Novel radiolabeled antibody conjugates

Advancing protein therapeutics through proximity-induced chemistry

Recent years have seen a remarkable growth in the field of protein-based medical treatments. Nevertheless, concerns have arisen regarding the cytotoxicity limitations, low affinity, potential immunogenicity, low stability, and challenges to modify these proteins. To overcome these obstacles, proximity-induced chemistry has emerged as a next-generation strategy for advancing protein therapeutics. This method allows site-specific modification of proteins with therapeutic agents, improving their effectiveness without extensive engineering. In addition, this innovative approach enables spatial control of the reaction based on proximity, facilitating the formation of irreversible covalent bonds between therapeutic proteins and their targets. This capability becomes particularly valuable in addressing challenges such as the low affinity frequently encountered between therapeutic proteins and their targets, as well as the limited availability of small molecules for specific protein targets. As a result, proximity-induced chemistry is reshaping the field of protein drug preparation and propelling the revolution in novel protein therapeutics.

A 33-residue peptide tag increases solubility and stability of Escherichia coli produced single-chain antibody fragments

Single-chain variable fragments (scFvs), composed of variable domains of heavy and light chains of an antibody joined by a linker, share antigen binding capacity with their parental antibody. Due to intrinsically low solubility and stability, only two Escherichia coli-produced scFvs have been approved for therapy. Here we report that a 33-residue peptide, termed P17 tag, increases the solubility of multiple scFvs produced in Escherichia coli SHuffle strain by up to 11.6 fold. Hydrophilic sequence, especially charged residues, but not the predicted α-helical secondary structure of P17 tag, contribute to the solubility enhancement. Notably, the P17 tag elevates the thermostability of scFv as efficiently as intra-domain disulfide bonds. Moreover, a P17-tagged scFv targeting hepatitis B virus surface proteins shows over two-fold higher antigen-binding affinity and virus-neutralizing activity than the untagged version. These data strongly suggest a type I intramolecular chaperone-like activity of the P17 tag. Hence, the P17 tag could benefit the research, production, and application of scFv.

Low solubility and stability of Escherichia coli produced single chain variable fragments (scFvs) restrict their applications. Here the authors report a 33-residue peptide tag which simultaneously increases the solubility and thermostability of multiple scFvs produced in Escherichia coli SHuffle strain.

Development and in vitro evaluation of new bifunctional 89Zr-chelators based on the 6-amino-1,4-diazepane scaffold for immuno-PET applications

INTRODUCTION

Combination of hydroxamate bearing side chains with the 6-amino-1,4-diazepane scaffold provides a promising strategy for fast and stable 89Zr-labeling of antibodies. Following this approach, we hereby present the development, labeling kinetics and in vitro complex stability of three resulting bifunctional chelator derivatives both stand-alone and coupled to a model protein in comparison to different linear deferoxamine (DFO) derivatives.

METHODS

The novel 89Zr-chelator Hy3ADA5 was prepared via amide-coupling of separately synthesized 6-amino-1,4-diazepane-6-pentanoic acid and hydroxamate-containing side chains. Two further bifunctional derivatives were synthesized by extending the resulting system with either a squaramide- or p-isothiocyanatophenyl moiety for simplified binding to proteins. After coupling to a model antibody and purification, the resulting immunoconjugates as well as the unbound chelator derivatives were 89Zr-labeled at room temperature (RT) and neutral pH. For comparison, different DFO derivatives were analogously coupled, purified and radiolabeled. In vitro complex stability of the resulting radioconjugates was investigated in phosphate buffered saline (PBS) and human serum at 37 °C over a period of 7 days.

RESULTS

89Zr-labeling of the novel unbound Hy3ADA5 derivatives indicated rapid complexation kinetics resulting in high radiochemical conversions (RCC) of 84-94% after 90 min. Similar or even faster radiolabeling with slightly increased maximum yields was obtained using the DFO-analogues. Initially, [89Zr]Zr-DFO*-p-Ph-NCS showed a delayed formation, nevertheless reaching almost quantitative complexation. Radiolabeling of the corresponding immunoconjugates Hy3ADA5-SA-mAb and Hy3ADA5-p-Ph-NCS-mAb resulted in 82.0 ± 1.1 and 89.2 ± 0.7% RCC, respectively after 90 min representing high but slightly lower labeling efficiency compared to the DFO- and DFO*-functionalized analogues. All examined radioimmunoconjugates showed very high in vitro complex stability both in human serum and PBS, providing no significant release of the radiometal. In the case of unbound chelators, however, the p-Ph-NCS-functionalized derivatives indicated considerable instability in human serum already after 1 h.

CONCLUSION

The novel chelator derivatives based on hydroxamate-functionalized 6-amino-1,4-diazepane revealed fast and high yielding 89Zr-labeling kinetics as well as high in vitro complex stability both stand-alone and coupled to an antibody. Therefore, Hy3ADA5 represents a promising tool for radiolabeling of biomolecules such as antibodies at mild conditions for immuno-PET applications.

Development of Dual Receptor Enhanced Pre-Targeting Strategy-A Novel Promising Technology for Immuno-Positron Emission Tomography Imaging

PET imaging has become an important diagnostic tool in the era of precise medicine. Various pre-targeting systems have been reported to address limitations associated with traditional immuno-PET. However, the application of these mono-receptor based pre-targeting (MRPT) strategies is limited to non-internalizable antibodies, and the tumor uptake is usually much lower than that in the corresponding immuno-PET. To circumvent these limitations, we develop the first Dual-Receptor Pre-Targeting (DRPT) system through entrapping the tumor-receptor-specific radioligand by the pre-administered antibody. Besides the similar ligation pathway happens in MRPT, incorporation of a tumor-receptor-specific peptide into the radioligand in DRPT enhances both concentration and retention of the radioligand on tumor, promoting its ligation with pre-administered mAb on cell-surface and/or internalized into tumor-cells. In this study, 64Cu based DRPT shows superior performance over corresponding MRPT and immuno-PET using internalizable antibodies. Besides, the compatibility of DRPT with short-lived and generator-produced 68Ga is demonstrated, leveraging its advantage in reducing radio-dose exposure. Furthermore, the feasibility of reducing the amount of the pre-administered antibody is confirmed, indicating the cost saving potential of DRPT. In summary, synergizing advantages of dual-receptor targeting and pre-targeting, we expect that this DRPT strategy can become a breakthrough technology in the field of antibody-based molecular imaging.

Labeling Antibodies

This introduction outlines general strategies for labeling proteins, with an emphasis on methods that are used primarily for labeling antibodies. It covers the specific site of modification, cross-linker options, types of labels, and postlabeling cleanup methodology, along with the advantages and disadvantages of each method. In general, polyclonal antibodies are more versatile and resistant to activity loss than are monoclonal antibodies. Greater care must be taken when labeling monoclonal antibodies to ensure a quality conjugate. The methods outlined here can be adapted for a variety of labels including multiple labels on the same immunoglobulin. The most important consideration when undertaking an antibody labeling experiment is to maintain the activity of the antibody. This is an empirical process and will often require additional experiments to optimize the label of a particular antibody. When successful, these reagents are very useful and adaptable biomolecules. This introduction provides the reader with methods and options for producing a variety of labeled immunological tools.

Novel single-chain variant of antibody against mesothelin established by phage library

Mesothelin (MSLN) shows increased expression in various cancer cells. For clinical application of antibodies as a positron emission tomography (PET) imaging reagent, a human shortened antibody is essential both for avoiding redundant immune responses and for providing rapid imaging. Therefore, we cloned a single‐chain fragment of variable regions (scFv) from a human‐derived gene sequence. This was achieved through the construction of a naïve phage library derived from human tonsil lymphocytes. Using a column with human recombinant MSLN, we carried out bio‐panning of phage‐variants by colony formation. We first obtained 120 clones that were subjected to selection in an ELISA using human recombinant MSLN as a solid phase antigen, and 15 phage clones of scFv with a different sequence were selected and investigated by flow cytometry (FCM). Then, six variants were selected and the individual scFv gene was synthesized in the V_L and V_H domains and expressed in Chinese hamster ovary cells. Mammalian cell‐derived human‐origin scFv clones were analyzed by FCM again, and one MSLN highly specific scFv clone was established. PET imaging by ⁸⁹Zr‐labeled scFv was done in mice bearing xenografts with MSLN‐expressing cancer cells, and tumor legions were successfully visualized. The scFv variant established in the present study may be potentially useful for cancer diagnosis by PET imaging.

A review of bispecific antibodies and antibody constructs in oncology and clinical challenges

Bispecific antibodies (bsAbs) are antibodies that bind two distinct epitopes to cancer.. For use in oncology, one bsAb has been approved and 57 bsAbs are in clinical trials, none of which has reached phase 3. These bsAbs show great variability in design and mechanism of action. The various designs are often linked to the mechanisms of actions. The majority of bsAbs engage immune cells to destroy tumor cells. However, some bsAbs are also used to deliver payloads to tumors or to block tumor signaling pathways. This review provides insight into the choice of construct for bsAbs, summarizes the clinical development of bsAbs in oncology and identifies subsequent challenges.

Bioorthogonally Applicable Fluorescence Deactivation Strategy for Receptor Kinetics Study and Theranostic Pretargeting Approaches

The availability of a receptor for theranostic pretargeting approaches was assessed by use of a new click-chemistry-based deactivatable fluorescence-quenching concept. The efficacy was evaluated in a cell-based model system featuring both membranous (available) and internalized (unavailable) receptor fractions of the clinically relevant receptor chemokine receptor 4 (CXCR4). Proof of concept was achieved with a deactivatable tracer consisting of a CXCR4-specific peptide functionalized with a Cy5 dye bearing a chemoselective azide handle (N3 -Cy5-AcTZ14011). Treatment with a Cy7 quencher dye (Cy7-DBCO) resulted in optically silent Cy7-[click]-Cy5-AcTZ14011. In situ, a >90 % FRET-based reduction of the signal intensity of N3 -Cy5-AcTZ14011 [KD =(222.4±25.2) nm] was seen within minutes after quencher addition. In cells, discrimination between the membranous and the internalized receptor fraction could be achieved through quantitative assessment of quenching/internalization kinetics. Similar evaluation of an activatable tracer variant based on the same targeting moiety (Cy5-S-S-Cy3-AcTZ14011) was unsuccessful in vitro. As such, using the described deactivatable approach to screen membrane receptors and their applicability in receptor-(pre-)targeted theranostics can become straightforward.

Prospective of 68Ga Radionuclide Contribution to the Development of Imaging Agents for Infection and Inflammation

During the last decade, the utilization of ⁶⁸Ga for the development of imaging agents has increased considerably with the leading position in the oncology. The imaging of infection and inflammation is lagging despite strong unmet medical needs. This review presents the potential routes for the development of ⁶⁸Ga-based agents for the imaging and quantification of infection and inflammation in various diseases and connection of the diagnosis to the treatment for the individualized patient management.

Synthesis of hydrophilic HYNIC-[1,2,4,5]tetrazine conjugates and their use in antibody pretargeting with99mTc

Pretargeted imaging is shown to be an attractive strategy to overcome disadvantages associated with traditional radioimmunoconjugates.

(90) Y/(177) Lu-labelled Cetuximab immunoconjugates: radiochemistry optimization to clinical dose formulation

Radiolabelled monoclonal antibodies (mAbs) are increasingly being utilized in cancer theranostics, which is a significant move toward tailored treatment for individual patients. Cetuximab is a recombinant, human-mouse chimeric IgG1 mAb that binds to the epidermal growth factor receptor with high affinity. We have optimized a protocol for formulation of clinically relevant doses (~2.22 GBq) of (90) Y-labelled Cetuximab and (177) Lu-labelled Cetuximab by conjugation of the mAb with a suitable bifunctional chelator, N-[(R)-2-amino-3-(paraisothiocyanato-phenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-N,N,N',N″,N″-pentaacetic acid (CHX-A″-DTPA). The radioimmunoconjugates demonstrated reasonably high specific activity (1.26 ± 0.27 GBq/mg for (90) Y-CHX-A″-DTPA-Cetuximab and 1.14 ± 0.15 GBq/mg for (177) Lu-CHX-A″-DTPA-Cetuximab), high radiochemical purity (>95%) and appreciable in vitro stability under physiological conditions. Preliminary biodistribution studies with both (90) Y-CHX-A″-DTPA-Cetuximab and (177) Lu-CHX-A″-DTPA-Cetuximab in Swiss mice bearing fibrosarcoma tumours demonstrated significant tumour uptake at 24-h post-injection (p.i.) (~16%ID/g) with good tumour-to-background contrast. The results of the biodistribution studies were further corroborated by ex vivo Cerenkov luminescence imaging after administration of (90) Y-CHX-A″-DTPA-Cetuximab in tumour-bearing mice. The tumour uptake at 24 h p.i. was significantly reduced with excess unlabelled Cetuximab, suggesting that the uptake was receptor mediated. The results of this study hold promise, and this strategy should be further explored for clinical translation.

Radioactive antibodies: a historical review of selective targeting and treatment of cancer

Radioactive antibodies have served as imaging and therapeutic agents for several decades, but recent developments raise enthusiasm that a new generation of cancer therapeutics and diverse molecular imaging agents for various cancers are more likely than ever before. This article traces the development of tumor-targeting antibodies labeled with diagnostic or therapeutic radionuclides, and describes the problems encountered and the clinical advances made. We also emphasize recent attempts to improve both molecular imaging and radioimmunotherapy with multistep pretargeting methods that separate the delivery of the tumor-binding, bispecific antibody given in the first step from the radionuclide carrier, which, in the second step, will localize to the "anti-carrier" binding arm of the pretargeted bispecific antibody.

(99m)Tc-bioorthogonal click chemistry reagent for in vivo pretargeted imaging

Metal-free click chemistry has become an important tool for pretargeted approaches in the molecular imaging field. The application of bioorthogonal click chemistry between a pretargeted trans-cyclooctene (TCO) derivatized monoclonal antibody (mAb) and a (99m)Tc-modified 1,2,4,5-tetrazine for tumor imaging was examined in vitro and in vivo. The HYNIC tetrazine compound was synthesized and structurally characterized, confirming its identity. Radiolabeling studies demonstrated that the HYNIC tetrazine was labeled with (99m)Tc at an efficiency of >95% and was radiochemically stable. (99m)Tc-HYNIC tetrazine reacted with the TCO-CC49 mAb in vitro demonstrating its selective reactivity. In vivo biodistribution studies revealed non-specific liver and GI uptake due to the hydrophobic property of the compound, however pretargeted SPECT imaging studies demonstrated tumor visualization confirming the success of the cycloaddition reaction in vivo. These results demonstrated the potential of (99m)Tc-HYNIC-tetrazine for tumor imaging with pretargeted mAbs.

Pretargeted Positron Emission Tomography Imaging That Employs Supramolecular Nanoparticles with in Vivo Bioorthogonal Chemistry

A pretargeted oncologic positron emission tomography (PET) imaging that leverages the power of supramolecular nanoparticles with in vivo bioorthogonal chemistry was demonstrated for the clinically relevant problem of tumor imaging. The advantages of this approach are that (i) the pharmacokinetics (PKs) of tumor-targeting and imaging agents can be independently altered via chemical alteration to achieve the desired in vivo performance and (ii) the interplay between the two PKs and other controllable variables confers a second layer of control toward improved PET imaging. In brief, we utilized supramolecular chemistry to synthesize tumor-targeting nanoparticles containing transcyclooctene (TCO, a bioorthogonal reactive motif), called TCO⊂SNPs. After the intravenous injection and subsequent concentration of the TCO⊂SNPs in the tumors of living mice, a small molecule containing both the complementary bioorthogonal motif (tetrazine, Tz) and a positron-emitting radioisotope ((64)Cu) was injected to react selectively and irreversibly to TCO. High-contrast PET imaging of the tumor mass was accomplished after the rapid clearance of the unreacted (64)Cu-Tz probe. Our nanoparticle approach encompasses a wider gamut of tumor types due to the use of EPR effects, which is a universal phenomenon for most solid tumors.

Safety and biodistribution of 111In-amatuximab in patients with mesothelin expressing cancers using single photon emission computed tomography-computed tomography (SPECT-CT) imaging

Amatuximab is a chimeric high-affinity monoclonal IgG1/k antibody targeting mesothelin that is being developed for treatment of mesothelin-expressing cancers. Considering the ongoing clinical development of amatuximab in these cancers, our objective was to characterize the biodistribution, and dosimetry of ¹¹¹Indium (¹¹¹In) radiolabelled amatuximab in mesothelin-expressing cancers. Between October 2011 and February 2013, six patients including four with malignant mesothelioma and two with pancreatic adenocarcinoma underwent Single Photon Emission Computed Tomography-Computed Tomography (SPECT/CT) imaging following administration of ¹¹¹In amatuximab. SPECT/CT images were obtained at 2–4 hours, 24–48 hours and 96–168 hours after radiotracer injection. In all patients, tumor to background ratios (TBR) consistently met or exceeded an uptake of 1.2 (range 1.2–62.0) which is considered the minimum TBR that can be visualized. TBRs were higher in tumors of patients with mesothelioma than pancreatic adenocarcinoma. ¹¹¹In-amatuximab uptake was noted in both primary tumors and metastatic sites. The radiotracer dose was generally well-tolerated and demonstrated physiologic uptake in the heart, liver, kidneys and spleen. This is the first study to show tumor localization of an anti-mesothelin antibody in humans. Our results show that ¹¹¹In-amatuximab was well tolerated with a favorable dosimetry profile. It localizes to mesothelin expressing cancers with a higher uptake in mesothelioma than pancreatic cancer.

Monovinyl sulfone β-cyclodextrin. A flexible drug carrier system

Cyclodextrins have been conjugated to target various receptors and have also been functionalized with carbohydrates for targeting specific organs. However, this approach is based on a rigid design that implies the ad hoc synthesis of each cyclodextrin-targeting agent conjugate. We hypothesized that: 1)a modular design that decouples the carrier function from the targeting function leads to a flexible system, 2) combining the reactivity of the vinyl sulfone group toward biomolecules that act as targeting agents with the ability of cyclodextrin to form complexes with a wide range of drugs may yield a versatile system that allows the targeting of different organs with different drugs, and 3) the higher reactivity of histidine residues toward the vinyl sulfone group can be exploited to couple the cyclodextrin to the targeting system with a degree of regioselectivity. As a proof of concept, we synthesized a monovinyl sulfone β-cyclodextrin (module responsible for the payload), which, after coupling to recombinant antibody fragments raised against Trypanosoma brucei (module responsible for targeting) and loading with nitrofurazone (module responsible for therapeutic action) resulted in an effective delivery system that targets the surface of the parasites and shows trypanocidal activity.

Prospective of ⁶⁸Ga-radiopharmaceutical development

Positron Emission Tomography (PET) experienced accelerated development and has become an established method for medical research and clinical routine diagnostics on patient individualized basis. Development and availability of new radiopharmaceuticals specific for particular diseases is one of the driving forces of the expansion of clinical PET. The future development of the ⁶⁸Ga-radiopharmaceuticals must be put in the context of several aspects such as role of PET in nuclear medicine, unmet medical needs, identification of new biomarkers, targets and corresponding ligands, production and availability of ⁶⁸Ga, automation of the radiopharmaceutical production, progress of positron emission tomography technologies and image analysis methodologies for improved quantitation accuracy, PET radiopharmaceutical regulations as well as advances in radiopharmaceutical chemistry. The review presents the prospects of the ⁶⁸Ga-based radiopharmaceutical development on the basis of the current status of these aspects as well as wide range and variety of imaging agents.

Radiofrequency ablation before intratumoral injection of (131)I-chTNT improves the tumor-to-normal tissue ratio in solid VX2 tumor

PURPOSE

This study was aimed to investigate whether the tumor necrosis induced by radiofrequency ablation (RFA) can improve the ratio of tumor-to-normal tissue (T/NT) after intratumoral injection of (131)I-chTNT.

MATERIALS AND METHOD

Eighteen New Zealand rabbits bearing VX2 tumor on the thigh were randomly divided into two treatment groups (control group: intratumoral injection of (131)I-chTNT alone; RFA group: RFA + intratumoral injection of (131)I-chTNT 3 days after RFA) and each group was further divided into three subgroups I, II, and III (1-2 cm, 2-3 cm, and 3-4 cm in maximum diameter, respectively), by the tumor size. SPECT was performed to evaluate the T/NT on days 1, 8, and 15 after (131)I-chTNT injection.

RESULTS

After treatment, all rabbits underwent the SPECT whole-body scan and the T/NT was analyzed. The results showed that T/NT in the RFA group (55.45±41.83) was significantly higher compared with the control group (7.23±5.61) (F=18.89, p=0.001). Meanwhile, a linear ascending trend was found for T/NT in the RFA group along with the follow-up time (r=0.47, p=0.01). The tumor size or the dose of (131)I-TNT injection had no significant effect on the variation of T/NT in both groups (p>0.05).

CONCLUSION

RFA before intratumoral injection of (131)I-chTNT can dramatically improve T/NT, demonstrating the potential application of this combination therapy.

Strategies to stabilize compact folding and minimize aggregation of antibody-based fragments

Monoclonal antibodies (mAbs) have proven to be useful for development of new therapeutic drugs and diagnostic techniques. To overcome the difficulties posed by their complex structure and folding, reduce undesired immunogenicity, and improve pharmacokinetic properties, a plethora of different Ab fragments have been developed. These include recombinant Fab and Fv segments that can display improved properties over those of the original mAbs upon which they are based. Antibody (Ab) fragments such as Fabs, scFvs, diabodies, and nanobodies, all contain the variable Ig domains responsible for binding to specific antigenic epitopes, allowing for specific targeting of pathological cells and/or molecules. These fragments can be easier to produce, purify and refold than a full Ab, and due to their smaller size they can be well absorbed and distributed into target tissues. However, the physicochemical and structural properties of the immunoglobulin (Ig) domain, upon which the folding and conformation of all these Ab fragments is based, can limit the stability of Ab-based drugs. The Ig domain is fairly sensitive to unfolding and aggregation when produced out of the structural context of an intact Ab molecule. When unfolded, Ab fragments may lose their specificity as well as establish non-native interactions leading to protein aggregation. Aggregated antibody fragments display altered pharmacokinetic and immunogenic properties that can augment their toxicity. Therefore, much effort has been placed in understanding the factors impacting the stability of Ig folding at two different levels: 1) intrinsically, by studying the effects of the amino acid sequence on Ig folding; 2) extrinsically, by determining the environmental conditions that may influence the stability of Ig folding. In this review we will describe the structure of the Ig domain, and the factors that impact its stability, to set the context for the different approaches currently used to achieve stable recombinant Ig domains when pursuing the development of Ab fragment-based biotechnologies.

The diversity of (68)Ga-based imaging agents

Development of new radiopharmaceuticals and their availability are crucial factors influencing the expansion of clinical nuclear medicine. The number of new (68)Ga-based imaging agents for positron emission tomography (PET) is increasing greatly. (68)Ga has been used for labeling of a broad range of molecules (small organic molecules, peptides, proteins, and oligonucleotides) as well as particles, thus demonstrating its potential to become a PET analog of the legendary generator-produced gamma-emitting (99m)Tc but with added value of higher sensitivity and resolution as well as quantitation and dynamic scanning. Further, the availability of technology for GMP-compliant automated tracer production can facilitate the introduction of new radiopharmaceuticals and enable standardized, harmonized multicenter studies to be conducted for regulatory approval. This chapter presents some examples of tracers for targeted, pretargeted, and nontargeted imaging with emphasis on the potential of (68)Ga to facilitate clinically practical PET development and to promote the PET technique worldwide for earlier and better diagnostics, and personalized medicine with the ultimate objective of improved therapeutic outcome.

Recombinant λ bacteriophage displaying nanobody towards third domain of HER-2 epitope inhibits proliferation of breast carcinoma SKBR-3 cell line

Phage display of many nanobodies via filamentous phage in combination with helper phage has been reported by many scientists. The aim of this study was to produce lambda (λ) bacteriophage displaying high-affinity nanobody against HER-2 expressing breast carcinoma cells. Bacteriophage λ is a temperate phage with inherent biological safety in mammalian cells. Here we report the construction of a recombinant λ phage that efficiently expresses specific nanobody towards third domain of HER-2 target on SKBR-3 and MCF-7 cell lines in vitro. We constructed recombinant λ phage particles containing a mammalian expression cassette, C-Myc tagged, encoding VHH gene of camelid anti HER-2 third domain epitope using λ ZAP-cytomegalic virus (CMV) vector. The SKBR-3, MCF-7 and human endometrial stem cells were treated by the nanobody displayed recombinant λ phage. The cell growth inhibition assay was performed by MTT Cell Viability Assay Kit. After the fourth round of biopanning there was a significant enrichment in the phage specifically binding to the antigen. The ratio of targeted phage increased approximately 1,000-fold in the fifth round. The nanobody expressed by λ ZAP-CMV-VHH phagemid cloned in λ bioparticles significantly inhibited the proliferation of HER-2 positive SKBR-3 and MCF-7 cells. Recombinant bacteriophage λ ZAP-CMV-VHH-cDNA could be used efficiently for construction of nanobodies to mortify HER-2 positive breast carcinoma cells as a nanomedical therapeutic.

Designing immunoconjugates for cancer therapy

INTRODUCTION

Antibody-drug conjugates (ADCs), as well as antibody conjugates of protein toxins (immunotoxins) and cytokines (immunocytokines), are showing clinical efficacy, with manageable toxicities, in cancer treatment.

AREAS COVERED

The utility of an ADC is governed by the antibody and the target, as well as by the drug-linker component of the conjugate. The conjugation site, conjugating group, drug/antibody ratios and site-specific conjugation for product homogeneity are all aspects to consider in optimizing the ADC and enhancing its therapeutic window. Immunotoxin and immunocytokine construction by recombinant methods can be modulated to improve efficacy and reduce toxicity. The Dock-and-Lock (DNL) platform technology provides a flexible approach to assemble mono- or bispecific constructs carrying multiple toxin or cytokine molecules for targeted therapy.

EXPERT OPINION

Conjugation chemistry and recombinant technologies have had a significant impact on the therapeutic prospects of immunoconjugates, particularly in hematopoietic diseases. Continued concerted efforts from different scientific disciplines are needed, together with newer treatment paradigms, for greater progress in the more challenging therapy of solid tumors.

PET tracers based on Zirconium-89

Positron emission tomography (PET) imaging with radiolabeled monoclonal antibodies has always been a dynamic area in molecular imaging. With decay half-life (3.3 d) well matched to the circulation half-lives of antibodies (usually on the order of days), (89)Zr has been extensively studied over the last decade. This review article will give a brief overview on (89)Zr isotope production, the radiochemistry generally used for (89)Zr-labeling, and the PET tracers that have been developed using (89)Zr. To date, (89)Zr-based PET imaging has been investigated for a wide variety of cancer-related targets, which include human epidermal growth factor receptor 2, epidermal growth factor receptor, prostate-specific membrane antigen, splice variant v6 of CD44, vascular endothelial growth factor, carbonic anhydrase IX, insulin-like growth factor 1 receptor, among others. With well-developed radiochemistry, commercial availability of chelating agents for (89)Zr labeling, increasingly widely available isotope supply, as well as successful proof-of-principle in pilot human studies, it is expected that PET imaging with (89)Zr-based tracers will be a constantly evolving and highly vibrant field in the near future.

Imaging the islet graft by positron emission tomography

Clinical islet transplantation is being investigated as a permanent cure for type 1 diabetes mellitus (T1DM). Currently, intraportal infusion of islets is the favoured procedure, but several novel implantation sites have been suggested. Noninvasive longitudinal methodologies are an increasingly important tool for assessing the fate of transplanted islets, their mass, function and early signs of rejection. This article reviews the approaches available for islet graft imaging by positron emission tomography and progress in the field, as well as future challenges and opportunities.

Synthesis of phosphine and antibody-azide probes for in vivo Staudinger ligation in a pretargeted imaging and therapy approach

The application of intact monoclonal antibodies (mAbs) as targeting agents in nuclear imaging and radioimmunotherapy is hampered by the slow pharmacokinetics of these molecules. Pretargeting with mAbs could be beneficial to reduce the radiation burden to the patient, while using the excellent targeting capacity of the mAbs. In this study, we evaluated the applicability of the Staudinger ligation as pretargeting strategy using an antibody-azide conjugate as tumor-targeting molecule in combination with a small phosphine-containing imaging/therapeutic probe. Up to 8 triazide molecules were attached to the antibody without seriously affecting its immunoreactivity, pharmacokinetics, and tumor uptake in tumor bearing nude mice. In addition, two (89)Zr- and (67/68)Ga-labeled desferrioxamine (DFO)-phosphines, a (177)Lu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-phosphine and a (123)I-cubyl phosphine probe were synthesized and characterized for their pharmacokinetic behavior in nude mice. With respect to the phosphine probes, blood levels at 30 min after injection were <5% injected dose per gram tissue, indicating rapid blood clearance. In vitro Staudinger ligation of 3.33 μM antibody-azide conjugate with 1 equiv of radiolabeled phosphine, relative to the azide, in aqueous solution resulted in 20-25% efficiency after 2 h. The presence of 37% human serum resulted in a reduced ligation efficiency (reduction max. 30% at 2 h), while the phosphines were still >80% intact. No in vivo Staudinger ligation was observed in a mouse model after injection of 500 μg antibody-azide, followed by 68 μg DFO-phosphine at t = 2 h, and evaluation in blood at t = 7 h. To explain negative results in mice, Staudinger ligation was performed in vitro in mouse serum. Under these conditions, a side product with the phosphine was formed and ligation efficiency was severely reduced. It is concluded that in vivo application of the Staudinger ligation in a pretargeting approach in mice is not feasible, since this ligation reaction is not bioorthogonal and efficient enough. Slow reaction kinetics will also severely restrict the applicability of Staudinger ligation in humans.

Bisphosphonates as radionuclide carriers for imaging or systemic therapy

Bisphosphonates (BP's), biologically stable analogs of naturally occurring pyrophosphates, became the treatment of choice for pathologic conditions characterized by increased osteoclast-mediated bone resorption, namely Paget's disease, osteoporosis and tumor bone disease. Moreover, the clinical success of BP's is also associated with their use in (99m)Tc-based radiopharmaceuticals for bone imaging. In addition to the successful delivery of (99m)Tc (γ-emitter) to bone, BP's have also been used to deliver β(-)-particle emitting radiometals (e.g.(153)Sm, (186/188)Re) for bone-pain palliation. The main goal of this Review is to update the most recent research efforts toward the synthesis, characterization and biological evaluation of novel BP-containing radiometal complexes and radiohalogenated compounds for diagnostic or therapeutic purposes. The structure and in vivo properties of those compounds will be discussed and compared to the clinically available ones, namely in terms of image quality and therapeutic effect. We will also mention briefly the use of BP's as carriers of multimodal nuclear and optical imaging probes.

Targeted radionuclide therapy in head and neck cancer

There is great potential for targeted radionuclide therapy (TRT) in the treatment of head and neck cancer. In recent years, developments in fields such as antigen screening, protein engineering, and cancer biology have facilitated the rational design of targeted pharmaceuticals, with monoclonal antibodies forming the most rapidly expanding category. TRT may be a promising way to improve targeted treatment, especially in head and neck cancer, because of the intrinsic radiosensitivity of this tumor type. TRT may also provide a good foundation on which to build rational biologic combination therapies. In the next few years the use of TRT may offer new opportunities for further improvement of the therapeutic ratio that potentially may obviate or reduce the need for conventional cytotoxics.

Dose intensified molecular targeted radiotherapy for cancer-lymphoma as a paradigm

Although most patients with locoregional cancer are cured by surgery, radiotherapy, chemotherapy, and combinations thereof, those with distant metastases are not despite systemic chemotherapy. These patients respond to local radiotherapy but generally need systemic therapy. Non-Hodgkin's lymphoma (NHL) provides a paradigm for the role of molecular targeted radiotherapy (MTRT) because these patients have multifocal disease in most cases. Although patients with NHL achieve remissions after multiple cycles of chemotherapy, less than one half of those with aggressive NHL are cured and almost none of those with low grade NHL. Furthermore, NHL, like other cancers, becomes chemoresistant, yet remains responsive to radiotherapy. MTRT, radiation targeted by molecules, is a good strategy for the treatment of multifocal and radiosensitive cancers. Radioimmunotherapy (RIT) is an MTRT approach using MAbs, or parts thereof, to target the radionuclide that delivers radiation. Two anti-CD20 monoclonal antibodies (MAbs), one labeled with (111)In for imaging or (90)Y for therapy and a second labeled with (131)I for imaging and therapy, have proven effective and safe for MTRT for NHL patients. The importance of the radiation is demonstrated in the data from the randomized pivotal trial of (90)Y-ibritumomab; response rates were distinctly better in the (90)Y-ibritumomab arm than in the rituximab arm. Furthermore, the efficacy of (131)I-tositumomab was greater than that of the same MAb alone in another pivotal trial. Although hematologic toxicity is dose limiting for MTRT, febrile neutropenia is uncommon. MTRT is also not associated with mucositis, hair loss, or persistent nausea or vomiting, unlike chemotherapy. Randomized trials of MTRT in different strategies have not been conducted, but there is evidence of better outcomes, particularly for strategies that provide dose intensification, such as pretargeted MTRT, multiple dosing ("fractionation"), and MTRT with stem cell transplantation (SCT). Pretargeted RIT separates delivery of the targeting molecule from radionuclide delivery, provides dose escalation, and is more effective than direct one-step RIT, although more complicated to implement. Improved drugs and strategies for MTRT have documented potential for better patient outcomes. Smaller radionuclide carriers, such as those used for pretargeted MTRT, should be incorporated into the management of patients with NHL and other cancers soon after the patients have proven incurable. Expected improvements using better drugs, strategies, and combinations with other drugs seem likely to make MTRT integral in the management of many patients with cancer and likely to lead to cures of NHL.

Identification of pancreatic cancer-specific cell-surface markers for development of targeting ligands

Pancreatic cancer is generally detected at later stages with a poor prognosis and a high-mortality rate. Development of theranostic imaging agents that noninvasively target pancreatic cancer by gene expression and deliver therapies directly to malignant cells could greatly improve therapeutic outcomes. Small-peptide ligands that bind cell-surface proteins and are conjugated to imaging moieties have demonstrated efficacy in cancer imaging. Identification of cancer-specific targets is a major bottleneck in the development of such agents. Herein, a method is presented that uses DNA microarray expression profiling of large sets of normal and cancer tissues to identify targets expressed in cancer but not expressed in relevant normal tissues. Identified targets are subsequently validated for protein expression using tissue microarray. Further validations are performed by quantifying expression in pancreatic cancer cells by quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR), by immunocytochemistry and immunohistochemistry, and by reviewing data and literature in public databases. Validated targets are selected for ligand development based on the existence of a known ligand or by known structure-activity relationships useful for development of novel ligands.

Initial characterization of an immunotoxin constructed from domains II and III of cholera exotoxin

Immunotoxins are antibody-toxin fusion proteins under development as cancer therapeutics. In early clinical trials, immunotoxins constructed with domains II and III of Pseudomonas exotoxin (termed PE38), have produced a high rate of complete remissions in Hairy Cell Leukemia and objective responses in other malignancies. Cholera exotoxin (also known as cholix toxin) has a very similar three-dimensional structure to Pseudomonas exotoxin (PE) and when domains II and III of each are compared at the primary sequence level, they are 36% identical and 50% similar. Here we report on the construction and activity of an immunotoxin made with domains II and III of cholera exotoxin (here termed CET40). In cell viability assays, the CET40 immunotoxin was equipotent to tenfold less active compared to a PE-based immunotoxin made with the same single-chain Fv. A major limitation of toxin-based immunotoxins is the development of neutralizing antibodies to the toxin portion of the immunotoxin. Because of structure and sequence similarities, we evaluated a CET40 immunotoxin for the presence of PE-related epitopes. In western blots, three-of-three anti-PE antibody preparations failed to react with the CET40 immunotoxin. More importantly, in neutralization studies neither these antibodies nor those from patients with neutralizing titers to PE38, neutralized the CET40-immunotoxin. We propose that the use of modular components such as antibody Fvs and toxin domains will allow a greater flexibility in how these agents are designed and deployed including the sequential administration of a second immunotoxin after patients have developed neutralizing antibodies to the first.

Imaging in targeted delivery of therapy to cancer

We review the current status of imaging as applied to targeted therapy with particular focus on antibody-based therapeutics. Antibodies have high tumor specificity and can be engineered to optimize delivery to, and retention within, the tumor. Whole antibodies can activate natural immune effector mechanisms and can be conjugated to beta- and alpha-emitting radionuclides, toxins, enzymes, and nanoparticles for enhanced therapeutic effect. Imaging is central to the development of these agents and is used for patient selection, performing dosimetry and assessment of response. gamma- and positron-emitting radionuclides may be used to image the distribution of antibody-targeted therapeutics While some radionuclides such as iodine-131 emit both beta and gamma radiation and are therefore suitable for both imaging and therapy, others are more suited to imaging or therapy alone. Hence for radionuclide therapy of neuroendocrine tumors, patients can be selected for therapy on the basis of gamma-emitting indium-111-octreotide imaging and treated with beta-emitting yttrium-90-octreotate. Positron-emitting radionuclides can give greater sensitivity that gamma-emitters but only a single radionuclide can be imaged at one time and the range of radionuclides is more limited. The multiple options for antibody-based therapeutic molecules, imaging technologies and therapeutic scenarios mean that very large amounts of diverse data are being acquired. This can be most effectively shared and progress accelerated by use of common data standards for imaging, biological, and clinical data.

Pretargeted radioimmunotherapy using anti-CD45 monoclonal antibodies to deliver radiation to murine hematolymphoid tissues and human myeloid leukemia

Radioimmunotherapy (RIT) for treatment of hematologic malignancies frequently fails because of disease recurrence. We therefore conducted pretargeted (P)RIT studies to augment the efficacy in mice of therapy using a pretargeted anti-human (h)CD45 antibody (Ab)-streptavidin (SA) conjugate followed by a biotinylated clearing agent and radiolabeled 1,4,7,10-tetraazacylodode cane N,N',N",N'''-tetraacetic (DOTA)-biotin. Tumor-to-blood ratios at 24 hours were 20:1 using pretargeted anti-hCD45 RIT and <1:1 with conventional RIT. In vivo imaging studies confirmed that the PRIT approach provided high-contrast tumor images with minimal blood-pool activity, whereas directly labeled anti-hCD45 Ab produced distinct tumor images but the blood pool retained a large amount of labeled Ab for a prolonged time. Therapy experiments showed that (90)Y-DOTA-biotin significantly prolonged survival of mice treated with pretargeted anti-hCD45 Ab-SA compared with mice treated with conventional RIT using (90)Y-labeled anti-hCD45 Ab at 200 muCi. Because human CD45 antigens are confined to xenograft tumor cells in this model, and all murine tissues are devoid of hCD45 and will not bind anti-hCD45 Ab, we also compared one-step and PRIT using an anti-murine (m)CD45 Ab where the target antigen is present on normal hematopoietic tissues. After 24 h, 27.3% +/- 2.8% of the injected dose of activity was delivered per gram (% ID/g) of lymph node using (131)I-A20-Ab compared with 40.0 +/- 5.4% ID/g for pretargeted (111)In-DOTA-biotin. These data suggest that pretargeted methods for delivering RIT may be superior to conventional RIT when targeting CD45 for the treatment of leukemia and may allow for the intensification of therapy, while minimizing toxicities.

[Contribution of radioimmunotherapy to the treatment of lymphoma]

Radioimmunotherapy (RIT) is an emerging treatment option for non-Hodgkin's lymphoma. Only Zevalin (Bayer Schering Pharma) radiolabeled with yttrium 90 ((90)Y) is approved in France for the treatment of adult patients with rituximab relapsed or refractory CD20+ follicular B-cell non-Hodgkin's lymphoma. This radioimmunotherapeutic agent consists of ibritumomab, a murine anti-CD20 monoclonal antibody, conjugated to the metal chelator tiuxetan for retention of the beta emitter (90)Y. This review presents the concept of RIT. The pharmacological characteristics of [(90)Y]-ibritumomab tiuxetan, the specificity of its preparation and its special precautions for use will be described. The other radionuclides and antibodies in development will also be mentioned.

Targeted radionuclide therapy

Targeted radionuclide therapy (TRT) seeks molecular and functional targets within patient tumor sites. A number of agents have been constructed and labeled with beta, alpha, and Auger emitters. Radionuclide carriers spanning a broad range of sizes; e.g., antibodies, liposomes, and constructs such as nanoparticles have been used in these studies. Uptake, in percent-injected dose per gram of malignant tissue, is used to evaluate the specificity of the targeting vehicle. Lymphoma (B-cell) has been the primary clinical application. Extension to solid tumors will require raising the macroscopic absorbed dose by several-fold over values found in present technology. Methods that may effect such changes include multistep targeting, simultaneous chemotherapy, and external sequestration of the agent. Toxicity has primarily involved red marrow so that marrow replacement can also be used to enhance future TRT treatments. Correlation of toxicities and treatment efficiency has been limited by relatively poor absorbed dose estimates partly because of using standard (phantom) organ sizes. These associations will be improved in the future by obtaining patient-specific organ size and activity data with hybrid SPECT/CT and PET/CT scanners.

Gene expression profiling-based identification of cell-surface targets for developing multimeric ligands in pancreatic cancer

Multimeric ligands are ligands that contain multiple binding domains that simultaneously target multiple cell-surface proteins. Due to cooperative binding, multimeric ligands can have high avidity for cells (tumor) expressing all targeting proteins and only show minimal binding to cells (normal tissues) expressing none or only some of the targets. Identifying combinations of targets that concurrently express in tumor cells but not in normal cells is a challenging task. Here, we describe a novel approach for identifying such combinations using genome-wide gene expression profiling followed by immunohistochemistry. We first generated a database of mRNA gene expression profiles for 28 pancreatic cancer specimens and 103 normal tissue samples representing 28 unique tissue/cell types using DNA microarrays. The expression data for genes that encode proteins with cell-surface epitopes were then extracted from the database and analyzed using a novel multivariate rule-based computational approach to identify gene combinations that are expressed at an efficient binding level in tumors but not in normal tissues. These combinations were further ranked according to the proportion of tumor samples that expressed the sets at efficient levels. Protein expression of the genes contained in the top ranked combinations was confirmed using immunohistochemistry on a pancreatic tumor tissue and normal tissue microarrays. Coexpression of targets was further validated by their combined expression in pancreatic cancer cell lines using immunocytochemistry. These validated gene combinations thus encompass a list of cell-surface targets that can be used to develop multimeric ligands for the imaging and treatment of pancreatic cancer.

Use of antibodies and immunoconjugates for the therapy of more accessible cancers

There are currently 6 unconjugated antibodies and 3 immunoconjugates approved for use in the United States in a variety of cancers, with a considerable number of new agents in clinical testing and preclinical development. Unconjugated antibodies alone can be effective, but more often, antibodies need to be combined with chemotherapy, which enhances the efficacy of the standard treatment. Immunoconjugates tend to be more effective than their unconjugated counterparts, but their increased toxicity often restricts when and how they are used. In order to improve efficacy, a number of immunoconjugates are being examined in settings where the disease is more easily accessible, such as leukemias, or within compartments that allow easier and more direct access to the tumor, such as in the peritoneal cavity or brain, or both locally and systemically, in adjuvant situations, where the disease burden has been reduced by some other means, and with the main goal of these treatments being to kill residual disease.

Recombinant antibodies for cancer therapy

BACKGROUND

Recombinant antibodies have evolved into successful therapeutics with 10 approved for cancer and more in the pipeline. Four of the top ten cancer therapy drugs are recombinant antibodies.

OBJECTIVES

To survey the current state-of-the-art highlighting the reasons for this success and looking ahead to the next generation of antibody therapy.

METHODS

An analysis was carried out to identify preclinical and clinical examples and the underlying concepts and mechanisms that have shown how to design better therapies.

RESULTS/CONCLUSIONS

Greater understanding of the molecular basis of cancer has led to improved antibodies and a greater selection of targets. Fine tuning of successful antibodies through modification of glycosylation, affinity, size and other parameters are paying dividends. Fc-engineering is likely to be predominant in the near future but conjugates, fragments and fusion proteins will continue to be developed and find their place in the arsenal of antibody therapeutics.

Improved therapeutic results by pretargeted radioimmunotherapy of non-Hodgkin's lymphoma with a new recombinant, trivalent, anti-CD20, bispecific antibody

We examined whether a pretargeting method using a new recombinant anti-CD20 bispecific antibody (bsMAb) followed by (90)Y-1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid ((90)Y-DOTA)-peptide could reduce hematologic toxicity yet improve therapeutic responses compared with conventional (90)Y-anti-CD20 IgG and a chemically conjugated bsMAb. TF4, a humanized, tri-Fab bsMAb with two Fabs binding CD20 and one Fab binding histamine-succinyl-glycine (HSG), developed by the dock and lock (DNL) method, was tested in nude mice with Ramos B-cell lymphomas. Optimal pretargeting required a 29-h interval between TF4 and (90)Y-DOTA-HSG, and 20-fold more moles of TF4. TF4 cleared more rapidly from the blood than anti-CD20 IgG, with early processing in the liver, spleen, and kidney. At 24 h, TF4 improved tumor uptake of (111)In-HSG-peptide 2.6-fold [13% versus 5% injected dose per gram (ID/g)] and enhanced tumor to blood ratios >45-fold (770 versus 17), compared with an anti-CD20 Fab x anti-HSG Fab chemical conjugate, and by 1.6-fold (9.0% versus 5.6% ID/g) and 1,600-fold (522 versus 0.32), respectively, compared with radiolabeled anti-CD20 IgG. A severe (>or=90%) and prolonged reduction of WBCs was observed at the maximum dose of (90)Y-anti-CD20 IgG, whereas pretargeting resulted in a <or=60% transient drop. TF4 pretargeting resulted in highly significant improvement in survival, curing 33% to 90% of the animals, even at relatively low doses, whereas most tumors progressed quickly without cures with (90)Y-anti-CD20 IgG. These results indicate an improved therapeutic index with pretargeted radioimmunotherapy (RAIT) using a DNL-constructed tri-Fab, bsMAb, compared with conventional therapy with directly radiolabeled antibody or with a chemically conjugated bsMAb. These encouraging results prompt testing these constructs for pretargeting RAIT in patients.

Paul Ehrlich's magic bullet concept: 100 years of progress

Exceptional advances in molecular biology and genetic research have expedited cancer drug development tremendously. The declared paradigm is the development of 'personalized and tailored drugs' that precisely target the specific molecular defects of a cancer patient. It is therefore appropriate to revisit the intellectual foundations of the development of such agents, as many have shown great clinical success. One hundred years ago, Paul Ehrlich, the founder of chemotherapy, received the Nobel Prize for Physiology or Medicine. His postulate of creating 'magic bullets' for use in the fight against human diseases inspired generations of scientists to devise powerful molecular cancer therapeutics.