P464: pulse dosing of potent and selective heterobifunctional mdm2 degrader kt-253 drives tumor regression and demonstrates differentiated pharmacology compared to p53/mdm2 small molecule inhibitors.

Background: The murine double minute 2 (MDM2) oncoprotein is a key E3 ubiquitin ligase that degrades the tumor-suppressor p53. Targeting of the MDM2/p53 interaction to stabilize p53 and induce apoptosis in wildtype (WT) p53 tumors is an emerging therapeutic approach in WT p53 hematologic and solid tumor malignancies. However, MDM2/p53 small molecule inhibitors (SMIs) induce a p53/MDM2 feedback loop, resulting in upregulation of MDM2 protein levels and p53 pathway inhibition thus limiting their biological activity and clinical application. KT-253 is a novel, highly potent heterobifunctional MDM2 degrader with >200-fold higher potency than MDM2 SMIs that can suppress p53-dependent MDM2 protein feedback upregulation. We have shown previously that because of its superior pharmacological profile, a single dose of KT-253 was sufficient to induce rapid apoptosis and sustained tumor regression, supporting an intermittent dosing schedule of KT-253. Aims: Here, we assessed how pulse dosing versus exposure matched fractionated doses of KT-253 drives efficacy in mouse xenograft models and characterized the underlying molecular mechanisms associated with the different dosing regimens. In addition, we compared the efficacy of KT-253 to the clinical equivalent dosing schedule of a p53/MDM2 SMI. Methods: Mice were treated with KT-253 on a 3-week dosing cycle, either with a “pulse” dose or various intermittent dosing regimens at doses matched for total AUC. MDM2 SMI DS-3032 was dosed at its clinically equivalent dose and regimen of 3 days on/11 days off. Pharmacodynamic effects were assessed at mRNA level by quantitative PCR and protein level by proteomics. Results: We show in the RS4;11 xenograft model that a pulse IV dose of KT-253 led to rapid and efficient MDM2 degradation, potent p53 induction, apoptosis and sustained tumor regression. In contrast, exposure matched fractionated doses of KT-253 administered on various frequent intermittent dosing schedules spanning a three-week cycle resulted in less efficient MDM2 degradation, modest activation of p53 downstream signaling, cell cycle arrest and tumor stasis. Furthermore, the p53/ MDM2 SMI DS-3032 led only to modest p53 activation and showed limited anti-tumor activity. The acute apoptotic mechanism of KT-253 observed with pulse dosing was confirmed across multiple cell lines and xenograft models. Our results suggest that acute and potent MDM2 degradation is necessary to induce irreversible apoptosis and tumor regression, which can only be achieved with a pulse dose of KT-253 and not with the repeat dosing regimens of MDM2/p53 SMIs. Summary/Conclusion: In summary, our data suggest that pulsatile, higher doses of the MDM2 degrader KT-253 are superior to more frequent lower doses by inducing rapid apoptosis in MDM2-dependent cancer cells. The acute pulse dosing regimen of KT-253 has the potential to demonstrate an improved efficacy and safety profile compared to the more frequent dosing of MDM2/p53 SMIs in the clinic. Keywords: Acute lymphoblastic leukemia, p53, Acute myeloid leukemia