Background: INO is effective in relapsed/refractory B-ALL and may have utility in eradicating MRD in patients (pts) in MRD-positive (MRD+ve) remission. Methods: This is a phase II trial of pts with B-ALL in complete remission (CR) with persistent MRD+ve or with MRD+ve relapse. Eligible pts were required to be MRD+ve at ≥0.01%. MRD negativity (MRD-ve) was defined as undetectable MRD by flow cytometry (sensitivity 10^-4) for Philadelphia (Ph)-negative (Ph-) B-ALL and undetectable MRD by both flow and PCR for BCR:ABL1 for Ph-positive (Ph+) B-ALL. INO was given at a dose of 0.6 mg/m² on D1 and 0.3 mg/m² on D8 of cycle 1 and 0.3 mg/m² on D1 and 8 of subsequent cycles (up to 6 total cycles, given every 21-28 days) along with ursodiol prophylaxis. Pts with Ph+ ALL received concomitant TKI. Results: 27 pts with MRD+ve B-ALL were treated between 11/2018 and 6/2022 (Table). 17 pts (63%) had Ph+ B-ALL (15 received ponatinib and 2 dasatinib), and 10 pts had Ph- B-ALL. 20 pts (74%) were in CR1, and 7 pts (26%) were in CR2 or beyond. 14 pts (52%) had received prior blinatumomab and 5 pts (18%) had prior stem cell transplantation (SCT). 18 pts (67%) became MRD-ve (responders), 16 after the 1st cycle and 2 after the 2nd cycle of INO. In the Ph+ group, 10/17 pts (59%) responded; another 3 pts achieved major molecular response. 8/10 (80%) pts with Ph- B-ALL responded. The median number of cycles of INO were 3 (range, 1-6). Rates of MRD response were higher in pts without prior blinatumomab (11/13 pts [80%]), compared with those with prior blinatumomab exposure (7/14 [50%]) (P=0.06). After a median follow-up of 21 months, 13/18 responding pts (72%) are in ongoing MRD-ve CR. Among the 18 responders, 6 pts underwent allogeneic stem cell transplant (ASCT) after a median of 3 cycles of INO (range, 1-4); all were in MRD-ve CR at the time of ASCT. 4/18 responders (22%) relapsed, 1 of whom relapsed after ASCT. Median RFS and OS for the whole group were not reached; estimated 2-year RFS and OS were 61% and 62%, respectively. 2-year OS for responders and non-responders were 76% and 50% (P=0.34), respectively. 2-year RFS for pts with or without prior blinatumomab was 23% and 91%, respectively; P=0.04. No grade 4 non-hematological toxicities were observed. 1 pt (who had a history of prior ASCT) had grade 3 sinusoidal obstruction syndrome. Conclusions: Low-dose, fractionated INO is effective at eradicating MRD in pts with B-ALL and can result in durable MRD-ve remissions. Clinical trial information: NCT03441061.