Biodistribution and in-vivo stability of a pet tracer for lung perfusion imaging

Introduction: 99mTc-Macroaggregated Albumin (MAA) is conventionally used for lung perfusion study using SPECT scanner. Earlier, in-house synthesis of Ga-MAA in hospital radiopharmacy has been reported. To assess the suitability of 68Ga-MAA as a PET lung perfusion agent, its normal biodistribution and in-vivo stability in swiss mice and Sprague Dawley rats are reported in the present study.

Methods: This study has been approved by the institutional animal ethics committee. Commercially available lyophilized MAA kits and eluate from 68Ge/68Ga generator were used. The macroaggregates were extracted and radiolabelled as per the previously standardized procedure. Radiochemical purity(RCP) of the preparation was determined using the TLC method. 68Ga-MAA (~100 µCi) was injected via the lateral tail vein. Biodistribution was performed in swiss mice and sprague dawley rats. Animals were sacrificed at 10 min, 30 min and 2 h post administration. Prior to the sacrifice, blood was collected from the retro-orbital vein of each animal. The animals were dissected and organs removed and counted in a well counter. The percentage Injected Dose (% ID) was determined for each organ. Imaging was also performed in mice using animal PET scanner at same time points after administration of 100 µCi of tracer.

Results: The average radiochemical yield and RCP of preparation was observed to be >95% and >99%, respectively. The %ID for each organ at different time points was shown in table. The average maximum and minimum % injected dose (ID) across all the time points in both mice and rats were noted to be > 90 % in lungs and <1 % in blood, liver and kidney. The animal imaging study also qualitatively corroborates with the findings of biodistribution study (figure).

Table: Biodistribution of 68Ga-MAA in Swiss Mice & Sprague Dawley Rats

Conclusions: Our results show that maximum uptake of the tracer was in the lungs with uniform distribution which is consistent with earlier published literature. The percentage uptake in the lung remains same for upto 2 h with no significant leaching indicating good in-vivo stability for upto 2 h post injection.

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