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Deadly Duo beat Resistant Breast Cancer

“Synergistic Effect of the γ-Secretase Inhibitor PF-03084014 and Docetaxel in Breast Cancer Models”

 

From Stem Cells Translational Medicine

Notch signaling, which plays a role in a variety of developmental processes by controlling cell fate decisions through the regulation of interactions between physically adjacent cells, has also been linked to tumourigenesis, especially breast cancer (Ranganathan et al).   Activation of this pathway leads to the γ-secretase-mediated cleavage of Notch and the release of the Notch intracellular domain (NICD), which translocates to the nucleus and activates a cascade of transcriptional events that mediate cellular proliferation, differentiation, and apoptosis. With regards to breast cancer, elevated levels of Notch signalling have been observed in  breast cancer tumour-initiating cells (TICs) compared with bulk tumour cells (Grudzien et alFarnie and Clarke and Wright et al) suggesting that Notch may be an effective target for anti-tumourigenic therapies. Indeed previous efforts to inhibit Notch signalling, by the blockade of Notch ligands Dll4 (Hoey et al) and the γ-secretase inhibitor (GSI) (Grudzien et al and Kondratyev et al], reduced TIC numbers, while the γ-secretase inhibitor PF-03084014 has been shown to be effective in haematological and breast xenograft models (Wei et al and Zhang et al). Now, researchers from the group of James G. Christensen at the Oncology Research Unit, Pfizer Global Research and Development, San Diego, USA have studied the synergistic effects of PF-03084014 with docetaxel, a common chemotherapeutic, in triple-negative breast cancer, finding that PF-03084014 significantly enhanced the antitumor activity of docetaxel and report on the mechanisms behind this effect (Zhang et al).

Initially, breast xenograft model MDA-MB-231Luc tumour-bearing mice were treated with docetaxel once per week for three cycles, and analysis of the remaining tumour mass suggested an activation of Notch signalling and the possible acquisition of EMT. The ALDH+ (proposed CSC marker) cell proportion of the tumour increased by double, while the CD133+/CD44+ subpopulation, more potential CSC markers, increased nearly 4-fold. Re-implantation of docetaxel-resistant residual cells led to more rapid disease relapse compared with those with initial tumour cells, tumours had increased levels of metastasis associated with them and mice had significantly shortened survival rates. However, the combination of PF-03084014 with docetaxel led to a significantly enhanced efficacy in the reduction of tumour volume compared to either drug alone in the MDA-MB-231Luc model, while good synergy was also observed in two other models: HCC1599 and AA1077 (patient-derived xenograft model). In all models, addition of PF-03084014 suppressed Notch signalling and increased apoptosis in tumour cells while IL-6 levels, which predict the severity of metastases and patient survival (Salgado et al), decreased where treatment with docetaxel had previously boosted levels. Other pro-tumourigenic markers were also decreased when synergising drugs; Survivin and ABCB1, while the genes changes associated with an EMT with docetaxel alone were also reversed.   Mammosphere formation, used as a measure of TIC/CSC number, was reduced with PF-03084014 alone or in combination with docetaxel, and PF-03084014 treatment also led to a 63% reduction in tumor size in in vivo serial transplantation assays. Excitingly, PF-03084014 with docetaxel inhibited tumour cell self-renewal nearly completely by these measures, concurrent with a drastic reduction in the ALDH+ cell population. Additionally, in the MDA-MB-231Luc model, PF-03084014 in combination with docetaxel also produced a significant decrease in CD133+/CD44+ cell numbers.  

Sorting of CD133+/CD44+ and CD133-/CD44- cell populations and subsequent injection into SCID Beige mice demonstrated that the positive fraction gave rise to tumours in most mice, even with as few as 20 cells, whereas the negative cell population gave rise to tumourigenesis in only one mouse implanted with 10,000 cells, suggesting that CD133+/CD44+ may represent a novel marker for CSCs in the breast.

Overall, this study suggests that synergism of the γ-secretase inhibitor PF-03084014 and the common chemotherapeutic docetaxel has the ability to eliminate CSCs that were previously resistant to chemotherapeutic assault. Additionally, a potentially important addition to the few markers specific for CSCs may have been identified. However, questions still remain over the effects that this may have on normal stem cell populations, but these findings do suggest that this useful drug may find its way to the clinic and may also work well in combination with other common chemotherapeutic regimes.

References

  • Farnie G, Clarke RB. Mammary stem cells and breast cancer: Role of notch signalling. Stem Cell Rev 2007;3:169 –175.
  • Grudzien P et al. Inhibition of notch signaling reduces the stem-like population of breast cancer cells and prevents mammosphere formation. Anticancer Res 2010;30:3853–3867.
  • Hoey T et al. DLL4 blockade inhibits tumor growth and reduces tumor-initiating cell frequency. Cell Stem Cell 2009;5:168 –177.
  • Kondratyev M et al. Gamma-secretase inhibitors target tumor-initiating cells in a mouse model of ERBB2 breast cancer. Oncogene 2012;31:93–103.
  • Ranganathan P et al. Notch signalling in solid tumours: A little bit of everything but not all the time. Nat Rev Cancer 2011;11:338 –351.
  • Salgado R et al. (2003) Circulating interleukin-6 predicts survival in patients with metastatic breast cancer. Int J Cancer 103:642–646.
  • Wei P et al. Evaluation of selective g -secretase inhibitor PF-03084014 for its antitumor efficacy and gastrointestinal safety to guide optimal clinical trial design. Mol Cancer Ther 2010;9:1618 –1628.
  • Wright M et al. Brca1 breast tumors contain distinct CD44+/CD24- and CD133+ cells with cancer stem cell characteristics. Breast Cancer Res 2008;10:R10.
  • Zhang CC et al. Biomarker and pharmacological evaluation of the g-secretase inhibitor PF-03084014 in breast cancer models. Clin Cancer Res 2012;18:5008– 5019.

STEM CELLS correspondent Stuart P. Atkinson reports on those studies appearing in current journals that are destined to make an impact on stem cell research and clinical studies.