A robust method for the preparation and purification of antibody/streptavidin conjugates

Construction and characterization of a truncated tissue factor‑coagulation‑based composite system for selective thrombosis in tumor blood vessels

The selective induction of tumor vascular thrombosis using truncated tissue factor (tTF) delivered via a target ligand is a promising novel antitumor strategy. In the present study, an anti-neuropilin-1 (NRP-1) monoclonal antibody (mAb)-streptavidin (SA):tTF-biotin (B) composite system was established. In this system, anti-NRP-1-mAb located tTF to the tumor vascular endothelial cell surface and induced vascular embolization. Due to their high binding affinity, SA and B were used to enhance thrombogenic activity. mAb was conjugated with SA using a coupling method with water-soluble 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysulfosuccinimide. Biotinylated tTF (tTF-B) was prepared using a B-labeling kit subsequent to the generation and purification of fusion protein tTF. Confocal microscopy and flow cytometry indicated that the anti-NRP-1-mAb-SA conjugate retained mAb targeting activity. The preservation of B-conjugate binding capacity was confirmed using a competitive ELISA, and factor X-activation analysis revealed that tTF-B retained the procoagulant activity exhibited by tTF. Live imaging was performed to assess mAb-SA distribution and tumor-targeting capability, and this yielded promising results. The results of in vivo studies in mice with subcutaneous xenografts demonstrated that this composite system significantly induced tumor vascular thrombosis and inhibited tumor growth, whereas these histological changes were not observed in normal organs.

Amine Landscaping to Maximize Protein-Dye Fluorescence and Ultrastable Protein-Ligand Interaction

Chemical modification of proteins provides great opportunities to control and visualize living systems. The most common way to modify proteins is reaction of their abundant amines with N-hydroxysuccinimide (NHS) esters. Here we explore the impact of amine number and positioning on protein-conjugate behavior using streptavidin-biotin, a central research tool. Dye-NHS modification of streptavidin severely damaged ligand binding, necessitating development of a new streptavidin-retaining ultrastable binding after labeling. Exploring the ideal level of dye modification, we engineered a panel bearing 1–6 amines per subunit: “amine landscaping.” Surprisingly, brightness increased as amine number decreased, revealing extensive quenching following conventional labeling. We ultimately selected Flavidin (fluorophore-friendly streptavidin), combining ultrastable ligand binding with increased brightness after conjugation. Flavidin enhanced fluorescent imaging, allowing more sensitive and specific cell labeling in tissues. Flavidin should have wide application in molecular detection, providing a general insight into how to optimize simultaneously the behavior of the biomolecule and the chemical probe.

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Labeling of streptavidin with small-molecule dyes impairs ligand binding

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K121R mutation rescues ligand stability after dye labeling

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Landscaping of protein amines optimizes brightness

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Fluorophore-friendly streptavidin improves imaging specificity and sensitivity

Antibody targeting facilitates effective intratumoral siRNA nanoparticle delivery to HER2-overexpressing cancer cells

The therapeutic potential of RNA interference (RNAi) has been limited by inefficient delivery of short interfering RNA (siRNA). Tumor-specific recognition can be effectively achieved by antibodies directed against highly expressed cancer cell surface receptors. We investigated the utility of linking an internalizing streptavidin-conjugated HER2 antibody to an endosome-disruptive biotinylated polymeric nanocarrier to improve the functional cytoplasmic delivery of siRNA in breast and ovarian cancer cells in vitro and in an intraperitoneal ovarian cancer xenograft model in vivo, yielding an 80% reduction of target mRNA and protein levels with sustained repression for at least 96 hours. RNAi-mediated site specific cleavage of target mRNA was demonstrated using the 5′ RLM-RACE (RNA ligase mediated-rapid amplification of cDNA ends) assay. Mice bearing intraperitoneal human ovarian tumor xenografts demonstrated increased tumor accumulation of Cy5.5 fluorescently labeled siRNA and 70% target gene suppression after treatment with HER2 antibody-directed siRNA nanocarriers. Detection of the expected mRNA cleavage product by 5′ RLM-RACE assay confirmed that suppression occurs via the expected RNAi pathway. Delivery of siRNA via antibody-directed endosomolytic nanoparticles may be a promising strategy for cancer therapy.

Antibody-drug conjugate characterization by chromatographic and electrophoretic techniques

Due to the inherent structure complexity and component heterogeneity of antibody drug conjugates (ADCs), separation technologies play a critical role in their characterization. In this review, we focus on chromatographic and electrophoretic approaches used to characterize ADCs with respect to drug-to-antibody ratio, drug distribution and conjugation sites, free small molecule drugs, charge variants, aggregates and fragments, etc. Chromatographic techniques including reversed-phase, ion exchange, size exclusion, hydrophobic interaction, two-dimensional liquid chromatography, and gas chromatography as well as capillary electrophoretic techniques including capillary electrophoresis sodium dodecyl sulfate, capillary zone electrophoresis and capillary isoelectric focusing are reviewed for their applications in the characterization of ADCs.

Synthesis and Evaluation of Astatinated N-[2-(Maleimido)ethyl]-3-(trimethylstannyl)benzamide Immunoconjugates

Effective treatment of metastasis is a great challenge in the treatment of different types of cancers. Targeted alpha therapy utilizes the short tissue range (50-100 μm) of α particles, making the method suitable for treatment of disseminated occult cancers in the form of microtumors or even single cancer cells. A promising radioactive nuclide for this type of therapy is astatine-211. Astatine-211 attached to tumor-specific antibodies as carrier molecules is a system currently under investigation for use in targeted alpha therapy. In the common radiolabeling procedure, astatine is coupled to the antibody arbitrarily on lysine residues. By instead coupling astatine to disulfide bridges in the antibody structure, the immunoreactivity of the antibody conjugates could possibly be increased. Here, the disulfide-based conjugation was performed using a new coupling reagent, maleimidoethyl 3-(trimethylstannyl)benzamide (MSB), and evaluated for chemical stability in vitro. The immunoconjugates were subsequently astatinated, resulting in both high radiochemical yield and high specific activity. The MSB-conjugate was shown to be stable with a long shelf life prior to the astatination. In a comparison of the in vivo distribution of the new immunoconjugate with other tin-based immunoconjugates in tumor-bearing mice, the MSB conjugation method was found to be a viable option for successful astatine labeling of different monoclonal antibodies.

Intracellular delivery system for antibody-Peptide drug conjugates

Antibodies armed with biologic drugs could greatly expand the therapeutic potential of antibody-drug conjugates for cancer therapy, broadening their application to disease targets currently limited by intracellular delivery barriers. Additional selectivity and new therapeutic approaches could be realized with intracellular protein drugs that more specifically target dysregulated pathways in hematologic cancers and other malignancies. A multifunctional polymeric delivery system for enhanced cytosolic delivery of protein drugs has been developed that incorporates endosomal-releasing activity, antibody targeting, and a biocompatible long-chain ethylene glycol component for optimized safety, pharmacokinetics, and tumor biodistribution. The pH-responsive polymeric micelle carrier, with an internalizing anti-CD22 monoclonal targeting antibody, effectively delivered a proapoptotic Bcl-2 interacting mediator (BIM) peptide drug that suppressed tumor growth for the duration of treatment and prolonged survival in a xenograft mouse model of human B-cell lymphoma. Antitumor drug activity was correlated with a mechanistic induction of the Bcl-2 pathway biomarker cleaved caspase-3 and a marked decrease in the Ki-67 proliferation biomarker. Broadening the intracellular target space by more effective delivery of protein/peptide drugs could expand the repertoire of antibody-drug conjugates to currently undruggable disease-specific targets and permit tailored drug strategies to stratified subpopulations and personalized medicines.

Protein–polymer therapeutics: a macromolecular perspective

The development of protein–polymer hybrids emerged several decades ago with the vision that their synergistic combination will offer macromolecular hybrids with manifold features to succeed as the next generation therapeutics.

Intracellular delivery and trafficking dynamics of a lymphoma-targeting antibody-polymer conjugate

Ratiometric fluorescence and cellular fractionation studies were employed to characterize the intracellular trafficking dynamics of antibody-poly(propylacrylic acid) (PPAA) conjugates in CD22+ RAMOS-AW cells. The HD39 monoclonal antibody (mAb) directs CD22-dependent, receptor-mediated uptake in human B-cell lymphoma cells, where it is rapidly trafficked to the lysosomal compartment. To characterize the intracellular-release dynamics of the polymer-mAb conjugates, HD39-streptavidin (HD39/SA) was dual-labeled with pH-insensitive Alexa Fluor 488 and pH-sensitive pHrodo fluorophores. The subcellular pH distribution of the HD39/SA-polymer conjugates was quantified as a function of time by live-cell fluorescence microscopy, and the average intracellular pH value experienced by the conjugates was also characterized as a function of time by flow cytometry. PPAA was shown to alter the intracellular trafficking kinetics strongly relative to HD39/SA alone or HD39/SA conjugates with a control polymer, poly(methacryclic acid) (PMAA). Subcellular trafficking studies revealed that after 6 h, only 11% of the HD39/SA-PPAA conjugates had been trafficked to acidic lysosomal compartments with values at or below pH 5.6. In contrast, the average intracellular pH of HD39/SA alone dropped from 6.7 ± 0.2 at 1 h to 5.6 ± 0.5 after 3 h and 4.7 ± 0.6 after 6 h. Conjugation of the control polymer PMAA to HD39/SA showed an average pH drop similar to that of HD39/SA. Subcellular fractionation studies with tritium-labeled HD39/SA demonstrated that after 6 h, 89% of HD39/SA was associated with endosomes (Rab5+) and lysosomes (Lamp2+), while 45% of HD39/SA-PPAA was translocated to the cytosol (lactate dehydrogenase+). These results demonstrate the endosomal-releasing properties of PPAA with antibody-polymer conjugates and detail their intracellular trafficking dynamics and subcellular compartmental distributions over time.

Protein heteroconjugation by the peroxidase-catalyzed tyrosine coupling reaction

Combining different proteins can integrate the functions of each protein to produce novel protein conjugates with wider ranges of applications. We have previously introduced a peptide containing tyrosine residues (Y-tag) at the C-terminus of Escherichia coli alkaline phosphatase (BAP). The tyrosine residues in the Y-tag were efficiently recognized by horseradish peroxidase (HRP) and were site-specifically cross-linked with each other to yield BAP homoconjugates. In this study, the HRP-catalyzed tyrosine coupling reaction was used for protein heteroconjugation. Streptavidin (SA) was selected as the conjugation partner for BAP. The Y-tag (GGGGY) was genetically introduced to the C-terminus of SA. Prior to heteroconjugation, the reactivity of the Y-tagged SA was examined. The Y-tagged SA cross-linked to form an SA homoconjugate upon HRP treatment, whereas wild-type SA remained essentially intact. In the heteroconjugation reaction of BAP and SA, the Y-tagged BAP and SA were efficiently cross-linked with each other upon HRP treatment. The functions of the BAP-SA conjugates were evaluated by measuring the BAP enzymatic activity on a biotin-coated plate. The BAP-SA conjugate tethered to the plate showed BAP enzymatic activity, indicating that both BAP and SA retained their functions following heteroconjugation. The BAP-SA conjugate prepared from both Y-tagged BAP and SA showed the highest enzymatic activity on the biotin-coated plates. This result illustrates the advantage of the protein conjugation reaction in which multiple numbers of proteins can be conjugated at the same time.

Anti-CD22 antibody targeting of pH-responsive micelles enhances small interfering RNA delivery and gene silencing in lymphoma cells

The application of small interfering RNA (siRNA) for cancer treatment is a promising strategy currently being explored in early phase clinical trials. However, efficient systemic delivery limits clinical implementation. We developed and tested a novel delivery system comprised of (i) an internalizing streptavidin-conjugated monoclonal antibody (mAb-SA) directed against CD22 and (ii) a biotinylated diblock copolymer containing both a positively charged siRNA condensing block and a pH-responsive block to facilitate endosome release. The modular design of the carrier facilitates the exchange of different targeting moieties and siRNAs to permit its usage in a variety of tumor types. The polymer was synthesized using the reversible addition fragmentation chain transfer (RAFT) technique and formed micelles capable of binding siRNA and mAb-SA. A hemolysis assay confirmed the predicted membrane destabilizing activity of the polymer under acidic conditions typical of the endosomal compartment. Enhanced siRNA uptake was demonstrated in DoHH2 lymphoma and transduced HeLa-R cells expressing CD22 but not in CD22 negative HeLa-R cells. Gene knockdown was significantly improved with CD22-targeted vs. nontargeted polymeric micelles. Treatment of DoHH2 cells with CD22-targeted polymeric micelles containing 15 nmol/l siRNA produced 70% reduction of gene expression. This CD22-targeted polymer carrier may be useful for siRNA delivery to lymphoma cells.

Reagents for astatination of biomolecules. 4. Comparison of maleimido-closo-decaborate(2-) and meta-[(211)At]astatobenzoate conjugates for labeling anti-CD45 antibodies with [(211)At]astatine

An investigation was conducted to compare the in vivo tissue distribution of a rat antimurine CD45 monoclonal antibody (30F11) and an irrelevant mAbs (CA12.10C12) labeled with (211)At using two different labeling methods. In the investigation, the mAbs were also labeled with (125)I to assess the in vivo stability of the labeling methods toward deastatination. One labeling method employed N-hydroxysuccinimidyl meta-[(211)At]astatobenzoate, [(211)At]1c, and N-hydroxysuccinimidyl meta-[(125)I]iodobenzoate, [(125)I]1b, in conjugation reactions to obtain the radiolabeled mAbs. The other labeling method involved conjugation of a maleimido-closo-decaborate(2-) derivative, 2, with sulfhydryl groups on the mAbs, followed by labeling of the mAb-2 conjugates using Na[(211)At]At or Na[(125)I]I and chloramine-T. Concentrations of the (211)At/(125)I pair of radiolabeled mAbs in selected tissues were examined in BALB/c mice at 1, 4, and 24 h post injection (pi). The co-injected anti-CD45 mAb, 30F11, labeled with [(125)I]1b and [(211)At]1c targeted the CD45-bearing cells in the spleen with the percent injected dose (%ID) of (125)I in that tissue being 13.31 ± 0.78; 17.43 ± 2.56; 5.23 ± 0.50; and (211)At being 6.56 ± 0.40; 10.14 ± 1.49; 7.52 ± 0.79 at 1, 4, and 24 h pi (respectively). However, better targeting (or retention) of the (125)I and (211)At was obtained for 30F11 conjugated with the closo-decaborate(2-), 2. The %ID in the spleen of (125)I (i.e., [(125)I]30F11-2) being 21.15 ± 1.33; 22.22 ± 1.95; 12.41 ± 0.75; and (211)At (i.e., [(211)At]30F11-2) being 22.78 ± 1.29; 25.05 ± 2.35; 17.30 ± 1.20 at 1, 4, and 24 h pi (respectively). In contrast, the irrelevant mAb, CA12.10C12, labeled with (125)I or (211)At by either method had less than 0.8% ID in the spleen at any time point, except for [(211)At]CA12.10C12-1c, which had 1.62 ± 0.14%ID and 1.21 ± 0.08%ID at 1 and 4 h pi. The higher spleen concentrations in that conjugate appear to be due to in vivo deastatination. Differences in (125)I and (211)At concentrations in lung, neck, and stomach indicate that the meta-[(211)At]benzoyl conjugates underwent deastatination, whereas the (211)At-labeled closo-decaborate(2-) conjugates were very stable to in vivo deastatination. In summary, using the closo-decaborate(2-) (211)At labeling approach resulted in higher concentrations of (211)At in target tissue (spleen) and higher stability to in vivo deastatination in this model. These findings, along with the simpler and higher-yielding (211)At-labeling method, provide the basis for using the closo-decaborate(2-) labeling reagent, 2, in our continued studies of the application of (211)At-labeled mAbs for conditioning in hematopoietic cell transplantation.

A comparative analysis of conventional and pretargeted radioimmunotherapy of B-cell lymphomas by targeting CD20, CD22, and HLA-DR singly and in combinations

Relapsed B-cell lymphomas are currently incurable with conventional chemotherapy and radiation treatments. Radiolabeled antibodies directed against B-cell surface antigens have emerged as effective and safe therapies for relapsed lymphomas. We therefore investigated the potential utility of both directly radiolabeled 1F5 (anti-CD20), HD39 (anti-CD22), and Lym-1 (anti-DR) antibodies (Abs) and of pretargeted radioimmunotherapy (RIT) using Ab-streptavidin (SA) conjugates, followed by an N-acetylgalactosamine dendrimeric clearing agent and radiometal-labeled DOTA-biotin, for treatment of lymphomas in mouse models using Ramos, Raji, and FL-18 human lymphoma xenografts. This study demonstrates the marked superiority of pretargeted RIT for each of the antigenic targets with more complete tumor regressions and longer mouse survival compared with conventional one-step RIT. The Ab-SA conjugate yielding the best tumor regression and progression-free survival after pretargeted RIT varied depending upon the lymphoma cell line used, with 1F5 Ab-SA and Lym-1 Ab-SA conjugates yielding the most promising results overall. Contrary to expectations, the best rates of mouse survival were obtained using optimal single Ab-SA conjugates rather than combinations of conjugates targeting different antigens. We hypothesize that clinical implementation of pretargeted RIT methods will provide a meaningful prolongation of survival for patients with relapsed lymphomas compared with currently available treatment strategies.

Preparation, characterization, and biological evaluation of a streptavidin-chimeric t84.66 conjugate for antibody pretargeting

In order to develop a new system for the antibody pretargeting of carcinoembryonic antigen (CEA)-positive cancers, a streptavidin (SA) conjugate of the monoclonal antibody (mAb) chimeric, T84.66, was synthesized and characterized. Antibody disulfide bonds were reduced with 1,4-dithiothreitol, and the resulting thiols were reacted with succinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate-derivatized streptavidin (SMCC-SA). The desired SA-cT84.66 conjugate was purified by iminobiotin affinity chromatography and size-exclusion high-performance liquid chromatography (HPLC). The molecular weight of the SA-cT84.66 conjugate (210 kDa) and immounoreactivity (100%) were confirmed by size-exclusion HPLC, and the conjugate bound three equivalents of (111)In-DOTA-biotin. SA-cT84.66-pretargeted (111)In-DOTA-biotin was evaluated in nude mice bearing LS174T human colon carcinoma xenografts. Tumor uptake of (111)In-DOTA-biotin peaked at 3.32% injected dose per gram after 15 minutes. Clearance from blood and normal organs was extremely rapid, and tumor-to-blood ratios were >20:1 after 24 hours. The specific tumor targeting and rapid whole-body clearance of SA-cT84.66-pretargeted (111)In-DOTA-biotin indicated that this system is promising for the imaging and therapy of CEA-positive cancers.

Comparative biodistributions of pretargeted radioimmunoconjugates targeting CD20, CD22, and DR molecules on human B-cell lymphomas

Pretargeted radioimmunotherapy (PRIT) using streptavidin (SA)-conjugated antibodies (Abs), followed by clearing agent and radiolabeled biotin is a promising method that can increase the effectiveness of RIT, while decreasing the toxicities associated with directly labeled Abs. Although CD20 has been the traditional target antigen for RIT of non-Hodgkin lymphoma (NHL), studies targeting HLA DR and CD22 have yielded promising results. Targeting all 3 antigens at once may further augment the effect of PRIT. This study compares the targeting of Ramos, Raji, and FL-18 lymphoma xenografts with either anti-CD20 Ab/SA (1F5/SA), anti-HLA DR Ab/SA (Lym-1/SA), anti-CD22 Ab/SA (HD39/SA), or all 3 conjugates in combination, followed 24 hours later by a biotin-N-acetyl-galactosamine clearing agent, and 3 hours after that by (111)In-DOTA-biotin. The Ab/SA conjugate yielding the best tumor uptake and tumor-to-normal organ ratios of radioactivity varied depending on the target antigen expression on the cell line used, with 1F5/SA and Lym-1/SA yielding the most promising results overall. Also, the best tumor-to-normal organ ratios of absorbed radioactivity were obtained using single conjugates optimized for target tumor antigen expression rather than the combination therapy. This study highlights the importance of screening the antigenic expression on lymphomas to select the optimal reagent for PRIT.

Immobilized iminobiotin on magnetic poly (vinyl alcohol) microspheres for single-step purification of streptavidin

Magnetic poly (vinyl alcohol) microspheres (MPVAMS) immobilized with iminobiotin as the ligand were prepared for the purification of streptavidin. Parameters of magnetic separation, including spacers, NHS-iminobiotin added, elution behavior, and sample treatment, were optimized to improve the purification efficiency to achieve the maximum recovery of the protein. Streptavidin was successfully purified 38-fold from culture supernatant in a single-step by iminobiotin-MPVAMS with an overall recovery of 90.15% and purity of 95.08%. Hence, this study effectively illustrated the favorable application of magnetic microcarriers for the purification of streptavidin.

A genetically engineered anti-CD45 single-chain antibody-streptavidin fusion protein for pretargeted radioimmunotherapy of hematologic malignancies

Acute myelogenous leukemia (AML) currently kills the majority of afflicted patients despite combination chemotherapy and hematopoietic cell transplantation (HCT). Our group has documented the promise of radiolabeled anti-CD45 monoclonal antibodies (Ab) administered in the setting of allogeneic HCT for AML, but toxicity remains high, and cure rates are only 25% to 30% for relapsed AML. We now show the superiority of pretargeted radioimmunotherapy (PRIT) compared with conventional radioimmunotherapy using a recombinant tetravalent single-chain Ab-streptavidin (SA) fusion protein (scFv(4)SA) directed against human CD45, administered sequentially with a dendrimeric N-acetylgalactosamine-containing clearing agent and radiolabeled 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic (DOTA)-biotin. The scFv(4)SA construct was genetically engineered by fusing Fv fragments of the human CD45-specific BC8 Ab to a full-length genomic SA gene and was expressed as a soluble tetramer in the periplasmic space of Escherichia coli. The fusion protein was purified to >95% homogeneity at an overall yield of approximately 50% using iminobiotin affinity chromatography. The immunoreactivity and avidity of the fusion protein were comparable with those of the intact BC8 Ab, and the scFv(4)SA construct bound an average of 3.9 biotin molecules out of four theoretically possible. Mouse lymphoma xenograft experiments showed minimal toxicity, excellent tumor-specific targeting of the fusion protein and radiolabeled DOTA-biotin in vivo, marked inhibition of tumor growth, and cured 100% of mice bearing CD45-expressing tumors. These promising results have prompted large-scale cGMP production of the BC8 fusion protein for clinical trials to be conducted in patients with hematologic malignancies.

Novel preparation and characterization of a trastuzumab???streptavidin conjugate for pre-targeted radionuclide therapy

INTRODUCTION

This study describes a novel and convenient route for the preparation of a trastuzumab-streptavidin conjugate such as might be used in a pre-targeting system and its in-vitro and in-vivo evaluation.

METHODS

Trastuzumab was irradiated with UV light in the presence of stannous ions to reduce a number of the disulfide bridges to free thiol groups. A range of irradiation times were studied in order to quantify the number of thiols produced and to optimize the reduction process. The conjugate was then prepared by reaction with succinimidyl 4-(N-maleimidomethy cyclohexane)-1-carboxylate (SMCC)-linked streptavidin.

RESULTS

Initial conjugation reactions in phosphate buffer were inefficient, producing low conjugate yields, but conjugation reactions in triethanolamine-based buffer showed greatly increased conjugation yields. A high purity product (approximately 100%) was obtained following purification by gel-filtration HPLC as determined by subsequent size exclusion HPLC analysis. The conjugate was shown to possess an essentially identical immunoreactivity to that of the native, unconjugated antibody and an unaltered biotin binding stoichiometry. Shedding and internalization by Her-2-expressing cells were low and the uptake in vivo by Her-2-expressing xenografts in nude mice was similar to that of labelled antibody.

CONCLUSION

Our results demonstrate a new, simple and effective method for the successful synthesis of antibody-streptavidin conjugates which could also be applied to many other heterodimeric protein conjugation reactions.

Comparison of anti-CD20 and anti-CD45 antibodies for conventional and pretargeted radioimmunotherapy of B-cell lymphomas

Radiolabeled anti-CD20 antibodies produce responses in 60% to 95% of patients with relapsed non-Hodgkin lymphoma (NHL); however, absorbed radiation ratios between tumors and normal organs are relatively low, and many patients have relapses. In this study we compared the abilities of anti-CD45 (BC8) and anti-CD20 (1F5) antibodies to target human Ramos lymphoma xenografts in athymic mice. When direct radioiodination was performed with conventional methods, BC8 delivered 2- to 4-fold more radioiodine to tumors than 1F5, with tumor-to-normal organ ratios as high as 20:1 using radiolabeled BC8 compared with a maximal ratio of 9.8:1 using radioiodinated 1F5. To optimize the biodistribution of radioactivity, we performed studies following a pretargeting method using streptavidin (SA)-conjugated BC8 and 1F5. Injection of a synthetic clearing agent decreased the circulating level of conjugates by 80% to 90% within 1 hour. Pretargeting with BC8-SA resulted in a 2- to 4-fold greater tumor uptake of radiolabeled biotin than with 1F5-SA, with maximal tumor-to-normal organ ratios of more than 80:1 and approximately 16:1, respectively. Therapy experiments demonstrated that 400 microCi (14.8 MBq) or more of yttrium-90-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-biotin cured 100% of mice treated with BC8-SA and more than 90% of mice pretargeted with 1F5-SA, with complete remission occurring 8 to 10 days sooner in mice receiving BC8-SA. After treatment with 200 microCi (7.4 MBq) (90)Y-DOTA-biotin, 70% of the mice treated with BC8-SA were cured, but no mice were cured using 1F5-SA. Doses up to 800 microCi (29.6 MBq) (90)Y-DOTA-biotin were delivered with minor toxicity using either antibody conjugate. These lymphoma xenograft data suggest that pretargeted radioimmunotherapy using either anti-CD20 or anti-CD45 conjugates is highly effective and minimally toxic.

A comparative evaluation of conventional and pretargeted radioimmunotherapy of CD20-expressing lymphoma xenografts

Radioimmunotherapy with anti-CD20 monoclonal antibodies is a promising new treatment approach for patients with relapsed B-cell lymphomas. However, the majority of patients treated with conventional radiolabeled anti-CD20 antibodies eventually have a relapse because the low tumor-to-blood and tumor-to-normal organ ratios of absorbed radioactivity limit the dose that can be safely administered without hematopoietic stem cell support. This study assessed the ability of a streptavidin-biotin "pretargeting" approach to improve the biodistribution of radioactivity in mice bearing Ramos lymphoma xenografts. A pretargeted streptavidin-conjugated anti-CD20 1F5 antibody was infused, followed 24 hours later by a biotinylated N-acetylgalactosamine-containing "clearing agent" and finally 3 hours later by (111)In-labeled DOTA-biotin. Tumor-to-blood ratios were 3:1 or more with pretargeting, compared with 0.5:1 or less with conventional (111)In-1F5. Tumor-to-normal organ ratios of absorbed radioactivity up to 56:1 were observed with pretargeting, but were 6:1 or less with conventional (111)In-1F5. Therapy experiments demonstrated that 400 microCi (14.8 MBq) or more of conventional (90)Y-1F5 was required to obtain major tumor responses, but this dose was associated with lethal toxicity in 100% of mice. In marked contrast, up to 800 microCi (29.6 MBq) (90)Y-DOTA-biotin could be safely administered by the pretargeting approach with only minor toxicity, and 89% of the mice were cured. These data suggest that anti-CD20 pretargeting shows great promise for improving current therapeutic options for B-cell lymphomas and warrants further preclinical and clinical testing.