Mismatch repair–deficient (MMRd) cancers have varied responses to immune-checkpoint blockade (ICB). We conducted a phase II clinical trial of the PD-1 inhibitor pembrolizumab in 24 patients with MMRd endometrial cancer (NCT02899793). Patients with mutational MMRd tumors (6 patients) had higher response rates and longer survival than those with epigenetic MMRd tumors (18 patients). Mutation burden was higher in tumors with mutational MMRd compared with epigenetic MMRd; however, within each category of MMRd, mutation burden was not correlated with ICB response. Pretreatment JAK1 mutations were not associated with primary resistance to pembrolizumab. Longitudinal single-cell RNA-seq of circulating immune cells revealed contrasting modes of antitumor immunity for mutational versus epigenetic MMRd cancers. Whereas effector CD8+ T cells correlated with regression of mutational MMRd tumors, activated CD16+ NK cells were associated with ICB-responsive epigenetic MMRd tumors. These data highlight the interplay between tumor-intrinsic and tumor-extrinsic factors that influence ICB response. Significance: The molecular mechanism of MMRd is associated with response to anti–PD-1 immunotherapy in endometrial carcinoma. Tumors with epigenetic MMRd or mutational MMRd are correlated with NK cell or CD8+ T cell–driven immunity, respectively. Classifying tumors by the mechanism of MMRd may inform clinical decision-making regarding cancer immunotherapy. This article is highlighted in the In This Issue feature, p. 247

Uterine serous carcinoma (USC) and carcinosarcomas (CSs) are rare, highly aggressive variants of endometrial cancer. No reliable tumor biomarkers are currently available to guide response to treatment or detection of early recurrence in USC/CS patients. Circulating tumor DNA (ctDNA) identified using ultrasensitive technology such as droplet digital polymerase chain reaction (ddPCR) may represent a novel platform for the identification of occult disease. We explored the use of personalized ctDNA markers for monitoring USC and CS patients. Tumor and plasma samples from USC/CS patients were collected at the time of surgery and/or during the treatment course for assessment of tumor-specific somatic structural variants (SSVs) by a clinical-grade next-generation sequencing (NGS) platform (i.e., Foundation Medicine) and a droplet digital PCR instrument (Raindance, ddPCR). The level of ctDNA was quantified by droplet digital PCR in plasma samples and correlated to clinical findings, including CA-125 serum and/or computed tomography (CT) scanning results. The genomic-profiling-based assay identified mutated “driver” target genes for ctDNA analysis in all USC/CS patients. In multiple patients, longitudinal ctDNA testing was able to detect the presence of cancer cells before the recurrent tumor was clinically detectable by either CA-125 or CT scanning. Persistent undetectable levels of ctDNA following initial treatment were associated with prolonged progression-free and overall survival. In a USC patient, CA-125 and TP53 mutations but not PIK3CA mutations become undetectable in the plasma at the time of recurrence, suggesting that more than one customized probe should be used for monitoring ctDNA. Longitudinal ctDNA testing using tumor-informed assays may identify the presence of residual tumors, predict responses to treatment, and identify early recurrences in USC/CS patients. Recognition of disease persistence and/or recurrence through ctDNA surveillance may allow earlier treatment of recurrent disease and has the potential to change clinical practice in the management of USC and CS patients. CtDNA validation studies in USC/CS patients prospectively enrolled in treatment trials are warranted.

Recurrent tumors that are resistant to conventional chemotherapy are a major challenge of ovarian cancer treatment. A better understanding of the underlying molecular mechanisms of chemoresistance is critical for developing more effective targeted therapies for ovarian cancer. In this study, we analyzed the transcriptomic profiles of thirteen pairs of matching primary and recurrent ovarian cancers to identify genes that were upregulated in the recurrent tumors. Among these genes, we identified sortilin-related receptor 1 (SORL1) and its role in promoting carboplatin resistance through regulating the stability of epidermal growth factor receptor (EGFR) and fibroblast growth receptor 4 (FGFR4) using ovarian cancer models in vitro and in vivo. We further identified that an anti-SORL1 antibody inhibited the pro-tumor functions of SORL1. Our data showed that a selective inhibitor of FGFR4, FGF401, can improve the therapeutic efficacy of carboplatin in a xenograft mouse model of ovarian cancer. This study has demonstrated the therapeutic potential of targeting the SORL1/FGFR4 pathway to improve the chemoresponse of patients with recurrent and/or resistant ovarian cancer.

Uterine serous carcinoma (USC) is a rare subset of endometrial cancer with a poor prognosis and high recurrence rate. Datopotamab deruxtecan (Dato-DXd) is a novel antibody–drug conjugate (ADC). The objective of this study was to evaluate the preclinical activity of Dato-DXd in USC in vitro against primary USC cell lines with various trophoblast cell-surface antigen 2 (TROP2) expression and in vivo in TROP2-overexpressing cell line–derived mice xenografts. USC primary tumor cell lines were treated with Dato-DXd and a control ADC (CTL ADC) to evaluate cell viability following exposure. Antibody-dependent cell-mediated cytotoxicity against TROP2-overexpressing and -nonexpressing cell lines was evaluated using a 4-hour chromium release assay. USC xenografts in mice were treated with Dato-DXd, CTL ADC, datopotamab, and vehicle to assess the in vivo effects via retro-orbital Dato-DXd administration. We found USC cell lines with TROP2 overexpression to be significantly more sensitive to killing induced by Dato-DXd compared with CTL ADC in vitro (e.g., IC50: 0.11 µmol/L vs. 30.07 µmol/L, P = 0.0074 and 0.11 µmol/L vs. 48.95 µmol/L, P = 0.0127, respectively). Dato-DXd induced antibody-dependent cell-mediated cytotoxicity in the presence of peripheral blood lymphocytes from healthy donors. TROP2-nonexpressing cell lines demonstrated minimal killing by Dato-DXd; however, when admixed with TROP2-overexpressing cells, a significant bystander effect was appreciated. In vivo, mice xenografts overexpressing TROP2 treated with Dato-DXd demonstrated tumor growth suppression and longer overall survival compared with CTL ADC–treated xenografts. These data demonstrate Dato-DXd to be highly active against TROP2-overexpressing USC in vitro and in vivo. Our preclinical activity results warrant future clinical trials for patients with advanced or recurrent USC. Significance: Targeted treatment of USC using the biomarker TROP2 represents a significant opportunity for further treatment options for patients already resistant to other lines of treatment. In this study, we present data showing preclinical evidence of effectiveness of this biomarker-targeted therapy in USC.

Uterine serous carcinoma (USC) is a rare, biologically aggressive variant of endometrial cancer with a high recurrence rate and poor prognosis. HER2 overexpression (3+ positivity) by IHC and/or FISH ERBB2 gene amplification is detected in approximately one-third of patients with USC. Clinical trials incorporating trastuzumab with standard chemotherapy have recently demonstrated improved progression-free and overall survival in advanced-stage or recurrent USC that overexpresses HER2. However, a large number of patients with USC eventually developed resistance to trastuzumab. Trastuzumab deruxtecan (T-DXd) is a novel HER2-directed antibody–drug conjugate with a topoisomerase I inhibitor payload recently approved by the Food and Drug Administration (FDA) for multiple tumor indications. Here, we investigated the in vitro and in vivo efficacy of T-DXd in primary USC cell lines and xenografts with different HER2 expression. T-DXd–induced cell growth suppression in HER2-overexpressing cell lines in vitro, increased early and late apoptosis as assessed by annexin and propidium iodide staining, and, similarly to trastuzumab, T-DXd–induced significant antibody-dependent cellular cytotoxicity in the presence of peripheral blood lymphocytes. While negligible activity was detected against USC cell lines with low HER2 expression, T-DXd demonstrated significant bystander killing against USC tumors with low/negligible HER2 when such cells were admixed with HER2 3+ tumor cells in vitro. T-DXd showed tumor growth suppression in in vivo USC PDX models that overexpress HER2 at 3+ levels, prolonging survival when compared with controls, with minimal toxicity. Future clinical trials are warranted in patients with USC failing trastuzumab treatment.

Datopotamab deruxtecan (Dato-DXd) is a novel antibody–drug conjugate (ADC) targeting trophoblast antigen-2 (TROP2), a cell surface glycoprotein highly expressed in many epithelial tumors, to deliver DXd, a potent topoisomerase I inhibitor. We evaluated TROP2 expression in primary endometrial cancer cell lines and the activity of Dato-DXd against endometrial cancer cell lines with different TROP2 expression in vitro and in vivo. TROP2 expression was assessed in nine primary tumor cell lines by flow cytometry. Cell viability after exposure to Dato-DXd was evaluated using flow cytometry–based assays to calculate the IC50. Bystander effect assay assessed the viability of TROP2-negative cells when cocultured with high TROP2-expressing cells. Fluorescent anti–phosphorylated histone H2AX antibody was used to demonstrate double-strand DNA breaks. Antibody-dependent cell cytotoxicity was tested in vitro using 4-hour chromium release assays. In vivo activity of Dato-DXd was evaluated against TROP2-positive endometrial cancer xenografts. A total of 78% (seven of nine) of the primary endometrial cancer cell lines expressed TROP2. Endometrial cancer cell lines expressing TROP2 were significantly more sensitive to Dato-DXd compared with control ADC. Dato-DXd–exposed, TROP2-positive endometrial cancer demonstrated increased double-strand DNA breaks compared with non-binding conjugate exposure. Dato-DXd mediated antibody-dependent cell cytotoxicity against TROP2-positive cell lines and induced significant bystander killing of TROP2-negative tumors when admixed with TROP2-positive tumors. In vivo, injection of Dato-DXd was well tolerated and demonstrated impressive tumor growth inhibition against chemotherapy-resistant poorly differentiated endometrial cancer xenografts (P < 0.0001). In conclusion, Dato-DXd is a novel ADC with remarkable preclinical activity against poorly differentiated endometrial cancer cell lines overexpressing TROP2. Clinical trials with Dato-DXd in patients with recurrent endometrial cancer are warranted. Significance: Targeted treatment of aggressive forms of endometrial cancer using the biomarker TROP2 is a significant opportunity for the development of treatments when patients are resistant to other lines of treatment. Here, we present data showing preclinical evidence of effectiveness of this biomarker-targeted therapy in endometrial cancer.