Boron neutron capture therapy of anterior chamber melanoma with p-boronophenylalanine.

Boron neutron capture therapy (BNCT) is a form of radiation therapy that requires selective uptake of boron by the tumor and irradiation with thermal neutrons. Phenylalanine is an amino acid precursor of melanin and when boronated (p-boronophenylalanine [BPA]) was found to be selectively taken up by Greene melanoma cells in the anterior chamber of rabbits. This tumor model was irradiated 24 hr after oral administration of BPA and was used for biodistribution studies that compared BPA and sodium pentaborate. Three groups were irradiated: group 1 (11 rabbits) received BPA followed by thermal neutron irradiation, group 2 (9 rabbits) received thermal neutron irradiation only, and group 3 (9 rabbits) served as unirradiated, undrugged control animals. Eight of the 11 tumors in group 1 were treated successfully; all tumors in groups 2 and 3 grew. Histopathologic examination did not reveal vascular or retina damage in group 1. These preliminary experiments confirm that newer boronated compounds, such as BPA, used in BNCT and improved neutron beams can provide selective irradiation of ocular melanomas.