Noninvasive in vivo quantification of adeno-associated virus serotype 9–mediated expression of the sodium/iodide symporter under hindlimb ischemia and neuraminidase desialylation in skeletal muscle using single-photon emission computed tomography/computed tomography

BACKGROUND: We propose micro single-photon emission computed tomography/computed tomography imaging of the hNIS (human sodium/ iodide symporter) to noninvasively quantify adeno-associated virus 9 (AAV9)-mediated gene expression in a murine model of peripheral artery disease. METHODS: AAV9-hNIS (2×10¹¹ viral genome particles) was injected into nonischemic or ischemic gastrocnemius muscles of C57Bl/6J mice following unilateral hindlimb ischemia ± the α-sialidase NA (neuraminidase). Control nonischemic limbs were injected with phosphate buffered saline or remained noninjected. Twelve mice underwent micro single-photon emission computed tomography/computed tomography imaging after serial injection of pertechnetate (^99mTcO₄⁻), a NIS substrate, up to 28 days after AAV9-hNIS injection. Twenty four animals were euthanized at selected times over 1 month for ex vivo validation. Forty-two animals were imaged with ^99mTcO₄⁻ ± the selective NIS inhibitor perchlorate on day 10, to ascertain specificity of radiotracer uptake. Tissue was harvested for ex vivo validation. A modified version of the U-Net deep learning algorithm was used for image quantification. RESULTS: As quantitated by standardized uptake value, there was a gradual temporal increase in ^99mTcO₄⁻ uptake in muscles treated with AAV9-hNIS. Hindlimb ischemia, NA, and hindlimb ischemia plus NA increased the magnitude of ^99mTcO₄⁻ uptake by 4- to 5-fold compared with nonischemic muscle treated with only AAV9-hNIS. Perchlorate treatment significantly reduced ^99mTcO₄⁻ uptake in AAV9-hNIS-treated muscles, demonstrating uptake specificity. The imaging results correlated well with ex vivo well counting (r²=0.9375; P<0.0001) and immunoblot analysis of NIS protein (r²=0.65; P<0.0001). CONCLUSIONS: Micro single-photon emission computed tomography/ computed tomography imaging of hNIS-mediated ^99mTcO₄⁻ uptake allows for accurate in vivo quantification of AAV9-driven gene expression, which increases under ischemic conditions or neuraminidase desialylation in skeletal muscle.