Preliminary studies of 99mTc-PQQ-NMDAR binding and effect of specificity binding by mannitol

Pyrroloquinoline quinone (PQQ) is a powerful neuroprotectant that specifically binds to brain NMDA receptors and inhibits excitotoxicity. Imaging this binding reaction in the brain remains a long sought goal in this field of study, and one of the primary challenges remaining is enabling soluble labeled PQQ to pass the blood–brain barrier (BBB). Previously, our group successfully labeled PQQ with Technetium-99m (99mTc), a metastable nuclear isomer used in radioactive isotope medical tests. In this work, we determined the specific binding of 99mTc-PQQ and NMDAR by radioligand receptor assay. Ebselen (EB) and MK-801 both effectively inhibited 99mTc-PQQ binding. We then investigated methods of opening the BBB using mannitol to enable entry to the brain by 99mTc-PQQ. Our results showed that 7.5 mL/kg of 20 % mannitol effectively opened the BBB and 20 min was the optimum treatment time. Competition studies showed that mannitol did not affect the specific binding between 99mTc-PQQ and NMDA receptors. Using this method, the amount of 99mTc-PQQ uptake and retention was increased most significantly in the hippocampus and cortex, and re-opening the BBB did not affect binding. Together, our results demonstrate that the use of mannitol to open the BBB may contribute significantly to improving image quality by increasing the uptake amount of a water-soluble agent in brain.