SYNTHESIS AND IN VITRO AND IN VIVO EVALUATION OF [¹¹C]METHYL-BIII277CL FOR IMAGING THE PCP-BINDING SITE OF THE NMDA RECEPTOR BY PET

ABSTRACT

A new benzomorphane derivative, [¹¹C]methyl-BIII277CL, was evaluated as a potential radiotracer for visualizing the PCP-binding site of the N-methyl-D-aspartate (NMDA) receptor by positron emission tomography (PET). Methyl-BIII277CL was prepared by reacting the desmethyl compound (BIII277CL) with dimethylsulfate. The pharmacological profile of methyl-BIII277CL was determined by in vitro receptor-screening assays. At a concentration of 100 nM, methyl-BIII277CL showed a significant interaction with the PCP-binding site of the NMDA receptor (79% inhibition of specific binding) and the σ1-binding site (46% inhibition). In displacement assays using mice cortical membranes, methyl-BIII277CL displayed a high affinity at the PCP-binding site of the NMDA receptor (K_i = 49 ± 14 nmol/L) and a 130-fold lower interaction with the σ1-binding site (K_i = 6.35 ± 0.26 µmol/L). For saturation experiments and in vivo studies, methyl-BIII277CL was radiolabeled with ¹¹C at the O-position of the desmethyl precursor (BIII277CL) using [¹¹C]methyliodide with a specific activity of 35–70 GBq/µmol at the end of synthesis (EOS). In saturation assays using rat whole brain membranes [¹¹C]methyl-BIII277CL showed a K_d of 6 ± 1 nmol/L and a B_max of 670 ± 154 fmol/mg protein. Biodistribution and PET studies in rats and pigs, however, indicated a lack of specific binding and unfavorable pharmacokinetics. Kinetic modeling using the 1-tissue compartment model demonstrated for [¹¹C]methyl-BIII277CL a low distribution volume (D_v = 0.98 mL/mL_tissue) and very high values for the kinetic parameters K₁ and k₂ (K₁ = 0.36 mL/mL_tissue/min and k₂ = 0.37 min⁻¹) in pig cortex. [¹¹C]methyl-BIII277CL, due to the lack of specificity in vivo, may not be a candidate for imaging the PCP-binding site of the NMDA receptor.

Key Words: :

• [¹¹C]Methyl-BIII277CL
• NMDA receptor
• PET
• Carbon-11