Kenneth M. Bradshaw

11B nuclear magnetic resonance studies of the interaction of borocaptate sodium with serum albumin.

The interaction between borocaptate sodium, Na2B12H11SH (BSH), and three types of serum albumin--bovine, human and dog (BSA, HSA and DSA)--has been investigated quantitatively using 11B NMR. The 11B chemical shifts and relaxation rates of BSH were studied with various concentrations of serum albumin (1-5%, w/v) at 295-310 degrees K. Correction of the longitudinal relaxation rate (R1) due to protein viscosity effects was accomplished. The corrected R1 values were analyzed mathematically using a saturation function and linear regression. The linewidths of 11B resonances, which are related to the spin-spin relaxation rates (R2), were also measured. The binding fractions (P), the number of binding sites (NBS), and the binding constants (Kb) of BSH at various concentrations of the three types of serum albumin (1-5%, w/v) were determined at 295 and 310 degrees K. We speculate that the nature of this interaction may be electrostatic.

In Vivo Pharmacokinetic Evaluation of Boron Compounds Using Magnetic Resonance Spectroscopy and Imaging

The success of Boron Neutron Capture Therapy (BNCT) will, in large degree, depend upon knowing the kinetics of a boron compound and treating when the optimum conditions have been reached. Before a potential boron compound is administered to a human patient for treatment, the pharmacokinetics of the boron compound should be determined first with an animal model and then with human studies. Until now, the accepted methods of conducting pharmacokinetic studies have all been invasive. Magnetic resonance (MR) spectroscopy and imaging is a new technology that has the potential of measuring the concentration of selected nuclei in tissue noninvasively. MR is presently being used to measure in-vivo concentration of chemical compounds containing 31P, 19F, and 23Na.(1,2) The MR techniques used to measure these compounds have been modified for boron, but have not been used in conducting time studies.(3–5) The in-vivo MR method proposed here provides a way to quantitate boron in tissue as a function of time and location.

Boron MRI in a Tumor Bearing Canine Model

Over the past several years, we have been involved in the development of a noninvasive magnetic resonance imaging (MRI) method for quantifying boron drug pharmacokinetics in tumor and other tissues of the canine head1–3. The underlying aims for this research are to provide a method to evaluate the suitability of boron compounds and to provide a potential drug prescreen as an aid to BNCT dosimetry calculations to be used in patient treatment planning.

First Boron MRI of a Human Patient with a Brain Tumor

Major challenges in the successful administration of BNCT include the development of the BNCT compound used to deliver 10B preferentially to the tumor cells and the analysis of the compound. To obtain treatment parameters this analysis must accurately reveal the distribution and concentration of the boron compound. This analysis can either be invasive or noninvasive. However, with invasive analysis we take the risk of disturbing the biological environment to the extent that the data will be biased. When performing invasive pharmacokinetic studies of a boron compound candidate, we also have the disadvantage that data sampling is often limited to one sample per animal or human experiment requiring multiple experiments per time point in order to have a statistically meaningful data set. We have introduced a method in which patient treatment parameters may be determined noninvasively. Furthermore, nonivasive data acquisition per experiment can be maximized for potential boron compound pharmacokinetic studies.

In Vivo NMR Evaluation of the Pharmacokinetics of Boronated Compounds in the Rat Model

The success of neutron capture therapy in the treatment of cancer is dependent on the development of boronated compounds that preferentially accumulate in tumor tissue and not in surrounding normal tissue. Researchers are investigating numerous new compounds for boron delivery to tumors. Evaluation of the performance of these compounds typically includes in vitro studies of drug uptake in tumor cell cultures followed by animal studies in which toxicity and tumor specificity are determined. Once toxicity issues are resolved, the drug proceeds to human clinical trials where tumor specificity and drug toxicity are again evaluated. The animal and human studies attempt to characterize the pharmacokinetic characteristics of the compounds by analyzing ex vivo tissue, blood, and urine samples. While these studies provide important information about drug accumulation and distribution in various tissues, they fail to provide an in vivo representation of the uptake and elimination of the boronated compound within a single animal or patient.

NMR Studies of the Interaction of Borocaptate Sodium with Serum Albumin

The interaction between BSH and serum albumin is of interest because it is related to the pharmacokinetics of BSH. Early research[1]suggested that covalent disulfide bridge might be involved in the interaction, but the idea was contradicted by subsequent studies[2, 3, 4]using11B Nuclear Magnetic Resonance (NMR) spectroscopy. NMR has been proved to be a very powerful technique to study the protein binding of a small molecule, such as BSH[5], and boron NMR is typically suitable to study the binding effect of the boronated agent. Both11B and10B have NMR activity, but11B has better NMR sensitivity (16.5% vs. 2% relative to1H) and a larger natural abundance (81% vs. 19%)[6]. Even though10B is the neutron active nucleus for BNCT, the more sensitive nucleus11B is more appropriate for NMR studies. Since the isotopic difference does not make any differences in the structure and binding effect of a compound, the result obtained from the11B NMR of BSH can hold for the10B enriched agent.

T1 Measurement to Study the Penetration of BNCT Agents Into Canine Tumors Caused by Blood Brain Barrier Damage

The blood brain barrier (BBB), maintained primarily by cerebral capillary endothelial cells, protects the brain by limiting the uptake and circulation of drugs1. It has been observed that the BBB is disrupted during brain tumor growth. The fenestrated capillaries allow large molecules, such as protein and blood products, to penetrate through the BBB and be taken up by tumors cells2. As a consequence, certain boron agents for Boron Neutron Capture Therapy (BNCT), such as borocaptate sodium (BSH)3, are able to penetrate the disrupted BBB and enter tumor tissues. The study of BBB disruption by tumor growth will assist in understanding the mechanism of this penetration and provide a background for BSH pharmacokinetics.

The Idaho Power Burst Facility/Boron Neutron Capture Therapy (PBF/BNCT) Program Overview

The Power Burst Facility/Boron Neutron Capture Therapy (PBF/BNCT) Program has been funded since 1988 to evaluate brain treatment using Na2B12H11SH (borocaptate sodium or BSH) and epithermal neutrons. The PBF/BNCT Program pursues this goal as a comprehensive, multidisciplinary, multiorganizational endeavor applying modern program management techniques. The initial focus was to: (1) establish a representative large animal model and (2) develop the generic analytical and measurement capabilities required to control treatment repeatability and determine critical treatment parameters independent of tumor type and body location. This paper will identify the PBF/BNCT Program elements and summarize the status of some of the developed capabilities.