Biodistribution studies of labeled tryptophan: a potential pancreas-scanning agent

The in vivo organ distributions of dl- and l-[side 3-14C]-tryptophan and dl-(75Se)-selenomethionine were studied in normal Long-Evans rats to determine which tryptophan isomer had the best pancreas/liver ratio. The commercially available radiochemical agents were injected into the tails of adult rats, and three animals were sacrificed for each compound at 15, 30, 45, 60, and 90 minutes. Samples of blood, pancreas, liver, kidney, spleen, and flank muscles were excised, weighed wet, digested, and counted by standard liquid scintillation technics. A standard fraction of the injected dose was also counted so that results could be reported as per cent injected dose per gram tissue (per cent ID/gm) and per whole organ. The spleen and muscle concentrations (per cent ID/gm) were indistinguishable for the three compounds at all times studied. The kidney concentration of dl-tryptophan was higher than that of l-tryptophan at 60 and 90 minutes, reflecting the renal excretion of the nonmetabolic isomer. The concentration of l-tryptophan was two to three times higher than that of dl-tryptophan and selenomethionine in the pancreas at the earlier times, but after 1 hour they became equivalent. All three compounds had equivalent concentrations in the liver, except for l-tryptophan at 15 minutes. We conclude that pancreas/liver ratios are much higher for tryptophan than for selenomethionine and that for l-tryptophan they are at least tenfold higher. Since the rat is able to partially metabolize d-tryptophan via an inversion step, differences between l- and dl-tryptophan might be even larger for humans. L-tryptophan labeled with a short-lived gamma-emitting nuclide (13N or 11C) should, therefore, be a much better radiopharmaceutical agent for pancreatic scintigraphy.