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11520 Background: Currently, there is no accepted standard chemotherapy regimen for recurrent osteosarcoma. We performed a retrospective study to evaluate the effectiveness, toxicity and clinical benefit of second line therapy with Gemcitabine (G) in combination with Docetaxel (D), in patients with relapsed high-grade osteosarcoma who have been previously treated with High-dose Ifosfamide (HD-IFO) during first-line treatment (ClinicalTrials ID: NCT04651179). Methods: Patients were eligible to the analysis according to the following criteria: episode of first relapse of osteosarcoma, previous first line treatment according to poor responders’ arm of ISG-AIEOP OS 2 Protocol based on methotrexate, cisplatin, doxorubicin and HD-IFO (3gr/m2/day, day 1-5) ± Mifamurtide, presence of measurable disease according to RECIST 1.1, treatment with at least two cycles of Gemcitabine 900 mg/m2/day, days 1 and 8 and Docetaxel 75 mg/m2 day 8 every 21 days. Primary objective: overall survival (OS) at 12 months; secondary objectives: overall response rate (ORR) (complete response + partial response), progression-free survival (PFS) at 6-12 months, toxicity and quality of life. Results: 14 patients were included in the analysis. All patients were previously treated according to poor responders’ arm of ISG-AIEOP OS 2 Protocol, 7/14 (50%) of them received Mifamurtide. Most of patients (12/14, 86%) received 4 GD cycles and the total number of cycles administered was 60. Median OS was 20 months (range 11-69); OS at 12 and 24 months was 84% (95% CI 65-100) and 51% (95% CI 22-79), respectively; ORR were 35,7% after six cycles. Median PFS was 12 months (range 2-69); 6-months and 12-months PFS were 86% (95% CI 67-100) and 56% (30-83) respectively. For 8/14 (57%) patients a surgical approach was feasible after 2 and 6 GD cycles. Previous use of Mifamurtide correlate with a better 6-months PFS rate (100% vs 57%, P = 0.0011). Also, amelioration of ECOG/Lansky score showed a positive correlation with both PFS (P = 0.01) and OS (P = 0.04). No extra-hematological toxicities grade ≥3 was observed according to CTCAE v4.03 criteria. Conclusions: This trial confirms that GD combination has an anti-tumor activity for relapsed high-grade osteosarcoma with a well-tolerated toxicity profile. Furthermore, GD guarantee a surgical tumor remotion for patients with a chemo-resistant disease such as patients previously treated with a poor responders’ arm improving their OS and PFS rate. Promising results for patients previously treated with Mifamurtide need to be confirmed in future trials. Clinical trial information: NCT04651179.

11501 Background: The role of dose intensification of chemotherapy in Ewing sarcoma (ES) is under evaluation in prospective trials. This is a controlled, randomized phase III study evaluating the impact on event-free survival (EFS) of two arms at different intensity of induction therapy in localized ES at onset. Methods: Newly diagnosed localized ES patients aged 2-40 were eligible. They were randomized to receive 4-courses induction therapy - 1 every 21 days - either with a standard arm (arm A) as per ISG/SSGIII protocol (Ferrari S, et at, Ann Oncol. 2011;22(5):1221) or with an intense arm B, consisting of vincristine 1,5mg/sqm+ doxorubicin 80mg/sqm+ifosfamide 9g/sqm for each course. After induction, patients underwent surgery and/or radiotherapy,followed by an adaptive treatment. Good responders received standard courses chemotherapy: arm A pts received 9 courses, while arm B pts received 5 courses. Poor responders in both arms received 4 courses followed by high-dose busulfan/melphalan+autologous stem cell rescue. The primary outcome measure was EFS for the 2 arms in the intention-to-treat population. Kaplan-Meier curves compared with log-rank test and Cox model were performed to assess differences between study arms. A secondary outcome was toxicity differences, assessed by means of the Fisher’s exact test. Initial sample size was 230 pts, type I error rate 5%, power 80%. Results: Between 2009 and 2019, 234 patients were randomized (arm A-115; arm B-119). M:F ratio was 1.8; median age 14 years (range 2-40); tumour site extremity in 55%, axial/pelvis in 45%; tumour volume < 200ml in 31% and ≥200ml in 69%. A good response was obtained in 56% in arm A and 60% in arm B. Median follow-up was 68 months. EFS was not significantly different between arms; HR: 0.85; 95% CI: 0,51-1,41, 5-year EFS (95% CI) was 73% (64-82%) in arm A and 75% (67-83%) in arm B ( p = 0.526). Good responders in arm A and in arm B and poor responders in arm B had comparable results: 5-year EFS (95% CI) was 80% (71-91%), 77% (67-88%), and 72% (59-86%), respectively, while poor responders in arm A showed a worse, not statistically significant (p = 0.164) performance (63%; 50-78%). Subgroup analyses showed similar outcome for age, tumour site and volume in both arms. Hematological, gastrointestinal, and cardiovascular grade ≥3 toxicities were more pronounced in arm B (p < 0.05). Conclusions: Intense induction therapy with arm B did not improve 5-year EFS when compared with the standard arm A. The higher toxicity observed in arm B than in arm A was counterbalanced, in good responders, by a similar outcome with a shorter treatment plan. For poor responders, with almost 30 patients per arm event-free and with < 48-month FUP, better 5-year EFS in arm B than in arm A was observed but needs further observation. Clinical trial information: NCT02063022.

11527 Background: Overexpression of ABCB1/P-glycoprotein (Pgp) predicts poor outcome in retrospective osteosarcoma series.Two prospective trials with Pgp expression and post-induction histologic response as stratification factors were activated in Italy (ISG/OS-2) and Spain (GEIS-33). Methods: Patients ≤ 40 years with extremity non-metastatic high-grade osteosarcoma were eligible. Analysisi of Pgp expression from diagnostic biopsy was centralized. Preoperatively, all patients received methotrexate, adriamycin, cisplatinum (MAP). Surgery was performed at week 8. All patients received a dose of adriamycin following surgery. In case of Pgp overexpression (Pgp+), mifamurtide (2 mg/m2 twice/week for 3 months then weekly for 6 months) was added after surgery, with 4 consecutive cycles of ifosfamide 3 gr/m2/day, day 1-5 (HDIFO) in case of poor histologic response (necrosis < 90%) to MAP. Patients without overexpression of Pgp (Pgp-) received MAP postoperatively, regardless the pathological response. From March 2013, an amendment increased high dose methotrexate cumulative dose from 60 g/m2 (5 cycles) to 120 mg/m2 (10 cycles). The post-amendment regimen was adopted in the observational prospective study by GEIS. Here we present the merged analysis of ISG/OS-2 patients treated post-amendment and GEIS-33. Results: From March 2013 to April 2018, 274 patients were included. Median age was 14 years (range 4-38), male/female: 163/111; 90 were Pgp-, 164 were Pgp+, 20 not evaluable. With a median follow-up of 48 months (1.3-78.5 months), the 3-year EFS and OS were 71.9% (95%CI 66-76.9) and 88% (95%CI: 83.2-91.5) respectively, with no inferior survival for Pgp positive patients and improved survival for good responders (Table). Conclusions: In this prospective uncontrolled study with a risk-adapted strategy for non-metastatic osteosarcoma, survival is superior to that of all ISG/GEIS previous series. The 3-year EFS of 71.9% compares favorably with other reports. Pgp+ patients performed well in this study, in which mifamurtide and HDIFO were added after a poor response to MAP. Clinical trial information: NCT01459484; NCT04383288. [Table: see text]