Development of subnanomolar radiofluorinated (2-pyrrolidin-1-yl)imidazo[1,2-b]pyridazine pan-Trk inhibitors as candidate PET imaging probes

Abstract

Dysregulation of tropomyosin receptor kinases (TrkA/B/C) expression and signalling is recognized as a hallmark of numerous neurodegenerative diseases including Parkinson's, Huntington's and Alzheimer's disease. TrkA/B/C is known to drive tumorogensis and metastatic potential in a wide range of neurogenic and non-neurogenic human cancers. The development of suitable positron emission tomography (PET) radioligands would allow an in vivo exploration of this versatile potential therapeutic target. Herein, the rational remodeling of the amide moiety of a 6-(2-(3-fluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazine-3-amide lead structure to accommodate efficient fluorine-18 labeling led to the identification of a series of fluorinated Trk inhibitors with picomolar IC₅₀. The ensuing representative radiolabeled inhibitors [¹⁸F]16 ([¹⁸F]-(±)-IPMICF6) and [¹⁸F]27 ([¹⁸F]-(±)-IPMICF10) constitute novel lead radioligands with about 2- to 3- orders of magnitude increased TrkB/C potencies compared to previous lead tracers and display favorable selectivity profiles and physicochemical parameters for translation into in vivo PET imaging agents.