E-Poster Presentation 33rd Lorne Cancer Conference 2021

Overcoming resistance to immunotherapy with BH3 mimetics (#183)

ZIZHEN MING 1 2 , Su Yin Lim 1 2 , Bernadette Pedersen 1 2 , Ashleigh Stewart 1 2 , Alexander M. Menzies 2 3 4 , Matteo S. Carlino 2 3 5 , Richard F. Kefford 1 2 4 , Richard A. Scolyer 2 3 6 , Georgina V. Long 2 3 4 , Helen Rizos 1 2
  1. Faculty of Medicine, Health and Human Sciences, Macquarie University, Sydney, NSW, Australia
  2. Melanoma Institute Australia, SYDNEY, New South Wales, Australia
  3. Sydney Medical School, The University of Sydney, SYDNEY, New South Wales, Australia
  4. Royal North Shore and Mater Hospitals, SYDNEY, New South Wales, Australia
  5. Westmead Hospital, SYDNEY, New South Wales, Australia
  6. Royal Prince Alfred Hospital, SYDNEY, New South Wales, Australia

Despite the remarkable improvements of immune checkpoint inhibitors in melanoma, 40% of patients will not respond to anti-PD1-based immunotherapies and 25% of responding patients will eventually progress within 24 months. Novel combination therapies are urgently sought to improve response rates and circumvent resistance. Defects in the apoptotic cell death machinery can promote tumourigenesis and impair tumour responses to anti-cancer therapy. The therapeutic potential of BH3-mimetic drugs targeting anti-apoptotic BCL-2 family proteins (BCL-2, BCL-XL and MCL-1) have been reported in different tumour types and are now progressing to the clinic.  

In our previous work investigating melanoma resistance to PD1 inhibition we confirmed that IFN-γ signalling was intact in 22/23 short-term melanoma cells derived from tumours progressing on anti-PD1 (PD1 PROGs), by demonstrating transcriptome response to exogenous IFN-γ exposure. However, only 2/7 (28%) PD1 PROG cell lines responded to IFN-γ treatment by undergoing marked (>20%) apoptosis.

In this study, we examined potential mechanisms of altered apoptotic regulation in response to IFN-γ in PD1 PROG cells. Significantly, we found that the IFN-γ stimulated the transcript and protein expression of BCL-2 family of anti-apoptotic MCL-1 and pro-apoptotic NOXA. In response to IFN-γ, MCL-1 was complexed preferentially with NOXA at the expense of binding to other pro-apoptotic proteins BIM and BAK.

To explore whether these alterations primed cells for apoptosis, we found that BH3 mimetics S63845 (MCL-1 inhibitor) or navitoclax (BCL-2/BCL-XL/BCL-W inhibitor) potently restored IFN-γ-mediated apoptosis in 7/8 (88%) PD1 PROG melanoma cells. Interestingly, navitoclax-induced cell death was positively associated with NOXA protein. Accordingly, NOXA knockout significantly diminished the sensitivity of cells to navitoclax, but not to S63845 in IFN-γ-primed cells.  We also confirmed the impact of BH3 mimetics in co-culture models of PD1 PROGs and paired autologous T cells (mixed in 1:1 ratio), with dramatically enhanced melanoma cell killing using S63845 (41%) and navitoclax (28%). Further work on in vivo response is currently under investigation in our group.