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Optimization of a Pretargeted Strategy for the PET Imaging of Colorectal Carcinoma via the Modulation of Radioligand Pharmacokinetics.

ABSTRACT: 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.

SUBMITTER: Zeglis BM

PROVIDER: S-EPMC4696756 | biostudies-literature | 2015 Oct

REPOSITORIES: biostudies-literature

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Optimization of a Pretargeted Strategy for the PET Imaging of Colorectal Carcinoma via the Modulation of Radioligand Pharmacokinetics.

Zeglis Brian M BM Brand Christian C Abdel-Atti Dalya D Carnazza Kathryn E KE Cook Brendon E BE Carlin Sean S Reiner Thomas T Lewis Jason S JS

Molecular pharmaceutics 20150831 10

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 effect ...[more]

PMID: 26287993

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Project description:UnlabelledThe specificity of antibodies have made immunoconjugates promising vectors for the delivery of radioisotopes to cancer cells; however, their long pharmacologic half-lives necessitate the use of radioisotopes with long physical half-lives, a combination that leads to high radiation doses to patients. Therefore, the development of targeting modalities that harness the advantages of antibodies without their pharmacokinetic limitations is desirable. To this end, we report the development of a methodology for pretargeted PET imaging based on the bioorthogonal Diels-Alder click reaction between tetrazine and transcyclooctene.MethodsA proof-of-concept system based on the A33 antibody, SW1222 colorectal cancer cells, and (64)Cu was used. The huA33 antibody was covalently modified with transcyclooctene, and a NOTA-modified tetrazine was synthesized and radiolabeled with (64)Cu. Pretargeted in vivo biodistribution and PET imaging experiments were performed with athymic nude mice bearing A33 antigen-expressing, SW1222 colorectal cancer xenografts.ResultsThe huA33 antibody was modified with transcyclooctene to produce a conjugate with high immunoreactivity, and the (64)Cu-NOTA-labeled tetrazine ligand was synthesized with greater than 99% purity and a specific activity of 9-10 MBq/μg. For in vivo experiments, mice bearing SW1222 xenografts were injected with transcyclooctene-modified A33; after allowing 24 h for accumulation of the antibody in the tumor, the mice were injected with (64)Cu-NOTA-labeled tetrazine for PET imaging and biodistribution experiments. At 12 h after injection, the retention of uptake in the tumor (4.1 ± 0.3 percent injected dose per gram), coupled with the fecal excretion of excess radioligand, produced images with high tumor-to-background ratios. PET imaging and biodistribution experiments performed using A33 directly labeled with either (64)Cu or (89)Zr revealed that although absolute tumor uptake was higher with the directly radiolabeled antibodies, the pretargeted system yielded comparable images and tumor-to-muscle ratios at 12 and 24 h after injection. Further, dosimetry calculations revealed that the (64)Cu pretargeting system resulted in only a fraction of the absorbed background dose of A33 directly labeled with (89)Zr (0.0124 mSv/MBq vs. 0.4162 mSv/MBq, respectively).ConclusionThe high quality of the images produced by this pretargeting approach, combined with the ability of the methodology to dramatically reduce nontarget radiation doses to patients, marks this system as a strong candidate for clinical translation.

Project description:The complementary nature of positron emission tomography (PET) and near-infrared fluorescence (NIRF) imaging makes the development of strategies for the multimodal PET/NIRF imaging of cancer a very enticing prospect. Indeed, in the context of colorectal cancer, a single multimodal PET/NIRF imaging agent could be used to stage the disease, identify candidates for surgical intervention, and facilitate the image-guided resection of the disease. While antibodies have proven to be highly effective vectors for the delivery of radioisotopes and fluorophores to malignant tissues, the use of radioimmunoconjugates labeled with long-lived nuclides such as 89Zr poses two important clinical complications: high radiation doses to the patient and the need for significant lag time between imaging and surgery. In vivo pretargeting strategies that decouple the targeting vector from the radioactivity at the time of injection have the potential to circumvent these issues by facilitating the use of positron-emitting radioisotopes with far shorter half-lives. Here, we report the synthesis, characterization, and in vivo validation of a pretargeted strategy for the multimodal PET and NIRF imaging of colorectal carcinoma. This approach is based on the rapid and bioorthogonal ligation between a trans-cyclooctene- and fluorophore-bearing immunoconjugate of the huA33 antibody (huA33-Dye800-TCO) and a 64Cu-labeled tetrazine radioligand (64Cu-Tz-SarAr). In vivo imaging experiments in mice bearing A33 antigen-expressing SW1222 colorectal cancer xenografts clearly demonstrate that this approach enables the non-invasive visualization of tumors and the image-guided resection of malignant tissue, all at only a fraction of the radiation dose created by a directly labeled radioimmunoconjugate. Additional in vivo experiments in peritoneal and patient-derived xenograft models of colorectal carcinoma reinforce the efficacy of this methodology and underscore its potential as an innovative and useful clinical tool.

Dataset Information

Optimization of a Pretargeted Strategy for the PET Imaging of Colorectal Carcinoma via the Modulation of Radioligand Pharmacokinetics.

Publications

Optimization of a Pretargeted Strategy for the PET Imaging of Colorectal Carcinoma via the Modulation of Radioligand Pharmacokinetics.

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