Longitudinal imaging of therapeutic enzyme expression after gene therapy for Fabry disease using Positron Emission Tomography and the radiotracer [18F]AGAL.

Abstract

Longitudinal, non-invasive, in vivo monitoring of therapeutic gene expression is an unmet need for gene therapy (GT). Positron Emission Tomography (PET) radiotracers designed to bind to therapeutic proteins may provide a sensitive imaging platform to guide treatment response and dose optimization in GT. Herein, we evaluated a novel PET tracer ([¹⁸F]AGAL) for targeting alpha galactosidase A (α-GalA), an enzyme deficient in Fabry disease. Gla knockout mice were subjected to either GT with an adeno-associated virus encoding the human a-Gal A (AAV_GLA) or recombinant α-GalA for enzyme replacement studies. PET imaging, ex vivo autoradiography, biochemical analyses and radiation dosimetry were performed. [¹⁸F]AGAL exhibited pH-dependent binding to a-GalA, suggesting recognition of the active enzyme residing within the acidified lysosomes. Imaging studies in the Fabry mouse model showed quick renal clearance with high radioactive uptake in the heart at 6 weeks that was sustained for 26 weeks after a single administration of AAV_GLA, indicating effective and durable transgene expression from GT. Good concordance was achieved between in vivo PET imaging and ex vivo quantification of α-GalA levels in biofluids and tissues. Biodistribution and dosimetry in non-human primate showed acceptable radiation exposure for multiple injections demonstrating its potential for translation to clinical trial use.