Longitudinal imaging of therapeutic enzyme expression after gene therapy for Fabry disease using positron emission tomography and the radiotracer [18F]AGAL

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 evaluate a novel PET tracer ([¹⁸F]AGAL) for targeting α-galactosidase A (GLA), an enzyme deficient in Fabry disease. Gla knockout mice were subjected to either GT with an adeno-associated virus encoding the human GLA (AAV_GLA) or recombinant GLA for enzyme replacement studies. PET imaging, ex vivo autoradiography, biochemical analyses and radiation dosimetry were performed. [¹⁸F]AGAL exhibited pH-dependent binding to GLA, 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 GLA 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.