Preparation and biodistribution of copper-67 complexes with tetradentate Schiff-base ligands

Uncharged, lipophilic, low molecular weight copper complexes labeled with generator-produced copper-62 are of interest as potential radiopharmaceutials for imaging the brain with positron emission tomography (PET). We report here the synthesis and biodistribution of a series of [67Cu]copper(II) complexes with tetradentate N2O2(2-)Schiff-base ligands. The compounds studied varied in lipophilicity from log P = 1.7 to log P = 3.6, where P is the octanol/water partition coefficient. In rat biodistribution studies the tracers were generally found to penetrate the blood-brain barrier following intravenous injection, but some far better than others. For closely related compounds brain uptake at 1 min postinjection increased with increasing lipophilicity, although log P was clearly not the sole determinant of high brain uptake. Substantial variations were also observed in the rate at which these various compounds are cleared from brain, with a few exhibiting the prolonged cerebral retention of tracer that would be desired for imaging with 62Cu and PET.