Cancer Patient-derived Xenograft Research Articles

Quantification of vascular networks in photoacoustic mesoscopy

Mesoscopic photoacoustic imaging (PAI) enables non-invasive visualisation of tumour vasculature. The visual or semi-quantitative 2D measurements typically applied to mesoscopic PAI data fail to capture the 3D vessel network complexity and lack robust ground truths for assessment of accuracy. Here, we developed a pipeline for quantifying 3D vascular networks captured using mesoscopic PAI and tested the preservation of blood volume and network structure with topological data analysis. Ground truth data of in silico synthetic vasculatures and a string phantom indicated that learning-based segmentation best preserves vessel diameter and blood volume at depth, while rule-based segmentation with vesselness image filtering accurately preserved network structure in superficial vessels. Segmentation of vessels in breast cancer patient-derived xenografts (PDXs) compared favourably to ex vivo immunohistochemistry. Furthermore, our findings underscore the importance of validating segmentation methods when applying mesoscopic PAI as a tool to evaluate vascular networks in vivo.

Genomic and transcriptomic analysis of a library of small cell lung cancer patient-derived xenografts

Access to clinically relevant small cell lung cancer (SCLC) tissue is limited because surgical resection is rare in metastatic SCLC. Patient-derived xenografts (PDX) and circulating tumor cell-derived xenografts (CDX) have emerged as valuable tools to characterize SCLC. Here, we present a resource of 46 extensively annotated PDX/CDX models derived from 33 patients with SCLC. We perform multi-omic analyses, using targeted tumor next-generation sequencing, RNA-sequencing, and immunohistochemistry to deconvolute the mutational landscapes, global expression profiles, and molecular subtypes of these SCLC models. SCLC subtypes characterized by transcriptional regulators, ASCL1, NEUROD1 and POU2F3 are confirmed in this cohort. A subset of SCLC clinical specimens, including matched PDX/CDX and clinical specimen pairs, confirm that the primary features and genomic and proteomic landscapes of the tumors of origin are preserved in the derivative PDX models. This resource provides a powerful system to study SCLC biology.

Circulating Tumour Cells Indicate the Presence of Residual Disease Post-Castration in Prostate Cancer Patient-Derived Xenograft Models

Castrate-resistant prostate cancer (CRPC) is the lethal form of prostate cancer. Epithelial mesenchymal plasticity (EMP) has been associated with disease progression to CRPC, and prostate cancer therapies targeting the androgen signalling axis, including androgen deprivation therapy (ADT), promote EMP. We explored effects of castration on EMP in the tumours and circulating tumour cells (CTCs) of patient-derived xenograft (PDX)-bearing castrated mice using human-specific RT-qPCR assays and immunocytochemistry. Expression of prostate epithelial cell marker KLK3 was below detection in most tumours from castrated mice (62%, 23/37 mice), consistent with its known up-regulation by androgens. Endpoint tumour size after castration varied significantly in a PDX model-specific pattern; while most tumours were castration-sensitive (BM18, LuCaP70), the majority of LuCaP105 tumours continued to grow following castration. By contrast, LuCaP96 PDX showed a mixed response to castration. CTCs were detected in 33% of LuCaP105, 43% of BM18, 47% of LuCaP70, and 54% of LuCaP96 castrated mice using RPL32 mRNA measurement in plasma. When present, CTC numbers estimated using human RPL32 expression ranged from 1 to 458 CTCs per ml blood, similar to our previous observations in non-castrated mice. In contrast to their non-castrated counterparts, there was no relationship between tumour size and CTC burden in castrated mice. Unsupervised hierarchical clustering of the gene expression profiles of CTCs collected from castrated and non-castrated mice revealed distinct CTC sub-groups within the pooled population that were classified as having mesenchymal, epithelial, or EMP hybrid gene expression profiles. The epithelial signature was only found in CTCs from non-castrated mice. Hybrid and mesenchymal signatures were detected in CTCs from both castrated and non-castrated mice, with an emphasis towards mesenchymal phenotypes in castrated mice. Post-castration serum PSA levels were either below detection or very low for all the CTC positive samples highlighting the potential usefulness of CTCs for disease monitoring after androgen ablation therapy. In summary, our study of castration effects on prostate cancer PDX CTCs showed that CTCs were often detected in the castrate setting, even in mice with no palpable tumours, and demonstrated the superior ability of CTCs to reveal residual disease over the conventional clinical biomarker serum PSA.

Establishing Patient-Derived Cancer Cell Cultures and Xenografts in Biliary Tract Cancer.

Biliary tract cancers (BTCs) are rare and show a dismal prognosis with limited treatment options. To improve our understanding of these heterogeneous tumors and develop effective therapeutic agents, suitable preclinical models reflecting diverse tumor characteristics are needed. We established and characterized new patient-derived cancer cell cultures and patient-derived xenograft (PDX) models using malignant ascites from five patients with BTC. Five patient-derived cancer cell cultures and three PDX models derived from malignant ascites of five patients with BTC, AMCBTC-01, -02, -03, -04, and -05, were established. To characterize the models histogenetically and confirm whether characteristics of the primary tumor were maintained, targeted sequencing and histopathological comparison between primary tissue and xenograft tumors were performed. From malignant ascites of five BTC patients, five patient-derived cancer cell cultures (100% success rate), and three PDXs (60% success rate) were established. The morphological characteristics of three primary xenograft tumors were compared with those of matched primary tumors, and they displayed a similar morphology. The mutated genes in samples (models, primary tumor tissue, or both) from more than one patient were TP53 (n=2), KRAS (n=2), and STK11 (n=2). Overall, the pattern of commonly mutated genes in BTC cell cultures was different from that in commercially available BTC cell lines. We successfully established the patient-derived cancer cell cultures and xenograft models derived from malignant ascites in BTC patients. These models accompanied by different genetic characteristics from commercially available models will help better understand BTC biology.

Integrative analysis of non-small cell lung cancer patient-derived xenografts identifies distinct proteotypes associated with patient outcomes

Non-small cell lung cancer (NSCLC) is the leading cause of cancer deaths worldwide. Only a fraction of NSCLC harbor actionable driver mutations and there is an urgent need for patient-derived model systems that will enable the development of new targeted therapies. NSCLC and other cancers display profound proteome remodeling compared to normal tissue that is not predicted by DNA or RNA analyses. Here, we generate 137 NSCLC patient-derived xenografts (PDXs) that recapitulate the histology and molecular features of primary NSCLC. Proteome analysis of the PDX models reveals 3 adenocarcinoma and 2 squamous cell carcinoma proteotypes that are associated with different patient outcomes, protein-phosphotyrosine profiles, signatures of activated pathways and candidate targets, and in adenocarcinoma, stromal immune features. These findings portend proteome-based NSCLC classification and treatment and support the PDX resource as a viable model for the development of new targeted therapies.

Immune cell infiltration pattern in non-small cell lung cancer PDX models is a model immanent feature and correlates with a distinct molecular and phenotypic make-up

BackgroundThe field of cancer immunology is rapidly moving towards innovative therapeutic strategies, resulting in the need for robust and predictive preclinical platforms reflecting the immunological response to cancer. Well characterized...

Anticancer effect of locally applicable aptamer-conjugated gemcitabine-loaded atelocollagen patch in pancreatic cancer patient-derived xenograft models.

We investigated the anticancer effect of the aptamer‐conjugated gemcitabine‐loaded atelocollagen patch in a pancreatic cancer patient–derived xenograft (PDX) model to propose a future potential adjuvant surgical strategy during curative pancreatic resection for pancreatic cancer. A pancreatic cancer PDX model was established. Animals were grouped randomly into a no‐treatment control group; treatment group treated with intraperitoneal gemcitabine injection (IP‐GEM) or aptamer‐conjugated gemcitabine (APT:GEM); and transplant with three kinds of patches: atelocollagen‐aptamer‐gemcitabine (patch I), atelocollagen‐inactive aptamer‐gemcitabine (patch II), and atelocollagen‐gemcitabine (patch III). Tumor volumes and response were evaluated based on histological analysis by H&E staining and Immunohistochemistry (IHC) was performed. Anticancer therapy–related toxicity was evaluated by hematologic findings. The patch I group showed the most significant reduction of tumor growth rate, compared with the no‐treatment group (p < 0.05). However, other treatment groups were not found to show significant reduction in tumor growth rate (0.05 < p < 0.1). There was no microscopic evidence suggesting potential toxicity, such as inflammation, nor necrotic changes in liver, lung, kidney, and spleen tissue. In addition, no leukopenia, anemia, or neutropenia was observed in the patch I group. This implantable aptamer‐drug conjugate system is thought to be a new surgical strategy to augment the oncologic significance of margin‐negative resection in treating pancreatic cancer in near future.

Targeting LRRC15 Inhibits Metastatic Dissemination of Ovarian Cancer.

Dissemination of ovarian cancer (OC) cells can lead to inoperable metastatic lesions in the bowel and omentum that cause patient death. Here we show that LRRC15, a type-I 15-leucine-rich repeat-containing membrane protein, highly overexpressed in OC bowel metastases compared to matched primary tumors and acts as a potent promoter of omental metastasis. Complementary models of OC demonstrated that LRRC15 expression leads to inhibition of anoikis-induced cell death and promotes adhesion and invasion through matrices that mimic omentum. Mechanistically, LRRC15 interacted with β1-integrin to stimulate activation of focal adhesion kinase (FAK) signaling. As a therapeutic proof of concept, targeting LRRC15 with the specific antibody-drug conjugate ABBV-085 in both early and late metastatic OC cell line xenograft models prevented metastatic dissemination, and these results were corroborated in metastatic patient-derived OC xenograft models. Furthermore, treatment of 3D-spheroid cultures of LRRC15-positive patient-derived ascites with ABBV-085 reduced cell viability. Overall, these data uncover a role for LRRC15 in promoting OC metastasis and suggest a novel and promising therapy to target OC metastases.

Liver Endothelium Promotes HER3-Mediated Cell Survival in Colorectal Cancer with Wild-Type and Mutant KRAS.

This body of work highlighted a potential strategy of using HER3 antibodies in combination with standard chemotherapy agents for treating patients with either KRAS wild-type or KRAS mutant metastatic colorectal cancer.

GLUT3/SLC2A3 Is an Endogenous Marker of Hypoxia in Prostate Cancer Cell Lines and Patient-Derived Xenograft Tumors.

The microenvironment of solid tumors is dynamic and frequently contains pockets of low oxygen levels (hypoxia) surrounded by oxygenated tissue. Indeed, a compromised vasculature is a hallmark of the tumor microenvironment, creating both spatial gradients and temporal variability in oxygen availability. Notably, hypoxia associates with increased metastasis and poor survival in patients. Therefore, to aid therapeutic decisions and better understand hypoxia’s role in cancer progression, it is critical to identify endogenous biomarkers of hypoxia to spatially phenotype oncogenic lesions in human tissue, whether precancerous, benign, or malignant. Here, we characterize the glucose transporter GLUT3/SLC2A3 as a biomarker of hypoxic prostate epithelial cells and prostate tumors. Transcriptomic analyses of non-tumorigenic, immortalized prostate epithelial cells revealed a highly significant increase in GLUT3 expression under hypoxia. Additionally, GLUT3 protein increased 2.4-fold in cultured hypoxic prostate cell lines and was upregulated within hypoxic regions of xenograft tumors, including two patient-derived xenografts (PDX). Finally, GLUT3 out-performs other established hypoxia markers; GLUT3 staining in PDX specimens detects 2.6–8.3 times more tumor area compared to a mixture of GLUT1 and CA9 antibodies. Therefore, given the heterogeneous nature of tumors, we propose adding GLUT3 to immunostaining panels when trying to detect hypoxic regions in prostate samples.

Lenvatinib inhibits the growth of gastric cancer patient-derived xenografts generated from a heterogeneous population

BackgroundLenvatinib is a multitargeted tyrosine kinase inhibitor that is being tested in combination with immune checkpoint inhibitors to treat advanced gastric cancer; however, little data exists regarding the efficacy of lenvatinib monotherapy. Patient-derived xenografts (PDX) are established by engrafting human tumors into immunodeficient mice. The generation of PDXs may be hampered by growth of lymphomas. In this study, we compared the use of mice with different degrees of immunodeficiency to establish PDXs from a diverse cohort of Western gastric cancer patients. We then tested the efficacy of lenvatinib in this system.MethodsPDXs were established by implanting gastric cancer tissue into NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG) or Foxn1nu (nude) mice. Tumors from multiple passages from each PDX line were compared histologically and transcriptomically. PDX-bearing mice were randomized to receive the drug delivery vehicle or lenvatinib. After 21 days, the percent tumor volume change (%Δvtumor) was calculated.Results23 PDX models were established from Black, non-Hispanic White, Hispanic, and Asian gastric cancer patients. The engraftment rate was 17% (23/139). Tumors implanted into NSG (16%; 18/115) and nude (21%; 5/24) mice had a similar engraftment rate. The rate of lymphoma formation in nude mice (0%; 0/24) was lower than in NSG mice (20%; 23/115; p < 0.05). PDXs derived using both strains maintained histologic and gene expression profiles across passages. Lenvatinib treatment (mean %Δvtumor: -33%) significantly reduced tumor growth as compared to vehicle treatment (mean %Δvtumor: 190%; p < 0.0001).ConclusionsNude mice are a superior platform than NSG mice for generating PDXs from gastric cancer patients. Lenvatinib showed promising antitumor activity in PDXs established from a diverse Western patient population and warrants further investigation in gastric cancer.

Cabozantinib Enhances Anti-PD1 Activity and Elicits a Neutrophil-Based Immune Response in Hepatocellular Carcinoma.

Immune checkpoint inhibitors combined with antiangiogenic agents produce benefits in the treatment of advanced hepatocellular carcinoma (HCC). We investigated the efficacy and immunomodulatory activity of cabozantinib alone and combined with anti-PD1 in experimental models of HCC, and explored the potential target population that might benefit from this combination. C57BL/6J mice bearing subcutaneous Hepa1-6 or Hep53.4 tumors received cabozantinib, anti-PD1, their combination, or placebo. Tumor and blood samples were analyzed by flow cytometry, IHC, transcriptome, and cytokine profiling. Cabozantinib-related effects were validated in a colorectal cancer patient-derived xenograft model. Transcriptomic data from three human HCC cohorts (cohort 1: n = 167, cohort 2: n = 57, The Cancer Genome Atlas: n = 319) were used to cluster patients according to neutrophil features, and assess their impact on survival. The combination of cabozantinib and anti-PD1 showed increased antitumor efficacy compared with monotherapy and placebo (P < 0.05). Cabozantinib alone significantly increased neutrophil infiltration and reduced intratumor CD8+PD1+ T-cell proportions, while the combination with anti-PD1 further stimulated both effects and significantly decreased regulatory T cell (Treg) infiltration (all P < 0.05). In blood, cabozantinib and especially combination increased the proportions of overall T cells (P < 0.01) and memory/effector T cells (P < 0.05), while lowering the neutrophil-to-lymphocyte ratio (P < 0.001 for combination). Unsupervised clustering of human HCCs revealed that high tumor enrichment in neutrophil features observed with the treatment combination was linked to less aggressive tumors with more differentiated and less proliferative phenotypes. Cabozantinib in combination with anti-PD1 enhanced antitumor immunity by bringing together innate neutrophil-driven and adaptive immune responses, a mechanism of action which favors this approach for HCC treatment.

MiR-486-5p Targets CD133+ Lung Cancer Stem Cells through the p85/AKT Pathway.

Despite improvements in therapies and screening strategies, lung cancer prognosis still remains dismal, especially for metastatic tumors. Cancer stem cells (CSCs) are endowed with properties such as chemoresistance, dissemination, and stem-like features, that make them one of the main causes of the poor survival rate of lung cancer patients. MicroRNAs (miRNAs), small molecules regulating gene expression, have a role in lung cancer development and progression. In particular, miR-486-5p is an onco-suppressor miRNA found to be down-modulated in the tumor tissue of lung cancer patients. In this study, we investigate the role of this miRNA in CD133+ lung CSCs and evaluate the therapeutic efficacy of coated cationic lipid-nanoparticles entrapping the miR-486-5p miRNA mimic (CCL-486) using lung cancer patient-derived xenograft (PDX) models. In vitro, miR-486-5p overexpression impaired the PI3K/Akt pathway and decreased lung cancer cell viability. Moreover, miR-486-5p overexpression induced apoptosis also in CD133+ CSCs, thus affecting the in vivo tumor-initiating properties of these cells. Finally, we demonstrated that in vivo CCL-486 treatment decreased CD133+ percentage and inhibited tumor growth in PDX models. In conclusion, we provided insights on the efficacy of a novel miRNA-based compound to hit CD133+ lung CSCs, setting the basis for new combined therapeutic strategies.

The EurOPDX Data Portal: an open platform for patient-derived cancer xenograft data sharing and visualization

BackgroundPatient-derived xenografts (PDX) mice models play an important role in preclinical trials and personalized medicine. Sharing data on the models is highly valuable for numerous reasons – ethical, economical, research cross validation etc. The EurOPDX Consortium was established 8 years ago to share such information and avoid duplicating efforts in developing new PDX mice models and unify approaches to support preclinical research. EurOPDX Data Portal is the unified data sharing platform adopted by the Consortium.Main bodyIn this paper we describe the main features of the EurOPDX Data Portal (https://dataportal.europdx.eu/), its architecture and possible utilization by researchers who look for PDX mice models for their research. The Portal offers a catalogue of European models accessible on a cooperative basis. The models are searchable by metadata, and a detailed view provides molecular profiles (gene expression, mutation, copy number alteration) and treatment studies. The Portal displays the data in multiple tools (PDX Finder, cBioPortal, and GenomeCruzer in future), which are populated from a common database displaying strictly mutually consistent views.(Short) ConclusionEurOPDX Data Portal is an entry point to the EurOPDX Research Infrastructure offering PDX mice models for collaborative research, (meta)data describing their features and deep molecular data analysis according to users’ interests.

Inhibition of growth of colorectal cancer patient-derived subcutaneous xenografts using combined Wnt signaling pathway inhibitor and 5‑fluorouracil

Purpose of the study. Was to analyze antitumor efficacy of the XAV 939 Wnt signaling pathway inhibitor and its combination with 5 fluorouracil in subcutaneous xenografts derived from patients with colorectal cancer.Materials and methods. Antitumor efficacy of the agents and their combination was studied in xenografts derived from patients with colorectal cancer and subcutaneously implanted in immunodeficient Balb/c Nude mice. All animals with tumors were divided into 4 groups (n = 5): group 1 received 5 fluorouracil 25 mg/kg, group 2 – XAV 939 25 mg/kg, group 3–5 fluorouracil and XAV 939 combination at the same dosages, group 4 was control. Criteria for the efficacy of the tested agents and their combination included tumor growth rate and tumor growth inhibition rate (TGI %).Results. The mean volumes of xenografts and tumor growth rate in the group receiving a combination of 5 fluorouracil and XAV 939 were 335.2 ± 40.7 mm3 , being lower than the averages of xenografts in controls – 609.3 ± 69.5 mm3 (p &lt; 0.05). The mean volumes of xenografts in the group receiving 5 fluorouracil monotherapy were 601.9 ± 45.5 mm3 , in the group with the XAV 939 monotherapy – 527.9 ± 258.6 mm3 . The highest TGI (44.99 %) was registered in the group receiving a combination of 5 fluorouracil and XAV 939.Conclusion. The study revealed the ability of combined XAV 939 Wnt signaling pathway inhibitor and 5 fluorouracil to inhibit the growth of subcutaneous xenografts derived from patients with colorectal cancer.

Abstract P5-13-14: Antitumor activity of patritumab deruxtecan (HER3-DXd), a HER3-directed antibody drug conjugate (ADC) across a diverse panel of breast cancer (BC) patient-derived xenografts (PDXs)

Abstract Background: HER3 is overexpressed in 30-50% of breast cancers and has been associated with poor prognosis. Patritumab deruxtecan (HER3-DXd; U3-1402) is a HER3-directed ADC with a potent topoisomerase I (TOP1) inhibitor payload. A phase 1/2 study of HER3-DXd (NCT02980341) demonstrated promising antitumor activity in hormone receptor positive (HR+) metastatic BC patients with clinical activity observed across baseline levels of HER3 protein or mRNA expression. A window of opportunity clinical trial is currently ongoing to evaluate the biological activity of HER3-DXd in patients with treatment naïve BC according to HER3 mRNA/protein expression levels (NCT04610528). Here, we aimed to describe the activity of HER3-DXd in PDX models to identify robust biomarkers of response. Methods: The antitumor activity of HER3-DXd was assessed in 21 BC PDX models (14 HR+ and 7 triple negative). HER3-DXd sensitivity was established as a complete response that lasted longer than 120 days, following 4 weekly doses of 10 mg/kg. HER3-DXd antitumor activity was compared to the antitumor activity of irinotecan (50 mg/kg dosed once weekly), which was evaluated according to modified RECIST criteria in a subset of 14 BC PDX models. PDX models of acquired resistance to HER3-DXd were generated by repeated treatment cycles. Pharmacodynamic (PD) experiments were conducted by collecting tumor samples from PDXs after a single dose of HER3-DXd or irinotecan. Baseline HER3 expression was assessed by immunohistochemistry (IHC) and Western blot. mRNA expression of 72 genes including ERBB3 and genes from the PAM50 signature were measured using the nCounter platform. Two-class unpaired significance analysis of microarrays (SAM), using a false-discovery rate&amp;lt;5%, identified differential gene expression across response groups. Genetic alterations harbored by PDX models were determined using the MSK-IMPACTTM targeted exome panel. Quantification of proliferation (% of Ki67-positive cells) and of DNA damage during the S-phase of the cell cycle (γH2AX nuclear foci in geminin-positive cells) was evaluated in untreated/treated PD samples by IHC or immunofluorescence (IF), respectively. Western blot was used to assess HER3-pathway downmodulation and induction of apoptosis. Results: Eight out of 21 (38%) PDXs were highly sensitive to HER3-DXd, and in 5/14 (36%) models HER3-DXd showed a superior antitumor activity when compared to irinotecan. We observed an enrichment of basal-like models amongst the non-relapsed PDXs, compared to the relapsed ones (6/8 (75%) vs. 3/13 (20%), p=0.0195). Baseline levels of HER3/ERBB3 were not associated with treatment response and a model of acquired-resistance did not exhibit a reduction in baseline HER3 expression. Interestingly, relapsed models showed increased expression of genes related with chemotherapy-resistance (MDM2, NAT1, MAPT, GRB7, BCL-2). Mechanistically, treatment with HER3-DXd did not reduce the level of Ki67-proliferating cells but resulted in a significantly higher induction of S-phase DNA damage measured as γH2AX nuclear foci in non-relapsed models, compared to relapsed ones. This was accompanied by a reduction of HER3 protein levels and downmodulation of pERK1/2 T202/Y204, along with activation of PARP cleavage. Conclusions: HER3-DXd exerts a potent antitumor response in BC PDXs, independently of baseline HER3/ERBB3 levels, in line with the clinical data of NCT02980341. Basal-like tumors were more sensitive to HER3-DXd than luminal B models. Mechanistically, our data suggests that treatment with HER3-DXd results in parallel HER3/ERK signaling downmodulation and induction of S-phase DNA damage, resulting in tumor cell death. Citation Format: Andreu Òdena, Laia Monserrat, Fara Brasó-Maristany, Marta Guzmán, Judit Grueso, Olga Rodríguez, Maurizio Scaltriti, Sarat Chandarlapaty, Yang Qiu, Kumiko Koyama, Mafalda Oliveira, Aleix Prat, Violeta Serra. Antitumor activity of patritumab deruxtecan (HER3-DXd), a HER3-directed antibody drug conjugate (ADC) across a diverse panel of breast cancer (BC) patient-derived xenografts (PDXs) [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 P5-13-14.

Abstract OT2-11-05: SERENA-6: A Phase III study to assess the efficacy and safety of AZD9833 (camizestrant) compared with aromatase inhibitors when given in combination with palbociclib or abemaciclib in patients with HR+/HER2- metastatic breast cancer with detectable ESR1m who have not experienced disease progression on first-line therapy

Abstract Background: More than two thirds of patients with metastatic breast cancer (mBC) have hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2−) tumors. Current guidelines recommend combining endocrine therapy (ET), such as an aromatase inhibitor (AI), with an inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6i) as first-line (1L) treatment (Cardoso et al. Ann Oncol 2020). However, drug resistance eventually develops, leading to disease progression. Mutations in the estrogen receptor (ER) alpha gene (ESR1m) result in constitutively active, estrogen-independent ER signaling which can reduce AI efficiency (Reinert et al. Front Oncol 2017). Clinically, ESR1m are associated with acquired resistance to AI as well as more aggressive disease features, including development of visceral metastasis (Gerratana et al. Eur J Cancer 2021). At the initial diagnosis of mBC, the frequency of ESR1m is low (approximately 3%) (Bidard et al. Ann Oncol 2019); however, this increases to 17−35% of patients when disease progresses on an AI + CDK4/6i (Bidard et al. Ann Oncol 2019; Goetz et al. J Clin Oncol 2020). Patients with ESR1m tumors have poor outcomes with subsequent lines of therapy. New approaches are needed to maximize time on 1L treatment with ET + CDK4/6i and prevent further clinical and radiological disease progression. AZD9833 (camizestrant) is a highly potent, next-generation oral selective ER degrader (ngSERD) and pure ER antagonist that has demonstrated antitumor activity in a wide range of ER+ breast cancer cell lines and patient-derived xenograft models, including those with wild type ESR1 (ESR1wt) and the most prevalent ESR1m, D538G and Y537S (Scott et al. AACR 2020; Lawsone et al. AACR 2020). The Phase I SERENA-1 study demonstrated that AZD9833 shows encouraging clinical activity as monotherapy or in combination with a CDK4/6i in heavily pre-treated patients with ER+/HER2− advanced breast cancer whose tumors are ESR1wt or ESR1m (Baird et al. SABCS 2020). SERENA-6 will assess the efficacy of switching patients from AI to AZD9833, while continuing CDK4/6i treatment, once ESR1m are detected but before overt disease progression. Study description: SERENA-6 is an ongoing, randomized, multicenter, double-blind, Phase III trial. Patients with HR+/HER2− mBC who have received at least 6 months of 1L AI (letrozole or anastrozole) + CDK4/6i (palbociclib or abemaciclib) and do not have clinical or radiological disease progression will be enrolled into Step 1, the ESR1m detection phase. During this phase, patients will be monitored regularly for the presence of ESR1m via central circulating tumor DNA analysis. Patients with detectable ESR1m and no overt disease progression (by RECIST v1.1 criteria) will be enrolled into Step 2 double-blind 1:1 randomization to either continue AI plus CDK4/6i, plus a placebo for AZD9833, or switch to AZD9833 (75 mg oral once daily), plus the same CDK4/6i plus a placebo for the AI. The primary endpoint will be investigator-assessed progression-free survival (PFS) per RECIST v1.1 criteria. A key secondary endpoint will be time to second progression or death on a subsequent therapy. Other secondary endpoints will include overall survival, chemotherapy-free survival, objective response rate, clinical benefit rate, patient-reported outcomes, and safety. Enrollment began in June 2021 and is expected at approximately 200 sites across 19 countries. Acknowledgments: We thank Julia Mawer, PhD, of Oxford PharmaGenesis, UK, for medical writing assistance, which was funded by AstraZeneca. Funding: The SERENA-6 trial is funded and overseen by AstraZeneca. Citation Format: François-Clément Bidard, Kevin Kalinsky, Massimo Cristofanilli, Giampaolo Bianchini, Stephen KL Chia, Wolfgang Janni, Cynthia X Ma, Erica L Mayer, Yeon Hee Park, Steven Fox, Xiaochun Liu, Andrew Walding, Cynthia Huang Bartlett, Nick C Turner. SERENA-6: A Phase III study to assess the efficacy and safety of AZD9833 (camizestrant) compared with aromatase inhibitors when given in combination with palbociclib or abemaciclib in patients with HR+/HER2- metastatic breast cancer with detectable ESR1m who have not experienced disease progression on first-line therapy [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 OT2-11-05.

Abstract P3-09-06: Targeting tumor subpopulations based on single-cell transcriptomics

Abstract Breast cancers that are hormone receptor negative, such that they do not express the estrogen receptor (ER), progesterone receptor (PR), or human epidermal growth factor receptor 2 (HER2), are known as triple-negative breast cancer (TNBC), and are predominantly of the basal-like subtype. These tumors aggressively metastasize, have limited effective therapeutic options, and are prevalent in patients of African descent. In these studies, we utilized a racially diverse set of 21 breast cancer patient-derived xenografts (PDXs) and 6 cell lines to identify new therapeutic combinations that may exhibit increased efficacy for patients based on ancestral heritage. Tumor subpopulation abundance was measured by single-cell RNA-sequencing (scRNAseq) a set of 13 PDXs, including some that had been subjected to long term carboplatin treatment or endocrine therapy in vivo. We identified two to four major cell types per PDX and sought to integrate this information with cytotoxic high throughput screens (HTS) of 516 clinically actionable agents. Among several interesting and overlapping findings from these multiscale models, BIRC5 (Survivin) was identified as a readily actionable target in the proliferative component of all TNBC models assayed. Analysis of public datasets found that BIRC5 expression in TNBC patient tumors was significantly correlated with reduced metastasis-free survival. In vitro and in vivo studies from our group and others found that YM155, which targets Survivin, was highly cytotoxic towards breast cancer cells. YM155 has been previously safely administered in Phase I/II clinical trials. To uncover putative dependency pathways that may be exploited to prevent acquired resistance to BIRC5 (Survivin) inhibition, we performed synergistic HTS assays whereby the efficacy of our 516 therapeutic library was assessed in the presence versus absence of YM155. Those drugs which were found to be more highly effective when given in combination with YM155 were then selected for ongoing in vitro and in vivo applications. Grant ID: 1R01CA246182-01A1 (NIH/NCI), CCR19608826 (Susan G. Komen Foundation), VCU School of Medicine VETAR, the Jeffress Trust, UL1TR002649 (CTSA/NCATS) Citation Format: Jacqueline M Grible, Tess Leftwich, Amy L Olex, Alex K Duong, Nicole Hairr, Narmeen S Rashid, Timothy M Smith, Lia Signaevskaia, David C Boyd, Mikhail G Dozmorov, Joshua C Harrell. Targeting tumor subpopulations based on single-cell transcriptomics [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 P3-09-06.

Abstract PD1-04: Estrogen-mediated mechanisms in estrogen receptor-positive breast cancer at the single cell level

Abstract Background: Estrogen typically promotes the progression of hormone-dependent breast cancer through activation of estrogen receptor (ER)-α encoded by ESR1. While estrogen-induced tumor suppression in ER+ breast cancer has been clinically observed as an unexpected outcome of aromatase inhibitor (AI)-resistance, the molecular mechanisms have not yet been fully defined. Methods: Characterization of estrogen regulation in the two ER+ breast cancer patient-derived xenograft (PDX) models with opposite responses to estrogen offered us an unprecedented opportunity to assess how 17β-estradiol (E2) modulates ER+ cancer. We succeeded in establishing estrogen-stimulating (SC31) and estrogen-suppressing (GS3) PDX models. In vivo tumor promotion or suppression by estrogen was confirmed through experiments by implanting E2 pellets in mice carrying SC31/GS3, and then single cell analysis was performed using SC31/GS3 tumors. To investigate whether GS3 would change its response to E2, we performed intermittent E2 treatment every 28 days (E2 pellet on/off every 28 days) and whole-genome RNA sequencing using GS3 tumors. Results: SC31 and GS3 behaved oppositely regarding estrogen-mediated tumor growth. Immunohistochemistry indicated that the number of ERα+ cells and Ki-67+ cells were increased in SC31 and decreased in GS3 after E2 treatment, but progesterone receptor+ cells appeared in both SC31 and GS3 after E2 treatment at the protein level. The E2-induced suppression of GS3 involves ERα, not ERβ, which was wild-type and not amplified. Single cell RNA sequencing analysis of these PDXs had revealed that E2 upregulated the expression of estrogen-regulated genes (e.g., PGR and AREG) in both SC31 and GS3. However, E2 treatment induced cell cycle promotion in SC31, while E2 induced cell cycle arrest in GS3. These gene-expression changes occurred in both ESR1+ cells and ESR1- cells, demonstrating for the first time the influence of estrogen on ESR1- cells in ER+breast tumors. This result suggests that 100% ER positivity is not essential for endocrine response. E2 also upregulated a tumor suppressor gene, IL24, only in GS3. More IL24+ cells were ESR1+ and in G1 phase than IL24- cells. Hallmark apoptosis gene sets were upregulated and the hallmark G2M checkpoint gene set was downregulated in IL24+ cells after E2 treatment. After three rounds of intermittent E2 treatment on GS3, an E2 independent growth developed. Approximately 60% of genes in the intermittent E2-treated sample had the same trend as E2-treated samples (mainly ER target genes), in which 40% of the genes behaved similarly to the placebo-treated sample (mainly cell cycle progression genes). Furthermore, lower levels of IL24 were linked to estrogen independence. Conclusions: Estrogen/ERα signaling increases the expression of estrogen-regulated genes, but it can modulate HR+ tumor growth in different manners. While E2 should activate ERα and regulate ESR1+ cells, our findings regarding ESR1- cells were new and suggested crosstalk between ESR1+ cells and ESR1- cells in both estrogen-stimulating and -suppressing ER+ tumors. Furthermore, our studies demonstrate the potential roles of tumor suppressor gene, IL24 in HR+ cancer. Our findings point to the need to identify biomarkers for patients with estrogen-suppressing tumors who can benefit from E2 treatment after AI resistance; measurements of ER and PR expression are insufficient. Expression of IL24 in AI-resistant tumors may be one such indicator for favorable response to E2 therapy. Citation Format: Hitomi Mori, Kohei Saeki, Gregory Chang, Jinhui Wang, Xiwei Wu, Pei-Yin Hsu, Noriko Kanaya, George Somlo, Masafumi Nakamura, Andrea Bild, Shiuan Chen. Estrogen-mediated mechanisms in estrogen receptor-positive breast cancer at the single cell level [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 PD1-04.

Inhibition of Musashi-1 enhances chemotherapeutic sensitivity in gastric cancer patient-derived xenografts.

Musashi-1 (MSI1), a neural RNA-binding protein, is considered a gastric and intestinal stem cell marker. Although the function of MSI1 in gastric cancer has attracted increasing interest, it is not known whether MSI1 can be used as a biomarker to monitor gastric cancer development and response to treatment. Here, the role of MSI1 in the chemotherapeutic sensitivity of gastric cancer was investigated. Patients with high MSI1 levels had poor outcomes, implicating the gene in the development and progression of the disease. We overexpressed and silenced MSI1 in the human gastric cancer cell lines MKN45 and HGC27, finding that knockdown reduced proliferation, invasion, and migration, while promoting apoptosis. A patient-derived xenograft gastric cancer model was constructed in which mice received chemical drugs, si-MSI1, or a drug-si-MSI1 combination. It was found that blocking MSI1 expression reduced gastric cancer drug tolerance. The combination treatment with si-MSI1 was superior to 5F-dUMP and cisplatin, either separately or in combination, indicating that including si-MSI1 was better than drug therapy alone. Transcriptome sequencing analysis showed that MSI1 altered cell cycle regulation and growth signal transduction, including that of blood vessel epicardial substance (BVES). These results suggest that MSI1 reduces the tolerance of gastric cancer to chemical drugs through modulation of MSI1/BVES signaling.