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 sigma-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 (Ki = 49 +/- 14 nmol/L) and a 130-fold lower interaction with the sigma1-binding site (Ki = 6.35 +/- 0.26 micromol/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/micromol at the end of synthesis (EOS). In saturation assays using rat whole brain membranes [11C]methyl-BIII277CL showed a Kd of 6 +/- 1 nmol/L and a Bmax 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 (Dv = 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 = 0.37min(-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.

PET and SPECT Imaging of the NMDA Receptor System: An Overview of Radiotracer Development

“…In vitro saturation experiments using [ 11 C]30 demonstrated a high affinity (K D =6 nM) but a weak B max /K D ratio of approximately 10 (B max =670 fmol/mg of protein). PET imaging in pig revealed for [ 11 C]30 a fast uptake with similar time activity curves in cortex and cerebellum and a homogenous distribution in the whole brain [90].…”

Recent Advances in the Development of NR2B Subtype-Selective NMDA Receptor Antagonists

“…However, retention of the hydroxyl adjacent to the nitrogen and in the R-configuration (49) was sufficient to obtain comparable potency to the lead compound 47. Further simplification of the aminopropan-2-ol in 49 to an ethanolamine group resulted in a compound (50) with no stereocenters and improved potency (K i = 110 nM) relative to the original lead 47.…”

“…[5,[42][43][44] In order to fully understand the contributions of each NMDA receptor subtype in neurological disorders, several attempts to image NMDA receptors in living systems using noninvasive imaging techniques such as positron emission tomography (PET) have been reported. Unfortunately, nonspecific NMDA radioligands for the glycine binding site [45][46][47][48][49] and the ion channel [50][51][52][53][54] have proven unsuccessful due to poor brain penetration. Several radiotracers of NR2B subtype-selective antagonists have also failed to meet the pharmacological requirements in vivo due to uniform distribution of tracer throughout brain structures.…”

27 Radiochemistry

“…65 But the most widely used conversion of labelled carbon dioxide is to [ 11 C]methyl iodide which can then be used in a wide variety of reactions leading to methyl labelled systems -chloroindoles, 66 substituted pyridines, 67,68 dihydroxy-benzene derivatives, 69 NЈ( 4 71 and a bewildering array of complex receptor antagonists known by their cryptic codes, AG957, 72 DR4446, 73 NS4194 74 and BIII277CL. 75…”