by Natsuko Kondo
My main biological research project for BNCT is working to aim to overcome glioblastoma (GBM). As you know, GBM is the most lethal tumor with a median overall survival of 14 months and is resistant to chemo-radiation therapy and immuno-therapy. In addition, GBM is so invasive and infiltrative tumor into the normal brain that it is impossible to control it with other particle radiation therapy using focused ion beams. BNCT has prolonged the survival time of malignant glioma patients . However, recurrences have occurred locally or remotely as cerebrospinal fluid dissemination (CSFD) after BNCT and complete cure is still not achieved .
One reason may come from endogenous property of glioma. For example, we found that small cell subtype of IDH1R132H mutation-negative glioblastoma more frequently developed CSFD after BNCT . Another reason may come from incomplete uptake of p-boronophenylalanine (BPA) with heterogeneous distribution of BPA in the tumor .
Recent studies have shown that glioma stem cells (GSCs), a small subpopulation of tumor cells, are responsible for tumor resistance to radiation and chemotherapy, and the stemness, quiescence and therapy resistance are maintained by GSC niches in the tumor microenvironment [4, 5]. However, BPA uptake in GSCs has been unknown. Therefore, we investigated whether BPA is taken up by GSCs using mass cytometry (Cytof) and a mouse orthotopic tumor model. In brief, we established two glioma stem like cells from two GBM patients (GSLCs) and induced differentiated cells (DCs) with fetal bovine serum. After exposure to BPA for 24 h at 25 ppm in 5% CO2 incubator, we immune-stained them with twenty stem cell markers, anti-Ki-67, anti-BPA and anti-CD98 (heterodimer that forms the large BPA transporter) antibodies and analyzed them with Cytof. The percentage of BPA+ or CD98+ cells with stem cell markers (Oct3/4, Nestin, SOX2, Musashi-1, PDGFRα, Notch2, Nanog, STAT3 and C-myc, among others) was 2–15 times larger among GSLCs than among DCs. Analyses of in vivo orthotopic tumor also indicated that 100% of SOX2+ or Nestin+ GSLCs were BPA+, whereas only 36.9% of glial fibrillary acidic protein (GFAP)+ DCs were BPA+. Therefore, GSLCs may take up BPA and could be targeted by BNCT . Then why does the recurrence occur although GSLCs can be eliminated with BPA-BNCT? Cell-cell interactions between residual tumors and tumor microenvironment maybe the key. Peri-hypoxic niches, one of the GSC niches, can dedifferentiate glioma differentiated cells into GSCs , and this may contribute to the resistance to BNCT. Now we are investigating these cell-cell interactions in vitro and in vivo.
To protect normal brain tissue is also an important issue because most glioma cases are re-irradiations in BNCT. Radiation brain necrosis (RN) is a late adverse event that often occurs after BNCT and Bevacizumab (anti-Vascular Endothelial Growth Factor antibody) is shown to avoid the progression of RN after BNCT . However, once bevacizumab is discontinued, the RN can occur . Other treatment strategy should also be studied. We developed a mouse RN model using proton beams, in which the restricted area in right hemisphere of mouse brain is irradiated . Recently, lipid mediators, lysophospholipids are shown to be involved in pathogenesis of cerebral or cardiac infarction . We are investigating the relationship of these lipid mediators and RN pathogenesis with the RN mouse model.
In conclusion, we edge toward efforts to achieve a complete cure with BNCT for glioma.
Natsuko Kondo MD, PhD
Particle Radiation Oncology Research Center
Institute for Integrated Radiation and Nuclear Science, Kyoto university
Member of the ISNCT’s Board of Councilors
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(image design: @Freepik)