Background: ¹³¹I-metaiodobenzylguanidine (MIBG) remains one of the most active agents for neuroblastoma. It is not clear if putative radiation sensitizers improve upon this activity. The primary aim of this trial was to identify the MIBG treatment regimen with highest response rate among: MIBG monotherapy (Arm A); MIBG/Vincristine/Irinotecan (Arm B); MIBG/Vorinostat (Arm C). The secondary aim was to compare toxicity across arms. Methods: We conducted a multicenter, randomized phase II trial. Patients 1-30 years with relapsed/refractory high-risk neuroblastoma were eligible with at least one MIBG-avid site and adequate autologous stem cells (ASCs). All patients received MIBG 18 mCi/kg on Day 1 and ASC on day 15. Patients on Arm A received only MIBG; patients on Arm B also received vincristine (2 mg/m²) IV on Day 0 and irinotecan (50 mg/m²) IV daily on Days 0-4; patients on Arm C also received vorinostat (180 mg/m²) orally once daily on days -1 to 12. The primary endpoint was response after one course according to NANT response criteria. The trial was designed as a pick-the-winner study with a maximum of 105 eligible and evaluable patients to ensure an 80% chance that the arm with highest response rate is selected, if that response rate is at least 15% higher than the other arms. Results: 114 patients enrolled. Three patients were ineligible and 6 eligible patients never received MIBG, leaving 105 eligible and evaluable patients (36 Arm A; 35 Arm B; and 34 Arm C; 55 boys; median age 6.5 years). 9 patients had received prior MIBG monotherapy, 65 prior irinotecan, and 7 prior vorinostat. After one course, the response rates (Partial Response or better) on Arms A, B, and C were 17% (95% CI 7-33%), 14% (5-31%), and 32% (18-51%). An additional 4, 4, and 7 patients met NANT Minor Response criteria [partial response in one disease category (e.g., bone marrow) and stable disease in other categories] on Arms A, B, and C, respectively. On Arms A, B, and C, rates of any grade 3+ non-hematologic toxicity were 19%, 49% and 32%; rates of grade 3+ diarrhea were 0%, 11%, 0%; and rates of grade 3+ febrile neutropenia were 6%, 11%, and 0%. Conclusions: The combination of vorinostat/MIBG had the highest response rate, with manageable toxicity. Vincristine and irinotecan do not improve the response rate to MIBG and are associated with increased toxicity. These data provide response rates for MIBG monotherapy in a contemporary patient population assessed with current response criteria. Clinical trial information: NCT02035137

U.S. National Institutes of Health Pharmaceutical/Biotech Company