Delivery of the alpha-emitting radioisotope bismuth-213 to solid tumors via single-chain Fv and diabody molecules

Targeted Alpha Therapy (TAT) with Single-Domain Antibodies (Nanobodies)

The persistent threat of cancer necessitates the development of improved and more efficient therapeutic strategies that limit damage to healthy tissues. Targeted alpha therapy (TαT), a novel form of radioimmuno-therapy (RIT), utilizes a targeting vehicle, commonly antibodies, to deliver high-energy, but short-range, alpha-emitting particles specifically to cancer cells, thereby reducing toxicity to surrounding normal tissues. Although full-length antibodies are often employed as targeting vehicles for TαT, their high molecular weight and the presence of an Fc-region lead to a long blood half-life, increased bone marrow toxicity, and accumulation in other tissues such as the kidney, liver, and spleen. The discovery of single-domain antibodies (sdAbs), or nanobodies, naturally occurring in camelids and sharks, has introduced a novel antigen-specific vehicle for molecular imaging and TαT. Given that nanobodies are the smallest naturally occurring antigen-binding fragments, they exhibit shorter relative blood half-lives, enhanced tumor uptake, and equivalent or superior binding affinity and specificity. Nanobody technology could provide a viable solution for the off-target toxicity observed with full-length antibody-based TαT. Notably, the pharmacokinetic properties of nanobodies align better with the decay characteristics of many short-lived α-emitting radionuclides. This review aims to encapsulate recent advancements in the use of nanobodies as a vehicle for TαT.

Clinical Advances and Perspectives in Targeted Radionuclide Therapy

Targeted radionuclide therapy has become increasingly prominent as a nuclear medicine subspecialty. For many decades, treatment with radionuclides has been mainly restricted to the use of iodine-131 in thyroid disorders. Currently, radiopharmaceuticals, consisting of a radionuclide coupled to a vector that binds to a desired biological target with high specificity, are being developed. The objective is to be as selective as possible at the tumor level, while limiting the dose received at the healthy tissue level. In recent years, a better understanding of molecular mechanisms of cancer, as well as the appearance of innovative targeting agents (antibodies, peptides, and small molecules) and the availability of new radioisotopes, have enabled considerable advances in the field of vectorized internal radiotherapy with a better therapeutic efficacy, radiation safety and personalized treatments. For instance, targeting the tumor microenvironment, instead of the cancer cells, now appears particularly attractive. Several radiopharmaceuticals for therapeutic targeting have shown clinical value in several types of tumors and have been or will soon be approved and authorized for clinical use. Following their clinical and commercial success, research in that domain is particularly growing, with the clinical pipeline appearing as a promising target. This review aims to provide an overview of current research on targeting radionuclide therapy.

Recent Achievements about Targeted Alpha Therapy-Based Targeting Vectors and Chelating Agents

One of the most rapidly growing options in the management of cancer therapy is Targeted Alpha Therapy (TAT) through which lethal α-emitting radionuclides conjugated to tumor-targeting vectors selectively deliver high amount of radiation to cancer cells.²²⁵Ac, ²¹²Bi, ²¹¹At, ²¹³Bi, and ²²³Ra have been investigated by plenty of clinical trials and preclinical researches for the treatment of smaller tumor burdens, micro-metastatic disease, and post-surgery residual disease. In order to send maximum radiation to tumor cells while minimizing toxicity in normal cells, a high affinity of targeting vectors to cancer tissue is essential. Besides that, the stable and specific complex between chelating agent and α-emitters was found as a crucial parameter. The present review was planned to highlight recent achievements about TAT-based targeting vectors and chelating agents and provide further insight for future researches.

Bismuth-213 for Targeted Radionuclide Therapy: From Atom to Bedside

In contrast to external high energy photon or proton therapy, targeted radionuclide therapy (TRNT) is a systemic cancer treatment allowing targeted irradiation of a primary tumor and all its metastases, resulting in less collateral damage to normal tissues. The α-emitting radionuclide bismuth-213 (213Bi) has interesting properties and can be considered as a magic bullet for TRNT. The benefits and drawbacks of targeted alpha therapy with 213Bi are discussed in this review, covering the entire chain from radionuclide production to bedside. First, the radionuclide properties and production of 225Ac and its daughter 213Bi are discussed, followed by the fundamental chemical properties of bismuth. Next, an overview of available acyclic and macrocyclic bifunctional chelators for bismuth and general considerations for designing a 213Bi-radiopharmaceutical are provided. Finally, we provide an overview of preclinical and clinical studies involving 213Bi-radiopharmaceuticals, as well as the future perspectives of this promising cancer treatment option.

Principles of Immunotherapy: Implications for Treatment Strategies in Cancer and Infectious Diseases

The advances in cancer biology and pathogenesis during the past two decades, have resulted in immunotherapeutic strategies that have revolutionized the treatment of malignancies, from relatively non-selective toxic agents to specific, mechanism-based therapies. Despite extensive global efforts, infectious diseases remain a leading cause of morbidity and mortality worldwide, necessitating novel, innovative therapeutics that address the current challenges of increasing antimicrobial resistance. Similar to cancer pathogenesis, infectious pathogens successfully fashion a hospitable environment within the host and modulate host metabolic functions to support their nutritional requirements, while suppressing host defenses by altering regulatory mechanisms. These parallels, and the advances made in targeted therapy in cancer, may inform the rational development of therapeutic interventions for infectious diseases. Although "immunotherapy" is habitually associated with the treatment of cancer, this review accentuates the evolving role of key targeted immune interventions that are approved, as well as those in development, for various cancers and infectious diseases. The general features of adoptive therapies, those that enhance T cell effector function, and ligand-based therapies, that neutralize or eliminate diseased cells, are discussed in the context of specific diseases that, to date, lack appropriate remedial treatment; cancer, HIV, TB, and drug-resistant bacterial and fungal infections. The remarkable diversity and versatility that distinguishes immunotherapy is emphasized, consequently establishing this approach within the armory of curative therapeutics, applicable across the disease spectrum.

Targeted alpha therapy using short-lived alpha-particles and the promise of nanobodies as targeting vehicle

Introduction: The combination of a targeted biomolecule that specifically defines the target and a radionuclide that delivers a cytotoxic payload offers a specific way to destroy cancer cells. Targeted radionuclide therapy (TRNT) aims to deliver cytotoxic radiation to cancer cells and causes minimal toxicity to surrounding healthy tissues. Recent advances using α-particle radiation emphasizes their potential to generate radiation in a highly localized and toxic manner because of their high level of ionization and short range in tissue.

Areas covered: We review the importance of targeted alpha therapy (TAT) and focus on nanobodies as potential beneficial vehicles. In recent years, nanobodies have been evaluated intensively as unique antigen-specific vehicles for molecular imaging and TRNT.

Expert opinion: We expect that the efficient targeting capacity and fast clearance of nanobodies offer a high potential for TAT. More particularly, we argue that the nanobodies’ pharmacokinetic properties match perfectly with the interesting decay properties of the short-lived α-particle emitting radionuclides Astatine-211 and Bismuth-213 and offer an interesting treatment option particularly for micrometastatic cancer and residual disease.

An evolved Mxe GyrA intein for enhanced production of fusion proteins

Expressing antibodies as fusions to the non-self-cleaving Mxe GyrA intein enables site-specific, carboxy-terminal chemical modification of the antibodies by expressed protein ligation (EPL). Bacterial antibody-intein fusion protein expression platforms typically yield insoluble inclusion bodies that require refolding to obtain active antibody-intein fusion proteins. Previously, we demonstrated that it was possible to employ yeast surface display to express properly folded single-chain antibody (scFv)-intein fusions, therefore permitting the direct small-scale chemical functionalization of scFvs. Here, directed evolution of the Mxe GyrA intein was performed to improve both the display and secretion levels of scFv-intein fusion proteins from yeast. The engineered intein was shown to increase the yeast display levels of eight different scFvs by up to 3-fold. Additionally, scFv- and green fluorescent protein (GFP)-intein fusion proteins can be secreted from yeast, and while fusion of the scFvs to the wild-type intein resulted in low expression levels, the engineered intein increased scFv-intein production levels by up to 30-fold. The secreted scFv- and GFP-intein fusion proteins retained their respective binding and fluorescent activities, and upon intein release, EPL resulted in carboxy-terminal azide functionalization of the target proteins. The azide-functionalized scFvs and GFP were subsequently employed in a copper-free, strain-promoted click reaction to site-specifically immobilize the proteins on surfaces, and it was demonstrated that the functionalized, immobilized scFvs retained their antigen binding specificity. Taken together, the evolved yeast intein platform provides a robust alternative to bacterial intein expression systems.

A novel recombinant protein of IP10-EGFRvIIIscFv and CD8(+) cytotoxic T lymphocytes synergistically inhibits the growth of implanted glioma in mice

The epidermal growth factor receptor (EGFR) mutant of EGFRvIII is highly expressed on glioma cells and has been thought to be an excellent target molecule for immunotherapy. IP-10 is a potent chemokine and can recruit CXCR3(+) T cells, including CD8(+) T cells that are important for the control of tumor growth. This study is aimed at investigating the therapeutic efficacy of a novel fusion protein of IP10-EGFRvIIIscFv (IP10-scFv) in combination with glioma lysate-pulsed DCs-activated CD8(+) cytotoxic T lymphocytes (CTLs) in a mouse model of glioma. A plasmid of pET-IP10-scFv was generated by linking mouse IP-10 gene with the DNA fragment for anti-EGFRvIIIscFv, a (Gly4Ser)3 flexible linker and a His-tag. The recombinant IP10-scFv in E. coli was purified by affinity chromatography and characterized for its anti-EGFRvIII immunoreactivity and chemotactic activity. C57BL/6 mice were inoculated with mouse glioma GL261 cells in the brain and treated intracranially with IP10-scFv and/or intravenously with CTL for evaluating the therapeutic effect. The glioma-specific immune responses were examined. The IP10-scFv retained anti-EGFRvIII immunoreactivity and IP-10-like chemotactic activity. Treatment with both IP10-scFv and CTL synergistically inhibited the growth of glioma and prolonged the survival of tumor-bearing mice, accompanied by increasing the numbers of brain-infiltrating lymphocytes (BILs) and the frequency of CXCR3(+)CD8(+) T cells, enhancing glioma-specific IFN-γ responses and cytotoxicity, and promoting glioma cell apoptosis in mice. Our novel data indicate that IP10-scFv and CTL have synergistic therapeutic effects on inhibiting the growth of mouse glioma in vivo.

Cell-targeting fusion constructs containing recombinant gelonin

Therapeutic agents capable of targeting tumor cells present as established tumors and micrometastases have already demonstrated their potential in clinical trials. Immunotoxins targeting hematological malignancies and solid tumors have additionally demonstrated excellent clinical activity. This review focuses on our design and characterization studies of constructs composed of recombinant gelonin toxin fused to either growth factors or single-chain antibodies targeting solid tumor cells, tumor vasculature or hematological malignancies. These agents demonstrate cytotoxicity at nanomolar or sub-nanomolar levels. All of these constructs display impressive selectivity and specificity for antigen-bearing target cells in vitro and in vivo and are excellent clinical trial candidates.

Impact of expression system on the function of the C6.5 diabody PET radiotracer

The ability of engineered antibodies to rapidly and selectively target tumors that express their target antigen makes them well suited for use as radioimaging tracers. The combination of molecular size and bivalent nature makes diabody molecules a particularly promising structure for use as radiotracers for diagnostic imaging. Previous data have demonstrated that the anti-HER2 C6.5 diabody (C6.5db) is an effective radiotracer in preclinical models of HER2-positive cancer. The aim of this study was to evaluate the impact on radiotracer performance, associated with expressing the C6.5db in the Pichia pastoris (P-C6.5db) system as compared to Escherichia coli (E. C6.5db). Glycosylation of P-C6.5db led to faster blood clearance and lower overall tumor uptake than seen with E. coli-produced C6.5db. However, P-C6.5db achieved high tumor/background ratios that are critical for effective imaging. Dosimetry measurements determined in this study for both (124)I-P-C6.5db and (124)I-E-C6.5db suggest that they are equivalent to other radiotracers currently being administered to patients.

Modulating antibody pharmacokinetics using hydrophilic polymers

INTRODUCTION

The use of hydrophilic polymers as a substitute for the Fc-domain in immuno- or non-immuno-based binding proteins is accelerating. Chemical PEGylation has led the way and is still the most advanced and clinically-approved approach. Hydrophilic polymers act by maintaining a flexible conformation and hydrogen bonding to a network of water molecules to acquire a larger hydrodynamic volume and apparent mass than their actual molecular mass suggest. The benefits are increased blood half-life and bioavailability, stability and reduced immunogenicity. In the case of PEG, there is also evidence of enhanced targeting and reduced side effects, but drawbacks include the fact that PEG is non-biodegradable.

AREAS COVERED

This report reviews the state of the art for antibody PEGylation in terms of approaches and effects. Additionally, non-biological (such as N-(2-hydroxypropyl)methacrylamide) and potentially superior biological alternatives (such as polysialylation) are described, ending with recombinant approaches (such as hydrophilic peptides and glyco-engineering), which promise to circumvent the need for chemical modification altogether.

EXPERT OPINION

The emergence of many small, antibody fragment-like mimics will drive the need for such technologies, and PEGylation is still the choice polymer due to its established use and track record. However, there will be a place for many alternative technologies if they can match the pharmacokinetics of PEG-conjugates and bring addition beneficial features such as easier production.

Evaluation of the anti-HER2 C6.5 diabody as a PET radiotracer to monitor HER2 status and predict response to trastuzumab treatment

PURPOSE

The rapid tumor targeting and pharmacokinetic properties of engineered antibodies make them potentially suitable for use in imaging strategies to predict and monitor response to targeted therapies. This study aims to evaluate C6.5 diabody (C6.5 db), a noncovalent anti-HER2 single-chain Fv dimer, as a radiotracer for predicting response to HER2-targeted therapies such as trastuzumab.

EXPERIMENTAL DESIGN

Immunodeficient mice bearing established HER2-positive tumor xenografts were injected with radioiodinated C6.5 db and imaged by PET/CT. Radiotracer biodistribution was quantified by biopsied tumor and normal tissues. Potential competition between trastuzumab and C6.5 db was examined in vitro by flow cytometry and coimmunoprecipitations.

RESULTS

Biodistribution analysis of mice bearing xenografts with varying HER2 density revealed that the tumor uptake of (125)I-C6.5 db correlates with HER2 tumor density. In vitro competition experiments suggest that the C6.5 db targets an epitope on HER2 that is distinct from that bound by trastuzumab. Treatment of mice affected with SK-OV-3 tumor with trastuzumab for 3 days caused a 42% (P = 0.002) decrease in tumor uptake of (125)I-C6.5 db. This is consistent with a dramatic decrease in the tumor PET signal of (124)I-C6.5 db after trastuzumab treatment. Furthermore, mice affected with BT-474 tumor showed an approximately 60% decrease (P = 0.0026) in C6.5 db uptake after 6 days of trastuzumab treatment. Immunohistochemistry of excised xenograft sections and in vitro flow cytometry revealed that the decreased C6.5 db uptake on trastuzumab treatment is not associated with HER2 downregulation.

CONCLUSIONS

These studies suggest that (124)I-C6.5 db-based imaging can be used to evaluate HER2 levels as a predictor of response to HER2-directed therapies.

Site-specific polysialylation of an antitumor single-chain Fv fragment

Protein pharmacokinetic modulation is becoming an important tool in the development of biotherapeutics. Proteins can be chemically or recombinantly modified to alter their half-lives and bioavailability to suit particular applications as well as improve side effect profiles. The most successful and clinically used approach to date is chemical conjugation with poly(ethylene glycol) polymers (PEGylation). Here, therapeutic protein half-life can be increased significantly while retaining biological function, reducing immunogenicity and cross-reaction. Naturally occurring alternatives to such synthetic polymers could have major advantages such as lower side effects due to biodegradability and metabolism. Polysialic acid (PSA) has been investigated as a pharmacokinetic modulatory biopolymer with many successful examples in preclinical and clinical development. Single-chain Fvs (scFvs) are a choice antibody format for human therapeutic antibody discovery. Because of their small size, they are rapidly eliminated from the circulation and often are rebuilt into larger proteins for drug development and a longer half-life. Here we show that chemical polysialylation can increase the half-life of an antiplacental alkaline (PLAP) and anticarcinoembryonic antigen (CEA) scFv (F1 and MFE-23, respectively) 3.4-4.9-fold, resulting in a 10.6-15.2-fold increase in blood exposure. Amine-directed coupling of the MFE-23 scFv reduced its immunoreactivity 20-fold which was resolved by site-specific polysialylation through an engineered C-terminal thiol residue. The site-specifically polysialylated MFE-23 scFv demonstrated up to 30-fold improved tumor uptake while displaying favorable tumor:normal tissue specificity. This suggests that engineering antibody fragments for site-specific polysialylation could be a useful approach to increase the half-life for a variety of therapeutic applications.

Recombinant single-chain variable fragment antibodies against extracellular epitopes of human multidrug resistance protein MRP3 for targeting malignant gliomas

Multidrug resistance protein 3 (MRP3), a multidrug resistance protein identified by serial analysis of gene expression as a glioblastoma multiforme (GBM)-associated molecule, is highly expressed in GBM, but not in normal brain cells. Thus, MRP3 is a candidate for GBM immunotargeting, but to date, no monoclonal antibody has been isolated that can target an extracellular MRP3 epitope. By phage display, we have isolated 3 recombinant, fully human, single-chain Fv (scFv) antibodies, M25, M58 and M89, which specifically react with the extracellular N-terminus of human MRP3. In ELISA, these scFvs reacted only with the peptide used for screening and not with other MRP3-derived peptides. Flow cytometric analysis revealed that these scFv fragments bind specifically to viable human GBM cells displaying different MRP3 expression levels, but not to MRP3-null cells. Furthermore, these scFv antibodies failed to react with tumor cells overexpressing other MRP proteins, including MRP1, MRP2, MRP4 and MRP5. M25 and M58 also bound to viable neurospheres. Iodogen-labeled scFvs demonstrated a yield of 56-76%. The immunoreactive fractions of the radiolabeled M25, M58 and M89 scFvs were 32, 52 and 69%, respectively. M25 exhibited 20% internalization into D2159MG neurospheres, M58, 33% into D54MG cells and M89, 26% into D247MG. Immunohistochemical evaluation of human gliomas to determine the localization of MRP3 antigen using scFvs M25 and M58 showed a dense cytoplasmic and membranous staining pattern. These Fv-based recombinant antibodies, which possess superior tumor penetration capabilities and selectively target tumor cells that express MRP3, may potentially be used in immunotherapy and diagnosis for brain tumors and other cancers.

Modulating the pharmacokinetics of therapeutic antibodies

With the advent of antibody fragments and alternative binding scaffolds, that are devoid of Fc-regions, strategies to increase the half-life of small proteins are becoming increasingly important. Currently, the established method is chemical PEGylation, but more elaborate approaches are being described such as polysialylation, amino acid polymers and albumin-binding derivatives. This article reviews the main strategies for pharmacokinetic enhancement, primarily chemical conjugates and recombinant fusions that increase apparent molecular weight or hydrodynamic radius or interact with serum albumin which itself has a long plasma half-life. We highlight the key chemical linkage methods that preserve antibody function and retain stability and look forward to the next generation of technologies which promise to make better quality pharmaceuticals with lower side effects. Although restricted to antibodies, all of the approaches covered can be applied to other biotherapeutics.

Effective treatment of established human breast tumor xenografts in immunodeficient mice with a single dose of the alpha-emitting radioisotope astatine-211 conjugated to anti-HER2/neu diabodies

PURPOSE

Successful radioimmunotherapy strategies depend on selecting radioisotopes with physical properties complementary to the biological properties of the targeting vehicle. Small, engineered antitumor antibody fragments are capable of rapid, highly specific tumor targeting in immunodeficient mouse models. We hypothesized that the C6.5 diabody, a noncovalent anti-HER2 single-chain Fv dimer, would be an ideal radioisotope carrier for the radioimmunotherapy of established tumors using the short-lived alpha-emitting radioisotope (211)At.

EXPERIMENTAL DESIGN

Immunodeficient nude mice bearing established HER2/neu-positive MDA-MB-361/DYT2 tumors treated with N-succinimidyl N-(4-[(211)At]astatophenethyl)succinamate ((211)At-SAPS)-C6.5 diabody. Additional cohorts of mice were treated with (211)At-SAPS T84.66 diabody targeting the carcinoembryonic antigen or (211)At-SAPS on a diabody specific for the Müllerian inhibiting substance type II receptor, which is minimally expressed on this tumor cell line.

RESULTS

A single i.v. injection of (211)At-SAPS C6.5 diabody led to a 30-day delay in tumor growth when a 20 muCi dose was administered and a 57-day delay in tumor growth (60% tumor-free after 1 year) when a 45 muCi dose was used. Treatment of mice bearing the same tumors with (211)At-SAPS T84.66 diabody at the same doses led to a delay in tumor growth, but no complete responses, likely due to substantially lower expression of this antigen on the MDA-MB-361/DYT2 tumors. In contrast, a dose of 20 muCi of (211)At-SAPS on the anti-Müllerian-inhibiting substance type II receptor diabody did not affect tumor growth rate, demonstrating specificity of the therapeutic effect.

CONCLUSIONS

These findings indicate that diabody molecules can be effective agents for targeted radioimmunotherapy of solid tumors using powerful, short-lived alpha-emitting radioisotopes.

Modulation of antibody pharmacokinetics by chemical polysialylation

Chemical coupling of a variety of polymers to therapeutic proteins has been studied as a way of improving their pharmacokinetics and pharmacodynamics in vivo. Conjugates have been shown to possess greater stability, lower immunogenicity, and a longer blood circulation time due to the chemicophysical properties of these hydrophilic long chain molecules. Naturally occurring colominic acid (polysialic acid, PSA) has been investigated as an alternative to synthetic polymers such as poly(ethylene glycol) (PEG) due to its lower toxicity and natural metabolism. Antibodies and their fragments are a good example of the types of proteins which benefit from pharmacokinetic engineering. Here, we chemically attached differing amounts and differing lengths of short (11 kDa) and longer (22 kDa) chain colominic acid molecules to the antitumor monoclonal antibody H17E2 Fab fragment. Different coupling ratios and lengths were seen to alter the electrophoretic mobility of the Fab fragment but have a minor effect on the antibody immunoreactivity toward the placental alkaline phosphatase (PLAP) antigen. Polysialylation generally increased Fab fragment blood half-life resulting in higher tumor uptake in a KB human tumor xenograft mouse model. One H17E2 Fab-PSA conjugate had over a 5-fold increase in blood exposure and over a 3-fold higher tumor uptake with only a marginal decrease in tumor/blood selectivity ratio compared to the unconjugated Fab. This conjugate also had a blood bioavailability approaching that of a whole immunoglobulin.

Therapy of human carcinoma xenografts with antibodies to EGFr and HER-2 conjugated to radionuclides emitting low-energy electrons

PURPOSE

Low-energy electrons (10-50 keV) can be effective and specific cytotoxic agents when delivered to the cell surface by antibodies, because their path length in tissue is comparable to a cell diameter. In this study, we have begun to evaluate the therapeutic potential of antibodies (Abs) conjugated to (111)In against carcinoma xenografts in nude mice.

METHODS

Abs to EGFr or HER-2 were labeled with (111)In to a high specific activity of approximately 1.48 GBq/mg (40 mCi/mg). They were injected into nude mice 5-6 days after inoculation of human carcinoma cells, either A431 or SK-OV-3, and tumor growth was monitored. In preliminary in vitro experiments, we calculated the cumulative decays per cell, estimated the centigray dose delivered to the nucleus, and related this to the fraction surviving.

RESULTS

Abs to both antigens provided significant protection in nude mouse xenograft models (p values ranging from <0.05 to <0.001). Some mice appeared to be cured, but most had delayed tumor growth. The specificity of the effect was demonstrated by testing non-reactive Abs labeled in the same way. The radioactivity was required, because unconjugated Abs had no therapeutic effect. The maximum tolerated dose was required in order for therapy to be effective, but most of the treated mice had no significant weight loss or other overt signs of toxicity.

CONCLUSION

Abs labeled with nuclides emitting low-energy electrons, such as (111)In, can be effective therapeutic agents against microscopic s.c. tumors. This strategy should be considered for clinical applications.

Targeted photodynamic therapy with multiply-loaded recombinant antibody fragments

Current photodynamic therapy (PDT) of cancer is limited by inefficiencies involved in specifically targeting photosensitizers to tumors. Although antibodies are being explored as targeting vehicles, they present significant challenges, particularly in terms of pharmacokinetics and drug-coupling. We describe here a novel and effective system to covalently attach multiple photosensitizer molecules (both preclinical, pyropheophorbide-a and clinically approved, verteporfin photosensitizers) to single-chain Fvs. Further, we demonstrate that not only do the resulting photoimmunoconjugates retain photophysical functionality, they are more potent than either free photosensitizer, effectively killing tumor cells in vitro and in vivo. For example, treatment of human breast cancer xenografts with a photoimmunoconjugate comprising an anti-HER-2 scFv linked to 8-10 molecules of pyropheophorbide-a leads to significant tumor regression. These results give an insight into the important features that make scFvs good carriers for PDT drugs and provide proof of concept of our unique approach to targeted photodynamic therapy (tPDT). This promises to significantly improve on current photodynamic therapies for the treatment of cancer.

Streptavidin in antibody pretargeting. 5. chemical modification of recombinant streptavidin for labeling with the alpha-particle-emitting radionuclides 213Bi and 211At

We are investigating the use of recombinant streptavidin (rSAv) as a carrier molecule for the short-lived alpha-particle-emitting radionuclides 213Bi ( t 1/2 = 45.6 min) and 211At ( t 1/2 = 7.21 h) in cancer therapy. To utilize rSAv as a carrier, it must be modified in a manner that permits rapid chelation or bonding with these short-lived radionuclides and also modified in a manner that diminishes its natural propensity for localization in the kidney. Modification for labeling with (213)Bi was accomplished by conjugation of rSAv with the DTPA derivative p-isothiocyanato-benzyl-CHX-A'' (CHX-A''), 3a. Modification for direct labeling with 211At was accomplished by conjugation of rSAv with an isothiocyanatophenyl derivative of a nido-carborane (nCB), 3b, or an isothiocyanatophenyl-dPEG/decaborate(2-) derivative, 3c. After conjugation of the chelating or bonding moiety, rSAv was further modified by reaction with an excess (50-100 equivalents) of succinic anhydride. Succinylation of the lysine amines has previously been shown to greatly diminish kidney localization. rSAv modified by conjugation with 3a and succinylated rapidly radiolabeled with 213Bi (<5 min), providing a 72% isolated yield. 211At labeling of modified rSAv was accomplished in aqueous solution using chloramine-T as the oxidant. Astatination of rSAv conjugated with 3b and succinylated occurred very rapidly (<1 min), providing a 50% isolated radiochemical yield. Astatination of rSAv conjugated with 3c and succinylated was also very rapid (<1 min) providing 66-71% isolated radiochemical yields. Astatination of succinylated rSAv, 2a, which did not have conjugated borane cage moieties, resulted in a much lower radiolabeling yield (18%). The 213Bi or 211At-labeled modified rSAv preparations were mixed with the corresponding 125 I-labeled rSAv, and dual-label in vivo distributions were obtained in athymic mice. The in vivo data show that 213Bi-labeled succinylated rSAv [ 213Bi] 6a has tissue concentrations similar to those of 125 I-labeled modified rSAv [ 125 I] 6b, suggesting that (213)Bi is quite stable toward release from the chelate in vivo. In vivo data also indicate that the (211)At-labeled rSAv conjugated with 3b or 3c and succinylated are stable to in vivo deastatination, whereas succinylated rSAv lacking a boron cage moiety is subject to some deastatination. The modified rSAv conjugated with nido-carborane derivative 3b has a higher retention in many tissues than rSAv without the carborane conjugated. Interestingly, the rSAv conjugated with 3c, which also contains an m-dPEG 12 moiety, has significantly decreased concentrations in blood and other tissues when compared with those of direct-labeled rSAv, suggesting that it may be a good candidate for further study. In conclusion, rSAv that has been modified with CHX-A'' and succinylated (i.e., 5a) may be useful as a carrier of 213Bi. The encouraging results obtained with the PEGylated decaborate(2-) derivative 3c and succinylated (i.e., 5c) suggests that its further study as a carrier of 211At in pretargeting protocols is warranted.

Fluorescence characterisation of multiply-loaded anti-HER2 single chain Fv-photosensitizer conjugates suitable for photodynamic therapy

We report the synthesis, spectroscopic properties and intracellular imaging of recombinant antibody single chain fragment (scFv) conjugates with photosensitizers used for photodynamic therapy of cancer (PDT). Two widely-studied photosensitizers have been selected: preclinical pyropheophorbide-a (PPa) and verteporfin (VP), which has been clinically approved for the treatment of acute macular degeneration (Visudyne). Pyropheophorbide-a and verteporfin have been conjugated to an anti-HER2 scFv containing on average ten photosensitizer molecules per scFv with a small contribution (<or=20%) from non-covalently bound molecules. Confocal fluorescence microscopy demonstrates good cellular uptake of PPa conjugate with the HER2-positive cell line, SKOV-3, while negligible cell uptake is demonstrated for the HER2-negative cell line, KB. For the VP conjugate, increased rate of cellular uptake and prolonged retention in SKOV-3 cells is observed compared to free photosensitizer. In clinical applications this could provide increased potency and desired selectivity towards malignant tissue, leaving surrounding healthy tissue unharmed and reducing skin photosensitivity. The present study highlights the usefulness of photosensitizer immunoconjugates with scFvs for targeted PDT.

[Alpha-radioimmunotherapy: a review of recent developments]

The use of heavy particles in the treatment of cancer is increasing remarkably, whether with external radiation or using a vector such as an antibody in radioimmunotherapy. Recent pre-clinical and clinical developments of alpha-radioimmunotherapy have provided more interesting information in parallel of the use of high Linear Energy Transfer (LET) external irradiation. This review aims at presenting recent advances of this therapeutic approach, and at detailing the biological specificities of this kind of radiation.

Potentiation of high-LET radiation by gemcitabine: targeting HER2 with trastuzumab to treat disseminated peritoneal disease

PURPOSE

Recent studies from this laboratory with (212)Pb-trastuzumab have shown the feasibility of targeted therapy for the treatment of disseminated peritoneal disease using (212)Pb as an in vivo generator of (212)Bi. The objective of the studies presented here was improvement of the efficacy of alpha-particle radioimmunotherapy using a chemotherapeutic agent.

EXPERIMENTAL DESIGN

In a series of experiments, a treatment regimen was systematically developed in which athymic mice bearing i.p. LS-174T xenografts were injected i.p. with gemcitabine at 50 mg/kg followed by (212)Pb radioimmunotherapy.

RESULTS

In a pilot study, tumor-bearing mice were treated with gemcitabine and, 24 to 30 h later, with 5 or 10 muCi (212)Pb-trastuzumab. Improvement in median survival was observed at 5 microCi (212)Pb-trastuzumab in the absence (31 days) or presence (51 days) of gemcitabine: 45 and 70 days with 10 microCi versus 16 days for untreated mice (P < 0.001). Multiple doses of gemcitabine combined with a single (212)Pb radioimmunotherapy (10 microCi) administration was then evaluated. Mice received three doses of gemcitabine: one before (212)Pb-trastuzumab and two afterwards. Median survival of mice was 63 versus 54 days for those receiving a single gemcitabine dose before radioimmunotherapy (P < 0.001), specifically attributable to (212)Pb-trastuzumab (P = 0.01). Extending these findings, one versus two treatment cycles was compared. A cycle consisted of sequential treatment with gemcitabine, 10 microCi (212)Pb radioimmunotherapy, then one or two additional gemcitabine doses. In the first cycle, three doses of gemcitabine resulted in a median survival of 90 versus 21 days for the untreated mice. The greatest benefit was noted after cycle 2 in the mice receiving 10 microCi (212)Pb-trastuzumab and two doses of gemcitabine with a median survival of 196.5 days (P = 0.005). Pretreatment of tumor-bearing mice with two doses of gemcitabine before (212)Pb radioimmunotherapy was also assessed with gemcitabine injected 72 and 24 h before (212)Pb-trastuzumab. The median survival was 56 and 76 days with one and two doses of gemcitabine versus 49 days without gemcitabine. The effect may not be wholly specific to trastuzumab because (212)Pb-HuIgG with two doses of gemcitabine resulted in a median survival of 66 days (34 days without gemcitabine).

CONCLUSIONS

Treatment regimens combining chemotherapeutics with high-LET targeted therapy may have tremendous potential in the management and care of cancer patients.

Technological advances in radioimmunotherapy

Radioimmunotherapy (RIT) is a method of selectively delivering radionuclides with toxic emissions to cancer cells, while reducing the dose to normal tissues. Although primary tumours can often be treated successfully with external beam radiotherapy or surgery, metastases often escape detection and treatment, leading to therapy failure, and these can be treated with systemic targeted therapies such as RIT. This review describes more recent developments in the field, including both technological developments from the laboratory and increasingly encouraging findings from clinical studies.

212Pb@C60 and Its Water-Soluble Derivatives: Synthesis, Stability, and Suitability for Radioimmunotherapy

Fullerenes could potentially play a valuable role in radioimmunotherapy by more stably encapsulating radionuclides, especially where conventional chelation chemistry is inadequate due to the physical and/or chemical properties of the radionuclide. One of the therapeutically useful radionuclides that requires improved containment in vivo is 212Pb (tau1/2 = 10.6 h), the beta-emitting parent to alpha-emitting 212Bi (tau1/2 = 60.6 min). Myelotoxicity resulting from the accumulation of 212Pb in the bone marrow has limited the use of this radionuclide despite its favorable decay characteristics. In this work, 212Pb@C60 and its malonic ester derivatives were prepared for the first time by allowing the 212Pb to recoil into C60 following alpha-decay from its parent, 0.15-s 216Po, generated in situ from the decay of 224Ra (tau1/2 = 15 days). Repeated washing of the organic phase containing the 212Pb@C60 malonic esters with challenge solutions containing cold Pb2+ ions demonstrated that some of the 212Pb could not be exchanged and was apparently inside of the fullerenes. Malonic esters of endohedral alpha-emitting 213Bi (tau1/2 = 45 min) fullerenes were prepared by an analogous procedure. Following acidification of the esters, a preliminary biodistribution study in mice was performed with the untargeted water-soluble radiofullerenes. It was found that 212Pb did not accumulate in bone after being administered as an endohedral fullerene, in contrast to results with polyhydroxylated radiofullerenes and conventional polyaminocarboxylate chelators for 212Pb. The results indicate that 212Pb is held more tightly in the fullerene than in other methods and suggest that fullerenes may have an important role in the targeted delivery of 212Pb.

[Conformational analysis of cysteine-containing peptides and prospects of their application to 213Bi chelating in antitumor therapy]

The method of conformational analysis was applied to the spatial structures of peptide analogues of phytochelatins and some fragments of metallothioneins: (Cys-Gly)3, (Cys-Gly)3Asp, (Cys-Gly)3Glu, (Cys-betaAla)3, (Cys-gammaGlu)3, and (Cys-Gly-Gly)3. All the possible low-energy conformations of the molecules were revealed and the role of intra- and inter-residual interactions in the formation of their spatial structures was determined. A different tendency of the molecules under study for acceptance of conformations favorable for binding bismuth ions was shown. Low-energy structures providing an optimum binding of bismuth ion were shown to be most frequent for (Cys-betaAla)3 peptide. Among the analogues of peptide fragments of the metallothioneins, lacking in natural peptides, low-energy pentapeptide CCXXC fragments (where X = Gln, Asn, Phe, Tyr, and Gly) were revealed. In the alpha-helical conformations of these pentapeptides, the distance between the sulfur atoms corresponds to that in Bi2S3. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2006, vol. 32, no. 3; see also http://www.maik.ru.

Mono and bivalent binding of a scFv and covalent diabody to murine laminin-1 using radioiodinated proteins and SPR measurements: effects on tissue retention in vivo

Phage display techniques identified a scFv, 15-9, which binds to murine laminin-1 and accumulated selectively in tumors. In this study, a covalent diabody was constructed by changing the amino acid residues at positions VH44 and VL100 to cysteine residues so that the diabody form could be stabilized via a disulfide bond. The covalent diabody was expressed in Pichia pastoris and purified by affinity chromatography. The binding properties were measured by surface plasmon resonance and solid phase binding of (125)I diabody and scFv. Data from the plasmon resonance method yielded calculated K(D)s of 4.4 x 10(-10) M for the covalent diabody and 9.9 x 10(-8) M for the scFv. K(D)s calculated from solid phase binding of radioiodinated proteins were 1.7-2.1 x 10(-10) M and 2.1-2.4 x 10(-8) M respectively. The rate of dissociation of (125)I scFv from solid phase laminin was independent of laminin concentration; however, the dissociation of the (125)I diabody was dependent both on the concentration of laminin and on the concentration of the diabody. Specifically, high concentrations of laminin yielded very slow rates of diabody dissociation indicating that bivalent attachments had formed. When higher amounts of diabody were used that essentially saturated the laminin sites with univalent binding, the dissociation rate was similar to that for the scFv indicating univalent binding. Biodistribution studies in tumor-bearing SCID mice showed that the covalent diabody improved the ratio of tumor/muscle 2 fold over that obtained with the scFv, although the absolute amount of protein bound to the tumor site was not significantly different for the two forms. The data also showed that retention of the diabody in the tumor and kidney, sites where laminin is present in high concentration, was much longer compared to that of scFv. These data are consistent with the hypothesis that both scFv and diabody forms bind to available laminin in vivo with similar association kinetics, but that in situations of high target concentration, the diabody can bind bivalently and is thus retained at the binding site much longer than the scFv.

Expression, purification, and in vitro refolding of a humanized single-chain Fv antibody against human CTLA4 (CD152)

A human-derived single-chain Fv (scFv) antibody fragment specific against human CTLA4 (CD152) was produced at high level in Escherichia coli. The scFv gene was cloned from a phagemid to the expression vector pQE30 with a N-terminal 6His tag fused in-frame, and expressed as a 29 kDa protein in E. coli as inclusion bodies. The inclusion body of scFv was isolated from E. coli lysate, solubilized in 8M urea with 10mM dithiothreitol, and purified by ion-exchange chromatography. Method for in vitro refolding of the scFv was established. The effects of refolding buffer composition, protein concentration and temperature on the refolding yield were investigated. The protein was renatured finally by dialyzing against 3mM GSH, 1mM GSSG, 150 mM NaCl, 1M urea, and 50 mM Tris-Cl (pH 8.0) for 48 h at 4 degrees C, and then dialyzed against phosphate-buffered saline (pH 7.4) to remove remaining denaturant. This refolding protocol generated up to a 70% yield of soluble protein. Soluble scFv was characterized for its specific antigen-binding activity by indirect cellular ELISA. The refolded scFv was functionally active and was able to bind specifically to CTLA4 (CD152). The epitopes recognized by refolded anti-CTLA4 scFv do not coincide with those epitopes recognized by CD80/CD86.

Alpha-particle radioimmunotherapy of disseminated peritoneal disease using a (212)Pb-labeled radioimmunoconjugate targeting HER2

These studies demonstrate the feasibility of targeted therapy for the treatment of disseminated peritoneal disease using (212)Pb-labeled Herceptin as an in vivo generator of (212)Bi. In vitro studies compare the potential of the bismuth radioisotopes, (213)Bi and (212)Bi, to that of (212)Pb. Overall, (212)Pb results in a higher therapeutic index than either bismuth radioisotope, requiring lower radioactivity (microCi) for effective cytotoxic response. A pilot radioimmunotherapy (RIT) experiment treating mice bearing 5 d LS-174T intraperitoneally (i.p.) xenografts determined a maximum tolerated dose (MTD) of 20-40 microCi with i.p. administration. A specific dose response was observed and 10 microCi was selected as the effective operating dose for future experiments. Median survival of tumor-bearing mice receiving 10 microCi increased from 19 to 56 days (p = 0.008). The efficacy of (212)Pb-Herceptin was also assessed in a human pancreatic carcinoma xenograft (Shaw; i.p.) animal model previously reported as unresponsive to 213Bi-Herceptin (p = 0.002). Multiple dosing of (212)Pb-Herceptin was evaluated in both animal models. The median survival of mice bearing 3 d LS-174T i.p. xenografts increased to 110 days, with up to 3 doses of (212)Pb-Herceptin given at approximately monthly intervals; however, there was no evidence of a correlation with the second and third doses (p = 0.98). No improvement in median survival was noted with a similar regimen in the Shaw xenograft model.

Fractionated locoregional low-dose radioimmunotherapy improves survival in a mouse model of diffuse-type gastric cancer using a 213Bi-conjugated monoclonal antibody

PURPOSE

Locoregional radioimmunotherapy of i.p. tumor cell dissemination of diffuse-type gastric cancer using the alpha-emitter 213Bi displayed good therapeutic results after a single application depending on the time interval between tumor cell inoculation and injection of the 213Bi-immunoconjugate. The aim of the present study was to compare single versus double i.p. injection of a tumor-specific antibody (d9MAb) conjugated with low activities of 213Bi in terms of therapeutic efficacy and toxicity.

EXPERIMENTAL DESIGN

Nude mice were inoculated i.p. with 1 x 10(7) human gastric cancer cells (HSC45-M2) expressing tumor-specific mutant d9-E-cadherin (d9-E-cad). After tumor cell inoculation, the mice were injected i.p. with a single injection at day 1 or 8, or double injections at days 1 and 8 or days 8 and 15 with 0.37, 0.74, or 1.48 MBq 213Bi-d9MAb. Therapeutic efficacy was determined by median survival, and toxicity was evaluated by leukocyte and platelet counts. The development of i.p. carcinomatosis was monitored by carcinoembryonic antigen concentrations in the serum of the mice.

RESULTS

The median survival of treated animals increased, depending on the time interval (days) between tumor cell inoculation and therapy, and the injected activity, from 22 days of untreated mice to 48 days (0.37 MBq, 1 day), 84 days (0.37 MBq, 1 and 8 days), 37 days (0.37 MBq, 8 days), 46 days (0.37 MBq, 8 and 15 days), 42 days (0.74 MBq, 8 days), 78 days (0.74 MBq, 8 and 15 days), and 44 days (1.48 MBq, 8 days). The injected activities did not reduce leukocyte and platelet counts. Carcinoembryonic antigen, which was not detectable in the serum of tumor-free mice, increased after tumor cell inoculation and tumor proliferation and decreased after each therapeutic application of 213Bi-d9MAb.

CONCLUSIONS

Double application of only 0.37 MBq of 213Bi-d9MAb at days 1 and 8 after tumor cell inoculation significantly prolonged median survival in nude mice suffering from i.p. tumor cell dissemination compared with a single injection. Even in an advanced stage of the disease, double injection of 0.74 MBq at days 8 and 15 was superior to a single injection of 1.48 MBq at day 8 without any sign of toxicity.

Cancer radioimmunotherapy with alpha-emitting nuclides

In lymphoid malignancies and in certain solid cancers such as medullary thyroid carcinoma, somewhat mixed success has been achieved when applying radioimmunotherapy (RIT) with beta-emitters for the treatment of refractory cases. The development of novel RIT with alpha-emitters has created new opportunities and theoretical advantages due to the high linear energy transfer (LET) and the short path length in biological tissue of alpha-particles. These physical properties offer the prospect of achieving selective tumoural cell killing. Thus, RIT with alpha-emitters appears particularly suited for the elimination of circulating single cells or cell clusters or for the treatment of micrometastases at an early stage. However, to avoid non-specific irradiation of healthy tissues, it is necessary to identify accessible tumoural targets easily and rapidly. For this purpose, a small number of alpha-emitters have been investigated, among which only a few have been used for in vivo preclinical studies. Another problem is the availability and cost of these radionuclides; for instance, the low cost and the development of a reliable actinium-225/bismuth-213 generator were probably determining elements in the choice of bismuth-213 in the only human trial of RIT with an alpha-emitter. This article reviews the literature concerning monoclonal antibodies radiolabelled with alpha-emitters that have been developed for possible RIT in cancer patients. The principal radio-immunoconjugates are considered, starting with physical and chemical properties of alpha-emitters, their mode of production, the possibilities and difficulties of labelling, in vitro studies and finally, when available, in vivo preclinical and clinical studies.

Probing ATP-dependent conformational changes in the multidrug resistance protein 1 (MRP1/ABCC1) in live tumor cells with a novel recombinant single-chain Fv antibody targeted to the extracellular N-terminus

The multidrug resistance protein 1 (MRP1/ABCC1) is an ATP-driven transporter that mediates the cellular extrusion of various chemotherapeutic agents. We have previously isolated a novel recombinant single-chain Fv antibody (A5scFv), which specifically targets the extracellular N-terminus of the human MRP1 expressed on the surface of live tumor cells. Fusion of A5scFv to Pseudomonas exotoxin revealed an immunotoxin that bound to the immobilized MRP1-derived peptide upon ELISA, but surprisingly failed to recognize MRP1 on the surface of live tumor cells. As these results suggested that the N-terminus of MRP1 has a limited accessibility to the extracellular space, we used the A5scFv antibody to probe for putative conformational changes that might occur in viable tumor cells upon ATP binding. A5scFv recognized viable MRP1-expressing cells with intact ATP pools, whereas ATP depletion resulted in the loss of A5scFv reactivity. Consistently, restoration of cellular ATP levels resulted in resumption of A5scFv binding to MRP1 in live tumor cells. Flow cytometric analysis confirmed that ATP-depleted cells accumulated significantly higher levels of the established substrate calcein AM, whereas after restoration of cellular ATP pools, cells displayed a much lower level of calcein AM accumulation. Moreover, pretreatment of MRP1-expressing cells with the membrane fluidizer benzyl alcohol resulted in a dramatic increase in A5scFv reactivity, suggesting that membrane fluidization results in the exposure of the N-terminus of MRP1 to the extracellular milieu. These results constitute the first extracellular probing of the putative conformational changes that MRP1 adopts in viable tumor cells upon ATP binding. Furthermore, although ATP binding occurs in the cytosolic nucleotide binding domains of MRP1, significant conformational changes are apparently propagated to the N-terminus residing at the extracellular compartment.

Arming antibodies: prospects and challenges for immunoconjugates

Immunoconjugates--monoclonal antibodies (mAbs) coupled to highly toxic agents, including radioisotopes and toxic drugs (ineffective when administered systemically alone)--are becoming a significant component of anticancer treatments. By combining the exquisite targeting specificity of mAbs with the enhanced tumor-killing power of toxic effector molecules, immunoconjugates permit sensitive discrimination between target and normal tissue, resulting in fewer toxic side effects than most conventional chemotherapeutic drugs. Two radioimmunoconjugates, ibritumomab tiuxetan (Zevalin) and tositumomab-131I (Bexxar), and one drug conjugate, gemtuzumab ozogamicin (Mylotarg), are now on the market. For the next generation of immunoconjugates, advances in protein engineering will permit greater control of mAb targeting, clearance and pharmacokinetics, resulting in significantly improved delivery to tumors of radioisotopes and potent anticancer drugs. Pre-targeting strategies, which separate the two functions of antibody-based localization and delivery or generation of the toxic agent into two steps, also promise to afford superior tumor targeting and therapeutic efficacy. Several challenges in optimizing immunoconjugates remain, however, including poor intratumoral mAb uptake, normal tissue conjugate exposure and issues surrounding drug potency and conditional release from mAb carriers. Nonetheless, highly promising results from preclinical models will continue to drive the clinical development of this therapeutic class.

Targeting of HER2 antigen for the treatment of disseminated peritoneal disease

The studies reported herein demonstrate the efficacy of alpha-particle-targeted radiation therapy of peritoneal disease with Herceptin as the targeting vehicle. Using the CHX-A-DTPA linker, Herceptin was radiolabeled with indium-111 and bismuth-213 with high efficiency without compromising immunoreactivity. A pilot radioimmunotherapy study treating mice bearing 5-day LS-174T (i.p.) xenografts, a low but uniform HER2 expressing, human colon carcinoma, with a single dose of (213)Bi-CHX-A"-Herceptin, proved disappointing. This defined the effect of tumor burden/size on tumor response to radioimmunotherapy with alpha-radiation. A more successful experiment with a lower tumor burden (3 days) in mice followed. A specific dose-response (P = 0.009) was observed, and although a maximum-tolerated dose was not determined, a dose of 500 to 750 muCi was selected as the operating dose for future experiments based on changes in animal weight. Median survival was increased from 20.5 days for the mock-treated mice to 43 and 59 days with 500 and 750 muCi, respectively. The therapeutic effectiveness of (213)Bi-CHX-A"-Herceptin was also evaluated in a second animal model for peritoneal disease with a human pancreatic carcinoma (Shaw). The results of this study were not as dramatic as with the former model, and higher doses were required to obtain an increase in survival of the mice (P = 0.001).

Quantitative immuno-positron emission tomography imaging of HER2-positive tumor xenografts with an iodine-124 labeled anti-HER2 diabody

Positron emission tomography (PET) provides an effective means of both diagnosing/staging several types of cancer and evaluating efficacy of treatment. To date, the only U.S. Food and Drug Administration-approved radiotracer for oncologic PET is (18)F-fluoro-deoxyglucose, which measures glucose accumulation as a surrogate for malignant activity. Engineered antibody fragments have been developed with the appropriate targeting specificity and systemic elimination properties predicted to allow for effective imaging of cancer based on expression of tumor associated antigens. We evaluated a small engineered antibody fragment specific for the HER2 receptor tyrosine kinase (C6.5 diabody) for its ability to function as a PET radiotracer when labeled with iodine-124. Our studies revealed HER2-dependent imaging of mouse tumor xenografts with a time-dependent increase in tumor-to-background signal over the course of the experiments. Radioiodination via an indirect method attenuated uptake of radioiodine in tissues that express the Na/I symporter without affecting the ability to image the tumor xenografts. In addition, we validated a method for using a clinical PET/computed tomography scanner to quantify tumor uptake in small-animal model systems; quantitation of the tumor targeting by PET correlated with traditional necropsy-based analysis at all time points analyzed. Thus, diabodies may represent an effective molecular structure for development of novel PET radiotracers.

CXCR4 and CXCL12 (SDF-1) in prostate cancer: inhibitory effects of human single chain Fv antibodies

PURPOSE

Metastasis is a major cause of morbidity in prostate cancer (PCa). Several studies have shown that the chemokine receptor CXCR4 and its ligand, CXCL12 (stromal cell-derived factor-1), regulate tumor cell metastasis to specific organs. Recently, it was demonstrated that CXCL12 enhances PCa cell adhesion, migration, and invasion, implicating CXCR4 in PCa metastasis. In this study, we examined the inhibitory effects of anti-CXCR4 antibodies on CXCL12-mediated PCa cell activities.

EXPERIMENTAL DESIGN

We developed fully human single chain Fv antibodies (scFv), Ab124 and Ab125, against CXCR4 using the yeast two-hybrid system. We performed immunofluorescent staining, flow cytometry, and ELISA-binding assays to measure scFv binding to PCa cells. We also examined the effects of scFv on CXCL12-mediated calcium mobilization, cell migration, and invasion.

RESULTS

Our results confirmed that PCa cell lines express cell-surface CXCR4. Real-time quantitative reverse transcription-PCR and immunohistochemical staining also verified that CXCR4 is expressed in primary cultures of prostate epithelial cells from adenocarcinomas and in human prostate tissues. Ab124 and Ab125 demonstrated specific binding to PCa cell lines by flow cytometry and in binding assays. Preincubation with scFv resulted in significant reduction of CXCL12-induced calcium mobilization in PC-3 and LNCaP cells. Ab124 and Ab125 also inhibited PCa cell migration toward CXCL12, as well as invasion through extracellular matrix gels.

CONCLUSIONS

Ab124 and Ab125 inhibit CXCL12-mediated cellular activities by binding the receptor CXCR4. Recombinant scFv are an efficient mode of targeting tumor antigens. Considering the high incidence of PCa, the development of fully human scFv may be a useful therapeutic approach in the prevention and treatment of PCa metastasis.

Targeted alpha therapy for cancer

Targeted alpha therapy (TAT) offers the potential to inhibit the growth of micrometastases by selectively killing isolated and preangiogenic clusters of cancer cells. The practicality and efficacy of TAT is tested by in vitro and in vivo studies in melanoma, leukaemia, colorectal, breast and prostate cancers, and by a phase 1 trial of intralesional TAT for melanoma. The alpha-emitting radioisotope used is Bi-213, which is eluted from the Ac-225 generator and chelated to a cancer specific monoclonal antibody (mab) or protein (e.g. plasminogen activator inhibitor-2 PAI2) to form the alpha-conjugate (AC). Stable alpha-ACs have been produced which have been tested for specificity and cytotoxicity in vitro against melanoma (9.2.27 mab), leukaemia (WM60), colorectal (C30.6), breast (PAI2, herceptin), ovarian (PAI2, herceptin, C595), prostate (PAI2, J591) and pancreatic (PAI2, C595) cancers. Subcutaneous inoculation of 1-1.5 million human cancer cells into the flanks of nude mice causes tumours to grow in all mice. Tumour growth is compared for untreated controls, nonspecific AC and specific AC, for local (subcutaneous) and systemic (tail vein or intraperitoneal) injection models. The 213Bi-9.2.27 AC is injected into secondary skin melanomas in stage 4 patients in a dose escalation study to determine the effective tolerance dose, and to measure kinematics to obtain the equivalent dose to organs. In vitro studies show that TAT is one to two orders of magnitude more cytotoxic to targeted cells than non-specific ACs, specific beta emitting conjugates or free isotopes. In vivo local TAT at 2 days post-inoculation completely prevents tumour formation for all cancers tested so far. Intra-lesional TAT can completely regress advanced sc melanoma but is less successful for breast and prostate cancers. Systemic TAT inhibits the growth of sc melanoma xenografts and gives almost complete control of breast and prostate cancer tumour growth. Intralesional doses up to 450 microCi in human patients are effective in regressing melanomas, with no concomitant complications. These results point to the application of local and systemic TAT in the management of secondary cancer. Results of the phase 1 clinical trial of TAT of subcutaneous, secondary melanoma indicate proof of the principle that TAT can make tumours in patients regress.

Radiobromination of anti-HER2/neu/ErbB-2 monoclonal antibody using the p-isothiocyanatobenzene derivative of the [76Br]undecahydro-bromo-7,8-dicarba-nido-undecaborate(1-) ion

The monoclonal humanized anti-HER2 antibody trastuzumab was radiolabeled with the positron emitter (76)Br (T(1/2) =16.2 h). Indirect labeling was performed using the p-isothiocyanatobenzene derivative of the [(76)Br]undecahydro-bromo-7,8-dicarba-nido-undecaborate(1-) ((76)Br-NBI) as a precursor molecule. (76)Br-NBI was prepared by bromination of the 7-(p-isothiocyanato-phenyl)dodecahydro-7,8-dicarba-nido-undecaborate(1-) ion (NBI) with a yield of 93-95% using Chloramine-T (CAT) as an oxidant. Coupling of radiobrominated NBI to antibody was performed without intermediate purification, in an "one pot" reaction. An overall labeling yield of 55.7 +/- 4.8% (mean +/- maximum error) was achieved when 300 microg of antibody was labeled. The label was stable in vitro in physiological and denaturing conditions. In a cell binding test, trastuzumab remained immunoreactive after labeling.

A Single Treatment of Yttrium-90-labeled CHX-A″–C6.5 Diabody Inhibits the Growth of Established Human Tumor Xenografts in Immunodeficient Mice

Antitumor diabody molecules are noncovalent single-chain Fv dimers that recapitulate the divalent binding properties of native IgG antibodies. Diabodies are capable of substantial accumulation in tumor xenografts expressing relevant antigens in immunodeficient mouse models. With a Mr of approximately 55,000, diabodies are rapidly cleared from the circulation, resulting in tumor-to-blood ratios that significantly exceed those achieved early after the administration of monoclonal antibodies. We have evaluated the therapeutic potential of the beta-emitting isotope yttrium-90 (t1/2, 64 hours) conjugated to the C6.5K-A diabody that specifically targets the HER2/neu human tumor-associated antigen. We have found that a single intravenous dose of 150 microCi (200 microg) 90Y-CHX-A"-C6.5K-A diabody substantially inhibits the growth rates of established MDA-361/DYT2 human breast tumor xenografts in athymic nude mice. In contrast, 300 microCi (300 microg) 90Y-CHX-A"-C6.5K-A diabody resulted in only a minor delay in the growth of SK-OV-3 human ovarian cancer xenografts. The maximum tolerated dose was also dependent on the tumor xenograft model used. These studies indicate that genetically engineered antitumor diabody molecules can be used as effective vehicles for radioimmunotherapy.

Feasibility of radioimmunotherapy of experimental pneumococcal infection

Streptococcus pneumoniae is an important cause of community-acquired pneumonia, meningitis, and bacteremia. The problem of pneumococcal disease is exacerbated by increasing drug resistance. Furthermore, patients with impaired immunity are at high risk for invasive pneumococcal infections. Thus, there is an urgent need for new approaches to antimicrobial therapy. Antibody therapies take advantage of the specificity and high affinity of the antigen-antibody interaction to deliver antibacterial compounds to a site of infection in the form of naked or conjugated antibodies. We have recently established that radioimmunotherapy (RIT) can be used to treat experimental fungal infections in mice. In the present study, we investigated the feasibility of applying a RIT approach to the treatment of S. pneumoniae infection by evaluating the susceptibility of S. pneumoniae to radiolabeled antibody in vitro and in an animal infection model. For the specific antibody carrier, we used human monoclonal antibody D11, which binds to pneumococcal capsular polysaccharide 8. We have selected the alpha particle emitter (213)Bi as the radionuclide for conjugation to the antibody. Incubation of serotype 8 S. pneumoniae with (213)Bi-D11 resulted in dose-dependent killing of bacteria. RIT of S. pneumoniae infection in C57BL/6 mice showed that 60% more mice survived in the (213)Bi-D11-treated group (80 micro Ci) than in the untreated group (P < 0.01). The treatment did not cause hematological toxicity, as demonstrated by platelet counts. This feasibility study establishes that RIT can be applied to the treatment of bacterial infections.

Experimental therapy of disseminated B-Cell lymphoma xenografts with 213Bi-labeled anti-CD74

A (213)Bi-labeled antibody to CD74 was evaluated as a therapeutic agent for B-cell lymphoma. The alpha-particle emission, with a half-life of 46 min, is appropriate for therapy of micrometastases. The labeled Ab retained full immunoreactivity, and was potent at single-cell kill of the Raji B-lymphoma cell line. Approximately 30 decays of cell-bound (213)Bi was required for a cell kill of 99%, and dosimetry calculations suggested that the cGy dose delivered was sufficient to produce the level of toxicity observed. A non-reactive control Ab, labeled similarly, also produced toxicity, due to decays occurring in the medium, but was approximately 3-fold less potent than the reactive Ab. In a SCID mouse xenograft micrometastatic model, Ab injection at day 2 or day 5 after tumor inoculation resulted in strong, specific suppression of tumor growth, with some apparent cures.

Radioimmunotherapy of Human Colon Carcinoma Xenografts Using a213Bi-Labeled Domain-Deleted Humanized Monoclonal Antibody

The data presented within this paper is the first report of a humanized domain-deleted monoclonal antibody (HuCC49DeltaCH2) to be utilized in a radioimmunotherapeutic (RIT) application with 213Bi. An initial study indicated that 111In-HuCC49DeltaCH2 targets the subcutaneously implanted human colon carcinoma xenograft, LS-174T, when injected via a peritoneal route. The HuCC49DeltaCH2 was then radiolabeled with 213Bi, an alpha-emitting radionuclide with a half-life of 45.6 minutes, and evaluated for therapeutic efficacy. Dose titration studies indicated that a single dose of 500-1000 microCi, when injected by an intraperitoneal route, resulted in the growth inhibition or regression of the tumor xenograft. The radioimmunotherapeutic effect was found to be dose-dependent. Specificity of the therapeutic efficacy was confirmed in a subsequent experiment with athymic mice bearing TAG-72 negative MIP (human colorectal) xenografts. A preliminary study was also performed to assess a multiple-dose administration of 213Bi-HuCC49DeltaCH2. Doses (500 microCi) were administered at 14-day intervals after tumor implantation. A reduction in volume and/or delay in tumor growth was evident following the second and third injections of 213Bi-HuCC49DeltaCH2. As further validation of the use of 213Bi-HuCC49DeltaCH2 for RIT, a study using 131I was conducted. The overall survival of mice receiving 213Bi-HuCC49DeltaCH2 was greater than those that received 131I-HuCC49DeltaCH2.

Targeting an extracellular epitope of the human multidrug resistance protein 1 (MRP1) in malignant cells with a novel recombinant single chain Fv antibody

Inherent and acquired multidrug resistance (MDR) is characterized by a simultaneous resistance to diverse anticancer drugs and is a major impediment towards curative chemotherapy of cancer. Hence one important goal is to develop strategies aimed at specific targeting of major anticancer drug efflux transporters of the ATP-binding cassette (ABC) superfamily including multidrug resistance protein 1 -MRP1 (ABCC1). To date, no monoclonal antibody has been isolated that can target an extracellular MRP1 epitope. Using a phage display approach, we have isolated a recombinant single-chain Fv (scFv) antibody that specifically reacts with the extracellular N-terminus of the human MRP1. Flow cytometric analysis revealed that this scFv fragment binds specifically to various viable human tumor cells that display variable MRP1 expression levels but not to MRP1 null cells. Furthermore, this scFv antibody failed to react with tumor cells that overexpress other members of the MRP family that have an extracellular N-terminus (MRP2 and MRP3) as well as with MRP4, MRP5, and breast cancer resistance protein. Flow cytometric analysis also showed a good correlation between the fluorescence intensity of the anti-MRP1 scFv antibody and MRP1 levels in viable tumor cells. These findings constitute the first successful isolation of a small recombinant scFv antibody directed to an extracellular epitope of the MRP1 in viable malignant cells. These novel small Fv-based recombinant antibodies that possess superior tumor penetration capabilities may possibly be used to selectively target drugs or tumor cells that express MRP-1.

Ionizing radiation delivered by specific antibody is therapeutic against a fungal infection

There is an urgent need for new antimicrobial therapies to combat drug resistance, new pathogens, and the relative inefficacy of current therapy in compromised hosts. Ionizing radiation can kill microorganisms quickly and efficiently, but this modality has not been exploited as a therapeutic antimicrobial strategy. We have developed methods to target ionizing radiation to a fungal cell by labeling a specific mAb with the therapeutic radioisotopes Rhenium-188 and Bismuth-213. Radiolabeled antibody killed cells of human pathogenic fungus Cryptococcus neoformans in vitro, thus converting an antibody with no inherent antifungal activity into a microbicidal molecule. Administration of radiolabeled antibody to mice with C. neoformans infection delivered 213Bi and 188Re to the sites of infection, reduced their organ fungal burden, and significantly prolonged their survival without apparent toxicity. This study establishes the principle that targeted radiation can be used for the therapy of an infectious disease, and suggests that it may have wide applicability as an antimicrobial strategy.

Construction and high-level expression of a single-chain Fv antibody fragment specific for acidic isoferritin in Escherichia coli

A functional single-chain Fv antibody fragment (scFv) specific for acidic isoferritin (AIF) was produced at high level in Escherichia coli. The variable regions of heavy chain (V(H)) and light chain (V(L)) from the hybridoma 4c9 were connected with a flexible linker using an assembly polymerase chain reaction. The construct of V(H)-linker-V(L) was inserted into a phagemid pCANTAB 5 E followed by selection with the Recombinant Phage Antibody System (RPAS). Anti-AIF scFv gene from the recombinant phagemid pCAN4c9 was subcloned into pET28a fused to N-terminal His-tag sequence in frame and overexpressed in E. coli BL21(DE3). With an on-column refolding procedure based on Ni-chelating chromatography, the active anti-AIF scFv was recovered efficiently from inclusion bodies with a refolding yield of approximate 75% confirmed by spectrophotometer. The activity of refolded scFv was determined through sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Western blotting and enzyme-linked immunosorbent assay. The results showed anti-AIF scFv retains the specific binding activity to AIF with an affinity constant of 7.29 x 10(-8) mol l(-1). The overall yield of anti-AIF scFv with bioactivity in E. coli flask culture was more than 60 mg l(-1).