Minimal Residual Disease (MRD) Assessment By Multiparametric Flow Cytometry Is Prognostic for Progression-Free Survival in Phase 1/1b Relapsed/Refractory Acute Myeloid Leukemia (AML) Patients Treated with Idasanutlin MDM2 Antagonist

Introduction. Despite considerable efforts towards novel therapeutics research and discovery, outcomes in AML remain poor. Although composite Complete Remission (cCR; CR, CRp, CRi) is routinely used as a measure of therapeutic clinical activity, cCR frequently does not translate to survival benefit. On this basis, the characterization and tracking of minimal residual disease (MRD) has emerged as a tool to help better define the depth of such cCR to offer prognostic utility on AML patients likely to experience survival benefit from a given experimental therapeutic.

The MDM2 antagonist idasanutlin has shown promising clinical activity in AML. Idasanutlin enhances p53 activity through antagonism of the MDM2:p53 interaction. Disruption of this protein:protein interaction inhibits MDM2 targeting of p53 for ubiquitination and degradation, thus stabilizing p53 protein to exert tumor suppressor transcriptional regulation and induction of apoptotic pathways.

Patients and Methods. Trial NP28679 (NCT01773408) is a Phase 1/1b study evaluating idasanutlin as monotherapy or in combination with cytarabine in relapsed or refractory AML patients with safety as primary and complete remission as secondary endpoints, respectively (Martinelli, EHA, 2016.) Duration of response was available as exploratory clinical data for a subset of patient. Patients' pre-treatment bone marrow aspirate specimens were evaluated by multiparametric flow cytometry using an 18 marker surface antigen-based panel. MRD assessment occurred per protocol recommendation at the time of hematological malignancy response assessment (HMRA) at Day 28 and for those patients initially experiencing cCR at each subsequent HMRA. Further flow cytometry analyses were conducted for expression changes of known p53 regulated proteins in CD45⁺(dim) blasts correlating with drug exposure, consistent with mechanistic engagement.

Results. PFS analyses for cCR patients versus non-CR Kaplan-Meier plots indicate the median for responders is 315d (95%CI: 282, NA) versus non-responders is 43.5d (95%CI: 30, NA). When MRD<1% versus >1% is applied as a cut-point, analytics show a statistical association with median PFS (log-rank p-value < .001) at 367d (95%CI: 219, NA) for <1%MRD versus >1% is 84d (95%CI: 30, NA.) Median MRD values for CR/CRp/CRi vs PR/HI vs PD are 0.42%, 1.79%, and 19.16%, respectively, again consistent with MRD serving as a surrogate for clinical activity (log rank test for trend p-value = .001) Additionally, multiparametric flow cytometry analysis of idasanutlin pharmacodynamic (PD) protein expression changes comparing pretreatment patient blood specimens to 24 hours following first dose administration indicates that increases in both p53 and MDM2 in CD45⁺(dim) blasts is associated with steady state drug exposure levels ([Spearman's Correlation =0.27; p=.013] and [Spearman's Correlation=0.24; p=.026,] respectively.) Interestingly, these changes in protein expression for p53 and MDM2 are associated with orthologous PD changes in serum protein macrophage inhibitory cytokine 1 (MIC-1) ([Spearman's Correlation=0.23; p=.035] and [Spearman's Correlation=0.22; p=.042,] respectively.) These PD changes are mechanistically consistent with enhanced p53 resulting from diminished MDM2 ubiquitination and degradation of p53.

Conclusions. In summary, the results presented here are consistent with multiparametric flow cytometry MRD assessment as an early indication aligning with cCR in relapsed/refractory AML patients treated with idasanutlin. Further, assessment for association with progression-free survival indicates that the lowest quartile patients by MRD flow cytometry measurement (<1%) are likely to exhibit a pronounced survival benefit over those patients with higher detectable levels of MRD (>1%). As such, the data presented here support inclusion of MRD assessment by flow cytometry as a prognostic indicator to provide guiding information for assignment of depth of AML patient response. We will continue to monitor this biomarker for diagnostic potential as a prognostic indicator of survival-based outcomes in future randomized clinical studies of idasanutlin.