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Numerous cancers, including prostate cancer (PCa), are addicted to transcription programs driven by specific genomic regions known as super-enhancers (SEs). The robust transcription of genes at such SEs is enabled by the formation of phase-separated condensates by transcription factors and coactivators with intrinsically disordered regions. The androgen receptor (AR), the main oncogenic driver in PCa, contains large disordered regions and is co-recruited with the transcriptional coactivator mediator complex subunit 1 (MED1) to SEs in androgen-dependent PCa cells, thereby promoting oncogenic transcriptional programs. In this work, we reveal that full-length AR forms foci with liquid-like properties in different PCa models. We demonstrate that foci formation correlates with AR transcriptional activity, as this activity can be modulated by changing cellular foci content chemically or by silencing MED1. AR ability to phase separate was also validated in vitro by using recombinant full-length AR protein. We also demonstrate that AR antagonists, which suppress transcriptional activity by targeting key regions for homotypic or heterotypic interactions of this receptor, hinder foci formation in PCa cells and phase separation in vitro. Our results suggest that enhanced compartmentalization of AR and coactivators may play an important role in the activation of oncogenic transcription programs in androgen-dependent PCa.

Transcription regulates key functions of living organisms in normal and disease states, including cell growth and development, embryonic and adult tissue organization, and tumor progression. Here we identify a novel mechanism of transcriptional regulation by an actin regulatory and signaling protein, Abelson Interactor 1 (ABI1). Using prostate cancer models, we uncover a reciprocal regulation between ABI1 and the Androgen Receptor (AR). ABI1 is a direct, androgen-regulated target; in turn, ABI1 interacts with AR and its splice variant ARv7, and co-regulates a subset of specific transcriptional targets. ABI1 directs transcription through transient yet well-defined interaction of its intrinsically disordered region with DNA. Clinical evaluation shows that the ABI1-DNA binding (through Exon 4 splicing) and ABI1-AR interaction are regulated during androgen deprivation therapy and prostate cancer progression, thus controlling tumor plasticity through connecting actin cytoskeleton and cellular signaling to transcriptional regulation. We propose ABI1 as epigenetic regulator of transcriptional homeostasis in AR-driven cancers.

Supplemental figures and legends in one document

PEG10 levels are elevated in chemotherapy-resistant cells.

Primary antibodies used in this study

The significance of lactate transporters has been recognized in various cancer types, but their role in urothelial carcinoma remains mostly unknown. The aim of this study was to investigate the functional importance of the monocarboxylate transporter (MCT) 4 in preclinical models of urothelial carcinoma and to assess its relevance in patient tumors. The association of MCT4 expression with molecular subtypes and outcome was determined in The Cancer Genome Atlas (TCGA) cohort and two independent cohorts of patients with urothelial carcinoma. Silencing of MCT4 was performed using siRNAs in urothelial carcinoma cell lines. Effects of MCT4 inhibition on cell growth, apoptosis, and production of reactive oxygen species (ROS) were assessed. Moreover, effects on lactate efflux were determined. The in vivo effects of MCT4 silencing were assessed in an orthotopic xenograft model. MCT4 expression was higher in the basal subtype. Decreased MCT4 methylation and increased RNA and protein expression were associated with worse overall survival (OS). Inhibition of MCT4 led to a reduction in cell growth, induction of apoptosis, and an increased synthesis of ROS. MCT4 inhibition resulted in intracellular accumulation of lactate. In vivo, stable knockdown of MCT4 reduced tumor growth. The expression of MCT4 in urothelial carcinoma is associated with features of aggressive tumor biology and portends a poor prognosis. Inhibition of MCT4 results in decreased tumor growth in vitro and in vivo. Targeting lactate metabolism via MCT4 therefore provides a promising therapeutic approach for invasive urothelial carcinoma, especially in the basal subtype.

Supplementary Data from Proteomic Screens for Suppressors of Anoikis Identify IL1RAP as a Promising Surface Target in Ewing Sarcoma

Supplementary Methods

Supplementary Table from Proteomic Screens for Suppressors of Anoikis Identify IL1RAP as a Promising Surface Target in Ewing Sarcoma

Probes used in the PCR assays

Figure legends for the supplemental figures and supplemental table 1

Slope analysis of subcutaneous B16 tumor initiation

Suppl. Table 1: Characteristics of the TCGA cohort; Suppl. Table 2: Characteristics of cohort A. CIS= carcinoma in situ; Suppl. Table 3: Characteristics of cohort B Suppl. Fig. 1. Correlation of degree of DNA methylation at CpG site cg10183885 in the SLC16A3 gene region (see Suppl. Table 4) and MCT4 mRNA expression; Suppl. Fig. 2. Correlation of gene expression and protein expression in cohort B. Suppl. Fig. 3. Representative H&E (upper panel) and immunohistochemical images in matched bladder cancer samples (each column is one patient). CD8 was used to stain T cells (middle panel) in comparison with MCT4 expression (lower panel). Suppl. Fig. 4. Protein expression of MCT4 in benign urothelium, primary tumor tissue and lymph node metastases (cohort A). Suppl. Fig. 5. Protein expression in primary tumors according to tumor stage (left) and lymph node involvement (right) (cohort A).; Suppl. Fig. 6. MCT4 mRNA expression correlates with molecular subtypes in the TCGA cohort using the updated TCGA nomenclature (n=408); Suppl. Fig. 7. MCT4 gene expression does not correlate with overall survival in a cohort of patients undergoing neoadjuvant chemotherapy; Suppl. Fig. 8. Expression of MCT4 in urothelial carcinoma cell lines detected by Western Blot (A) and qPCR (B); Suppl. Fig. 9. Effect of MCT4 silencing on cell growth in hypoxic conditions determined by MTT assay. Suppl. Fig. 10. Effect of inhibition of MCT4 using siRNA on cell growth of the MCT4 negative cell line RT4 determined by MTT assay; Suppl. Fig 11. Effect of MCT4 silencing on cell growth in glucose free conditions determined by MTT assay; Suppl.Fig. 12. Effect of different concentrations of MCT4 specific antisense olignucleotide (ASO) on cell growth determined by MTT assay. Suppl. Fig. 13. Oxygen consumption rates at baseline (A), following glucose injection (B) and following oligomycin injection (C) in cell lines treated with control siRNA or MCT4 specific siRNAs.

Cancer cells must overcome anoikis (detachment-induced death) to successfully metastasize. Using proteomic screens, we found that distinct oncoproteins upregulate IL1 receptor accessory protein (IL1RAP) to suppress anoikis. IL1RAP is directly induced by oncogenic fusions of Ewing sarcoma, a highly metastatic childhood sarcoma. IL1RAP inactivation triggers anoikis and impedes metastatic dissemination of Ewing sarcoma cells. Mechanistically, IL1RAP binds the cell-surface system Xc− transporter to enhance exogenous cystine uptake, thereby replenishing cysteine and the glutathione antioxidant. Under cystine depletion, IL1RAP induces cystathionine gamma lyase (CTH) to activate the transsulfuration pathway for de novo cysteine synthesis. Therefore, IL1RAP maintains cyst(e)ine and glutathione pools, which are vital for redox homeostasis and anoikis resistance. IL1RAP is minimally expressed in pediatric and adult normal tissues, and human anti-IL1RAP antibodies induce potent antibody-dependent cellular cytotoxicity of Ewing sarcoma cells. Therefore, we define IL1RAP as a new cell-surface target in Ewing sarcoma, which is potentially exploitable for immunotherapy.Significance:Here, we identify cell-surface protein IL1RAP as a key driver of metastasis in Ewing sarcoma, a highly aggressive childhood sarcoma. Minimal expression in pediatric and adult normal tissues nominates IL1RAP as a promising target for immunotherapy.See related commentary by Yoon and DeNicola, p. 2679.This article is highlighted in the In This Issue feature, p. 2659

Information about methods and data that further support the claims made in this article. This includes, but is not limited to TEM8 distribution in normal tissues, CAR T cell normalization, additional control groups and CAR toxicity studies.

Table S1-S5 Figures S1-10

Supplemental Tables

Purpose:After cisplatin-based neoadjuvant chemotherapy (NAC), 60% of patients with muscle-invasive bladder cancer (MIBC) still have residual invasive disease at radical cystectomy. The NAC-induced biological alterations in these cisplatin-resistant tumors remain largely unstudied.Experimental Design:Radical cystectomy samples were available for gene expression analysis from 133 patients with residual invasive disease after cisplatin-based NAC, of whom 116 had matched pre-NAC samples. Unsupervised consensus clustering (CC) was performed and the consensus clusters were investigated for their biological and clinical characteristics. Hematoxylin & Eosin and IHC on tissue microarrays were used to confirm tissue sampling and gene expression analysis.Results:Established molecular subtyping models proved to be inconsistent in their classification of the post-NAC samples. Unsupervised CC revealed four distinct consensus clusters. The CC1-Basal and CC2-Luminal subtypes expressed genes consistent with a basal and a luminal phenotype, respectively, and were similar to the corresponding established pretreatment molecular subtypes. The CC3-Immune subtype had the highest immune activity, including T-cell infiltration and checkpoint molecule expression, but lacked both basal and luminal markers. The CC4-Scar–like subtype expressed genes associated with wound healing/scarring, although the proportion of tumor cell content in this subtype did not differ from the other subtypes. Patients with CC4-Scar–like tumors had the most favorable prognosis.Conclusions:This study expands our knowledge on MIBC not responding to cisplatin by suggesting molecular subtypes to understand the biology of these tumors. Although these molecular subtypes imply consequences for adjuvant treatments, this ultimately needs to be tested in clinical trials.

NOTCH2-inactivating antibody inhibited tumor progression in vitro and in vivo.

L2 2G and L2 3G CAR T cell normalization and functional testing.

NOTCH2 silencing inhibited tumor progression through decreased cell proliferation and invasion.

Supplemental figures and box plots

NOTCH2-induced cell invasion was blocked by downstream inhibition of canonical NOTCH2 signaling.

Purpose: Recent molecular analyses of bladder cancer open the door to significant advances in targeted therapies. NOTCH has been identified as a tumor suppressor in bladder cancer, but prior reports have focused on NOTCH1. Here we hypothesized that NOTCH2 is an oncogene suitable for therapeutic targeting in bladder cancer.Experimental design: We studied genomic aberrations of NOTCH, compared survival and tumor progression according to NOTCH2 expression levels, and studied NOTCH2 function in vitro and vivo.Results: We report a high rate of NOTCH2 copy number gain in bladder cancer. High NOTCH2 expression was identified especially in the basal subtype and in mesenchymal tumors. NOTCH2 activation correlated with adverse disease parameters and worse prognosis by immunohistochemistry. Forced overexpression of the intracellular domain of NOTCH2 (N2ICD) induced cell growth and invasion by cell-cycle progression, maintenance of stemness and epithelial-to-mesenchymal transition (EMT). These effects were abrogated by silencing of CSL, indicating that the effects were mediated through the canonical NOTCH signaling pathway. In an orthotopic xenograft model, forced overexpression of N2ICD increased growth, invasion, and metastasis. To explore the potential for therapeutic targeting of NOTCH2, we first silenced the receptor with shRNA and subsequently treated with a specific inhibitory antibody. Both interventions decreased cell growth, invasion, and metastasis in vitro and in the orthotopic xenograft model.Conclusions: We have demonstrated that NOTCH2 acts as an oncogene that promotes bladder cancer growth and metastasis through EMT, cell-cycle progression, and maintenance of stemness. Inhibition of NOTCH2 is a rational novel treatment strategy for invasive bladder cancer. Clin Cancer Res; 22(12); 2981–92. ©2016 AACR.

NOTCH2 overexpression promoted bladder cancer progression through increased proliferation and invasion.

The lists of antibodies, primers and patients characteristics and detailed information of methods were described in Supplementary Material and Methods.

Purpose:Neuroendocrine (NE) bladder carcinoma is a rare and aggressive variant. Molecular subtyping studies have found that 5% to 15% of muscle-invasive bladder cancer (MIBC) have transcriptomic patterns consistent with NE bladder cancer in the absence of NE histology. The clinical implications of this NE-like subtype have not been explored in depth.Experimental Design:Transcriptome-wide expression profiles were generated for MIBC collected from 7 institutions and clinical-use of Decipher Bladder. Using unsupervised clustering, we generated a clustering solution on a prospective training cohort (PTC; n = 175), developed single-sample classifiers to predict NE tumors, and evaluated the resultant models on a testing radical cystectomy (RC) cohort (n = 225). A random forest model was finalized and applied to 5 validation cohorts (n = 1302). Uni- and multivariable survival analyses were used to characterize clinical outcomes.Results:In the training cohort (PTC), hierarchical clustering using an 84-gene panel showed a cluster of 8 patients (4.6%) with highly heterogeneous expression of NE markers in the absence of basal or luminal marker expression. NE-like tumors were identified in 1% to 6.6% of cases in validation cohorts. Patients with NE-like tumors had significantly worse 1-year progression-free survival (65% NE-like vs. 82% overall; P = 0.046) and, after adjusting for clinical and pathologic factors, had a 6.4-fold increased risk of all-cause mortality (P = 0.001). IHC confirmed the neuronal character of these tumors.Conclusions:A single-patient classifier was developed that identifies patients with histologic urothelial cancer harboring a NE transcriptomic profile. These tumors represent a high-risk subgroup of MIBC, which may require different treatment.

Background Receptor activator of NF kappa B (RANK) and its ligand have an essential role in T-cell regulation and the development of bone metastases. The role of RANK expression in muscle-invasive bladder cancer (MIBC) is unknown. Objective To assess the relevance of RANK expression in patients with MIBC. Design, setting, and participants Expression of RANK was assessed via immunohistochemistry of benign urothelium, MIBC tissue, and lymph node metastases from 153 patients undergoing radical cystectomy. Expression data from The Cancer Genome Atlas (TCGA) cohort were analyzed for potential associations with molecular subtypes and outcome. Outcome measurements and statistical analysis RANK expression was correlated with clinical and pathological parameters and to individual data for the clinical course of MIBC. Results and limitations Expression of RANK was significantly higher in both primary tumors (p = 0.02) and lymph node metastases (p = 0.01) compared to normal urothelium. In tumor tissue, RANK expression was significantly lower in patients with locally advanced disease and lymph node involvement compared to those with organ-confined disease (p = 0.0009) and node-negative MIBC (p = 0.0002). In univariable and multivariable analyses, high expression of RANK was associated with a longer time to recurrence (p = 0.0005 and 0.01) and better cancer-specific (p = 0.0004 and 0.007) and overall survival (p = 0.002 and 0.04). High expression of RANK was associated with better outcome for patients with luminal infiltrated tumors in the TCGA cohort. Conclusions RANK expression is increased in bladder cancer tissue compared to benign urothelium, with higher expression in organ-defined compared to locally advanced disease. High RANK expression indicates a favorable prognosis in MIBC. The prognostic role differs in tumors of different molecular subtypes. Patient summary Expression of a protein involved in bone turnover regulation (RANK) is higher in bladder cancer tissue than in benign bladder tissue. However, high levels of RANK on tumor cells indicate favorable prognosis for patients with bladder cancer that invades the muscle layer of the bladder.

Prostate cancer is the most common cancer in men and it is estimated that over 350,000 men worldwide die of prostate cancer every year. There remains an unmet clinical need to improve how clinically significant prostate cancer is diagnosed and develop new treatments for advanced disease. Aberrant glycosylation is a hallmark of cancer implicated in tumour growth, metastasis, and immune evasion. One of the key drivers of aberrant glycosylation is the dysregulated expression of glycosylation enzymes within the cancer cell. Here, we demonstrate using multiple independent clinical cohorts that the glycosyltransferase enzyme GALNT7 is upregulated in prostate cancer tissue. We show GALNT7 can identify men with prostate cancer, using urine and blood samples, with improved diagnostic accuracy than serum PSA alone. We also show that GALNT7 levels remain high in progression to castrate-resistant disease, and using in vitro and in vivo models, reveal that GALNT7 promotes prostate tumour growth. Mechanistically, GALNT7 can modify O-glycosylation in prostate cancer cells and correlates with cell cycle and immune signalling pathways. Our study provides a new biomarker to aid the diagnosis of clinically significant disease and cements GALNT7-mediated O-glycosylation as an important driver of prostate cancer progression.

Over the past decade, genomic and transcriptomic analyses have uncovered promising tumor antigens including immunotherapeutic targets in bladder cancer (BCa). Conventional tumor antigens are proteins expressed on the plasma membrane of tumor cells such as EGFR, FGFR3, and ERBB2 in BCa, which can be targeted by antibodies or similar epitope-specific binding reagents. The cellular proteome consists of ∼100,000 proteins but the expression of these proteins is rarely unique to tumor cells. Many tumor-associated proteins are post-translationally modified with phosphorylation, glycosylation, ubiquitination, or SUMOylation moieties. Although these modifications expand the complexity, they potentially offer novel targeting opportunities across tumor sub-populations. Experimental targeting of cancer-specific post-translational modifications (PTMs) has shown encouraging results in pre-clinical models of BCa, which could potentially overcome issues with inherent intra-tumor heterogeneity due to simultaneous expression on different proteins. Here, we review current knowledge on post-translational modifications in BCa and highlight recent efforts in experimental targeting strategies.

Bladder cancer is a heterogeneous disease classified into two broad molecular subtype categories, basal and luminal, with critical treatment and prognostic implications. Recent studies have shown the utility of immunohistochemistry in predicting bladder cancer molecular subtypes, with a two-marker approach using GATA3 and CK5/6 showing over 80% reliability. In the current study, we calculated the accuracy of uroplakin II (UPII), a marker of urothelial differentiation, with different scores (0:1%, 1+: 1-10%, 2+: 10-50%, 3+:50%) to predict RNA-based luminal versus basal subtypes in a cohort of muscle-invasive bladder cancer-received neoadjuvant chemotherapy followed by radical cystectomy. The 1% cutoff of the UPII stain predicts the luminal subtype with the sensitivity and specificity of 95% and 56%, respectively. With a UPII cutoff of 10%, the sensitivity and specificity were 93% and 81%, respectively, and with a UPII cutoff of 50%, the sensitivity and specificity were 91% and 96%, respectively. The prediction performance of UPII was better than either GATA3 or CK5/6. There was no significant difference in prognoses between UPII 0-2+ and UPII 3+ patients in this cohort. The current study shows that evaluating the staining proportion score of UPII can accurately predict basal and luminal subtypes of muscle-invasive bladder cancer.

Paternally expressed gene 10 (PEG10) has been associated with neuroendocrine muscle-invasive bladder cancer (MIBC), a subtype of the disease with the poorest survival. In this work, we further characterized the expression pattern of PEG10 in The Cancer Genome Atlas database of 412 patients with MIBC, and found that, compared with other subtypes, PEG10 mRNA level was enhanced in neuroendocrine-like MIBC and highly correlated with other neuroendocrine markers. PEG10 protein level also associated with neuroendocrine markers in a tissue microarray of 82 cases. In bladder cancer cell lines, PEG10 expression was induced in drug-resistant compared with parental cells, and knocking down of PEG10 resensitized cells to chemotherapy. Loss of PEG10 increased protein levels of cell-cycle regulators p21 and p27 and delayed G1–S-phase transition, while overexpression of PEG10 enhanced cancer cell proliferation. PEG10 silencing also lowered levels of SLUG and SNAIL, leading to reduced invasion and migration. In an orthotopic bladder cancer model, systemic treatment with PEG10 antisense oligonucleotide delayed progression of T24 xenografts. In summary, elevated expression of PEG10 in MIBC may contribute to the disease progression by promoting survival, proliferation, and metastasis. Targeting PEG10 is a novel potential therapeutic approach for a subset of bladder cancers.

Proteoglycans are proteins that are modified with glycosaminoglycan chains. Chondroitin sulfate proteoglycans (CSPGs) are currently being exploited as targets for drug-delivery in various cancer indications, however basic knowledge on how CSPGs are internalized in tumor cells is lacking. In this study we took advantage of a recombinant CSPG-binding lectin VAR2CSA (rVAR2) to track internalization and cell fate of CSPGs in tumor cells. We found that rVAR2 is internalized into cancer cells via multiple internalization mechanisms after initial docking on cell surface CSPGs. Regardless of the internalization pathway used, CSPG-bound rVAR2 was trafficked to the early endosomes in an energy-dependent manner but not further transported to the lysosomal compartment. Instead, internalized CSPG-bound rVAR2 proteins were secreted with exosomes to the extracellular environment in a strictly chondroitin sulfate-dependent manner. In summary, our work describes the cell fate of rVAR2 proteins in tumor cells after initial binding to CSPGs, which can be further used to inform development of rVAR2-drug conjugates and other therapeutics targeting CSPGs.

Purpose: After cisplatin-based neoadjuvant chemotherapy (NAC), 60% of patients with muscle-invasive bladder cancer (MIBC) still have residual invasive disease at radical cystectomy. The NAC-induced biological alterations in these cisplatin-resistant tumors remain largely unstudied. Experimental Design: Radical cystectomy samples were available for gene expression analysis from 133 patients with residual invasive disease after cisplatin-based NAC, of whom 116 had matched pre-NAC samples. Unsupervised consensus clustering (CC) was performed and the consensus clusters were investigated for their biological and clinical characteristics. Hematoxylin & Eosin and IHC on tissue microarrays were used to confirm tissue sampling and gene expression analysis. Results: Established molecular subtyping models proved to be inconsistent in their classification of the post-NAC samples. Unsupervised CC revealed four distinct consensus clusters. The CC1-Basal and CC2-Luminal subtypes expressed genes consistent with a basal and a luminal phenotype, respectively, and were similar to the corresponding established pretreatment molecular subtypes. The CC3-Immune subtype had the highest immune activity, including T-cell infiltration and checkpoint molecule expression, but lacked both basal and luminal markers. The CC4-Scar–like subtype expressed genes associated with wound healing/scarring, although the proportion of tumor cell content in this subtype did not differ from the other subtypes. Patients with CC4-Scar–like tumors had the most favorable prognosis. Conclusions: This study expands our knowledge on MIBC not responding to cisplatin by suggesting molecular subtypes to understand the biology of these tumors. Although these molecular subtypes imply consequences for adjuvant treatments, this ultimately needs to be tested in clinical trials.

Purpose: Neuroendocrine (NE) bladder carcinoma is a rare and aggressive variant. Molecular subtyping studies have found that 5% to 15% of muscle-invasive bladder cancer (MIBC) have transcriptomic patterns consistent with NE bladder cancer in the absence of NE histology. The clinical implications of this NE-like subtype have not been explored in depth. Experimental Design: Transcriptome-wide expression profiles were generated for MIBC collected from 7 institutions and clinical-use of Decipher Bladder. Using unsupervised clustering, we generated a clustering solution on a prospective training cohort (PTC; n = 175), developed single-sample classifiers to predict NE tumors, and evaluated the resultant models on a testing radical cystectomy (RC) cohort (n = 225). A random forest model was finalized and applied to 5 validation cohorts (n = 1302). Uni- and multivariable survival analyses were used to characterize clinical outcomes. Results: In the training cohort (PTC), hierarchical clustering using an 84-gene panel showed a cluster of 8 patients (4.6%) with highly heterogeneous expression of NE markers in the absence of basal or luminal marker expression. NE-like tumors were identified in 1% to 6.6% of cases in validation cohorts. Patients with NE-like tumors had significantly worse 1-year progression-free survival (65% NE-like vs. 82% overall; P = 0.046) and, after adjusting for clinical and pathologic factors, had a 6.4-fold increased risk of all-cause mortality (P = 0.001). IHC confirmed the neuronal character of these tumors. Conclusions: A single-patient classifier was developed that identifies patients with histologic urothelial cancer harboring a NE transcriptomic profile. These tumors represent a high-risk subgroup of MIBC, which may require different treatment.

MNK is a serine/threonine kinase signaling node in the MAPK pathway, which is phosphorylated and activated by ERK. MNK phosphorylates eIF4E to promote translation of a subset of mRNAs that encode oncogenically relevant proteins including c-Myc, Cyclin D1 and PD-L1. We conducted a phase 1B study [NCT04261218] of tomivosertib (T), an orally administered inhibitor of MNK, to assess target engagement in metastatic breast cancer tissue and tolerability and pharmacokinetics of co-administration with paclitaxel (P). There are no prior reports of co-administration of a MNK inhibitor with a cytotoxic agent. All patients underwent pre-treatment baseline and on-treatment (day ~7) core biopsy of metastatic disease during the T monotherapy lead-in phase; subsequently patients continued oral T 100mg bid (without or with meal) with the addition of IV P at 80mg/m2 day 1 and 8 every 21 days. Results: We report here interim data for the first 14 patients with MBC refractory to standard treatments. Pharmacokinetics: T plasma levels were 21+/- 40 ng/ml throughout the day, and were not influenced by infusion of P. Peak P levels following infusion were 600 +/- 220 ng/ml and fell to below detection limit by 24 hours post infusion, similar to prior reports for single-agent P. Pharmacodynamics: The main endpoint was assessment of T -induced eIF4E phosphorylation in MBC biopsy tissue, estimated by immunostaining with an antibody specific for eIF4E phosphorylated at S209. Phospho-eIF4E staining intensity was generally high in the pre-treatment samples (indicating baseline activation of eIF4E in MBC). In contrast, the on- T biopsies revealed undetectable phopho-eIF4E staining in all patients. These results were confirmed by mass cytometry analysis, which also included a characterization of the immune microenvironment. We performed ribo-seq analysis on 2 mg flash frozen cancer specimens, and this provided preliminary evidence for decreased translation of mRNAs encoding proteins relevant to neoplasia including TMSB4X and MUCL1 on exposure to T. Preliminary analysis of comparison of proteomic data for each patient, comparing baseline and on-drug biopsies, revealed upregulation of proteins involved in fatty acid catabolic processes, consistent with prior experimental work (PMID 32712434). Safety and Clinical: Of 14 initial patients, 1 left the study for adverse reaction (nausea), 1 left study at physician’s discretion, and 5 left the study for progressive disease. There were no indications of increased P toxicity, and T was generally well tolerated in keeping with prior single agent phase I data. Two patients showed a decrease (36% and 45%) in size of metastases, but others showed disease progression, despite evidence for absence of phospho-eIF4E. The results indicate feasibility of a future phase 2 study of the T - P combination. (Supported by a Dream Team grant from Stand Up To Cancer - Canada) Citation Format: Cristiano Ferrario, John Mackey, Karen Gelmon, Poul HB Sorensen, Htoo Zarni Oo, Gregg B. Morin, Sonia del Rincon, Mehdi Amiri, Premal Patel, Harvey W. Smith, Nahum Sonenberg, Michael N. Pollak. Phase 1b trial of the MNK1/2 inhibitor tomivosertib combined with paclitaxel in patients with metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT521.