The molecular determinates involved in the progression of myxoid liposarcoma to increased cellularity/round cell change are poorly understood. We studied the PI3K/Akt pathway in myxoid and round cell liposarcomas using a tissue microarray composed of 165 tumors from 111 patients, and mutational analysis of PIK3CA in 44 cases. Activating PIK3CA mutations were found in 6/44 cases, 14%; mutations were more frequent in round cell vs. myxoid tumors (5/15, 33% vs. 1/29, 3%; p=0.013). Complete loss of PTEN, an alternative mechanism for PI3K/Akt activation, was found in 13/111 (12%) cases and was mutually exclusive with PIK3CA mutation. Strong IGF1R expression was demonstrated in 14/39 (36%) of round cell and 11/58 (19%) of myxoid tumors (p=0.062). Activation of the PI3K pathway was confirmed using immunohistochemical analysis for downstream targets phospho-S6 ribosomal protein and phospho-4EBP1. Phospho-4EBP1 was increased in round cell tumors compared to myxoid tumors (24/30, 80% vs. 25/44, 57%; p=0.038) or tumors with treatment effect (10/24, 42%; p=0.02). Phospho-S6 was highly expressed in both myxoid and round cell tumors (29/47, 62% and 14/30, 47%, respectively; p=0.2). In tumors with PIK3CA mutation, any IGF1R expression, or loss of PTEN expression, phospho-4EBP1 was more frequently elevated compared to tumors without a known activating event in the PI3K pathway (55/72; 76% vs. 3/8, 38%; p=0.033). These findings suggest that activation of the PI3K/Akt pathway via activating mutation of PIK3CA, loss of PTEN, or IGF1R expression play a role in round cell transformation. The PI3K/Akt pathway may therefore provide a therapeutic target in round cell liposarcoma.

11505 Background: is a randomized, phase II non-comparative trial evaluating the efficacy of neoadjuvant checkpoint blockade [nivolumab (N) or ipilimumab/ nivolumab (I/N)] in patients (pts) with surgically resectable retroperitoneal DDLPS or extremity/truncal UPS treated with concurrent neoadjuvant radiation therapy (RT, UPS only). Methods: Primary endpoint was pathologic (path) response. Secondary endpoints were safety, RECIST response, recurrence-free survival, overall survival and patient-reported outcomes. Biospecimens (tumor, blood, fecal microbiome) at baseline, on therapy, and at time of surgery were collected and will be assessed for immune-based prognostic biomarkers. We assessed correlation between radiographic and pathologic response by linear regression. Correlative analyses includes assessment of tumor PD-L1 expression, characterization of tumor immune infiltrates by multiplex immunohistochemistry, and transcriptomic and genomic analyses. Results: Of the 25 pts enrolled; 24 are evaluable for response (14 DDLPS, 9 UPS). Clinical activity was variable by histologic subtype and treatment with RT. Median path response in the UPS cohort was 95% [95% CI 85–99] and was similar between the N/RT and I/N/RT groups (Table). Median path response in the DDLPS cohort was 22.5% [95% CI 85–99; Table]. Median change in tumor size (radiographic response) was -4% and +9% in the UPS and DDLPS cohorts, respectively. There was no correlation between path response and radiographic response (R2 0.0309; p = 0.43). Of 8 pts with path response ≥ 85%, there was 1 partial response, 5 stable disease and 2 progressive disease by RECIST criteria. There was 1 delay to surgery due to grade 3 hyperbilirubinemia (Arm B). There was no difference in toxicity between N/RT and I/N/RT. Conclusions: N/RT and I/N/RT have significant clinical activity in UPS; more than expected compared to historic controls. Toxicity profiles were as expected and the majority of patients underwent resection without delay. Larger studies evaluating N/RT in UPS are warranted given the significant path response in this cohort. RECIST was not associated with path response and better markers of on-treatment clinical activity are needed. Correlative analyses that may guide combination strategies are ongoing and will be presented at the meeting. Clinical trial information: NCT03307616 .

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Malignant peripheral nerve sheath tumors (MPNSTs) are soft tissue sarcomas that frequently harbor genetic alterations in polycomb repressor complex 2 (PRC2) components-SUZ12 and EED. Here, we show that PRC2 loss confers a dedifferentiated early neural-crest phenotype which is exclusive to PRC2-mutant MPNSTs and not a feature of neurofibromas. Neural crest phenotype in PRC2 mutant MPNSTs was validated via cross-species comparative analysis using spontaneous and transgenic MPNST models. Systematic chromatin state profiling of the MPNST cells showed extensive epigenomic reprogramming or chromatin states associated with PRC2 loss and identified gains of active enhancer states/super-enhancers on early neural crest regulators in PRC2-mutant conditions around genomic loci that harbored repressed/poised states in PRC2-WT MPNST cells. Consistently, inverse correlation between H3K27me3 loss and H3K27Ac gain was noted in MPNSTs. Epigenetic editing experiments established functional roles for enhancer gains on DLX5-a key regulator of neural crest phenotype. Consistently, blockade of enhancer activity by bromodomain inhibitors specifically suppressed this neural crest phenotype and tumor burden in PRC2-mutant PDXs. Together, these findings reveal accumulation of dedifferentiated neural crest like state in PRC2-mutant MPNSTs that can be targeted by enhancer blockade.

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