Strategically Timing Inhibition of Phosphatidylinositol 3-Kinase to Maximize Therapeutic Index in Estrogen Receptor Alpha-Positive, PIK3CA-Mutant Breast Cancer

Clin Cancer Res. 2016 May 1;22(9):2250-60. doi: 10.1158/1078-0432.CCR-15-2276. Epub 2016 Jan 5.


Purpose: Phosphatidylinositol 3-kinase (PI3K) inhibitors are being developed for the treatment of estrogen receptor α (ER)-positive breast cancer in combination with antiestrogens. Understanding the temporal response and pharmacodynamic effects of PI3K inhibition in ER(+) breast cancer will provide a rationale for treatment scheduling to maximize therapeutic index.

Experimental design: Antiestrogen-sensitive and antiestrogen-resistant ER(+) human breast cancer cell lines and mice bearing PIK3CA-mutant xenografts were treated with the antiestrogen fulvestrant, the PI3K inhibitor GDC-0941 (pictilisib; varied doses/schedules that provided similar amounts of drug each week), or combinations. Cell viability, signaling pathway inhibition, proliferation, apoptosis, tumor volume, and GDC-0941 concentrations in plasma and tumors were temporally measured.

Results: Treatment with the combination of fulvestrant and GDC-0941, regardless of dose/schedule, was significantly more effective than that with single-agent treatments in fulvestrant-resistant tumors. Short-term, complete PI3K inhibition blocked cell growth in vitro more effectively than chronic, incomplete inhibition. Longer-term PI3K inhibition hypersensitized cells to growth factor signaling upon drug withdrawal. Different schedules of GDC-0941 elicited similar tumor responses. While weekly high-dose GDC-0941 with fulvestrant continuously suppressed PI3K signaling for 72 hours, inducing a bolus of apoptosis and inhibiting proliferation, PI3K reactivation upon GDC-0941 washout induced a proliferative burst. Fulvestrant with daily low-dose GDC-0941 metronomically suppressed PI3K for 6 to 9 hours/day, repeatedly inducing small amounts of apoptosis and temporarily inhibiting proliferation, followed by proliferative rebound compared with fulvestrant alone.

Conclusions: Continuous and metronomic PI3K inhibition elicits robust anticancer effects in ER(+), PIK3CA-mutant breast cancer. Clinical exploration of alternate treatment schedules of PI3K inhibitors with antiestrogens is warranted. Clin Cancer Res; 22(9); 2250-60. ©2016 AACRSee related commentary by Toska and Baselga, p. 2099.

MeSH terms

  • Animals
  • Apoptosis / drug effects
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / metabolism
  • Cell Line, Tumor
  • Cell Proliferation / drug effects
  • Cell Survival / drug effects
  • Class I Phosphatidylinositol 3-Kinases / antagonists & inhibitors*
  • Class I Phosphatidylinositol 3-Kinases / genetics*
  • Drug Resistance, Neoplasm / drug effects
  • Estradiol / analogs & derivatives
  • Estradiol / pharmacology
  • Estrogen Receptor Modulators / pharmacology
  • Estrogen Receptor alpha / genetics*
  • Estrogen Receptor alpha / metabolism
  • Female
  • Fulvestrant
  • Humans
  • Indazoles / pharmacology
  • MCF-7 Cells
  • Mice
  • Mice, Inbred NOD
  • Mutation / drug effects
  • Mutation / genetics*
  • Phosphoinositide-3 Kinase Inhibitors*
  • Protein Kinase Inhibitors / pharmacology*
  • Signal Transduction / drug effects
  • Sulfonamides / pharmacology
  • Therapeutic Index


  • 2-(1H-indazol-4-yl)-6-(4-methanesulfonylpiperazin-1-ylmethyl)-4-morpholin-4-ylthieno(3,2-d)pyrimidine
  • Estrogen Receptor Modulators
  • Estrogen Receptor alpha
  • Indazoles
  • Phosphoinositide-3 Kinase Inhibitors
  • Protein Kinase Inhibitors
  • Sulfonamides
  • Fulvestrant
  • Estradiol
  • Class I Phosphatidylinositol 3-Kinases
  • PIK3CA protein, human