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

Cell-based Relay Delivery Strategy in Biomedical Applications

The relay delivery strategy is a two-step targeting approach based on two distinct modules in which the first step with an initiator is to artificially create a target/environment which can be targeted by the follow-up effector. This relay delivery concept creates opportunities to amplify existing or create new targeted signals through deploying initiators to enhance the accumulation efficiency of the following effector at the disease site. As the "live" medicines, cell-based therapeutics possess inherent tissue/cell homing abilities and favorable feasibility of biological and chemical modifications, endowing them the great potential in specifically interacting with diverse biological environments. All these unique capabilities make cellular products great candidates that can serve as either initiators or effectors for relay delivery strategies. In this review, we survey recent advances in relay delivery strategies with a specific focus on the roles of various cells in developing relay delivery systems.

Development of a Versatile and Modular Linker for Antibody-Drug Conjugates Based on Oligonucleotide Strand Pairing

Linker design is crucial to the success of antibody-drug conjugates (ADCs). In this work, we developed a modular linker format for attaching molecular cargos to antibodies based on strand pairing between complementary oligonucleotides. We prepared antibody-oligonucleotide conjugates (AOCs) by attaching 18-mer oligonucleotides to an anti-HER2 antibody through thiol-maleimide chemistry, a method generally applicable to any immunoglobulin with interchain disulfide bridges. The hybridization of drug-bearing complementary oligonucleotides to our AOCs was rapid, stoichiometric, and sequence-specific. AOCs loaded with cytotoxic payloads were able to selectively target HER2-overexpressing cell lines such as SK-BR-3 and N87, with in vitro potencies similar to that of the marketed ADC Kadcyla (T-DM1). Our results demonstrated the potential of utilizing AOCs as a highly versatile and modular platform, where a panel of well-characterized AOCs bearing DNA, RNA, or various nucleic acid analogs, such as peptide nucleic acids, could be easily paired with any cargo of choice for a wide range of diagnostic or therapeutic applications.

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.

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.

Pretargeting in nuclear imaging and radionuclide therapy: Improving efficacy of theranostics and nanomedicines

Pretargeted nuclear imaging and radiotherapy have recently attracted increasing attention for diagnosis and treatment of cancer with nanomedicines. This is because it conceptually offers better imaging contrast and therapeutic efficiency while reducing the dose to radiosensitive tissues compared to conventional strategies. In conventional imaging and radiotherapy, a directly radiolabeled nano-sized vector is administered and allowed to accumulate in the tumor, typically on a timescale of several days. In contrast, pretargeting is based on a two-step approach. First, a tumor-accumulating vector carrying a tag is administered followed by injection of a fast clearing radiolabeled agent that rapidly recognizes the tag of the tumor-bound vector in vivo. Therefore, pretargeting circumvents the use of long-lived radionuclides that is a necessity for sufficient tumor accumulation and target-to-background ratios using conventional approaches. In this review, we give an overview of recent advances in pretargeted imaging strategies. We will critically reflect on the advantages and disadvantages of current state-of-the-art conventional imaging approaches and compare them to pretargeted strategies. We will discuss the pretargeted imaging concept and the involved chemistry. Finally, we will discuss the steps forward in respect to clinical translation, and how pretargeted strategies could be applied to improve state-of-the-art radiotherapeutic approaches.

New insights into the pretargeting approach to image and treat tumours

Tumour pretargeting is a promising strategy for cancer diagnosis and therapy allowing for the rational use of long circulating, highly specific monoclonal antibodies (mAbs) for both non-invasive cancer radioimmunodetection (RID) and radioimmunotherapy (RIT). In contrast to conventional RID/RIT where the radionuclides and oncotropic vector molecules are delivered as presynthesised radioimmunoconjugates, the pretargeting approach is a multistep procedure that temporarily separates targeting of certain tumour-associated antigens from delivery of diagnostic or therapeutic radionuclides. In principle, unlabelled, highly tumour antigen specific mAb conjugates are, in a first step, administered into a patient. After injection, sufficient time is allowed for blood circulation, accumulation at the tumour site and subsequent elimination of excess mAb conjugates from the body. The small fast-clearing radiolabelled effector molecules with a complementary functionality directed to the prelocalised mAb conjugates are then administered in a second step. Due to its fast pharmacokinetics, the small effector molecules reach the malignant tissue quickly and bind the local mAb conjugates. Thereby, corresponding radioimmunoconjugates are formed in vivo and, consequently, radiation doses are deposited mainly locally. This procedure results in a much higher tumour/non-tumour (T/NT) ratio and is favourable for cancer diagnosis and therapy as it substantially minimises the radiation damage to non-tumour cells of healthy tissues. The pretargeting approach utilises specific non-covalent interactions (e.g. strept(avidin)/biotin) or covalent bond formations (e.g. inverse electron demand Diels-Alder reaction) between the tumour bound antibody and radiolabelled small molecules. This tutorial review descriptively presents this complex strategy, addresses the historical as well as recent preclinical and clinical advances and discusses the advantages and disadvantages of different available variations.

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.

In vivo demonstration of an active tumor pretargeting approach with peptide nucleic acid bioconjugates as complementary system

A novel, promising strategy for cancer diagnosis and therapy is the use of a pretargeting approach. For this purpose, the non-natural DNA/RNA analogues Peptide Nucleic Acids (PNAs) are ideal candidates as in vivo recognition units due to their high metabolic stability and lack of unspecific accumulation. In the pretargeting approach, an unlabeled, highly specific antibody-PNA conjugate has sufficient time to target a tumor before administration of a small fast-clearing radiolabeled complementary PNA that hybridizes with the antibody-PNA conjugate at the tumor site. Herein, we report the first successful application of this multistep process using a PNA-modified epidermal growth factor receptor (EGFR) specific antibody (cetuximab) and a complementary 99mTc-labeled PNA. In vivo studies on tumor bearing mice demonstrated a rapid and efficient in vivo hybridization of the radiolabeled PNA with the antibody-PNA conjugate. Decisively, a high specific tumor accumulation was observed with a tumor-to-muscle ratio of >8, resulting in a clear visualization of the tumor by single photon emission computed tomography (SPECT).

Design, Preparation, and Characterization of PNA-Based Hybridization Probes for Affibody-Molecule-Mediated Pretargeting

In radioimmunotherapy, the contrast between tumor and normal tissue can be improved by using a pretargeting strategy with a primary targeting agent, which is conjugated to a recognition tag, and a secondary radiolabeled molecule binding specifically to the recognition tag. The secondary molecule is injected after the targeting agent has accumulated in the tumor and is designed to have a favorable biodistribution profile, with fast clearance from blood and low uptake in normal tissues. In this study, we have designed and evaluated two complementary peptide nucleic acid (PNA)-based probes for specific and high-affinity association in vivo. An anti-HER2 Affibody-PNA chimera, Z(HER2:342)-SR-HP1, was produced by a semisynthetic approach using sortase A catalyzed ligation of a recombinantly produced Affibody molecule to a PNA-based HP1-probe assembled using solid-phase chemistry. A complementary HP2 probe carrying a DOTA chelator and a tyrosine for dual radiolabeling was prepared by solid-phase synthesis. Circular dichroism (CD) spectroscopy and UV thermal melts showed that the probes can hybridize to form a structured duplex with a very high melting temperature (T(m)), both in HP1:HP2 and in Z(HER2:342)-SR-HP1:HP2 (T(m) = 86-88 °C), and the high binding affinity between Z(HER2:342)-SR-HP1 and HP2 was confirmed in a surface plasmon resonance (SPR)-based binding study. Following a moderately fast association (1.7 × 10(5) M(-1) s(-1)), the dissociation of the probes was extremely slow and <5% dissociation was observed after 17 h. The equilibrium dissociation constant (K(D)) for Z(HER2:342)-SR-HP1:HP2 binding to HER2 was estimated by SPR to be 212 pM, suggesting that the conjugation to PNA does not impair Affibody binding to HER2. The biodistribution profiles of (111)In- and (125)I-labeled HP2 were measured in NMRI mice, showing very fast blood clearance rates and low accumulation of radioactivity in kidneys and other organs. The measured radioactivity in blood was 0.63 ± 0.15 and 0.41 ± 0.15%ID/g for (125)I- and (111)In-HP2, respectively, at 1 h p.i., and at 4 h p.i., the kidney accumulation of radioactivity was 0.17 ± 0.04%ID/g for (125)I-HP2 and 3.83 ± 0.39%ID/g for (111)In-HP2. Taken together, the results suggest that a PNA-based system has suitable biophysical and in vivo properties and is a promising approach for pretargeting of Affibody molecules.

Pretargeted imaging and radioimmunotherapy of cancer using antibodies and bioorthogonal chemistry

Selective delivery of radionuclides to tumors may be accomplished using a two-step approach, in which in the first step the tumor is pretargeted with an unlabeled antibody construct and in the second step the tumor is targeted with a radiolabeled small molecule. This results in a more rapid clearance of the radioactivity from normal tissues due to the fast pharmacokinetics of the small molecule as compared to antibodies. In the last decade, several pretargeting approaches have been tested, which have shown improved tumor-to-background ratios and thus improved imaging and therapy as compared to directly labeled antibodies. In this review, we will discuss the strategies and applications in (pre-)clinical studies of pretargeting concepts based on the use of bispecific antibodies, which are capable of binding to both a target antigen and a radiolabeled peptide. So far, three generations of the bispecific antibody-based pretargeting approach have been studied. The first clinical studies have shown the feasibility and potential for these pretargeting systems to detect and treat tumor lesions. However, to fully integrate the pretargeting approach in clinic, further research should focus on the best regime and pretargeting protocol. Additionally, recent developments in the use of bioorthogonal chemistry for pretargeting of tumors suggest that this chemical pretargeting approach is an attractive alternative strategy for the detection and treatment of tumor lesions.

Bioorthogonal chemistry: implications for pretargeted nuclear (PET/SPECT) imaging and therapy

Due to their rapid and highly selective nature, bioorthogonal chemistry reactions are attracting a significant amount of recent interest in the radiopharmaceutical community. Over the last few years, reactions of this type have found tremendous utility in the construction of new radiopharmaceuticals and as a method of bioconjugation. Furthermore, reports are beginning to emerge in which these reactions are also being applied in vivo to facilitate a novel pretargeting strategy for the imaging and therapy of cancer. The successful implementation of such an approach could lead to dramatic improvements in image quality, therapeutic index, and reduced radiation dose to non-target organs and tissues. This review will focus on the potential of various bioorthogonal chemistry reactions to be used successfully in such an approach.

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".

A self-assembling short oligonucleotide duplex suitable for pretargeting

Monoclonal antibodies (mAbs) have naturally evolved as suitable, high affinity and specificity targeting molecules. However, the large size of full-length mAbs yields poor pharmacokinetic properties. A solution to this issue is the use of a multistep administration approach, in which the slower clearing mAb is administered first and allowed to reach the target site selectively, followed by administration of a rapidly clearing small molecule carrier of the cytotoxic or imaging ligand, which bears a cognate receptor for the mAb. Here, we introduce a novel pretargetable RNA based system comprised of locked nucleic acids (LNA) and 2'O-Methyloligoribonucleotides (2'OMe-RNA). The duplex shows fast hybridization, high melting temperatures, excellent affinity, and high nuclease stability in plasma. Using a prototype model system with rituximab conjugated to 2'OMe-RNA (oligo), we demonstrate that LNA-based complementary strand (c-oligo) effectively hybridizes with rituximab-oligo, which is slowly circulating in vivo, despite the high clearance rates of c-oligo.

Improvement of radioimmunotherapy using pretargeting

During the past two decades, considerable research has been devoted to radionuclide therapy using radiolabeled monoclonal antibodies and receptor binding agents. Conventional radioimmunotherapy (RIT) is now an established and important tool in the treatment of hematologic malignancies such as Non-Hodgkin lymphoma. For solid malignancies, the efficacy of RIT has not been as successful due to lower radiosensitivity, difficult penetration of the antibody into the tumor, and potential excessive radiation to normal tissues. Innovative approaches have been developed in order to enhance tumor absorbed dose while limiting toxicity to overcome the different limitations due to the tumor and host characteristics. Pretargeting techniques (pRIT) are a promising approach that consists of decoupling the delivery of a tumor monoclonal antibody (mAb) from the delivery of the radionuclide. This results in a much higher tumor-to-normal tissue ratio and is favorable for therapy as well and imaging. This includes various strategies based on avidin/streptavidin-biotin, DNA-complementary DNA, and bispecific antibody-hapten bindings. pRIT continuously evolves with the investigation of new molecular constructs and the development of radiochemistry. Pharmacokinetics improve dosimetry depending on the radionuclides used (alpha, beta, and Auger emitters) with prediction of tumor response and host toxicities. New constructs such as the Dock and Lock technology allow production of a variety of mABs directed against tumor-associated antigens. Survival benefit has already been shown in medullary thyroid carcinoma. Improvement in delivery of radioactivity to tumors with these pretargeting procedures associated with reduced hematologic toxicity will become the next generation of RIT. The following review addresses actual technical and clinical considerations and future development of pRIT.

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.

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.