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

Pretargeting: A Path Forward for Radioimmunotherapy

Pretargeted radioimmunodiagnosis and radioimmunotherapy aim to efficiently combine antitumor antibodies and medicinal radioisotopes for high-contrast imaging and high-therapeutic-index (TI) tumor targeting, respectively. As opposed to conventional radioimmunoconjugates, pretargeted approaches separate the tumor-targeting step from the payload step, thereby amplifying tumor uptake while reducing normal-tissue exposure. Alongside contrast and TI, critical parameters include antibody immunogenicity and specificity, availability of radioisotopes, and ease of use in the clinic. Each of the steps can be optimized separately; as modular systems, they can find broad applications irrespective of tumor target, tumor type, or radioisotopes. Although this versatility presents enormous opportunity, pretargeting is complex and presents unique challenges for clinical translation and optimal use in patients. The purpose of this article is to provide a brief historical perspective on the origins and development of pretargeting strategies in nuclear medicine, emphasizing 2 protein delivery systems that have been extensively evaluated (i.e., biotin-streptavidin and hapten-bispecific monoclonal antibodies), as well as radiohaptens and radioisotopes. We also highlight recent innovations, including pretargeting with bioorthogonal chemistry and novel protein vectors (such as self-assembling and disassembling proteins and Affibody molecules). We caution the reader that this is by no means a comprehensive review of the past 3 decades of pretargeted radioimmunodiagnosis and pretargeted radioimmunotherapy. But we do aim to highlight major developmental milestones and to identify benchmarks for success with regard to TI and toxicity in preclinical models and clinically. We believe this approach will lead to the identification of key obstacles to clinical success, revive interest in the utility of radiotheranostics applications, and guide development of the next generation of pretargeted theranostics.

Novel Methods to Improve the Efficiency of Radioimmunotherapy for Non-Hodgkin Lymphoma

Radioimmunotherapy (RIT) is a novel strategy for treating non-Hodgkin lymphoma (NHL). Several studies have shown the promising results of using RIT in NHL, which have led to FDA approval for two RIT agents in treating low grade NHL. In spite of these favorable results in low-grade NHL, most of the aggressive or relapsed/refractory NHL subjects experience relapses following RIT. Although more aggressive treatments such as myeloablative doses of RIT followed by stem cell transplantation appear to be able to provide a longer survival for some patients these approaches are associated with significant treatment-related adverse events and challenging to deliver in most centers. Therefore, it seems reasonable to develop treatment approaches that enhance the efficiency of RIT, while reducing its toxicity. In this paper, novel methods that improve the efficiency of RIT and reduce its toxicity through various mechanisms are reviewed. Further clinical development of these methods could expand the NHL patient groups eligible for receiving RIT, and even extend the use of RIT to new indications and disease groups in future.

Comparative Analysis of Bispecific Antibody and Streptavidin-Targeted Radioimmunotherapy for B-cell Cancers

Streptavidin (SA)-biotin pretargeted radioimmunotherapy (PRIT) that targets CD20 in non-Hodgkin lymphoma (NHL) exhibits remarkable efficacy in model systems, but SA immunogenicity and interference by endogenous biotin may complicate clinical translation of this approach. In this study, we engineered a bispecific fusion protein (FP) that evades the limitations imposed by this system. Briefly, one arm of the FP was an anti-human CD20 antibody (2H7), with the other arm of the FP an anti-chelated radiometal trap for a radiolabeled ligand (yttrium[Y]-DOTA) captured by a very high-affinity anti-Y-DOTA scFv antibody (C825). Head-to-head biodistribution experiments comparing SA-biotin and bispecific FP (2H7-Fc-C825) PRIT in murine subjects bearing human lymphoma xenografts demonstrated nearly identical tumor targeting by each modality at 24 hours. However, residual radioactivity in the blood and normal organs was consistently higher following administration of 1F5-SA compared with 2H7-Fc-C825. Consequently, tumor-to-normal tissue ratios of distribution were superior for 2H7-Fc-C825 (P < 0.0001). Therapy studies in subjects bearing either Ramos or Granta subcutaneous lymphomas demonstrated that 2H7-Fc-C825 PRIT is highly effective and significantly less myelosuppressive than 1F5-SA (P < 0.0001). All animals receiving optimal doses of 2H7-Fc-C825 followed by ⁹⁰Y-DOTA were cured by 150 days, whereas the growth of tumors in control animals progressed rapidly with complete morbidity by 25 days. In addition to demonstrating reduced risk of immunogenicity and an absence of endogenous biotin interference, our findings offer a preclinical proof of concept for the preferred use of bispecific PRIT in future clinical trials, due to a slightly superior biodistribution profile, less myelosuppression, and superior efficacy. Cancer Res; 76(22); 6669-79. ©2016 AACR.

Addressing challenges of heterogeneous tumor treatment through bispecific protein-mediated pretargeted drug delivery

Tumors are frequently characterized by genomically and phenotypically distinct cancer cell subpopulations within the same tumor or between tumor lesions, a phenomenon termed tumor heterogeneity. These diverse cancer cell populations pose a major challenge to targeted delivery of diagnostic and/or therapeutic agents, as the conventional approach of conjugating individual ligands to nanoparticles is often unable to facilitate intracellular delivery to the full spectrum of cancer cells present in a given tumor lesion or patient. As a result, many cancers are only partially suppressed, leading to eventual tumor regrowth and/or the development of drug-resistant tumors. Pretargeting (multistep targeting) approaches involving the administration of 1) a cocktail of bispecific proteins that can collectively bind to the entirety of a mixed tumor population followed by 2) nanoparticles containing therapeutic and/or diagnostic agents that can bind to the bispecific proteins accumulated on the surface of target cells offer the potential to overcome many of the challenges associated with drug delivery to heterogeneous tumors. Despite its considerable success in improving the efficacy of radioimmunotherapy, the pretargeting strategy remains underexplored for a majority of nanoparticle therapeutic applications, especially for targeted delivery to heterogeneous tumors. In this review, we will present concepts in tumor heterogeneity, the shortcomings of conventional targeted systems, lessons learned from pretargeted radioimmunotherapy, and important considerations for harnessing the pretargeting strategy to improve nanoparticle delivery to heterogeneous tumors.

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.

Structure-based design of a streptavidin mutant specific for an artificial biotin analogue

For a multistep pre-targeting method using antibodies, a streptavidin mutant with low immunogenicity, termed low immunogenic streptavidin mutant No. 314 (LISA-314), was produced previously as a drug delivery tool. However, endogenous biotins (BTNs) with high affinity (Kd < 10(-10) M) for the binding pocket of LISA-314 prevents access of exogenous BTN-labelled anticancer drugs. In this study, we improve the binding pocket of LISA-314 to abolish its affinity for endogenous BTN species, therefore ensuring that the newly designed LISA-314 binds only artificial BTN analogue. The replacement of three amino acid residues was performed in two steps to develop a mutant termed V212, which selectively binds to 6-(5-((3aS,4S,6aR)-2-iminohexahydro-1H-thieno[3,4-d]imidazol-4-yl)pentanamido)hexanoic acid (iminobiotin long tail, IMNtail). Surface plasmon resonance results showed that V212 has a Kd value of 5.9 × 10(-7) M towards IMNtail, but no binding affinity for endogenous BTN species. This V212/IMNtail system will be useful as a novel delivery tool for anticancer therapy.

Structure-based design and synthesis of a bivalent iminobiotin analog showing strong affinity toward a low immunogenic streptavidin mutant

The streptavidin/biotin interaction has been widely used as a useful tool in research fields. For application to a pre-targeting system, we previously developed a streptavidin mutant that binds to an iminobiotin analog while abolishing affinity for natural biocytin. Here, we design a bivalent iminobiotin analog that shows 1000-fold higher affinity than before, and determine its crystal structure complexed with the mutant protein.

Advances in the treatment of hematologic malignancies using immunoconjugates

Monoclonal antibody therapy has revolutionized cancer treatment by significantly improving patient survival both in solid tumors and hematologic malignancies. Recent technological advances have increased the effectiveness of immunotherapy leading to its broader application in diverse treatment settings. Immunoconjugates (ICs) consist of a cytotoxic effector covalently linked to a monoclonal antibody that enables the targeted delivery of its therapeutic payload to tumors based on cell-surface receptor recognition. ICs are classified into 3 groups based on their effector type: immunotoxins (protein toxin), radioimmunoconjugates (radionuclide), and antibody drug conjugates (small-molecule drug). Optimization of each individual component of an IC (antibody, linker, and effector) is essential for therapeutic efficacy. Clinical trials have been conducted to investigate the effectiveness of ICs in hematologic malignancies both as monotherapy and in multiagent regimens in relapsed/refractory disease as well as frontline settings. These studies have yielded encouraging results particularly in lymphoma. ICs comprise an exciting group of therapeutics that promise to play an increasingly important role in the management of hematologic malignancies.

N-acetylgalactosamino dendrons as clearing agents to enhance liver targeting of model antibody-fusion protein

Dendrimer clearing agents represent a unique class of compounds for use in multistep targeting (MST) in radioimmunotherapy and imaging. These compounds were developed to facilitate the removal of excess tumor-targeting monoclonal antibody (mAb) prior to administration of the radionuclide to minimize exposure of normal tissue to radiation. Clearing agents are designed to capture the circulating mAb, and target it to the liver for metabolism. Glycodendrons are ideally suited for MST applications as these highly branched compounds are chemically well-defined, thus advantageous over heterogeneous macromolecules. Previous studies have described glycodendron 3 as a clearing agent for use in three-step MST protocols, and early in vivo assessment of 3 showed promise. However, synthetic challenges have hampered its availability for further development. In this report we describe a new sequence of chemical steps which enables the straightforward synthesis and analytical characterization of this class of dendrons. With accessibility and analytical identification solved, we sought to evaluate both lower and higher generation dendrons for hepatocyte targeting as well as clearance of a model protein. We prepared a series of clearing agents where a single biotin is connected to glycodendrons displaying four, eight, sixteen or thirty-two α-thio-N-acetylgalactosamine (α-SGalNAc) units, resulting in compounds with molecular weights ranging from 2 to 17 kDa, respectively. These compounds were fully characterized by LCMS and NMR. We then evaluated the capacity of these agents to clear a model (131)I-labeled single chain variable fragment antibody-streptavidin ((131)I-scFv-SAv) fusion protein from blood and tissue in mice, and compared their clearing efficiencies to that of a 500 kDa dextran-biotin conjugate. Glycodendrons and dextran-biotin exhibited enhanced blood clearance of the scFv-SAv construct. Biodistribution analysis showed liver targeting/uptake of the scFv-SAv construct to be 2-fold higher for compounds 1 to 4, as well as for the 500 kDa dextran, over saline. Additionally, the data suggest the glycodendrons clear through the liver, whereas the dextran through reticuloendothelial system (RES) metabolism.

A new Tri-Fab bispecific antibody for pretargeting Trop-2-expressing epithelial cancers

RS7 is an internalizing anti-Trop-2 pancarcinoma antibody capable of targeting most epithelial cancers. Because pretargeting strategies could improve the tumor localization of radionuclides, a new anti-Trop-2 × antihapten bispecific antibody for pretargeting, based on humanized RS7, was prepared and evaluated with a radiolabeled hapten-peptide in vitro and in vivo to determine whether its internalization properties would interfere with pretargeting.

METHODS

The anti-Trop-2 × antihapten bispecific antibody, TF12, was prepared using the modular dock-and-lock method. TF12 and humanized RS7 binding was assessed by cell binding assays and fluorescence-activated cell sorting analysis in a variety of human carcinoma cell lines. The internalization of TF12 was evaluated in vitro using a fluorescent TF12 conjugate or hapten-peptide and (111)In-labeled TF12 and RS7. The biodistribution of TF12 and its use as a pretargeting agent with an (111)In-labeled hapten-peptide were assessed in several human epithelial cancer xenografts. Dose optimization was examined in 2 tumor models.

RESULTS

TF12 internalizes, but a substantial fraction remained accessible on the tumor surface. Fluorescence-activated cell sorting analysis showed only a minor change in fluorescent signal when the tumor was probed with a fluorescent hapten-peptide over 4 h, and microscopy showed substantial membrane staining when reassessed at 24 h after TF12 exposure. Only 40.1% of (111)In-TF12 was internalized after 24 h. In vivo, excellent tumor localization of the (111)In-labeled peptide was observed in several tumor models.

CONCLUSION

TF12 was retained sufficiently on the cell surface in several epithelial cancers, thereby making it suitable for pretargeted imaging and therapy of various Trop-2-expressing carcinomas.

A pretargeted nanoparticle system for tumor cell labeling

Nanoparticle-based cancer diagnostics and therapeutics can be significantly enhanced by selective tissue localization, but the strategy can be complicated by the requirement of a targeting ligand conjugated on nanoparticles, that is specific to only one or a limited few types of neoplastic cells, necessitating the development of multiple nanoparticle systems for different diseases. Here, we present a new nanoparticle system that capitalizes on a targeting pretreatment strategy, where a circulating fusion protein (FP) selectively prelabels the targeted cellular epitope, and a biotinylated iron oxide nanoparticle serves as a secondary label that binds to the FP on the target cell. This approach enables a single nanoparticle formulation to be used with any one of existing fusion proteins to bind a variety of target cells. We demonstrated this approach with two fusion proteins against two model cancer cell lines: lymphoma (Ramos) and leukemia (Jurkat), which showed 72.2% and 91.1% positive labeling, respectively. Notably, TEM analysis showed that a large nanoparticle population was endocytosed via attachment to the non-internalizing CD20 epitope.

Engineering an antibody with picomolar affinity to DOTA chelates of multiple radionuclides for pretargeted radioimmunotherapy and imaging

INTRODUCTION

In pretargeted radioimmunotherapy (PRIT), a bifunctional antibody is administered and allowed to pre-localize to tumor cells. Subsequently, a chelated radionuclide is administered and captured by cell-bound antibody while unbound hapten clears rapidly from the body. We aim to engineer high-affinity binders to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelates for use in PRIT applications.

METHODS

We mathematically modeled antibody and hapten pharmacokinetics to analyze hapten tumor retention as a function of hapten binding affinity. Motivated by model predictions, we used directed evolution and yeast surface display to affinity mature the 2D12.5 antibody to DOTA, reformatted as a single chain variable fragment (scFv).

RESULTS

Modeling predicts that for high antigen density and saturating bsAb dose, a hapten-binding affinity of 100 pM is needed for near-maximal hapten retention. We affinity matured 2D12.5 with an initial binding constant of about 10 nM to DOTA-yttrium chelates. Affinity maturation resulted in a 1000-fold affinity improvement to biotinylated DOTA-yttrium, yielding an 8.2±1.9 picomolar binder. The high-affinity scFv binds DOTA complexes of lutetium and gadolinium with similar picomolar affinity and indium chelates with low nanomolar affinity. When engineered into a bispecific antibody construct targeting carcinoembryonic antigen, pretargeted high-affinity scFv results in significantly higher tumor retention of a (111)In-DOTA hapten compared to pretargeted wild-type scFv in a xenograft mouse model.

CONCLUSIONS

We have engineered a versatile, high-affinity, DOTA-chelate-binding scFv. We anticipate it will prove useful in developing pretargeted imaging and therapy protocols to exploit the potential of a variety of radiometals.

Conventional and pretargeted radioimmunotherapy using bismuth-213 to target and treat non-Hodgkin lymphomas expressing CD20: a preclinical model toward optimal consolidation therapy to eradicate minimal residual disease

Radioimmunotherapy (RIT) with α-emitting radionuclides is an attractive approach for the treatment of minimal residual disease because the short path lengths and high energies of α-particles produce optimal cytotoxicity at small target sites while minimizing damage to surrounding normal tissues. Pretargeted RIT (PRIT) using antibody-streptavidin (Ab-SA) constructs and radiolabeled biotin allows rapid, specific localization of radioactivity at tumor sites, making it an optimal method to target α-emitters with short half-lives, such as bismuth-213 (²¹³Bi). Athymic mice bearing Ramos lymphoma xenografts received anti-CD20 1F5(scFv)(4)SA fusion protein (FP), followed by a dendrimeric clearing agent and [²¹³Bi]DOTA-biotin. After 90 minutes, tumor uptake for 1F5(scFv)₄SA was 16.5% ± 7.0% injected dose per gram compared with 2.3% ± .9% injected dose per gram for the control FP. Mice treated with anti-CD20 PRIT and 600 μ Ci [²¹³Bi]DOTA-biotin exhibited marked tumor growth delays compared with controls (mean tumor volume .01 ± .02 vs. 203.38 ± 83.03 mm³ after 19 days, respectively). The median survival for the 1F5(scFv)₄SA group was 90 days compared with 23 days for the control FP (P < .0001). Treatment was well tolerated, with no treatment-related mortalities. This study demonstrates the favorable biodistribution profile and excellent therapeutic efficacy attainable with ²¹³Bi-labeled anti-CD20 PRIT.

Pretargeted radioimmunotherapy for hematologic and other malignancies

Radioimmunotherapy (RIT) has emerged as one of the most promising treatment options, particularly for hematologic malignancies. However, this approach has generally been limited by a suboptimal therapeutic index (target-to-nontarget ratio) and an inability to deliver sufficient radiation doses to tumors selectively. Pretargeted RIT (PRIT) circumvents these limitations by separating the targeting vehicle from the subsequently administered therapeutic radioisotope, which binds to the tumor-localized antibody or is quickly excreted if unbound. A growing number of preclinical proof-of-principle studies demonstrate that PRIT is feasible and safe and provides improved directed radionuclide delivery to malignant cells compared with conventional RIT while sparing normal cells from nonspecific radiotoxicity. Early phase clinical studies corroborate these preclinical findings and suggest better efficacy and lesser toxicities in patients with hematologic and other malignancies. With continued research, PRIT-based treatment strategies promise to become cornerstones to improved outcomes for cancer patients despite their complexities.

Improving the efficacy of radioimmunotherapy for non-Hodgkin lymphomas

Approximately 66,000 Americans develop non-Hodgkin lymphoma (NHL) each year. Although the use of unlabeled antibodies such as rituximab has significantly improved survival when combined with standard chemotherapy regimens, approximately two-thirds of lymphoma patients eventually develop disease recurrence and die of their disease. Novel treatments are urgently needed to cure these patients. One strategy involves the use of radiolabeled immunoconjugates that specifically localize radiation delivery to sites of lymphoma while minimizing toxicity to normal tissues. A growing number of studies support the contention that radiolabeled antibody therapy can improve overall survival of lymphoma patients and lead to durable remissions, with probable cures, in many patients. Various approaches for enhancing the effectiveness of radioimmunoconjugates have been studied, including: use in newly diagnosed lymphoma patients, combination with chemotherapy or other monoclonal antibodies, use with hematopoietic stem cell transplantation, multistep pretargeting strategies to further minimize toxicity, and simultaneous targeting of multiple B-cell antigens. This article summarizes the current knowledge supporting the use of radioimmunotherapy, an underused but effective treatment modality in NHL patients.

Pretargeting CD45 enhances the selective delivery of radiation to hematolymphoid tissues in nonhuman primates

Pretargeted radioimmunotherapy (PRIT) is designed to enhance the directed delivery of radionuclides to malignant cells. Through a series of studies in 19 nonhuman primates (Macaca fascicularis), the potential therapeutic advantage of anti-CD45 PRIT was evaluated. Anti-CD45 PRIT demonstrated a significant improvement in target-to-normal organ ratios of absorbed radiation compared with directly radiolabeled bivalent antibody (conventional radioimmunotherapy [RIT]). Radio-DOTA-biotin administered 48 hours after anti-CD45 streptavidin fusion protein (FP) [BC8 (scFv)(4)SA] produced markedly lower concentrations of radiation in nontarget tissues compared with conventional RIT. PRIT generated superior target:normal organ ratios in the blood, lung, and liver (10.3:1, 18.9:1, and 9.9:1, respectively) compared with the conventional RIT controls (2.6:1, 6.4:1, and 2.9:1, respectively). The FP demonstrated superior retention in target tissues relative to comparable directly radiolabeled bivalent anti-CD45 RIT. The time point of administration of the second step radiolabeled ligand (radio-DOTA-biotin) significantly impacted the biodistribution of radioactivity in target tissues. Rapid clearance of the FP from the circulation rendered unnecessary the addition of a synthetic clearing agent in this model. These results support proceeding to anti-CD45 PRIT clinical trials for patients with both leukemia and lymphoma.

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.

Generation of flow cytometry data files with a potentially infinite number of dimensions

Immunophenotypic characterization of B-cell chronic lymphoproliferative disorders (B-CLPD) is associated with the use of increasingly larger panels of multiple combinations of 3 to > or =6 monoclonal antibodies (Mab), data analysis being separately performed for each of the different stained sample aliquots. Here, we describe and validate an automated method for calculation of flow cytometric data from several multicolor stainings of the same cell sample--i.e., the merging of data from different aliquots stained with partially overlapping combinations of Mab reagents (focusing on > or =1 cell populations)--into one data file as if it concerned a single "super" multicolor staining. Evaluation of the performance of the method described was done in a group of 60 B-CLPD studied at diagnosis with 18 different reagents in a panel containing six different 3- and 4-color stainings, which systematically contained CD19 for the identification of B-cells. Our results show a high degree of correlation and agreement between originally measured and calculated data about cell surface stainings, providing a basis for the use of this approach for the generation of flow cytometric data files containing information about a virtually infinite number of stainings for each individual cellular event measured in a sample, using a limited number of fluorochrome stainings.

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

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

A novel bispecific, trivalent antibody construct for targeting pancreatic carcinoma

Preclinical and clinical studies have demonstrated the application of radiolabeled mAb-PAM4 for nuclear imaging and radioimmunotherapy of pancreatic carcinoma. We have now examined the ability of a novel PAM4-based, bispecific monoclonal antibody (mAb) construct, TF10, to pretarget a radiolabeled peptide for improved imaging and therapy. TF10 is a humanized, bispecific mAb, divalent for mAb-PAM4 and monovalent for mAb-679, reactive against the histamine-succinyl-glycine hapten. Biodistribution studies and nuclear imaging of the radiolabeled TF10 and/or TF10-pretargeted hapten-peptide (IMP-288) were conducted in nude mice bearing CaPan1 human pancreatic cancer xenografts. (125)I-TF10 cleared rapidly from the blood, with levels decreasing to <1% injected dose per gram (ID/g) by 16 hours. Tumor uptake was 3.47 +/- 0.66% ID/g at this time point with no accumulation in any normal tissue. To show the utility of the pretargeting approach, (111)In-IMP-288 was administered 16 hours after TF10. At 3 hours postadministration of radiolabeled peptide, imaging showed intense uptake within the tumors and no evidence of accretion in any normal tissue. No targeting was observed in animals given only the (111)In-peptide. Tumor uptake of the TF10-pretargeted (111)In-IMP-288 was 24.3 +/- 1.7% ID/g, whereas for (111)In-IMP-288 alone it was only 0.12 +/- 0.002% ID/g at 16 hours. Tumor/blood ratios were significantly greater for the pretargeting group ( approximately 1,000:1 at 3 hours) compared with (111)In-PAM4-IgG ( approximately 5:1 at 24 hours; P < 0.0003). Radiation dose estimates suggested that TF10/(90)Y-peptide pretargeting would provide a greater antitumor effect than (90)Y-PAM4-IgG. Thus, the results suggest that TF10 pretargeting may provide improved imaging for early detection, diagnosis, and treatment of pancreatic cancer as compared with directly radiolabeled PAM4-IgG.

Radioimmunotherapy-based conditioning regimens for stem cell transplantation

Radioimmunotherapy (RIT) combines the mechanism of action and targeting capability of monoclonal antibodies with the tumoricidal effect of radiation and has shown promising results in the treatment of various hematologic malignancies. Based on RIT's efficacy and safety profile, many investigators have evaluated its use in transplant conditioning regimens with the goal of improving long-term disease control with limited toxicity. In lymphoma, two basic transplant approaches targeting CD20 have emerged: (1) myeloablative doses of RIT with or without chemotherapy, and (2) standard nonmyeloablative doses of RIT combined with high-dose chemotherapy. Myeloablative RIT has been shown to be feasible and efficacious using escalated doses of iodine 131-tositumomab, yttrium 90-ibritumomab tiuxetan, and (131)I-rituximab with or without chemotherapy followed by autologous stem cell transplant (ASCT). The second approach predominantly has used standard doses of (90)Y-ibritumomab tiuxetan or (131)I-tositumomab plus BEAM chemotherapy (carmustine [BCNU], etoposide, cytarabine, melphalan) followed by ASCT. RIT targeting CD45, CD33, and CD66 prior to allogeneic transplantation also has been evaluated for the treatment of acute leukemia. Overall RIT-based transplant conditioning for lymphoma and leukemia has been shown to be safe, effective, and feasible with ongoing randomized trials currently underway to definitively establish its place in the treatment of hematologic malignancies.

Eradication of disseminated leukemia in a syngeneic murine leukemia model using pretargeted anti-CD45 radioimmunotherapy

We describe the use of pretargeted radioimmunotherapy (PRIT) using an anti-murine CD45 antibody-streptavidin (SA) conjugate followed by radiobiotin to deliver radiation selectively to murine hematolymphoid tissues, which may potentially augment the efficacy and decrease the toxicity of radioimmunotherapy for disseminated murine leukemia. Biodistribution and therapeutic results demonstrated high target organ to nontarget organ ratios of radioactivity and significant long-term survival in leukemic mice using PRIT. These data suggest that anti-CD45 PRIT using an anti-CD45-SA conjugate in a syngeneic murine model of disseminated leukemia may be more effective and less toxic than directly labeled monoclonal antibodies.

Pretargeted Radioimmunotherapy for B-Cell Lymphomas

Relapsed or treatment refractory B-cell lymphomas are currently incurable with conventional chemotherapy and radiation treatments. High-dose chemoradiotherapy and stem cell transplantation can cure some patients with relapsed or refractory lymphoma, but the majority of such patients die of progressive disease. We have investigated the potential utility of pretargeted radioimmunotherapy using monoclonal antibody-streptavidin, immunoconjugates, and fusion proteins in combination with N-acetylgalactosamine dendrimeric clearing agent and radiometal-labeled 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid biotin for treatment of lymphomas using mouse and primate models. We have targeted a variety of cell surface antigens, including CD20, CD22, CD45, and HLA-DR, using conventional and pretargeted radioimmunotherapy. These studies showed the marked superiority of pretargeted radioimmunotherapy for each of the antigenic targets in terms of superior biodistributions, more complete tumor regressions, and longer survival. We are optimistic that this novel approach will provide a meaningful prolongation of survival for patients with relapsed or refractory lymphomas.

Evaluation of CD20, CD22, and HLA-DR Targeting for Radioimmunotherapy of B-Cell Lymphomas

Despite the promise of radioimmunotherapy using anti-CD20 antibodies (Ab) for the treatment of relapsed patients with indolent non-Hodgkin lymphoma (NHL), most patients treated with conventional doses of (131)I-tositumomab or (90)Y-ibritumomab eventually relapse. We did comparative assessments using conventional radioimmunotherapy targeting CD20, CD22, and HLA-DR on human Ramos, Raji, and FL-18 lymphoma xenografts in athymic mice to assess the potential for improving the efficacy of radioimmunotherapy by targeting other NHL cell surface antigens. Results of biodistribution studies showed significant differences in tumor localization consistent with variable antigenic expression on the different lymphoma cell lines. Interestingly, the radioimmunoconjugate that yielded the best tumor-to-normal organ ratios differed in each tumor model. We also explored administering all three (111)In-1,4,7,10-tetra-azacylododecane N,N',N'',N'''-tetraacetic acid antibodies in combination, but discovered, surprisingly, that this approach did not augment the localization of radioactivity to tumors compared with the administration of the best single radiolabeled Ab alone. These data suggest that conventional radioimmunotherapy using anti-CD20, anti-HLA-DR, or anti-CD22 Abs is effective when used singly and provides targeted uptake of radiolabel into the tumor that is dependent on the levels of antigen expression. Improvements in tumor-to-normal organ ratios of radioactivity cannot be achieved using directly labeled Abs in combination but may be afforded by novel pretargeting methods.