The work was to investigate radioiodinated tetrahydropalmatine and its biodistribution in mice. Tetrahy-dropalmatine was labeled with 131I using the chloramine-T method and the labeled compound were characterized by polyamide TLC. The animals were sacrificed at different times after radiopharmaceutical i.v. administration. The interested tissues samples were collected

and percent injected dose per gram (%ID·g-1) was calculated for each sample. The labeling yield of 131I-tetrahydropalmatine was 76% and its RCPs were 97.3%

95.4%

and 96.8% after 1

7 and 20 days

respectively. Biodistribution in mice demonstrated that 131I-tetrahydropalmatine was extensive

and it was me-tabolized mainly in liver and kidney

which contained of 14.35% and 6.55% ID·g-1at 5 min

respectively

with 3.26% and 1.20% ID·g-1at 4h

respectively. Comparatively high 131I-tetrahydropalmatine was found in intestine and fat

but clearance was slow

3.91%and 3.05% at 5 min and decreased to 0.79% and 0.37% at 4 h. The results also showed that 131I-tetrahydropalmatine could well penetrate the blood-brain barrier to attain a maximal level in brain tissue within 5～10 min

but it mostly was cleaned out after 2 h. There was no significant difference in brain regions despite of highest biodistribution in parietal lobe. In conclusion

131I-tetrahydropalmatine was stable and it was metabolized mainly in liver and kidney

but there was no significant difference in brain regions.