Biodistribution and Subcellular Localization of an Unnatural Boron-Containing Amino Acid (Cis-ABCPC) by Imaging Secondary Ion Mass Spectrometry for Neutron Capture Therapy of Melanomas and Gliomas

Abstract

The development of new boron-delivery agents is a high priority for improving the effectiveness of boron neutron capture therapy. In the present study, 1-amino-3-borono-cyclopentanecarboxylic acid (cis-ABCPC) as a mixture of its L- and D- enantiomers was evaluated in vivo using the B16 melanoma model for the human tumor and the F98 rat glioma as a model for human gliomas. A secondary ion mass spectrometry (SIMS) based imaging instrument, CAMECA IMS 3F SIMS Ion Microscope, was used for quantitative imaging of boron at 500 nm spatial resolution. Both in vivo and in vitro studies in melanoma models demonstrated that boron was localized in the cytoplasm and nuclei with some cell-to-cell variability. Uptake of cis-ABCPC in B16 cells was time dependent with a 7.5:1 partitioning ratio of boron between cell nuclei and the nutrient medium after 4 hrs. incubation. Furthermore, cis-ABCPC delivered boron to cells in all phases of the cell cycle, including S-phase. In vivo SIMS studies using the F98 rat glioma model revealed an 8:1 boron partitioning ratio between the main tumor mass and normal brain tissue with a 5:1 ratio between infiltrating tumor cells and contiguous normal brain. Since cis-ABCPC is water soluble and can cross the blood-brain-barrier via the L-type amino acid transporters (LAT), it may accumulate preferentially in infiltrating tumor cells in normal brain due to up-regulation of LAT in high grade gliomas. Once trapped inside the tumor cell, cis-ABCPC cannot be metabolized and remains either in a free pool or bound to cell matrix components. The significant improvement in boron uptake by both the main tumor mass and infiltrating tumor cells compared to those reported in animal and clinical studies of p-boronophenylalanine strongly suggest that cis-ABCPC has the potential to become a novel new boron delivery agent for neutron capture therapy of gliomas and melanomas.

Advancements in Tumor Targeting Strategies for Boron Neutron Capture Therapy

“…Additionally, ABCPC is capable of penetrating the nucleus and delivering twice as much boron to T98G human glioblastoma cells compared to BPA [62]. Similarly, treatment of F98 glioma bearing rats with ABCPC achieved a T/N ratio of 5 between infiltrating tumor cells and contiguous normal brain, and this level of selectivity is significant since previous studies with BPA report infiltrating tumor/normal brain tissue ratios of 1.5-2.0:1 [38].…”

“…The most promising candidate, 1-amino-3-boronocyclopentanecarboxylic acid (ABCPC, Fig. 2), has been shown to achieve a T/B ratio of 8, T/N ratio of 21, and achieved intracellular boron accumulation in cancer cells of 28±7 ppm in a B-16 melanoma mouse model [38,62]. Additionally, ABCPC is capable of penetrating the nucleus and delivering twice as much boron to T98G human glioblastoma cells compared to BPA [62].…”

“…Unnatural Amino Acids L-type amino acid transporter upregulation in tumor increases uptake; accumulation occurs since cell cannot metabolize [38].…”

“…To put the severity of these diagnoses into perspective, consider the 2-and 5-year survival rates of newly diagnosed GBM are 10 and 1%, respectively [37]. High grade glioma patients enrolled in BNCT clinical trials have affirmed that BNCT is tolerated well, has comparable (or fewer) side effects than conventional radiation therapy, typically requires only 1-2 treatment sessions, and the median survival times are comparable to standard of care (radiation and temozolomide treatment) [15,38]. Clinical findings such as this encourage further investigation of Tumor Pre-treated with 10 BNCT as a therapeutic option.…”

Bioanalytics in Quantitive (Bio)imaging/Mapping of Metallic Elements in Biological Samples

“…Therefore, SIMS (bio)imaging has the possibility of filling a niche for the analysis of low-molecular-weight compounds and biomolecules at high spatial resolution for biomedical studies. (Bio)imaging by the SIMS method can be applied to a wide range of biological tissues including whole brain sections, the aorta, liver, hair, and kidney Gillen et al, 1999;Kurczy et al, 2008;Locker, 2014;McDonnell et al, 2005;Nygren et al, 2005Nygren et al, , 2007Solon et al, 2010) The SIMS method can be applied to the (bio)imaging of metallic elements (alkali and also heavy) in different biological samples such as organelles, organs, and plants (Chandra et al, 2013;Derue et al, 2006;Heard et al, 2001;Nygren et al, 2005Nygren et al, , 2014Smith et al, 2013;Wang et al, 2014;Wu and Becker, 2012). However, analyzing elements in isolation offers only limited functional information about any given sample.…”