e17048 Background: Standard treatment for high-grade non-muscle-invasive bladder cancer (HGNMIBC) is transurethral resection of the bladder tumor followed by intravesical Bacillus Calmette-Guérin (BCG) immunotherapy. Up to 40% of patients with HGNMIBC will fail intravesical BCG therapy. A promising therapeutic perspective is represented by OncoTherad immunotherapy. OncoTherad is a nanostructured inorganic phosphate complex associated to glycosidic protein developed by University of Campinas/Brazil that exhibits antitumor properties. The aims of this study were to evaluate the efficacy and safety of OncoTherad immunotherapy for BCG-refractory or relapsed HGNMIBC. Methods: We conducted a prospective, single-center (Municipal Hospital of Paulinia, São Paulo, Brazil), single-arm phase I/ II study of OncoTherad immunotherapy in 29 (18 male, 11 female) patients with BCG-unresponsive HGNMIBC (≥ 1 previous course of BCG intravesical therapy). The schedule was initiated with weekly intravesical (120 mg/mL) and intramuscular (25 mg/mL) OncoTherad treatment for 6 weeks, followed by one every other week application for 3 months and, one monthly application until the end of the treatment (24 months). Follow-up was performed with systematic mapping biopsies of the bladder, cystoscopy and ultrasound. The primary endpoint was pathological complete response (pCR) and recurrence-free survival (RFS). The recurrence was defined as histology proven tumor recurrence (any grade) and monitored at 3-month intervals. Secondary endpoints were time to disease recurrence and safety response. Results: The median age of the 29 patients was 64 years (range 34-94). At baseline pTis, pTaG2-3, pT1G2-3 occurred in 10%, 59% and 31% of patients respectively. OncoTherad treatment showed pCR rates (95% CI) of 100% at 3, 6 and 9 months, 89,6% (26/29) at 12 months and, 89,6% (26/29) at 24 months. A 24-months RFS rate in all patients was 79,3%. Also, the median time to disease recurrence for patients was 459 days (15,3 months; 95% CI) at 24-months follow-up. 95% of adverse events were Grade 1 or 2. The most commonly reported treatment-related adverse events were dysuria (51,7%), cystitis (34,5%), pruritus (44,8%), rash (27,6%), arthralgia (27,6%) and fatigue (27,6%). Conclusions: In conclusion, OncoTherad seems a safe and effective treatment option for BCG-unresponsive HGNMIBC patients and may provide benefit for preventing tumor recurrence. Clinical trial information: RBR-6swqd2.