysis demonstrated no germline DDR mutations among the trial subjects, but somatic DDR mutations at baseline and acquired during treatment were common. Three subjects remain on study treatment. Conclusions: The study exceeded its target response rate. This is the second trial to demonstrate a benefit of PARP inhibition with low-dose TMZ in SCLC (see Farago Cancer Discovery 2019). A phase 3 study is appropriate to confirm the benefit of this approach compared to currently approved options. Clinical trial information: NCT03672773.

Circulating tumor DNA (ctDNA) mutations associate with response in patients (pts) with extensive-stage small cell lung cancer (ES-SCLC) treated with talazoparib (TALA) and temozolomide (TMZ)

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Background: Poly (ADP-ribose) polymerase (PARP) inhibition in combination with TMZ is a promising treatment strategy for ES-SCLC. In SCLC models, TALA, a potent PARP inhibitor, exhibits cytotoxic effects by impairing PARP proteins 1/2 and trapping PARP on DNA while TMZ potentiates antitumor response by contributing to genomic instability (Wainberg 2016). A prior analysis of ctDNA in 15 pts treated on trial with TALA and TMZ suggested that mutations in DNA damage repair (DDR) genes occurred with this combination and may associate with response (Mulroy ASCO 2021). Methods: Pts with relapsed or refractory ES-SCLC were treated with TALA 0.75 mg po daily with TMZ 37.5 mg/m2 po on days 1-5 of 28-day cycles in a phase 2 clinical trial (UCLA/TRIO-US L-07, NCT03672773). ctDNA was collected and assessed based on allele frequency and plasma copy number at baseline and every 8 weeks during treatment with the Guardant360 assay (Redwood City, CA). DDR status was defined as a mutation known or likely to result in aberrant expression of ATM or BRCA1/2 (other DDR genes not detected by assay) (Pearl 2015). Germline DDR mutations were evaluated with matched-normal (PBMC) whole exome sequencing (WES) with archival specimens by Tempus (Chicago, IL). Response to treatment was defined by RECIST 1.1 criteria. Fisher’s exact tests were used to compare proportions of patients, with P-values <0.05 considered statistically significant ( www.r-project.org , Vienna, AU). Results: For 27 pts with evaluable response, 78 ctDNA samples were collected. The most common baseline somatic alterations were mutations in TP53 (23 pts), RB1 (8 pts), ATM (5 pts), and BRCA2 (5 pts). There were no patients with germline DDR mutations. Overall, 22/27 (81.5%) had disease control (DC), including 11 with confirmed partial responses (PR) and 11 with stable disease while 5 had progressive disease. All those with PRs and ctDNA burden >0.2% at baseline experienced a ctDNA decrease at 8 weeks of treatment. DDR mutations were found in 18/27 (66.7%) pts. Of those with ≥ 1 follow-up ctDNA time point collected, 13/17 (76.4%) pts had at least one new mutation detected while on treatment, most commonly in ATM (6 pts). The appearance of new mutations associated with DC (P=0.042) and with a trend towards improved progression free survival (PFS, 5.9 m vs 3.6 m, P=0.099). All 5 pts with DDR mutations present at baseline had DC with TALA and TMZ, and 9/11 (81.8%) of those with PR had DDR mutations detected at some point during the trial, although the trend toward DC enrichment with DDR mutations did not maintain statistical significance (P=0.24). Conclusions: Mutations in DDR genes occur on treatment with TALA and TMZ and may associate with disease control. Validation in a larger cohort will be pursued.

Tucatinib has Selective Activity in HER2-Positive Cancers and Significant Combined Activity with Approved and Novel Breast Cancer-Targeted Therapies

Pharmacologically targeting the HER2 oncoprotein with therapeutics such as the mAb, trastuzumab, provides clinical benefit for patients with HER2-positive (HER2+) cancers. However, a significant number of patients eventually progress on these therapies. Efforts to overcome therapeutic resistance through combination therapy with small-molecule inhibitors of HER2 have been limited by toxicities associated with off-target activity and/or limited efficacy. In this preclinical study, we explore single-agent and combined activity of tucatinib, a novel HER2-selective small-molecule inhibitor. Tucatinib demonstrated potent, selective activity in a panel of 456 human cancer cell lines, with activity restricted to cell lines (breast and non-breast) with HER2-amplification, including models of acquired resistance to trastuzumab. Within the HER2+ population, tucatinib response tracked strongly with HER2-driven signaling. Single-agent tucatinib induced tumor regressions in xenograft models of HER2+ breast cancer and combination with trastuzumab induced a complete and sustained blockade of HER2/PI3K/AKT signaling. Efficacy of the tucatinib/trastuzumab combination matched that induced by current standard-of-care trastuzumab/pertuzumab/docetaxel combination, with the exception that the chemotherapy-sparing tucatinib/trastuzumab combination did not require a dosing holiday to achieve the same efficacy. In xenograft models of HER2+ breast cancer that also express estrogen receptor (ER; HER2+/ER+), tucatinib showed combined efficacy with inhibitors of CDK4/6 and ER, indicating potential novel therapeutic strategies for difficult-to-treat subtypes of HER2+ breast cancer. These data support expanded clinical investigations of tucatinib as a combination partner for other novel and approved targeted therapies for HER2-driven malignancies.

Abstract PD2-07: Impact of using cross-platform gene expression profiling technologies and computational methods for intrinsic breast cancer subtyping in PALOMA-2 and PALLET

Background Intrinsic breast cancer molecular subtyping (IBCMS) provides significant prognostic information for patients (pts) with breast cancer (BC) treated with chemotherapy, human epidermal growth factor receptor 2 (HER2) targeted therapies, and endocrine therapies (ETs). Classifying tumors into intrinsic subtypes to determine optimal treatment is often applied using PAM50, commercially known as Prosigna. Meanwhile, Absolute Assignment of Breast Cancer Intrinsic Molecular Subtypes (AIMS) computational method was trained to predict PAM50-based IBCMS. As the PAM50 algorithm was developed to capture the major subtypes in a general pt population, clinicopathologic distribution of the study cohort and technology platform calibration should be considered in IBCMS analyses. This study compared different next-generation sequencing technologies and methodologic approaches of PAM50 on tumor samples from 2 randomized trials of postmenopausal women with estrogen receptor-positive (ER+)/HER2-negative (HER2-) BC. Methods PALOMA-2 is a double-blind, randomized study of first-line palbociclib (PAL) + letrozole (LET) for ER+/HER2- advanced BC (ABC). Tumor samples from consented pts were subtyped using the validated RUO PAM50 assay (ruoProsigna, NanoString); results were compared with published subtype results using AIMS on EdgeSeq Oncology Biomarker Panel (HTG Molecular Diagnostics). PALLET is a phase 2, randomized trial of PAL+LET as neoadjuvant therapy in pts with ER+ HER2- BC. Baseline frozen tumor biopsies underwent whole transcriptome mRNA-sequencing (mRNA-seq). IBCMS was performed using AIMS; PAM50 subtyping was performed on data normalised with subgroup-specific gene centering and microarray-RNA-sequencing calibration. Results In PALOMA-2, 222 pts had both ruoProsigna and AIMS data; an overall 54% agreement rate between methods was observed, with 46% (56/121) of Luminal B (LumB) subtype by ruoProsigna assigned as Luminal A (LumA) by AIMS and 67% (6/9) of basal-like by ruoProsigna as HER2-enriched (HER2-E) by AIMS (Table 1). In PALLET, 224 pts had mRNA-seq data; a 69% agreement between the two approaches (AIMS and PAM50) was observed, with only 4% (2/49) of LumB assigned as LumA by AIMS but 17% (26/156) and 16% (25/156) of LumA considered LumB or normal-like by AIMS, respectively. Progression-free survival (PFS) by ruoProsigna-derived subtype in PALOMA-2 showed that PAL+LET benefited all pts but those with a basal-like subtype (Table 2). With AIMS, PAL+LET provided a PFS benefit in pts with LumA and LumB subtypes, but was less effective in the HER2-E subtype. Conclusion Intrinsic subtyping has potential clinical utility. PAL+ET should be considered for ER+/HER2- ABC, except possibly in pts with a basal-like tumor, consistent with previous reports. A standardized clinical PAM50 assay and bioinformatics approach should be used as discrepancies in gene expression platforms and algorithms lead to different results and could misguide treatment decisions. Clinical trial identification: Pfizer (NCT01740427)

Table 1.Intrinsic Subtyping by IBCMS MethodsPALOMA-2PALLETMethodruoProsignaPAM50 mRNAseqAIMS BasalHER2LumALumBGrand TotalBasalHER2LumALumBNormalGrand TotalBasal-like, n (%)1 (11)NANANA13 (75)0001 (8)4HER2-E, n (%)6 (67)6 (30)6 (8)13 (10)311 (25)3 (100)8 (5)6 (12)1 (8)19LumA, n (%)NA2 (10)60 (83)56 (46)1180097 (62)2 (4)099LumB, n (%)2 (22)12 (60)3 (4)52 (43)690026 (17)41 (84)067Normal-like, n (%)NANA3 (4)NA30025 (16)010 (83)35Grand Total9 (100)20 (100)72 (100)121 (100)2224 (100)3 (100)156 (100)49 (100)12 (100)224NA=Not available

Table 2.Median PFS statistics by subtype in PALOMA-2PAL+LET PFS, monthsPBO+LET PFS, monthsHazard Ratio(95% CI)P ValueruoProsignaBasal-like8.2 (n=5)3.6 (n=4)0.39 (0.09-1.77)0.206HER2-E11.0 (n=12)5.1 (n=8)0.41 (0.15-1.11)0.071LumA37.2 (n=52)13.6 (n=20)0.42 (0.21-0.84)0.011LumB27.6 (n=79)13.8 (n=42)0.63 (0.40-1.00)0.049AIMSBasal-likeNANANANAHER2-E16.4 (n=21)8.4 (n=10)0.82 (0.32-2.1)0.684LumA30.6 (n=84)16.5 (n=34)0.56 (0.33-0.95)0.029LumB19.3 (n=41)8.8 (n=28)0.39 (0.23-0.67)&lt;0.001NA=Not available; PBO=placebo

Citation Format: Maggie Cheang, Mitch Dowsett, Mothaffar Rimawi, Stephen Johnston, Samuel Jacobs, Judith Bliss, Katherine Pogue-Geile, Lucy Kilburn, Zhou Zhu, Eugene F. Schuster, Hui Xiao, Lisa Swaim, Shibing Deng, Dongrui R. Lu, Eric Gauthier, Jennifer Tursi, Dennis J. Slamon, Hope S. Rugo, Richard S. Finn, Yuan Liu. Impact of using cross-platform gene expression profiling technologies and computational methods for intrinsic breast cancer subtyping in PALOMA-2 and PALLET [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD2-07.

Genetic polymorphisms and correlation with treatment induced cardiotoxicity and prognosis in breast cancer patients

BACKGROUND

Cardiac toxicity is a serious complication of HER2-directed therapies and anthracyclines. HER2 codon 655 and SLC28A3 gene polymorphisms have been reported to be associated with cardiac toxicity from anti-HER2 and anthracycline therapy, respectively. Association of the polymorphism at HER2 codon 655 with prognosis has also been reported.

METHODS

Whole blood samples from patients treated on a randomized adjuvant breast cancer trial (BCIRG-006) that compared anthracycline-based chemotherapy to trastuzumab plus either anthracycline or non-anthracycline chemotherapy were tested for genetic polymorphisms in HER2 codon 655 and SLC28A3 Genotypes were correlated with cardiac function and disease-free survival (DFS) outcomes.

RESULTS

Of 3,222 patients enrolled in BCIRG-006, 662 patient samples were successfully genotyped for the rs1136201 allele in HER2 (codon 655): 424 (64%) were AA, 30 (4.5%) were GG, and 208 (31%) were AG genotype. Additionally, 665 patient samples were successfully genotyped for the rs7853758 allele in the SLC28A3 gene: 19 (3%) were AA, 475 (71%) were GG, and 171 (26%) were AG genotype. Follow up time was 10 years. No correlation between DFS, cardiac event rate or mean left ventricular ejection fraction (LVEF) and rs1136201 genotype was seen in the trastuzumab treated or non-trastuzumab treated patients. Moreover, mean LVEF and cardiac event rates were similar in all rs7853758 genotype groups treated with anthracycline-based therapy.

CONCLUSIONS

In the largest study to date to evaluate whether two polymorphisms are associated with DFS and/or cardiac toxicity in HER2 positive breast cancer treated with trastuzumab and/or anthracyclines, we observed no correlation.

Results of TRIO-14, a phase II, multicenter, randomized, placebo-controlled trial of carboplatin-paclitaxel versus carboplatin-paclitaxel-ganitumab in newly diagnosed epithelial ovarian cancer

PURPOSE

Insulin-like growth factor (IGF) signaling is implicated in pathogenesis and chemotherapy resistance of epithelial ovarian cancer (EOC). We explored efficacy and safety of adding ganitumab, a monoclonal antibody targeting IGF-1R, to carboplatin/paclitaxel (CP) chemotherapy in patients with primary EOC.

DESIGN

Patients were randomly assigned to receive CP/ganitumab (18 mg/kg q3w) or CP/placebo for 6 cycles followed by 6 cycles of single agent ganitumab/placebo maintenance therapy as front-line therapy. Primary endpoint was progression free survival. Secondary endpoints were time to progression and overall survival. Pretreatment samples were prospectively collected for retrospective biomarker analyses.

RESULTS

170 patients enrolled. 165 patients assessable for toxicity. Median PFS was 15.7 months with CP/ganitumab and 16.7 months with CP/placebo (HR 1.23; 95% CI 0.82-1.83, P = 0.313). All grade neutropenia (84.1% vs 71.4%), thrombocytopenia (75.3% vs 57.1%) and hyperglycemia (15.9% vs 2.6%) were more common in the ganitumab group compared to the placebo group. Ganitumab/placebo related serious adverse events were reported in 26.1% of the patients with ganitumab and in 6.5% with placebo. Non-progression related fatal events were more common with ganitumab (5 versus 2 patients). The ganitumab group experienced more dose delays which resulted in lower relative dose intensity of chemotherapy in the experimental group. In an exploratory model IGFBP2 expression was predictive of ganitumab response (treatment interaction; PFS, P = 0.03; OS, P = 0.01).

CONCLUSION

Addition of ganitumab to CP chemotherapy in primary EOC did not improve PFS. Our results do not support further study of ganitumab in unselected EOC patients.

A careful reassessment of anthracycline use in curable breast cancer

It has been over three decades since anthracyclines took their place as the standard chemotherapy backbone for breast cancer in the curative setting. Though the efficacy of anthracycline chemotherapy is not debatable, potentially life-threatening and long-term risks accompany this class of agents, leading some to question their widespread use, especially when newer agents with improved therapeutic indices have become available. Critically assessing when to incorporate an anthracycline is made more relevant in an era where molecular classification is enabling not only the development of biologically targeted therapeutics but also is improving the ability to better select those who would benefit from cytotoxic agents. This comprehensive analysis will present the problem of overtreatment in early-stage breast cancer, review evidence supporting the use of anthracyclines in the pre-taxane era, analyze comparative trials evaluating taxanes with or without anthracyclines in biologically unselected and selected patient populations, and explore published work aimed at defining anthracycline-sensitive tumor types.

Activation of the IFN Signaling Pathway is Associated with Resistance to CDK4/6 Inhibitors and Immune Checkpoint Activation in ER-Positive Breast Cancer

PURPOSE

Cyclin-dependent kinase 4 (CDK4) and CDK6 inhibitors (CDK4/6i) are highly effective against estrogen receptor-positive (ER⁺)/HER2^- breast cancer; however, intrinsic and acquired resistance is common. Elucidating the molecular features of sensitivity and resistance to CDK4/6i may lead to identification of predictive biomarkers and novel therapeutic targets, paving the way toward improving patient outcomes.

EXPERIMENTAL DESIGN

Parental breast cancer cells and their endocrine-resistant derivatives (EndoR) were used. Derivatives with acquired resistance to palbociclib (PalboR) were generated from parental and estrogen deprivation-resistant MCF7 and T47D cells. Transcriptomic and proteomic analyses were performed in palbociclib-sensitive and PalboR lines. Gene expression data from CDK4/6i neoadjuvant trials and publicly available datasets were interrogated for correlations of gene signatures and patient outcomes.

RESULTS

Parental and EndoR breast cancer lines showed varying degrees of sensitivity to palbociclib. Transcriptomic analysis of these cell lines identified an association between high IFN signaling and reduced CDK4/6i sensitivity; thus an "IFN-related palbociclib-resistance Signature" (IRPS) was derived. In two neoadjuvant trials of CDK4/6i plus endocrine therapy, IRPS and other IFN-related signatures were highly enriched in patients with tumors exhibiting intrinsic resistance to CDK4/6i. PalboR derivatives displayed dramatic activation of IFN/STAT1 signaling compared with their short-term treated or untreated counterparts. In primary ER⁺/HER2^- tumors, the IRPS score was significantly higher in lumB than lumA subtype and correlated with increased gene expression of immune checkpoints, endocrine resistance, and poor prognosis.

CONCLUSIONS

Aberrant IFN signaling is associated with intrinsic resistance to CDK4/6i. Experimentally, acquired resistance to palbociclib is associated with activation of the IFN pathway, warranting additional studies to clarify its involvement in resistance to CDK4/6i.

Cycling cancer persister cells arise from lineages with distinct programs

Non-genetic mechanisms have recently emerged as important drivers of cancer therapy failure¹, where some cancer cells can enter a reversible drug-tolerant persister state in response to treatment². Although most cancer persisters remain arrested in the presence of the drug, a rare subset can re-enter the cell cycle under constitutive drug treatment. Little is known about the non-genetic mechanisms that enable cancer persisters to maintain proliferative capacity in the presence of drugs. To study this rare, transiently resistant, proliferative persister population, we developed Watermelon, a high-complexity expressed barcode lentiviral library for simultaneous tracing of each cell's clonal origin and proliferative and transcriptional states. Here we show that cycling and non-cycling persisters arise from different cell lineages with distinct transcriptional and metabolic programs. Upregulation of antioxidant gene programs and a metabolic shift to fatty acid oxidation are associated with persister proliferative capacity across multiple cancer types. Impeding oxidative stress or metabolic reprogramming alters the fraction of cycling persisters. In human tumours, programs associated with cycling persisters are induced in minimal residual disease in response to multiple targeted therapies. The Watermelon system enabled the identification of rare persister lineages that are preferentially poised to proliferate under drug pressure, thus exposing new vulnerabilities that can be targeted to delay or even prevent disease recurrence.

Ribociclib plus fulvestrant for postmenopausal women with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer in the phase III randomized MONALEESA-3 trial: updated overall survival

BACKGROUND

Ribociclib plus fulvestrant demonstrated significant progression-free survival (PFS) and overall survival (OS) benefits in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2-) advanced breast cancer (ABC). Here we present a new landmark in survival follow-up for a phase III cyclin-dependent kinases 4 and 6 inhibitor clinical trial in patients with ABC (median, 56.3 months).

PATIENTS AND METHODS

This phase III, randomized, double-blind, placebo-controlled trial was conducted at 174 sites (30 countries). Patients were men and postmenopausal women (age ≥18 years) with histologically/cytologically confirmed HR+/HER2- ABC. Patients could have received ≤1 line of endocrine therapy (ET) but no chemotherapy for ABC. Patients, assigned 2:1, were stratified by the presence/absence of liver/lung metastases and previous ET. Patients received intramuscular fulvestrant (500 mg, day 1 of each 28-day cycle plus day 15 of cycle 1) with oral ribociclib (600 mg/day, 3 weeks on, 1 week off) or placebo. Efficacy analyses were by intention to treat. Safety was assessed in patients receiving ≥1 dose study treatment. OS was a secondary endpoint. MONALEESA-3 is registered with ClinicalTrials.gov (NCT02422615; no longer enrolling).

RESULTS

Between 18 June 2015 and 10 June 2016, 726 patients were randomly assigned (484, ribociclib; 242, placebo). At data cut-off (30 October 2020), median OS (mOS) was 53.7 months (ribociclib) versus 41.5 months (placebo) [hazard ratio (HR), 0.73; 95% confidence interval (CI) 0.59-0.90]. Subgroup analyses were consistent with overall population. In the first-line setting, most patients in the ribociclib arm (∼60%) lived longer than median follow-up; mOS was 51.8 months in the placebo arm (HR, 0.64; 95% CI 0.46-0.88). In the second-line setting, mOS was 39.7 months (ribociclib) versus 33.7 months (placebo) (HR, 0.78; 95% CI 0.59-1.04). No apparent drug-drug interaction between ribociclib and fulvestrant or new safety signals were observed.

CONCLUSIONS

This analysis reported extended OS follow-up in MONALEESA-3. mOS was ∼12 months longer in patients with HR+/HER2- ABC treated with ribociclib plus fulvestrant compared with fulvestrant monotherapy.

Updated results of tucatinib versus placebo added to trastuzumab and capecitabine for patients with pretreated HER2+ metastatic breast cancer with and without brain metastases (HER2CLIMB)

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Background: Tucatinib (TUC) is an oral tyrosine kinase inhibitor (TKI) highly specific for HER2. TUC is approved for use in combination with trastuzumab (T) and capecitabine (C) in patients (pts) with and without brain metastases (BM) who have received 1 or more prior anti-HER2–based regimens in the metastatic setting. In the primary analysis from the pivotal HER2CLIMB trial, the addition of TUC to T and C in pts with HER2+ metastatic breast cancer showed a statistically significant and clinically meaningful prolongation of progression-free (PFS) (HR = 0.54 [95% CI: 0.42, 0.71]; P < 0.001) and overall survival (OS) (HR = 0.66 [95% CI: 0.50, 0.88]; P = 0.005) (Murthy, et al. NEJM 2020). TUC in combination with T and C was well tolerated with few discontinuations other than for disease progression. Based on these data, the protocol was amended for unblinding of sites to treatment assignment to allow for crossover from the placebo arm to receive TUC in combination with T and C. Methods: HER2CLIMB (NCT02614794) is a global, randomized, double-blind, placebo-controlled trial in pts with unresectable locally advanced or metastatic HER2+ breast cancer previously treated with T, pertuzumab, and T-emtansine (T-DM1), including pts with untreated, treated stable, or treated and progressing BM. Overall 612 pts were randomized 2:1 to receive TUC 300 mg BID or placebo, each in combination with T and C. Randomization was stratified by BM, ECOG performance status, and geographic region. Protocol prespecified analysis of OS, PFS (by investigator assessment) and safety in the total study population will be performed at approximately 2 years from the last patient randomized. Results: Updated Kaplan-Meier time-to-event analysis of OS and PFS with hazard ratios and 95% confidence intervals for TUC arm vs placebo arm will be presented overall, as well as for OS in the prespecified subgroups reported previously (Murthy, et al. NEJM 2020). Safety and tolerability assessments will include frequency of adverse events by severity, dose modifications and discontinuation of study medications. Conclusions: Conclusions will be presented in the presentation. Clinical trial information: NCT02614794 .

Circulating tumor DNA (ctDNA) mutations may predict treatment response in extensive-stage small cell lung cancer (ES-SCLC) treated with talazoparib and temozolomide (TMZ)

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Background: Poly (ADP-ribose) polymerase (PARP) inhibition in combination with TMZ is a promising treatment strategy for ES-SCLC. In SCLC models, talazoparib, a potent PARP inhibitor, exhibits cytotoxic effects by inhibiting PARP proteins 1/2 and trapping PARP on DNA while TMZ potentiates antitumor response by contributing to genomic instability (Wainberg 2016). Prior ctDNA studies in SCLC have suggested that treatment precipitates the appearance of DNA repair alterations (Nong 2018), but it is unknown whether homologous recombination deficiency (HRD) predicts for treatment response with this combination. Methods: Patients (pts) with relapsed or refractory ES-SCLC were treated with oral talazoparib 0.75 mg daily on 28-day cycles and oral TMZ 37.5 mg/m2 on days 1-5 in a phase 2 clinical trial (UCLA/TRIO-US L-07, NCT03672773). ctDNA was collected and assessed based on allele frequency and plasma copy number at baseline and every 8 weeks during treatment with the Guardant360 assay (Redwood City, CA). HRD was defined as a deletion or missense mutation known or likely to result in aberrant expression of ATM or BRCA1/2 (other HRD genes not detected by assay). Response to treatment was defined by RECIST 1.1 criteria. Fisher’s exact tests were used to compare proportions of patients with P-values < 0.05 considered statistically significant ( www.r-project.org , Vienna, AU). Results: For 15 evaluable pts in the first Simon stage of this trial, 45 ctDNA samples were collected. The most common baseline genetic alterations were mutations in TP53 (14 pts), BRCA2 (5 pts), ATM (4 pts), and RB1 (3 pts). Of those with > 1 ctDNA timepoint collected, 10/11 (90.9%) pts had ≥1 new mutation (range 1-19) detected after receiving treatment (range 8-35 weeks), most commonly in ATM (5 pts). Overall, 5 pts had confirmed partial responses (PR), 7 had stable disease, and 3 had progressive disease. Disease control (DC) was associated with the presence of new mutations (P = 0.022) and was more common in those with HRD, with DC in 9/10 (90.0%) HRD pts vs 3/5 (60.0%) pts without HRD. All those with PRs experienced a ctDNA nadir at 8 weeks of treatment with nearly all (4/5, 80.0%) exhibiting HRD, 2 at baseline and 2 at 8 weeks of treatment. Conclusions: Mutations in DNA repair genes occur on treatment with talazoparib and TMZ and may associate with disease control. With a response rate of 33% in the first Simon stage of this trial, the TRIO-US L-07 trial exploring the combination of talazoparib and TMZ will be assessed in 13 additional patients, after which additional ctDNA analyses will be performed on the cohort as a whole. Clinical trial information: NCT03672773.

Overall survival (OS) with palbociclib (PAL) + fulvestrant (FUL) in women with hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2–) advanced breast cancer (ABC): Updated analyses from PALOMA-3

1000

Background: In PALOMA-3, a randomized, double-blind, placebo-controlled, phase 3 study, PAL+FUL significantly prolonged progression-free survival (PFS) compared with placebo (PBO) + FUL (1-sided P<0.0001). The final protocol-specified OS analysis, which was conducted with a median follow-up of 44.8 months (mo), showed improved OS with PAL+FUL vs PBO+FUL (median OS, 34.9 vs 28.0 mo; hazard ratio, 0.814 [95% CI, 0.644–1.029]; 1-sided P=0.0429). Here, we report the results from an OS analysis with a longer median follow-up of 73.3 mo. Methods: A total of 521 patients (pts) with HR+/HER2– ABC who had progressed on prior endocrine therapy were randomized 2:1 to PAL (125 mg/d orally, 3/1 week schedule) + FUL (500 mg intramuscular injection) or PBO+FUL. Investigator-assessed PFS was the primary endpoint; OS was a key secondary endpoint. An ad hoc OS analysis was performed when 393 events (75% of the total population) were observed. Circulating tumor DNA (ctDNA) analysis was conducted among pts who consented for this study. Results: Improvement in OS continues to be observed with longer follow-up, with a hazard ratio of 0.806 (95% CI, 0.654–0.994; 1-sided nominal P=0.0221). The 5-year OS rate was 23.3% (95% CI, 18.7–28.2) with PAL+FUL and 16.8% (95% CI, 11.2–23.3) with PBO+FUL. Favorable OS with PAL+FUL vs PBO+FUL was observed in most subgroups except among pts who were endocrine resistant or had prior chemotherapy for ABC. No new safety signals were identified. Eighteen pts remain on study treatment, including 15 (4.3%) on PAL+FUL and 3 (1.7%) on PBO+FUL. A post-study cyclin-dependent kinase 4/6 inhibitor was received by 20 pts (7.5%) in the PAL+FUL arm and 32 pts (22.2%) in the PBO+FUL arm. ctDNA analyses of tumor mutation profiles (ie, ESR1, PIK3CA, RB1) at the end of treatment and their effect on OS will also be presented. Conclusions: The clinically meaningful improvement in OS with PAL+FUL was maintained with >6 years of median follow-up in pts with HR+/HER2– ABC who had progressed on prior endocrine treatment. Pfizer (NCT01942135) Clinical trial information: NCT01942135 .[Table: see text]

Updated overall survival (OS) results from the phase III MONALEESA-3 trial of postmenopausal patients (pts) with HR+/HER2- advanced breast cancer (ABC) treated with fulvestrant (FUL) ± ribociclib (RIB)

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Background: The Phase III MONALEESA-3 trial (NCT02422615) previously demonstrated a statistically significant improvement in OS with RIB, a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i), plus FUL compared with placebo (PBO) plus FUL as first-line (1L) or second-line (2L) treatment in postmenopausal pts with HR+/HER2− ABC (median, not reached vs 40.0 mo; hazard ratio [HR], 0.72; 95% CI, 0.57-0.92, P =.00455). This analysis was final per the protocol; following the unblinding of the study, pts still on study treatment in the PBO arm were allowed to cross over to the RIB arm. We report an exploratory analysis of OS after an additional median 16.9 mo of follow-up, allowing for further characterization of long-term survival benefits of RIB. Methods: Postmenopausal pts with HR+/HER2− ABC were randomized 2:1 to receive RIB + FUL or PBO + FUL in 1L and 2L settings. Updated OS was evaluated by Cox proportional hazards model and summarized using Kaplan-Meier methods. Additional postprogression endpoints such as progression-free survival 2 (PFS2), time to chemotherapy (CT), and CT-free survival were also evaluated and summarized. Results: At the data cutoff (Oct 30, 2020), the median follow-up was 56.3 mo (min, 52.7 mo) and 68 (14.0%) and 21 (8.7%) patients were still on treatment in the RIB vs PBO arms, respectively. With this extended follow-up, RIB + FUL continued to demonstrate an OS benefit vs PBO + FUL (median, 53.7 vs 41.5 mo; HR, 0.73; 95% CI, 0.59-0.90). RIB + FUL had prolonged OS vs PBO + FUL in the 1L (median, not reached vs 51.8 mo; HR, 0.64; 95% CI, 0.46-0.88) and 2L subgroups (median, 39.7 vs 33.7 mo; HR, 0.78; 95% CI, 0.59-1.04). Subgroup analyses also showed a consistent OS benefit compared with the intent-to-treat (ITT) population for most subgroups. PFS2, time to CT, and CT-free survival for the ITT population favored RIB + FUL (Table). Among pts who discontinued study treatment, 81.9% and 86.4% received a next-line subsequent antineoplastic therapy, while 14.0% and 30.0% received a CDK4/6i as any subsequent line in the RIB vs PBO arms, respectively. No new safety signals were observed. Conclusions: The previously demonstrated robust and clinically meaningful OS benefit with RIB + FUL compared with PBO + FUL was maintained after almost 5 years of follow-up in postmenopausal pts with HR+/HER2− ABC. The OS benefit of RIB was observed in the 1L and 2L subgroups, which further supports the use of RIB in these populations. The results also demonstrated a significant delay in the use of subsequent CT with RIB vs PBO. Clinical trial information: NCT02422615 .[Table: see text]

Clinical evaluation of BCL-2/XL levels pre- and post- HER2-targeted therapy

Our previous pre-clinical work defined BCL-2 induction as a critical component of the adaptive response to lapatinib-mediated inhibition of HER2. To determine whether a similar BCL-2 upregulation occurs in lapatinib-treated patients, we evaluated gene expression within tumor biopsies, collected before and after lapatinib or trastuzumab treatment, from the TRIO-B-07 clinical trial (NCT#00769470). We detected BCL2 mRNA upregulation in both HER2+/ER- as well as HER2+/ER+ patient tumors treated with lapatinib or trastuzumab. To address whether mRNA expression correlated with protein expression, we evaluated pre- and post-treatment tumors for BCL-2 via immunohistochemistry. Despite BCL2 mRNA upregulation within HER2+/ER- tumors, BCL-2 protein levels were undetectable in most of the lapatinib- or trastuzumab-treated HER2+/ER- tumors. BCL-2 upregulation was evident within the majority of lapatinib-treated HER2+/ER+ tumors and was often coupled with increased ER expression and decreased proliferation. Comparable BCL-2 upregulation was not observed within the trastuzumab-treated HER2+/ER+ tumors. Together, these results provide clinical validation of the BCL-2 induction associated with the adaptive response to lapatinib and support evaluation of BCL-2 inhibitors within the context of lapatinib and other HER2-targeted receptor tyrosine kinase inhibitors.

Spatial proteomic characterization of HER2-positive breast tumors through neoadjuvant therapy predicts response

The addition of HER2-targeted agents to neoadjuvant chemotherapy has dramatically improved pathological complete response (pCR) rates in early-stage, HER2-positive breast cancer. Nonetheless, up to 50% of patients have residual disease after treatment, while others are likely overtreated. Here, we performed multiplex spatial proteomic characterization of 122 samples from 57 HER2-positive breast tumors from the neoadjuvant TRIO-US B07 clinical trial sampled pre-treatment, after 14-21 d of HER2-targeted therapy and at surgery. We demonstrated that proteomic changes after a single cycle of HER2-targeted therapy aids the identification of tumors that ultimately undergo pCR, outperforming pre-treatment measures or transcriptomic changes. We further developed and validated a classifier that robustly predicted pCR using a single marker, CD45, measured on treatment, and showed that CD45-positive cell counts measured via conventional immunohistochemistry perform comparably. These results demonstrate robust biomarkers that can be used to enable the stratification of sensitive tumors early during neoadjuvant HER2-targeted therapy, with implications for tailoring subsequent therapy.

Abstract PD12-06: Treatment-related amenorrhea with T-DM1 plus pertuzumab (KP) is lower than with docetaxel/carboplatin/trastuzumab/pertuzumab (TCHP) in the phase III neoadjuvant KRISTINE trial

Background: Cytotoxic chemotherapy (CT) in combination with trastuzumab and pertuzumab (HP) is standard of care for patients (pts) diagnosed with HER2-positive early breast cancer (EBC). While highly effective, the toxicity associated with CT is challenging. In KRISTINE/TRIO-021, neoadjuvant T-DM1 was combined with pertuzumab (KP) and compared to standard TCHP. Pts. received six cycles of neoadjuvant treatment followed by adjuvant therapy (KP or HP). Pts. in the KP arm were allowed to receive standard adjuvant CT. Pathologic complete response (pCR) rate was significantly lower with KP versus TCHP and more pts. had disease progression prior to surgery with KP, resulting in a meaningfully lower event-free survival rate vs. the control (85.3% vs 94.2%). However, 3-year invasive disease-free survival was numerically similar in both arms. Neoadjuvant KP demonstrated less toxicity than standard CT, although treatment discontinuation was higher post-surgery. Association of KP and TCHP with treatment-related amenorrhea (TRA) in premenopausal EBC pts. has not been ascertained. Methods: All pts. with premenopausal status at study entry (those not meeting the menopause definition based on National Comprehensive Cancer Network Guidelines v3, 2012) and with menstrual period documented within 3 months of randomization, were independently evaluated for presence or absence of TRA by two reviewers. TRA, a prespecified exploratory endpoint of KRISTINE, was defined as cessation of menstruation for &gt;12 months in the absence of treatment with ovarian suppression or other interventions that can induce amenorrhea. Pts. were followed from the time of study entry through the 3-year follow up period after surgery. For cases with inconsistent determination between the two reviewers, a third reviewer adjudicated. TRA rates were calculated per arm, hormone-receptor (HR) status, adjuvant CT and age group. Proportions were compared by estimating the odds ratio (OR) and the 95% confidence interval. Results: Of 444 pts. enrolled, 205 were excluded based on being post-menopausal per NCCN guidelines. Of 239 pts. remaining, 56 were excluded due to insufficient data. The median age of pts. included was 40 years (range: 22-53) for TCHP and 42.5 (range: 23-52) for KP. TRA was observed in 55% (50/91) of pts. treated with TCHP compared to 30% (28/92) treated with KP (OR=2.79; 95% CI 1.52-5.12). In pts. with HR-positive EBC, TRA occurred in 62% with TCHP vs 35% for KP (OR=2.998; 95% CI 1.44-6.25). In those with HR-negative EBC, TRA was observed in 42% with TCHP vs. 21% with KP (OR=2.77; 95% CI 0.88-8.72). In the KP arm, TRA was observed in 38% (8/21) of pts. treated with standard adjuvant CT vs. 28% (20/71) of those that did not (OR 1.57; 95% CI 0.57-4.36). For women age ≤ 40, the rate of TRA was 38% with TCHP vs. 17% with KP (OR=3.00; 95% CI 1.05-8.60). For those &gt; 40 years, TRA was observed in 74% treated with TCHP vs. 39% of those with KP (OR=4.50; 95% CI 1.88-10.73). Conclusion: The rate of TRA with standard TCHP is nearly double that observed with KP, suggesting that targeted CT with an antibody-drug conjugate regimen is associated with less gonadal toxicity. Rates of TRA are higher in women over the age of 40 for each treatment arm however KP is associated with lower rate of TRA in each age group. Association of TRA with efficacy outcomes (pCR, iDFS) will be presented.

Citation Format: Sara A Hurvitz, Rodrigo Fresco, Karen Afenjar, Daniil Stroyakovskiy, Chiun-Sheng Huang, Hans Wildiers, Kyung Hae Jung, Jean-François Boileau, Mario Campone, Miguel Martín, Vicente Valero, Joseph A. Sparano, W. Fraser Symmans, Peter A. Fasching, Alastair M. Thompson, Nadia Harbeck, Vanesa López-Valverde, Chunyan Song, Thomas Boulet, Eleonora Restuccia, Dennis J. Slamon. Treatment-related amenorrhea with T-DM1 plus pertuzumab (KP) is lower than with docetaxel/carboplatin/trastuzumab/pertuzumab (TCHP) in the phase III neoadjuvant KRISTINE trial [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD12-06.

Frontiers in HER2-positive breast cancer in 2020

PURPOSE OF REVIEW

The field of HER2-positive breast cancer has seen tremendous advances in the last 2 years with largest number of new drugs in decades. The present review aims to summarize the cutting-edge research of the past 2 years and future directions.

RECENT FINDINGS

This review will go over four new drugs, three of which have gained FDA approval within the past 18 months, in the treatment of HER2-positive breast cancer. We will go over early and mature clinical data on these therapeutics and ongoing clinical trials further exploring their role in the treatment of patients with advanced HER2-positive breast cancer and HER2 low breast cancer. Will also discuss ongoing trials using immunotherapy and CDK4/6 inhibitors in the advanced HER2-positive setting.

SUMMARY

: The therapies described in this review have quickly become standard of care for patients with HER2-positive breast cancer. Furthermore, they have the potential to change the landscape of breast cancer therapy further to include even patients with HER2 low breast cancer.

Large-Scale Characterization of Drug Responses of Clinically Relevant Proteins in Cancer Cell Lines

Perturbation biology is a powerful approach to modeling quantitative cellular behaviors and understanding detailed disease mechanisms. However, large-scale protein response resources of cancer cell lines to perturbations are not available, resulting in a critical knowledge gap. Here we generated and compiled perturbed expression profiles of ∼210 clinically relevant proteins in >12,000 cancer cell line samples in response to ∼170 drug compounds using reverse-phase protein arrays. We show that integrating perturbed protein response signals provides mechanistic insights into drug resistance, increases the predictive power for drug sensitivity, and helps identify effective drug combinations. We build a systematic map of "protein-drug" connectivity and develop a user-friendly data portal for community use. Our study provides a rich resource to investigate the behaviors of cancer cells and the dependencies of treatment responses, thereby enabling a broad range of biomedical applications.

Analysis in epithelial ovarian cancer identifies KANSL1 as a biomarker and target gene for immune response and HDAC inhibition

OBJECTIVE

There is an immunoreactive subtype of ovarian cancer with a favorable prognosis, but the majority of ovarian cancers have limited immune reactivity. The reason for this is poorly understood. This study aimed to approach this question by identifying prognostically relevant genes whose prognostic mRNA expression levels correlated with a genomic event.

METHODS

Expression microarray and 5-year survival data on 170 ovarian tumors and aCGH data on 45 ovarian cancer cell lines were used to identify amplified/deleted genes associated with prognosis. Three immune-response genes were identified mapping to epigenetically modified chromosome 6p21.3. Genes were searched for roles in epigenetic modification, identifying KANSL1. Genome-wide association studies were searched to identify genetic variants in KANSL1 associated with altered immune profile. Sensitivity to HDAC inhibition in cell lines with KANSL1 amplification/rearrangement was studied.

RESULTS

Expression of 196 genes was statistically significantly associated with survival, and expression levels correlated with copy number variations for 82 of them. Among these, 3 immune-response genes (HCP5, PSMB8, PSMB9) clustered together at epigenetically modified chromosome 6p21.3 and their expression was inversely correlated to epigenetic modification gene KANSL1. KANSL1 is amplified/rearranged in ovarian cancer, associated with lymphocyte profile, a biomarker for response to HDAC inhibition, and may drive expression of immune-response genes.

CONCLUSION

This study identifies 82 genes with prognostic relevance and genomic alteration in ovarian cancer. Among these, immune-response genes have correlated expression which is associated with 5-year survival. KANSL1 may be a master gene altering immune-response gene expression at 6p21.3 and drive response to HDAC inhibitors. Future research should investigate KANSL1 and determine whether targeting it alters the immune profile of ovarian cancer and improves survival, HDAC inhibition, and/or immunotherapy response.

Pathologic and molecular responses to neoadjuvant trastuzumab and/or lapatinib from a phase II randomized trial in HER2-positive breast cancer (TRIO-US B07)

In this multicenter, open-label, randomized phase II investigator-sponsored neoadjuvant trial with funding provided by Sanofi and GlaxoSmithKline (TRIO-US B07, Clinical Trials NCT00769470), participants with early-stage HER2-positive breast cancer (N = 128) were recruited from 13 United States oncology centers throughout the Translational Research in Oncology network. Participants were randomized to receive trastuzumab (T; N = 34), lapatinib (L; N = 36), or both (TL; N = 58) as HER2-targeted therapy, with each participant given one cycle of this designated anti-HER2 therapy alone followed by six cycles of standard combination chemotherapy with the same anti-HER2 therapy. The primary objective was to estimate the rate of pathologic complete response (pCR) at the time of surgery in each of the three arms. In the intent-to-treat population, we observed similar pCR rates between T (47%, 95% confidence interval [CI] 30-65%) and TL (52%, 95% CI 38-65%), and a lower pCR rate with L (25%, 95% CI 13-43%). In the T arm, 100% of participants completed all protocol-specified treatment prior to surgery, as compared to 69% in the L arm and 74% in the TL arm. Tumor or tumor bed tissue was collected whenever possible pre-treatment (N = 110), after one cycle of HER2-targeted therapy alone (N = 89), and at time of surgery (N = 59). Higher-level amplification of HER2 and hormone receptor (HR)-negative status were associated with a higher pCR rate. Large shifts in the tumor, immune, and stromal gene expression occurred after one cycle of HER2-targeted therapy. In contrast to pCR rates, the L-containing arms exhibited greater proliferation reduction than T at this timepoint. Immune expression signatures increased in all arms after one cycle of HER2-targeted therapy, decreasing again by the time of surgery. Our results inform approaches to early assessment of sensitivity to anti-HER2 therapy and shed light on the role of the immune microenvironment in response to HER2-targeted agents.

Overall survival results from the randomized phase 2 study of palbociclib in combination with letrozole versus letrozole alone for first-line treatment of ER+/HER2- advanced breast cancer (PALOMA-1, TRIO-18)

PURPOSE

Palbociclib is a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor, approved in combination with endocrine therapy for the treatment of women and men with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer (HR+/HER2- ABC). In the phase 2, open-label, PALOMA-1 trial, palbociclib plus letrozole significantly prolonged progression-free survival (PFS) versus letrozole alone (hazard ratio, 0.488; 95% CI 0.319‒0.748; P = 0.0004; median PFS, 20.2 vs 10.2 months, respectively) in postmenopausal women with estrogen receptor-positive (ER+)/HER2- ABC. Here, we present the final overall survival (OS) and updated safety results.

METHODS

Postmenopausal women with ER+/HER2- ABC were randomized 1:1 to receive either palbociclib (125 mg/day, 3/1 schedule) plus letrozole (2.5 mg/day, continuous) or letrozole alone (2.5 mg/day, continuous). The primary endpoint was investigator-assessed PFS; secondary endpoints included OS and safety.

RESULTS

A total of 165 patients were randomized. At the data cutoff date of December 30, 2016 (median duration of follow-up, 64.7 months), the stratified hazard ratio for OS was 0.897 (95% CI 0.623-1.294; P = 0.281); median OS in the palbociclib plus letrozole and letrozole alone arms was 37.5 and 34.5 months, respectively. The median time from randomization to first subsequent chemotherapy use was longer with palbociclib plus letrozole than letrozole alone (26.7 and 17.7 months, respectively). The most frequently reported adverse event in the palbociclib plus letrozole arm was neutropenia (any grade, 75%; grade 3 or 4, 59%).

CONCLUSIONS

Palbociclib plus letrozole treatment led to a numerical but not statistically significant improvement in median OS. Pfizer Inc (NCT00721409).

Targeting activated PI3K/mTOR signaling overcomes acquired resistance to CDK4/6-based therapies in preclinical models of hormone receptor-positive breast cancer

Background

Combined targeting of CDK4/6 and ER is now the standard of care for patients with advanced ER+/HER2− breast cancer. However, acquired resistance to these therapies frequently leads to disease progression. As such, it is critical to identify the mechanisms by which resistance to CDK4/6-based therapies is acquired and also identify therapeutic strategies to overcome resistance.

Methods

In this study, we developed and characterized multiple in vitro and in vivo models of acquired resistance to CDK4/6-based therapies. Resistant models were screened by reverse phase protein array (RPPA) for cell signaling changes that are activated in resistance.

Results

We show that either a direct loss of Rb or loss of dependence on Rb signaling confers cross-resistance to inhibitors of CDK4/6, while PI3K/mTOR signaling remains activated. Treatment with the p110α-selective PI3K inhibitor, alpelisib (BYL719), completely blocked the progression of acquired CDK4/6 inhibitor-resistant xenografts in the absence of continued CDK4/6 inhibitor treatment in models of both PIK3CA mutant and wild-type ER+/HER2− breast cancer. Triple combination therapy against PI3K:CDK4/6:ER prevented and/or delayed the onset of resistance in treatment-naive ER+/HER2− breast cancer models.

Conclusions

These data support the clinical investigation of p110α-selective inhibitors of PI3K, such as alpelisib, in patients with ER+/HER2− breast cancer who have progressed on CDK4/6:ER-based therapies. Our data also support the investigation of PI3K:CDK4/6:ER triple combination therapy to prevent the onset of resistance to the combination of endocrine therapy plus CDK4/6 inhibition.