We report here on a surprising in vivo
synergy of NVP-BKM120 in combination with Olaparib for the treatment of BRCA1
mutant breast tumors, that suggests an important role of PI3Kα in the DNA damage response. Kumar et al. (40
) showed that PI3K-β is required for the recruitment of NBS1 to DNA double-strand breaks (DSBs) and for the assembly of repair foci in response to ionizing radiation. It was shown previously that loss of PTEN, frequently seen in TNBC, leads not only to activation of the PI3K pathway, but also to an accumulation of DNA DSBs (41
). In addition NVP-BKM120 enhances production of poly-ADP-ribose and phosphorylation of H2AX, suggesting increased DNA damage when the PI3K pathway is inhibited in the context of a BRCA1
mutation. In vivo
H2AX phosphorylation in tumors increased when mice were treated with the combination of NVP-BKM120 and Olaparib during the period of response (day 10), and was highest at the time of treatment failure (), suggestive of a progressive accumulation of unrepaired DNA DSBs, which would contribute to the reliance on PARP activity for DNA damage repair and would explain the sensitivity to combined PARP and PI3K-inhibtion.
Of particular interest was our observation that, in spite of the increase in phosphorylation of H2AX in response to NVP-BKM120, both, NVP-BKM120 and depletion of PI3Kα, greatly reduced Rad51 incorporation into foci in cells treated with radiation. These results suggest that Class IA PI3K catalytic activity is required for recruitment of Rad51 into sites of DNA damage and raise the possibility that the increase in DNA-PK phosphorylation is a feedback response to this failure to form proper DNA damage repair complexes. BRCA1
is known to play a role in recruitment of Rad51 to sites of DNA damage (42
) and thus it is possible that in BRCA1
defective cells, a PI3K dependent pathway becomes more critical for this recruitment. Clearly additional studies will be needed to understand the interactions between PI3K, Rad51 and DNA-PK in DNA repair processes.
Regulated PARP activity allows for DNA damage repair required for the maintenance of genomic stability. However, massive PARP-activation leads to depletion of its substrate NAD+
and consecutively depletion of ATP in an effort to replenish NAD+
, resulting in energy loss and eventually cell death. Activation of PI3Kα leads to increased energy production via glycolysis. Glycolysis and poly (ADP) ribosylation both consume NAD+
, and may compete for NAD+
available in the cytosol. Such metabolic competition makes sense for decisions on the fate of cells: If energy supply and glycolysis are high, the amount of NAD+
diverted into poly (ADP) ribosylation is limited, and cell death as a consequence of massive PARP-activation is avoided. Conversely, if glucose supply and glycolytic activity are low, NAD is consumed by PARP and the ensuing massive poly (ADP) ribosylation may lead to cell death (43
). PARP-inhibition spares NAD+
which becomes available for glycoloysis and can protect cells from death, such as myocardial or CNS ischemia (44
), sepsis (46
), or pancreatic islet cell damage (47
). Consistent with this model we saw in vivo
enhancement of glucose uptake (Fig. S3
) and phosphorylation of AKT in response to Parp-inhibition, which was reversed by addition of the PI3K-inhibitor (). Thus, a possible explanation for the in vivo
synergy of PI3K and Parp-inhibitors is that PI3K-inhibition reverses the pro-survival effect of PARP-inhibition and thereby makes these drugs more effective, a combination that one would predict to be particularly effective in cancers with defects in homologous recombination (HR) such as BRCA1
-related breast and ovarian cancers.
Finally, it is noteworthy that the in vivo approach allowed us to make several observations that could not be made in vitro: Much greater efficacy of the NVP-BKM120/Olaparib combination was observed in vivo than in vitro, suggesting that tumor microenvironment and metabolism may be important. Sequential tumor biopsies allowed us to monitor target inhibition in combination with tumormetrics allowed us to discover a potent synergy of PI3K inhibitor NVP-BKM120 with PARP inhibitor Olaparib to treat BRCA1-related breast cancer that may warrant exploration in an early phase clinical trial.