IC‐P‐196: ADVANCING THE ACCURACY OF AUTOMATED FDG‐PET MEASUREMENTS USING HIGH‐DIMENSIONAL IMAGE NORMALIZATION

a potential approach for the treatment of AD and a BACE radioligand could be valuable for assessing target occupancy and therapeutic responses to BACE inhibition. Methods: Mdr1a/b (P-gp 3 and 1 deficient, termed P-gp KO), FVB wild type (wt), BACE1 knockout (KO), and BACE1 wt mice, were purchased fromTaconic.Mdr1a/b andFVBWTmicewere 8weeks old,whileBACE1KO and BACE1 WT mice were two years old when used in the studies. The unlabeled compounds and [3 H]Compound-A (75.3Ci/mmol) were synthesized in house. Autoradiography was performed using frozen brain slices. In vitro binding assays were done with brain membranes or homogenates. In vivo brain occupancy was performed in Mdr1a/b, FVB and BACE1 KO mice by i.p. administration of BACE inhibitor or vehicle and then followed by i.v. dosing of the radioligand. Results: In mouse brain homogenates, [3 H]Compound-A showed binding site densities (Bmax) from 6.5 nM to 8.4 nM and high binding affinity (K d1⁄4 0.5 nM), yielding good binding potentials (Bmax/K d). In autoradiography of BACE1 wt mice, [3 H]Compound-A binding sites were widely distributed across the brain with different binding densities in various regions, and the non-displaceable bindingwas low and homogenous among all the brain slices. In contrast, [3 H]Compound-A failed to show specific binding in the brain slices of BACE1 KO mice. Furthermore, the in vivo brain occupancy studies demonstrated that [3 H]Compound-A binding to Mdr1a/b brains was dose-dependent and displaceable by pre-administration of a selective, brain penetrant BACE inhibitor. In FVB mouse brains, [3 H]Compound-A did not show specific brain uptake following i.v. administration of [3 H]CompoundA. Conclusions: These results demonstrate that [3 H]Compound-A binding is specific to BACE which is widely distributed in mouse brain. In vivo brain uptake of [3 H]-Compound-A only occurred in P-gp KO mice, not in wild type and BACE1 KO mice. Together, these data suggest that a compound with no P-gp liability could be developed into a PET tracer for BACE.