Synthesis of PET radioligands as potential probes for imaging COX-2 in neuroinflammation

Objectives Cyclooxygenase-2 (COX-2) expression is elevated in inflammation.[1] PET radioligands for imaging COX-2 should prove useful for studying neuroinflammation but none yet exists.[2] Substituted pyrimidines, such as 6-methoxy-2-(4-(methylsulfonyl)phenyl)-N-(thiophen-2-ylmethyl)pyrimidin-4-amine (MC1), show many desirable properties for development as PET radioligands.[3] Here we aimed to prepare and label such pyrimidines for evaluation in monkey as PET radioligands for COX-2.

Methods A set of five substituted pyrimidines, including MC1, was synthesized by modifying published procedures.[3] These ligands were tested for affinity to human blood COX-2 and COX-1. MC1 was selected for labeling with carbon-11 by treating the desmethyl-precursor 1 and base in DMF with [¹¹C]MeI. [¹¹C]MC1 was separated by HPLC and formulated for intravenous injection. The 6-fluoromethoxy analog (MC8) was similarly labeled by treating 1 and base with [¹⁸F]FCD₂Br. PET experiments were performed with [¹¹C]MC1 administered intravenously to two monkeys at baseline and after treatment with MC1 (3 mg/kg, i.v.). Radioligand arterial input function and plasma free fraction (f_P) were measured to calculate total volume of distribution (V_T) and V_T/f_P.

Results The prepared pyrimidines showed affinities for COX-2 in the nM range. MC1 showed one of the highest affinities (IC₅₀ = 3 nM) and highest selectivities (> 3000-fold) over binding to COX-2. [¹¹C]MC1 and [¹⁸F]MC8 were obtained in useful yields and high specific activities. Following administration of [¹¹C]MC1, radioactivity peaked in brain at ~ 2.9 SUV at 2 min and then reduced by about 60% at ~ 40 min. On average, MC1 pretreatment reduced brain V_T by 44% and V_T/f_P by 15%. f_P values at baseline were 0.71 and 0.82%, and 0.49 and 0.52% at MC1 pretreatment.

Conclusions [¹¹C]MC1 enters monkey brain adequately. Further evaluation of [¹¹C]MC1 to verify the presence of COX-2 specific signal in monkey brain and its magnitude is in progress. Evaluation of [¹⁸F]MC8 is ongoing.

Research Support [¹¹C]MC1 enters monkey brain adequately. Further evaluation of [¹¹C]MC1 to verify the presence of COX-2 specific signal in monkey brain and its magnitude is in progress. Evaluation of [¹⁸F]MC8 is ongoing.