The development of novel PET/NIRF agents for hyperparathyroidism management

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Abstract Affecting >100,000 persons each year in the United States, primary hyperparathyroidism (PHPT) is a common disorder that leads to significant morbidity. For PHPT patients, surgically removing the affected parathyroid gland is the only curative option. The current state of the art is to use preoperative imaging to locate abnormal parathyroid glands, aiming to perform a focused surgical approach that can limit unnecessary dissection. In addition, localization can reveal abnormal locations of the parathyroid glands outside the neck which can prevent fruitless neck exploration and guide alternative surgical approaches. Despite the progress, current imaging techniques have several limitations on detecting small glands (especially in unusual locations) and multiple abnormal parathyroids (occurs in 15-20% of cases). This application proposes to develop new imaging agents that can efficiently detect parathyroid glands preoperatively, and be used intraoperatively to guide exploration. In detail, we will construct innovative positron emission tomography (PET) agents targeting markers specific for parathyroid glands, and PET/near-infrared fluorescence (NIRF) dual modality agents that seamlessly integrate parathyroid detection and image-guided surgery. This project is built upon our recent advances on radiofluorination, PET probe development and dual modality imaging. In detail, calcium sensing receptor (CSR) is a Class C G-protein coupled receptor that is highly expressed in the parathyroid gland and the kidneys. The development of CSR-specific pharmaceuticals for imaging could potentially lead to novel PET agents that are highly specific for parathyroid (kidneys are located far away from our area of interest, which will not interfere with PHPT detection). The use of a dual tracer (optical and PET) would not only introduce high sensitivity for parathyroid detection, but also facilitate subsequent gland localization during surgery. There are three aims for this project. Aim 1 will develop PET agents for parathyroid imaging based on novel ligands of CSR. Aim 2 will develop a robust radiofluorination method to generate 18F labeled NIRF dyes that could localize at parathyroid gland selectively. Aim 3 will evaluate our lead agents in rodent model bearing transplanted human hyperparathyroidism tissue. The lead agent will be further characterized in nonhuman primates. In our preliminary study, we have successfully introduced 18F (radio tag) into cinacalcet (a small molecule targeting CSR) using photoredox radiofluorination. The resulting agent could be preferentially taken up by parathyroid gland in rats. Initial study in non-human primate also suggested the great potential of using 18F- cinacalcet for parathyroid imaging. In aim 2, we successfully generated 18F labeled NIRF dyes for specific parathyroid and thyroid gland targeting. This approach would address the clinical need by integrating PET with image-guided surgery. In summary, the overarching goal of this multi-disciplinary research is to develop novel imaging agents that are highly sensitive and specific for parathyroid, which would greatly benefit the management of patients with primary hyperparathyroidism.