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

Synthesis, Solid State Structure, and Cytotoxic Activity of a Complex Dimer of Yttrium with Anthranilic Acid against Cancer Cells

This paper presents the synthesis and isolation of a new binuclear complex of yttrium with anthranilic acid (HA). The complex [Y2(HA)6(H2O)4] Cl6.2C2H5OH (C1) was obtained as single crystals that its X-ray analysis revealed its triclinic P-1 space group in addition to anti-prismatic geometry around each of the yttrium ions. In the complex, the anthranilic acid ligands are bidentate, zwitter ionic and neutral, and the yttrium ions' charge is only compensated by six chloride ions. The cytotoxicity of this complex against human breast cancer MDA-MB-231 cells, prostate cancer PC-3 cells, and bladder cancer T-24 cells was evaluated. This yttrium complex displayed more cytotoxic activity against the bladder cancer cells with an IC50 value of 307.7 μg/ml (223 μM). On the other hand, the activities of complex C1 against the MDA-MB-231 and PC-3 cells were less significant respectively with IC50 values of 1097 μg/ml (796 μM) and 921 μg/ml (669 μM).

Dosimetry of a Novel 111Indium-Labeled Anti-P-Cadherin Monoclonal Antibody (FF-21101) in Non-Human Primates

P-cadherin is associated with a wide range of tumor types, making it an attractive therapeutic target. FF-21101 is a human-mouse chimeric monoclonal antibody (mAb) directed against human P-cadherin, which has been radioconjugated with indium-111 (111In) utilizing a DOTA chelator. We investigated the biodistribution of FF-21101(111In) in cynomolgus macaques and extrapolated the results to estimate internal radiation doses of 111In- and yttrium-90 (90Y)-FF-21101 for targeted radioimmunotherapy in humans. Whole-body planar and SPECT imaging were performed at 0, 2, 24, 48, 72, 96, and 120 h post-injection, using a dual-head gamma camera. Volumes of interest of identifiable source organs of radioactivity were defined on aligned reference CT and serial SPECT images. Organs with the highest estimated dose values (mSv/MBq) for FF-21101(111In) were the lungs (0.840), spleen (0.816), liver (0.751), kidneys (0.629), and heart wall (0.451); and for FF-21101(90Y) dose values were: lungs (10.49), spleen (8.21), kidneys (5.92), liver (5.46), and heart wall (2.61). FF-21101(111In) exhibits favorable biodistribution in cynomolgus macaques and estimated human dosimetric characteristics. Data obtained in this study were used to support the filing of an investigational new drug application with the FDA for a Phase I clinical trial.

Optimization of Radiolabeling of a [90Y]Y-Anti-CD66-Antibody for Radioimmunotherapy before Allogeneic Hematopoietic Cell Transplantation

For patients with acute myeloid leukemia, myelodysplastic syndrome, or acute lymphoblastic leukemia, allogeneic hematopoietic cell transplantation (HCT) is a potentially curative treatment. In addition to standard conditioning regimens for HCT, high-dose radioimmunotherapy (RIT) offers the unique opportunity to selectively deliver a high dose of radiation to the bone marrow while limiting side effects. Modification of a CD66b-specific monoclonal antibody (mAb) with a DTPA-based chelating agent should improve the absorbed dose distribution during therapy. The stability and radioimmunoreactive fraction of the radiolabeled mAbs were determined. Before RIT, all patients underwent dosimetry to determine absorbed doses to bone marrow, kidneys, liver, and spleen. Scans were performed twenty-four hours after therapy for quality control. A radiochemical purity of >95% and acceptable radioimmunoreactivity was achieved. Absorbed organ doses for the liver and kidney were consequently improved compared to reported historical data. All patients tolerated RIT well with no treatment-related acute adverse events. Complete remission could be observed in 4/5 of the patients 3 months after RIT. Two patients developed delayed liver failure unrelated to the radioimmunotherapy. The improved conjugation and radiolabeling procedure resulted in excellent stability, radiochemical purity, and CD66-specific radioimmunoreactivity of ⁹⁰Y-labeled anti-CD66 mAb. RIT followed by conditioning and HCT was well tolerated. Based on these promising initial data, further prospective studies of [⁹⁰Y]Y-DTPA-Bn-CHX-A″-anti-CD66-mAb-assisted conditioning in HCT are warranted.

Mo(vi) complexes of amide-imine conjugates for tuning the selectivity of fluorescence recognition of Y(iii) vs. Pb(ii)

Two amide-imine conjugates, viz. 3-methyl-benzoic acid (4-diethylamino-2-hydroxy-benzylidene)-hydrazide (L1) and 3-methyl-benzoic acid (2-hydroxy-naphthalen-1-ylmethylene)-hydrazide (L2), have been prepared and used for a further synthesis of Mo(vi) complexes (M1 and M2, respectively). Single crystal X-ray diffraction analysis confirmed their structures. Interestingly, M1 selectively recognizes Y3+ and Pb2+ at two different wavelengths, whereas M2 selectively interacts with Y3+ with a significantly high binding constant, 1.3 × 105 M-1. The proposed sensing mechanism involves the displacement of Mo(vi) by Y3+/Pb2+ from respective Mo(vi) complexes. The TCSPC experiment also substantiates the "turn-on" fluorescence process. A logic gate has been constructed utilizing the fluorescence recognition of cations by M1. DFT studies corroborated the cation-probe interactions and allowed exploring the orbital energy parameters.

Perspectives on metals-based radioimmunotherapy (RIT): moving forward

Radioimmunotherapy (RIT) is FDA-approved for the clinical management of liquid malignancies, however, its use for solid malignancies remains a challenge. The putative benefit of RIT lies in selective targeting of antigens expressed on the tumor surface using monoclonal antibodies, to systemically deliver cytotoxic radionuclides. The past several decades yielded dramatic improvements in the quality, quantity, recent commercial availability of alpha-, beta- and Auger Electron-emitting therapeutic radiometals. Investigators have created new or improved existing bifunctional chelators. These bifunctional chelators bind radiometals and can be coupled to antigen-specific antibodies. In this review, we discuss approaches to develop radiometal-based RITs, including the selection of radiometals, chelators and antibody platforms (i.e. full-length, F(ab')2, Fab, minibodies, diabodies, scFv-Fc and nanobodies). We cite examples of the performance of RIT in the clinic, describe challenges to its implementation, and offer insights to address gaps toward translation.

PET and SPECT Imaging of the EGFR Family (RTK Class I) in Oncology

The human epidermal growth factor receptor family (EGFR-family, other designations: HER family, RTK Class I) is strongly linked to oncogenic transformation. Its members are frequently overexpressed in cancer and have become attractive targets for cancer therapy. To ensure effective patient care, potential responders to HER-targeted therapy need to be identified. Radionuclide molecular imaging can be a key asset for the detection of overexpression of EGFR-family members. It meets the need for repeatable whole-body assessment of the molecular disease profile, solving problems of heterogeneity and expression alterations over time. Tracer development is a multifactorial process. The optimal tracer design depends on the application and the particular challenges of the molecular target (target expression in tumors, endogenous expression in healthy tissue, accessibility). We have herein summarized the recent preclinical and clinical data on agents for Positron Emission Tomography (PET) and Single Photon Emission Tomography (SPECT) imaging of EGFR-family receptors in oncology. Antibody-based tracers are still extensively investigated. However, their dominance starts to be challenged by a number of tracers based on different classes of targeting proteins. Among these, engineered scaffold proteins (ESP) and single domain antibodies (sdAb) show highly encouraging results in clinical studies marking a noticeable trend towards the use of smaller sized agents for HER imaging.

The emerging role of radionuclide molecular imaging of HER2 expression in breast cancer

Targeting of human epidermal growth factor type 2 (HER2) using monoclonal antibodies, antibody-drug conjugates and tyrosine kinase inhibitors extends survival of patients with HER2-expressing metastatic breast cancer. High expression of HER2 is a predictive biomarker for such specific treatment. Accurate determination of HER2 expression level is necessary for stratification of patients to targeted therapy. Non-invasive in vivo radionuclide molecular imaging of HER2 has a potential of repetitive measurements, addressing issues of heterogeneous expression and conversion of HER2 status during disease progression or in response to therapy. Imaging probes based of several classes of targeting proteins are currently in preclinical and early clinical development. Both preclinical and clinical data suggest that the most promising are imaging agents based on small proteins, such as single domain antibodies or engineered scaffold proteins. These agents permit a very specific high-contrast imaging at the day of injection.

Imaging Diagnostic and Therapeutic Targets: Human Epidermal Growth Factor Receptor 2

Since the approval of trastuzumab, a humanized monoclonal antibody against the extracellular domain of human epidermal growth factor receptor 2 (HER2), 3 other HER2-targeting agents have gained regulatory approval: lapatinib, pertuzumab, and trastuzumab-emtansine. These agents have revolutionized the management of HER2-positive breast cancer, highlighting the concept that targeted therapies are successful when patients exhibit tumor-selective expression of a molecular target-in this case, HER2. However, response prediction and innate or acquired resistance remain serious concerns. Predictive biomarkers of a response-which could help in the selection of patients who might benefit from a selected targeted therapy-are currently lacking. Molecular imaging with anti-HER2 probes allows the noninvasive, whole-body assessment of HER2 tumor burden and has the potential to improve patient selection, optimize the dose and schedule, and rationalize assessment of the response to anti-HER2 therapies. Furthermore, unlike biopsy-based HER2 assessment, this approach can reveal inter- or intratumoral heterogeneity as well as variations in HER2 expression over time. This review summarizes the available literature and the current status of molecular imaging as a tool for the assessment of HER2 (target) expression or the prediction of an early treatment response in early and advanced HER2-positive breast cancer.

Targeted radionuclide therapy with A 177Lu-labeled anti-HER2 nanobody

RIT has become an attractive strategy in cancer treatment, but still faces important drawbacks due to poor tumor penetration and undesirable pharmacokinetics of the targeting vehicles. Smaller radiolabeled antibody fragments and peptides feature highly specific target accumulation, resulting in low accumulation in healthy tissue, except for the kidneys. Nanobodies are the smallest (MW < 15 kDa) functional antigen-binding fragments that are derived from heavy chain-only camelid antibodies.

Here, we show that the extend of kidney retention of nanobodies is predominantly dictated by the number of polar residues in the C-terminal amino acid tag. Three nanobodies were produced with different C-terminal amino-acid tag sequences (Myc-His-tagged, His-tagged, and untagged). Dynamic planar imaging of Wistar rats with ¹¹¹In-DTPA-nanobodies revealed that untagged nanobodies showed a 70 % drop in kidney accumulation compared to Myc-His-tagged nanobodies at 50 min p.i.. In addition, coinfusion of untagged nanobodies with the plasma expander Gelofusin led to a final reduction of 90 %. Similar findings were obtained with different ¹⁷⁷Lu-DTPA-2Rs15d nanobody constructs in HER2^pos tumor xenografted mice at 1 h p.i.. Kidney accumulation decreased 88 % when comparing Myc-His-tagged to untagged 2Rs15d nanobody, and 95 % with a coinfusion of Gelofusin, without affecting the tumor targeting capacity. Consequently, we identified a generic method to reduce kidney retention of radiolabeled nanobodies. Dosimetry calculations of Gelofusin-coinfused, untagged ¹⁷⁷Lu-DTPA-2Rs15d revealed a dose of 0.90 Gy/MBq that was delivered to both tumor and kidneys and extremely low doses to healthy tissues. In a comparative study, ¹⁷⁷Lu-DTPA-Trastuzumab supplied 6 times more radiation to the tumor than untagged ¹⁷⁷Lu-DTPA-2Rs15d, but concomitantly also a 155, 34, 80, 26 and 4180 fold higher radioactivity burden to lung, liver, spleen, bone and blood.

Most importantly, nanobody-based targeted radionuclide therapy in mice bearing small estiblashed HER2^pos tumors led to an almost complete blockade of tumor growth and a significant difference in event-free survival between the treated and the control groups (P < 0.0001). Based on histology analyses, no evidence of renal inflammation, apoptosis or necrosis was obtained.

In conclusion, these data highlight the importance of the amino acid composition of the nanobody's C-terminus, as it has a predominant effect on kidney retention. Moreover, we show successful nanobody-based targeted radionuclide therapy in a xenograft model and highlight the potential of radiolabeled nanobodies as a valuable adjuvant therapy candidate for treatment of minimal residual and metastatic disease.

166Ho and 90Y labeled 6D2 monoclonal antibody for targeted radiotherapy of melanoma: comparison with 188Re radiolabel

INTRODUCTION

An approach to radioimmunotherapy (RIT) of metastatic melanoma is the targeting of melanin pigment with monoclonal antibodies (mAbs) to melanin radiolabeled with therapeutic radionuclides. The proof of principle experiments were performed using a melanin-binding antibody 6D2 of IgM isotype radiolabeled with a β emitter (188)Re and demonstrated the inhibition of tumor growth. In this study we investigated the efficacy of 6D2 antibody radiolabeled with two other longer lived β emitters (90)Y and (166)Ho in treatment of experimental melanoma, with the objective to find a possible correlation between the efficacy and half-life of the radioisotopes which possess high energy β (E(max)>1.5 MeV) emission properties.

METHODS

6D2 was radiolabeled with longer lived β emitters (90)Y and (166)Ho in treatment of experimental melanoma in A2058 melanoma tumor-bearing nude mice. The immunoreactivity of the radiolabeled 6D2 mAb, its in vitro binding to the MNT1 human melanoma cells, the biodistribution and therapy in A2058 human melanoma bearing nude mice as well as dosimetry calculations were performed.

RESULTS

When labeled with the longer lived (90)Y radionuclide, the 6D2 mAb did not produce any therapeutic effect in tumor bearing mice while the reduction of the tumor growth by (166)Ho-6D2 was very similar to the previously reported therapy results for (188)Re-6D2. In addition, (166)Ho-labeled mAb produced the therapeutic effect on the tumor without any toxic effects while the administration of the (90)Y-labeled radioconjugate was toxic to mice with no appreciable anti-tumor effect.

CONCLUSIONS

(166)Ho-labeled mAb to melanin produced some therapeutic effect on the tumor without any toxic effects while the administration of the (90)Y-labeled radioconjugate was toxic to mice with no appreciable anti-tumor effect. We concluded that the serum half-life of the 6D2 carrier antibody matched well the physical half-life of (166)Ho to deliver the tumoricidal absorbed dose to the tumor. Further investigation of this radionuclide for RIT of melanoma is warranted.

Growth Impairment of Small-Cell Cancer by Targeting Pro-Vasopressin with MAG-1 Antibody

Previously we demonstrated that human small-cell lung cancer (SCLC) seems to universally express the vasopressin gene, and this leads to the presence of a cell surface marker representing the entire pro-hormone precursor. In this study, we show this marker can be targeted with MAG-1, a mouse monoclonal antibody against a C-terminal moiety on pro-vasopressin. In vitro targeting of cell lines derived from primary and recurrent disease demonstrates attachment of antibody to the cell surface followed by internalization. In vivo targeting with ⁹⁹Tc-labeled Fab fragments of MAG-1 shows selective attachment to xenografts. In vivo treatment of tumors from classical cell line, NCI H345, with either ~1.65 mCi (~1.65 mg)/kg body weight (BW) of ⁹⁰Yttrium-labeled MAG-1, or ~1.65 mg/kg BW native MAG-1, delivered every second day for 6 days produced similar reductions in the growth rate to ~50% (p < 0.03). When dosing with native MAG-1 was escalated to daily amounts of ~3.3 mg/kg BW over 16 days, tumor growth rates fell to ~33% of saline controls (p < 0.005). Examination of tumors treated with this higher dosing demonstrated the presence in several of extensive apoptosis. Normal tissues seemed to be unaffected. A larger dosage of MAG-1 (~6.6 mg/kg BW) given daily for 14 days was used to treat xenografts of the variant cell line NCI H82 representing recurrent disease. This treatment decreased the rate of increase in tumor size by half, and doubling time ~3-fold. Increases in cleaved PARP supported increased apoptosis with antibody treatment. We believe these data provide evidence that the growth rate of SCLC tumors can be extensively reduced by treatment with MAG-1 antibody, and that a humanized form of this antibody could, in future, be potentially used for targeting therapy onto recurrent SCLC in patients.

Discovery of internalizing antibodies to tumor antigens from phage libraries

Phage antibody technology can be used to generate human antibodies to essentially any antigen. Many therapeutic target antigens are cell surface receptors, which can be challenging targets for antibody generation. In addition, for many therapeutic applications, one needs antibodies that not only bind the cell surface receptor but also are internalized into the cell upon binding. This allows use of the antibody to deliver a range of payloads into the cell to achieve a therapeutic effect. In this chapter, we describe how human phage antibody libraries can be selected directly on tumor cell lines to generate antibodies that bind cell surface receptors and which upon binding are rapidly internalized into the cell. Specific protocols show how to (1) directly select cell binding and internalizing antibodies from human phage antibody libraries, (2) screen the phage antibodies in a high-throughput flow cytometry assay for binding to the tumor cell line used for selection, (3) identify the antigen bound by the phage antibody using immunoprecipitation and mass spectrometry, and (4) direct cell binding and internalizing selections to a specific tumor antigen by sequential selection on a tumor cell line followed by selection on yeast displaying the target tumor antigen on the yeast surface.

Native MAG-1 antibody almost destroys human breast cancer xenografts

A native form of mouse monoclonal IgG1 antibody called MAG-1, which recognizes an epitope on provasopressin, has been found to shrink and produce extensive necrosis of human breast tumor xenografts in nu/nu mice. We examined the ability of (90)Yttrium-labeled and native MAG-1 to affect the growth in nu/nu mice of cancer xenografts that were estrogen-responsive (from MCF-7 cells) and triple-negative (from MDA-MB231 cells). The growth rates of treated cells were compared to those receiving saline vehicle and those receiving (90)Yttrium-labeled and native forms of the ubiquitous antibody, MOPC21. Short-term treatments (4 doses over 6 days) not only with (90)Yttrium-MAG-1 but also native MAG-1 produced large reductions in size of rapidly growing tumors of both types, while both (90)Yttrium- MOPC21 and native MOPC21 had no effect. Native and (90)Yttrium-MAG-1 effects were similar, and arrested tumors recommenced growing soon after treatments stopped. Increasing native MAG-1 treatment to single dosing for 16 consecutive days shrank tumors of both types with no regrowth apparent over a 20-day post-treatment period of observation. Pathological examination of such tumors revealed they had undergone very extensive (>66%) necrosis.

Radiolabeled affibody-albumin bioconjugates for HER2-positive cancer targeting

Affibody molecules have received significant attention in the fields of molecular imaging and drug development. However, Affibody scaffolds display an extremely high renal uptake, especially when modified with chelators and then labeled with radiometals. This unfavorable property may impact their use as radiotherapeutic agents in general and as imaging probes for the detection of tumors adjacent to kidneys in particular. Herein, we present a simple and generalizable strategy for reducing the renal uptake of Affibody molecules while maintaining their tumor uptake. Human serum albumin (HSA) was consecutively modified by 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono-N-hydroxysuccinimide ester (DOTA-NHS ester) and the bifunctional cross-linker sulfosuccinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate (Sulfo-SMCC). The HER2 Affibody analogue, Ac-Cys-Z(HER2:342), was covalently conjugated with HSA, and the resulting bioconjugate DOTA-HSA-Z(HER2:342) was further radiolabeled with ⁶⁴Cu and ¹¹¹In and evaluated in vitro and in vivo. Radiolabeled DOTA-HSA-Z(HER2:342) conjugates displayed a significant and specific cell uptake into SKOV3 cell cultures. Positron emission tomography (PET) investigations using ⁶⁴Cu-DOTA-HSA-Z(HER2:342) were performed in SKOV3 tumor-bearing nude mice. High tumor uptake values (>14% ID/g at 24 and 48 h) and high liver accumulations but low kidney accumulations were observed. Biodistribution studies and single-photon emission computed tomography (SPECT) investigations using ¹¹¹In-DOTA-HSA-Z(HER2:342) validated these results. At 24 h post injection, the biodistribution data revealed high tumor (16.26% ID/g) and liver (14.11% ID/g) uptake but relatively low kidney uptake (6.06% ID/g). Blocking studies with coinjected, nonlabeled Ac-Cys-Z(HER2:342) confirmed the in vivo specificity of HER2. Radiolabeled DOTA-HSA-Z(HER2:342) Affibody conjugates are promising SPECT and PET-type probes for the imaging of HER2 positive cancer. More importantly, DOTA-HSA-Z(HER2:342) is suitable for labeling with therapeutic radionuclides (e.g., ⁹⁰Y or ¹⁷⁷Lu) for treatment studies. The approach of using HSA to optimize the pharmacokinetics and biodistribution profile of Affibodies may be extended to the design of many other targeting molecules.

Radioimmunotherapy of solid tumors: searching for the right target

Radioimmunotherapy of solid tumors remains a challenge despite the tremendous success of ⁹⁰Y ibritumomab tiuxetan (Zevalin) and ¹³¹I Tositumomab (Bexxar) in treating non-Hodgkin's lymphoma. For a variety of reasons, clinical trials of radiolabeled antibodies against solid tumors have not led to responses equivalent to those seen against lymphoma. In contrast, promising responses have been observed with unlabeled antibodies that target solid tumor receptors associated with cellular signaling pathways. These observations suggest that anti-tumor efficacy of the carrier antibody might be critical to achieving clinical responses. Here, we review and compare tumor antigens targeted by radiolabeled antibodies and unlabeled antibodies used in immunotherapy. The review shows that the trend for radiolabeled antibodies under pre-clinical development is to also target antigens associated with signaling pathways that are essential for the growth and survival of the tumor.

Antibody vectors for imaging

Noninvasive molecular imaging approaches include nuclear, optical, magnetic resonance imaging, computed tomography, ultrasound, and photoacoustic imaging, which require accumulation of a signal delivered by a probe at the target site. Monoclonal antibodies are high affinity molecules that can be used for specific, high signal delivery to cell surface molecules. However, their long circulation time in blood makes them unsuitable as imaging probes. Efforts to improve antibodies pharmacokinetics without compromising affinity and specificity have been made through protein engineering. Antibody variants that differ in antigen binding sites and size have been generated and evaluated as imaging probes to target tissues of interest. Fast clearing fragments, such as single-chain variable fragment (scFv; 25 kDa), with 1 antigen-binding site (monovalent) demonstrated low accumulation in tumors because of the low exposure time to the target. Using scFv as building block to produce larger, bivalent fragments, such as scFv dimers (diabodies, 50 kDa) and scFv-fusion proteins (80 kDa minibodies and 105 kDa scFv-Fc), resulted in higher tumor accumulation because of their longer residence time in blood. Imaging studies with these fragments after radiolabeling have demonstrated excellent, high-contrast images in gamma cameras and positron emission tomography scanners. Several studies have also investigated antibody fragments conjugated to fluorescence (near infrared dyes), bioluminescence (luciferases), and quantum dots for optical imaging and iron oxides nanoparticles for magnetic resonance imaging. However, these studies indicate that there are several factors that influence successful targeting and imaging. These include stability of the antibody fragment, the labeling chemistry (direct or indirect), whether critical residues are modified, the number of antigen expressed on the cell, and whether the target has a rapid recycling rate or internalizes upon binding. The preclinical data presented are compelling, and it is evident that antibody-based molecular imaging tracers will play an important future role in the diagnosis and management of cancer and other diseases.

Construction of non-covalent single-chain Fv dimers for hepatocellular carcinoma and their biological functions

AIM

To create new diabodies with improved binding activity to antigen of the variable light - variable heavy (VH-VL) oriented single-chain Fv dimers genes (scFv).

METHODS

The linker between VH and VL genes was shortened to 3-5 amino acid residues and cloned into the vector pCANTAB5E. The recombinant plasmids were transformed into TG1 cells and sequenced. The positive transformed cells were infected by M13K07 helper phage to form human recombinant phage antibodies. Expressed products were identified by SDS-PAGE, Western blotting, size exclusion gel chromatography (SEC), ELISA and immunohistochemistry.

RESULTS

Three scFv (scFv-3, scFv-4, scFv-5) were constructed successfully with binding ability to hepatocellular carcinoma 3.5-6 fold greater than their parental scFv. The single-chain Fv dimer (scFv-5, termed BDM3) with the best binding ability was successfully expressed in Yeast pichlia, as shown by. SDS-PAGE and Western blotting. SEC results suggested the molecular weight of the expressed products was about 61 kDa. Expressed products showed significantly stronger binding to hepatocellular carcinoma cells than scFv, still having 50% binding activity even after 16 h incubation as 37°C. The purified dimers were bound specifically to the tumor antigen of HCC.

CONCLUSION

we have generated scFv dimers by shortening a series of linkers to 3-5 amino acid residues in VH-linker-VL orientation, resulting in highly stable and affinity-improved dimeric molecules. These will become an attractive targeting moiety in immunotherapeutic and diagnostic applications for HCC.

68Ga-labelled recombinant antibody variants for immuno-PET imaging of solid tumours

PURPOSE

Recombinant antibodies isolated from human antibody libraries have excellent affinities and high target specificity. As full-length IgGs are cleared inadequately slowly from the circulation, the aim of this work was to figure out which kind of recombinant antibody fragment proves to be appropriate for imaging epithelial cell adhesion molecule (EpCAM)-expressing tumours with the short-living radioisotope (68)Ga.

METHODS

In order to combine the promising tumour targeting properties of antibodies with (68)Ga, four antibody variants with the same specificity and origin only differing in molecular weight were constructed for comparison. Therefore, the binding domains of a single-chain fragment variable (scFv) isolated from a human naïve antibody library were modified genetically to construct the respective full-length IgG, the tria- and diabody variants. These molecules were conjugated with the bifunctional chelating agent N,N'-bis[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid (HBED-CC) to enable (68)Ga labelling at ambient temperature and compared in biodistribution and immuno-PET imaging experiments.

RESULTS

The antibody variants with identical specificity proved to have the correct molecular weight, high binding affinity and specificity to their antigen, EpCAM. Radiometal complexation was efficiently performed at room temperature leading to (68)Ga-labelled antibodies with unchanged binding properties compared to the original antibody variants. The best targeting properties were obtained with the scFv and especially with the diabody. The triabody showed higher absolute tumour uptake but only moderate clearance from circulation.

CONCLUSION

The antibody variants differed considerably in normal organ uptake, clearance from circulation and tumour accumulation. The data demonstrate the feasibility of imaging solid tumours with the (68)Ga-labelled diabody format. This type of recombinant protein might be a promising carrier even for the short-lived radiometal (68)Ga to support e.g. the management of immunotherapy which may provide important information regarding receptor expression of solid tumours.

New coupling strategy for radionuclide labeling of synthetic polymers

We have developed a radiolabeling strategy for synthetic polymers based on the formation of azo dye usable for both covalent and chelating labeling modalities under mild conditions. Poly[N-(2-hydroxypropyl)methacrylamide] and poly(N-isopropyl acrylamide) were used as model polymers. N-methacryloyl tyrosinamide was introduced into the polymers and the phenolic moiety was then reacted with diazotized chelator precursors. The conjugates were radiolabeled with both the covalently bound (iodine-125) and chelated (indium-111) radionuclides in high yields and sufficient in vitro stability of the labels was proven.

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.

Synthesis and chemoselective intramolecular crosslinking of a HER2-binding affibody

The human epidermal growth factor receptor HER2 has emerged as an important target for molecular imaging of breast cancer. This article presents the design and synthesis of a HER2-targeting affibody molecule with improved stability and tumor targeting capacity, and with potential use as an imaging agent. The 58 aa three-helix bundle protein was assembled using solid-phase peptide synthesis, and a chemoselective ligation strategy was used to establish an intramolecular thioether bond between the side chain thiol group of a cysteine residue, positioned in the loop between helices I and II, and a chloroacetyl group on the side chain amino group of the C-terminal lysine residue. The tethered protein offered an increased thermal stability, with a melting temperature of 64 degrees C, compared to 54 degrees C for the linear control. The ligation did not have a major influence on the HER2 binding affinity, which was 320 and 380 pM for the crosslinked and linear molecules, respectively. Biodistribution studies were performed both in normal and tumor-bearing mice to evaluate the impact of the crosslinking on the in vivo behavior and on the tumor targeting performance. The distribution pattern was characterized by a low uptake in all organs except kidney, and rapid clearance from blood and normal tissue. Crosslinking of the protein resulted in a significantly increased tumor accumulation, rendering the tethered HER2-binding affibody molecule a valuable lead in the development of superior HER2 imaging agents.

Design, synthesis and biological evaluation of a multifunctional HER2-specific Affibody molecule for molecular imaging

PURPOSE

The purpose of this study was to design and evaluate a novel platform for labelling of Affibody molecules, enabling both recombinant and synthetic production and site-specific labelling with (99m)Tc or trivalent radiometals.

METHODS

The HER2-specific Affibody molecule PEP05352 was made by peptide synthesis. The chelator sequence SECG (serine-glutamic acid-cysteine-glycine) was anchored on the C-terminal to allow (99m)Tc labelling. The cysteine can alternatively serve as a conjugation site of the chelator DOTA for indium labelling. The resulting (99m)Tc- and (111)In-labelled Affibody molecules were evaluated both in vitro and in vivo.

RESULTS

Both conjugates retained their capacity to bind to HER2 receptors in vitro and in vivo. The tumour to blood ratio in LS174T xenografts was 30 at 4 h post-injection for both conjugates. Biodistribution data showed that the (99m)Tc-labelled Affibody molecule had a fourfold lower kidney accumulation compared with the (111)In-labelled Affibody molecule while the accumulation in other organs was similar. Gamma camera imaging of the conjugates could clearly visualise the tumours 4 h after injection.

CONCLUSION

Incorporation of the C-terminal SECG sequence in Affibody molecules provides a general multifunctional platform for site-specific labelling with different nuclides (technetium, indium, gallium, cobalt or yttrium) and for a flexible production (chemical synthesis or recombinant).

Therapeutic antibodies: successes, limitations and hopes for the future

With more than 20 molecules in clinical use, monoclonal antibodies have finally come of age as therapeutics, generating a market value of $11 billion in 2004, expected to reach $26 billion by 2010. While delivering interesting results in the treatment of several major diseases including autoimmune, cardiovascular and infectious diseases, cancer and inflammation, clinical trials and research are generating a wealth of useful information, for instance about associations of clinical responses with Fc receptor polymorphisms and the infiltration and recruitment of effector cells into targeted tissues. Some functional limitations of therapeutic antibodies have come to light such as inadequate pharmacokinetics and tissue accessibility as well as impaired interactions with the immune system, and these deficiencies point to areas where additional research is needed. This review aims at giving an overview of the current state of the art and describes the most promising avenues that are being followed to create the next generation of antibody-based therapeutic agents.

Engineering therapeutic monoclonal antibodies

During last two decades, the chimerization and humanization of monoclonal antibodies (mAbs) have led to the approval of several for the treatment of cancer, autoimmune diseases, and transplant rejection. Additional approaches have been used to further improve their in vivo activity. These include combining them with other modalities such as chemotherapy and redesigning them for improved pharmacokinetics, effector function, and signaling activity. The latter has taken advantage of new insights emerging from an increased understanding of the cellular and molecular mechanisms that are involved in the interaction of immunoglobulin G with Fc receptors and complement as well as the negative signaling resulting from the hypercrosslinking of their target antigens. Hence, mAbs have been redesigned to include mutations in their Fc portions, thereby endowing them with enhanced or decreased effector functions and more desirable pharmacokinetic properties. Their valency has been increased to decrease their dissociation rate from cells and enhance their ability to induce apoptosis and cell cycle arrest. In this review we discuss these redesigned mAbs and current data concerning their evaluation both in vitro and in vivo.

Influence of valency and labelling chemistry on in vivo targeting using radioiodinated HER2-binding Affibody molecules

PURPOSE

HER2 is a transmembrane tyrosine kinase, which is overexpressed in a number of carcinomas. The Affibody molecule Z(HER2:342) is a small (7 kDa) affinity protein binding to HER2 with an affinity of 22 pM. The goal of this study was to evaluate the use of ((4-hydroxyphenyl)ethyl)maleimide (HPEM) for radioiodination of Z(HER2:342) and to compare the targeting properties of monomeric and dimeric forms of Z(HER2:342).

METHODS

The biodistribution of different radioiodinated derivatives of Z(HER2:342) was studied in BALB/C nu/nu mice bearing HER2-expressing SKOV-3 xenografts. Biodistributions of (125)I-PIB-Z(HER2:342) and site-specifically labelled (125)I-HPEM-Z(HER2:342)-C were compared. Biodistributions of monomeric (131)I-HPEM-Z(HER2:342)-C and dimeric (125)I-HPEM-(Z(HER2:342))(2)-C were evaluated using a paired-label method.

RESULTS

(125)I-HPEM-Z(HER2:342)-C had the same level of tumour accumulation as (125)I-PIB-Z(HER2:342), but fourfold lower renal retention of radioactivity. The monomeric form of Z(HER2:342) provided better tumour targeting than the dimeric form.

CONCLUSION

Favourable biodistribution of (131)I-HPEM-Z(HER2:342)-C makes it a promising candidate for radionuclide therapy.

Ligand-based vascular targeting of disease

This review illustrates the basic principles of ligand-based vascular targeting and presents some of the most advanced results obtained in this field, not only in terms of biopharmaceuticals, which are currently being investigated in clinical and preclinical studies, but also in terms of enabling technologies that facilitate target and ligand discovery. Whereas most of the vascular targeting research activities have so far concentrated on tumoral angiogenesis, the development of non-oncological applications has recently gained momentum and is likely to become an important area of modern pharmaceutical research.

Evaluation of a maleimido derivative of CHX-A'' DTPA for site-specific labeling of affibody molecules

Affibody molecules are a new class of small targeting proteins based on a common three-helix bundle structure. Affibody molecules binding a desired target may be selected using phage-display technology. An Affibody molecule Z HER2:342 binding with subnanomolar affinity to the tumor antigen HER2 has recently been developed for radionuclide imaging in vivo. Introduction of a single cysteine into the cysteine-free Affibody scaffold provides a unique thiol group for site-specific labeling of recombinant Affibody molecules. The recently developed maleimido-CHX-A'' DTPA was site-specifically conjugated at the C-terminal cysteine of Z HER2:2395-C, a variant of Z HER2:342, providing a homogeneous conjugate with a dissociation constant of 56 pM. The yield of labeling with (111)In was >99% after 10 min at room temperature. In vitro cell tests demonstrated specific binding of (111)In-CHX-A'' DTPA-Z 2395-C to HER2-expressing cell-line SKOV-3 and good cellular retention of radioactivity. In normal mice, the conjugate demonstrated rapid clearance from all nonspecific organs except kidney. In mice bearing SKOV-3 xenografts, the tumor uptake of (111)In-CHX-A'' DTPA-Z 2395-C was 17.3 +/- 4.8% IA/g and the tumor-to-blood ratio 86 +/- 46 (4 h postinjection). HER2-expressing xenografts were clearly visualized 1 h postinjection. In conclusion, coupling of maleimido-CHX-A'' DTPA to cysteine-containing Affibody molecules provides a well-defined uniform conjugate, which can be rapidly labeled at room temperature and provides high-contrast imaging of molecular targets in vivo.

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.

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.

Comparative evaluation of capsular polysaccharide-specific IgM and IgG antibodies and F(ab')2 and Fab fragments as delivery vehicles for radioimmunotherapy of fungal infection

PURPOSE

The applicability of radioimmunotherapy with organism-specific monoclonal antibodies to treatment of infectious disease in experimental models has been recently shown for fungal, bacterial, and viral infections. To identify the best delivery vehicle for radioimmunotherapy of human pathogenic fungus Cryptococcus neoformans (CN), we have done comparative evaluation of capsular polysaccharide-specific antibodies with IgG1 and IgM isotypes and F(ab')2 and Fab fragments.

EXPERIMENTAL DESIGN

18B7 IgG1 and 13F1 IgM and their isotype-matching controls were radiolabeled with 188Re, and their binding to 24067 and H99 CN strains was evaluated by doing Scatchard and kinetics analyses. The doses delivered during in vitro radioimmunotherapy were estimated using a cellular dosimetry algorithm. The biodistribution of 188Re-labeled 18B7 and 13F1 and of 111In-labeled 18B7 and its F(ab')2 and Fab fragments was done in A/JCr mice systemically infected with 24067 CN strain.

RESULTS

18B7 IgG1 showed superior to 13F1 IgM binding to 24067 CN (Ka=1.7x10(9) mol/L(-1) and 5.4x10(7) mol/L(-1), respectively). Substantial killing of 24067 and H99 CN cells was achieved with 1 microCi 188Re-18B7 (55 cGy dose), whereas no killing was observed for 1 microCi 188Re-13F1 (2 cGy dose). In vivo 188Re-18B7 localized specifically in the lungs of CN-infected mice, whereas uptake of 188Re-13F1 was nonspecific. 111In-F(ab')2 fragments showed higher uptake in the lungs and lower in the liver at the 48-h time point in comparison with intact 111In-18B7.

CONCLUSIONS

Comparative evaluation of IgG and IgM and of F(ab')2 and Fab fragments as potential delivery vehicles for radioimmunotherapy of cryptococcal infection strongly suggests that affinity for the target antigen is an important prerequisite for successful targeting of infection in vivo and that in vitro affinity measurements may predict the in vivo efficacy of candidate monoclonal antibodies.

Recombinant full-length human IgG1s targeting hormone-refractory prostate cancer

We have used a naive human single-chain fragment variable (scFv) library as a source of random shape repertoire to directly probe the altered surface chemistry of tumor cells. We reported previously the identification of more than 90 internalizing phage monoclonal antibodies targeting prostate cancer cells, including those that are hormone refractory. In this report, we describe the conversion of a panel of those scFvs into full-length human immunoglobulins (IgGs) and show that tumor specificity is retained. We have further shown that antibodies isolated from a naive phage display library can nevertheless be of high affinity towards target tumor cells. In addition, full-length IgGs retain the functionality of parental scFvs including the ability to rapidly enter target cells through receptor-mediated endocytosis and thereby to mediate efficient and specific intracellular payload delivery to tumor cells. We have used recombinant IgGs to immunoprecipitate target antigens and analyzed their molecular composition by mass spectrometry. We have identified one target antigen as activated leukocyte cell adhesion molecule (ALCAM)/MEMD/CD166 and have further studied tissue specificity of this internalizing ALCAM epitope by immunohistochemistry. Our study shows that cell type-specific internalizing human antibody can be readily identified from a naive phage antibody display library, characterized with regards to sequence, affinity, tissue specificity, and antigen identity, and modified genetically and chemically to generate various forms of targeted therapeutics.

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.

Radionuclide therapy of HER2-positive microxenografts using a 177Lu-labeled HER2-specific Affibody molecule

A radiolabeled anti-HER2 Affibody molecule (Z(HER2:342)) targets HER2-expressing xenografts with high selectivity and gives good imaging contrast. However, the small size (approximately 7 kDa) results in rapid glomerular filtration and high renal accumulation of radiometals, thus excluding targeted therapy. Here, we report that reversible binding to albumin efficiently reduces the renal excretion and uptake, enabling radiometal-based nuclide therapy. The dimeric Affibody molecule (Z(HER2:342))(2) was fused with an albumin-binding domain (ABD) conjugated with the isothiocyanate derivative of CHX-A''-DTPA and labeled with the low-energy beta-emitter (177)Lu. The obtained conjugate [CHX-A''-DTPA-ABD-(Z(HER2:342))(2)] had a dissociation constant of 18 pmol/L to HER2 and 8.2 and 31 nmol/L for human and murine albumin, respectively. The radiolabeled conjugate displayed specific binding to HER2-expressing cells and good cellular retention in vitro. In vivo, fusion with ABD enabled a 25-fold reduction of renal uptake in comparison with the nonfused dimer molecule (Z(HER2:342))(2). Furthermore, the biodistribution showed high and specific uptake of the conjugate in HER2-expressing tumors. Treatment of SKOV-3 microxenografts (high HER2 expression) with 17 or 22 MBq (177)Lu-CHX-A''-DTPA-ABD-(Z(HER2:342))(2) completely prevented formation of tumors, in contrast to mice given PBS or 22 MBq of a radiolabeled non-HER2-binding Affibody molecule. In LS174T xenografts (low HER2 expression), this treatment resulted in a small but significant increase of the survival time. Thus, fusion with ABD improved the in vivo biodistribution, and the results highlight (177)Lu-CHX-A''-DTPA-ABD-(Z(HER2:342))(2) as a candidate for treatment of disseminated tumors with a high level of HER2 expression.

Synthetic affibody molecules: a novel class of affinity ligands for molecular imaging of HER2-expressing malignant tumors

The Affibody molecule Z(HER2:342-pep2), site-specifically and homogeneously conjugated with a 1,4,7,10-tetra-azacylododecane-N,N',N'',N'''-tetraacetic acid (DOTA) chelator, was produced in a single chemical process by peptide synthesis. DOTA-Z(HER2:342-pep2) folds spontaneously and binds HER2 with 65 pmol/L affinity. Efficient radiolabeling with >95% incorporation of (111)In was achieved within 30 min at low (room temperature) and high temperatures (up to 90 degrees C). Tumor uptake of (111)In-DOTA-Z(HER2:342-pep2) was specific for HER2-positive xenografts. A high tumor uptake of 23% injected activity per gram tissue, a tumor-to-blood ratio of >7.5, and high-contrast gamma camera images were obtained already 1 h after injection. Pretreatment with Herceptin did not interfere with tumor targeting, whereas degradation of HER2 using the heat shock protein 90 inhibitor 17-allylamino-geldanamycin before administration of (111)In-DOTA-Z(HER2:342-pep2) obliterated the tumor image. The present results show that radiolabeled synthetic DOTA-Z(HER2:342-pep2) has the potential to become a clinically useful radiopharmaceutical for in vivo molecular imaging of HER2-expressing carcinomas.

Imaging of HER2-expressing tumours using a synthetic Affibody molecule containing the 99mTc-chelating mercaptoacetyl-glycyl-glycyl-glycyl (MAG3) sequence

PURPOSE

Expression of human epidermal growth factor receptor type 2 (HER2) in malignant tumours possesses well-documented prognostic and predictive value. Non-invasive imaging of expression can provide valuable diagnostic information, thereby influencing patient management. Previously, we reported a phage display selection of a small (about 7 kDa) protein, the Affibody molecule Z(HER2:342), which binds HER2 with subnanomolar affinity, and demonstrated the feasibility of targeting of HER2-expressing xenografts using radioiodinated Z(HER2:342). The goal of this study was to develop a method for (99m)Tc labelling of Z(HER2:342) using the MAG3 chelator, which was incorporated into Z(HER2:342) using peptide synthesis, and evaluate the targeting properties of the labelled conjugate.

METHODS

MAG3-Z(HER2:342) was assembled using Fmoc/tBu solid phase peptide synthesis. Biochemical characterisation of the agent was performed using RP-HPLC, ESI-MS, biosensor studies and circular dichroism. A procedure for (99m)Tc labelling in the presence of sodium/potassium tartrate was established. Tumour targeting was evaluated by biodistribution study and gamma camera imaging in xenograft-bearing mice. Biodistribution of (99m)Tc-MAG3-Z(HER2:342) and (125)I-para-iodobenzoate -Z(HER2:342) was compared 6 h p.i.

RESULTS

Synthetic MAG3-Z(HER2:342) possessed an affinity of 0.2 nM for HER2 receptors. The peptide was labelled with (99m)Tc with an efficiency of about 75-80%. Labelled (99m)Tc-MAG3-Z(HER2:342) retained capacity to bind specifically HER2-expressing SKOV-3 cells in vitro. (99m)Tc-MAG3-Z(HER2:342) showed specific tumour targeting with a contrast similar to a radioiodinated analogue in mice bearing LS174T xenografts. Gamma camera imaging demonstrated clear and specific visualisation of HER2 expression.

CONCLUSION

Incorporation of a mercaptoacetyl-containing chelating sequence during chemical synthesis enabled site-specific (99m)Tc labelling of the Z(HER2:342) Affibody molecule with preserved targeting capacity.

In vitro display technologies reveal novel biopharmaceutics

Display technologies are fundamental to the isolation of specific high-affinity binding proteins for diagnostic and therapeutic applications in cancer, neurodegenerative, and infectious diseases as well as autoimmune and inflammatory disorders. Applications extend into the broad field of antibody (Ab) engineering, synthetic enzymes, proteomics, and cell-free protein synthesis. Recently, in vitro display technologies have come to prominence due to the isolation of high-affinity human antibodies by phage display, the development of novel scaffolds for ribosome display, and the discovery of novel protein-protein interactions. In vitro display represents an emerging and innovative technology for the rapid isolation and evolution of high-affinity peptides and proteins. So far, only one clinical drug candidate produced by in vitro display technology has been approved by the FDA for use in humans, but several are in clinical or preclinical testing. This review highlights recent advances in various engineered biopharmaceutical products isolated by in vitro display with a focus on the commercial developments.

Antibodies for angiogenesis inhibition, vascular targeting and endothelial cell transcytosis

The endothelium is increasingly recognized as a target for biomedical intervention, not only for its accessibility to molecular agents coming from the blood-stream, but also for the active role played by endothelial cell proliferation to support diseases such as cancer, blinding ocular disorders and chronic inflammatory conditions. The notion that solid tumors cannot grow beyond a size of few millimeters without inducing the proliferation of new blood vessels has stimulated the search for mediators of angiogenesis and for inhibitors of this process, culminating in the approval of a humanized monoclonal antibody to VEGF-A for oncology applications. In parallel, researchers have begun to consider imaging and therapeutic strategies based on the selective delivery of bioactive agents to the new blood vessels, mediated by monoclonal antibody derivatives. Recently, the field of vascular targeting research has been extended to the investigation of molecular agents that may mediate endothelial cell transcytosis, in the hope to overcome this body barrier for drug delivery. This article reviews some of the most significant advances in these areas, and outlines future challenges and opportunities.

Therapeutic advantage of pretargeted radioimmunotherapy using a recombinant bispecific antibody in a human colon cancer xenograft

PURPOSE

To assess if pretargeting, using a combination of a recombinant bispecific antibody (bsMAb) that binds divalently to carcinoembryonic antigen (CEA) and monovalently to the hapten histamine-succinyl-glycine and a (90)Y-peptide, improves therapeutic efficacy in a human colon cancer-nude mouse xenograft compared with control animals given (90)Y-humanized anti-CEA immunoglobulin G (IgG).

EXPERIMENTAL DESIGN

Clearance and biodistribution were monitored by whole-body readings and necropsy. Animals were monitored for 34 weeks with a determination of residual disease and renal pathology in survivors. Hematologic toxicity was assessed separately in non-tumor-bearing NIH Swiss mice.

RESULTS

Hematologic toxicity was severe at doses of 100 to 200 microCi of (90)Y-IgG, yet mild in the pretargeted animals given 500 or 700 microCi of the (90)Y-peptide. Evidence of end-stage renal disease was found at 900 microCi of the pretargeted (90)Y-peptide whereas animals given 700 microCi showed only mild renal pathology, similar to that seen in control animals given (90)Y-IgG. Biodistribution data indicated that the average amount of tumor radioactivity by a 700-microCi dose of the pretargeted peptide over a 96-hour period was increased 2.5-fold (48 microCi/g) compared with 150 microCi of (90)Y-IgG (18.9 microCi/g). At these doses, survival (i.e., time to progression to 2.5 cm(3)) was significantly improved (P < 0.04) compared with (90)Y-IgG, with ablation of about one third of the tumors, whereas viable tumor was present in all of the (90)Y-IgG-treated animals.

CONCLUSION

Pretargeting increases the amount of radioactivity delivered to colorectal tumors sufficiently to improve the therapeutic index and responses as compared with conventional radioimmunotherapy.

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.