There is a pressing need for new and more efficient boron delivery agents to tumor cells in Boron Neutron Capture Therapy (BNCT) of glioblastoma multiforme (GBM). A class of boronated unnatural cyclic amino acids has shown a remarkable selectivity to tumors in animal models, which is far superior to currently used agents in clinical BNCT. The boron analogue of one of these amino acids, 1-amino-3- boronocyclopentanecarboxylic acid, has shown a tumor to blood ratio of 8 and a tumor to normal brain ratio of nearly 21. This compound has also shown a remarkable difference between racemic mixtures of cis- and trans- isomers in tumor targeting of boron. This implies that further separation of the L- and D- forms of this compound may potentially enhance tumor targeting to an even higher degree than that provided by the isomeric mixture. Since all of these measurements were made in homogenized tumor and normal tissues, the basic understanding of the subcellular location of boron and differences between the isomers is currently missing. A team of experts is assembled in the current proposal for understanding subcellular delivery and cell cycle preferences of 10 boronated unnatural amino acids (three parent molecules and all of their isomers) in T98G human glioblastoma and F-98 rat glioma cell lines with the secondary ion mass spectrometry (SIMS) based technique of ion microscopy. SIMS is capable of quantitatively imaging elemental (isotopic) gradients in cells and tissues at 500 nm spatial resolution and has been developed to a remarkable precision in BNCT for boron imaging in cryogenically prepared cells. The boronated unnatural amino acids and their isomers will be evaluated for their boron delivery characteristics at subcellular resolution with SIMS. From these in vitro studies of 3 parent molecules and their isomers (a total of 10 compounds), we will select each year one boronated unnaturated amino acid from each parent molecule. This selected molecule will be tested in F98 rat glioma model of GBM for its ability to selectively deliver boron atoms to infiltrating tumor cells in the normal brain. SIMS analysis in cryogenically prepared tissues will provide the boron- targeting information of the main tumor mass, the normal brain, and the infiltrating tumor cells in the normal brain. SIMS is currently the only technique capable of direct measurements of boron in infiltrating tumor cells. Since infiltrating tumor cells in the normal brain are the main targets of clinical BNCT, these unique SIMS observations will provide the most compelling observations highly relevant to GBM on the boron-delivery characteristics of boronated unnatural amino acids. By the end of the third year of the proposed grant, a most promising candidate drug will be selected for testing in clinical BNCT. From the point of view of fundamental chemical biology, as well as BNCT, the proposed subcellular SIMS studies of isomeric forms of boronated unnatural amino acids offer a unique opportunity to produce invaluable new information.
Currently, there is no viable therapy for brain cancer, glioblastoma multiforme (GBM). The boron neutron capture therapy (BNCT) is under development for the treatment of GBM. The subcellular/single cell isotopic imaging technique of SIMS ion microscopy will be used for understanding mechanistic aspects of boronated unnatural amino acids and their isomers in delivering boron to tumor cells in cell culture and animal models of GBM. SIMS is currently the only technique for reliably measuring the partitioning of boron atoms between the infiltrating glioblastoma cells and the normal brain. These are essential observations for the success of BNCT.
