Optimization of a Pretargeted Strategy for the PET Imaging of Colorectal Carcinoma via the Modulation of Radioligand Pharmacokinetics.
Pretargeted PET imaging has emerged as an effective strategy for merging the exquisite selectivity of antibody-based targeting vectors with the rapid pharmacokinetics of radiolabeled small molecules. We previously reported the development of a strategy for the pretargeted PET imaging of colorectal cancer based on the bioorthogonal inverse electron demand Diels-Alder reaction between a tetrazine-bearing radioligand and a transcyclooctene-modified huA33 immunoconjugate. Although this method effectively delineated tumor tissue, its clinical potential was limited by the somewhat sluggish clearance of the radioligand through the gastrointestinal tract. Herein, we report the development and in vivo validation of a pretargeted strategy for the PET imaging of colorectal carcinoma with dramatically improved pharmacokinetics. Two novel tetrazine constructs, Tz-PEG7-NOTA and Tz-SarAr, were synthesized, characterized, and radiolabeled with (64)Cu in high yield (>90%) and radiochemical purity (>99%). PET imaging and biodistribution experiments in healthy mice revealed that although (64)Cu-Tz-PEG7-NOTA is cleared via both the gastrointestinal and urinary tracts, (64)Cu-Tz-SarAr is rapidly excreted by the renal system alone. On this basis, (64)Cu-Tz-SarAr was selected for further in vivo evaluation. To this end, mice bearing A33 antigen-expressing SW1222 human colorectal carcinoma xenografts were administered huA33-TCO, and the immunoconjugate was given 24 h to accumulate at the tumor and clear from the blood, after which (64)Cu-Tz-SarAr was administered via intravenous tail vein injection. PET imaging and biodistribution experiments revealed specific uptake of the radiotracer in the tumor at early time points (5.6 ± 0.7 %ID/g at 1 h p.i.), high tumor-to-background activity ratios, and rapid elimination of unclicked radioligand. Importantly, experiments with longer antibody accumulation intervals (48 and 120 h) yielded slight decreases in tumoral uptake but also concomitant increases in tumor-to-blood activity concentration ratios. This new strategy offers dosimetric benefits as well, yielding a total effective dose of 0.041 rem/mCi, far below the doses produced by directly labeled (64)Cu-NOTA-huA33 (0.133 rem/mCi) and (89)Zr-DFO-huA33 (1.54 rem/mCi). Ultimately, this pretargeted PET imaging strategy boasts a dramatically improved pharmacokinetic profile compared to our first generation system and is capable of clearly delineating tumor tissue with high image contrast at only a fraction of the radiation dose created by directly labeled radioimmunoconjugates.
Department of Chemistry and Biochemistry, Hunter College and the Graduate Center of the City University of New York , New York, New York 10021, United States.
Brian M
BM
Zeglis
Department of Radiology, Memorial Sloan Kettering Cancer Center , New York, New York 10065, United States.
Christian
C
Brand
Department of Radiology, Memorial Sloan Kettering Cancer Center , New York, New York 10065, United States.
Dalya
D
Abdel-Atti
Department of Radiology, Memorial Sloan Kettering Cancer Center , New York, New York 10065, United States.
Kathryn E
KE
Carnazza
Department of Chemistry and Biochemistry, Hunter College and the Graduate Center of the City University of New York , New York, New York 10021, United States.
Brendon E
BE
Cook
Department of Radiology, Memorial Sloan Kettering Cancer Center , New York, New York 10065, United States.
Sean
S
Carlin
Department of Radiology, Memorial Sloan Kettering Cancer Center , New York, New York 10065, United States.
Thomas
T
Reiner
Department of Radiology, Memorial Sloan Kettering Cancer Center , New York, New York 10065, United States.|Program in Molecular Pharmacology and Chemistry, Memorial Sloan Kettering Cancer Center , New York, New York 10065, United States.
Jason S
JS
Lewis
D000818:Animals; D045744:Cell Line, Tumor / Q000637:transplantation; D015179:Colorectal Neoplasms / Q000000981:diagnostic imaging*; D003301:Copper Radioisotopes / Q000493:pharmacokinetics; D005260:Female; D006801:Humans; D018796:Immunoconjugates / Q000493:pharmacokinetics; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics*
D003301:Copper Radioisotopes; D018796:Immunoconjugates; D019275:Radiopharmaceuticals
Click Chemistry; Colorectal Cancer; Inverse Electron Demand Diels−Alder Reaction; PET; Positron Emission Tomography; Pretargeting; Tetrazine; Transcyclooctene; huA33