Synthesis and structure-activity relationships of 5,6,7-substituted pyrazolopyrimidines: discovery of a novel TSPO PET ligand for cancer imaging

Abstract

Focused library synthesis and structure-activity relationship development of 5,6,7-substituted pyrazolopyrimidines led to the discovery of 2-(5,7-diethyl-2-(4-(2-fluoroethoxy)phenyl)pyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide (6b), a novel translocator protein (TSPO) ligand exhibiting a 36-fold enhancement in affinity compared to another pyrazolopyrimidine-based TSPO ligand, 6a (DPA-714). Radiolabeling with fluorine-18 ((18)F) facilitated production of 2-(5,7-diethyl-2-(4-(2-[(18)F]fluoroethoxy)phenyl)pyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide ((18)F-6b) in high radiochemical yield and specific activity. In vivo studies of (18)F-6b were performed which illuminated this agent as an improved probe for molecular imaging of TSPO-expressing cancers.

Figure 1

PET imaging of preclinical glioma using ¹⁸F-6b.

Scheme 1. Synthetic methodology utilized to generate a focused library of 5,6,7-substituted pyrazolopyrimidines and ¹⁸F-6b^α

^αReagents and conditions: (a) NaOH, NaI, microwave, 80 °C, 40 min, 80% EtOH in H₂O, 70%; (b) hydrazine, AcOH, microwave, 90 °C, 40 min, EtOH, 42%; (c) microwave, 140–190 °C, 45min, EtOH, 10–90%; (d) HTPB, microwave, 110 °C, 40 min, aqueous HBr; (e) NaH, 2-fluoroethyl-4-methylbenzenesulfonate, microwave, 120 °C, 30 min, dry THF. (f) ethylene di(p-toluenesulfonate), NaH, microwave, 120 °C, 30 min, dry THF, 45%. (g) Fluoride-18 ion, K⁺-K_2.2.2/K₂CO₃, 99 °C, 20 min, dry DMSO.