Clinical Response to Idasanutlin (RG7388) in Acute Myeloid Leukemia Patients Is Associated with Pre-Treatment MDM2 Protein Expression in Leukemic Blasts and Leukemic Stem Cells

Introduction. Despite years of translational research and discovery, outcomes in Acute Myeloid Leukemia remain poor. The MDM2 antagonist idasanutlin has shown promising clinical activity in solid tumors and acute leukemias. Idasanutlin enhances the activity of the tumor suppressor, p53, through antagonism of MDM2:p53 interaction. Such protein:protein disruption abolishes MDM2 targeting of p53 for ubiquitination and degradation. Subsequent stabilization of the p53 protein allows it to exert tumor suppressor transcriptional regulation and induction of apoptotic pathways. Identification of patients in whom functional p53 activation drives efficacy may translate into improved outcomes for patients treated with idasanutlin.

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 clinical response as the primary endpoint. To identify biomarkers of response, patients' pre-treatment peripheral blood specimens were evaluated for MDM2 protein expression levels in leukemic blasts and leukemic stem cells. Association of MDM2 percent cell positivity with clinical outcomes was evaluated using intracellular flow cytometry gated on CD45dim leukemic blasts and CD45dim/CD117+/CD34+ leukemic stem cells.

Results. We observed in a proof-of-principle training data set that MDM2 expression in leukemic blasts was associated with patients more likely to exhibit Complete Remission (CR, CRp, CRi) versus Progressive Disease (PD, HI, PR) [n=61; Wilcoxon p = .0041 (AUC = .74; 95%CI[.57, .91])] TP53 mutational status alone was not tightly associated with patient response (Fisher's Exact Test p = .19 [AUC = .60; 95%CI [.52,.67.])] When MDM2 protein expression and TP53 mutational status were analyzed as co-variates, an enhanced association with patient CR was observed (AUC = .76; 95% CI [.59, .93.]) A separate group of patients treated with an optimized idasanutlin formulation was utilized as a validation set. Association of CR with MDM2 expression in blasts was also shown in this separate patient population [n=24; Wilcoxon p = .0052 (AUC = .82; 95% CI [.64, 1.00.]) Comparable results were observed for proof-of-principle and validation set analyses of response association with MDM2 protein expression in AML patients CD45dim/CD117+/CD34+ stem cell subpopulations.

Conclusions. In summary, training and validation sets reveal that MDM2 protein expression in leukemic blasts and stem cells are associated with idasanutlin-induced CR in patients with AML. We will continue to monitor this potentially predictive biomarker in future randomized clinical studies of idasanutlin. On a broader level, the data presented here support the concept that leukemic blasts may be "oncogene-addicted" to MDM2 in AML, i.e. that AML tumor cells may rely on a dominant oncogene for growth and survival, such that inhibition of the function of this specific oncogene is sufficient to halt the neoplastic phenotype. Thus, the concept of oncogene addiction may apply not only to mutated kinases but also to ubiquitin ligases such as MDM2.