Aza-BODIPY: A New Vector for Enhanced Theranostic Boron Neutron Capture Therapy Applications

Abstract

Boron neutron capture therapy (BNCT) is a radiotherapeutic modality based on the nuclear capture of slow neutrons by stable ¹⁰B atoms followed by charged particle emission that inducing extensive damage on a very localized level (<10 μm). To be efficient, a sufficient amount of ¹⁰B should accumulate in the tumor area while being almost cleared from the normal surroundings. A water-soluble aza-boron-dipyrromethene dyes (BODIPY) fluorophore was reported to strongly accumulate in the tumor area with high and BNCT compatible Tumor/Healthy Tissue ratios. The clinically used ¹⁰B-BSH (sodium borocaptate) was coupled to the water-soluble aza-BODIPY platform for enhanced ¹⁰B-BSH tumor vectorization. We demonstrated a strong uptake of the compound in tumor cells and determined its biodistribution in mice-bearing tumors. A model of chorioallantoic membrane-bearing glioblastoma xenograft was developed to evidence the BNCT potential of such compound, by subjecting it to slow neutrons. We demonstrated the tumor accumulation of the compound in real-time using optical imaging and ex vivo using elemental imaging based on laser-induced breakdown spectroscopy. The tumor growth was significantly reduced as compared to BNCT with ¹⁰B-BSH. Altogether, the fluorescent aza-BODIPY/¹⁰B-BSH compound is able to vectorize and image the ¹⁰B-BSH in the tumor area, increasing its theranostic potential for efficient approach of BNCT.

Keywords: 10B-BSH; BNCT; NIR-I; SWIR; aza-BODIPY; boron compound; in ovo model; optical imaging; theranostic.

Figure 1

(a) General structure of boron-dipyrromethene dyes (BODIPY), (b) aza-BODIPY, and (c) aza-SWIR-BSH-01, (d) absorption and emission spectra of aza-SWIR-BSH-01 in DMSO [29].

Figure 2

Human glioblastoma U-87 MG cells cultured in 2D incubated with aza-SWIR-BSH-01 (red signal) for 3 h (a), and control cells (b). Nuclei were labeled with Hoechst (blue signal). Corresponding phase-contrast pictures are depicted in black and white color. Scale bars represent 20 µm.

Figure 3

Boron neutron capture therapy (BNCT) experiment in vitro. U-87 MG (a) and U-251 MG (b) cells were incubated with ¹⁰B-BSH (grey) or aza-SWIR-BSH-01 (white) during 2 h before neutron exposure, and re-seeding for colony assay. Control condition (neutron alone) is indicated in black. The results are represented as the mean of 3 independent experiments ± standard deviation (S.D.).

Figure 4

In vivo distribution and behavior of aza-SWIR-BSH-01 in mice-bearing subcutaneous U-87 MG tumors. The non-invasive images were taken from T0 until 48 h (a). Tumors are indicated with an arrow. (b) The distributions were observed at 24 h (green) and 48 h (blue) post-injection. (c) Remoted tumor observed at 24 h and 48 h post-injection revealed a higher tumor accumulation at 24 h. (d) Tumor/Skin and (e) Tumor/Muscle ratios from ex vivo analysis.

Figure 5

In ovo model of tumor for evaluation of theranostic aza-SWIR-BSH-01 compound. (a) Presentation of the in ovo tumor model for BNCT application. (b) Color images of tumor before and after administration of aza-SWIR-BSH-01 (pale blue color), indicating the presence of the compound at the tumor site. (c) Tumor development measured at day 16, i.e., 6 days after the addition of ¹⁰B-BSH and aza-SWIR-BSH-01 for 1 h followed by neutron exposure. (d) Laser-induced breakdown spectroscopy (LIBS) elemental imaging of boron from tumor sections collected at day 16 showing the presence of remaining boron in tumors treated with aza-SWIR-BSH-01. (e) 2D fluorescence imaging of aza-SWIR-BSH-01 distribution before and until 24 h post-administration onto glioma tumors implanted on chorioallantoic membrane (CAM). (f) Non-invasive measurement of aza-SWIR-BSH-01 fluorescence in tumors with time. Results are expressed as the tumor fluorescence mean ± S.D (n = 4).