Project EVOLVE: An international analysis of postimmunotherapy lineage switch, an emergent form of relapse in leukemia

• Lineage switch, an emergent form of acute leukemia relapse following antigen-targeted therapy, primarily in B-cell ALL, has dismal outcomes.

• Lineage switch arises rapidly following immune targeted therapy, highlighting the importance of enhanced methods of detection and treatment.

Lineage switch (LS), defined as the immunophenotypic transformation of acute leukemia, has emerged as a mechanism of relapse following antigen-targeted immunotherapy which is associated with dismal outcomes. Through an international collaborative effort, we identified cases of LS following a host of antigen-targeted therapies (e.g., CD19, CD22, CD38 and CD7), described how LS was diagnosed, reviewed treatment approaches, and analyzed overall outcomes for this form of post-immunotherapy relapse. Collectively, 75 cases of LS were evaluated, including 53 (70.7%) cases of B-ALL to AML, 17 (22.7%) cases of B-ALL to mixed phenotypic acute leukemia (MPAL)/acute leukemias of ambiguous lineage (ALAL), and 5 (6.7%) cases of rare LS presentation (i.e., T-cell ALL to AML). An additional 10 cases with incomplete changes in immunophenotype, referred to as "lineage drift" were also described. With a primary focus on the 70 cases of LS from B-ALL to AML or MPAL/ALAL, LS emerged at a median of 1.5 months (range, 0-36.5 months) post-immunotherapy, with 81.4% presenting with LS within the first 6 months from the most proximal immunotherapy. While the majority involved KMT2A rearrangements (n=45, 64.3%), other rare cytogenetic and/or molecular alterations were uniquely observed. Treatment outcomes were generally poor with < 40% remission rates. The median overall survival following LS diagnosis was 4.8 months. Outcomes were similarly poor for those with rare immunophenotypes of LS or "lineage drift." This global initiative robustly categorizes lineage changes post-immunotherapy and, through enhanced understanding, establishes a foundation for improving outcomes of LS.