Identification of p53 Aberration-Dependent as Well as Non-p53-Mediated Resistance to MDM2 Inhibitors in Acute Myelogenous Leukemia.

MDM2 inhibitors are under development as novel therapeutic agents in cancers and multiple such compounds have entered early-phase clinical trials. AML is characterized by a relatively low incidence of p53 mutations, thus constituting a tumor type that may have high response rates to MDM2 inhibitors.

We have measured apoptosis induction following ex-vivo treatment with the MDM2 inhibitors MI63 and MI-219 in highly purified AML blasts from 80 patients using Annexin-V FACS-based readouts at 40 hours post treatment. IC₅₀ values for apoptosis induction were calculated using dose response curves and the program XLfit. Data were integrated with p53 exons 5–9 sequence data and p53 immunoblotting data before and after MDM2 inhibitor treatment.

Results: AML blast sensitivities to treatment with MI219 fell into three classes: sensitive with IC₅₀ values <2μM (22/80=28% of all cases), partial resistance with IC₅₀ values ≥2 to <10μM (32/80=40% of all cases) and resistant with IC₅₀ >10μM (26/80=32% of all cases). All eight cases with p53 exons 5–9 sequence mutations were resistant to MI-219. p53 immunoblotting analysis before and after MI219 treatment revealed that the vast majority of resistant cases with wild-type p53 exons 5–9 displayed wild-type p53 induction.

Conclusion: AML is characterized by a substantial proportion of cases with partial or complete resistance to MDM2 inhibitor treatment. Further, given that the majority of these cases display wild-type p53 induction after MDM2 inhibitor treatment, it can be concluded that post-p53 network defects confer blocks to apoptosis induction in resistant AML blasts. To identify and catalogue these defects we have begun expression array-based analysis of transcriptome differences in sensitive versus resistant cases, SNP 6.0 array-based analyses of genomic aberrations in sensitive versus resistant cases and targeted molecule interrogations.

In summary, our data provide important supportive information regarding the planned initiation of human clinical trials with MDM2 inhibitors in AML. Such trials may benefit from ex-vivo testing of AML blasts for sensitivity to MDM2 inhibitors as part of pre-enrollment patient selection.