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2029 Background: Glioblastoma (GBM) gene expression subtypes have been described in last years, data in homogeneously treated patients is lacking. Methods: Clinical, molecular and immunohistochemistry (IHC) analysis from patients with newly diagnosed GBM homogeneously treated with standard radiochemotherapy were studied. Samples were classified based on the expression profiles into three different subtypes (classical, mesenchymal, proneural) using Support Vector Machine (SVM), the K-nearest neighbor (K-NN) and the single sample Gene Set Enrichment Analysis (ssGSEA) classification algorithms provided by GlioVis web application. Results: GLIOCAT Project recruited 432 patients from 6 catalan institutions, all of whom received standard first-line treatment (2004 -2015). Best paraffin tissue samples were selected for RNAseq and reliable data were obtained from 124. 82 cases (66%) were classified into the same subtype by all three classification algorithms. SVM and ssGEA algorithms obtain more similar results (87%). No differences in clinical variables were found between the 3 GBM subtypes. Proneural subtype was enriched with IDH1 mutated and G-CIMP positive tumors. Mesenchymal subtype (SVM) was enriched in unmethylated MGMT tumors (p = 0.008), and classical (SVM) in methylated MGMT tumors (p = 0.008). Long survivors ( > 30 months) were rarely classified as mesenchymal (0-7.5%) and were more frequently classified as Proneural (23.1-26.). Clinical (age, resection, KPS) and molecular ( IDH1, MGMT) known prognostic factors were confirmed in this serie. Overall, no differences in prognosis were observed between 3 subtypes, but a trend to worse survival in mesenchymal was observed in K-NN (9.6 vs 15 ). Mesenchymal subtype presented less expression of Olig2 (p < 0.001) and SOX2 (p = 0.003) by IHC, but more YLK-40 expression (p = 0.023, SVM). On the other hand, classical subtype expressed more Nestin (p = 0.004) compared to the other subtypes (K-NN). Conclusions: In our study we have not found correlation between glioblastoma expression subtype and outcome. This large serie provides reproducible data regarding clinical-molecular-immunohistochemistry features of glioblastoma genetic subtypes.

2038 Background: The pRB-dependent cell cycle checkpoint is altered in the vast majority of anaplastic oligodendrogliomas (AO), either by homozygous deletion or by hypermethylation of CDKN2A and/or CDKN2B, or by amplification and/or overexpression of CDK4. Palbociclib is an oral inhibitor of CDK4 and 6 that has already been shown to be highly active in breast cancer. Methods: We conducted a multicenter, open-label, phase II trial evaluating efficacy and safety of Palbociclib in patients with AO that progressed to radiotherapy and more than one chemotherapy regimen containing Temozolomide and/or Lomustine. Inclusion criteria included: histologically and molecularly confirmed grade III oligodendroglioma (WHO 2016 classification, IDH1/2 mutation and 1p/19 codeletion were mandatory), recurrence after radiotherapy and 1 or 2 chemotherapy regimens and conserved RB protein expression by immunohistochemistry (IHC). Patients were treated with Palbociclib 125 mg/daily 3 weeks on/1off. The primary objective of the study was progression-free survival at 6 months (6M-PFS). Results: Between October 2015 and September 2018, 34 patients were enrolled across ten hospitals. The study was stopped early secondary to lack of efficacy, with 74% of evaluable patients progressing within 6 months. Number of patients alive and free from progression at 6 months after the enrollment was 9 (26%) out of the first 34 patients, below the minimum number required (18 out of 40) to consider Palbociclib as an active drug in this population. With a median follow-up of 11.2 months, the median PFS was 3 months (95% CI: 2.5-3.5 months). Median overall survival (OS) was 23.1 months (95% CI: 17.2-25 months). There were no partial or complete responses and only 11 patients (32%) achieved stable disease as best response. Palbociclib was well tolerated with neutropenia (Grade 3 or 4: 40%) and thrombocytopenia (Grade 3 or 4: 15%) as the most common adverse effects (AEs). Both AEs had no significant impact since there were no episodes of febrile neutropenia or bleeding. Conclusions: Despite the good tolerance and drug exposure, Palbociclib monotherapy did not show favorable activity in recurrent AO. Clinical trial information: NCT02530320.