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e23564 Background: Activity of temozolomide (TEM) and irinotecan (IRI) in recurrent Ewing sarcoma (EWS) was demonstrated. Few data are available on TEMIRI use upfront. Biological predictive factors are lacking. Methods: This multi-institutional retrospective study (NCT03542097) included 59 patients with EWS. 8 patients with very high risk (HR) EWS (multivisceral ± bone marrow) received TEMIRI (TEM 100 mg/m2/day oral, and IRI 40 mg/m2/day intravenous, days 1-5, every 21 days) upfront, 51 patients after relapse (28% in 1st line, 72% ≥ 2nd line). Overall response rate (ORR: CR+PR), SD, and PD, progression-free (PFS) and overall survival (OS) were assessed. The relationship between pre-treatment expression of MGMT and ABCG2 (when FFPE tissue available) with ORR, PFS and OS was evaluated. Results: Median age was 27 years (range 4-62 years): 47 patients (80%) were adults (≥18 years), 35 (61%) had ECOG 0 and 42 (71%) presented with multivisceral disease (+ bone marrow in 5). MGMT was positive in 16/30 (53%), ABCG2 in 4/33 (12%). ORR for upfront TEMIRI (n = 8) was 50% (CR + PR = 1 + 3), with SD 50%, while in recurrent EWS (n = 49, 2 patients no measurable by RECIST) ORR was 31% (CR + PR = 4 + 11), SD 38%, and PD 31%. A better ORR was observed in adult (p = 0.008) & ECOG 0 (p = 0.001) patients; MGMT & ABCG2 expression did not influence ORR. 6-mos PFS was 87% after TEMIRI upfront, 43% at recurrence (p = 0.06), and 65% vs 28% (p = 0.02) for ECOG 0 vs ECOG 1-2, respectively. 6-mos PFS was 62% in MGMT+ vs 33% in MGMT- (p = 0.4) and 75% in ABCG2+ vs 50% in ABCG2- (p = 0.7). Median time to progression (TTP) with upfront TEMIRI was 9 months (range 5-28 months), with 1 patient with ongoing CR at 56 months; median TTP at relapse was 3 months (1-29 months). MGMT and ABCG2 expression did not influence 1-yr OS, whereas ECOG (0 vs 1-2) did (88% vs 31% p < 0.001). Grade 3-4 diarrhea, neutropenia, and thrombocytopenia incidence was < 5%, 1 patient with recurrent kidney EWS died of acute liver failure. Conclusions: TEMIRI showed promising activity in a very unfavorable cohort of EWS patients, with manageable toxicity. Performance status correlated with 6-month PFS & OS whereas MGMT & ABCG2 did not. Clinical trial information: NCT03542097 .