Radiolabeling of MAG3-morpholino oligomers with 188Re at high labeling efficiency and specific radioactivity for tumor pretargeting

DNA-Targeted Complexes of Tc and Re for Biomedical Applications

Due to their favorable chemical features, Re and Tc complexes have been widely used for the development of new therapeutic agents and imaging probes to solve problems of biomedical relevance. This review provides an update of the most relevant research efforts towards the development of novel cancer theranostic agents using Re and Tc-based compounds interacting with specific DNA structures. This includes a variety of homometallic complexes, namely those containing M(CO)3 (M=Re, Tc) moieties, that exhibit different modes of interaction with DNA, such as covalent binding, intercalation, groove binding or G-quadruplex DNA binding. Additionally, heterometallic complexes, designed to potentiate synergistic effects of different metal centers to improve DNA-targeting, cytotoxicity and fluorescence properties, are also reviewed. Particular attention is also given to 99m Tc- and 188 Re-labeled oligonucleotides that have been widely explored to develop imaging and therapeutic radiopharmaceuticals through the in vivo hybridization with a specific complementary DNA or RNA target sequence to provide useful molecular tools in precision medicine for cancer diagnosis and treatment. Finally, the need for further improvement of DNA-targeted Re and Tc-based compounds as potential therapeutic and diagnostic agents is highlighted, and future directions are discussed.

Bifunctional chelators for radiorhenium: past, present and future outlook

Targeted radionuclide therapy (TRNT) is an ever-expanding field of nuclear medicine that provides a personalised approach to cancer treatment while limiting toxicity to normal tissues. It involves the radiolabelling of a biological targeting vector with an appropriate therapeutic radionuclide, often facilitated by the use of a bifunctional chelator (BFC) to stably link the two entities. The radioisotopes of rhenium, 186Re (t 1/2 = 90 h, 1.07 MeV β-, 137 keV γ (9%)) and 188Re (t 1/2 = 16.9 h, 2.12 MeV β-, 155 keV γ (15%)), are particularly attractive for radiotherapy because of their convenient and high-abundance β--particle emissions as well as their imageable γ-emissions and chemical similarity to technetium. As a transition metal element with multiple oxidation states and coordination numbers accessible for complexation, there is great opportunity available when it comes to developing novel BFCs for rhenium. The purpose of this review is to provide a recap on some of the past successes and failings, as well as show some more current efforts in the design of BFCs for 186/188Re. Future use of these radionuclides for radiotherapy depends on their cost-effective availability and this will also be discussed. Finally, bioconjugation strategies for radiolabelling biomolecules with 186/188Re will be touched upon.

A Revisit to the Pretargeting Concept-A Target Conversion

Pretargeting is often used as a tumor targeting strategy that provides much higher tumor to non-tumor ratios than direct-targeting using radiolabeled antibody. Due to the multiple injections, pretargeting is investigated less than direct targeting, but the high T/NT ratios have rendered it more useful for therapy. While the progress in using this strategy for tumor therapy has been regularly reviewed in the literature, this review focuses on the nature and quantitative understanding of the pretargeting concept. By doing so, it is the goal of this review to accelerate pretargeting development and translation to the clinic and to prepare the researchers who are not familiar with the pretargeting concept but are interested in applying it. The quantitative understanding is presented in a way understandable to the average researchers in the areas of drug development and clinical translation who have the basic concept of calculus and general chemistry.

Radiolabeling of Nucleic Acid Aptamers for Highly Sensitive Disease-Specific Molecular Imaging

Aptamers are short single-stranded DNA or RNA oligonucleotide ligand molecules with a unique three-dimensional shape, capable of binding to a defined molecular target with high affinity and specificity. Since their discovery, aptamers have been developed for various applications, including molecular imaging, particularly nuclear imaging that holds the highest potential for the clinical translation of aptamer-based molecular imaging probes. Their easy laboratory production without any batch-to-batch variations, their high stability, their small size with no immunogenicity and toxicity, and their flexibility to incorporate various functionalities without compromising the target binding affinity and specificity make aptamers an attractive class of targeted-imaging agents. Aptamer technology has been utilized in nuclear medicine imaging techniques, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), as highly sensitive and accurate biomedical imaging modalities towards clinical diagnostic applications. However, for aptamer-targeted PET and SPECT imaging, conjugation of appropriate radionuclides to aptamers is crucial. This review summarizes various strategies to link the radionuclides to chemically modified aptamers to accomplish aptamer-targeted PET and SPECT imaging.

Use of Morpholino Oligomers for Pretargeting

Differing from the conventional direct-targeting strategy in which a probe or payload is directly loaded onto a targeting molecule that binds to the native target, pretargeting is an improved targeting strategy. It converts the native target to an artificial target specific for a secondary targeting molecule loaded with the probe or payload (effector). The effector is small and does not accumulate in normal tissues, which accelerates the targeting process and generates high target to nontarget ratios. DNA/cDNA analogs can serve as the recognition pair, i.e., the artificial target and the secondary targeting effector. Morpholino oligomers are so far the most investigated and the most successful DNA/cDNA analog recognition pairs for pretargeting. Herein, we describe the pretargeting principles, the pretargeting strategy using Morpholino oligomers, and the preclinical success so far achieved.

Evaluation of (188)Re-labeled NGR-VEGI protein for radioimaging and radiotherapy in mice bearing human fibrosarcoma HT-1080 xenografts

Vascular endothelial growth inhibitor (VEGI) is an anti-angiogenic protein, which includes three isoforms: VEGI-174, VEGI-192, and VEGI-251. The NGR (asparagine-glycine-arginine)-containing peptides can specifically bind to CD13 (Aminopeptidase N) receptor which is overexpressed in angiogenic blood vessels and tumor cells. In this study, a novel NGR-VEGI fusion protein was prepared and labeled with (188)Re for radioimaging and radiotherapy in mice bearing human fibrosarcoma HT-1080 xenografts. Single photon emission computerized tomography (SPECT) imaging results revealed that (188)Re-NGR-VEGI exhibits good tumor-to-background contrast in CD13-positive HT-1080 tumor xenografts. The CD13 specificity of (188)Re-NGR-VEGI was further verified by significant reduction of tumor uptake in HT-1080 tumor xenografts with co-injection of the non-radiolabeled NGR-VEGI protein. The biodistribution results demonstrated good tumor-to-muscle ratio (4.98 ± 0.25) of (188)Re-NGR-VEGI at 24 h, which is consistent with the results from SPECT imaging. For radiotherapy, 18.5 MBq of (188)Re-NGR-VEGI showed excellent tumor inhibition effect in HT-1080 tumor xenografts with no observable toxicity, which was confirmed by the tumor size change and hematoxylin and eosin (H&E) staining of major mouse organs. In conclusion, these data demonstrated that (188)Re-NGR-VEGI has the potential as a theranostic agent for CD13-targeted tumor imaging and therapy.

Aptamers as radiopharmaceuticals for nuclear imaging and therapy

Today, radiopharmaceuticals belong to the standard instrumentation of nuclear medicine, both in the context of diagnosis and therapy. The majority of radiopharmaceuticals consist of targeting biomolecules which are designed to interact with a disease-related molecular target. A plethora of targeting biomolecules of radiopharmaceuticals exists, including antibodies, antibody fragments, proteins, peptides and nucleic acids. Nucleic acids have some significant advantages relative to proteinaceous biomolecules in terms of size, production, modifications, possible targets and immunogenicity. In particular, aptamers (non-coding, synthetic, single-stranded DNA or RNA oligonucleotides) are of interest because they can bind a molecular target with high affinity and specificity. At present, few aptamers have been investigated preclinically for imaging and therapeutic applications. In this review, we describe the use of aptamers as targeting biomolecules of radiopharmaceuticals. We also discuss the chemical modifications which are needed to turn aptamers into valuable (radio-)pharmaceuticals, as well as the different radiolabeling strategies that can be used to radiolabel oligonucleotides and, in particular, aptamers.

A feasible approach to evaluate the relative reactivity of NHS-ester activated group with primary amine-derivatized DNA analogue and non-derivatized impurity

Synthetic DNA analogues with improved stability are widely used in life science. The 3'and/or 5' equivalent terminuses are often derivatized by attaching an active group for further modification, but a certain amount of non-derivatized impurity often remains. It is important to know to what extent the impurity would influence further modification. The reaction of an NHS ester with primary amine is one of the most widely used options to modify DNA analogues. In this short communication, a 3'-(NH2-biotin)-derivatized morpholino DNA analogue (MORF) was utilized as the model derivatized DNA analogue. Inclusion of a biotin concomitant with the primary amine at the 3'-terminus allows for the use of streptavidin to discriminate between the products from the derivatized MORF and non-derivatized MORF impurity. To detect the MORF reaction with NHS ester, S-acetyl NHS-MAG3 was conjugated to the DNA analogue for labeling with (99m)Tc, a widely used nuclide in the clinic. It was found that the non-derivatized MORF also reacted with the S-acetyl NHS-MAG3. Radiolabeling of the product yielded an equally high labeling efficiency. Nevertheless, streptavidin binding indicated that under the conditions of this investigation, the non-derivatized MORF was five times less reactive than the amine-derivatized MORF.

Quantitative Correlation of in Vivo Properties with in Vitro Assay Results: The in Vitro Binding of a Biotin-DNA Analogue Modifier with Streptavidin Predicts the in Vivo Avidin-Induced Clearability of the Analogue-Modified Antibody

Quantitative prediction of in vivo behavior using an in vitro assay would dramatically accelerate pharmaceutical development. However, studies quantitatively correlating in vivo properties with in vitro assay results are rare because of the difficulty in quantitatively understanding the in vivo behavior of an agent. We now demonstrate such a correlation as a case study based on our quantitative understanding of the in vivo chemistry. In an ongoing pretargeting project, we designed a trifunctional antibody (Ab) that concomitantly carried a biotin and a DNA analogue (hereafter termed MORF). The biotin and the MORF were fused into one structure prior to conjugation to the Ab for the concomitant attachment. Because it was known that avidin-bound Ab molecules leave the circulation rapidly, this design would theoretically allow complete clearance by avidin. The clearability of the trifunctional Ab was determined by calculating the blood MORF concentration ratio of avidin-treated Ab to non-avidin-treated Ab using mice injected with these compounds. In theory, any compromised clearability should be due to the presence of impurities. In vitro, we measured the biotinylated percentage of the Ab-reacting (MORF-biotin)⊃-NH2 modifier, by addition of streptavidin to the radiolabeled (MORF-biotin)⊃-NH2 samples and subsequent high-performance liquid chromatography (HPLC) analysis. On the basis of our previous quantitative understanding, we predicted that the clearability of the Ab would be equal to the biotinylation percentage measured via HPLC. We validated this prediction within a 3% difference. In addition to the high avidin-induced clearability of the trifunctional Ab (up to ∼95%) achieved by the design, we were able to predict the required quality of the (MORF-biotin)⊃-NH2 modifier for any given in vivo clearability. This approach may greatly reduce the steps and time currently required in pharmaceutical development in the process of synthesis, chemical analysis, in vitro cell study, and in vivo validation.

Differentiation between temporary and real non-clearability of biotinylated IgG antibody by avidin in mice

Although an increasing number of antibody conjugates are being used in the clinic, there remain many unmet needs in antibody targeting. Normal tissue background is one of the key issues that limits the therapeutic efficacy and the detection sensitivity. Background reduction coupled with dose increase may provide the required target accumulation of the label or toxin at an acceptable normal tissue background. However, the knowledge about the in vivo interaction between antibody and a clearing agent is currently inadequate for designing a rational clearance regimen or system. The current investigation focuses on the clearability of antibody for background reduction, an important topic to antibody targeting in general. The investigation employs pretargeting as a research tool and avidin as a model clearing agent. By comparing the effects of natural clearance at a longer post-injection time and avidin clearance, we demonstrated that avidin clearance is much more effective. By directly attaching avidin to a biotinylated antibody prior to injection, we found that the biotinylated antibody in blood, once bound to the clearing agent, can be removed from the circulation immediately and completely, while the real non-clearable antibody without biotin stays. The study of multiple avidin injections confirmed that the presence of clearable biotinylated antibodies after an avidin injection is due to their temporary inaccessibility and subsequent return from tissue compartments. The collective clearance efficiency of 91% by three avidin injections indicates a continuous IV infusion would be recommended to remove all of the biotinylated IgG molecules. In conclusion, the use of antibody pretargeting as a tool in this study has improved understanding of the incomplete clearance by avidin and can aid in overcoming this obstacle.

Biodistribution of (99m)tc tricarbonyl glycine oligomers

^99mTc tricarbonyl glycine monomers, trimers, and pentamers were synthesized and evaluated for their radiolabeling and in vivo distribution characteristics. We synthesized a ^99mTc-tricarbonyl precursor with a low oxidation state (I). ^99mTc(CO)₃(H₂O)₃⁺ was then made to react with monomeric and oligomeric glycine for the development of bifunctional chelating sequences for biomolecules. Labeling yields of ^99mTc-tricarbonyl glycine monomers and oligomers were checked by high-performance liquid chromatography. The labeling yields of ^99mTc-tricarbonyl glycine and glycine oligomers were more than 95%. We evaluated the characteristics of ^99mTc-tricarbonyl glycine oligomers by carrying out a lipophilicity test and an imaging study. The octanol-water partition coefficient of ^99mTc tricarbonyl glycine oligomers indicated hydrophilic properties. Single-photon emission computed tomography imaging of ^99mTc-tricarbonyl glycine oligomers showed rapid renal excretion through the kidneys with a low uptake in the liver, especially of ^99mTc tricarbonyl triglycine. Furthermore, we verified that the addition of triglycine to prototype biomolecules (AGRGDS and RRPYIL) results in the improvement of radiolabeling yield. From these results, we conclude that triglycine has good characteristics for use as a bifunctional chelating sequence for a ^99mTc-tricarbonyl- based biomolecular imaging probe.

Comparison between two labeled agents in mice using a coinjection-ratio approach in contrast to a conventional group approach

INTRODUCTION

The differences between two agents often need to be accurately defined in vivo. Usually they are injected respectively into two groups of subjects. However, if the two agents do not interact with each other in vivo, a coinjection would serve the same purpose. We believe some individual differences in biodistribution may be circumvented through this approach by calculating organ level ratios.

METHODS

A model system of MORF/cMORF pretargeting (MORF/cMORF is a complementary pair of DNA analogues) was employed in connection with an on-going tumor therapeutic project. Human LS174T cells were implanted into the flank of severely immuno-compromised NOD-scid IL2rg(null) mice. The tumor was confirmed to express TAG-72 antigens. At 16 days post tumor inoculation, mice received IV 60 μg of MORF-conjugated CC49 (an antiTAG-72 antibody), followed 2 days later by a low-mass-dose IV coinjection containing 2.5 μg of (90)Y-cMORF and 2.5 μg of (99m)Tc-cMORF. At 3 h post radioactivity injection, the distribution of (99m)Tc was imaged on a SPECT/CT camera and then organs were excised and counted for (90)Y and (99m)Tc. Because the two labeled cMORFs do not react or interact with each other in vivo, the two groups of (90)Y and (99m)Tc data enabled a conventional group comparison. In a new effort, (90)Y/(99m)Tc ratios were calculated. Student's t-test and retrospective power analysis were performed for both approaches. In the new approach, the ratios were set at 1 as the null hypothesis.

RESULTS

The Student's t-test in the conventional group approach indicated that the two labeled cMORFs distributed similarly, but significant differences were observed in salivary gland and large intestines. The coinjection-ratio approach certainly did not subvert the results of the conventional approach but revealed subtler differences. The P values were reduced, the powers were increased in most organs, and more significant differences were observed. The increased sensitivity was due to the reduced CV%s (SD/average*100%) of the (90)Y/(99m)Tc ratios. Therefore, some individual differences were circumvented and notably the ratio approach differentiated individual differences into ratio-correctable and ratio-uncorrectable.

CONCLUSIONS

Although the conventional approach is reliable, the coinjection-ratio approach using organ level ratios is more sensitive and therefore is recommended whenever possible. In addition, it differentiates individual differences into "coinjection correctable" and "coinjection uncorrectable".

Intraperitoneal injection is not always a suitable alternative to intravenous injection for radiotherapy

Abstract Intraperitoneal (IP) injection is frequently reported to be as effective as intravenous (IV) injection. Because it allows administering a larger volume with more radioactivity, we have investigated this route and the possibility of using it to circumvent the volume constraint we earlier experienced with pretargeting radiotherapy. Using (99m)Tc as the label, the pharmacokinetics (PK) of the cMORF effector (a DNA analogue) was evaluated after IP or IV injection in normal mice by necropsy and SPECT/CT imaging. In another experiment, nude mice bearing tumors were used and they received MORF-CC49 pretargeting antibody IV 2 days earlier than labeled cMORF IV or IP. Tumor accumulations of cMORF were measured at 6 hours after its injections. The absorbed radiation doses for (188)Re or (90)Y pretargeting were estimated using the (99m)Tc data and a self-absorbed model. Although the absorbed radiation doses to other organs were comparable, the dose to intestines after IP injection was 30-fold higher than IV injection due to the slow entry into the circulation. It had reached such a level as high as the dose to the kidneys that cleared the radioactivity and usually were at the highest level. Nevertheless, the slow entry did not reduce the tumor accumulation. In conclusion, using IP in place of IV led to an unacceptably high absorbed radiation dose to the intestines although the tumor accumulation was not compromised. This effect may be applicable to other radiotherapeutic agents as well.

Design and evaluation of new Tc-99m-labeled lactam bridge-cyclized alpha-MSH peptides for melanoma imaging

The purpose of this study was to examine the melanoma targeting and imaging properties of new (99m)Tc-labeled lactam bridge-cyclized alpha-melanocyte stimulating hormone (α-MSH) peptides using bifunctional chelating agents. MAG3-GGNle-CycMSH(hex), AcCG3-GGNle-CycMSH(hex), and HYNIC-GGNle-CycMSH(hex) peptides were synthesized, and their melanocortin-1 (MC1) receptor binding affinities were determined in B16/F1 melanoma cells. The biodistribution of (99m)Tc-MAG3-GGNle-CycMSH(hex), (99m)Tc-AcCG3-GGNle-CycMSH(hex), (99m)Tc(CO)3-HYNIC-GGNle-CycMSH(hex), and (99m)Tc(EDDA)-HYNIC-GGNle-CycMSH(hex) were determined in B16/F1 melanoma-bearing C57 mice at 2 h postinjection to select a lead peptide for further evaluation. The melanoma targeting and imaging properties of (99m)Tc(EDDA)-HYNIC-GGNle-CycMSH(hex) were further examined because of its high melanoma uptake and fast urinary clearance. The IC50 values of MAG3-GGNle-CycMSH(hex), AcCG3-GGNle-CycMSH(hex), and HYNIC-GGNle-CycMSH(hex) were 1.0 ± 0.05, 1.2 ± 0.19, and 0.6 ± 0.04 nM in B16/F1 melanoma cells, respectively. Among these four (99m)Tc-peptides, (99m)Tc(EDDA)-HYNIC-GGNle-CycMSH(hex) exhibited the highest melanoma uptake (14.14 ± 4.90% ID/g) and fastest urinary clearance (91.26 ± 1.96% ID) at 2 h postinjection. (99m)Tc(EDDA)-HYNIC-GGNle-CycMSH(hex) showed high tumor to normal organ uptake ratios except for the kidneys. The tumor/kidney uptake ratios of (99m)Tc(EDDA)-HYNIC-GGNle-CycMSH(hex) were 2.50 and 3.55 at 4 and 24 h postinjection. The melanoma lesions were clearly visualized by SPECT/CT using (99m)Tc(EDDA)-HYNIC-GGNle-CycMSH(hex) as an imaging probe at 2 h postinjection. Overall, high melanoma uptake coupled with fast urinary clearance of (99m)Tc(EDDA)-HYNIC-GGNle-CycMSH(hex) highlighted its potential for metastatic melanoma detection in the future.

Detection of Aspergillus fumigatus pulmonary fungal infections in mice with (99m)Tc-labeled MORF oligomers targeting ribosomal RNA

PURPOSE

Invasive aspergillosis is a major cause of infectious morbidity and mortality in immunocompromised patients. The fungus Aspergillus fumigatus (A. fumigatus) is the primary causative agent of invasive aspergillosis. However, A. fumigatus infections remain difficult to diagnose particularly in the early stages due to the lack of a rapid, sensitive and specific diagnostic approach. In this study, we investigated (99m)Tc labeled MORF oligomers targeting fungal ribosomal RNA (rRNA) for the imaging detection of fungal infections.

PROCEDURES

Three phosphorodiamidate morpholino (MORF) oligomer (a DNA analogue) probes were designed: AGEN, complementary to a sequence of the fungal 28S ribosomal RNA (rRNA) of Aspergillus, as a genus-specific probe; AFUM, complementary to the 28S rRNA sequence of A. fumigatus, as a fungus species-specific probe; and cMORF, irrelevant to all fungal species, as a control probe. The probes were conjugated with Alexa Fluor 633 carboxylic acid succinimidyl ester (AF633) for fluorescence imaging or with NHS-mercaptoacetyl triglycine (NHS-MAG3) for nuclear imaging with (99m)Tc and then evaluated in vitro and in vivo.

RESULTS

The specific binding of AGEN and AFUM to fungal total RNA was confirmed by dot blot hybridization while specific binding of AGEN and AFUM in fixed and live A. fumigatus was demonstrated by both fluorescent in situ hybridization (FISH) analysis and accumulation in live cells. SPECT imaging of BALB/c mice with pulmonary A. fumigatus infections and administered (99m)Tc labeled AGEN and AFUM showed immediate and obvious accumulation in the infected lungs, while no significant accumulation of the control (99m)Tc-cMORF in the infected lung was observed. Compared to non-infected mice, with sacrifice at 1h, the accumulation of (99m)Tc-AGEN and (99m)Tc-AFUM in the lungs of mice infected with A. fumigatus was 2 and 2.7 fold higher respectively.

CONCLUSIONS

In vivo targeting fungal ribosomal RNA with (99m)Tc labeled MORF probes AGEN and AFUM may be useful for A. fumigatus infection imaging and may provide a new strategy for the noninvasive diagnosis of invasive aspergillosis and other fungal infections.

Pretargeting vs. direct targeting of human betalox5 islet cells subcutaneously implanted in mice using an anti-human islet cell antibody

INTRODUCTION

We previously demonstrated MORF/cMORF pretargeting of human islets and betalox 5 cells (a human beta cell line) transplanted subcutaneously in mice with the anti-human islet antibody, HPi1. We now compare pretargeting with direct targeting in the beta cell transplant model to evaluate the degree to which target/non-target (T/NT) ratios may be improved by pretargeting.

METHODS

Specific binding of an anti-human islet antibody HPi1 to the beta cells transplanted subcutaneously in mice was examined against a negative control antibody. We then compared pretargeting by MORF-HPi1 plus 111In-labeled cMORF to direct targeting by 111In-labeled HPi1.

RESULTS

HPi1 binding to betalox5 human cells in the transplant was shown by immunofluorescence. Normal organ 111In backgrounds by pretargeting were always lower, although target accumulations were similar. More importantly, the transplant to pancreas and liver ratios was, respectively, 26 and 10 by pretargeting as compared to 9 and 0.6 by direct targeting.

CONCLUSIONS

Pretargeting greatly improves the T/NT ratios, and based on the estimated endocrine to exocrine ratio within a pancreas, pretargeting may be approaching the sensitivity required for successful imaging of human islets within this organ.

90Y labeled phosphorodiamidate morpholino oligomer for pretargeting radiotherapy

While (188)Re has been used successfully in mice for tumor radiotherapy by MORF/cMORF pretargeting, previous radiolabeling of the amine-derivatized cMORF with (90)Y, a longer physical half-life nuclide, was not very successful. After developing a method involving a prepurification heating step during conjugation that increases labeling efficiency and label stability, the biodistribution of (90)Y-DOTA-Bn-SCN-cMORF ((90)Y-DOTA-cMORF) was measured in normal mice and in MORF-CC49 pretargeted mice that bear LS174T tumors. Absorbed radiation doses were then estimated and compared to those estimated for (188)Re. The pharmacokinetics of the (90)Y-DOTA-cMORF in normal mice and in the pretargeted nude mice was similar to that observed previously with (99m)Tc- and (188)Re-MAG(3)-cMORFs. While the (90)Y-DOTA-cMORF cleared rapidly from normal tissues, tumor clearance was very slow and tumor radioactivity accumulation was constant for at least 7 days such that the tumor/blood (T/B) ratio increased linearly from 6 to 25 over this period. Therefore, by extrapolation, normal tissue toxicities following administration of therapeutic doses of (90)Y may be comparable to that observed for (188)Re in which the T/B increased from 5 to 20. In conclusion, radiolabeling of DOTA-cMORF with (90)Y was improved by introducing a prepurification heating step during conjugation. The (90)Y-DOTA-cMORF provided a similar T/B ratio and biodistribution to that of (188)Re-MAG(3)-cMORF and was retained well in the tumor pretargeted with MORF-CC49. Because of the longer physical half-life, the T/NT absorbed radiation dose ratios were improved in most organs and especially in blood.

Radiolabeled oligonucleotides for antisense imaging

Oligonucleotides radiolabeled with isotopes emitting γ-rays (for SPECT imaging) or positrons (for PET imaging) can be useful for targeting messenger RNA (mRNA) thereby serving as non-invasive imaging tools for detection of gene expression in vivo (antisense imaging). Radiolabeled oligonucleotides may also be used for monitoring their in vivo fate, thereby helping us better understand the barriers to its delivery for antisense targeting. These developments have led to a new area of molecular imaging and targeting, utilizing radiolabeled antisense oligonucleotides. However, the success of antisense imaging relies heavily on overcoming the barriers for its targeted delivery in vivo. Furthermore, the low ability of the radiolabeled antisense oligonucleotide to subsequently internalize into the cell and hybridize with its target mRNA poses additional challenges in realizing its potentials. This review covers the advances in the antisense imaging probe development for PET and SPECT, with an emphasis on radiolabeling strategies, stability, delivery and in vivo targeting.

A preclinical 188Re tumor therapeutic investigation using MORF/cMORF pretargeting and an antiTAG-72 antibody CC49

The utility of MORF/cMORF pretargeting for the radiotherapy of cancer requires further validation in tumored mice before clinical trials. We now report on a therapeutic study in mice pretargeted with MORF-CC49 (an anti-TAG-72 antibody CC49 conjugated with MORF, a phosphorodiamidate morpholino oligomer) and then targeted by 188Re-cMORF (a 188Re labeled complementary MORF). Before the dose-escalating therapeutic study, a pretargeting study in LS174T tumored mice was performed at tracer levels. By both necropsy and imaging, the tracer study showed that the whole body radioactivity was largely restricted to tumor in the mice pretargeted 48 h earlier with MORF-CC49 and the tumor radioactivity was retained over 90 h. After decay correction, a best-fit to the biodistribution provided the areas under the radioactivity curves (AUCs) used for the radiation dose estimates. The tumor to normal organ AUC ratios in all cases were greater than unity and ranged from 3 (kidneys) to 48 (muscle). Tumor growth was inhibited in the therapy study. At the highest 188Re dose of 1.40 mCi, a complete but temporary tumor remission was evident in 3 out of the 5 animals. Histological examination of tissues from these animals showed no evidence of cytotoxicity to normal tissues but obvious radiation damage to tumor. In conclusion, effective radiotherapy was achieved in a mouse model by MORF/cMORF pretargeting using 188Re as the therapeutic radionuclide and CC49 as the pretargeting antibody.

Adding a clearing agent to pretargeting does not lower the tumor accumulation of the effector as predicted

Clearing agents are often used in pretargeting despite the potential for decreased tumor accumulation of the effector. However, according to the authors' semiempirical model, a clearing agent should not necessarily decrease tumor accumulation. In this study, the authors have added a clearing step to their model-morpholino phosphorodiamidate oligomer (MORF)/complement MORF (cMORF) pretargeting system-to confirm this prediction. The CC49 antibody was conjugated with both biotin and an 18 mer MORF. The influence of avidin on antibody clearance was first evaluated in normal mice in which each animal received 30 μg of MORF-CC49-biotin, 0-70 μg of avidin 1 day later, and 1.2 μg of ⁹⁹(m)Tc-cMORF 3 hours later, with sacrifice at 3 hours. Thereafter, a pretargeting study in mice bearing an LS174T tumor was performed at a 34 μg avidin dosage. In normal mice, the blood level of ⁹⁹(m)Tc-cMORF fell by 60% at an avidin dosage of 10 μg or higher. In tumored mice, avidin produced a similar reduction in blood but had no influence on tumor level, which remained at 6.30% ID/g as predicted. In conclusion, in addition to the expected reduced effector levels in blood and normal tissues, a reduction in tumor accumulation was avoided when adding a clearing agent as predicted.

Human islet cell MORF/cMORF pretargeting in a xenogeneic murine transplant model

Noninvasive measurement of human islet cell mass in pancreas or following islet transplantation by nuclear imaging has yet to be achieved. It has been shown using mouse tumor models that pretargeting imaging strategies are sensitive and can greatly increase target to nontarget signal ratios. The objective now is to demonstrate the specific pretargeting of human islet cells in mice. Our pretargeting strategy uses an anti-human islet cell antibody HPi1, conjugated to a phosphorodiamidate morpholino oligomer (MORF) that binds specifically to a (99m)Tc labeled complementary MORF (cMORF). Sensitivity and specificity of the pretargeting were first validated in culture using a human beta cell line (betalox5) and a negative control human cell line (HEK293). Pretargeting was then used to target and visualize these two cell lines and human islets transplanted subcutaneously in NOD-scid IL2rγ(null) mice. In culture, (99m)Tc accumulation on the betalox5 cells pretargeted by MORF-HPi1 was 100-fold higher than on untreated betalox5 cells or following treatment with native HPi1 and much higher than on the MORF-HPi1 pretargeted control HEK293 cells. Small animal imaging readily localized the transplanted betalox5 cells and human islets, but not the HEK293 cells. Ex vivo counting demonstrated 3-fold higher (99m)Tc accumulation in the transplanted betalox5 cells and human islets than in the control HEK293 cells. The target accumulation was also shown to increase linearly with increased numbers of the implanted betalox5 cells. These results demonstrate specific binding of radioactivity and successful imaging of human betalox5 cells and human islets transplanted in mice. Thus MORF/cMORF pretargeting may be useful to measure noninvasively human islet cell mass within the pancreas or following islet transplantation.

Unexpected side products in the conjugation of an amine-derivatized morpholino oligomer with p-isothiocyanate benzyl DTPA and their removal

In connection with pretargeting, an amine-derivatized morpholino phosphorodiamidate oligomer (NH(2)-cMORF) was conjugated conventionally with p-isothiocyanate benzyl-DTPA (p-SCN-Bn-DTPA). However, after (111)In radiolabeling, unexpected label instability was observed. To understand this instability, the NH(2)-cMORF and, as control, the native cMORF without the amine were conjugated in the conventional manner. Surprisingly, the (111)In labeling of the native cMORF conjugate was equally effective as that of the NH(2)-cMORF conjugate (>95%) despite the absence of the amine group. Furthermore, heating the radiolabeled NH(2)-cMORF and native cMORF conjugates resulted in a 35% loss and a complete loss of the label, respectively. Since the (111)In labeled DTPA is known to be stable, the instability in both cases must be due to some unstable association of DTPA to the cMORF, presumably unstable association to some endogenous sites in cMORF. Based on this assumption, a postconjugation-prepurification heating step was introduced, and labeling efficiency and stability were again investigated. By introducing the heating step, the side products were dissociated, and after purification and labeling, the NH(2)-cMORF conjugate provided a stable label and high labeling efficiency with no need for postlabeling purification. The biodistribution of this radiolabeled conjugate in normal mice showed significantly lower backgrounds compared with the labeled unstable native cMORF conjugate. In conclusion, the conventional conjugation procedure to attach the p-SCN-Bn-DTPA to NH(2)-cMORF resulted in side product(s) that were responsible for the (111)In label instability. Adding a postconjugation-prepurification heating step dissociated the side products, improved the label stability and lowered tissue backgrounds in mice.

Tumor pretargeting in mice using MORF conjugated CC49 antibody and radiolabeled complimentary cMORF effector

AIM

Using the antiCEA antibody MN14, a LS174T mouse tumor model has been successfully targeted with (⁹⁹m)Tc for imaging and ¹⁸⁸Re for radiotherapy by phosphorodiamidate morpholino oligomers (MORF)/complementary MORF (cMORF) pretargeting strategy. This investigation evaluated the antiTAG-72 antibody CC49 as an alternative to MN14 for this application.

METHODS

Both CC49 and MN14 were labeled with ¹¹¹In via SCN-benzyl-DTPA and their biodistributions were compared to that of MN14 labeled via DTPA anhydride. Since the accessibility of the antibody to the effector is required for optimization of pretargeting, the internalization of both MORF-CC49 and MORF-MN14 antibodies in LS174T cells were evaluated in culture. In addition, the accessible concentration of MORF-CC49 antibody in tumor was determined in a series of pretargeting studies with escalating dosages of the [(⁹⁹m)Tc]cMORF effector. Finally, using these results and our semi-empirical model, an imaging study was performed under optimal pretargeting conditions.

RESULTS

The biodistribution of ¹¹¹In to trace the MN14 antibody depended significantly on the labeling method. Furthermore, both MORF-CC49 and MORF-MN14 antibodies showed rapid internalization in culture. Fortunately, the accessibility in tumor was found to be less seriously reduced in vivo. In a pretargeting study under optimal conditions, both by imaging and by necropsy, the [(⁹⁹m)Tc]cMORF effector accumulated predominantly in the tumor of pretargeted mice. Normal tissue accumulations were minimal except in kidneys, liver, and a segment of intestines.

CONCLUSION

MORF pretargeting with CC49 was equally successful in the LS174T tumor model to the MORF pretargeting with MN14. The MORF-CC49 antibody may therefore be considered for future investigations toward early clinical trials.

A Brief Review of Chelators for Radiolabeling Oligomers

The chemical modification of oligomers such as DNA, PNA, MORF, LNA to attach radionuclides for nuclear imaging and radiotherapy applications has become a field rich in innovation as older methods are improved and new methods are introduced. This review intends to provide a brief overview of several chelators currently in use for the labeling of oligomers with metallic radionuclides such as ^99mTc, ¹¹¹In and ¹⁸⁸Re. While DNA and its analogs have been radiolabeled with important radionuclides of nonmetals such as ³²P, ³⁵S, ¹⁴C, ¹⁸F and ¹²⁵I, the labeling methods for these isotopes involve covalent chemistry that is quite distinct from the coordinate-covalent chelation chemistry described herein. In this review, we provide a summary of the several chelators that have been covalently conjugated to oligomers for the purpose of radiolabeling with metallic radionuclides by chelation and including details on the conjugation, the choice of radionuclides and labeling methods.

Preparation of (111)In-DTPA morpholino oligomer for low abdominal accumulation

An ability to quantitate the beta cell mass by noninvasive nuclear imaging will be very useful in the prevention, diagnosis, and treatment of diabetes. However, to be successful, radioactivity from the pancreas must not be obscured by the background radioactivity in the abdomen. Pretargeting offers the promise of achieving high target organ to normal tissue ratios. In preparation for pancreas imaging studies by pretargeting using morpholino oligomers (MORF/cMORF), it was necessary to develop a simple and efficient method to radiolabel the cMORF effector. Because we have shown that labeling the cMORF with (111)In via DTPA reduces excretion into the intestines compared to labeling with (99m)Tc via MAG(3), the conjugation of DTPA to cMORF was investigated for (111)In labeling. The amine-derivatized cMORF was conjugated with DTPA using 1-ethyl-3(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) as an alternative to the conventional cyclic anhydride. The conjugation efficiency (represented by the number of DTPA groups attached per cMORF) was investigated by changing the EDC, DTPA, and cMORF molar ratios. Different open columns were considered for the purification of DTPA-cMORF. Before conjugation, each cMORF molecule was confirmed to have an amine by trinitrobenzene sulfonic acid (TNBS) assay using the omega-amino butyric acid as positive standard and the non-amine derivatized cMORF as negative standard. The average number of DTPA groups per cMORF was 0.15-0.20 following the conjugation over a cMORF/DTPA molar ratio of 0.5-5 and over a cMORF/EDC molar ratio of 20-60. The conjugation efficiency was lower than expected probably due to steric hindrance. A 1 x 50cm P-4 column using ammonium acetate as eluting buffer provided an adequate separation of DTPA-cMORF from free DTPA. The (111)In labeling efficiency by transchelation from acetate exceeded 95%, thus avoiding the need for postlabeling purification. Despite the lower than expected conjugation efficiency in which no more than one-fifth of the cMORF were DTPA-derivatized, a specific radioactivity of at least 300microCi/microg or 1.90Ci/micromol of cMORF was achieved. In conclusion, a protocol is described for (111)In-DTPA-cMORF that provides the high specific activity favorable to beta cell imaging because of the low mass fraction of beta cells in pancreas (1-2%) and obviates the need for postlabeling purification.

Replacing 99mTc with 111In improves MORF/cMORF pretargeting by reducing intestinal accumulation

PURPOSE

To reduce accumulation in the abdomen by MORF/cMORF pretargeting, 111In was compared to 99mTc as the radiolabel.

PROCEDURES

After receiving either 99mTc (MAG3)-cMORF or 111In (DTPA)-cMORF, normal mice were imaged and killed for pharmacokinetics. Thereafter, tumored mice were pretargeted withMORF-antibody, 48 h later were given an injection of 99mTc- or 111In-cMORF, and finally were imaged repeatedly.

RESULTS

The cMORF biodistribution in both normal and pretargeted tumored mice was influenced by its radiolabel. While excretion of both 99mTc-cMORF and 111In-cMORF was rapid and mainly through the kidneys, about 2% of 99mTc accumulated in the intestines compared toessentially no intestinal accumulation for 111In at any time. Tumor accumulation was unchanged.

CONCLUSION

In applications of MORF/cMORF pretargeting intended to image organs deep within the abdomen such as the pancreas, radiolabeling with 111In may be superior to 99mTc.

An experimental and theoretical evaluation of the influence of pretargeting antibody on the tumor accumulation of effector

In treating tumors by pretargeting, the antitumor antibody and the cytotoxic effector (e.g., toxins and radioactivity) are separately administered. Therefore, pretargeting is more complicated with many variables. We are conducting studies to understand the influence of each variable using a novel recognition pair of mutually complementary phosphorodiamidate morpholino oligomers (MORF/cMORF). Earlier we developed a semi-empirical model capable of accurately predicting the behavior of a radiolabeled cMORF effector with variations in dosages and timing. We have now extended the model to predict the effector behavior, in particular, its maximum percent tumor accumulation (MPTA) in mice pretargeted with three different MORF-conjugated antibodies (MN14, B72.3, and CC49). The MN14 and the CC49 target different antigens in the same tumor, whereas the CC49 and the B72.3 target the same antigen but with very different tumor accumulation. By comparing the pretargeting results of these three antibodies with our prediction, we confirmed that the MPTA of the radiolabeled cMORF effector in the LS174T tumor is independent of the antibodies. In conclusion, the MPTA cannot be improved through the use of different pretargeting antibodies, although different antibodies may improve the maximum absolute tumor accumulation, the heterogeneity, and/or the tumor-to-normal tissue ratios of the effector. This conclusion will apply equally well to effectors carrying a fluorescent probe, an anticancer agent, or a radioactive imaging agent.

Synthesis and in vitro characterization of a dendrimer-MORF conjugate for amplification pretargeting

Amplification pretargeting can play an important role in molecular imaging by significantly increasing the accumulation of signal in target tissues. Multiple-step amplification pretargeting offers the potential to greatly improve target localization of effector molecules through the intermediate use of polymers conjugated with multiple copies of complementary oligomers. In this study, PAMAM dendrimer generation 3 (G3) was conjugated with multiple copies of a phosphorodiamidate morpholino (MORF) oligomer. Characterization of the conjugate by native-PAGE and SE-HPLC demonstrated that the conjugation was successful. The average numbers of MORF groups in the G3-MORF conjugate, both attached and accessible to the (99m)Tc labeled complementary MORF (cMORF), were determined. The antitumor antibody CC49 was conjugated with both MORF and cMORF (collectively (c)MORF) at an average of about one group per molecule. Nine of the 32 carboxyl groups of the dendrimer were modified with MORF, of which 90% were accessible in solution to (99m)Tc-cMORF. After purification, the G3-MORF was radiolabeled with tracer (99m)Tc-labeled cMORF (i.e., G3-MORF/(99m)Tc-cMORF) and added to the antibody CC49 previously conjugated with cMORF (i.e., CC49-cMORF/G3-MORF/(99m)Tc-cMORF), the complex demonstrated a single peak on SE-HPLC as evidence of complete hybridization between G3-MORF/(99m)Tc-cMORF and CC49-cMORF. The CC49-(c)MORF were bound to both Protein G and Protein L coated plates, and G3-MORF was added to hybridize with CC49-cMORF before the (99m)Tc-cMORF was added to test amplification pretargeting. In comparison to conventional pretargeting without the G3-MORF, the signal was amplified about 6 and 14 times, respectively, showing that the G3-MORF participated in amplifying the signal. Further amplification studies using the CC49-(c)MORF for LS174T tumor cells in tissue culture also demonstrated clear evidence of signal amplification.

Pretargeting CWR22 prostate tumor in mice with MORF-B72.3 antibody and radiolabeled cMORF

PURPOSE

We have now applied our MORF/cMORF pretargeting technology to the targeting of CWR22 prostate tumor in nude mice.

METHODS

The antiTAG-72 antibody B72.3 was conjugated with an 18 mer MORF while the cMORF was radiolabeled with (99m)Tc. The specific binding of the antibody to the CWR22 cells was first confirmed in an assay placing the radiolabeled B72.3 antibody in competition with increasing concentrations of native B72.3. Thereafter, a group of four CWR22 tumored mice intravenously received the MORF-B72.3 and, 3 days later, the (99m)Tc-cMORF, and were killed at 3 h postradioactivity injection. The dosage of the labeled cMORF was selected on the basis of previous experience in LS174T tumored mice. As controls, four animals received only the radiolabeled cMORF and another four received only the (111)In-B72.3. The maximum percent tumor accumulation (MPTA) of the labeled cMORF was subsequently determined by a dosage study of labeled cMORF. Both a multipinhole SPECT image and a planar gamma camera image were obtained of a representative mouse.

RESULTS

The CWR22 tumor was confirmed to be TAG-72-positive. The MPTA of the labeled cMORF in the CWR22 tumor was 2.22%ID/g compared to only 0.12%ID/g in control mice without pretargeting. Both the planar and tomographic images confirmed the success of the CWR22 pretargeting.

CONCLUSIONS

The MORF/cMORF pretargeting approach has been successfully applied to tumor targeting of the prostate xenograft CWR22. However, the MPTA in this tumor model is lower than that in the LS174T tumor model investigated earlier, possibly due to a lower tumor blood supply.

A novel pretargeting method for measuring antibody internalization in tumor cells

A novel pretargeting method has been developed to quantitate antibody cellular internalization. In this study, the antibody was conjugated with a phosphorodiamidate morpholino oligomer (MORF) specific for the complementary MORF (cMORF) as an effector. Half the tumor cells were incubated with the MORF-antibody (pretargeting group) and the other half with the same MORF-antibody at the same concentration but radiolabeled (direct targeting group). After incubation, the same dosage of radiolabeled cMORF was added to the wells of the pretargeting group. The radioactivity of the direct targeting cells represented the sum of both internalized and cell-surface-bound antibodies, whereas the radioactivity of the pretargeting cells resulted only from the surface-bound antibodies, as the radiolabeled cMORF does not penetrate the cell surface. Therefore, the difference in radioactivity accumulation between pretargeting and direct targeting provides the internalized fraction. In this example, the internalization of a MORF conjugated anti-prostate-specific membrane antigen antibody, 3C6, in LNCaP cells was examined, and the average cell-surface residence time was determined as 2 hours. This method of measuring antibody internalization is directly applicable to pretargeting applications but can be a universal alternative to the conventional acid-wash method, with the advantage of leaving the cell membrane undamaged.

Predicting the biodistribution of radiolabeled cMORF effector in MORF-pretargeted mice

PURPOSE

Pretargeting with phosphorodiamidate morpholino oligomers (MORFs) involves administration of a MORF-conjugated anti-tumor antibody such as MN14 as a pretargeting agent before that of the radiolabeled complementary MORF (cMORF) as the effector. The dosages of the pretargeting agent and effector, the pretargeting interval, and the detection time are the four pretargeting variables. The goal of this study was to develop a semiempirical description capable of predicting the biodistribution of the radiolabeled effector in pretargeted mice and then to compare predictions with experimental results from pretargeting studies in tumored animals in which the pretargeting interval and the detection time were both fixed but the dosages of both the effector and the pretargeting agent were separately varied.

METHODS

Pretargeting studies in LS174T tumored mice were performed using the anti-CEA antibody MN14 conjugated with MORF and the cMORF radiolabeled with (99m)Tc. A description was developed based on our previous observations in the same mouse model of the blood and tumor levels of MORF-MN14, accessibility of MORF-MN14 to labeled cMORF, the tumor accumulation of labeled cMORF relative to MORF-MN14 levels therein, and the kidney accumulation of labeled cMORF. The predicted values were then compared with the experimental values.

RESULTS

The predicted biodistribution of the radiolabeled effector and the experimental data were in gratifying agreement in normal organs, suggesting that the description of the pretargeting process was reliable. The tumor accumulations occasionally fell outside two standard deviations of that predicted, but after tumor size correction, good agreement between predicted and experimental values was observed here as well.

CONCLUSION

A semiempirical description of the biodistribution of labeled cMORF was capable of predicting the biodistribution of the radiolabeled effector in the pretargeted tumored mouse model, demonstrating that the underlying pretargeting concepts are correct. We believe that the approach described herein may be applied to any of the alternative pretargeting approaches and animal tumor models currently under investigation. Furthermore, appreciation of the concepts may provide a rationale for selecting dosages and timings in human pretargeting studies as an alternative to pure empirical means.

Labeling biomolecules with radiorhenium: a review of the bifunctional chelators

For radiotherapy, biomolecules such as intact antibodies, antibody fragments, peptides, DNAs and other oligomers have all been labeled with radiorhenium ((186)Re and (188)Re). Three different approaches have been employed that may be referred to as direct, indirect and integral labeling. Direct labeling applies to proteins and involves the initial reduction of endogenous disulfide bridges to provide chelation sites. Indirect labeling can apply to most biomolecules and involves the initial attachment of an exogenous chelator. Finally, integral labeling is a special case applying only to small molecules in which the metallic radionuclide serves to link two parts of a biomolecule together in forming the labeled complex. While the number of varieties for the direct and integral radiolabeling approaches is rather limited, a fairly large and diverse number of chelators have been reported in the case of indirect labeling. Our objective herein is to provide an overview of the various chelators that have been used in the indirect labeling of biomolecules with radiorhenium, including details on the labeling procedures, the stability of the radiolabel and, where possible, the influence of the label on biological properties.

Successful radiotherapy of tumor in pretargeted mice by 188Re-radiolabeled phosphorodiamidate morpholino oligomer, a synthetic DNA analogue

PURPOSE

Pretargeting has been attracting increasing attention as a drug delivery approach. We recently proposed Watson-Crick pairing of phosphorodiamidate morpholino oligomers (MORF) for the recognition system in tumor pretargeting. MORF pretargeting involves the initial i.v. injection of a MORF-conjugated antitumor antibody and the subsequent i.v. injection of the radiolabeled complement. Our laboratory has reported on MORF pretargeting for diagnosis using (99m)Tc as radiolabel. We now report on the use of MORF pretargeting for radiotherapy in a mouse tumor model using (188)Re as the therapeutic radiolabel.

EXPERIMENTAL DESIGN

An initial tracer study was done to estimate radiation dose, and was followed by the radiotherapy study at 400 muCi per mouse with three control groups (untreated, MORF antibody alone, and (188)Re complementary MORF alone).

RESULTS

Tracer study indicated rapid tumor localization of (188)Re and rapid clearance from normal tissues with a tumor area under the curve (AUC) about four times that of kidney and blood (the normal organs with highest radioactivity). Tumor growth in the study group ceased 1 day after radioactivity injection, whereas tumors continued to grow at the same rate among the three control groups. At sacrifice on day 5, the average net tumor weight in the study group was significantly lower at 0.68 +/- 0.29 g compared with the three control groups (1.24 +/- 0.31 g, 1.25 +/- 0.39 g, and 1.35 +/- 0.41 g; Ps < 0.05), confirming the therapeutic benefit observed by tumor size measurement.

CONCLUSIONS

MORF pretargeting has now been shown to be a promising approach for tumor radiotherapy as well as diagnosis.

Methods for MAG3 conjugation and 99mTc radiolabeling of biomolecules

The chelator mercaptoacetyltriglycine (MAG3) forms a single stable chelate with technetium-99m (99mTc) oxotechnetate. The bifunctional N-hydroxysuccinimidyl ester of mercaptoacetyltriglycine with S-acetyl protection of the sulfhydryl group may be used to conjugate MAG3 to primary amine functionalized biomolecules for the purpose of radiolabeling with 99mTc for gamma detection or single photon emission computed tomography imaging (SPECT). We report here an improved MAG3 conjugation and 99mTc radiolabeling method capable of generating high radiochemical yield and high specific radioactivity. Post-labeling purification will not be needed if the protocol is followed as presented. Apart from the preparation of reagents, the conjugation and purification requires about 4 h, while the labeling with 99mTc requires about an additional 30 min.