Breaking Chemoresistance and Radioresistance with [213Bi]anti-CD45 Antibodies in Leukemia Cells

Mechanisms of resistance to high and low linear energy transfer radiation in myeloid leukemia cells

Low linear energy transfer (LET) ionizing radiation (IR) is an important form of therapy for acute leukemias administered externally or as radioimmunotherapy. IR is also a potential source of DNA damage. High LET IR produces structurally different forms of DNA damage and has emerged as potential treatment of metastatic and hematopoietic malignancies. Therefore, understanding mechanisms of resistance is valuable. We created stable myeloid leukemia HL60 cell clones radioresistant to either γ-rays or α-particles to understand possible mechanisms in radioresistance. Cross-resistance to each type of IR was observed, but resistance to clustered, complex α-particle damage was substantially lower than to equivalent doses of γ-rays. The resistant phenotype was driven by changes in: apoptosis; late G2/M checkpoint accumulation that was indicative of increased genomic instability; stronger dependence on homology-directed repair; and more robust repair of DNA double-strand breaks and sublethal-type damage induced by γ-rays, but not by α-particles. The more potent cytotoxicity of α-particles warrants their continued investigation as therapies for leukemia and other cancers.

A nephron-based model of the kidneys for macro-to-micro α-particle dosimetry

Targeted α-particle therapy is a promising treatment modality for cancer. Due to the short path-length of α-particles, the potential efficacy and toxicity of these agents is best evaluated by microscale dosimetry calculations instead of whole-organ, absorbed fraction-based dosimetry. Yet time-integrated activity (TIA), the necessary input for dosimetry, can still only be quantified reliably at the organ or macroscopic level. We describe a nephron- and cellular-based kidney dosimetry model for α-particle radiopharmaceutical therapy, more suited to the short range and high linear energy transfer of α-particle emitters, which takes as input kidney or cortex TIA and through a macro to micro model-based methodology assigns TIA to micro-level kidney substructures. We apply a geometrical model to provide nephron-level S-values for a range of isotopes allowing for pre-clinical and clinical applications according to the medical internal radiation dosimetry (MIRD) schema. We assume that the relationship between whole-organ TIA and TIA apportioned to microscale substructures as measured in an appropriate pre-clinical mammalian model also applies to the human. In both, the pre-clinical and the human model, microscale substructures are described as a collection of simple geometrical shapes akin to those used in the Cristy-Eckerman phantoms for normal organs. Anatomical parameters are taken from the literature for a human model, while murine parameters are measured ex vivo. The murine histological slides also provide the data for volume of occupancy of the different compartments of the nephron in the kidney: glomerulus versus proximal tubule versus distal tubule. Monte Carlo simulations are run with activity placed in the different nephron compartments for several α-particle emitters currently under investigation in radiopharmaceutical therapy. The S-values were calculated for the α-emitters and their descendants between the different nephron compartments for both the human and murine models. The renal cortex and medulla S-values were also calculated and the results compared to traditional absorbed fraction calculations. The nephron model enables a more optimal implementation of treatment and is a critical step in understanding toxicity for human translation of targeted α-particle therapy. The S-values established here will enable a MIRD-type application of α-particle dosimetry for α-emitters, i.e. measuring the TIA in the kidney (or renal cortex) will provide meaningful and accurate nephron-level dosimetry.

Actinium-225 in targeted alpha-particle therapeutic applications

Alpha particle-emitting isotopes are being investigated in radioimmunotherapeutic applications because of their unparalleled cytotoxicity when targeted to cancer and their relative lack of toxicity towards untargeted normal tissue. Actinium- 225 has been developed into potent targeting drug constructs and is in clinical use against acute myelogenous leukemia. The key properties of the alpha particles generated by 225Ac are the following: i) limited range in tissue of a few cell diameters; ii) high linear energy transfer leading to dense radiation damage along each alpha track; iii) a 10 day halflife; and iv) four net alpha particles emitted per decay. Targeting 225Ac-drug constructs have potential in the treatment of cancer.

A method to predict response of cell populations to cocktails of chemotherapeutics and radiopharmaceuticals: validation with daunomycin, doxorubicin, and the alpha particle emitter (210)Po

There is considerable interest in the use of α-emitting radionuclides in radioimmunotherapy. However, the high toxicity of α-emitting radionuclides often does not permit administration of high activities for fear of normal tissue toxicity. Accordingly, targeting procedures need to be optimized for improved tumor control and minimized normal tissue toxicity. To guide design of effective cocktails of α-emitting radiopharmaceuticals and chemotherapy drugs, approaches that can predict biological response of a cell population on a cell-by-cell basis are needed.

METHODS

Cells were concomitantly treated with the α-particle emitting radiochemical (210)Po-citrate and daunomycin, or with (210)Po-citrate and doxorubicin. The responses of the treated cell populations were measured with a colony forming assay. The nonuniform cellular incorporation of the radiochemical and drugs was determined simultaneously on a cell-by-cell basis using flow cytometry. Monte Carlo methods were used to simulate cell survival on the basis of individual cell incorporation of each cytotoxic agent and validated by direct comparison with the experimental clonogenic cell survival.

RESULTS

Both daunomycin and doxorubicin enhanced the toxicity of the α-particles with a magnitude greater than expected based on single-agent toxicities. Cell survival obtained by Monte Carlo simulation was in good agreement with clonogenic cell survival for the combination treatments.

CONCLUSION

Flow cytometry assisted Monte Carlo simulations can be used to predict toxicity of cocktails of α-emitting radiopharmaceuticals and chemotherapy drugs in a manner that takes into account the effects of nonuniform distributions of agents within cell populations.

(212)Pb-radioimmunotherapy induces G(2) cell-cycle arrest and delays DNA damage repair in tumor xenografts in a model for disseminated intraperitoneal disease

In preclinical studies, targeted radioimmunotherapy using (212)Pb-TCMC-trastuzumab as an in vivo generator of the high-energy α-particle emitting radionuclide (212)Bi is proving an efficacious modality for the treatment of disseminated peritoneal cancers. To elucidate mechanisms associated with this therapy, mice bearing human colon cancer LS-174T intraperitoneal xenografts were treated with (212)Pb-TCMC-trastuzumab and compared with the nonspecific control (212)Pb-TCMC-HuIgG, unlabeled trastuzumab, and HuIgG, as well as untreated controls. (212)Pb-TCMC-trastuzumab treatment induced significantly more apoptosis and DNA double-strand breaks (DSB) at 24 hours. Rad51 protein expression was downregulated, indicating delayed DNA double-strand damage repair compared with (212)Pb-TCMC-HuIgG, the nonspecific control. (212)Pb-TCMC-trastuzumab treatment also caused G(2)-M arrest, depression of the S phase fraction, and depressed DNA synthesis that persisted beyond 120 hours. In contrast, the effects produced by (212)Pb-TCMC-HuIgG seemed to rebound by 120 hours. In addition, (212)Pb-TCMC-trastuzumab treatment delayed open chromatin structure and expression of p21 until 72 hours, suggesting a correlation between induction of p21 protein and modification in chromatin structure of p21 in response to (212)Pb-TCMC-trastuzumab treatment. Taken together, increased DNA DSBs, impaired DNA damage repair, persistent G(2)-M arrest, and chromatin remodeling were associated with (212)Pb-TCMC-trastuzumab treatment and may explain its increased cell killing efficacy in the LS-174T intraperitoneal xenograft model for disseminated intraperitoneal disease.

(211)At-antiCD33 in NMRI nu/nu mice. Biodistribution, in vivo stability and radiotoxicity

The aim of this study is to verify the in vivo stability, to determine the biodistribution and to estimate the unspecific radiotoxicity of an (211)At-labelled CD33-antibody ((211)At-antiCD33) in mice with a view to therapeutic application in treating leukaemia.

ANIMALS, METHODS

(211)At was produced via the (209)Bi(a,2n)(211)At reaction and was linked via 3-(211)At-succinimidyl-benzoate to the antiCD33-antibody. The biodistribution and the in vivo stability in serum were determined after i.v.-injection in NMRI nu/nu-mice. For toxicity experiments, mice received either three times 315-650 kBq (211)At-antiCD33 or unlabelled antibody and NaCl-solution respectively.

RESULTS

(211)At-antiCD33 showed a characteristic biodistribution complying with the unspecific antibody retention in the reticular endothelial system. The largest proportion of radioactivity remained in blood and blood-rich tissues with a minor accumulation in the thyroid and stomach. After 21 h, >85% of activity in serum still represented intact antibody. Mice showed no difference in unspecific toxicity of (211)At-labelled antibodies over six months compared to those treated with unlabelled antibody and NaCl-solution respectively, with regard to histopathologic lesions, survival time, behaviour and haemograms.

CONCLUSION

The radiolabelling method yielded adequate in vivo stability of (211)At-antiCD33. Biodistribution with rapid elimination of free (211)At via kidneys and urine complies with requirements for targeted therapy. Activity doses potentially required for treatment do not elicit radiotoxicity to normal organs in mice. Further development is required to enhance the apparent specific activity and to verify the efficacy in an adequate animal model before phase I clinical studies in leukaemia can be envisaged.

Synthesis and evaluation of a bifunctional chelate for development of Bi(III)-labeled radioimmunoconjugates

A new bifunctional ligand C-DEPA was designed and synthesized as a component for antibody-targeted radiation therapy (radioimmunotherapy, RIT) of cancer. C-DEPA was conjugated to a tumor targeting antibody, trastuzumab, and the corresponding C-DEPA-trastuzumab conjugate was evaluated for radiolabeling kinetics with (205/6)Bi. C-DEPA-trastuzumab conjugate rapidly bound (205/6)Bi, and (205/6)Bi-C-DEPA-trastuzumab conjugate was stable in human serum for 72 h. The in vitro radiolabeling kinetics and serum stability data suggest that C-DEPA is a potential chelate for preclinical RIT applications using (212)Bi and (213)Bi.

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.

Renal uptake of bismuth-213 and its contribution to kidney radiation dose following administration of actinium-225-labeled antibody

Clinical therapeutic studies using (225)Ac-labeled antibodies have begun. Of major concern is renal toxicity that may result from the three alpha-emitting progeny generated following the decay of (225)Ac. The purpose of this study was to determine the amount of (225)Ac and non-equilibrium progeny in the mouse kidney after the injection of (225)Ac-huM195 antibody and examine the dosimetric consequences. Groups of mice were sacrificed at 24, 96 and 144 h after injection with (225)Ac-huM195 antibody and kidneys excised. One kidney was used for gamma ray spectroscopic measurements by a high-purity germanium (HPGe) detector. The second kidney was used to generate frozen tissue sections which were examined by digital autoradiography (DAR). Two measurements were performed on each kidney specimen: (1) immediately post-resection and (2) after sufficient time for any non-equilibrium excess (213)Bi to decay completely. Comparison of these measurements enabled estimation of the amount of excess (213)Bi reaching the kidney (γ-ray spectroscopy) and its sub-regional distribution (DAR). The average absorbed dose to whole kidney, determined by spectroscopy, was 0.77 (SD 0.21) Gy kBq(-1), of which 0.46 (SD 0.16) Gy kBq(-1) (i.e. 60%) was due to non-equilibrium excess (213)Bi. The relative contributions to renal cortex and medulla were determined by DAR. The estimated dose to the cortex from non-equilibrium excess (213)Bi (0.31 (SD 0.11) Gy kBq(-1)) represented ∼46% of the total. For the medulla the dose contribution from excess (213)Bi (0.81 (SD 0.28) Gy kBq(-1)) was ∼80% of the total. Based on these estimates, for human patients we project a kidney-absorbed dose of 0.28 Gy MBq(-1) following administration of (225)Ac-huM195 with non-equilibrium excess (213)Bi responsible for approximately 60% of the total. Methods to reduce renal accumulation of radioactive progeny appear to be necessary for the success of (225)Ac radioimmunotherapy.

Involvement of tyrosine phosphatase CD45 in apoptosis

CD45 is a transmembrane molecule with phosphatase activity expressed in all nucleated haematopoietic cells and plays a major role in immune cells. It is a protein tyrosine phosphatase that is essential for antigen-receptor-mediated signal transduction by regulating Src family members that initiate TCR signaling. CD45 is being attributed a new emerging role as an apoptosis regulator. Cross-linking of the extracellular portion of the CD45 by monoclonal antibodies and by galectin-1, can induce apoptosis in T and B cells. Interestingly, this phosphatase has also been involved in nuclear apoptosis induced by mitochondrial perturbing agents. Furthermore, it is involved in apoptosis induced by HIV-1. CD45 defect is implicated in various diseases such as severe-combined immunodeficiency disease (SCID), acquired immunodeficiency syndrome (AIDS), lymphoma and multiple myelomas. The understanding of the mechanisms by which CD45 regulates apoptosis would be very useful in disease treatment.

Differential gene expression triggered by highly cytotoxic alpha-emitter-immunoconjugates in gastric cancer cells

Immunoconjugates composed of the alpha-emitter (213)Bi and the monoclonal antibody d9MAb specifically target HSC45-M2 gastric cancer cells expressing mutant d9-E-cadherin. These conjugates efficiently killed tumor cells in a nude mouse peritoneal carcinomatosis model. To elucidate the molecular responses of HSC45-M2 cells to alpha-emitter irradiation, whole genome gene expression profiling was performed. For that purpose HSC45-M2 cells were incubated with lethal doses of (213)Bi-d9MAb. RNA was isolated at 6, 24 and 48 h after irradiation, transcribed into cDNA and hybridized to whole genome microarrays. Results of microarray analysis were validated using RTQ-PCR showing correspondence of approximately 90%. Following incubation with (213)Bi-d9MAb, 682-1125 genes showed upregulation and 666-1278 genes showed downregulation at one time point, each. Eight genes appeared upregulated and 12 genes downregulated throughout. Molecular functions and biological processes of differentially expressed genes were categorized according to the PANTHER database. Following (213)Bi-d9MAb irradiation also a time-dependent shift in terms of overrepresentation of biological processes was observed. Among the genes showing continuous upregulation, COL4A2, NEDD9 and C3 have not been associated with the cellular response to high LET radiation so far. The same holds true for WWP2, RFX3, HIST4H4 and JADE1 that showed continuous downregulation. According to PANTHER, three of the consistently upregulated (ITM2C, FLJ11000, MSMB) and downregulated (HCG9, GAS2L3, FLJ21439) genes, respectively, have not been associated with any biological process or molecular function so far. Thus, these findings revealed interesting new targets for selective elimination of tumor cells and new insights regarding response of tumor cells to alpha-emitter exposure.

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

Radioimmunotherapy (RIT) for treatment of hematologic malignancies frequently fails because of disease recurrence. We therefore conducted pretargeted (P)RIT studies to augment the efficacy in mice of therapy using a pretargeted anti-human (h)CD45 antibody (Ab)-streptavidin (SA) conjugate followed by a biotinylated clearing agent and radiolabeled 1,4,7,10-tetraazacylodode cane N,N',N",N'''-tetraacetic (DOTA)-biotin. Tumor-to-blood ratios at 24 hours were 20:1 using pretargeted anti-hCD45 RIT and <1:1 with conventional RIT. In vivo imaging studies confirmed that the PRIT approach provided high-contrast tumor images with minimal blood-pool activity, whereas directly labeled anti-hCD45 Ab produced distinct tumor images but the blood pool retained a large amount of labeled Ab for a prolonged time. Therapy experiments showed that (90)Y-DOTA-biotin significantly prolonged survival of mice treated with pretargeted anti-hCD45 Ab-SA compared with mice treated with conventional RIT using (90)Y-labeled anti-hCD45 Ab at 200 muCi. Because human CD45 antigens are confined to xenograft tumor cells in this model, and all murine tissues are devoid of hCD45 and will not bind anti-hCD45 Ab, we also compared one-step and PRIT using an anti-murine (m)CD45 Ab where the target antigen is present on normal hematopoietic tissues. After 24 h, 27.3% +/- 2.8% of the injected dose of activity was delivered per gram (% ID/g) of lymph node using (131)I-A20-Ab compared with 40.0 +/- 5.4% ID/g for pretargeted (111)In-DOTA-biotin. These data suggest that pretargeted methods for delivering RIT may be superior to conventional RIT when targeting CD45 for the treatment of leukemia and may allow for the intensification of therapy, while minimizing toxicities.

Enhancement of radiosensitivity by roscovitine pretreatment in human non-small cell lung cancer A549 cells

Roscovitine has been reported to have anti-proliferative properties and is in process of undergoing clinical trials. In addition to its intrinsic anticancer properties, it has recently been suggested that roscovitine may also enhance the activity of traditional chemo- and radio- therapies in certain cancer cell lines. The purpose of this study was to define the activity of roscovitine in increasing radiosensitivity of human non-small cell lung cancer (NSCLC) cell line A549 cells in vitro. A549 cells were exposed to ionizing radiation (IR) of gamma-ray with or without roscovitine pretreatment. Clonogenic assay was performed and cell cycle and apoptosis were analyzed by flow cytometry. Expression of PARP, Ku70 and Ku80 proteins was detected by Western blot. The active form of caspase-3 positive cells were measured by flow cytometry. Our results showed that roscovitine caused dose-dependent apoptosis in A549 cells. Pretreatment with minimally toxic concentration of roscovitine significantly radiosensitized A549 cells by inhibiting colony formation. We then examined potential mechanisms that may contribute to the enhanced radiation response induced by roscovitine. Our results showed that the combination treatment significantly induced apoptosis in A549 cells compared to roscovitine or IR treatment alone. Meanwhile, in the co-treatment group, the percentage of cells with the active form of caspase-3 was markedly increased, while roscovitine or IR alone had little effect. Roscovitine decreased S phase cells when used alone or in sequential combination with IR. Furthermore, this combination treatment blocked DNA repair process after IR, indicated by down regulation of Ku70 and Ku80 proteins, while the singly used treatment did not. Taken together, these results suggest that roscovitine has the potential to act as a radio-sensitizer in A549 cells by promoting caspase-3 activity and increasing apoptosis, affecting cell cycle distribution and impairing DNA repair process.

Realizing the potential of the Actinium-225 radionuclide generator in targeted alpha particle therapy applications

Alpha particle-emitting isotopes have been proposed as novel cytotoxic agents for augmenting targeted therapy. Properties of alpha particle radiation such as their limited range in tissue of a few cell diameters and their high linear energy transfer leading to dense radiation damage along each alpha track are promising in the treatment of cancer, especially when single cells or clusters of tumor cells are targeted. Actinium-225 (225 Ac) is an alpha particle-emitting radionuclide that generates 4 net alpha particle isotopes in a short decay chain to stable 209 Bi, and as such can be described as an alpha particle nanogenerator. This article reviews the literature pertaining to the research, development, and utilization of targeted 225 Ac to potently and specifically affect cancer.

Radioimmunotherapy of infection with Bi-labeled antibodies

Bismuth-213 ((213)Bi) (physical half-life 46 min) is a beta-emitter (97%) and an alpha-emitter (3%) which decays to short lived alpha-emitter Polonium-213 and could therefore be used as an in vivo generator of alpha particles with the energy of around 8 MeV. (213)Bi has been successfully used during the last decade in both clinical and pre-clinical work for radioimmunotherapy (RIT) of cancer with (213)Bi-labeled monoclonal antibodies (mAbs). RIT has been proposed as a novel techonology for treatment of infectious diseases. (213)Bi-labeled mAbs have been successfully used for treatment of experimental fungal, bacterial and viral infections with transient or none hematologic toxicity. The mechanisms of RIT of infection with (213)Bi-labeled mAbs include "direct" killing of cells and induction of apoptosis. In vivo RIT results in decrease of inflammation in infected organs. Among the delivery vehicles for RIT of infection whole IgG1 mAbs seem to be the most suitable in terms of the highest uptake in the target organs and the lowest - in normal tissues. RIT with alpha-emitter (213)Bi involves the application of established technology developed for the treatment of malignancies to infectious diseases. The development of RIT for infectious diseases is potentially easier than its application to tumor therapy given antigenic and tissue perfusion differences between sites of microbial infection and tumor infiltration. Nevertheless, considerable pre-clinical and clinical development work is likely to be required to learn how to use RIT for infection optimally.

DNA-ligase IV and DNA-protein kinase play a critical role in deficient caspases activation in apoptosis-resistant cancer cells by using doxorubicin

Resistance toward cytotoxic drugs is one of the primary causes for therapeutic failure in cancer therapy. DNA repair mechanisms as well as deficient caspases activation play a critical role in apoptosis resistance of tumor cells toward anticancer drug treatment. Here, we discovered that deficient caspases activation in apoptosis-resistant cancer cells depends on DNA-ligase IV and DNA-protein kinase (DNA-PK), playing crucial roles in the nonhomologous end joining (NHEJ) pathway, which is the predominant pathway for DNA double-strand break repair (DNA-DSB-repair) in mammalian cells. DNA-PK(+/+) as well as DNA-ligase IV (+/+) cancer cells were apoptosis resistant and deficient in activation of caspase-3, caspase-9, and caspase-8 and in cleavage of poly(ADP-ribose) polymerase after doxorubicin treatment. Inhibition of NHEJ by knocking out DNA-PK or DNA-ligase IV restored caspases activation and apoptosis sensitivity after doxorubicin treatment. In addition, inhibition of caspases activation prevented doxorubicin-induced apoptosis but could not prevent doxorubicin-induced DNA damage, indicating that induction of DNA damage is independent of caspases activation. However, caspases activation depends on induction of DNA damage left unrepaired by NHEJ-DNA-DSB-repair. We conclude that DNA damage left unrepaired by DNA-ligase IV or DNA-PK might be the initiator for caspases activation by doxorubicin in cancer cells. Failure in caspases activation using doxorubicin depends on loss of DNA damage and is due to higher rates of NHEJ-DNA-DBS-repair.

Cancer stem cell targeting using the alpha-particle emitter, 213Bi: mathematical modeling and feasibility analysis

There is increasing recognition that treatment failure in cancer may be associated with the failure to sterilize a small subpopulation of tumor cells that have been characterized as tumor stem cells. Defined as cells that are able to self-renew and also to replenish a phenotypically diverse tumor-cell population, such cells are also considered resistant to chemotherapy. These characteristics are optimal for targeting by using alpha-particle-emitting radionuclides. Because of their high-energy deposition density per track, alpha-particles are capable of targeting single cells or small clusters of cells with minimal normal organ toxicity. The DNA damage induced by alpha-particles is largely irreparable and, therefore, alpha-particle-induced damage is minimally susceptible to resistance mechanisms. In this work, theoretical modeling was performed to examine the potential of alpha-emitter targeting of such small clusters of cancer stem cells. Critical parameters influencing efficacy and toxicity were identified and their relationship elucidated. The results identify specific activity, antigen site density, and number of target cells as critical parameters for effective cell killing and demonstrate substantial efficacy gains by targeting a smaller number of stem cells, as opposed to the entire tumor-cell population.