Structural modification of receptor-binding technetium-99m complexes in order to improve brain uptake

Low brain uptake is a generally accepted problem in developing technetium-99m brain receptor imaging agents. For a class of potential 5-HT2A receptor-binding agents we tried to improve the original low brain uptake of 0.4% injected dose (ID) in rats 5 min p.i. by modifying the lipophilic properties of the molecules. Because of the presence of a protonable nitrogen. which according to the pK(a) value leads to ionization of the molecule at blood pH, the pK(a) value was considered to be the parameter most suitable for adjustment of lipophilicity. Insertion of ether-oxygen in the molecule of five candidates lowers the apparent pK(a) value from 10.0 to 8.3 and dramatically increases the brain uptake to 1.3% ID at 5 min, The direct relationship between brain uptake and apparent pK(a) cannot be simply explained by the increase in the pK(a)-governed proportion of the neutral species.