Synthesis and in vivo biodistribution of BPA-Gd-DTPA complex as a potential MRI contrast carrier for neutron capture therapy.

p-boronophenylalanine (BPA) conjugated Gd-DTPA complex (3) was synthesized from the active methyne compound 6, the allylic carbonate 7, and BPA by the palladium-catalyzed allylation reaction followed by the DCC coupling reaction. The in vivo biodistribution of complex 3 was evaluated by prompt gamma-ray analysis and alpha-autoradiography using the tumor-bearing rats. High accumulation of gadolinium was observed in the kidney and the %ID values were 0.17 and 0.088 at 20 and 60 min after injection of 3, respectively. The accumulation was also observed in the tumor and the %ID values were 0.010 and 0.0025 at 20 and 60 min after injection, respectively. The visualization experiment of boron distribution in the tumor-bearing rat by alpha-autoradiography indicates that boron was accumulated in the tumor and the intestines at 20 min after injection.