Synergistic targeting of estrogen-receptor positive breast cancers by MDM2 inhibition in combination with endocrine therapy or CDK4/6 inhibition

Here we characterise the response of models of ER-positive breast cancer to treatment with the small molecule MDM2 inhibitor NVP-CGM097, a dihydroisoquinolinone derivative currently evaluated in a phase I clinical trial. We show that NVP-CGM097 reduces tumour cell viability of in vitro and in vivo models of endocrine sensitive, endocrine resistant and palbociclib (CDK4/6 inhibitor) resistant p53 wildtype (p53wt) ER-positive breast cancer. NVP-CGM097 synergises with both fulvestrant and palbociclib in models of therapy resistance. Importantly, we identify the key mechanisms of the synergistic interactions between NVP-CGM097 and endocrine therapy, which occurs through the inhibition of E2F Targets and G2M Checkpoint signalling and induction of senescence, rather than depending upon upregulation of p53 dependent apoptotic pathways. Moreover, we find these same pathways are synergistically targeted during the combination treatment of ER positive breast cancer models with NVP-CGM097 and palbociclib. This indicates the genuine potential of MDM2 inhibition as therapy in advanced ER-positive breast cancer as combination endocrine therapy and CDK4/6 inhibitor treatment becomes embedded as standard of care. Overall design: Experimental Design: In vitro and in vivo models were treated with NVP-CGM097 alone and in combination with fulvestrant or palbociclib. We perform cell viability, cell cycle, apoptosis and senescence assays to evaluate antitumor effects in treatment naïve and therapy resistant ER positive cell lines. We further assess the drug effects in patient-derived xenograft (PDX) models of endocrine-sensitive and -resistant breast cancer. Results: We demonstrate that NVP-CGM097 inhibits tumour cells in multiple clinically relevant models of ER-positive breast cancer. Activation of p53 by NVP-CGM097 leads to cell cycle arrest and increased apoptosis in p53-wildtype breast cancer cell lines, and reduces tumour growth in a PDX model. NVP-CGM097 in combination with fulvestrant or palbociclib do not enhance apoptosis, but instead yields profound downregulation of cell cycle-related transcriptional programmes. Cell lines resistant to fulvestrant or palbociclib become senescent upon treatment with combination therapy, and a fulvestrant resistant PDX model reduces tumour growth with combined NVP-CGM097 and fulvestrant treatment.