Abstract
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Objectives In this study, generation 6 (G6) PAMAM dendrimer was conjugated to a DOTA-NHS molecule and a technique to radiolabel this conjugate was optimized so as to enable PET molecular imaging during targeted delivery of drugs. Biodistribution studies in normal rats and tumor bearing mice were also performed so as to evaluate the fate of radiolabeled polymers in the body.
Methods G6 PAMAM dendrimer was conjugated to DOTA-NHS ester. Sephadex G-25 and MALDI-tof technique was used to purify and characterize the formed conjugate respectively. In vitro cytotoxicity of DOTA-G6 PAMAM conjugate was evaluated by MTT assay. The formed conjugate was further radiolabeled with 68Ga. Radiolabeling efficiency was determined by radio ITLC and the radiolabeled conjugate was subjected to various quality control parameters. The absorbed radiation dose to human organs has also been estimated following intravenous administration of [68Ga]DOTA-G6 PAMAM conjugates using normal biodistribution data obtained from Wistar rats. The biodistribution data obtained from rats was extrapolated to humans by using OLINDA software. The organ biodistribution was also estimated in tumor bearing balb/c mice at different time intervals and the organ's biodistribution data was supported by animal PET imaging in tumor bearing mice.
Results DOTA-G6 PAMAM conjugate showed negligible cytotoxic effect for up to the concentration of 1.0 μg/mL at 24 h. At maximum concentration of 100.0 μg/mL, almost 70.0% cells were viable. The radiolabeling efficiency of DOTA conjugated G6-PAMAM dendrimer with Gallium-68 (68Ga) was found to be 95.0%. In vitro and serum stability assay showed that the complex moieties on the surface of dendrimers remained stable up to 2 hrs (=94.0% labeling efficiency). The plasma protein binding was found to be 7.7 ± 3.2%. From rat data, it is estimated that a 200.0 MBq injection of [68Ga]DOTA-G6 PAMAM dendrimers might result in an estimated absorbed dose of 1.4 mSv to the whole body. The image interpretation indicated that the tracer had significant uptake in the tumor tissue.
Conclusions The present study highlights the importance of dendrimer based radio imaging of angiogenesis as a novel approach for molecular imaging of carcinogenesis. The specificity of these radiolabeled conjugates can further be improved by conjugating peptides or antibodies with these conjugates. However, to demonstrate the translational relevance of radiolabeled dendrimer conjugates, further pre-clinical and clinical studies are required to be conducted.