Comparison of technetium-99m aminoalkyl diaminodithiol (DADT) analogs as potential brain blood flow imaging agents

N-ethyl piperidinyl diaminodithiol (NEP-DADT), complexed with 99mTc has been developed as an agent for the measurement of brain blood flow using SPECT. Studies in patients have shown that 99mTc NEP-DADT enters rapidly into the brain, but also clears rapidly (t1/2 = 17 min). In this study nine new aminoalkyl DADT derivatives were synthesized, labeled with 99mTc and tested in mice with the aim of developing an agent with increased retention in the brain. In addition, relationships between chemical properties of the derivatives and their in vivo localization were investigated. The results were as follows: (a) the R-group and its isomeric configuration has a profound influence on the biodistribution; (b) 99mTc aminoalkyl DADT derivatives with apparent pKa values of greater than 6.9 show poor brain uptake (less than 0.40% dose at 5 min); (c) lengthening of the chain between the DADT moiety and the amino-R group from ethyl to hexyl generally increases the apparent pKa and consequently lowers brain uptake; (d) a correlation (r = 0.71) exists between initial brain uptake and the octanol-buffer partition coefficient; (e) 99mTc-4'-methyl NEP-DADT has the highest partition coefficient, relatively high uptake, and longest retention in the mouse brain. This complex has characteristics suited for brain blood flow measurements.