First Results of an Open Label Phase II Study to Evaluate the Efficacy and Safety of Inotuzumab Ozogamicin for Induction Therapy Followed By a Conventional Chemotherapy Based Consolidation and Maintenance Therapy in Patients Aged 56 Years and Older with Acute Lymphoblastic Leukemia (INITIAL-1 trial)

Introduction: Acute lymphoblastic leukemia (ALL) represents around 5% of all newly diagnosed leukemia in patients between 55 and 70 years of age. Despite recent advances especially in younger patients, the prognosis of older patients with ALL remains dismal, even after moderately intensive chemotherapy. Due to increasing toxicity and infection rates in older patients, dose intensification of induction treatment often is no justifiable option. Consequently, new treatment options are needed to improve the survival of older ALL patients.

Methods: This open label phase II study of the German Multicenter Study Group on Adult Acute Lymphoblastic Leukemia (GMALL) is currently activated in 14 centers in Germany. Patients aged >55 years with newly diagnosed acute B lymphoblastic leukemia, with the exception of Philadelphia-chromosome or BCR-ABL positive ALL

or Burkitt's or mixed phenotype acute leukemia, are eligible. Leukemic blasts must have CD22 surface expression of at least 20%. No previous ALL-specific treatment, with the exception of corticosteroids and/or single dose vincristine and/or up to 3 doses of cyclophosphamide (cycloph.) plus standard prephase treatment are allowed. The 1^st induction cycle consists of inotuzumab ozogamicin (InO) 0.8 mg/m² on day1 and 0.5 mg/m² on days 8 and 15 together with dexamethasone 10 mg/m² (days 7-8, days 14-17) and 1 intrathecal injection of methotrexate (MTX), cytarabine (AraC) and dexamethasone (Dexa). The 2^nd and 3^rd induction cycle consist of InO 0.5 mg/m² on days 1, 8 and 15 plus intrathecal injection of MTX/AraC/Dexa. Response evaluation is scheduled after each cycle. Patients achieving a complete remission are offered to receive 5 conventional consolidation therapies (3 x ID-MTX/asparaginase; 2 x ID-AraC) and one reinduction therapy (idarubicine/AraC/cycloph./Dexa) in combination with rituximab (for CD20+ ALL), followed by a maintenance therapy with 6-mercaptopurine/MTX. The primary endpoint is is event free survival (EFS) at 12-months follow-up. An event is any of the following: persisting bone marrow blasts after 2 cycles of InO, relapse or death. An event rate of ≤40% at 12 months follow-up is considered to qualify the experimental treatment as very promising for additional testing. Under the assumption of one-sided type I error of 5% and 80% power, 42 evaluable patients were needed for primary endpoint analysis. The INITIAL-1 trial is registered with ClinicalTrials.gov, identifier: NCT03460522.

Results: As of July 2020, 31 patients have been included, with induction results available for 29 patients. Median age at initial diagnosis was 64 years (range 56-80 years). Twenty-five patients were diagnosed with a common- and 4 with a pro-B lymphoblastic leukemia. Median CD22 expression on leukemic blasts was 70% (range 21-99%). Due to suspected therapy related liver toxicities, 1 patient received 1 induction cycle and 1 patient 2 induction cycles (both were in remission after the 1^st induction). All other patients completed induction therapy and achieved complete remission (CR/CRi) mainly after the 1^st induction. Results of minimal residual disease (MRD) measured by PCR are available for 23 patients, with 17 being MRD-negative after induction. So far, 4 events have been reported (2 deaths in remission and 1 relapsed ALL in the 1^st year of treatment; one relapsed disease in the 2^nd year). With a median follow-up of 242 days, the probability of OS at 1 year is 82.4 %. Two patients received an allogeneic stem cell transplantation in ongoing 1^st remission. With regard to adverse events (AEs) during induction therapy 1, 2 and 3, most common AEs ≥CTC 3 reported were leukocytopenia (in 64%, 33% and 13% of all cases, respectively), anemia (54%, 28%, 13%), thrombocytopenia (68%, 17%, 26%) and elevation of liver enzymes (31%, 22%, 20%).

Conclusion: Replacement of conventional induction chemotherapy by InO is feasible, results in promising remission rates, and may reduce the risk of early morbidity and lethality, particular in older patients with acute B lymphoblastic leukemia.