Stevens, Alexandra M.

Abstract Lineage switch (LS), defined as the immunophenotypic transformation of acute leukemia, has emerged as a mechanism of relapse after antigen-targeted immunotherapy, which is associated with dismal outcomes. Through an international collaborative effort, we identified cases of LS after a host of antigen-targeted therapies (eg, CD19, CD22, CD38, and CD7), described how LS was diagnosed, reviewed treatment approaches, and analyzed overall outcomes for this form of postimmunotherapy relapse. Collectively, 75 cases of LS were evaluated, including 53 (70.7%) cases of B-cell acute lymphoblastic leukemia (B-ALL) transforming to acute myeloid leukemia (AML), 17 (22.7%) cases of B-ALL transforming to mixed phenotypic acute leukemia (MPAL)/acute leukemias of ambiguous lineage (ALAL), and 5 (6.7%) cases of rare LS presentation (ie, 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) after immunotherapy, with 81.4% presenting with LS within the first 6 months from the most proximal immunotherapy. Although most involved KMT2A rearrangements (n = 45, 64.3%), other rare cytogenetic and/or molecular alterations were uniquely observed. Treatment outcomes were generally poor, with remission rates of <40%. The median overall survival after 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 after immunotherapy and, through enhanced understanding, establishes a foundation for improving outcomes of LS.

Introduction: Survival of children with relapsed/refractory (r/r) acute myeloid leukemia (AML), myelodysplastic syndromes (MDS) and juvenile myelomonocytic leukemia (JMML) remains dismal, and successful treatment approaches are limited. Myeloid leukemias frequently have high telomerase expression, and neoplastic progenitor cells have relatively shorter telomeres compared to normal tissues due to the balance between frequent malignant cell division and compensatory increased telomerase activity. In one study, the 5-year overall survival (OS) in children with AML and lower telomerase activity was 88% compared to 43% for those with higher activity, suggesting potential prognostic and therapeutic importance of telomerase in this population (Verstovsek Cancer 2003). Imetelstat is a first-in-class telomerase inhibitor approved by the FDA in 2024 for the treatment of adult patients with low-to-intermediate-1 risk myelodysplastic syndromes (LR-MDS) with transfusion-dependent anemia who are r/r/ineligible for erythropoiesis-stimulating agents. . Imetelstat has also been shown to drive AML cells into ferroptosis (Bruedigam Nature Cancer 2024). The Children's Oncology Group (COG) previously conducted the ADVL1112 Phase 1 study of imetelstat monotherapy in children with r/r solid tumors or lymphomas (Thompson CCR 2013). Imetelstat was administered intravenously on Days 1 and 8 of a 21-day cycle. The recommended phase 2 dose (RP2D) was 268 mg/m2. Imetelstat was well-tolerated with no grade >3 non-hematologic toxicities and one dose-limiting thrombocytopenia observed in a subject treated at the RP2D. In preclinical studies, imetelstat was active against leukemia stem cells in pediatric AML patient derived xenograft models (Barwe J Clin Med, 2022). Methods: PEPN2312 is an open-label, multi-center phase 1 study of imetelstat in combination with fludarabine/high dose cytarabine (FLA) for patients with AML in second or greater relapse, refractory to re-induction after first relapse, or r/r MDS or JMML (NCT06247787). The primary objective of this study is to estimate the maximum tolerated dose (MTD) and/or RP2D of imetelstat in combination with FLA in this patient population. Secondary objectives include toxicity assessment, pharmacokinetics, clinical response/minimal residual disease assessments per standard criteria, and OS. Exploratory analyses will be conducted to analyze telomerase activity in peripheral blood mononuclear cells, conduct pharmacodynamic studies, and evaluate any changes in cytogenetic and mutational abnormalities. Eligible patients must be between the ages of 1 to 18 at the time of study enrollment with Lansky/Karnofsky performance status of ≥50. Patients with leukemia must have platelet count >25,000/µL and hemoglobin >8 g/dL (transfusions allowed). Patients known to have a congenital bone marrow failure syndrome are excluded. This study will use the Rolling 6 design to estimate the MTD/RP2D. The starting dose of imetelstat (DL1) is 212 mg/m2 IV over 2 hours (80% of the single-agent RP2D) given on days 1 and 8 of a 28-day cycle in combination with FLA administered days 2-6. Premedication for infusion reactions with an antihistamine and corticosteroid are required prior to each dose of imetelstat. Patients may receive up to 2 cycles of therapy in the absence of progressive disease or unacceptable toxicity as graded by CTCAE version 5.0. Response assessment will be performed via morphology and central flow cytometry analysis for patients with AML and standard response criteria for patients with JMML or MDS (Niemeyer Haematologica 2015; Zeidan Blood 2023). One dose escalation to 268 mg/m2 (DL2) will be explored if DL1 is deemed safe. Intra-patient dose escalation is not permitted. Pharmacokinetic sampling will be performed during cycle 1. Once the MTD/RP2D is determined, further assessment of safety and preliminary activity of imetelstat and FLA chemotherapy will occur in an expansion cohort of up to 6 additional patients treated at the MTD/RP2D. PEPN2312 was activated in July 2024 and will enroll at 21 participating COG/PEP-CTN sites.

Introduction: Lineage switch (LS) refers to the transformation of acute leukemia from one cell lineage to another (e.g., lymphoid (ALL) to myeloid (AML)). This rare phenomenon, distinct from treatment induced malignancy (i.e., t-AML) and mixed phenotype acute leukemia (MPAL), is increasingly being observed following cell surface antigen targeting. Given challenges in the diagnosis and treatment of LS, Project EVOLVE was developed to globally identify cases of LS following immunotherapy. Methods: A protocol and data collection form for this IRB exempt study was widely disseminated to capture deidentified LS case information across multiple centers and cancer consortia. This included collection of data from published reports of LS, with authors contacted to obtain additional information. For this analysis, LS was defined by the emergence of cell-surface markers warranting reclassification of the original leukemia as a different lineage derivation based on the immunophenotype, including cases of AML emerging alongside the original ALL. “Confirmed” LS cases included those where retention of original cytogenetic or molecular aberrations and/or the clonal immunoglobulin rearrangement patterns were verified to confirm the clonal relationship. “Suspected” LS cases included those where samples to confirm clonality marker retention could not be obtained, but the clinical presentation was consistent with LS. The term “transition” was applied when immunophenotypic shifts were transient. Cases with new cytogenetic abnormalities unrelated to the original diagnosis and compatible with t-AML were excluded. Statistical analysis was primarily descriptive. Data cut-off was July 26, 2023. Results: A total of 58 cases were identified, including 19 cases referenced in prior publications. Of the 58, 11 cases were excluded from this initial analysis, including: 4 cases of t-AML, 2 cases of T-ALL to AML, and 5 “transition” cases. Collectively, 47 cases of LS (43 “confirmed”; 4 “suspected”), spanning across 3 continents and 8 countries, were analyzed. (Fig. 1A) Cases included transition from B-ALL to AML in 36 (76.6%) and to MPAL/biphenotypic/ambiguous lineage in 11 (23.4%). Two likely had MPAL at diagnosis and a third with chronic myeloid leukemia. The median age at initial diagnosis was 8.4 years (range, 1 day-76.5 years); median age at LS presentation was 11.0 years (range 0.4-77.3 years). Twenty-three (48.9%) were male and 19 (40.4%) had prior stem cell transplant (SCT). Baseline cytogenetics showed KMT2A rearrangement in 27 (57.4%). Baseline myeloid antigen co-expression was seen in 22 of 23 patients with submitted data, but variation in degree of expression across patients and antigens was substantial. The most proximal immunotherapy prior to development of LS was CAR T-cells in 23 (48.9%) and blinatumomab in 24 (51.1%). The median time from the most proximal immunotherapy to development of LS was 1.6 months (range, 7 days-36.5 months). In 7 (29.2%) patients receiving blinatumomab, LS developed during the infusion. LS presented as isolated marrow relapse in 27 (57.4%), isolated CNS in 2 (4.3%), isolated non-CNS extramedullary disease in 3 (6.4%), combined relapse in 14 (29.8%) and unreported in 1. First line treatment for LS was chemotherapy induction in 31 (66.0%), palliative care/no therapy in 8 (17.0%), and alternative therapies in the remaining 8 (17.0%). In 35 (74.5%) patients, remission induction was the goal of first line therapy. Fifteen (31.9%) achieved a complete remission (CR) of whom 2 were treated with palliative intent. Across all patients, 8 (17.0%) are alive in remission, the longest being 4.7 years from LS diagnosis; 2 (4.3%) are alive with active LS and 37 (78.7%) died from LS, their original disease, or complications of treatment of LS (Fig 1B). For those alive in CR (n=8), 7 (87.5%) proceeded to a consolidative SCT for treatment of LS and 1 patient, an octogenarian remains in remission after AML directed therapy. Conclusions: LS is emerging as an important mechanism of immune escape following immunotherapy. Project EVOLVE provides the largest dataset of patients with LS to date. This systematic, international effort reveals substantial complexity and variability in diagnosing and managing LS. Unfortunately, outcomes following LS are grim, underscoring the critical need to identify risk factors of and optimal therapies for this leukemia transformation.