Pharmacokinetics of Chlorin e₆-Cobalt Bis(Dicarbollide) Conjugate in Balb/c Mice with Engrafted Carcinoma

Abstract

The necessary precondition for efficient boron neutron capture therapy (BNCT) is control over the content of isotope ¹⁰B in the tumor and normal tissues. In the case of boron-containing porphyrins, the fluorescent part of molecule can be used for quantitative assessment of the boron content. Study Objective: We performed a study of the biodistribution of the chlorin e₆-Cobalt bis(dicarbollide) conjugate in carcinoma-bearing Balb/c mice using ex vivo fluorescence imaging, and developed a mathematical model describing boron accumulation and release based on the obtained experimental data. Materials and Methods: The study was performed on Balb/c tumor-bearing mice (CT-26 tumor model). A solution of the chlorin e₆-Cobalt bis(dicarbollide) conjugate (CCDC) was injected into the blood at a dose of 10 mg/kg of the animal's weight. Analysis of the fluorescence signal intensity was performed at several time points by spectrofluorimetry in blood and by laser scanning microscopy in muscle, liver, and tumor tissues. The boron content in the same samples was determined by mass spectroscopy with inductively coupled plasma. Results: Analysis of a linear approximation between the fluorescence intensity and boron content in the tissues demonstrated a satisfactory value of approximation reliability with a Spearman's rank correlation coefficient of r = 0.938, p < 0.01. The dynamics of the boron concentration change in various organs, calculated on the basis of the fluorescence intensity, enabled the development of a model describing the accumulation of the studied compound and its distribution in tissues. The obtained results reveal a high level of correspondence between the model and experimental data.

Keywords: MS-ICP; boron content; boron neutron capture therapy; chlorin e6 derivatives; fluorescent microscopy; simple multichamber model.

Figure 1

Structure of the studied chlorin e₆-Cobalt bis(dicarbollide) conjugate (CCDC).

Figure 2

An example of the selection of homogeneous regions of interest (ROIs) in a fluorescence image of a tumor growing into the muscular tissue. (a) Reflected light image; (b) spectral image of the same area. Image size: 354 × 354 μm. Dashed line depicts the boundary between tumor cells and muscle fibers that was identified using morphological features observed in the reflected light image of the scanned area. (c) Fluorescence spectra in ROIs selected in panel (b). Dashed lines in panel (c) restrict the 648–687 nm range that was used to calculate fluorescent intensity of CCDC in ROIs.

Figure 3

Fluorescent images of animal’s tissue samples in 1, 3, 6, and 24 h after the CCDC injection at a dose of 10 mg/kg. λ_ex = 514 nm. λ_em = 648–687 nm. Image size is 354 × 354 μm.

Figure 4

Correlation analysis of the CCDC fluorescence intensity (Ifl) and boron concentration [B] in the studied tissues. Joined data obtained for tumor, muscle, and liver are presented. Spearman’s rank correlation coefficient r = 0.938; significance level p < 0.01.

Figure 5

Scheme of a simple multichamber model.

Figure 6

Pharmacokinetics of CCDC injected into the blood at a dose of 10 mg/kg. Dots denote experimental data. Lines denote the results of the solution of the mathematical model.