Abstract
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Objectives: Dysfunction of microtubules (MT) is clinically validated as a major pathogenesis of neurodegenerative disorders, brain malignancies, and brain injuries. Therefore, PET imaging of MT with radiolabeled blood brain barrier (BBB) penetrating MT targeted ligands may offer a direct and sensitive approach for early diagnosis, monitoring disease progression and treatment effects, and assessing the clinical potential of therapeutics and treatments. We recently developed [11C]MPC-68271 and [11C]HD-8002 as pioneer brain penetrating PET ligands for imaging MT in rodents and non-human primates. The major metabolic pathway of these molecules is reported to be via the initial labeling site, O-methyl in comparison to the N-methyl group. In this presentation, we report the radiosynthesis of N-CH3-MPC-6827 and N-11CH3-HD-800, via labeling the N-methyl group, and compared their in vivo binding with corresponding O-11CH3 analogues via microPET imaging and biodistribution methods.
Methods: Synthesis of O-[11C]MPC-6827 and O-[11C]HD-800 were achieved based on previous reports1,2 Precursors for N-11CH3 labeling were synthesized by coupling 4-chloro-2-methylquinazoline and 4-chloro-5-methylfuro[2,3-d]pyrimidine with anisole. The N-11CH3 analogues were synthesized by reacting the corresponding N-desmethyl precursors with [11C]CH3I-NaH/DMF system in a GE-FX2MeI/FX2M module. In vivo microPET/CT imaging and post-PET standard biodistribution experiments of N-CH3 and O-CH3 radiolabeled MPC-6827 and HD-800 were performed in white male mice (n=3). Blocking experiments were performed (n=3) using pretreatment with nonradioactive ligands, 30 min prior to radiotracer administration.
Results: Synthesis of O-[11C]MPC-6827 and O-[11C]HD-800 were achieved in 40+5% (n=>50). Radiosynthesis of N-[11C]MPC-6827 and N-[11C]HD-800 were achieved in 7+3 % (n=6). All tracers were obtained in >99% radiochemical purity with a molar activity of 2+0.5 Ci/μmol. MicroPET data show BBB penetration, retention of activity in brain with high specific binding while blocking with cold ligands. Biodistribution showed high brain uptake with both N-11CH3 labeled MT tracers with brain to muscle ratio ~ 4. Heart, pancreas, lung and spleen exhibit similar radioactive uptake to that of O-11CH3 labeled tracers.
Conclusions: Our studies show that the N-11CH3 labeled MT PET ligands demonstrated similar in vivo binding characteristic of the corresponding O-11CH3 labeled tracers [11C]MPC-6827 and [11C]HD-800 respectively. Further metabolic analysis experiments of these tracers in rodents are underway. Research support: CTSA WFSM TIP (UL1TR001420), Diane Goldberg Foundation (NYSPI/CUMC). References 1. Kumar JSD et al. Radiosynthesis and In vivo evaluation of [11C]MPC-6827, the first brain penetrant microtubule PET ligand, J. Med. Chem. 2018, 61(5), 2118 2. Solingapuram Sai K K et al., Radiosynthesis and evaluation of [11C]HD-800, a high affinity brain penetrant PET tracer for imaging microtubules, ACS Medicinal Chemistry Letters, 2018, 9 (5), 452-456.