99mTc-Macroaggregated Albumin (MAA) stability when used in the SNMMI Procedure Guideline for Adult Solid-Meal Gastric-Emptying scintigraphy

Introduction: The Society of Nuclear Medicine and Molecular Imaging (SNMMI) and the American Neurogastroenterological and Motility Society have developed a standardized protocol for solid gastric emptying (SGE) studies (Ziessman H et al, JNM 2007). The standardized meal includes egg whites tagged with ^99mTc Sulfur Colloid. Recent supply issues in sulfur colloid necessitated an alternative radiopharmaceutical to tag the egg whites. ^99mTc MAA has previously been reported to be used for radiolabeling egg whites for SGE studies (Notghi A et al, Nucl Med Commun 2018). However, there is little documentation on the stability of ^99mTc MAA binding to the egg whites in-vivo over the full four-hour study time recommended by the standardized SNMMI guidelines.

Methods: ^99mTc MAA, 0.5 mCi, was added to 125 milliliters of liquid egg whites (e.g. Eggbeaters or an equivalent generic liquid egg white) that was poured into a container, the mixture was then microwaved for two minutes, stirred and microwaved for one more minute as per standard SGE clinical protocol. Once cooking was completed, the eggs were then cut up using a fork to simulate chewing until the eggs became a paste-like substance. Approximately 0.5 g of egg was transferred into 1 mL centrifuge tubes, and 0.5 mL of simulated gastric juices was added to each tube. Simulated gastric juices were comprised of 0.26% Hydrochloric acid and 0.20% Sodium Chloride in water, with a pH between 1-3. The tubes were then vortexed for 30 seconds and spun in a centrifuge for two minutes at 1000 rpm. A 30-gauge needle attached to a 1 mL syringe was used to remove the supernatant from the solid egg pellets; care was taken to avoid getting solid egg product in the transfer. The supernatant was then transferred to a clean centrifuge tube. The activity of the supernatant and solid egg pellet was measured separately using an Atomlab 500 Wipe Test Counter. The supernatant was added back into the corresponding egg pellet and vortexed for about 30 seconds. The centrifuge tube was placed into a heating block set to approximately 37 ° C for about one hour. This process of measuring the supernatant and solid egg pellet radiotracer activity followed by heating with simulated gastric juices was repeated every hour over the course of 4 hours; this amount of time was chosen since it best replicated the process and duration for imaging after consumption of the meal obtained during a clinical SGE protocol.

Results: The average ^99mTc MAA binding results over the four hours of heating was > 92% and the package insert does not indicate a minimum tag for the prepared kit. The kit was prepared per the FDA package insert and passed the 90% radiochemical purity minimum as established by USP 825 guidelines. Table 1 presents the total counts of radioactivity in egg white (solid) and supernatant (liquid) and binding percentage of ^99mTc MAA to the liquid egg whites from four different samples taken at zero, one, two, three, and four-hour time points and demonstrates that the ^99mTc MAA does stay bound. Table 2 presents average, standard deviation, confidence intervals of the four samples at the zero, one, two, three and four-hour time points.

Conclusions: The results from this study demonstrate that over 90% of the ^99mTc MAA remains bound to the egg whites over the course of the four-hour standard SGE protocol. Our radiology department administers over 300 SGE clinical studies annually, often with about a one-month lead time to be scheduled. To increase availability of SGE for patients during limited ^99mTc sulfur colloid availability, we have demonstrated the rationale for utilizing ^99mTc MAA as a substitute for ^99mTc sulfur colloid.

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