Abstract

Background

PTEN phosphatase tumor suppressor function is lost in many cancers, and is associated with increased dependency on the PI3K pathway.  PTEN-deficient cancers often require p110β/PI3Kβ for growth and survival. AZD8186, a potent inhibitor of PI3Kβ/d, inhibits AKT phosphorylation and cell proliferation of PTEN-null tumour cells in vitro and in vivo. To learn more about the role of PI3Kb in PTEN null tumours we have studied the consequences of inhibiting PI3Kb on non-canonical pathways and the associations with anti-tumour effects in PTEN-deficient models.

Method

We used two parallel approaches. First, a hypothesis free RNAseq analyses, and second, a hypothesis led combination screen measuring long term (>14day) suppression of cell growth by AZD8186 and various inhibitors targeting feedback signalling. The effects of drug treatment were assessed using various biomarkers of PI3K pathway, stress and metabolic signalling, as well as ¹⁸F-FDG-PET imaging.

Results

Both approaches revealed the importance of inhibiting mTOR signaling in combination with AZD8186 to get optimal anti-tumour effects both in vitro and in vivo. RNAseq analysis identified downregulation of genes associated with PI3K signalling and lipid/cholesterol biosynthesis, as well as upregulation of PDK pathway activity. These changes were also confirmed at the protein level both in vitro and in vivo. Combining AZD8186 with Vistusertib (mTORC1/2 inhibitor, also known as AZD2014), resulted in increased depth and/or time of PI3K pathway suppression, enhanced impact on metabolic pathways, and improved efficacy.

Conclusion

In addition to the inhibition of signalling associated with PI3K/AKT/mTOR pathway, inhibiting PI3Kβ can also suppress the expression of enzymes associated with cholesterol biosynthesis as well as decreased glucose uptake. These metabolic changes elevate the levels of cellular stress. Combining AZD8186 with mTOR inhibitors in PTEN-deficient tumours enhances these effects and drives increased anti-tumour effects.