The authors thank Dr. Jacob Houghton for helpful conversations.&#160;The authors would also like to thank the NIH for their generous funding (R00CA178205 and U01CA221046).

<ol>
	<li>Goldenberg, D. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Targeted+Therapy+of+Cancer+with+Radiolabeled+Antibodies.">Targeted Therapy of Cancer with Radiolabeled Antibodies.</a> Journal of Nuclear Medicine. 43 (5), 693-713 (2002).</li><li>Goldenberg, D. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Radioimmunotherapy+of+B-cell+Lymphomas+with+Iodine-131-labeled+LL2+Monoclonal+Antibody.">Radioimmunotherapy of B-cell Lymphomas with Iodine-131-labeled LL2 Monoclonal Antibody.</a> Journal of Clinical Oncology. 9 (4), 548-564 (1991).</li><li>Kaminski, M. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Radioimmunotherapy+with+131I-Tositumomab+for+Relapsed+or+Refractory+B-cell+non-Hodgkin+Lymphoma:+Updated+Results+and+Long-Term+Follow-Up+of+the+University+of+Michigan+Experience.">Radioimmunotherapy with 131I-Tositumomab for Relapsed or Refractory B-cell non-Hodgkin Lymphoma: Updated Results and Long-Term Follow-Up of the University of Michigan Experience.</a> Blood. 96 (4), 1259-1266 (2000).</li><li>Davies, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Radioimmunotherapy+for+B-cell+Lymphoma:+Y90+Ibritumomab+Tiuxetan+and+I131+Tositumomab.">Radioimmunotherapy for B-cell Lymphoma: Y90 Ibritumomab Tiuxetan and I131 Tositumomab.</a> Oncogene. 26, 3614 (2007).</li><li>Rajendran, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+Radiation+Dose+Estimation+for+Myeloablative+Radioimmunotherapy+for+Relapsed+or+Recurrent+Mantle+Cell+Lymphoma+Using+131I+Tositumomab+to+That+of+Other+Types+of+Non-Hodgkin's+Lymphoma.">Comparison of Radiation Dose Estimation for Myeloablative Radioimmunotherapy for Relapsed or Recurrent Mantle Cell Lymphoma Using 131I Tositumomab to That of Other Types of Non-Hodgkin's Lymphoma.</a> Cancer Biotherapy and Radiopharmaceuticals. 19 (6), 738-745 (2004).</li><li>Rajendran, J. G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-Dose+131I-Tositumomab+(Anti-CD20)+Radioimmunotherapy+for+Non-Hodgkin's+Lymphoma:+Adjusting+Radiation+Absorbed+Dose+to+Actual+Organ+Volumes.">High-Dose 131I-Tositumomab (Anti-CD20) Radioimmunotherapy for Non-Hodgkin's Lymphoma: Adjusting Radiation Absorbed Dose to Actual Organ Volumes.</a> Journal of Nuclear Medicine. 45 (6), 1059-1064 (2004).</li><li>Larson, S. M., Carrasquillo, J. A., Cheung, N. -. K. V., Press, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Radioimmunotherapy+of+Human+tumours.">Radioimmunotherapy of Human tumours.</a> Nature Reviews Cancer. 15 (6), 347-360 (2015).</li><li>Kelly, M. P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tumor+Targeting+by+a+Multivalent+Single-Chain+Fv+(scFv)+Anti-Lewis+Y+Antibody+Construct.">Tumor Targeting by a Multivalent Single-Chain Fv (scFv) Anti-Lewis Y Antibody Construct.</a> Cancer Biotherapy &#38; Radiopharmaceuticals. 23 (4), 411-423 (2008).</li><li>Yazaki, P. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tumor+Targeting+of+Radiometal+Labeled+Anti-CEA+Recombinant+T84.66+Diabody+and+T84.66+Minibody:+Comparison+to+Radioiodinated+Fragments.">Tumor Targeting of Radiometal Labeled Anti-CEA Recombinant T84.66 Diabody and T84.66 Minibody: Comparison to Radioiodinated Fragments.</a> Bioconjugate Chemistry. 12 (2), 220-228 (2001).</li><li>van Duijnhoven, S. M. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diabody+Pretargeting+with+Click+Chemistry+In+Vivo.">Diabody Pretargeting with Click Chemistry In Vivo.</a> Journal of Nuclear Medicine. 56 (9), 1422-1428 (2015).</li><li>Altai, M., Membreno, R., Cook, B., Tolmachev, V., Zeglis, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pretargeted+Imaging+and+Therapy.">Pretargeted Imaging and Therapy.</a> Journal of Nuclear Medicine. 58 (10), 1553-1559 (2017).</li><li>Goldenberg, D. M., Chatal, J. -. F., Barbet, J., Boerman, O., Sharkey, R. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cancer+Imaging+and+Therapy+with+Bispecific+Antibody+Pretargeting.">Cancer Imaging and Therapy with Bispecific Antibody Pretargeting.</a> Update on cancer therapeutics. 2 (1), 19-31 (2007).</li><li>Rossin, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In-Vivo+Chemistry+for+Pretargeted+Tumor+Imaging+in+Live+Mice.">In-Vivo Chemistry for Pretargeted Tumor Imaging in Live Mice.</a> Angewandte Chemie International Edition. 49 (19), 3375-3378 (2010).</li><li>Gestin, J. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-Step+Targeting+of+Xenografted+Colon+Carcinoma+Using+a+Bispecific+Antibody+and+188Re-Labeled+Bivalent+Hapten:+Biodistribution+and+Dosimetry+Studies.">Two-Step Targeting of Xenografted Colon Carcinoma Using a Bispecific Antibody and 188Re-Labeled Bivalent Hapten: Biodistribution and Dosimetry Studies.</a> Journal of Nuclear Medicine. 42 (1), 146-153 (2001).</li><li>Green, D. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparative+Analysis+of+Bispecific+Antibody+and+Streptavidin-Targeted+Radioimmunotherapy+for+B-cell+Cancers.">Comparative Analysis of Bispecific Antibody and Streptavidin-Targeted Radioimmunotherapy for B-cell Cancers.</a> Cancer Research. 76 (22), 6669 (2016).</li><li>Sharkey, R. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+a+Streptavidin&#8722;Anti-Carcinoembryonic+Antigen+Antibody,+Radiolabeled+Biotin+Pretargeting+Method+for+Radioimmunotherapy+of+Colorectal+Cancer.+Studies+in+a+Human+Colon+Cancer+Xenograft+Model.">Development of a Streptavidin&#8722;Anti-Carcinoembryonic Antigen Antibody, Radiolabeled Biotin Pretargeting Method for Radioimmunotherapy of Colorectal Cancer. Studies in a Human Colon Cancer Xenograft Model.</a> Bioconjugate Chemistry. 8 (4), 595-604 (1997).</li><li>Knox, S. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phase+II+Trial+of+Yttrium-90-DOTA-Biotin+Pretargeted+by+NR-LU-10+Antibody/Streptavidin+in+Patients+with+Metastatic+Colon+Cancer.">Phase II Trial of Yttrium-90-DOTA-Biotin Pretargeted by NR-LU-10 Antibody/Streptavidin in Patients with Metastatic Colon Cancer.</a> Clinical Cancer Research. 6 (2), 406 (2000).</li><li>Schubert, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Novel+Tumor+Pretargeting+System+Based+on+Complementary+L-Configured+Oligonucleotides.">Novel Tumor Pretargeting System Based on Complementary L-Configured Oligonucleotides.</a> Bioconjugate Chemistry. 28 (4), 1176-1188 (2017).</li><li>Forero, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phase+1+Trial+of+a+Novel+Anti-CD20+Fusion+Protein+in+Pretargeted+Radioimmunotherapy+for+B-cell+Non-Hodgkin+Lymphoma.">Phase 1 Trial of a Novel Anti-CD20 Fusion Protein in Pretargeted Radioimmunotherapy for B-cell Non-Hodgkin Lymphoma.</a> Blood. 104 (1), 227 (2004).</li><li>Kalofonos, H. P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+of+Tumor+in+Patients+with+Indium-111-Labeled+Biotin+and+Streptavidin-Conjugated+Antibodies:+Preliminary+Communication.">Imaging of Tumor in Patients with Indium-111-Labeled Biotin and Streptavidin-Conjugated Antibodies: Preliminary Communication.</a> Journal of Nuclear Medicine. 31 (11), 1791-1796 (1990).</li><li>Zeglis, B. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Pretargeted+PET+Imaging+Strategy+Based+on+Bioorthogonal+Diels-Alder+Click+Chemistry.">A Pretargeted PET Imaging Strategy Based on Bioorthogonal Diels-Alder Click Chemistry.</a> Journal of Nuclear Medicine. 54 (8), 1389-1396 (2013).</li><li>Meyer, J. -. P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=18F-Based+Pretargeted+PET+Imaging+Based+on+Bioorthogonal+Diels-Alder+Click+Chemistry.">18F-Based Pretargeted PET Imaging Based on Bioorthogonal Diels-Alder Click Chemistry.</a> Bioconjugate Chemistry. 27 (2), 298-301 (2016).</li><li>Rossin, R., L&#228;ppchen, T., vanden Bosch, S. M., Laforest, R., Robillard, M. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diels-Alder+Reaction+for+Tumor+Pretargeting:+In+Vivo+Chemistry+Can+Boost+Tumor+Radiation+Dose+Compared+with+Directly+Labeled+Antibody.">Diels-Alder Reaction for Tumor Pretargeting: In Vivo Chemistry Can Boost Tumor Radiation Dose Compared with Directly Labeled Antibody.</a> Journal of Nuclear Medicine. 54 (11), 1989-1995 (2013).</li><li>Zeglis, B. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optimization+of+a+Pretargeted+Strategy+for+the+PET+Imaging+of+Colorectal+Carcinoma+via+the+Modulation+of+Radioligand+Pharmacokinetics.">Optimization of a Pretargeted Strategy for the PET Imaging of Colorectal Carcinoma via the Modulation of Radioligand Pharmacokinetics.</a> Molecular Pharmaceutics. 12 (10), 3575-3587 (2015).</li><li>Agard, N. J., Prescher, J. A., Bertozzi, C. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Strain-Promoted+[3+++2]+Azide&#8722;Alkyne+Cycloaddition+for+Covalent+Modification+of+Biomolecules+in+Living+Systems.">A Strain-Promoted [3 + 2] Azide&#8722;Alkyne Cycloaddition for Covalent Modification of Biomolecules in Living Systems.</a> Journal of the American Chemical Society. 126 (46), 15046-15047 (2004).</li><li>Blackman, M. L., Royzen, M., Fox, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tetrazine+Ligation:+Fast+Bioconjugation+Based+on+Inverse-Electron-Demand+Diels&#8722;Alder+Reactivity.">Tetrazine Ligation: Fast Bioconjugation Based on Inverse-Electron-Demand Diels&#8722;Alder Reactivity.</a> Journal of the American Chemical Society. 130 (41), 13518-13519 (2008).</li><li>Houghton, J. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Establishment+of+the+In+Vivo+Efficacy+of+Pretargeted+Radioimmunotherapy+Utilizing+Inverse+Electron+Demand+Diels-Alder+Click+Chemistry.">Establishment of the In Vivo Efficacy of Pretargeted Radioimmunotherapy Utilizing Inverse Electron Demand Diels-Alder Click Chemistry.</a> Molecular Cancer Therapeutics. 16 (1), 124-133 (2017).</li><li>Membreno, R., Cook, B. E., Fung, K., Lewis, J. S., Zeglis, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Click-Mediated+Pretargeted+Radioimmunotherapy+of+Colorectal+Carcinoma.">Click-Mediated Pretargeted Radioimmunotherapy of Colorectal Carcinoma.</a> Molecular Pharmaceutics. 15 (4), 1729-1734 (2018).</li><li>Reiner, T., Lewis, J. S., Zeglis, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Harnessing+the+Bioorthogonal+Inverse+Electron+Demand+Diels-Alder+Cycloaddition+for+Pretargeted+PET+Imaging.">Harnessing the Bioorthogonal Inverse Electron Demand Diels-Alder Cycloaddition for Pretargeted PET Imaging.</a> Journal of Visualized Experiments. (96), e52335 (2015).</li><li>Cook, B. E., Membreno, R., Zeglis, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dendrimer+Scaffold+for+the+Amplification+of+In+Vivo+Pretargeting+Ligations.">Dendrimer Scaffold for the Amplification of In Vivo Pretargeting Ligations.</a> Bioconjugate Chemistry. 29 (8), 2734-2740 (2018).</li><li>Cheal, S. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Theranostic+Pretargeted+Radioimmunotherapy+of+Colorectal+Cancer+Xenografts+in+Mice+Using+Picomolar+Affinity+Y-86-+or+Lu-177-DOTA-Bn+Binding+scFv+C825/GPA33+IgG+Bispecific+Immunoconjugates.">Theranostic Pretargeted Radioimmunotherapy of Colorectal Cancer Xenografts in Mice Using Picomolar Affinity Y-86- or Lu-177-DOTA-Bn Binding scFv C825/GPA33 IgG Bispecific Immunoconjugates.</a> European journal of nuclear medicine and molecular imaging. 43 (5), 925-937 (2016).</li><li>Meyer, J. -. P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bioorthogonal+Masking+of+Circulating+Antibody-TCO+Groups+Using+Tetrazine-Functionalized+Dextran+Polymers.">Bioorthogonal Masking of Circulating Antibody-TCO Groups Using Tetrazine-Functionalized Dextran Polymers.</a> Bioconjugate Chemistry. 29 (2), 538-545 (2018).</li><li>Houghton, J. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pretargeted+Immuno-PET+of+Pancreatic+Cancer:+Overcoming+Circulating+Antigen+and+Internalized+Antibody+to+Reduce+Radiation+Doses.">Pretargeted Immuno-PET of Pancreatic Cancer: Overcoming Circulating Antigen and Internalized Antibody to Reduce Radiation Doses.</a> Journal of Nuclear Medicine. 57 (3), 453-459 (2016).</li><li>Kein&#228;nen, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pretargeting+of+Internalizing+Trastuzumab+and+Cetuximab+with+a+(18)F-Tetrazine+Tracer+in+Xenograft+Models.">Pretargeting of Internalizing Trastuzumab and Cetuximab with a (18)F-Tetrazine Tracer in Xenograft Models.</a> European Journal of Nuclear Medicine and Molecular Imaging Research. 7, 95 (2017).</li><li>Billaud, E. M. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Micro-flow+Photosynthesis+of+New+Dienophiles+for+Inverse-Electron-Demand+Diels-Alder+Reactions.+Potential+Applications+for+Pretargeted+In+Vivo+PET+Imaging.">Micro-flow Photosynthesis of New Dienophiles for Inverse-Electron-Demand Diels-Alder Reactions. Potential Applications for Pretargeted In Vivo PET Imaging.</a> Chemical Science. 8 (2), 1251-1258 (2017).</li></ol>

The authors have nothing to disclose.

One of the strengths of this approach to in vivo pretargeting &#8212; especially in relation to strategies predicated on bispecific antibodies and radiolabeled haptens &#8212; is its modularity: trans-cyclooctene moieties can be appended to any antibody, and tetrazine radioligands can be radiolabeled with an extraordinary variety of radionuclides without impairing their ability to react with their click partners. Yet the adaptation of this approach to other antibody/antigen system is not as simple as duplicating the protocol described here. Of course, any attempt to create a new mAb-TCO immunoconjugate or a novel tetrazine-bearing radioligand should be accompanied by the appropriate chemical and biological characterization assays, including tests for stability and reactivity. But beyond this, there are two variables that are particularly important to explore and optimize: (1) the mass of mAb-TCO immunoconjugate administered and (2) the interval time between the injection of the mAb-TCO and the administration of the radioligand. Both factors can dramatically influence the in vivo behavior of the pretargeting system. For example, the use of overly high doses of immunoconjugate or interval periods that are too short can result in undesirably high activity concentrations in the blood due to click reactions between the radioligand and immunoconjugate remaining in circulation. Conversely, employing masses of immunoconjugate that are too low or overly long interval periods can needlessly reduce activity concentrations in the tumor due to a failure to saturate the antigen or the inexorable (though slow) isomerization of the trans-cyclooctene to inactive cis-cyclooctene. Along these lines, performing biodistribution experiments using a range of masses of immunoconjugate and pretargeting intervals can be extraordinarily helpful. Of course, it is also recommended that appropriate controls be run in tandem with any in vivo experiments. These include &#8212; but are not limited to &#8212; experiments featuring antigen-negative cell lines, blocking cohorts that receive a vast excess of unconjugated antibody, the administration of the radioligand alone, the injection of the radioligand following a TCO-lacking immunoconjugate, and in vivo pretargeting using a non-specific, isotype control TCO-bearing immunoconjugate.
Alternatively, imaging experiments can be used for optimization if the therapeutic radionuclide emits positrons or &#8216;imageable&#8217; photons or if an &#8216;imageable&#8217; isotopologue of the therapeutic radionuclide is available. Ultimately, the sets of variables that provide the best balance of high tumoral activity concentrations and high tumor-to-background activity concentration ratios should be selected for subsequent longitudinal therapy studies. In the case presented here, 100 &#181;g of huA33-TCO was injected with an interval of 24 h. Dosimetry calculations &#8212; particularly those that allow for the calculation of tumor doses and therapeutic ratios &#8212; can also be helpful during the process of optimization.
It is important to note that even the promising [177Lu]Lu-DOTA-PEG7-Tz/huA33-TCO system that has been developed could benefit from additional optimization. A comparison between the dosimetry data from this approach to PRIT and traditional RIT with a 177Lu-labeled variant of huA33 reveals that the tumor dose of PRIT lies below that of traditional RIT. Furthermore, the effective dose of the PRIT system (0.054 mSv/MBq) is only slightly lower than that of traditional RIT (0.068 mSv/MBq).
Two remedies to these issues are currently being explored. First, a dendritic scaffold has been developed capable of increasing the number of TCOs appended to each antibody30. In the context of pretargeted PET imaging, this approach dramatically boosts tumoral activity concentrations, and analogous experiments with [177Lu]Lu-DOTA-PEG7-Tz are underway. Second, the use of tetrazine-bearing clearing agents may be useful in the context of PRIT. The administration of clearing agents prior to the injection of the radioligand has been exploited in a variety of pretargeting methodologies as a way to decrease the concentration of residual immunoconjugate in the blood and thus reduce activity concentrations in healthy organs23,31. The use of clearing agents is not without its drawbacks, though; the most notable of which is increasing the complexity of an already admittedly complicated therapeutic modality. Nonetheless, researchers at Memorial Sloan Kettering Cancer Center recently published a compelling report on the creation a Tz-labeled dextran clearing agent for pretargeted PET imaging, and data on the use of this construct in conjunction with [177Lu]Lu-DOTA-PEG7-Tz and huA33-TCO are forthcoming32. Another approach to maximizing the dosimetric benefits of PRIT is the use of radionuclides with shorter physical half-lives. This has proven highly effective for imaging; however, therapeutic radionuclides with short physical half-lives are few and far between.
Finally, we would be remiss if we failed to properly address some of the intrinsic limitations of pretargeting based on the inverse electron demand Diels-Alder reaction. The first of these problems is inherent to all approaches to in vivo pretargeting: the antibody employed cannot be internalized upon binding to the target tissue. This, of course, is essential, as the antibody must remain accessible to the radioligand rather than sequestered in an intracellular compartment. This limitation is admittedly hard to circumvent, though it has been shown recently that antibodies with slow-to-moderate rates of internalizing can be used for in vivo pretargeting33,34. Second, the slow in vivo isomerization of reactive trans-cyclooctene to inactive cis-cyclooctene has the potential to limit the length of the interval between the administration of the TCO-bearing immunoconjugate and the injection of the radioligand. Critically, intervals of up to 120 h have still provided excellent results in the context of both pretargeted PET imaging and PRIT. However, the use of these longer intervals is almost always accompanied by slight reductions in tumoral activity concentrations, a result which may stem from this isomerization. In order to address this issue, several laboratories have attempted to create more stable trans-cyclooctenes without compromising reactivity, while others have tried to develop entirely new dienophiles capable of reacting with tetrazine35. In the coming years, it is our hope that these chemical developments will be leveraged for PRIT.
In the end, PRIT based on the inverse electron demand Diels-Alder ligation is undeniably an emergent and somewhat immature technology. However, we are nonetheless encouraged by the preclinical results we have obtained and excited for the clinical promise of this strategy. We sincerely hope that this protocol encourages others to explore and optimize this approach and thus fuel its journey from the laboratory to the clinic.

Radioimmunotherapy (RIT) &#8212; the use of antibodies for the delivery of therapeutic radionuclides to tumors &#8212; has long been an enticing approach to the treatment of cancer1,2. Indeed, this promise has been underscored by the United States Food and Drug Administration&#8217;s approval of two radioimmunoconjugates for the treatment of Non-Hodgkin&#8217;s Lymphoma: 90Y-ibritumomab tiuxetan and 131I-tositumomab3,4. Yet even from its earliest days, the clinical prospects of RIT have been hampered by a critical complication: high radiation dose rates to healthy tissues5,6. Generally speaking, radioimmunoconjugates for RIT are labeled with long-lived radionuclides (e.g., 131I [t&#189; = 8.0 days] and 90Y [t&#189; = 2.7 days]) with physical half-lives that dovetail well with the long pharmacokinetic half-lives of immunoglobulins. This is essential, as it ensures that sufficient radioactivity remains once the antibody has reached its optimal biodistribution after several days of circulation. However, this combination of long residence times in the blood and long physical half-lives inevitably results in the irradiation of healthy tissues, thereby reducing therapeutic ratios and limiting the efficacy of therapy7. Several strategies have been explored to circumvent this problem, including the use of truncated antibody fragments such as Fab, Fab&#39;, F(ab&#39;)2, minibodies, and nanobodies8,9,10. One of the most promising and fascinating, yet undeniably complex, alternative approaches is in vivo pretargeting11.
In vivo pretargeting is an approach to nuclear imaging and therapy that seeks to harness the exquisite affinity and selectivity of antibodies while skirting their pharmacokinetic drawbacks11,12,13. To this end, the radiolabeled antibody used in traditional radioimmunotherapy is deconstructed into two components: a small molecule radioligand and an immunoconjugate that can bind both a tumor antigen and the aforementioned radioligand. The immunoconjugate is injected first and given a &#8216;head start&#8217;, often several days, during which it accumulates in the target tissue and clears from the blood. Subsequently, the small molecule radioligand is administered and either combines with the immunoconjugate at the tumor or rapidly clears from the body. In essence, in vivo pretargeting relies upon performing radiochemistry within the body itself. By reducing the circulation of the radioactivity, this approach simultaneously reduces radiation doses to healthy tissues and facilitates the use of radionuclides (e.g., 68Ga, t&#189; = 68 min211; As, t&#189; = 7.2 h) with half-lives that are typically considered incompatible with antibody-based vectors.
Starting in the late 1980s, a handful of different approaches to in vivo pretargeting have been developed, including strategies based on bispecific antibodies, the interaction between streptavidin and biotin, and the hybridization of complementary oligonucleotides14,15,16,17,18. Yet each has been held back to varying degrees by complications, most famously the potent immunogenicity of streptavidin-modified antibodies19,20. Over the last five years, our group and others have developed an approach to in vivo pretargeting based on the rapid and bioorthogonal inverse electron demand Diels-Alder ligation between trans-cyclooctene (TCO) and tetrazine (Tz)21,22,23,24. The most successful of these strategies have employed a TCO-modified antibody and a Tz-bearing radioligand, as TCO is typically more stable in vivo than its Tz partner (Figure 1)25,26. As in other pretargeting methodologies, the mAb-TCO immunoconjugate is administered first and given time to clear from circulation and accumulate in tumor tissue. Subsequently, the small molecule Tz radioligand is injected, after which it either clicks with the immunoconjugate within the target tissue or clears rapidly from the body. This in vivo pretargeting strategy has proven highly effective for PET and SPECT imaging with several different antibody/antigen systems, consistently producing images with high contrast and enabling the use of short-lived radionuclides such as 18F (t&#189; = 109 min) and 64Cu (t1/2 = 12.7 h)21,22,24. More recently, the efficacy of click-based pretargeted radioimmunotherapy (PRIT) has been demonstrated in murine models of pancreatic ductal adenocarcinoma (PDAC) and colorectal carcinoma27,28. To this end, the therapeutic radionuclide 177Lu (&#946;max = 498 keV, t1/2 = 6.7 days) was employed in conjunction with two different antibodies: 5B1, which targets carbohydrate antigen 19.9 (CA19.9) ubiquitously expressed in PDAC, and huA33, which targets A33, a transmembrane glycoprotein expressed in &#62;95% of colorectal cancers. In both cases, this approach to 177Lu-PRIT yielded high activity concentrations in tumor tissue, created a dose-dependent therapeutic effect, and simultaneously reduced activity concentrations in healthy tissues compared to traditional directly-labeled radioimmunoconjugates.
In this article, we describe protocols for PRIT using a 177Lu-DOTA-labeled tetrazine radioligand ([177Lu]Lu-DOTA-PEG7-Tz) and a TCO-modified variant of the huA33 antibody (huA33-TCO). More specifically, we describe the construction of huA33-TCO (Figure 2), the synthesis and radiolabeling of [177Lu]Lu-DOTA-PEG7-Tz (Figure 3 and Figure 4), and the performance of in vivo biodistribution and longitudinal therapy studies in murine models of colorectal carcinoma. Furthermore, in the representative results and discussion, we present a sample data set, address possible strategies for the optimization of this approach, and consider this strategy in the wider context of in vivo pretargeting and PRIT. Finally, it is important to note that while we have chosen to focus on pretargeting using huA33-TCO and [177Lu]Lu-DOTA-PEG7-Tz in this protocol, this strategy is highly modular and can be adapted to suit a wide range of antibodies and radionuclides.

<table>
	<tbody>
		<tr>
			<td>(E)-Cyclooct-4-enyl 2,5-dioxo-1-pyrrolidinyl carbonate (TCO-NHS)</td>
			<td>Sigma-Aldrich</td>
			<td>764523</td>
			<td>Store at -80 &#176;C</td>
		</tr>
		<tr>
			<td>2,5-Dioxo-1-pyrrolidinyl 5-[4-(1,2,4,5-tetrazin-3-yl)benzylamino]-5-oxopentanoate (Tz-NHS)</td>
			<td>Sigma-Aldrich</td>
			<td>764701</td>
			<td>Store at -80 &#176;C</td>
		</tr>
		<tr>
			<td>Acetonitrile (MeCN)</td>
			<td>Fisher Scientific</td>
			<td>A998-4</td>
			<td></td>
		</tr>
		<tr>
			<td>Ammonium Acetate (NH4OAc)</td>
			<td>Fisher Scientific</td>
			<td>A639-500</td>
			<td></td>
		</tr>
		<tr>
			<td>Boc-PEG7-amine (O-(2-Aminoethyl)-O&#8242;-[2-(Boc-amino)ethyl]hexaethylene glycol)</td>
			<td>Sigma-Aldrich</td>
			<td>70023</td>
			<td>Store at -20 &#176;C</td>
		</tr>
		<tr>
			<td>Dichloromethane (DCM)</td>
			<td>Fisher Scientific</td>
			<td>D143-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Dimethyl sulfoxide (DMSO), anhydrous</td>
			<td>Fisher Scientific</td>
			<td>D12345</td>
			<td></td>
		</tr>
		<tr>
			<td>EMD Millipore Amicon Ultra-2 Centrifugal Filter Unit</td>
			<td>Fisher Scientific</td>
			<td>UFC205024</td>
			<td></td>
		</tr>
		<tr>
			<td>GE Healthcare Disposable PD-10 Desalting Columns</td>
			<td>Fisher Scientific</td>
			<td>45-000-148</td>
			<td></td>
		</tr>
		<tr>
			<td>N,N-Dimethylformamide (DMF), anhydrous</td>
			<td>Fisher Scientific</td>
			<td>AC610941000</td>
			<td></td>
		</tr>
		<tr>
			<td>Phosphate Buffered Saline (PBS)</td>
			<td>Fisher Scientific</td>
			<td>70-011-044</td>
			<td>10x Concentrated</td>
		</tr>
		<tr>
			<td>p-SCN-Bn-DOTA</td>
			<td>Macrocyclics</td>
			<td>B-205</td>
			<td>Store at -20 &#176;C</td>
		</tr>
		<tr>
			<td>Triethylamine (TEA)</td>
			<td>Fisher Scientific</td>
			<td>AC157911000</td>
			<td></td>
		</tr>
		<tr>
			<td>Trifluoroacetic Acid (TFA)</td>
			<td>Fisher Scientific</td>
			<td>A116-50</td>
			<td></td>
		</tr>
		<tr>
			<td>Tumor measuring device</td>
			<td>Peira TM900</td>
			<td>Peira TM900</td>
			<td></td>
		</tr>
	</tbody>
</table>

pretargeted radioimmunotherapy based on the inverse electron demand diels-alder reaction

While radioimmunotherapy (RIT) is a promising approach for the treatment of cancer, the long pharmacokinetic half-life of radiolabeled antibodies can result in high radiation doses to healthy tissues. Perhaps not surprisingly, several different strategies have been developed to circumvent this troubling limitation. One of the most promising of these approaches is pretargeted radioimmunotherapy (PRIT). PRIT is predicated on decoupling the radionuclide from the immunoglobulin, injecting them separately, and then allowing them to combine in vivo at the target tissue. This approach harnesses the exceptional tumor-targeting properties of antibodies while skirting their pharmacokinetic drawbacks, thereby lowering radiation doses to non-target tissues and facilitating the use of radionuclides with half-lives that are considered too short for use in traditional radioimmunoconjugates. Over the last five years, our laboratory and others have developed an approach to in vivo pretargeting based on the inverse electron-demand Diels-Alder (IEDDA) reaction between trans-cyclooctene (TCO) and tetrazine (Tz). This strategy has been successfully applied to pretargeted positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging with a variety of antibody-antigen systems. In a pair of recent publications, we have demonstrated the efficacy of IEDDA-based PRIT in murine models of pancreatic ductal adenocarcinoma and colorectal carcinoma. In this protocol, we describe protocols for PRIT using a 177Lu-DOTA-labeled tetrazine radioligand ([177Lu]Lu-DOTA-PEG7-Tz) and a TCO-modified variant of the colorectal cancer targeting huA33 antibody (huA33-TCO). More specifically, we will describe the construction of huA33-TCO, the synthesis and radiolabeling of [177Lu]Lu-DOTA-PEG7-Tz, and the performance of in vivo biodistribution and longitudinal therapy studies in murine models of colorectal carcinoma.

The conjugation of TCO to huA33 is predicated on the coupling between the amine-reactive TCO-NHS and the lysine residues on the surface of the immunoglobulin. This method is highly robust and reproducible and reliably yields a degree-of-labeling of 2-4 TCO/mAb. In this case, MALDI-ToF mass spectrometry was employed to confirm a degree of labeling of approximately 4.0 TCO/mAb; a similar value was obtained using a fluorophore-modified tetrazine as a reporter24. The synthesis of the tetrazine ligand is performed in three steps: (1) the coupling of Tz-NHS to a mono-Boc-protected PEG linker (2) the deprotection of this intermediate to yield Tz-PEG7-NH2, and (3) the formation of a thiourea linkage between p-SCN-Bn-DOTA and Tz-PEG7-NH2. This procedure is relatively facile and affords Tz-PEG7-DOTA in an overall yield of ~75%. Each of the intermediates has been characterized by HRMS and 1H-NMR; this data is presented in Table 1.
Moving on to the radiolabeling, 177Lu3+ is typically obtained from commercial suppliers as a chloride salt [177Lu]LuCl3 in 0.5 M HCl. The radiolabeling of Tz-PEG7-DOTA with 177Lu to yield the radioligand [177Lu]Lu-DOTA-PEG7-Tz is very straightforward: in just 20 min, the reaction is complete, producing the desired product in &#62;99% radiochemical purity as determined by radio-iTLC. Typically, no further purification is necessary prior to formulation. A survey of the literature on Tz/TCO-based pretargeting suggests that a Tz:mAb molar ratio of ~1:1 produces the best in vivo data10. As a result, it is not essential to obtain the radioligand in the highest possible molar activity. For example, biodistribution experiments discussed here employ [177Lu]Lu-DOTA-PEG7-Tz with a molar activity of ~12 GBq/&#181;mol. For longitudinal therapy studies, in contrast, [177Lu]Lu-DOTA-PEG7-Tz with higher molar activity was used in order to facilitate the administration of larger doses of radioactivity without changing the number of injected moles of tetrazine.
As will be addressed further in the discussion, biodistribution experiments are of paramount importance to understanding and optimizing any approach to PRIT. In this case, biodistribution experiments were conducted to determine the optimal interval time between the administration of the immunoconjugate and the injection of the radioligand. To this end, we employed athymic nude mice bearing subcutaneous A33 antigen-expressing SW1222 xenografts on their right shoulder. These animals received 100 &#181;g (0.67 nmol) of huA33-TCO 24, 48, 72, or 120 h prior to the injection of [177Lu]Lu-DOTA-PEG7-Tz (9.14 MBq, 0.74 nmol). Figure 5 shows that all injection intervals produce high activity concentrations in the tumor tissue as well as low activity concentrations in healthy organs. The 24 h injection interval affords the highest tumoral uptake at 120 h post-injection: 21.2 &#177; 2.9%ID/g. Each set of conditions also produces impressive tumor-to-organ activity concentration ratios. Pretargeting with a 24 h interval, for example, yields tumor-to-blood, tumor-to-liver, and tumor-to-muscle ratios of 20 &#177; 5, 37 &#177; 7, and 184 &#177; 30, respectively, 120 h after the administration of the radioligand. Based on these findings, a 24 h interval was chosen for the subsequent longitudinal therapy study (see below).
For the in vivo longitudinal therapy study, cohorts (n = 10) of athymic nude mice bearing subcutaneous SW1222 xenografts on their right flank were administered huA33-TCO (100 ug, 0.67 nmol) 24 hours prior to injection of [177Lu]Lu-DOTA-PEG7-Tz. Three different experimental cohorts were employed, receiving 18.7, 37, or 55.5 MBq of [177Lu]Lu-DOTA-PEG7-Tz (corresponding to molar activities of 24, 45, and 70 GBq/&#181;mol). In addition, two control cohorts received one half of the PRIT regimen: either huA33-TCO (100 ug, 0.67 nmol) without [177Lu]Lu-DOTA-PEG7-Tz or [177Lu]Lu-DOTA-PEG7-Tz (55.5 MBq, 0.74 nmol) without huA33-TCO. These are essential controls to ensure that the therapeutic response is not elicited by either the immunoconjugate or radioligand alone. The volumes of the tumors were measured every 3 days for the first three weeks of the study and then once per week until the conclusion of the experiment (70 days, 10 half-lives of 177Lu). As seen in Figure 6, there is a stark difference in the response of the experimental cohorts compared to the control groups. While the tumors in the mice receiving only one component of the PRIT strategy continue to grow unchecked, the tumors of the mice receiving the full PRIT regimen stop growing and ultimately shrink to volumes well below those measured at the beginning of the study. Importantly, no toxic side effects were observed, and all animals maintained a weight within 20% of their initial mass (Figure 7A). A Kaplan-Meier plot of the data provides an even more striking visualization of the study: while all mice in the control cohorts had to be euthanized within a few weeks, the mice of the experimental cohorts had a perfect record of survival at the end of the investigation (Figure 7B).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59041/59041fig1.jpg" /> 
Figure 1: Cartoon schematic of pretargeted radioimmunotherapy based on the inverse electron demand Diels-Alder reaction. This figure has been modified from reference #28. Reprinted (adapted) with permission from Membreno, R., Cook, B. E., Fung, K., Lewis, J. S., &#38; Zeglis, B. M. Click-Mediated Pretargeted Radioimmunotherapy of Colorectal Carcinoma. Molecular Pharmaceutics. 15 (4), 1729-1734 (2018). Copyright 2018 American Chemical Society. <a href="https://www.jove.com/files/ftp_upload/59041/59041fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59041/59041fig2.jpg" /> 
Figure 2: Schematic of the construction of huA33-TCO. <a href="https://www.jove.com/files/ftp_upload/59041/59041fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59041/59041fig3.jpg" /> 
Figure 3: Schematic of the synthesis of Tz-PEG7-DOTA. <a href="https://www.jove.com/files/ftp_upload/59041/59041fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59041/59041fig4.jpg" /> 
Figure 4: (A) Schematic of the radiolabeling of [ 177Lu]Lu-DOTA-PEG7-Tz; (B) Representative radio-iTLC chromatogram demonstrating the &#62;98% radiochemical purity of [177Lu]Lu-DOTA-PEG7-Tz. <a href="https://www.jove.com/files/ftp_upload/59041/59041fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/59041/59041fig5.jpg" /> 
Figure 5: Biodistribution in of in vivo pretargeting with huA33-TCO and [177Lu]-DOTA-PEG7-Tz in athymic nude mice bearing subcutaneous SW1222 human colorectal cancer tumors using pretargeting intervals of 24 (purple), 48 (green), 72 (orange), or 120 (blue) hours. Data with standard errors from cohorts of n = 4; Statistical analysis was performed by an unpaired Student&#39;s t-test, **p &#60; 0.01. This figure has been modified from reference #28. Reprinted (adapted) with permission from Membreno, R., Cook, B. E., Fung, K., Lewis, J. S., &#38; Zeglis, B. M. Click-Mediated Pretargeted Radioimmunotherapy of Colorectal Carcinoma. Molecular Pharmaceutics. 15 (4), 1729-1734 (2018). Copyright 2018 American Chemical Society. <a href="https://www.jove.com/files/ftp_upload/59041/59041fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/59041/59041fig6.jpg" /> 
Figure 6: Longitudinal therapy study of 5 groups of mice (n = 10 each) bearing subcutaneous SW1222 tumors depicted in average tumor volume as a function of time (A); and tumor volume normalized to initial volume as a function of time (B). The control groups received either the immunoconjugate without the radioligand (blue) or the radioligand without the immunoconjugate (red). The three treatment groups received huA33-TCO (100 &#181;g, 0.6 nmol) followed 24 h later by either 18.5 (green), 37.0 (purple), or 55.5 (orange) MBq (~0.8 nmol in each case) of [177Lu]-DOTA-PEG7-Tz. By log-rank (Mantel-Cox) test, survival was significant (p &#60; 0.0001) for all treatment groups. This figure has been modified from reference #28. Reprinted (adapted) with permission from Membreno, R., Cook, B. E., Fung, K., Lewis, J. S., &#38; Zeglis, B. M. Click-Mediated Pretargeted Radioimmunotherapy of Colorectal Carcinoma. Molecular Pharmaceutics. 15 (4), 1729-1734 (2018). Copyright 2018 American Chemical Society. <a href="https://www.jove.com/files/ftp_upload/59041/59041fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/59041/59041fig7.jpg" /> 
Figure 7: Weight curves for animals during the longitudinal therapy study of 5 groups of mice (n = 10 each) bearing subcutaneous SW1222 tumors (A); the corresponding Kaplan-Meier survival curve (B). The control groups received either the immunoconjugate without the radioligand (blue) or the radioligand without the immunoconjugate (red). The three treatment groups received huA33-TCO (100 &#181;g, 0.6 nmol) followed 24 h later by either 18.5 (green), 37.0 (purple), or 55.5 (orange) MBq (~0.8 nmol in each case) of [177Lu]-DOTA-PEG7-Tz. This figure has been modified from reference #28. Reprinted (adapted) with permission from Membreno, R., Cook, B. E., Fung, K., Lewis, J. S., &#38; Zeglis, B. M. Click-Mediated Pretargeted Radioimmunotherapy of Colorectal Carcinoma. Molecular Pharmaceutics. 15 (4), 1729-1734 (2018). Copyright 2018 American Chemical Society. <a href="https://www.jove.com/files/ftp_upload/59041/59041fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td rowspan="2">Compound</td>
			<td>1H-NMR Shifts</td>
			<td rowspan="2">HRMS (ESI)</td>
		</tr>
		<tr>
			<td>500 MHz, DMSO</td>
		</tr>
		<tr>
			<td>Tz-PEG7-NHBoc</td>
			<td>10.52 (s, 1H), 8.50 (m, 3H), 7.82 (t, 1H), 7.46 (d, 2H), 6.69 (t, 1H), 4.33 (d, 2H), 3.42 (m, 22H), 3.33 (t, 2H), 3.31 (t, 2H), 3.12 (q, 2H), 2.99 (q, 2H), 2.12 (t, 2H), 2.03 (t, 2H), 2.12 (t, 2H), 1.70 (q, 2H), 1.29 (s, 9H)</td>
			<td>m/z calcd. for C35H57N7O11Na: 774.4005; found: 774.4014.</td>
		</tr>
		<tr>
			<td>Tz-PEG7-NH2</td>
			<td>10.58 (s, 1H), 8.46 (m, 2H), 7.87 (t, 1H), 7.75 (d, 2H), 7.52 (d, 1H), 4.40 (d, 2H), 3.60-3.50 (m, 26H), 3.40 (t, 2H), 3.32 (bs, 2H), 3.20 (q, 2H), 2.99 (bs, 2H), 2.19 (t, 2H), 2.12 (t, 2H), 1.79 (q, 2H).</td>
			<td>m/z calcd. for C30H50N7O9: 652.3670; found: 652.3676.</td>
		</tr>
		<tr>
			<td>Tz-PEG7-DOTA</td>
			<td>10.57 (s, 1H), 9.63 (bs, 1H), 8.45 (m, 3H), 7.86 (m, 1H), 7.73 (bs, 1H), 7.54 (d, 2H), 7.41 (m, 2H), 7.19 (m, 2H), 6.54 (bs, 1H),&#160; 4.40 (d, 2H), 4.00-3.20 (m, 55H), 3.20 (q, 4H), 2.54 (s, 1H), 2.18 (t, 3H), 2.10 (t, 3H), 1.76 (q, 2H).</td>
			<td>m/z calcd. for C50H76N11O15S: 1202.56; found: 1203.5741.</td>
		</tr>
	</tbody>
</table>
Table 1. Characterization data for the organic compounds described in this protocol.

Watch this Scientific Journal Video about Pretargeted Radioimmunotherapy Based on the Inverse Electron Demand Diels-Alder Reaction at JoVE.com

Pretargeted Radioimmunotherapy Based on the Inverse Electron Demand Diels-Alder Reaction

This protocol describes the synthesis and characterization of a trans-cyclooctene (TCO)-modified antibody and a 177Lu-labeled tetrazine (Tz) radioligand for pretargeted radioimmunotherapy (PRIT). In addition, it details the use of these two constructs for in vivo biodistribution and longitudinal therapy studies in a murine model of colorectal cancer.

While radioimmunotherapy (RIT) is a promising approach for the treatment of cancer, the long pharmacokinetic half-life of radiolabeled antibodies can ...

pretargeted-radioimmunotherapy-based-on-inverse-electron-demand-diels

Nanyang Technological University

Research

JoVE Journal

Chemistry

10.0K Views.  Hunter College of the City University of New York. This protocol describes the synthesis and characterization of a trans-cyclooctene (TCO)-modified antibody and a 177Lu-labeled tetrazine (Tz) radioligand for pretargeted radioimmunotherapy (PRIT). In addition, it details the use of these two constructs for in vivo biodistribution and longitudinal therapy studies in a murine model of colorectal cancer.

Video: Pretargeted Radioimmunotherapy Based on the Inverse Electron Demand Diels-Alder Reaction

Microwave-assisted Intramolecular Dehydrogenative Diels-Alder Reactions for the Synthesis of Functionalized Naphthalenes/Solvatochromic Dyes

Functionalized naphthalenes have applications in a variety of research fields ranging from the synthesis of natural or biologically active molecules to the preparation of new organic dyes. Although numerous strategies have been reported to access naphthalene scaffolds, many procedures still present limitations in terms of incorporating functionality, which in turn narrows the range of available substrates. The development of versatile methods for direct access to substituted naphthalenes is therefore highly desirable.
The Diels-Alder (DA) cycloaddition reaction is a powerful and attractive method for the formation of saturated and unsaturated ring systems from readily available starting materials. A new microwave-assisted intramolecular dehydrogenative DA reaction of styrenyl derivatives described herein generates a variety of functionalized cyclopenta[b]naphthalenes that could not be prepared using existing synthetic methods. When compared to conventional heating, microwave irradiation accelerates reaction rates, enhances yields, and limits the formation of undesired byproducts.
The utility of this protocol is further demonstrated by the conversion of a DA cycloadduct into a novel solvatochromic fluorescent dye via a Buchwald-Hartwig palladium-catalyzed cross-coupling reaction. Fluorescence spectroscopy, as an informative and sensitive analytical technique, plays a key role in research fields including environmental science, medicine, pharmacology, and cellular biology. Access to a variety of new organic fluorophores provided by the microwave-assisted dehydrogenative DA reaction allows for further advancement in these fields.

Microwave-assisted intramolecular dehydrogenative Diels-Alder (DA) reactions provide concise access to functionalized cyclopenta[b]naphthalene building blocks. The utility of this methodology is demonstrated by one-step conversion of the dehydrogenative DA cycloadducts into novel solvatochromic fluorescent dyes via Buchwald-Hartwig palladium-catalyzed cross-coupling reactions.

Functionalized naphthalenes have applications in a variety of research fields ranging from the synthesis of natural or biologically active molecules to ...

Determining the Ice-binding Planes of Antifreeze Proteins by Fluorescence-based Ice Plane Affinity

Antifreeze proteins (AFPs) are expressed in a variety of cold-hardy organisms to prevent or slow internal ice growth. AFPs bind to specific planes of ice through their ice-binding surfaces. Fluorescence-based ice plane affinity (FIPA) analysis is a modified technique used to determine the ice planes to which the AFPs bind. FIPA is based on the original ice-etching method for determining AFP-bound ice-planes. It produces clearer images in a shortened experimental time. In FIPA analysis, AFPs are fluorescently labeled with a chimeric tag or a covalent dye then slowly incorporated into a macroscopic single ice crystal, which has been preformed into a hemisphere and oriented to determine the a- and c-axes. The AFP-bound ice hemisphere is imaged under UV light to visualize AFP-bound planes using filters to block out nonspecific light. Fluorescent labeling of the AFPs allows real-time monitoring of AFP adsorption into ice. The labels have been found not to influence the planes to which AFPs bind. FIPA analysis also introduces the option to bind more than one differently tagged AFP on the same single ice crystal to help differentiate their binding planes. These applications of FIPA are helping to advance our understanding of how AFPs bind to ice to halt its growth and why many AFP-producing organisms express multiple AFP isoforms.

Antifreeze proteins (AFPs) bind to specific planes of ice to prevent or slow ice growth. Fluorescence-based ice plane affinity (FIPA) analysis is a modification of the original ice-etching method for determination of AFP-bound ice planes. AFPs are fluorescently labeled, incorporated into macroscopic single ice crystals, and visualized under UV light.

Antifreeze proteins (AFPs) are expressed in a variety of cold-hardy organisms to prevent or slow internal ice growth. AFPs bind to specific planes of ice ...

Amide Coupling Reaction for the Synthesis of Bispyridine-based Ligands and Their Complexation to Platinum as Dinuclear Anticancer Agents

Amide coupling reactions can be used to synthesize bispyridine-based ligands for use as bridging linkers in multinuclear platinum anticancer drugs. Isonicotinic acid, or its derivatives, are coupled to variable length diaminoalkane chains under an inert atmosphere in anhydrous DMF or DMSO with the use of a weak base, triethylamine, and a coupling agent, 1-propylphosphonic anhydride. The products precipitate from solution upon formation or can be precipitated by the addition of water. If desired, the ligands can be further purified by recrystallization from hot water. Dinuclear platinum complex synthesis using the bispyridine ligands is done in hot water using transplatin. The most informative of the chemical characterization techniques to determine the structure and gross purity of both the bispyridine ligands and the final platinum complexes is 1H NMR with particular analysis of the aromatic region of the spectra (7-9 ppm). The platinum complexes have potential application as anticancer agents and the synthesis method can be modified to produce trinuclear and other multinuclear complexes with different hydrogen bonding functionality in the bridging ligand.

This protocol describes the use of amide coupling reactions of isonicotinic acid and diaminoalkanes to form bridging ligands suitable for use in the synthesis of multinuclear platinum complexes, which combine aspects of the anticancer drugs BBR3464 and picoplatin.

Amide coupling reactions can be used to synthesize bispyridine-based ligands for use as bridging linkers in multinuclear platinum anticancer drugs. ...

An Inverse Analysis Approach to the Characterization of Chemical Transport in Paints

The ability to directly characterize chemical transport and interactions that occur within a material (i.e., subsurface dynamics) is a vital component in understanding contaminant mass transport and the ability to decontaminate materials. If a material is contaminated, over time, the transport of highly toxic chemicals (such as chemical warfare agent species) out of the material can result in vapor exposure or transfer to the skin, which can result in percutaneous exposure to personnel who interact with the material. Due to the high toxicity of chemical warfare agents, the release of trace chemical quantities is of significant concern. Mapping subsurface concentration distribution and transport characteristics of absorbed agents enables exposure hazards to be assessed in untested conditions. Furthermore, these tools can be used to characterize subsurface reaction dynamics to ultimately design improved decontaminants or decontamination procedures. To achieve this goal, an inverse analysis mass transport modeling approach was developed that utilizes time-resolved mass spectroscopy measurements of vapor emission from contaminated paint coatings as the input parameter for calculation of subsurface concentration profiles. Details are provided on sample preparation, including contaminant and material handling, the application of mass spectrometry for the measurement of emitted contaminant vapor, and the implementation of inverse analysis using a physics-based diffusion model to determine transport properties of live chemical warfare agents including distilled mustard (HD) and the nerve agent VX.

In this paper, a procedure for quantifying the mass transport parameters of chemicals in various materials is presented. This process involves employing an inverse-analysis based diffusion model to vapor emission profiles recorded by real-time, mass spectrometry in high vacuum.

The ability to directly characterize chemical transport and interactions that occur within a material (i.e., subsurface dynamics) is a vital component in ...

1,3,5-Triphenylbenzene and Corannulene as Electron Receptors for Lithium Solvated Electron Solutions

The authors report on conductivity studies carried out on lithium solvated electron solutions (LiSES) prepared using two types of polyaromatic hydrocarbons (PAH), namely 1,3,5-triphenylbenzene and corannulene, as electron receptors. The solid PAHs were first dissolved in tetrahydrofuran (THF) to form a solution. Metallic lithium was then dissolved into these PAH/THF solutions to yield either blue or greenish blue solutions, colors which are indicative of the presence of solvated electrons. Conductivity measurements at ambient temperature carried out on 1,3,5-triphenylbenzene-based LiSES, denoted by LixTPB(THF)24.7 (x = 1, 2, 3, 4), showed an increase of conductivity with increase of Li:PAH ratio from x = 1 to 2. However, the conductivity gradually decreased upon further increasing the ratio. Indeed the conductivity of LixTPB(THF)24.7 for x = 4 is even lower than for x = 1. Such behavior is similar to that of the previously reported LiSES prepared from biphenyl and naphthalene. Conductivity versus temperature measurements on corannulene-based LiSES, denoted by LixCor(THF)247 (x = 1, 2, 3, 4, 5), showed linear relationships with negative slopes, indicating a metallic behavior similar to biphenyl and naphthalene-based LiSES.

The authors report on conductivity studies carried out on lithium solvated electron solutions (LiSES) prepared using 1,3,5-triphenylbenzene (TPB) and corannulene as electron receptors.

The authors report on conductivity studies carried out on lithium solvated electron solutions (LiSES) prepared using two types of polyaromatic ...

The Preparation and Properties of Thermo-reversibly Cross-linked Rubber Via Diels-Alder Chemistry

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ethylene-propylene rubber, grafted with maleic anhydride, is thermo-reversibly cross-linked in two steps. The pending anhydride moieties are first modified with furfurylamine to graft furan groups to the rubber backbone. These pendant furan groups are then cross-linked with a bis-maleimide via a Diels-Alder coupling reaction. Both reactions can be performed under a broad range of experimental conditions and can easily be applied on a large scale. The material properties of the resulting Diels-Alder cross-linked rubbers are similar to a peroxide-cured ethylene/propylene/diene rubber (EPDM) reference. The cross-links break at elevated temperatures (&#62; 150 &#176;C) via the retro-Diels-Alder reaction and can be reformed by thermal annealing at lower temperatures (50-70 &#176;C). Reversibility of the system was proven with infrared spectroscopy, solubility tests and mechanical properties. Recyclability of the material was also shown in a practical way, i.e., by cutting a cross-linked sample into small parts and compression molding them into new samples displaying comparable mechanical properties, which is not possible for conventionally cross-linked rubbers.

A simple two-step approach involving rubber modification and cross-linking yields fully reworkable, elastic rubber products.

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ...

A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. Conducting the reduction step in a microcentrifuge tube or small volume conical tube facilitates a proposed nucleation, growth, densification, fusion, equilibration model for gel formation, with final gel geometry smaller than the initial reaction volume. This method takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step, and as a consequence of reagent concentrations. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. After rinsing and freeze drying, the resulting aerogel structure is examined with scanning electron microscopy, X-ray diffractometry, and nitrogen gas adsorption. The synthesis method and characterization techniques result in a close correspondence of aerogel ligament sizes. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

A rapid, direct solution-based reduction synthesis method to obtain Au, Pd, and Pt aerogels is presented.

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various ...

Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot electrons upon SPR irradiation drive reductive deposition of Pd on colloidal AuNR in the presence of [PdCl4]2-. Plasmon-driven reduction of secondary metals potentiates covalent, sub-wavelength deposition at targeted locations coinciding with electric field &#8220;hot-spots&#8221; of the plasmonic substrate using an external field (e.g., laser). The process described herein details a solution-phase deposition of a catalytically-active noble metal (Pd) from a transition metal halide salt (H2PdCl4) onto aqueously-suspended, anisotropic plasmonic structures (AuNR). The solution-phase process is amenable to making other bimetallic architectures. Transmission UV-vis monitoring of the photochemical reaction, coupled with ex situ XPS and statistical TEM analysis, provide immediate experimental feedback to evaluate properties of the bimetallic structures as they evolve during the photocatalytic reaction. Resonant plasmon irradiation of AuNR in the presence of [PdCl4]2- creates a thin, covalently-bound Pd0 shell without any significant dampening effect on its plasmonic behavior in this representative experiment/batch. Overall, plasmonic photodeposition offers an alternative route for high-volume, economical synthesis of optoelectronic materials with sub-5 nm features (e.g., heterometallic photocatalysts or optoelectronic interconnects).

A protocol for anisotropic photodeposition of Pd onto aqueously-suspended Au nanorods via localized surface plasmon excitation is presented.

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot ...

A Salt-Templated Synthesis Method for Porous Platinum-based Macrobeams and Macrotubes

The synthesis of high surface area porous noble metal nanomaterials generally relies on time consuming coalescence of pre-formed nanoparticles, followed by rinsing and supercritical drying steps, often resulting in mechanically fragile materials. Here, a method to synthesize nanostructured porous platinum-based macrotubes and macrobeams with a square cross section from insoluble salt needle templates is presented. The combination of oppositely charged platinum, palladium, and copper square planar ions results in the rapid formation of insoluble salt needles. Depending on the stoichiometric ratio of metal ions present in the salt-template and the choice of chemical reducing agent, either macrotubes or macrobeams form with a porous nanostructure comprised of either fused nanoparticles or nanofibrils. Elemental composition of the macrotubes and macrobeams, determined with x-ray diffractometry and x-ray photoelectron spectroscopy, is controlled by the stoichiometric ratio of metal ions present in the salt-template. Macrotubes and macrobeams may be pressed into free standing films, and the electrochemically active surface area is determined with electrochemical impedance spectroscopy and cyclic voltammetry. This synthesis method demonstrates a simple, relatively fast approach to achieve high-surface area platinum-based macrotubes and macrobeams with tunable nanostructure and elemental composition that may be pressed into free-standing films with no required binding materials.

A synthesis method to obtain porous platinum-based macrotubes and macrobeams with a square cross section through chemical reduction of insoluble salt-needle templates is presented.

The synthesis of high surface area porous noble metal nanomaterials generally relies on time consuming coalescence of pre-formed nanoparticles, followed ...

Quantitative Atomic-Site Analysis of Functional Dopants/Point Defects in Crystalline Materials by Electron-Channeling-Enhanced Microanalysis

A novel elemental and chemical analysis scheme based on electron-channeling phenomena in crystalline materials is introduced, where the incident high-energy electron beam is rocked with the submicrometric pivot point fixed on a specimen. This method enables us to quantitatively derive the site occupancies and site-dependent chemical information of impurities or intentionally doped functional elements in a specimen, using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy attached to a scanning transmission electron microscope, which is of significant interest to current materials science, particularly related to nanotechnologies. This scheme is applicable to any combination of elements even when the conventional Rietveld analysis by X-ray or neutron diffraction occasionally fails to provide the desired results because of limited sample sizes and close scattering factors of neighboring elements in the periodic table. In this methodological article, we demonstrate the basic experimental procedure and analysis method of the present beam-rocking microanalysis.

We provide a general outline of quantitative microanalysis methods for estimating the site occupancies of impurities and their chemical states by taking advantage of electron-channeling phenomena under incident electron beam-rocking conditions, which reliably extract information from minority species, light elements, oxygen vacancies, and other point/line/planar defects.