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

Continuing progress in radioimmunotherapy for hematologic malignancies

Radioimmunotherapy (RIT) involves combining a cytotoxic radionuclide with an antibody (Ab) targeting a tumor antigen. Compared with conventional therapies, RIT improves the therapeutic efficacy of Ab and ameliorates toxicity. This comprehensive review describes the current advancements and future prospects in RIT for treating hematologic malignancies based on recent investigations. Although β-particle RITs targeting CD20 are effective with low toxicity in patients with relapsed/refractory indolent or transformed non-Hodgkin's lymphoma, these treatments have not gained popularity because of the increasing availability of new therapies. RIT using single-domain antibodies is expected to improve tumor penetrance and reduce radiation exposure to non-target organs. To enhance RIT efficacy, α-particle RIT and pretargeted radioimmunotherapy (PRIT) are currently being developed. Alpha-particle RIT demonstrates substantial antitumor activity and reduced bystander effects due to its high linear energy transfer and short particle range. PRIT may increase the tumor-to-whole body dose ratio.

Glypican-3-Targeted 227Th -Therapy Reduces Tumor Burden in an Orthotopic Xenograft Murine Model of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a significant cause of morbidity and mortality worldwide, with limited therapeutic options for advanced disease. Targeted α-therapy is an emerging class of targeted cancer therapy in which α-particle-emitting radionuclides, such as ²²⁷Th, are delivered specifically to cancer tissue. Glypican-3 (GPC3) is a cell surface glycoprotein highly expressed on HCC. In this study, we describe the development and in vivo efficacy of a ²²⁷Th-labeled GPC3-targeting antibody conjugate (²²⁷Th-octapa-αGPC3) for treatment of HCC in an orthotopic murine model. Methods: The chelator p-SCN-Bn-H₄octapa-NCS (octapa) was conjugated to a GPC3-targeting antibody (αGPC3) for subsequent ²²⁷Th radiolabeling (octapa-αGPC3). Conditions were varied to optimize radiolabeling of ²²⁷Th. In vitro stability was evaluated by measuring the percentage of protein-bound ²²⁷Th by γ-ray spectroscopy. An orthotopic athymic Nu/J murine model using HepG2-Red-FLuc cells was developed. Biodistribution and blood clearance of ²²⁷Th-octapa-αGPC3 were evaluated in tumor-bearing mice. The efficacy of ²²⁷Th-octapa-αGPC3 was assessed in tumor-bearing animals with serial measurement of serum α-fetoprotein at 23 d after injection. Results: Octapa-conjugated αGPC3 provided up to 70% ²²⁷Th labeling yield in 2 h at room temperature. In the presence of ascorbate, at least 97.8% of ²²⁷Th was bound to αGPC3-octapa after 14 d in phosphate-buffered saline. In HepG2-Red-FLuc tumor-bearing mice, highly specific GPC3 targeting was observed, with significant ²²⁷Th-octapa-αGPC3 accumulation in the tumor over time and minimal accumulation in normal tissue. Twenty-three days after treatment, a significant reduction in tumor burden was observed in mice receiving a 500 kBq/kg dose of ²²⁷Th-octapa-αGPC3 by tail-vein injection. No acute off-target toxicity was observed, and no animals died before termination of the study. Conclusion:²²⁷Th-octapa-αGPC3 was observed to be stable in vitro; maintain high specificity for GPC3, with favorable biodistribution in vivo; and result in significant antitumor activity without significant acute off-target toxicity in an orthotopic murine model of HCC.

Where do we stand with radioimmunotherapy for acute myeloid leukemia?

INTRODUCTION

Despite the approval of several new drugs, deaths from acute myeloid leukemia (AML) remain common. Because of well-defined cell surface antigens, easy accessibility, and radiosensitivity of leukemia cells, there is long-standing interest in radiolabeled antibodies (radioimmunotherapy [RIT]) to complement or replace existing treatments and improve outcomes in AML.

AREAS COVERED

Targeting primarily CD33, CD45, or CD66, early RIT efforts have focused on β-emitters, including iodine-131 (131I) and yttrium-90, mostly to intensify conditioning therapy before allogeneic hematopoietic cell transplantation (HCT). An 131I-labeled CD45 antibody (Iomab-B [apamistamab-I131]) is currently studied in the registration-type phase 3 SIERRA trial (NCT02665065) for this purpose. Of growing interest as therapeutic payloads are α-particle emitting radionuclides such as actinium-225 (225Ac) or astatine-211 (211At) since they deliver substantially higher decay energies over a much shorter distance than β-emitters, rendering them more suitable for precise, potent, and efficient target cell killing while minimizing toxicity to surrounding bystander cells, possibly allowing use outside of HCT. Clinical efforts with 211At-labeled CD45 antibodies and 225Ac-labeled CD33 antibodies (e.g. 225Ac-lintuzumab [Actimab-A]) are ongoing.

EXPERT OPINION

A first anti-AML RIT may soon become available. This might propel further work to develop RIT-based treatments for AML, with many such efforts already ongoing.

A deep learning model (FociRad) for automated detection of γ-H2AX foci and radiation dose estimation

DNA double-strand breaks (DSBs) are the most lethal form of damage to cells from irradiation. γ-H2AX (phosphorylated form of H2AX histone variant) has become one of the most reliable and sensitive biomarkers of DNA DSBs. However, the γ-H2AX foci assay still has limitations in the time consumed for manual scoring and possible variability between scorers. This study proposed a novel automated foci scoring method using a deep convolutional neural network based on a You-Only-Look-Once (YOLO) algorithm to quantify γ-H2AX foci in peripheral blood samples. FociRad, a two-stage deep learning approach, consisted of mononuclear cell (MNC) and γ-H2AX foci detections. Whole blood samples were irradiated with X-rays from a 6 MV linear accelerator at 1, 2, 4 or 6 Gy. Images were captured using confocal microscopy. Then, dose-response calibration curves were established and implemented with unseen dataset. The results of the FociRad model were comparable with manual scoring. MNC detection yielded 96.6% accuracy, 96.7% sensitivity and 96.5% specificity. γ-H2AX foci detection showed very good F1 scores (> 0.9). Implementation of calibration curve in the range of 0-4 Gy gave mean absolute difference of estimated doses less than 1 Gy compared to actual doses. In addition, the evaluation times of FociRad were very short (< 0.5 min per 100 images), while the time for manual scoring increased with the number of foci. In conclusion, FociRad was the first automated foci scoring method to use a YOLO algorithm with high detection performance and fast evaluation time, which opens the door for large-scale applications in radiation triage.

Therapeutic Applications of Pretargeting

Targeted therapies, such as radioimmunotherapy (RIT), present a promising treatment option for the eradication of tumor lesions. RIT has shown promising results especially for hematologic malignancies, but the therapeutic efficacy is limited by unfavorable tumor-to-background ratios resulting in high radiotoxicity. Pretargeting strategies can play an important role in addressing the high toxicity profile of RIT. Key to pretargeting is the concept of decoupling the targeting vehicle from the cytotoxic agent and administrating them separately. Studies have shown that this approach has the ability to enhance the therapeutic index as it can reduce side effects caused by off-target irradiation and thereby increase curative effects due to higher tolerated doses. Pretargeted RIT (PRIT) has been explored for imaging and treatment of different cancer types over the years. This review will give an overview of the various targeted therapies in which pretargeting has been applied, discussing PRIT with alpha- and beta-emitters and as part of combination therapy, plus its use in drug delivery systems.

Scale-up of high specific activity 186gRe production using graphite-encased thick 186W targets and demonstration of an efficient target recycling process

Production of high specific activity 186gRe is of interest for development of theranostic radiopharmaceuticals. Previous studies have shown that high specific activity 186gRe can be obtained by cyclotron irradiation of enriched 186W via the 186W(d,2n)186gRe reaction, but most irradiations were conducted at low beam currents and for short durations. In this investigation, enriched 186W metal targets were irradiated at high incident deuteron beam currents to demonstrate production rates and contaminants produced when using thick targets. Full-stopping thick targets, as determined using SRIM, were prepared by uniaxial pressing of powdered natural abundance W metal or 96.86% enriched 186W metal encased between two layers of graphite flakes for target material stabilization. An assessment of structural integrity was made on each target preparation. To assess the performance of graphite-encased thick 186W metal targets, along with the impact of encasing on the separation chemistry, targets were first irradiated using a 22 MeV deuteron beam for 10 min at 10, 20, and 27 μA, with an estimated nominal deuteron energy of 18.7 MeV on the 186W target material (after energy degradation correction from top graphite layer). Gamma-ray spectrometry was performed post EOB on all targets to assess production yields and radionuclidic byproducts. The investigation also evaluated a method to recover and recycle enriched target material from a column isolation procedure. Material composition analyses of target materials, pass-through/wash solutions and recycling process isolates were conducted with SEM, FTIR, XRD, EDS and ICP-MS spectrometry. To demonstrate scaled-up production, a graphite-encased 186W target made from recycled 186W was irradiated for ~2 h with 18.7 MeV deuterons at a beam current of 27 μA to provide 0.90 GBq (24.3 mCi) of 186gRe, decay-corrected to the end of bombardment. ICP-MS analysis of the isolated 186gRe solution provided data that indicated the specific activity of 186gRe in this scaled-up production run was 2.6±0.5 GBq/μg (70±10 Ci/mg).

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.

Radiolabeled anti-CD45 antibody with reduced-intensity conditioning and allogeneic transplantation for younger patients with advanced acute myeloid leukemia or myelodysplastic syndrome

We treated patients under age 50 years with iodine-131 ((131)I)-anti-CD45 antibody combined with fludarabine and 2 Gy total body irradiation to create an improved hematopoietic cell transplantation (HCT) strategy for advanced acute myeloid leukemia or high-risk myelodysplastic syndrome patients. Fifteen patients received 332 to 1561 mCi of (131)I, delivering an average of 27 Gy to bone marrow, 84 Gy to spleen, and 21 Gy to liver. Although a maximum dose of 28 Gy was delivered to the liver, no dose-limiting toxicity was observed. Marrow doses were arbitrarily capped at 43 Gy to avoid radiation-induced stromal damage; however, no graft failure or evidence of stromal damage was observed. Twelve patients (80%) developed grade II graft-versus-host disease (GVHD), 1 patient developed grade III GVHD, and no patients developed grade IV GVHD during the first 100 days after HCT. Of the 12 patients with chronic GVHD data, 10 developed chronic GVHD, generally involving the skin and mouth. Six patients (40%) are surviving after a median of 5.0 years (range, 4.2 to 8.3 years). The estimated survival at 1 year was 73% among the 15 treated patients. Eight patients relapsed, 7 of whom subsequently died. The median time to relapse among these 8 patients was 54 days (range, 26 to 1364 days). No cases of nonrelapse mortality were observed in the first year after transplantation. However, 2 patients died in remission from complications of chronic GVHD and cardiomyopathy, at 18 months and 14 months after transplantation, respectively. This study suggests that patients may tolerate myeloablative doses >28 Gy delivered to the liver using (131)I-anti-CD45 antibody in addition to standard reduced-intensity conditioning. Moreover, the arbitrary limit of 43 Gy to the marrow may be unnecessarily conservative, and continued escalation of targeted radioimmunotherapy doses may be feasible to further reduce relapse.

Bioorthogonal chemistry: implications for pretargeted nuclear (PET/SPECT) imaging and therapy

Due to their rapid and highly selective nature, bioorthogonal chemistry reactions are attracting a significant amount of recent interest in the radiopharmaceutical community. Over the last few years, reactions of this type have found tremendous utility in the construction of new radiopharmaceuticals and as a method of bioconjugation. Furthermore, reports are beginning to emerge in which these reactions are also being applied in vivo to facilitate a novel pretargeting strategy for the imaging and therapy of cancer. The successful implementation of such an approach could lead to dramatic improvements in image quality, therapeutic index, and reduced radiation dose to non-target organs and tissues. This review will focus on the potential of various bioorthogonal chemistry reactions to be used successfully in such an approach.

Radiolabeled antibodies directed at CD45 for conditioning prior to allogeneic transplantation in acute myeloid leukemia and myelodysplastic syndrome

While allogeneic hematopoietic cell transplantation (HCT) may offer the best chance of cure for patients suffering from aggressive hematological malignancies such as acute myeloid leukemia, acute lymphoblastic leukemia, and myelodysplastic syndrome, successful outcomes for the subgroup of patients with high-risk disease remain disappointing and lag behind those of lower-risk patients. Because relatively high rates of relapse are an important contributor to these poor outcomes, efforts have explored approaches to increase the cytotoxic effects of treatment. Relapse rates have been shown to improve with the addition of increased doses of total body irradiation (TBI) and/or the introduction of additional chemotherapy to a HCT conditioning regimen. However, the increase in TBI dose and/or additional chemotherapy has also been associated with a significant increase in life-threatening toxicities, resulting in no change in overall survival. Radioimmunotherapy (RIT) has been employed as an adjunct to HCT where targeted delivery of radiation may allow for further escalation of therapy to reduce relapse with minimal toxicity. In this review we describe these efforts, including the benefits of escalating the dose of radiation to sites of hematologic disease prior to HCT, the various cellular targets for antibody-mediated delivery of radiation, as well as the rationale for incorporation of various radionuclides such as alpha emitters and beta emitters into the preparative regimen prior to HCT. Lastly, newer novel approaches such as pretargeted RIT (PRIT) are described as a method to further increase delivery of targeted radiation to hematological tissues while sparing noninvolved organs.

Anti-CD45 pretargeted radioimmunotherapy using bismuth-213: high rates of complete remission and long-term survival in a mouse myeloid leukemia xenograft model

Pretargeted radioimmunotherapy (PRIT) using an anti-CD45 antibody (Ab)-streptavidin (SA) conjugate and DOTA-biotin labeled with β-emitting radionuclides has been explored as a strategy to decrease relapse and toxicity. α-emitting radionuclides exhibit high cytotoxicity coupled with a short path length, potentially increasing the therapeutic index and making them an attractive alternative to β-emitting radionuclides for patients with acute myeloid leukemia. Accordingly, we have used (213)Bi in mice with human leukemia xenografts. Results demonstrated excellent localization of (213)Bi-DOTA-biotin to tumors with minimal uptake into normal organs. After 10 minutes, 4.5% ± 1.1% of the injected dose of (213)Bi was delivered per gram of tumor. α-imaging demonstrated uniform radionuclide distribution within tumor tissue 45 minutes after (213)Bi-DOTA-biotin injection. Radiation absorbed doses were similar to those observed using a β-emitting radionuclide ((90)Y) in the same model. We conducted therapy experiments in a xenograft model using a single-dose of (213)Bi-DOTA-biotin given 24 hours after anti-CD45 Ab-SA conjugate. Among mice treated with anti-CD45 Ab-SA conjugate followed by 800 μCi of (213)Bi- or (90)Y-DOTA-biotin, 80% and 20%, respectively, survived leukemia-free for more than 100 days with minimal toxicity. These data suggest that anti-CD45 PRIT using an α-emitting radionuclide may be highly effective and minimally toxic for treatment of acute myeloid leukemia.

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.

Dose intensity of preparative regimens for acute myeloid leukemia - one-size-fits-all or tailor-made?

Both toxicity and leukemia eradication correlate with the intensity of the transplant preparative regimen. How much any specific patient benefits from increased dose intensity depends, in part, on his/her ability to tolerate higher-dose therapy and the status of the leukemia being treated. Newer tools that include not only age and performance status, but also comorbidity indices, are useful for predicting the ability of a patient to tolerate therapy and should allow for the development of models predicting risk/benefit ratios for the use of more or less intense preparative regimens for any specific patient. If attempts are made to build such models, recent transplant data should be used, since advances in ancillary transplant measures (including better approaches to infection management and treatment of graft-vs-host disease) have led to improved transplant outcomes over the last decade. Ultimately, the goal should be to create preparative regimens with greater antileukemic intensity but with minimal extramedullary toxicity.

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.

Recombinant bispecific monoclonal antibodies prepared by the dock-and-lock strategy for pretargeted radioimmunotherapy

The selective delivery of therapeutic radionuclides is a promising approach for treating cancer. Antibody-targeted radionuclides are of particular interest, with 2 products approved for the treatment of certain forms of non-Hodgkin lymphoma. However, for many other cancers, radioimmunotherapy has been ineffective, being limited by prolonged exposure to the highly radiosensitive bone marrow. An alternative approach, known as pretargeting, separates radionuclide from the antibody, allowing the radiation to be delivered on a small molecule that can quickly and efficiently migrate into the tumor, and then rapidly clear from the body with minimal retention in tissues. Several pretargeting methods have been developed that differ in the way they selectively capture the radionuclide. This review focuses on the development of a novel form of bispecific monoclonal antibody (bsMAb) pretargeting that uses a unique radiolabeled hapten-peptide system that can be modified to bind numerous therapeutic and imaging radionuclides. Together with a specialized recombinant humanized bsMAb prepared with by a technique known as the Dock-and-Lock method, this pretargeting procedure has been examined in many different animal models, showing a high level of sensitivity and specificity for localizing tumors, and improved efficacy with less hematologic toxicity associated with directly radiolabeled IgG. The bsMAb is a tri-Fab structure, having 2 binding arms for the tumor antigen and 1 capable of binding a hapten-peptide. Preclinical studies were preformed to support the clinical use of a bsMAb and a hapten-peptide bearing a single DOTA moiety (IMP-288). A phase 0 trial found an (131)I-tri-Fab bsMAb, TF2, that targets carcinoembryonic antigen was stable in vivo, quickly clears from the blood, and localizes known tumors. The first-in-patient pretargeting experience with the (111)In-IMP-288 also observed rapid clearance and low tissue (kidney) retention, as well as localization of tumors, providing initial promising evidence for developing these materials for radioimmunotherapy.

Optimising the conditioning regimen for acute myeloid leukaemia

The conditioning regimen administered prior to allogeneic transplantation for acute myeloid leukaemia (AML) must be sufficiently immunosuppressive to ensure engraftment, and contributes to the anti-leukaemic impact of the procedure. A broad spectrum of regimens have been studied, varying in their intensity, whether high-dose or reduced intensity, and in the agents used, containing total body irradiation (TBI) plus cyclophosphamide, fludarabine, busulfan and/or anti-thymocyte globulin. Over the past 2 decades, research has influenced the way conditioning regimens are applied. Newer research shows that targeted radiotherapy using an anti-CD45 antibody should be able to reduce toxicity, improve tumour cell kill and thereby improve results.

Endoradiotherapy in cancer treatment--basic concepts and future trends

Endoradiotherapy represents an alternative therapeutic method in cancer treatment with advantageous features compared to chemotherapy and radiation therapy. Intelligent dose delivery concepts using small drugs, peptides or antibodies as radionuclide carriers enable the verification of a selective accumulation in the tumour lesion and to reduce radiation toxicity for the peripheral organs. The development of endoradiotherapeutic agents, especially chelator-conjugated biomolecules, for example ibritumomab tiuxetan or DOTATOC, gains importance due to the stable complexation of versatile radiometals, such as (90)Y or (177)Lu. The rational design of novel target binding sides and their grafting into a drug scaffold is a highly promising strategy, which may promote further implication in endoradiotherapy. This review highlights the basic concepts of endoradiotherapy and discusses the potential of targeted therapy and the properties of energy-rich particles emitted by radionuclides for tumour therapy.

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.