10037

Background: Patients with relapsed/refractory (R/R) high-risk neuroblastoma (HR-NB) have dismal prognosis with chemotherapy-only salvage regimens. Naxitamab, a humanized anti-GD2 monoclonal antibody (mAb), when combined with Irinotecan and Temozolomide (I/T), has demonstrated clinically meaningful efficacy in R/R HR-NB patients (NCT03189706). We aimed to investigate the potential synergy of naxitamab with other cytotoxics. Methods: In this retrospective analysis, we examined patients treated at SJD with chemo-refractory disease, who received naxitamab in combination with chemotherapeutics other than I/T. Each cycle comprised naxitamab 2.25 mg/kg/day IV over 30 minutes, days 2, 4, 9 and 11 (total 9 mg/kg or 270 mg/m2 per cycle), and GM-CSF 250 mg/m2/day subcutaneously, days 6-10 combined with either a) cyclophosphamide (250mg/m2) and topotecan (0.75mg/m2) D1 to D5 (C/T), b) ifosfamide (1500mg/m2), etoposide (100mg/m2) D1-D3 and carboplatin (400mg/m2) D1 (ICE) or c) doxorubicin 37.5mg/m2 D1-D2 and cyclophosphamide (250mg/m2) D1-D3 (C/D). Toxicity was measured by CTCAE v4.0 and responses by INRC. Results: Twenty-nine patients received naxitamab plus GM-CSF with C/T (n=23), ICE (n=6) and C/D (n=1). One patient received both ICE and C/T. Seventeen patients had a variable number of prior relapses (1 n=14, 2 n=1, and 3 n=2). Twelve patients had refractory disease, most having received additional therapy post-induction. All but 4 patients had received naxitamab and I/T (n=20), and/or ICE (n=3/n=1), and dinutuximab-beta and I/T (n=1). Toxicities of the new regimens included myelosuppression expected with chemotherapy, and pain and hypertension expected with naxitamab. Hemorrhagic cystitis occurred in 2 patients treated with C/T, BK infection documented in one. A total of 113 cycles (median 2; 1-8) were administered, outpatient. Out of the 30 treatments, 5 achieved complete response (CR), 2 partial response (PR), and 2 mixed response (MR), providing an objective response (OR) of 30%. Eight patients achieved stable disease (SD) and 13 progressed on treatment. Disease control rate (DCR) (OR or SD) after 2 cycles was 56.6%. Patients who never responded with prior chemo-immunotherapy progressed through the new combinations. Contrary, patients who showed initial stabilization with I/T or ICE but ultimately progressed, switching drugs resulted in 60% DCR and 30% OR. Most favourable OR (75%) was seen in patients who achieved CR with prior chemoimmunotherapy and relapsed (3 out of 4). Conclusions: Naxitamab-based chemo-immunotherapy with regimens other than I/T exhibited similar and manageable toxicity profiles. Switching chemotherapeutics provided objective responses only to patients who had previously shown efficacy with I/T chemo-immunotherapy.