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^99mTc-ENS: A New Radiopharmaceutical for Aerosol Lung Scintigraphy. Comparison Between Different Freeze-Dried Formulations

Radioisotope Laboratory and Departments of Physics and Mathematics, School of Pharmacy and Biochemistry, University of Buenos Aires, Buenos Aires
Bacon Laboratories SAIC, Buenos Aires, Argentina

Correspondence: For correspondence or reprints contact: Bioch. Gabriela P. Calmanovici, Radioisotope Laboratory, School of Pharmacy and Biochemistry, University of Buenos Aires, Junin 956, Buenos Aires, 1113 Argentina.

ABSTRACT

Exogenous natural surfactant (ENS) labeled with ^99mTc(^99mTc-ENS is) a new radiopharmaceutical for pulmonary aerosol scintigraphy. In this study, different freeze-dried formulations were evaluated to develop a suitable and long-storage method for the ENS, the nonradioactive precursor of this radiopharmaceutical. Methods: Two freeze-dried formulations were evaluated: the sterile ENS suspension-stannous chloride altogether lyophilized (chlorlioENS) and the lyophilized sterile ENS suspension with the addition of stannous chloride as a solid drug (li0ENS). These precursors were stored at room temperature for 3 mo and then labeled with ^99mTc. For comparative purposes, the sterile ENS suspension with the addition of stannous chloride labeled with ^99mTc(^99mTc-chlorENS) was also studied. The quality controls for each radiopharmaceutical were performed by an ascending paper chromatography to determine the labeling yield percentages. The study was performed in 30 female Sprague Dawley rats, which inhaled each radiopharmaceutical by nebulization. Twenty-five minutes after the aerosol inhalation, the animals were killed to extract their organs and measure their activity in a gamma spectrometer. The data are given as the percentage of activity concentration (C%) for each organ. Results: The physicochemical properties of lioENS were adequate for a freeze-dried product. The labeling yields for ^99mTc-lioENS and for ^99mTc-chlorENS were always greater than 95% even after nebulization. The results of the biologic distribution studies showed that the activity concentration found in lungs for these radiopharmaceuticals were 95.7% ± 2.6% and 96.7% ± 2.6% respectively, results that do not differ statistically. On the other hand, the activity concentration found in lungs for the ^99mTc-chlorlioENS (31.3% ± 11.1%) and its labeling yield percentages (<10%) are statistically different (P < 0.05) from the results obtained with the two radiopharmaceuticals mentioned above. Conclusion: Taking into account the lioENS physicochemical properties, its long shelf life and that ^99mTc-lioENS shows the same radiochemical and radiopharmacological behavior of the ^99mTc-chlorENS, it can be concluded that the ^99mTc-lioENS can be used for aerosol lung scintigraphy.

Key Words: exogenous natural surfactant • ^99mTc-labeled exogenous natural surfactant • pulmonary aerosol scintigraphy • freeze drying