Poster Presentation 38th Lorne Cancer Conference 2026

Prioritizing PI3K pathway inhibitors for PTEN-mutant breast cancer (#251)

Kaveesha Senasinghe 1 , Sylvia Mahara 2 3 , Kelvin Yip 4 , Antonella Papa 1 4
  1. The University of Adelaide, South Australian immunoGENomics Cancer Institute, Adelaide, South Australia, Australia
  2. Department of Anatomy and Developmental Biology, Monash Biomedicine Discovery Institute, Monash University, Clayton, Victoria, Australia
  3. Neil Beauglehall Department of Medical Oncology Research, Cabrini Health, Malvern, Victoria, Australia
  4. Cancer Program, Monash Biomedicine Discovery Institute, Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria, Australia

PTEN is among the most frequently inactivated tumour suppressor genes in human cancer. PTEN alterations occur in all breast cancer (BC) subtypes including 15% of luminal and 30% of basal like BC. Functionally, PTEN is a dual lipid and protein phosphatase which dephosphorylates the phospholipid PIP3, thereby reducing its cytosolic pool. This terminates the PI3K-AKT-mTOR pathway which is a master regulator of cellular growth, proliferation and survival. PI3K is the key lipid kinase phosphorylating PIP2 to PIP3, and resulting in the activation of AKT, a critical downstream effector of this pathway.

Loss of PTEN drives resistance to PI3K targeted therapies such as alpelisib (BYL719), which is FDA and TGA approved for the treatment of PI3K-mutant metastatic BC. Consistent with this, our lab previously demonstrated that mammary organoids harboring PTEN and PI3K mutations were resistant to BYL719 yet remained sensitive to AKT inhibition (via MK2206), underscoring a unique dependency of these cells to the oncogenic signal initiated by AKT (1). Supporting this, the phase III CAPItello-291trial reported that AKT inhibition significantly improved outcomes in patients with hormone receptor-positive, advanced BC harboring AKT pathway alterations (2). Based on these observations, we hypothesized that AKT inhibition is more effective than PI3K inhibition, in the treatment of PTEN and PI3K mutant BC.

Here we monitored tumour response to targeted PI3K or AKT inhibition in a PTEN and PI3K mutant mouse model of advanced breast cancer. Consistent with our ex vivo data in mammary organoids (1), a statistically significant reduction in tumour burden was only observed in response to MK2206 treatments, but not BYL719. To completely eradicate mammary tumours driven by PTEN and PI3K mutations, we next performed transcriptomic analysis and targeted assays on residual MK2206-treated tumours and derived cells. We found that deregulation of cell cycle regulatory networks define a major mechanism of resistance to AKT inhibition, underscoring the therapeutic potential of targeting cell cycle machinery in combination with AKT to overcome resistance in breast cancer.

References:

  1. Yip HYK. et al. (2020) ‘Control of glucocorticoid receptor levels by PTEN establishes a failsafe mechanism for tumor suppression’, Molecular Cell, 80(2), pp. 279-295.
  2. Turner, NC et al. (2023) ‘Capivasertib in Hormone Receptor–Positive Advanced Breast Cancer’, The New England Journal of Medicine, 388 (22).