Open-Label, Multicenter, Phase I/II, First-in-Human Trial of TK216: A First-Generation EWS::FLI1 Fusion Protein Antagonist in Ewing Sarcoma.

PURPOSE: Ewing Sarcoma (ES), a rare cancer with a pathognomonic translocation resulting in the Ewing sarcoma gene (EWS)::FLI1 oncoprotein, has a poor prognosis in the relapsed/refractory (R/R) setting. Tokalas (TK)216 was designed to bind EWS::FLI1 proteins directly, disrupt protein-protein interactions, and inhibit transcription factor function. TK216 plus vincristine showed synergistic activity in preclinical tumor models. To our knowledge, we report the results of a first-in-class, first-in-human phase I/II trial of TK216 in R/R ES. PATIENTS AND METHODS: TK216 was administered intravenously as a continuous infusion to patients with R/R ES in 11 cohorts. The dosing duration of 7 days was later extended to 10, 14, and 28 days. Vincristine could be added on day 1 after cycle 2, per investigators' choice. The trial used a 3 + 3 design with an expansion cohort at the recommended phase II dose (RP2D). RESULTS: A total of 85 patients with a median age of 27 years (range, 11-77) were enrolled. The maximum tolerated dose for the 14-day infusion of TK216, 200 mg/m2 once daily, was determined in cohort 9 and selected as the RP2D. The median previous number of systemic therapies regimens was three (range, 1-10). The most frequent-related adverse events in patients treated at the RP2D included neutropenia (44.7%), anemia (29.4%), leukopenia (29.4%), febrile neutropenia (15.3%), thrombocytopenia (11.8%), and infections (17.6%). In cohorts 9 and 10, two patients had a complete response, one had a partial response, and 14 had stable disease; the 6-month progression-free survival was 11.9%. There were no responses among the eight patients in cohort 11. CONCLUSION: TK216 administered as 14-day continuous infusion with or without vincristine was well tolerated and showed limited activity at the RP2D in R/R ES.

Mapping the Single-cell Differentiation Landscape of Osteosarcoma.

PURPOSE: The genetic intratumoral heterogeneity observed in human osteosarcomas (OS) poses challenges for drug development and the study of cell fate, plasticity, and differentiation, processes linked to tumor grade, cell metastasis, and survival. EXPERIMENTAL DESIGN: To pinpoint errors in OS differentiation, we transcriptionally profiled 31,527 cells from a tissue-engineered model that directsMSCs toward adipogenic and osteoblastic fates. Incorporating pre-existing chondrocyte data, we applied trajectory analysis and non-negative matrix factorization (NMF) to generate the first human mesenchymal differentiation atlas. RESULTS: This 'roadmap' served as a reference to delineate the cellular composition of morphologically complex OS tumors and quantify each cell's lineage commitment. Projecting a bulk RNA-seq OS dataset onto this roadmap unveiled a correlation between a stem-like transcriptomic phenotype and poorer survival outcomes. CONCLUSIONS: Our study quantifies OS differentiation and lineage, a prerequisite to better understanding lineage-specific differentiation bottlenecks that might someday be targeted therapeutically.

Enzalutamide induces cytotoxicity in desmoplastic small round cell tumor independent of the androgen receptor.

Desmoplastic Small Round Cell Tumor (DSRCT) is a rare, pediatric cancer caused by the EWSR1::WT1 fusion protein. DSRCT predominantly occurs in males, which comprise 80-90% of the patient population. While the reason for this male predominance remains unknown, one hypothesis is that the androgen receptor (AR) plays a critical role in DSRCT and elevated testosterone levels in males help drive tumor growth. Here, we demonstrate that AR is highly expressed in DSRCT relative to other fusion-driven sarcomas and that the AR antagonists enzalutamide and flutamide reduce DSRCT growth. However, despite these findings, which suggest an important role for AR in DSRCT, we show that DSRCT cell lines form xenografts in female mice at the same rate as male mice and AR depletion does not significantly alter DSRCT growth in vitro. Further, we find that AR antagonists reduce DSRCT growth in cells depleted of AR, establishing an AR-independent mechanism of action. These findings suggest that AR dependence is not the reason for male predominance in DSRCT and that AR-targeted therapies may provide therapeutic benefit primarily through an AR-independent mechanism that requires further elucidation.

Outcomes of Late-Line Systemic Treatment in GIST: Does Sequence Matter?

Ripretinib and avapritinib have demonstrated activity in the late-line treatment of gastrointestinal stomal tumors (GISTs). We investigated whether patients previously treated with ripretinib benefit from avapritinib, and vice versa. Patients diagnosed with metastatic/unresectable GIST and treated with both drugs at two institutions in 2000-2021 were included. Patients were grouped by drug sequence: ripretinib-avapritinib (RA) or avapritinib-ripretinib (AR). Radiographic response was evaluated using RECIST 1.1. Kaplan-Meier and log-rank tests were used to compare time-to-progression (TTP) and overall survival (OS). Thirty-four patients (17 per group) were identified, with a median age of 48 years. The most common primary site was the small bowel (17/34, 50%), followed by the stomach (10/34, 29.4%). Baseline characteristics and tumor mutations were not significantly different between groups. Response rates (RRs) for ripretinib were 18% for RA and 12% for AR; RRs for avapritinib were 12% for AR and 18% for RA. Median TTPs for ripretinib were 3.65 months (95%CI 2-5.95) for RA and 4.73 months (1.87-15.84) for AR. Median TTPs for avapritinib were 5.39 months (2.86-18.99) for AR and 4.11 months (1.91-11.4) for RA. Median OS rates following RA or AR initiation were 29.63 (95%CI 13.8-50.53) and 33.7 (20.03-50.57) months, respectively. Both ripretinib and avapritinib were efficacious in the late-line treatment of GIST, with no evidence that efficacy depended on sequencing.

Enzalutamide Induces Cytotoxicity in Desmoplastic Small Round Cell Tumor Independent of the Androgen Receptor.

Desmoplastic Small Round Cell Tumor (DSRCT) is a rare, pediatric cancer caused by the EWSR1::WT1 fusion protein. DSRCT predominantly occurs in males, which comprise 80-90% of the patient population. While the reason for this male predominance remains unknown, one hypothesis is that the androgen receptor (AR) plays a critical role in DSRCT and elevated testosterone levels in males help drive tumor growth. Here, we demonstrate that AR is highly expressed in DSRCT relative to other fusion-driven sarcomas and that the AR antagonists enzalutamide and flutamide reduce DSRCT growth. However, despite these findings, which suggest an important role for AR in DSRCT, we show that DSRCT cell lines form xenografts in female mice at the same rate as male mice and AR depletion does not significantly alter DSRCT growth in vitro. Further, we find that AR antagonists reduce DSRCT growth in cells depleted of AR, establishing an AR-independent mechanism of action. These findings suggest that AR dependence is not the reason for male predominance in DSRCT and that AR-targeted therapies may provide therapeutic benefit primarily through an AR-independent mechanism that requires further elucidation.

Mapping the Single-cell Differentiation Landscape of Osteosarcoma.

The genetic and intratumoral heterogeneity observed in human osteosarcomas (OS) poses challenges for drug development and the study of cell fate, plasticity, and differentiation, processes linked to tumor grade, cell metastasis, and survival. To pinpoint errors in OS differentiation, we transcriptionally profiled 31,527 cells from a tissue-engineered model that directs MSCs toward adipogenic and osteoblastic fates. Incorporating pre-existing chondrocyte data, we applied trajectory analysis and non-negative matrix factorization (NMF) to generate the first human mesenchymal differentiation atlas. This 'roadmap' served as a reference to delineate the cellular composition of morphologically complex OS tumors and quantify each cell's lineage commitment. Projecting these signatures onto a bulk RNA-seq OS dataset unveiled a correlation between a stem-like transcriptomic phenotype and poorer survival outcomes. Our study takes the critical first step in accurately quantifying OS differentiation and lineage, a prerequisite to better understanding global differentiation bottlenecks that might someday be targeted therapeutically.

Dissociation protocols used for sarcoma tissues bias the transcriptome observed in single-cell and single-nucleus RNA sequencing.

BACKGROUND: Single-cell RNA-seq has emerged as an innovative technology used to study complex tissues and characterize cell types, states, and lineages at a single-cell level. Classification of bulk tumors by their individual cellular constituents has also created new opportunities to generate single-cell atlases for many organs, cancers, and developmental models. Despite the tremendous promise of this technology, recent evidence studying epithelial tissues and diverse carcinomas suggests the methods used for tissue processing, cell disaggregation, and preservation can significantly bias gene expression and alter the observed cell types. To determine whether sarcomas - tumors of mesenchymal origin - are subject to the same technical artifacts, we profiled patient-derived tumor explants (PDXs) propagated from three aggressive subtypes: osteosarcoma (OS), Ewing sarcoma (ES), desmoplastic small round cell tumor (DSRCT). Given the rarity of these sarcoma subtypes, we explored whether single-nuclei RNA-seq from more widely available archival frozen specimens could accurately be identified by gene expression signatures linked to tissue phenotype or pathognomonic fusion proteins. RESULTS: We systematically assessed dissociation methods across different sarcoma subtypes. We compared gene expression from single-cell and single-nucleus RNA-sequencing of 125,831 whole-cells and nuclei from ES, DSRCT, and OS PDXs. We detected warm dissociation artifacts in single-cell samples and gene length bias in single-nucleus samples. Classic sarcoma gene signatures were observed regardless of the dissociation method. In addition, we showed that dissociation method biases could be computationally corrected. CONCLUSIONS: We highlighted transcriptional biases, including warm dissociation and gene-length biases, introduced by the dissociation method for various sarcoma subtypes. This work is the first to characterize how the dissociation methods used for sc/snRNA-seq may affect the interpretation of the molecular features in sarcoma PDXs.

Impact of Biomarker-Matched Therapies on Outcomes in Patients with Sarcoma Enrolled in Early-Phase Clinical Trials (SAMBA 101).

PURPOSE: Developing new therapeutics for any of the more than 100 sarcoma subtypes presents a challenge. After progression from standard therapies, patients with sarcoma may be referred for enrollment in early-phase trials. This study aimed to investigate whether enrollment in biomarker-matched early-phase clinical trials leads to better outcomes for patients with advanced sarcoma. EXPERIMENTAL DESIGN: In this retrospective analysis, investigational treatment characteristics and longitudinal survival outcomes were analyzed in patients with biopsy-confirmed sarcoma enrolled in early-phase trials at MD Anderson Cancer Center from May 2006 to July 2021. RESULTS: Five hundred eighty-seven patients were included [405 soft tissue, 122 bone, 60 gastrointestinal stromal tumor (GIST); median of three prior lines of therapy]. Most common subtypes were leiomyosarcoma (17.2%), liposarcoma (14.0%), and GIST (10.2%). Molecular testing was available for 511 patients (87.1%); 221 patients (37.6%) were treated in matched trials. Overall response rate was 13.1% matched compared with 4.9% in unmatched (P < 0.001); the clinical benefit rate at 6 months was 43.9% vs. 19.9% (P < 0.001). Progression-free survival was longer for patients in matched trials (median, 5.5 vs. 2.4 months; P < 0.001), and overall survival was also superior for patients in matched trials (median, 21.5 vs. 12.3 months; P < 0.001). The benefit of enrollment in matched trials was maintained when patients with GIST were excluded from the analysis. CONCLUSIONS: Enrollment in biomarker-matched early-phase trials is associated with improved outcomes in heavily pretreated patients with metastatic sarcoma. Molecular testing of tumors from patients with advanced sarcoma and enrollment in matched trials is a reasonable therapeutic strategy.

Retrospective evaluation of the role of gemcitabine-docetaxel in well-differentiated and dedifferentiated liposarcoma.

OBJECTIVE: Well-differentiated (WDLPS) and dedifferentiated liposarcoma (DDLPS) account for the majority of liposarcomas. Although gemcitabine-docetaxel is used as second-line treatment in soft tissue sarcomas, its efficacy in WDLPS/DDLPS is not established. This study retrospectively analyzed the efficacy of gemcitabine regimens in WDLPS/DDLPS. METHODS: All patients with WDLPS or DDLPS who received gemcitabine-based chemotherapy at our institution between September 2002 and January 2021 were included. Response was evaluated by an independent radiologist using RECIST 1.1. The Kaplan-Meier method was used to estimate distributions of survival outcomes and log-rank tests were used to compare survival outcomes between subgroups. RESULTS: Sixty-five WDLPS/DDLPS patients were included. Seven patients (10.8%) received a gemcitabine-based regimen more than once, totaling 72 treatments. The median age at the start of treatment was 66 years (range 32-80 years). Sixty-five (90.3%) regimens were gemcitabine-docetaxel, and 7 (9.7%) were gemcitabine alone. Majorities of treatments were for disease that was recurrent/metastatic (86.1%), was abdominal/retroperitoneal (83.3%), and had DDLPS components (88.9%), while 25.0% of treatments were for multifocal disease. The overall response rate was 9.7% (7/72). All responses were in patients with documented DDLPS. The median time to progression was 9.2 months (95% CI 5.3-12.3 months). The median overall survival from the start of therapy was 18.8 months (95% CI 13.1-32.4 months). CONCLUSION: Gemcitabine-docetaxel is an efficacious second-line treatment for DDLPS. Though cross study comparisons are not advisable, response to gemcitabine-docetaxel compares favorably to current standard options trabectedin and eribulin. This combination is a valid comparator arm for future second-line trials in DDLPS.

Examining Stripes on a Herd of Zebras: Impact of Genomic Matching for Ultrarare Sarcomas in Phase 1 Clinical Trials (SAMBA 102).

PURPOSE: Recently, the Connective Tissue Oncology Society published consensus guidelines for recognizing ultrarare sarcomas (URS), defined as sarcomas with an incidence ≤1 per 1,000,000. We assessed the outcomes of 56 patients with soft tissue, and 21 with bone sarcomas, enrolled in Phase 1 trials. EXPERIMENTAL DESIGN: In this Sarcoma-Matched Biomarker Analysis (SAMBA-102 study), we reviewed records from patients on Phase 1 trials at the University of Texas MD Anderson Cancer Center between January 2013 and June 2021. RESULTS: Among 587 sarcomas, 106 (18.1%) were classified as URS. Fifty (47%) were male, and the median age was 44.3 years (range, 19-82). The most common subtypes were alveolar soft part sarcoma (ASPS), chordoma, dedifferentiated chondrosarcoma, and sclerosing epithelioid fibrosarcoma. Compared with common sarcomas, median OS was similar 16.1 months [95% confidence interval (CI), 13.6-17.5] versus 16.1 (95% CI, 8.2-24.0) in URS (P = 0.359). Objective response to treatment was higher in URS 13.2% (n = 14/106) compared with common sarcomas 6.9% (n = 33/481; P = 0.029). Median OS for those treated on matched trials was 27.3 months (95% CI, 1.9-52.7) compared with 13.4 months (95% CI, 6.3-20.6) for those not treated on matched trials (P = 0.291). Eight of 33 (24%) molecularly matched treatments resulted in an objective response, whereas 6 of 73 unmatched treatments (8.2%) resulted in an objective response (P = 0.024). Clinical benefit rate was 36.4% (12/33) in matched trials versus 26.0% (19/73) in unmatched trials (P = 0.279). CONCLUSIONS: The results demonstrate the benefit of genomic selection in Phase 1 trials to help identify molecular subsets likely to benefit from targeted therapy.

The immune landscape of undifferentiated pleomorphic sarcoma.

Introduction: Undifferentiated pleomorphic sarcoma (UPS) can be associated with a relatively dense immune infiltration. Immune checkpoint inhibitors (anti-PD1, anti-PDL1, and anti-CTLA4) are effective in 20% of UPS patients. We characterize the immune microenvironment of UPS and its association with oncologic outcomes. Material and methods: Surgically resected UPS samples were stained by immunohistochemistry (IHC) for the following: tumor-associated immune cells (CD3, CD8, CD163, CD20), immune checkpoints (stimulatory: OX40, ICOS; inhibitory: PD-L1, LAG3, IDO1, PD1), and the adenosine pathway (CD73, CD39). Sections were reviewed for the presence of lymphoid aggregates (LA). Clinical data were retrospectively obtained for all samples. The Wilcoxon rank-sum and Kruskal-Wallis tests were used to compare distributions. Correlations between biomarkers were measured by Spearman correlation. Univariate and multivariate Cox models were used to identify biomarkers associated with overall survival (OS) and disease-free survival (DFS). Unsupervised clustering was performed, and Kaplan-Meier curves and log-rank tests used for comparison of OS and DFS between immune clusters. Results: Samples analyzed (n=105) included 46 primary tumors, 34 local recurrences, and 25 metastases. LA were found in 23% (n=10/43), 17% (n=4/24), and 30% (n=7/23) of primary, recurrent, and metastatic samples, respectively. In primary UPS, CD73 expression was significantly higher after preoperative radiation therapy (p=0.009). CD39 expression was significantly correlated with PD1 expression (primary: p=0.002, recurrent: p=0.004, metastatic: p=0.001), PD-L1 expression (primary: p=0.009), and CD3+ cell densities (primary: p=0.016, recurrent: p=0.043, metastatic: p=0.028). In recurrent tumors, there was a strong correlation between CD39 and CD73 (p=0.015), and both were also correlated with CD163+ cell densities (CD39 p=0.013; CD73 p<0.001). In multivariate analyses, higher densities of CD3+ and CD8+ cells (Cox Hazard Ratio [HR]=0.33; p=0.010) were independently associated with OS (CD3+, HR=0.19, p<0.001; CD8+, HR= 0.33, p=0.010) and DFS (CD3+, HR=0.34, p=0.018; CD8+, HR=0.34, p= 0.014). Unsupervised clustering of IHC values revealed three immunologically distinct clusters: immune high, intermediate, and low. In primary tumors, these clusters were significantly associated with OS (log-rank p<0.0001) and DFS (p<0.001). Conclusion: We identified three immunologically distinct clusters of UPS Associated with OS and DFS. Our data support further investigations of combination anti-PD-1/PD-L1 and adenosine pathway inhibitors in UPS.

The androgen receptor is a therapeutic target in desmoplastic small round cell sarcoma.

Desmoplastic small round cell tumor (DSRCT) is an aggressive, usually incurable sarcoma subtype that predominantly occurs in post-pubertal young males. Recent evidence suggests that the androgen receptor (AR) can promote tumor progression in DSRCTs. However, the mechanism of AR-induced oncogenic stimulation remains undetermined. Herein, we demonstrate that enzalutamide and AR-directed antisense oligonucleotides (AR-ASO) block 5α-dihydrotestosterone (DHT)-induced DSRCT cell proliferation and reduce xenograft tumor burden. Gene expression analysis and chromatin immunoprecipitation sequencing (ChIP-seq) were performed to elucidate how AR signaling regulates cellular epigenetic programs. Remarkably, ChIP-seq revealed novel DSRCT-specific AR DNA binding sites adjacent to key oncogenic regulators, including WT1 (the C-terminal partner of the pathognomonic fusion protein) and FOXF1. Additionally, AR occupied enhancer sites that regulate the Wnt pathway, neural differentiation, and embryonic organ development, implicating AR in dysfunctional cell lineage commitment. Our findings have direct clinical implications given the widespread availability of FDA-approved androgen-targeted agents used for prostate cancer.

Multi-site desmoplastic small round cell tumors are genetically related and immune-cold.

Desmoplastic small round cell tumor (DSRCT) is a highly aggressive soft tissue sarcoma that is characterized by the EWSR1-WT1 fusion protein. Patients present with hundreds of tumor implants in their abdominal cavity at various sites. To determine the genetic relatedness among these sites, exome and RNA sequencing were performed on 22 DSRCT specimens from 14 patients, four of whom had specimens from various tissue sites. Multi-site tumors from individual DSRCT patients had a shared origin and were highly related. Other than the EWSR1-WT1 fusion, very few secondary cancer gene mutations were shared among the sites. Among these, ARID1A, was recurrently mutated, which corroborates findings by others in DSRCT patients. Knocking out ARID1A in JN-DSRCT cells using CRISPR/CAS9 resulted in significantly lower cell proliferation and increased drug sensitivity. The transcriptome data were integrated using network analysis and drug target database information to identify potential therapeutic opportunities in EWSR1-WT1-associated pathways, such as PI3K and mTOR pathways. Treatment of JN-DSRCT cells with the PI3K inhibitor alpelisib and mTOR inhibitor temsirolimus reduced cell proliferation. In addition, the low mutation burden was associated with an immune-cold state in DSRCT. Together, these data reveal multiple genomic and immune features of DSRCT and suggest therapeutic opportunities in patients.

Experimental models of undifferentiated pleomorphic sarcoma and malignant peripheral nerve sheath tumor.

Undifferentiated pleomorphic sarcoma (UPS) and malignant peripheral nerve sheath tumor (MPNST) are aggressive soft tissue sarcomas that do not respond well to current treatment modalities. The limited availability of UPS and MPNST cell lines makes it challenging to identify potential therapeutic targets in a laboratory setting. Understanding the urgent need for improved treatments for these tumors and the limited cellular models available, we generated additional cell lines to study these rare cancers. Patient-derived tumors were used to establish 4 new UPS models, including one radiation-associated UPS-UPS271.1, UPS511, UPS0103, and RIS620, one unclassified spindle cell sarcoma-USC060.1, and 3 new models of MPNST-MPNST007, MPNST3813E, and MPNST4970. This study examined the utility of the new cell lines as sarcoma models by assessing their tumorigenic potential and mutation status for known sarcoma-related genes. All the cell lines formed colonies and migrated in vitro. The in vivo tumorigenic potential of the cell lines and corresponding xenografts was determined by subcutaneous injection or xenograft re-passaging into immunocompromised mice. USC060.1 and UPS511 cells formed tumors in mice upon subcutaneous injection. UPS0103 and RIS620 tumor implants formed tumors in vivo, as did MPNST007 and MPNST3813E tumor implants. Targeted sequencing analysis of a panel of genes frequently mutated in sarcomas identified TP53, RB1, and ATRX mutations in a subset of the cell lines. These new cellular models provide the scientific community with powerful tools for detailed studies of tumorigenesis and for investigating novel therapies for UPS and MPNST.

Targeting the IGF/PI3K/mTOR pathway and AXL/YAP1/TAZ pathways in primary bone cancer.

Primary bone cancers (PBC) belong to the family of mesenchymal tumors classified based on their cellular origin, extracellular matrix, genetic regulation, and epigenetic modification. The three major PBC types, Ewing sarcoma, osteosarcoma, and chondrosarcoma, are frequently aggressive tumors, highly metastatic, and typically occur in children and young adults. Despite their distinct origins and pathogenesis, these sarcoma subtypes rely upon common signaling pathways to promote tumor progression, metastasis, and survival. The IGF/PI3K/mTOR and AXL/YAP/TAZ pathways, in particular, have gained significant attention recently given their ties to oncogenesis, cell fate and differentiation, metastasis, and drug resistance. Naturally, these pathways - and their protein constituents - have caught the eye of the pharmaceutical industry, and a wide array of small molecule inhibitors and antibody drug-conjugates have emerged. Here, we review how the IGF/PI3K/mTOR and AXL/YAP/TAZ pathways promote PBC and highlight the drug candidates under clinical trial investigation.

Correlation of nuclear pIGF-1R/IGF-1R and YAP/TAZ in a tissue microarray with outcomes in osteosarcoma patients.

Osteosarcoma (OS) is a genetically diverse bone cancer that lacks a consistent targetable mutation. Recent studies suggest the IGF/PI3K/mTOR pathway and YAP/TAZ paralogs regulate cell fate and proliferation in response to biomechanical cues within the tumor microenvironment. How this occurs and their implication upon osteosarcoma survival, remains poorly understood. Here, we show that IGF-1R can translocate into the nucleus, where it may act as part of a transcription factor complex. To explore the relationship between YAP/TAZ and total and nuclear phosphorylated IGF-1R (pIGF-1R), we evaluated sequential tumor sections from a 37-patient tissue microarray by confocal microscopy. Next, we examined the relationship between stained markers, clinical disease characteristics, and patient outcomes. The nuclear to cytoplasmic ratios (N:C ratio) of YAP and TAZ strongly correlated with nuclear pIGF-1R (r = 0.522, p = 0.001 for each pair). Kaplan-Meier analyses indicated that nuclear pIGF-1R predicted poor overall survival, a finding confirmed in the Cox proportional hazards model. Though additional investigation in a larger prospective study will be required to validate the prognostic accuracy of these markers, our results may have broad implications for the new class of YAP, TAZ, AXL, or TEAD inhibitors that have reached early phase clinical trials this year.

Real-world use of palbociclib monotherapy in retroperitoneal liposarcomas at a large volume sarcoma center.

Palbociclib has been evaluated in early phase trials for well-differentiated liposarcoma (WDLPS) and dedifferentiated liposarcoma (DDLPS) patients, with reported median progression-free survival (PFS) of 18 weeks. Here, we report on real-world use and surgical outcomes associated with palbociclib treatment. We retrospectively reviewed 61 consecutive patients with retroperitoneal WDLPS (n = 14) or DDLPS (n = 47) treated with palbociclib monotherapy between 1 March 2016 and 28 February 2021 at The University of Texas MD Anderson Cancer Center. At palbociclib initiation, median age was 64 (interquartile range [IQR] 56-72). In WDLPS and DDLPS cohorts, the median number of prior systemic treatments was 0 (IQR 0-0) and 2 (IQR 0-4), respectively. Median number of prior surgeries was 2 (WDLPS IQR 1-2.75) and 2 (DDLPS IQR 1-3). Median PFS was 9.2 (WDLPS IQR 3.9-21.9) and 2.6 months (DDLPS IQR 2.0-6.1), with median time on treatment of 7.4 months (WDLPS IQR 3.5-14.2) and 2.7 months (DDLPS IQR 2.0-5.7). Twelve patients ultimately underwent surgical resection. Resections were macroscopically complete (R0/R1) in half (n = 6/12), among whom only one patient experienced relapse after resection (median follow-up 7.5 months). All patients who underwent macroscopically incomplete resections progressed after surgery with median time to progression of 3.3 months (IQR 2.3-4.4). Surgery after palbociclib treatment was not associated with improved overall survival. Efficacy of palbociclib monotherapy for patients with advanced WDLPS and DDLPS is disappointing. While palbociclib may have been used to delay surgery, there was no clear benefit from treatment and few patients achieved prolonged tumor control.