Experimental pretargeting studies of cancer with a humanized anti-CEA x murine anti-[In-DTPA] bispecific antibody construct and a (99m)Tc-/(188)Re-labeled peptide

Pretargeting: A Path Forward for Radioimmunotherapy

Pretargeted radioimmunodiagnosis and radioimmunotherapy aim to efficiently combine antitumor antibodies and medicinal radioisotopes for high-contrast imaging and high-therapeutic-index (TI) tumor targeting, respectively. As opposed to conventional radioimmunoconjugates, pretargeted approaches separate the tumor-targeting step from the payload step, thereby amplifying tumor uptake while reducing normal-tissue exposure. Alongside contrast and TI, critical parameters include antibody immunogenicity and specificity, availability of radioisotopes, and ease of use in the clinic. Each of the steps can be optimized separately; as modular systems, they can find broad applications irrespective of tumor target, tumor type, or radioisotopes. Although this versatility presents enormous opportunity, pretargeting is complex and presents unique challenges for clinical translation and optimal use in patients. The purpose of this article is to provide a brief historical perspective on the origins and development of pretargeting strategies in nuclear medicine, emphasizing 2 protein delivery systems that have been extensively evaluated (i.e., biotin-streptavidin and hapten-bispecific monoclonal antibodies), as well as radiohaptens and radioisotopes. We also highlight recent innovations, including pretargeting with bioorthogonal chemistry and novel protein vectors (such as self-assembling and disassembling proteins and Affibody molecules). We caution the reader that this is by no means a comprehensive review of the past 3 decades of pretargeted radioimmunodiagnosis and pretargeted radioimmunotherapy. But we do aim to highlight major developmental milestones and to identify benchmarks for success with regard to TI and toxicity in preclinical models and clinically. We believe this approach will lead to the identification of key obstacles to clinical success, revive interest in the utility of radiotheranostics applications, and guide development of the next generation of pretargeted theranostics.

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.

Antibody fragment and targeted colorectal cancer therapy: A global systematic review

BACKGROUND AND AIMS

Antibody-based therapeutics have been evidenced promising for the treatment of colorectal cancer patients. However, the size and long circulating half-lives of antibodies can limit their reproducible manufacture in clinical studies. Consequently, in novel therapeutic approaches conventional antibodies are minimized and engineered to produce fragments like Fab, scFv, nanobody, bifunctional antibody, bispecific antibody, minibody and diabody to preserve their high affinity and specificity to target pharmaceutical nanoparticle conjugates. This systematic review for the first time aimed to elucidate the role of various antibody fragments in colorectal cancer treatment.

METHOD

A systematic literature search in web of sciences, PubMed, Scopus, Google scholar and ProQuest was conducted. Reference lists of the articles were reviewed to identify the relevant papers. The full text search included articles published in English during 1990-2021.

RESULTS

Most the 53 included studies were conducted in vitro and in most conducted studies single-chain antibodies were among the most used antibody fragments. Most antibodies targeted CEA in the treatment of colorectal cancer. Moreover, a large number of studies observed apoptosis induction and tumor growth inhibition. In addition, few studies implicated the role of the innate immune system as an indirect mechanisms of tumor growth by enhancing NK-cell killing.

CONCLUSION

Antibody-based therapy was demonstrated to be of a great promise in the treatment of colorectal cancer rather than common treatments such as radiotherapy, chemotherapy, and surgical operations. This type of specified cancer treatment can also induce the activation of innate and specific immune system to eradicate tumor cells.

Principles and Current Clinical Landscape of Multispecific Antibodies against Cancer

Building upon the resounding therapeutic success of monoclonal antibodies, and supported by accelerating progress in engineering methods, the field of multispecific therapeutic antibodies is growing rapidly. Over 140 different molecules are currently in clinical testing, with excellent results in recent phase 1-3 clinical trials for several of them. Multivalent bispecific IgG-modified formats predominate today, with a clear tendency for more target antigens and further increased valency in newer constructs. The strategies to augment anticancer efficacy are currently equally divided between disruption of multiple surface antigens, and additional redirection of cytotoxic T or NK lymphocytes against the tumor. Both effects complement other modern modalities, such as tyrosine kinase inhibitors and adoptive cell therapies, with which multispecifics are increasingly applied in combination or merged, for example, in the form of antibody producing CAR-T cells and oncolytics. While mainly focused on B-cell malignancies early on, the contemporary multispecific antibody sector accommodates twice as many trials against solid compared to hematologic cancers. An exciting emerging prospect is the targeting of intracellular neoantigens using T-cell receptor (TCR) fusion proteins or TCR-mimic antibody fragments. Considering the fact that introduction of PD-(L)1 inhibitors only a few years ago has already facilitated 5-year survival rates of 30-50% for per se highly lethal neoplasms, such as metastatic melanoma and non-small-cell lung carcinoma, the upcoming enforcement of current treatments with "next-generation" immunotherapeutics, offers a justified hope for the cure of some advanced cancers in the near future.

Therapeutic Applications of Pretargeting

Targeted therapies, such as radioimmunotherapy (RIT), present a promising treatment option for the eradication of tumor lesions. RIT has shown promising results especially for hematologic malignancies, but the therapeutic efficacy is limited by unfavorable tumor-to-background ratios resulting in high radiotoxicity. Pretargeting strategies can play an important role in addressing the high toxicity profile of RIT. Key to pretargeting is the concept of decoupling the targeting vehicle from the cytotoxic agent and administrating them separately. Studies have shown that this approach has the ability to enhance the therapeutic index as it can reduce side effects caused by off-target irradiation and thereby increase curative effects due to higher tolerated doses. Pretargeted RIT (PRIT) has been explored for imaging and treatment of different cancer types over the years. This review will give an overview of the various targeted therapies in which pretargeting has been applied, discussing PRIT with alpha- and beta-emitters and as part of combination therapy, plus its use in drug delivery systems.

Technetium-99m radiochemistry for pharmaceutical applications

Technetium-99m (^99m Tc) is a widely used radionuclide, and the development of ^99m Tc imaging agents continues to be in demand. This overview discusses basic principles of ^99m Tc radiopharmaceutical preparation and design and focuses on the ^99m Tc radiochemistry relevant to its pharmaceutical applications. The ^99m Tc complexes are described based on the most typical examples in each category, keeping up with the state-of-the-art in the field. In addition, the main current strategies to develop targeted ^99m Tc radiopharmaceuticals are summarized.

Selection of an optimal cysteine-containing peptide-based chelator for labeling of affibody molecules with (188)Re

Affibody molecules constitute a class of small (7 kDa) scaffold proteins that can be engineered to have excellent tumor targeting properties. High reabsorption in kidneys complicates development of affibody molecules for radionuclide therapy. In this study, we evaluated the influence of the composition of cysteine-containing C-terminal peptide-based chelators on the biodistribution and renal retention of (188)Re-labeled anti-HER2 affibody molecules. Biodistribution of affibody molecules containing GGXC or GXGC peptide chelators (where X is G, S, E or K) was compared with biodistribution of a parental affibody molecule ZHER2:2395 having a KVDC peptide chelator. All constructs retained low picomolar affinity to HER2-expressing cells after labeling. The biodistribution of all (188)Re-labeled affibody molecules was in general comparable, with the main observed difference found in the uptake and retention of radioactivity in excretory organs. The (188)Re-ZHER2:V2 affibody molecule with a GGGC chelator provided the lowest uptake in all organs and tissues. The renal retention of (188)Re-ZHER2:V2 (3.1 ± 0.5 %ID/g at 4 h after injection) was 55-fold lower than retention of the parental (188)Re-ZHER2:2395 (172 ± 32 %ID/g). We show that engineering of cysteine-containing peptide-based chelators can be used for significant improvement of biodistribution of (188)Re-labeled scaffold proteins, particularly reduction of their uptake in excretory organs.

Development of an imaging-guided CEA-pretargeted radionuclide treatment of advanced colorectal cancer: first clinical results

Background:

Radiolabelled antibody targeting of cancer is limited by slow blood clearance. Pretargeting with a non-radiolabelled bispecific monoclonal antibody (bsMAb) followed by a rapidly clearing radiolabelled hapten peptide improves tumour localisation. The primary goals of this first pretargeting study in patients with the anti-CEACAM5 × anti-hapten (HSG) bsMAb, TF2, and the radiolabelled hapten-peptide, IMP288, were to assess optimal pretargeting conditions and safety in patients with metastatic colorectal cancer (CRC).

Methods:

Different dose schedules were studied in four cohorts of five patients: (1) shortening the interval between the bsMAb and peptide administration (5 days vs 1 day), (2) escalating the TF2 dose (from 75 to 150 mg), and (3) reducing the peptide dose (from 100 to 25 μg). After confirmation of tumour targeting by ¹¹¹In-IMP288, patients were treated with a bsMAb/¹⁷⁷Lu-IMP288 cycle.

Results:

Rapid and selective tumour targeting of the radiolabelled peptide was visualised within 1 h, with high tumour-to-tissue ratios (>20 at 24 h). Improved tumour targeting was achieved with a 1-day interval between the administration of the bsMAb and the peptide and with the 25-μg peptide dose. High ¹⁷⁷Lu-IMP288 doses (2.5–7.4 GBq) were well tolerated, with some manageable TF2 infusion reactions, and transient grades 3–4 thrombocytopaenia in 10% of the patients who received ¹⁷⁷Lu-IMP288.

Conclusion:

This phase I study demonstrates for the first time that pretargeting with bsMAb TF2 and radiolabelled IMP288 in patients with CEA-expressing CRC is feasible and safe. With this pretargeting method, tumours are specifically and rapidly targeted.

Quantitative immuno-SPECT monitoring of pretargeted radioimmunotherapy with a bispecific antibody in an intraperitoneal nude mouse model of human colon cancer

The prospects for using pretargeted immuno-SPECT to monitor the response to pretargeted radioimmunotherapy were examined. In this study, a bispecific anticarcinoembryonic antigen (CEACAM5; CD66e) × antihapten monoclonal antibody, TF2, was used in combination with a small (1.5 kD) peptide, IMP288, labeled with (111)In and (177)Lu.

METHODS

First, tumor uptake of (111)In-IMP288 and (177)Lu-IMP288, as determined by immuno-SPECT, was validated by ex vivo counting. Two groups of female BALB/c nude mice had LS174T tumors implanted in the peritoneal cavity. They received intravenous injections of TF2, followed by 10 MBq of (111)In-IMP288 or 90 MBq of (177)Lu-IMP288. A control group of non-tumor-bearing mice received TF2 and (111)In-IMP288. One hour after the radiolabeled IMP288 was given, small-animal SPECT/CT images were acquired, and subsequently animals were dissected. Furthermore, a survival study was performed in 3 groups of 10 mice with intraperitoneal tumors: mice received TF2 and (177)Lu-IMP288 (60 MBq), nonpretargeted (177)Lu-IMP288 (60 MBq), or phosphate-buffered saline. Immuno-SPECT scans were acquired directly after therapy and at 14 and 45 d after therapy. Tumor growth was analyzed in the successive scans in each animal.

RESULTS

(111)In- and (177)Lu-labeled IMP288 had similar in vivo distribution. The activity measured in the pretargeted immuno-SPECT images correlated well with the uptake measured in the dissected tumors (Pearson r = 0.99, P < 0.05). In the therapy study, the SPECT images showed rapid and selective tumor targeting with high tumor-to-background contrast (30 ± 12) as early as 1 h after injection. The successive images of the treated mice showed delayed tumor growth in the pretargeted radioimmunotherapy group, corresponding with their prolonged survival.

CONCLUSION

Pretargeted immuno-SPECT with TF2 and (111)In- or (177)Lu-IMP288 can be used to predict and confirm tumor targeting and monitor the therapeutic effect of pretargeted radioimmunotherapy.

Imaging small human prostate cancer xenografts after pretargeting with bispecific bombesin-antibody complexes and targeting with high specific radioactivity labeled polymer-drug conjugates

PURPOSE

Pretargeting with bispecific monoclonal antibodies (bsMAb) for tumor imaging was developed to enhance target to background activity ratios. Visualization of tumors was achieved by the delivery of mono- and divalent radiolabeled haptens. To improve the ability to image tumors with bsMAb, we have combined the pretargeting approach with targeting of high specific activity radiotracer labeled negatively charged polymers. The tumor antigen-specific antibody was replaced with bombesin (Bom), a ligand that binds specifically to the growth receptors that are overexpressed by many tumors including prostate cancer. Bomanti- diethylenetriaminepentaacetic acid (DTPA) bispecific antibody complexes were used to demonstrate pretargeting and imaging of very small human prostate cancer xenografts targeted with high specific activity ¹¹¹In- or ⁹⁹mTc-labeled negatively charged polymers.

METHODS

Bispecific antibody complexes consisting of intact anti-DTPA antibody or Fab′ linked to Bom via thioether bonds (Bom-bsCx or Bom-bsFCx, respectively) were used to pretarget PC-3 human prostate cancer xenografts in SCID mice. Negative control mice were pretargeted with Bom or anti-DTPA Ab. 111In-Labeled DTPA-succinyl polylysine (DSPL) was injected intravenously at 24 h (7.03 ± 1.74 or 6.88 ± 1.89 MBq ¹¹¹In-DSPL) after Bom-bsCx or 50 ± 5.34 MBq of ⁹⁹mTc-DSPL after Bom-bsFCx pretargeting, respectively. Planar or single photon emission computed tomography (SPECT)/CT gamma images were obtained for up to 3 h and only planar images at 24 h. After imaging, all mice were killed and biodistribution of 111In or 99mTc activities were determined by scintillation counting.

RESULTS

Both planar and SPECT/CT imaging enabled detection of PC-3 prostate cancer lesions less than 1-2 mm in diameter in 1-3 h post 111In-DSPL injection. No lesions were visualized in Bom or anti-DTPA Ab pretargeted controls. 111In-DSPL activity in Bom-bsCx pretargeted tumors (1.21 ± 0.36 %ID/g) was 5.4 times that in tumors pretargeted with Bom or anti-DTPA alone (0.22 ± 0.08, p = 0.001). PC-3 xenografts pretargeted with Bom-bsFCx and targeted with ⁹⁹mTc-DSPL were visualizable by 1-3 h. Exquisite tumor uptake at 24 h (6.54 ± 1.58 %ID/g) was about 15 times greater than that of Bom pretargeted controls (0.44 ± 0.17, p = 0.002).

CONCLUSION

Pretargeting prostate cancer with Bom-bsCx or Bom-bsFCx enabled fast delivery of high specific radioactivity ¹¹¹In- or ⁹⁹mTc-labeled polymer-drug conjugates resulting in visualization of lesions smaller than 1- 2 mm in diameter within 3 h.

Pretargeted molecular imaging and radioimmunotherapy

Pretargeting is a multi-step process that first has an unlabeled bispecific antibody (bsMAb) localize within a tumor by virtue of its anti-tumor binding site(s) before administering a small, fast-clearing radiolabeled compound that then attaches to the other portion of the bsMAb. The compound's rapid clearance significantly reduces radiation exposure outside of the tumor and its small size permits speedy delivery to the tumor, creating excellent tumor/nontumor ratios in less than 1 hour. Haptens that bind to an anti-hapten antibody, biotin that binds to streptavidin, or an oligonucleotide binding to a complementary oligonucleotide sequence have all been radiolabeled for use by pretargeting. This review will focus on a highly flexible anti-hapten bsMAb platform that has been used to target a variety of radionuclides to image (SPECT and PET) as well as treat tumors.

Cancer radioimmunotherapy

Targeting of radionuclides with antibodies, or radioimmunotherapy, has been an active field of research spanning nearly 50 years, evolving with advancing technologies in molecular biology and chemistry, and with many important preclinical and clinical studies illustrating the benefits, but also the challenges, which all forms of targeted therapies face. There are currently two radiolabeled antibodies approved for the treatment of non-Hodgkin lymphoma, but radioimmunotherapy of solid tumors remains a challenge. Novel antibody constructs, focusing on treatment of localized and minimal disease, and pretargeting are all promising new approaches that are currently under investigation.

Pretargeted immuno-PET of CEA-expressing intraperitoneal human colonic tumor xenografts: a new sensitive detection method

BACKGROUND

In this study, pretargeted immuno-positron-emission tomography [PET] with a bispecific monoclonal anti-carcinoembryonic antigen [CEA] (CEACAM5) × anti-hapten antibody (bispecific monoclonal antibody [bsmAb]) and a small (1.5 kD) peptide labeled with 68Ga was compared to fludeoxyglucose [18F-FDG]-PET for detecting intraperitoneal [i.p.] CEA-expressing human colonic tumor xenografts in nude mice.

METHODS

Two groups of female BALB/c nude mice were inoculated with LS174T human colonic tumor cells i.p. One group received 5 MBq 18F-FDG, and the other received intravenous injections of the bsmAb, followed 16 h later with 5 MBq of 68Ga-labeled peptide. One hour after the radiolabeled peptide or FDG was given, micro-PET/computed tomography images were acquired. Thereafter, the uptake of the 68Ga or 18F in dissected tissue was determined.

RESULTS

Within 1 h, high uptake of the 68Ga-labeled peptide in the tumor lesions (23.4 ± 7.2% ID/g) and low background activity levels were observed (e.g., tumor-to-intestine ratio, 58 ± 22). This resulted in a clear visualization of all intra-abdominal tumor lesions ≥ 10 μL and even some tumors as small as 5 μL (2 mm diameter). 18F-FDG efficiently localized in the tumors (8.7 ± 3.1% ID/g) but also showed physiological uptake in various normal tissues (e.g., tumor-to-intestine ratio, 3.9 ± 1.1).

CONCLUSIONS

Pretargeted immuno-PET with bsmAb and a 68Ga-labeled peptide could be a very sensitive imaging method for imaging colonic cancer, disclosing occult lesions.

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.

Quantitative and specific molecular imaging of cancer with labeled engineered monoclonal antibody fragments

The high target specificity of antibodies and related constructs makes them excellent scaffolds for molecular-imaging probes. Quantitative data on biodistribution and pharmacokinetics can be acquired by radiolabeling these agents. Such studies demonstrate prolonged circulation times and resulting nonspecific accumulation with high background signal using antibody-based agents. Antibody fragments demonstrate more rapid clearance, but lower tumor uptake. Optical labeling of antibodies provides a basis for developing activatable probes that can image antigens with very high specificity, potentially allowing for the simultaneous visualization of multiple targets. While radioimmunoimaging provides valuable whole-body, quantitative information, activatable optical antibody-based agents could generate real-time diagnostic and prognostic information about near-surface lesions at high-spatial and temporal resolution without requiring ionizing radiation.

Affinity enhancement pretargeting: synthesis and testing of a 99mTc-labeled bivalent MORF

Pretargeting with bivalent effectors capable of bridging antitumor antibodies (affinity enhancement pretargeting) has been reported to provide superior results by affinity enhancement. Phosphorodiamidate morpholinos (MORFs) and other DNA analogues used for pretargeting are ideally suited as bivalent effectors since they are easily synthesized and the distance between binding regions, a determinant of binding, may be adjusted simply by lengthening the chain. We have shown by surface plasmon resonance that bivalent MORFs will provide superior affinity enhancement provided that suitable spacing exists between the binding regions. The goals of this study were to synthesize a bivalent MORF with a MAG(3) group attached for technetium-99m ((99m)Tc) radiolabeling, investigate whether the bivalent MORF showed improved cell accumulation in culture compared to its corresponding monovalent MORF and compare biodistributions in normal mice and in pretargeted tumored mice. An excess of an amine derivatized 18 mer MORF with 6 nonbinding bases for spacing was reacted with Fmoc-l-beta-homoglutamic acid to form duplexes via their carboxylate groups and, after deprotection, conjugated with NHS-MAG(3) to attach the chelator. The anti-CEA antibody MN14 was conjugated with a 12 mer complementary MORF (i.e., cMORF). The binding behavior between radiolabeled monovalent and bivalent MORFs was compared in LS174T tumor cells at 4 degrees C pretargeted with MN14-cMORF. Biodistributions of radiolabeled monovalent and bivalent MORFs at 3 h postadministration were measured in normal mice and in tumor mice pretargeted with MN14-cMORF. In the pretargeted cells in culture, the accumulation of the bivalent MORF was significantly higher than the monovalent MORF (p = 0.002), thus providing strong evidence for affinity enhancement. In normal mice, whole body clearance of the bivalent and monovalent MORFs was equally rapid. In tumored mice, tumor accumulation of the radiolabeled bivalent MORF was significantly higher than that of the monovalent MORF. In conclusion, a bivalent MAG(3)-MORF was successfully synthesized and radiolabeled with (99m)Tc. While a pharmacokinetic effect for the higher tumor accumulations in pretargeted mice of the radiolabeled bivalent MORF cannot be excluded, the results may be best explained by affinity enhancement. Thus two monovalent MORFs were covalently conjugated into a bivalent MORF effector to improve tumor targeting by both pharmacokinetics and affinity enhancement influences.

Improved cancer therapy and molecular imaging with multivalent, multispecific antibodies

Antibodies are highly versatile proteins with the ability to be used to target diverse compounds, such as radionuclides for imaging and therapy, or drugs and toxins for therapy, but also can be used unconjugated to elicit therapeutically beneficial responses, usually with minimal toxicity. This update describes a new procedure for forming multivalent and/or multispecific proteins, known as the dock-and-lock (DNL) technique. Developed as a procedure for preparing bispecific antibodies capable of binding divalently to a tumor antigen and monovalently to a radiolabeled hapten-peptide for pretargeted imaging and therapy, this methodology has the flexibility to create a number of other biologic agents of therapeutic interest. A variety of constructs, based on anti-CD20 and CD22 antibodies, have been made, with results showing that multispecific antibodies have very different properties from the respective parental monospecific antibodies. The technique is not restricted to antibody combination, but other biologics, such as interferon-alpha2b, have been prepared. These types of constructs not only allow small biologics to be sustained in the blood longer, but also to be selectively targeted. Thus, DNL technology is a highly flexible platform that can be used to prepare many different types of agents that could further improve cancer detection and therapy.

Pretargeted radioimmunotherapy for hematologic and other malignancies

Radioimmunotherapy (RIT) has emerged as one of the most promising treatment options, particularly for hematologic malignancies. However, this approach has generally been limited by a suboptimal therapeutic index (target-to-nontarget ratio) and an inability to deliver sufficient radiation doses to tumors selectively. Pretargeted RIT (PRIT) circumvents these limitations by separating the targeting vehicle from the subsequently administered therapeutic radioisotope, which binds to the tumor-localized antibody or is quickly excreted if unbound. A growing number of preclinical proof-of-principle studies demonstrate that PRIT is feasible and safe and provides improved directed radionuclide delivery to malignant cells compared with conventional RIT while sparing normal cells from nonspecific radiotoxicity. Early phase clinical studies corroborate these preclinical findings and suggest better efficacy and lesser toxicities in patients with hematologic and other malignancies. With continued research, PRIT-based treatment strategies promise to become cornerstones to improved outcomes for cancer patients despite their complexities.

Improved 18F labeling of peptides with a fluoride-aluminum-chelate complex

We reported previously the feasibility to radiolabel peptides with fluorine-18 ((18)F) using a rapid one-pot method that first mixes (18)F(-) with Al(3+) and then binds the (Al(18)F)(2+) complex to a NOTA ligand on the peptide. In this report, we examined several new NOTA ligands and determined how temperature, reaction time, and reagent concentration affected the radiolabeling yield. Four structural variations of the NOTA ligand had isolated radiolabeling yields ranging from 5.8% to 87% under similar reaction conditions. All of the Al(18)F NOTA complexes were stable in vitro in human serum, and those that were tested in vivo also were stable. The radiolabeling reactions were performed at 100 degrees C, and the peptides could be labeled in as little as 5 min. The IMP467 peptide could be labeled up to 115 GBq/micromol (3100 Ci/mmol), with a total reaction and purification time of 30 min without chromatographic purification.

Recombinant bispecific monoclonal antibodies prepared by the dock-and-lock strategy for pretargeted radioimmunotherapy

The selective delivery of therapeutic radionuclides is a promising approach for treating cancer. Antibody-targeted radionuclides are of particular interest, with 2 products approved for the treatment of certain forms of non-Hodgkin lymphoma. However, for many other cancers, radioimmunotherapy has been ineffective, being limited by prolonged exposure to the highly radiosensitive bone marrow. An alternative approach, known as pretargeting, separates radionuclide from the antibody, allowing the radiation to be delivered on a small molecule that can quickly and efficiently migrate into the tumor, and then rapidly clear from the body with minimal retention in tissues. Several pretargeting methods have been developed that differ in the way they selectively capture the radionuclide. This review focuses on the development of a novel form of bispecific monoclonal antibody (bsMAb) pretargeting that uses a unique radiolabeled hapten-peptide system that can be modified to bind numerous therapeutic and imaging radionuclides. Together with a specialized recombinant humanized bsMAb prepared with by a technique known as the Dock-and-Lock method, this pretargeting procedure has been examined in many different animal models, showing a high level of sensitivity and specificity for localizing tumors, and improved efficacy with less hematologic toxicity associated with directly radiolabeled IgG. The bsMAb is a tri-Fab structure, having 2 binding arms for the tumor antigen and 1 capable of binding a hapten-peptide. Preclinical studies were preformed to support the clinical use of a bsMAb and a hapten-peptide bearing a single DOTA moiety (IMP-288). A phase 0 trial found an (131)I-tri-Fab bsMAb, TF2, that targets carcinoembryonic antigen was stable in vivo, quickly clears from the blood, and localizes known tumors. The first-in-patient pretargeting experience with the (111)In-IMP-288 also observed rapid clearance and low tissue (kidney) retention, as well as localization of tumors, providing initial promising evidence for developing these materials for radioimmunotherapy.

Pretargeted immuno-positron emission tomography imaging of carcinoembryonic antigen-expressing tumors with a bispecific antibody and a 68Ga- and 18F-labeled hapten peptide in mice with human tumor xenografts

(18)F-Fluorodeoxyglucose ((18)F-FDG) is the most common molecular imaging agent in oncology, with a high sensitivity and specificity for detecting several cancers. Antibodies could enhance specificity; therefore, procedures were developed for radiolabeling a small ( approximately 1451 Da) hapten peptide with (68)Ga or (18)F to compare their specificity with (18)F-FDG for detecting tumors using a pretargeting procedure. Mice were implanted with carcinoembryonic antigen (CEA; CEACAM5)-expressing LS174T human colonic tumors and a CEA-negative tumor, or an inflammation was induced in thigh muscle. A bispecific monoclonal anti-CEA x anti-hapten antibody was given to mice, and 16 hours later, 5 MBq of (68)Ga- or (18)F-labeled hapten peptides were administered intravenously. Within 1 hour, tissues showed high and specific targeting of (68)Ga-IMP-288, with 10.7 +/- 3.6% ID/g uptake in the tumor and very low uptake in normal tissues (e.g., tumor-to-blood ratio of 69.9 +/- 32.3), in a CEA-negative tumor (0.35 +/- 0.35% ID/g), and inflamed muscle (0.72 +/- 0.20% ID/g). (18)F-FDG localized efficiently in the tumor (7.42 +/- 0.20% ID/g) but also in the inflamed muscle (4.07 +/- 1.13% ID/g) and in several normal tissues; thus, pretargeted (68)Ga-IMP-288 provided better specificity and sensitivity. Positron emission tomography (PET)/computed tomography images reinforced the improved specificity of the pretargeting method. (18)F-labeled IMP-449 distributed similarly in the tumor and normal tissues as the (68)Ga-labeled IMP-288, indicating that either radiolabeled hapten peptide could be used. Thus, pretargeted immuno-PET does exceptionally well with short-lived radionuclides and is a highly sensitive procedure that is more specific than (18)F-FDG-PET. Mol Cancer Ther; 9(4); 1019-27. (c)2010 AACR.

Advances in Radioimmunotherapy in the Age of Molecular Engineering and Pretargeting

Now that radioimmunotherapy is an approved method for the treatment of certain types of non-Hodgkin's lymphoma, investigators are turning to new approaches to further improve radionuclide targeting in hopes of expanding the use of this technology. A number of innovative recombinant proteins have been developed with more favorable pharmacokinetic and targeting properties than standard whole IgG, which conceivably could improve the therapeutic index for cancer treatment. Pretargeting methods also are coming of age, with preclinical and early clinical studies in a variety of cancers illustrating how this alternative approach can enhance the therapeutic window several-fold of what has been possible with directly radiolabeled IgG. This review will discuss some of these promising new developments.

Radionuclide-Based Cancer Imaging Targeting the Carcinoembryonic Antigen

Carcinoembryonic antigen (CEA), highly expressed in many cancer types, is an important target for cancer diagnosis and therapy. Radionuclide-based imaging techniques (gamma camera, single photon emission computed tomography [SPECT] and positron emission tomography [PET]) have been extensively explored for CEA-targeted cancer imaging both preclinically and clinically. Briefly, these studies can be divided into three major categories: antibody-based, antibody fragment-based and pretargeted imaging. Radiolabeled anti-CEA antibodies, reported the earliest among the three categories, typically gave suboptimal tumor contrast due to the prolonged circulation life time of intact antibodies. Subsequently, a number of engineered anti-CEA antibody fragments (e.g. Fab’, scFv, minibody, diabody and scFv-Fc) have been labeled with a variety of radioisotopes for CEA imaging, many of which have entered clinical investigation. CEA-Scan (a ^99mTc-labeled anti-CEA Fab’ fragment) has already been approved by the United States Food and Drug Administration for cancer imaging. Meanwhile, pretargeting strategies have also been developed for CEA imaging which can give much better tumor contrast than the other two methods, if the system is designed properly. In this review article, we will summarize the current state-of-the-art of radionuclide-based cancer imaging targeting CEA. Generally, isotopes with short half-lives (e.g. ¹⁸F and ^99mTc) are more suitable for labeling small engineered antibody fragments while the isotopes with longer half-lives (e.g. ¹²³I and ¹¹¹In) are needed for antibody labeling to match its relatively long circulation half-life. With further improvement in tumor targeting efficacy and radiolabeling strategies, novel CEA-targeted agents may play an important role in cancer patient management, paving the way to “personalized medicine”.

A Divalent Hapten-Peptide Induces Apoptosis in Human Non–Hodgkin Lymphoma Cell Lines Targeted by Anti-CD20 × Anti-Hapten Bispecific Antibodies

PURPOSE

Bispecific antibody (bsMAb) pretargeting procedures use divalent hapten-peptides to stabilize the binding of the hapten-peptide on tumor cells by a process known as the affinity enhancement system. The goal of this study was to determine if a divalent hapten-peptide could induce apoptosis by cross-linking bsMAb bound to CD20.

METHODS

Three forms of bsMAbs were prepared by coupling the IgG, F(ab')2, or Fab' of a humanized anti-CD20 antibody to a Fab' of a murine antibody directed against the hapten histamine-succinyl-glycine (HSG). A recombinant bsMAb with divalent binding to CD20 and monovalent binding to HSG was also examined. Induction of apoptosis on SU-DHL-6, RL, and Ramos cells was examined by propidium iodide staining, caspase-3 activation, and mitochondrial membrane potential collapse, and compared with induction by cross-linking an anti-CD20 IgG with an antispecies antibody.

RESULTS

The various forms of bsMAb had differing baseline levels of apoptosis in the absence of the divalent HSG peptide. The addition of the divalent HSG peptide significantly increased the level of apoptosis seen with the Fab'xFab' bsMAb by 2.2- to 3.9-fold, as well as the F(ab')2xFab', IgGxFab', and the recombinant bsMAbs by approximately 1.5-fold.

CONCLUSIONS

The addition of a divalent HSG peptide to various forms of bispecific anti-CD20 MAbs could enhance apoptotic signaling in several lymphoma cells. This effect was more consistently measured when the orientation of the anti-hapten-binding arm of the bsMAb was well defined, such as in the Fab'xFab' and recombinant forms of bsMAb.

Novel radiolabeled antibody conjugates

This article reviews the development of radioimmunoconjugates as a new class of cancer therapeutics. Numerous conjugates involving different antigen targets, antibody forms, radionuclides and methods of radiochemistry have been studied in the half-century since radioactive antibodies were first used in model systems to selectively target radiation to tumors. Whereas directly conjugated antibodies, fragments and subfragments have shown promise preclinically, the same approaches have not gained success in patients except in radiosensitive hematological neoplasms, or in settings involving minimal or locoregional disease. The separation of tumor targeting from the delivery of the therapeutic radionuclide in a multistep process called pretargeting has the potential to overcome many of the limitations of conventional, or one-step, radioimmunotherapy, with initial preclinical and clinical data showing increased sensitivity, specificity and higher radiation doses delivered. Our particular focus in pretargeting is the use of bispecific, trimeric (three Fab's) constructs made by a new antibody engineering method termed 'dock-and-lock.

Cancer Imaging and Therapy with Bispecific Antibody Pretargeting

This article reviews recent preclinical and clinical advances in the use of pretargeting methods for the radioimmunodetection and radioimmunotherapy of cancer. Whereas directly-labeled antibodies, fragments, and subfragments (minibodies and other constructs) have shown promise in both imaging and therapy applications over the past 25 years, their clinical adoption has not fulfilled the original expectations due to either poor image resolution and contrast in scanning or insufficient radiation doses delivered selectively to tumors for therapy. Pretargeting involves the separation of the localization of tumor with an anticancer antibody from the subsequent delivery of the imaging or therapeutic radionuclide. This has shown improvements in both imaging and therapy by overcoming the limitations of conventional, or 1-step, radioimmunodetection or radioimmunotherapy. We focus herein on the use of bispecific antibodies followed by radiolabeled peptide haptens as a new modality of selective delivery of radionuclides for the imaging and therapy of cancer. Our particular emphasis in pretargeting is the use of bispecific trimeric (3 Fab's) recombinant constructs made by a modular method of antibody and protein engineering of fusion molecules called Dock and Lock (DNL).

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.

Irreversibly binding anti-metal chelate antibodies: Artificial receptors for pretargeting

Antibodies against metal chelates may potentially be used in biomedical applications such as targeted imaging and therapy of cancer. Highly specific monoclonal antibodies can be developed, but their binding strength needs to be maximized for them to be of practical use. In general, the half-life for dissociation of an antibody-ligand complex is more than an order of magnitude lower than the half-lifetimes for decay of medically useful radiometal ions. Practically speaking, the metal chelate-based ligand will not be bound to its receptor long enough for all of the bound radiometal to decay. A novel approach to this problem is a combination of synthetic chemistry and site-directed mutagenesis, to position a mildly reactive group on the metal chelate adjacent to a complementary reactive group on the antibody when the complex is formed. The partners are chosen to be sufficiently unreactive so that they coexist with other molecules in living systems without undergoing reaction. When the antibody-chelate complex is formed the effective local concentrations of the two groups can be non-physically large, so that a permanent link is formed in the complex even though no reaction occurs when the partners are free in solution.

Improving the delivery of radionuclides for imaging and therapy of cancer using pretargeting methods

The article reviews the background and current status of pretargeting for cancer imaging and therapy with radionuclides. Pretargeting procedures were introduced approximately 20 years ago as an alternative to directly radiolabeled antibodies. Because they were multistep processes, they were met with resistance but have since progressed to simple and improved procedures that could become the next generation of imaging and therapy with radionuclides. The separation of the radiolabeled compound from the antibody-targeting agent affords pretargeting procedures considerable flexibility in the radiolabeling process, providing opportunities for molecular imaging using gamma- or positron-emitting radionuclides and a variety of beta- and alpha-emitting radionuclides of therapeutic applications. Pretargeting methods improve tumor/nontumor ratios, exceeding that achieved with directly radiolabeled Fab' fragments, particularly within just a few hours of the radionuclide injection. In addition, tumor uptake exceeds that of a Fab' fragment by as much as 10-fold, giving pretargeting a greatly enhanced sensitivity for imaging. Advances in molecular biology have led to the development of novel binding proteins that have further improved radionuclide delivery in these systems. Studies in a variety of hematologic and solid tumor models have shown advantages of pretargeting compared with directly radiolabeled IgG for therapy, and there are several clinical studies under way that are also showing promising results. Thus, the next generation of targeting agents will likely employ pretargeting approaches to optimize radionuclide delivery for a wide range of applications.

Pretargeting of Carcinoembryonic Antigen–Expressing Cancers with a Trivalent Bispecific Fusion Protein Produced in Myeloma Cells

PURPOSE

To characterize a novel trivalent bispecific fusion protein and evaluate its potential utility for pretargeted delivery of radionuclides to tumors.

EXPERIMENTAL DESIGN

hBS14, a recombinant fusion protein that binds bispecifically to carcinoembryonic antigen (CEA) and the hapten, histamine-succinyl-glycine (HSG), was produced by transgenic myeloma cells and purified to near homogeneity in a single step using a novel HSG-based affinity chromatography system. Biochemical characterization included size-exclusion high-performance liquid chromatography (SE-HPLC), SDS-PAGE, and isoelectric focusing. Functional characterization was provided by BIAcore and SE-HPLC. The efficacy of hBS14 for tumor pretargeting was evaluated in CEA-expressing GW-39 human colon tumor-bearing nude mice using a bivalent HSG hapten (IMP-241) labeled with (111)In.

RESULTS

Biochemical analysis showed that single-step affinity chromatography provided highly purified material. SE-HPLC shows a single protein peak consistent with the predicted molecular size of hBS14. SDS-PAGE analysis shows only two polypeptide bands, which are consistent with the calculated molecular weights of the hBS14 polypeptides. BIAcore showed the bispecific binding properties and suggested that hBS14 possesses two functional CEA-binding sites. This was supported by SE-HPLC immunoreactivity experiments. All of the data suggest that the structure of hBS14 is an 80 kDa heterodimer with one HSG and two CEA binding sites. Pretargeting experiments in the mouse model showed high uptake of radiopeptide in the tumor, with favorable tumor-to-nontumor ratios as early as 3 hours postinjection.

CONCLUSIONS

The results indicate that hBS14 is an attractive candidate for use in a variety of pretargeting applications, particularly tumor therapy with radionuclides and drugs.

Pretargeting of Carcinoembryonic Antigen–Expressing Tumors with a Biologically Produced Bispecific Anticarcinoembryonic Antigen × Anti-Indium–Labeled Diethylenetriaminepentaacetic Acid Antibody

PURPOSE

The aim of these studies was to develop a pretargeting strategy for CEA-expressing cancers using biologically produced bispecific monoclonal antibodies (bsMAb). The bsMAbs used in this system have affinity for the carcinoembryonic antigen on the one hand, and for indium-labeled diethylenetriaminepentaacetic acid (DTPA), on the other.

EXPERIMENTAL DESIGN

Stable quadroma clones producing bsMAb MN-14xDTIn-1 were isolated. LS174T tumor-bearing mice were injected with 1 to 100 microg of bsMAb followed by 1 to 60 ng of an (111)In-labeled bivalent peptide [Ac-Phe-Lys(DTPA)-Tyr-Lys(DTPA)-NH2]. Mice were killed at 24 hours postinjection and the biodistribution of the radiolabel was determined. The biodistribution of diDTPA labeled with four different radionuclides ((111)In, 99mTc, nonresidualizing 125I, and residualizing 125I) was determined at various time points postinjection following pretargeting of LS174T tumors with bsMAb MN-14xDTIn-1.

RESULTS

Optimal tumor targeting was observed when tumors were pretargeted with 10 microg of bsMAb MN-14xDTIn-1 and when 6 ng of a radiolabeled peptide was given 72 hours later. The uptake of the four radiolabels in LS174T tumors at 4 hours postinjection was similar. However, at later time points, the (111)In-label and residualizing 125I-label were better retained in the tumor than the nonresidualizing 125I label. Although the absolute uptake in the tumor (in terms of percentage of injected dose per gram of tissue) was 5-fold lower than the uptake obtained with directly labeled MN-14, the pretargeting strategy revealed much higher tumor-to-blood ratios due to the rapid clearance of the radiolabel from the circulation as compared with (111)In-MN-14 (445 +/- 90 and 5.3 +/- 1.1, respectively, at 72 hours postinjection).

CONCLUSIONS

Effective targeting of carcinoembryonic antigen-expressing tumors was achieved with a newly produced bispecific antibody. The (111)In-labeled L-amino acid peptide and 125I-D-amino acid peptide were better retained in the tumor than the 99mTc- and 125I-L-amino acid peptide. Very high tumor-to-blood ratios were obtained due to rapid background clearance.

Comparison of IgG and F(ab')2 fragments of bispecific anti-RCCxanti-DTIn-1 antibody for pretargeting purposes

PURPOSE

An effective pretargeting strategy was developed for renal cell carcinoma (RCC) based on a biologically produced bispecific monoclonal antibody: anti-RCCxanti-DTPA(In) (bsMAb: G250xDTIn-1). Tumour uptake of a (111)In-labelled bivalent peptide after pretargeting with bsMAb G250xDTIn-1 was relatively high compared with that in other pretargeting systems using chemically coupled F(ab')(2) fragments. Here, we investigated the effect of the bsMAb form in the pretargeting strategy.

METHODS

To determine the optimal interval between the administration of each of the bsMAb forms and the (111)In-labelled bivalent peptide, the biodistribution of the radioiodinated bsMAb forms was studied in athymic mice with subcutaneous SK-RC-1 RCC tumours. Since tumour targeting of the radiolabelled peptide depends on the bsMAb form and dose, a bsMAb dose escalation study was carried out for both bsMAb forms. Under optimised conditions, the biodistribution of the (111)In label in mice with pretargeted RCC was determined from 4 h up to 7 days p.i.

RESULTS

The optimal interval between the two administrations was 72 h for the bsMAb IgG and 4 h for the bsMAb F(ab')(2). The optimal bsMAb dose for intact IgG was 67 pmol and the optimal bsMAb F(ab')(2) dose was 200 pmol. Targeting of the pretargeted RCC with 4 pmol (111)In-labelled bivalent peptide revealed high tumour uptake with both bsMAb forms.

CONCLUSION

With the pretargeting strategy, using either bsMAb IgG or bsMAb F(ab')(2), very efficient peptide targeting of the tumour was obtained. Uptake and retention of the radiolabel in the tumour with the pretargeting approach are not affected by the bsMAb form used.

Improved targeting of pancreatic cancer: experimental studies of a new bispecific antibody, pretargeting enhancement system for immunoscintigraphy

PURPOSE

The early detection and diagnosis of pancreatic cancer remains a major clinical challenge in which imaging procedures have a central role. The purpose of this study was to evaluate a pretargeting method with a bispecific PAM4 (bsPAM4; anti-MUC1) antibody for radioimmunoscintigraphy of experimental human pancreatic cancer.

EXPERIMENTAL DESIGN

A bispecific F(ab')(2) antibody was generated from chimeric PAM4 Fab' and murine 734 (anti-indium-diethylenetriaminepentaacetic acid) Fab' fragments and then used in conjunction with 2 peptide haptens ((111)In-IMP-156 and (99m)Tc-IMP-192). Biodistribution studies and radioimmunoscintigraphic imaging properties of the radiolabeled bsPAM4, and pretargeted, radiolabeled peptides were examined in the CaPan1 human pancreatic tumor grown as s.c. xenografts in athymic nude mice. Tumor uptake and tumor:nontumor ratios were compared with a nontargeting irrelevant anti-CD20, bispecific rituximab, radiolabeled peptides alone, and with directly labeled PAM4.

RESULTS

Biodistribution results indicated significantly greater tumor uptake of radiolabeled peptides at 3 h after injection when pretargeting was performed with bsPAM4 as compared with the bispecific rituximab [20.2 +/- 5.5 percentage of injected dose per gram of tissue (%ID/g) versus 0.9 +/- 0.1%ID/g, respectively, for (111)In-IMP-156, and 16.8 +/- 4.8%ID/g versus 1.1 +/- 0.2%ID/g, respectively, for (99m)Tc-IMP-192]. Similar results were obtained at the 24-h time point. Tumor:nontumor ratios were >30 for all of the tissues except the kidneys, where a ratio of 7.8 +/- 2.8 was observed. By immunoscintigraphy, tumors could be visualized as early as 30 min after injection of the radiolabeled peptide.

CONCLUSIONS

These studies demonstrate the feasibility of using the pretargeted, bispecific antibody technology for nuclear imaging of pancreatic cancer. The advantage of pretargeted bsPAM4 antibody as an imaging platform is the high specificity for pancreatic cancer as compared with the physicochemical parameters identified by current imaging technologies.

Advances in pretargeting biotechnology

A major focus of current drug research is to improve drug targeting to internal target sites such as to solid tumors or specific organs. The objective of drug targeting, especially for cancer chemotherapy and radioimmunotherapy, is to enhance the effectiveness of the drug by concentrating it at the target site and minimizing its effects in nontarget sites. Although tumor targeting has been obtained with large long-circulating radiolabeled antibody molecules, normal organ activity, especially in the blood kidneys, liver, and bone marrow is a significant problem. Over the last 20 years, studies to improve the therapeutic use of antibodies have included the use of antibody fragments, chase molecules, metabolizable linkers, antibody-directed enzyme prodrugs (ADEPT), local delivery, and pretargeting. Here, we will review the most interesting recent advances in pretargeting biotechnology.

Investigations of 99mTc morpholino pretargeting in mice

This laboratory is exploring the use of morpholinos (MORFs), synthetic DNA analogues, for nuclear medicine applications, including pretargeting. The anti-CEA antibody MN14 was conjugated with an 18 mer MORF and with diethylenetriaminepentaacetic acid (DTPA) for 111In labelling. In a dual label pretargeting study, tumour-bearing nude mice received different doses of (MN14-DTPA-111In+MN14-MORF) followed, at various times after i.v. injection, by 0.15 microg complementary MORF (cMORF) radiolabelled with 99mTc via MAG(3). Animals were killed 3 h thereafter and tissues were counted for both radionuclides. The 99mTc-cMORF was also administered to tumour bearing mice that, 2 days previously, had received different doses of unlabelled MN14-MORF IgG or, as control, unlabelled Sandoglobulin IgG-MORF (Sandoz-MORF). Tumour uptake was higher at all time points for the labelled antibody itself versus labelled cMORF (8-10 vs 1.3-2.3%ID/g, respectively) in part due to the rapid clearance of cMORF through the kidneys. However, target to non-target ratios were superior for pretargeting at all time points and in all tissue except blood and kidneys. By pretargeting alone, these ratios were highest in all tissues for 15 microg compared to higher MN14-MORF dosages and in all cases were superior to that of the Sandoz-MORF control. The superior target to non-target ratios for pretargeting can be partially explained through calculations based on both radiolabels: after 24 h, only 0-6% of MORF on MN14 was bound by 99mTc-cMORF in liver and spleen suggesting that the antibody is sequestered in these organs and 'invisible' to labelled MORF. Fortunately, this was not the case in tumours in which 50-60% was bound. It is concluded that pretargeting using MORFs provided encouraging results in one mouse model/anti-tumour antibody system. The advantages of pretargeting in this model were evident in the superior target to non-target ratios obtained over conventional imaging.

Use of galactosylated-streptavidin as a clearing agent with 111In-labeled, biotinylated antibodies to enhance tumor/non-tumor localization ratios

Optimal tumor imaging using radiolabeled antibodies (Abs) depends on obtaining the highest possible tumor/non-tumor localization ratios. To increase this ratio, in a mouse xenograft model system, we induced rapid blood clearance of the Ab after extensive penetration of a solid tumor, at 24 hr after Ab injection. By using galactosylated streptavidin (gal-SA) as a clearing agent for biotinylated Abs, and by using an 111In-DTPA (diethylenetriaminepentaacetic acid) label, clearance was directed to hepatocytes (as opposed to Kupffer cells), and the radiolabel was excreted by the hepatocytes into bile, thereby reducing accumulation in the liver. In this study, we directly compared this approach with the use of 99mTc-F(ab)2 fragments, using the same Ab to carcinoembryonic antigen (CEA), with a colon carcinoma xenograft. The gal-SA clearance method produced substantially higher tumor/non-tumor localization ratios for all tissues except the liver, and even for the liver the disadvantage of the gal-SA clearance method was small. We also tested the gal-SA clearance method with a xenograft model of human B-cell lymphoma, using anti-CD22. High tumor/non-tumor ratios were obtained, as previously described with carcinomas of the lung and colon. Therefore, this approach appears to be a generally applicable strategy to obtain relatively high tumor/non-tumor ratios.