P583: PATIENTS AT HIGH RISK OF RELAPSE POST-TRANSPLANT: A PHASE 1 STUDY DESIGN WITH A NOVEL TREATMENT STRATEGY USING THE ESTIMAND FRAMEWORK

Abstract

Background: The risk of disease relapse in patients (pts) allografted for acute myeloid leukemia (AML) is high and remains the main cause of transplant failure. The great majority of pts destined to relapse post-allogeneic stem cell transplant (SCT) will do so within the first 12 months and survival after early relapse is dismal. Therefore, strategies to prevent relapse are urgently required. A potent graft-versus-leukemia (GvL) effect is exerted after SCT, and strategies that may augment a GVL effect have the potential to reduce relapse risk post-SCT. Siremadlin (HDM201) is a novel investigational MDM2 inhibitor with single-agent (SA) anti-AML activity. MDM2 inhibitors, including siremadlin, possess potent immunomodulatory effects (IME) in murine solid tumor and AML models. The post-allogeneic SCT setting is ideal for investigating the IME of MDM2 inhibition on enhancing GvL effect. Here we present a phase Ib/II proof-of-concept study designed to assess safety and preliminary efficacy of a treatment strategy with siremadlin as monotherapy and, subsequently, in combination with donor lymphocyte infusions (DLIs) for AML in post-SCT setting using the estimand framework (ICH E9 (R1) addendum). Aims: The objective of the planned treatment strategy is to enhance GvL post-SCT by siremadlin as monotherapy and in combination with DLIs. Due to the risk of Graft-vs-Host Disease (GvHD) with DLI early after SCT, siremadlin will be given as priming monotherapy to prevent early relapse prior to starting siremadlin/DLI combination therapy. SA siremadlin maintenance after the combination therapy is intended to prolong and enhance the GvL reaction to eradicate residual leukemic blasts. Methods: The estimand framework mandates a careful definition of the experimental intervention according to the clinical question of interest. In this trial, the experimental intervention follows a treatment strategy: Patients (pts) with AML in complete remission (CR) post-SCT but at high risk for relapse based on pre-SCT risk factors will start with siremadlin priming monotherapy. Pts who tolerate the siremadlin dose with no evidence of GvHD are eligible to start siremadlin/DLI combination therapy. After combination therapy, pts may continue with SA siremadlin maintenance. Depending on the treatment journey, pts may receive only a part of the intervention’s sequence (e.g., pts in siremadlin priming monotherapy who are not eligible for DLI combination will continue the priming part). The clinical questions addressed in this trial are to determine the recommended dose (RD) of siremadlin across all parts of the treatment strategy and to assess preliminary efficacy regardless of the parts of the treatment strategy pts received. The doses of interest were preselected based on SA studies and in pts who received siremadlin after SCT. Results: The proposed approach is tailored to investigate safety and efficacy of the treatment strategy. The RD of siremadlin will be determined over 2 parts: A Bayesian logistic regression model will guide dose finding of siremadlin during priming (Cycle 1) and a Bayesian time-to-DLT model will assess the siremadlin dose for combination. Efficacy will be assessed by proportion of pts remaining in CR for at least 6 months. Image:Summary/Conclusion: In clinical practice, treatment decisions follow a strategy. The estimand framework guided the design of a clinical study in AML post SCT to conceptualize the assessment of a treatment strategy. Accordingly, safety and efficacy of the treatment strategy as a whole will be assessed taking into account different patient journeys.