Accumulation and toxic effects of boron-containing drugs in SW-620 models

Accumulation and toxic effects of boron-containing drugs in SW-620 models According to the World Health Organization, colorectal cancer is one of the three leading causes of death among other tumors. Boron neutron capture therapy (BNCT) is a promising method for the treatment of oncological diseases. It is an experimental method of radiation therapy which is based on the capture reaction of a thermal neutron by an isotope ¹⁰B and results in the ¹⁰B(n, α)⁷Li fission reaction. The high linear energy transfer from alpha particle and ⁷Li nucleus has a short path length, thus the energy released is limited to the size of one cell. Previously, BNCT was actively used to treat patients with glioblastoma, melanoma, head and neck cancer, and other malignant neoplasms of various localization. The aim of this work is to determine the safe concentrations of boronphenylalanine (BPA) and sodium borocaptate (BSH) drugs for the SW-620 cell line and to assess the ¹⁰B biodistribution after administration of these compaunds to immunodeficient SCID mice with heterotopic SW-620 xenografts. To evaluate the possibility of using BNCT as a therapy for patients with colorectal cancer, we carried out preliminary in vitro and in vivo studies on the model of human colorectal adenocarcinoma SW-620. Firstly, the cytotoxicity of boron compounds BPA and BSH at ¹⁰B concentrations of 10-1280 μg/ml was investigated using MTT test. All animal experiments were conducted in accordance with the principles of humane treatment of animals in compliance with the directive of the European Community (86/609 / EEC) and correspond to the principles of the Guide for the Care and Use of Laboratory Animals (NIH USA, No 85-23, rev. 1985). To determine the toxicity of BPA and BSH in standard dosages, drugs were intravenously injected into the retroorbital sinus: BPA at a concentration of 350 mg/kg and BSH at 100 mg/ kg. The control group of animals were injected intravenously with sodium chloride 0.9%. After 2 months of observation planned euthanasia was done, organs (kidneys, liver, lungs, brain and heart) were sampled for pathomorphological study. Biodistribution of ¹⁰B in the tumor and organs of interest was performed on mice with SW-620 heterotopic xenograft. Boron containing drugs were injected at the dosages described earlier intravenously into the retroorbital sinus. Euthanasia was performed 1, 2, 3, and 4 hours after injection, tumor tissue, blood, brain, liver and kidneys were sampled. The boron concentration was analyzed by inductively coupled plasma atomic emission spectrometry (ICP AES). As a result of the MTT test, we revealed that incubation of SW-620 cells with BPA and BSH for 24 and 48 hours at ¹⁰B concentrations of 10-320 μg/ml had no cytotoxic effect. When incubated for 48 hours with BPA at a concentration of 320 μg/ml, survival rate did not differ significantly from control. The maximum safe concentration for BSH can also be considered 320 μg/ml of ¹⁰B, since the cell survival in the experimental groups was 96 and 95.5% for 24 and 48 hours of incubation with the drug, respectively. The first significant cytotoxic effects were noted during incubation with both boron-containing drugs at a ¹⁰B concentration of 640 μg/ml. For the BPA group, the percentage of survived cells was 65 (incubation for 24 hours) and 51 (48 hours), BSH had less cytotoxic effect: cell survival decreased to 87% and 80.5% at points 24 and 48 hours, respectively (See Fig. 1). Intravenous administration of BPA at a concentration of 350 mg/kg and BSH at a concentration of 100 mg/kg to SCID mice turned out to be safe: no pathological reactions were detected within 2 months of observation, all animals were alive. According to the pathomorphological study, no macroscopic and structural changes in the organs were found (See Fig. 2). The biodistribution of ¹⁰B was assessed in SCID mice with subcutaneous xenografts SW-620 after intravenous administration of boron drugs at standard concentrations. As a result of the analysis, we found out that after intravenous administration of BPA at a safe dosage, the maximum concentration of ¹⁰B in the tumor was recorded at the time points of 1 and 2 hours (8.0 and 8.7 |rg/g, respectively). In this case, the concentration ratio of ¹⁰B in the tumor and blood samples was determined as 0.6 for the 1 hour point and 1.7 for the 2 hour point. We also demonstrated that the kidneys were the organ with the highest boron accumulation throughout the experiment (See Fig. 3). Thus, the time point of 2 hours can be considered potentially the most suitable for BNCT, but the concentration of boron in the tumor tissue is not sufficient. For BSH the maximum concentration was recorded at the time point of 1 hour (5.7 μg/g). The liver, kidneys and blood contained the highest concentration of ¹⁰B (See Fig. 4). Thus, apparently, further investigations on increasing the dose of the drugs and choosing different ways of boron agents administration can increase the accumulation of ¹⁰B by the tumor and reduce its concentration in the surrounding tissues and blood. The use of additional drugs that improve the penetration of boron into the tumor may also be effective and should be taken into account. The paper contains 4 Figures and 25 References. Acknowledgments: The authors are grateful to Veronika R. Kasatova for the help in preparation of illustrative material. The Authors declare no conflict of interest.

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