The authors thank Prof. Thomas Reiner, Dr. Jacob Houghton, and Dr. Serge Lyaschenko for helpful conversations.&#160;

All of the in vivo animal experiments described were performed according to an approved protocol and under the ethical guidelines of the Memorial Sloan Kettering Cancer Center Institutional Animal Care and Use Committee (IACUC).
1. Conjugation of DFO-NCS to J591
<ol>
	<li>In a 1.7 ml microcentrifuge tube, prepare a 2-5 mg/ml solution of J591 in 1 ml of either 1x phosphate-buffered saline (pH 7.4) or 0.5 M HEPES buffer (pH 7.4).</li>
	<li>Dissolve DFO-NCS in dry DMSO at a concentrati...

While the construction, radiolabeling, and imaging of 89Zr-DFO-labled radioimmunoconjugates is generally a rather straightforward procedure, it is important to keep a few key considerations in mind during each step of the process. For example, perhaps the most likely cause for concern during the conjugation step of the procedure is the aggregation of the antibody during the conjugation reaction. This problem is most often a product of poor mixing of the conjugation reaction after the addition of the DFO-NCS st...

Due to their remarkable sensitivity, affinity, and selectivity, antibodies have long been considered promising vectors for the delivery of radioisotopes to cancer cells. However, their application in positron emission tomography (PET) imaging has been hampered by the lack of a suitable positron-emitting radioisotope for their labeling.1-3 One of the most critical considerations in the design of radioimmunoconjugates is matching the physical decay half-life of the radioisotope to the in vivo pharmacokinetics of the antibody. More specifically, antibodies often have relatively long, multi-day biological half-lives and therefore must be labeled with radioisotopes with comparable physical half-lives. For PET imaging applications, antibodies have traditionally been radiolabeled with 64Cu (t1/2 = 12.7 hr), 86Y (t1/2 = 14.7 hr), or 124I (t1/2 = 4.18 d).4,5 However, each of these radioisotopes possesses significant limitations that hamper their suitability for clinical imaging. While radioimmunoconjugates labeled with 86Y and 64Cu have proven promising in preclinical investigations, both isotopes possess physical half-lives that are too short to be effective for imaging in humans. 124I, in contrast, has a nearly ideal physical half-life for imaging with antibodies, but it is expensive and has suboptimal decay characteristics that lead to relatively low resolution clinical images. Furthermore, 124I-labeled radioimmunoconjugates can be subject to dehalogenation in vivo, a process which can lower tumor-to-background activity ratios.6,7
The drive to find a positron-emitting radioisotope to supplant 64Cu, 86Y, and 124I in radioimmunoconjugates has fueled the recent surge in research on 89Zr-labeled antibodies.8-12 The reason for the advent of 89Zr is straightforward: the radiometal possesses near-ideal chemical and physical properties for use in diagnostic PET radioimmunoconjugates.13 89Zr is produced via the 89Y(p,n)89Zr reaction on a cyclotron using a commercially available and 100% naturally abundant 89Y target.14,15 The radiometal has a positron yield of 23%, decays with a half-life of 78.4 hr, and emits positrons with the relatively low energy of 395.5 keV (Figure 1).13,16,17 It is important to note that 89Zr also emits a high energy, 909 keV &#947;-ray with 99% efficiency. While this emission does not interfere energetically with the emitted 511 keV photons, it does require extra consideration with regard to transport, handling, and dosimetry. Despite this caveat, these decay characteristics ultimately mean that 89Zr not only has a more favorable half-life for imaging with antibodies than 86Y and 64Cu but can also produce higher resolution images than 124I, which emits positrons with higher energies of 687 and 975 keV as well as a number of photons with energies within 100-150 keV of the 511 keV positron-created photons.13 Moreover, 89Zr is also safer to handle, less expensive to produce, and residualizes in tumors more effectively than its radioiodine counterpart.18,19 One potential limitation of 89Zr is that it does not have a therapeutic isotopologue, e.g., 86Y (PET) vs. 90Y (therapy). This precludes the construction of chemically identical, surrogate imaging agents that can be employed as dosimetric scouts for their therapeutic counterparts. That said, investigations suggest that 89Zr-labeled antibodies do have potential as imaging surrogates for 90Y- and 177Lu-labeled immunoconjugates.20,21
From a chemical standpoint, as a Group IV metal, 89Zr exists as a +4 cation in aqueous solution. The Zr4+ ion is highly charged, relatively large (effective ionic radius = 0.84 &#197;), and can be classified as a &#8220;hard&#8221; cation. As such, it exhibits a preference for ligands bearing up to eight hard, anionic oxygen donors. Easily the most common chelator used in 89Zr-labeled radioimmunoconjugates is desferrioxamine (DFO), a siderophore-derived, acyclic chelator bearing three hydroxamate groups. The ligand stably coordinates the Zr4+ cation quickly and cleanly at RT at biologically relevant pH levels, and the resulting Zr-DFO complex remains stable over the course of multiple days in saline, blood serum, and whole blood.22 Computational studies strongly suggest that DFO forms a hexacoordinate complex with Zr4+ in which the metal center is coordinated to the three neutral and three anionic oxygen donors of the ligand as well as two exogenous water ligands (Figure 2).23,24 The in vivo behavior of radioimmunoconjugates employing the 89Zr-DFO conjugation scaffold has generally been excellent. However, in some cases, imaging and acute biodistribution studies have revealed elevated activity levels in the bones of mice injected with 89Zr-labeled antibodies, data that suggests that the osteophilic 89Zr4+ cation is released from the chelator in vivo and subsequently mineralizes in the bone.25 Recently, a number of investigations into the development of novel 89Zr4+ chelators particularly ligands with eight oxygen donors have appeared in the literature.24,26,27 Nevertheless, at present, DFO is the most widely employed chelator in 89Zr-labeled radioimmunoconjugates by a wide margin. A variety of different bioconjugation strategies have been employed to attach DFO to antibodies, including bioorthogonal click chemistry, the reaction of thiol-reactive DFO constructs with cysteines in the antibody, and the reaction of activated ester-bearing DFO constructs with lysines in the antibody.4,28-30 Easily the most common strategy, however, has been the use of an isothiocyanate-bearing derivative of DFO, DFO-NCS (Figure 2).22 This commercially available bifunctional chelator robustly and reliably forms stable, covalent thiourea linkages with the lysines of the antibody (Figure 3).
Over the past few years, a wide variety of 89Zr-DFO-labeled radioimmunoconjugates have been reported in the literature. Preclinical investigations have been especially abundant, featuring antibodies ranging from the more well-known cetuximab, bevacizumab, and trastuzumab to more esoteric antibodies such as the CD105-targeting TRC105 and fPSA-targeting 5A10.30-36 More recently, a small number of early-phase clinical trials using 89Zr-DFO-labeled antibodies have emerged in the literature. Specifically, groups in the Netherlands have published trials employing 89Zr-DFO-cmAb U36, 89Zr-DFO-ibritumomab tiuxetan, and 89Zr-DFO-trastuzumab.21,32,37 In addition, a range of other clinical trials with 89Zr-labeled radioimmunoconjugates are currently underway, including investigations here at Memorial Sloan Kettering Cancer Center using the PSMA-targeting 89Zr-DFO-J591 for prostate cancer imaging and the HER2-targeting 89Zr-DFO-trastuzumab for breast cancer imaging.23,30 In addition, while radiolabeled antibodies remain the most common 89Zr-labeled radiopharmaceuticals, the radiometal has also increasingly been employed with other vectors, including peptides, proteins, and nanomaterials.38-43
The modularity of this 89Zr-DFO labeling methodology is a tremendous asset. The repertoire of biomarker-targeting antibodies is ever-expanding, and the interest in performing in vivo PET imaging using these constructs is growing apace. As a result, we believe that the development of more standardized practices and protocols could benefit the field. An excellent written experimental protocol for DFO-NCS conjugation and 89Zr radiolabeling has already been published by Vosjan, et al.22 We feel that the visual demonstration provided by this work could further help investigators new to these techniques. In the protocol at hand, the prostate-specific membrane antigen targeting antibody J591 will be used as a model system to illustrate (1) the bioconjugation of the bifunctional chelator DFO-isothiocyanate to an antibody, (2) the radiosynthesis and purification of the 89Zr-DFO-mAb radioimmunoconjugate, and (3) in vivo PET imaging with a 89Zr-DFO-mAb radioimmunoconjugate in a murine model of cancer.23,44,45

<table border="0" cellpadding="0" cellspacing="0" width="676">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:217px;">Name of the Material/Equipment</td>
			<td style="width:183px;">Company</td>
			<td style="width:99px;">Catalog Number</td>
			<td style="width:179px;">Comments</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">p-SCN-Bn-DFO</td>
			<td>Macrocyclics</td>
			<td>B-705</td>
			<td>Store at -80 &#176;C</td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;">[89Zr]Zr-oxalate</td>
			<td>Various, including Perkin-Elmer</td>
			<td>-</td>
			<td>Caution: Radioactive material</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">PD-10 Desalting Columns</td>
			<td>GE Healthcare</td>
			<td>17-0851-01&#160;</td>
			<td>Store at room temperature</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Amicon Ultra-4 Centrifugal Filter Units</td>
			<td>EMD Millipore</td>
			<td>UFC805024</td>
			<td>Store at room temperature</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Silica Gel Impregnanted RadioTLC Paper</td>
			<td>Agilent Technologies</td>
			<td>SGI0001</td>
			<td>Cut into strips 0.5 cm wide</td>
		</tr>
	</tbody>
</table>

the bioconjugation and radiosynthesis of 89zr-dfo-labeled antibodies

The exceptional affinity, specificity, and selectivity of antibodies make them extraordinarily attractive vectors for tumor-targeted PET radiopharmaceuticals. Due to their multi-day biological half-life, antibodies must be labeled with positron-emitting radionuclides with relatively long physical decay half-lives. Traditionally, the positron-emitting isotopes 124I (t1/2 = 4.18 d), 86Y (t1/2 = 14.7 hr), and 64Cu (t1/2 = 12.7 hr) have been used to label antibodies for PET imaging. More recently, however, the field has witnessed a dramatic increase in the use of the positron-emitting radiometal 89Zr in antibody-based PET imaging agents. 89Zr is a nearly ideal radioisotope for PET imaging with immunoconjugates, as it possesses a physical half-life (t1/2 = 78.4 hr) that is compatible with the in vivo pharmacokinetics of antibodies and emits a relatively low energy positron that produces high resolution images. Furthermore, antibodies can be straightforwardly labeled with 89Zr using the siderophore-derived chelator desferrioxamine (DFO). In this protocol, the prostate-specific membrane antigen targeting antibody J591 will be used as a model system to illustrate (1) the bioconjugation of the bifunctional chelator DFO-isothiocyanate to an antibody, (2) the radiosynthesis and purification of a 89Zr-DFO-mAb radioimmunoconjugate, and (3) in vivo PET imaging with an 89Zr-DFO-mAb radioimmunoconjugate in a murine model of cancer.

The first step in this protocol the conjugation of DFO-NCS to the antibody is typically quite robust and reliable. Generally, the purified, chelator-modified immunoconjugate can be obtained in &#62; 90% yield, and using 3 molar equivalents of DFO-NCS in the initial conjugation reaction will yield a degree-of-labeling of the chelator of approximately 1.0-1.5 DFO/mAb. The 89Zr radiolabeling and purification steps of the procedure are likewise straightforward. At the concentrations outlined in the protocol above,...

Watch this Scientific Journal Video about The Bioconjugation and Radiosynthesis of 89Zr-DFO-labeled Antibodies at JoVE.com

The Bioconjugation and Radiosynthesis of 89Zr-DFO-labeled Antibodies

Due to its multi-day radioactive half-life and favorable decay properties, the positron-emitting radiometal 89Zr is extremely well-suited for use in antibody-based radiopharmaceuticals for PET imaging. In this protocol, the bioconjugation, radiosynthesis, and preclinical application of 89Zr-labeled antibodies will be described.

The Bioconjugation and Radiosynthesis of 89Zr-DFO-labeled Antibodies

The exceptional affinity, specificity, and selectivity of antibodies make them extraordinarily attractive vectors for tumor-targeted PET ...

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16.8K Views.  Memorial Sloan Kettering Cancer Center. Due to its multi-day radioactive half-life and favorable decay properties, the positron-emitting radiometal 89Zr is extremely well-suited for use in antibody-based radiopharmaceuticals for PET imaging. In this protocol, the bioconjugation, radiosynthesis, and preclinical application of 89Zr-labeled antibodies will be described. 