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Radioimmunotherapy (RIT) with anti-CD123 monoclonal antibody CSL360 modified with nuclear translocation sequence (NLS) peptides and labeled with the Auger electron-emitter, 111In (111In-NLS-CSL360) was studied in the prevalent NOD/SCID mouse AML engraftment assay. Significant decreases in CD123+ leukemic cells and impairment of leukemic stem cell self-renewal were achieved with high doses of RIT. However, NOD/SCID mice were very radiosensitive to these doses. At low non-toxic treatment doses, 111In-NLS-CSL360 demonstrated a trend towards improved survival associated with decreased spleen/body weight ratio, an indicator of leukemia burden, and almost complete eradication of leukemia from the bone marrow in some mice.

Introduction: The biological consequences of absorbed radiation doses are ill-defined for radiopharmaceuticals, unlike for external beam radiotherapy (EBRT). A reliable assay that assesses the biological consequences of any radionuclide is much needed. Here, we evaluated the cell-free plasmid DNA assay to determine the relative biological effects of radionuclides such as Auger electron-emitting [ 67 Ga]GaCl 3 or [ 111 In]InCl 3 compared to EBRT., Methods: Supercoiled pBR322 plasmid DNA (1.25 or 5 ng/μL) was incubated with 0.5 or 1 MBq [ 67 Ga]GaCl 3 or [ 111 In]InCl 3 for up to 73 h or was exposed to EBRT ( 137 Cs; 5 Gy/min; 0-40 Gy). The induction of relaxed and linear plasmid DNA, representing single and double strand breaks, respectively, was assessed by gel electrophoresis. Chelated forms of 67 Ga were also investigated using DOTA and THP. Topological conversion rates for supercoiled-to-relaxed (k sr x ) or relaxed-to-linear (k rl x ) DNA were obtained by fitting a kinetic model., Results: DNA damage increased both with EBRT dose and incubation time for [ 67 Ga]GaCl 3 and [ 111 In]InCl 3 . Damage caused by [ 67 Ga]GaCl 3 decreased when chelated. [ 67 Ga]GaCl 3 proved more damaging than [ 111 In]InCl 3 ; 1.25 ng/μL DNA incubated with 0.5 MBq [ 67 Ga]GaCl 3 for 2 h led to a 70% decrease of intact plasmid DNA as opposed to only a 19% decrease for [ 111 In]InCl 3 . For both EBRT and radionuclides, conversion rates were slower for 5 ng/μL than 1.25 ng/μL plasmid DNA. DNA damage caused by 1 Gy EBRT was the equivalent to damage caused by 0.5 MBq unchelated [ 67 Ga]GaCl 3 and [ 111 In]InCl 3 after 2.05 ± 0.36 and 9.3 ± 0.77 h of incubation, respectively., Conclusions: This work has highlighted the power of the plasmid DNA assay for a rapid determination of the relative biological effects of radionuclides compared to external beam radiotherapy. It is envisaged this approach will enable the systematic assessment of imaging and therapeutic radionuclides, including Auger electron-emitters, to further inform radiopharmaceutical design and application., (Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.)

Introduction: Thallium-201 is a radionuclide that has previously been used clinically for myocardial perfusion scintigraphy. Although in this role it has now been largely replaced by technetium-99 m radiopharmaceuticals, thallium-201 remains attractive in the context of molecular radionuclide therapy for cancer micrometastases or single circulating tumour cells. This is due to its Auger electron (AE) emissions, which are amongst the highest in total energy and number per decay for AE-emitters. Currently, chemical platforms to achieve this potential through developing thallium-201-labelled targeted radiopharmaceuticals are not available. Here, we describe convenient methods to oxidise [ 201 Tl]Tl(I) to chelatable [ 201 Tl]Tl(III) and identify challenges in stable chelation of thallium to support future synthesis of effective [ 201 Tl]-labelled radiopharmaceuticals., Methods: A plasmid pBR322 assay was carried out to determine the DNA damaging properties of [ 201 Tl]Tl(III). A range of oxidising agents (ozone, oxygen, hydrogen peroxide, chloramine-T, iodogen, iodobeads, trichloroisocyanuric acid) and conditions (acidity, temperature) were assessed using thin layer chromatography. Chelators EDTA, DTPA and DOTA were investigated for their [ 201 Tl]Tl(III) radiolabelling efficacy and complex stability., Results: Isolated plasmid studies demonstrated that [ 201 Tl]Tl(III) can induce single and double-stranded DNA breaks. Iodo-beads, iodogen and trichloroisocyanuric acid enabled more than 95% conversion from [ 201 Tl]Tl(I) to [ 201 Tl]Tl(III) under conditions compatible with future biomolecule radiolabelling (mild pH, room temperature and post-oxidation removal of oxidising agent). Although chelation of [ 201 Tl]Tl(III) was possible with EDTA, DTPA and DOTA, only radiolabeled DOTA showed good stability in serum., Conclusions: Decay of [ 201 Tl]Tl(III) in proximity to DNA causes DNA damage. Iodobeads provide a simple, mild method to convert thallium-201 from a 1+ to 3+ oxidation state and [ 201 Tl]Tl(III) can be chelated by DOTA with moderate stability. Of the well-established chelators evaluated, DOTA is most promising for future molecular radionuclide therapy using thallium-201; nevertheless, a new generation of chelating agents offering resistance to reduction and dissociation of [ 201 Tl]Tl(III) complexes is required., (Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.)

Introduction: Treatment of glioblastomas (GBM) using the Auger electron emitting compound [ 125 I]5-Iodo-2'-deoxyuridine ([ 125 I]I-UdR), combined with the thymidylate synthase inhibitor methotrexate (MTX) and concomitant chemotherapy with temozolomide (TMZ) has recently shown very promising therapeutic effects in vitro and in vivo in animals. The aim of the current study was to investigate if the therapeutic effects of this multimodal treatment strategy could be further increased by the thymidylate synthase inhibitor, 5-fluoro-2'-deoxyuridine (F-UdR), in comparison to MTX, and if the co-treatment should be given in a neoadjuvant or adjuvant setting., Methods: A patient-derived GBM cancer stem cell (CSC)-enriched cell line, grown as neurospheres, was employed to evaluate DNA-incorporation of [ 125 I]I-UdR, determined by a DNA precipitation assay, using either pre-treatment or co-treatment with MTX or F-UdR. The therapeutic effects in the CSC-enriched cell line after exposure to various combinations of MTX, F-UdR, TMZ and [ 125 I]I-UdR were also investigated by a CellTiter-Blue assay., Results: The highest general increase in [ 125 I]I-UdR incorporation was observed with F-UdR co-treatment, which resulted in approx. 2.5-fold increase in the DNA-associated activity. Also the cell viability was significantly decreased when F-UdR was combined with [ 125 I]I-UdR compared to [ 125 I]I-UdR alone at all activity concentrations tested. MTX was redundant when combined with 400 and 500 Bq/ml [ 125 I]I-UdR. TMZ was effective in combination with either [ 125 I]I-UdR alone or with both thymidylate synthase inhibitors combined with 50-100 Bq/ml [ 125 I]I-UdR., Conclusions: Overall, our study revealed a higher incorporation and therapeutic effect of [ 125 I]I-UdR when GBM cells were co-treated with F-UdR compared to MTX. The therapeutic effects were further increased when TMZ was combined with [ 125 I]I-UdR in combination with the thymidylate synthase inhibitors., Advances in Knowledge and Implications for Patient Care: Auger electron therapy in combination with thymidylate synthase inhibition and concomitant chemotherapy has the potential to become a future therapeutic treatment option for patients with glioblastoma., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Production of medical radionuclides with ISOL facilities is a unique production method that may provide access to preclinical quantities of some rare and potent radionuclides for nuclear medicine. Particularly attention over the past years was focused on several promising candidates for Targeted Radionuclides Therapy (TRT). With this review, we provide some perspectives of using the TRIUMF ISOL facility (ISAC) to produce medical radionuclides for TRT application and highlight our current effort to collect of 165 Er and 155 Tb for Auger Therapy and SPECT imaging, respectively., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Introduction: Our objective was to evaluate the effectiveness and normal tissue toxicity of nimotuzumab labeled with the Auger electron (AE)-emitter, 111 In ([ 111 In]In-Bn-DTPA-nimotuzumab) for radioimmunotherapy (RIT) of human triple-negative breast cancer (TNBC) or trastuzumab-resistant HER2-positive BC tumors overexpressing epidermal growth factor receptors (EGFR) in athymic mice., Methods: Normal tissue toxicity was studied in non-tumor-bearing Balb/c mice i.v. administered 9.0 or 28.6 MBq (3 mg/kg) of [ 111 In]In-Bn-DTPA-nimotuzumab, unlabeled nimotuzumab (3 mg/kg) or normal saline. A complete blood cell count (CBC) and serum alanine aminotransferase (ALT) and creatinine (Cr) were measured at 14 days. Body weight was monitored. RIT studies were performed in CD-1 athymic mice engrafted s.c. with MDA-MB-468 human TNBC tumors or TrR1 HER2-positive but trastuzumab-resistant BC tumors. Mice were i.v. administered two amounts (15.5 MBq; 3 mg/kg) of [ 111 In]In-Bn-DTPA-nimotuzumab separated by 14 days. Control mice received unlabeled Bn-DTPA-nimotuzumab (3 mg/kg) or anti-HER2 [ 111 In]In-Bn-DTPA-trastuzumab or normal saline. Tumor growth and body weight were measured for 6 weeks. A tumor growth index (TGI) and body weight index (BWI) were calculated to compare the tumor size and body weight post-treatment with the pre-treatment values. A tumor doubling ratio (TDR) was calculated for each treatment group compared to control mice receiving normal saline., Results: There was no loss of body weight or decreased red blood cells (RBC) or platelets (PLT) or increased serum ALT or Cr in Balb/c mice administered 9.0 or 28.6 MBq (3 mg/kg) of [ 111 In]In-Bn-DTPA-nimotuzumab compared to mice treated with unlabeled Bn-DTPA-nimotuzumab (3 mg/kg) or normal saline. There was a significant decrease in white blood cell (WBC) counts in Balb/c mice receiving 28.6 MBq but not 9.0 MBq of [ 111 In]In-Bn-DTPA-nimotuzumab. Based on these results, an administered amount of 15.5 MBq (3 mg/kg) was selected for RIT studies. Administration of two amounts (15.5 MBq; 3 mg/kg) separated by 14 days to CD-1 athymic mice with s.c. MDA-MB-468 xenografts strongly inhibited tumor growth. The TDR for mice treated with [ 111 In]In-Bn-DTPA-nimotuzumab was 2.15 compared to control mice receiving normal saline. In contrast, treatment with unlabeled Bn-DTPA-nimotuzumab or [ 111 In]In-Bn-DTPA-trastuzumab had no significant effect on tumor growth (TDR = 0.96 and 1.08, respectively). RIT with [ 111 In]In-Bn-DTPA-nimotuzumab also strongly inhibited the growth of TrR1 tumors in athymic mice (TDR = 2.13) compared to unlabeled Bn-DTPA-nimotuzumab (TDR = 0.91). There were no losses in body weight over 6 weeks in tumor bearing mice receiving [ 111 In]In-Bn-DTPA-nimotuzumab, unlabeled Bn-DTPA-nimotuzumab, [ 111 In]In-Bn-DTPA-trastuzumab or normal saline., Conclusions: [ 111 In]In-Bn-DTPA-nimotuzumab was effective for treatment of TNBC or trastuzumab-resistant HER2-positive human BC tumors in mice that overexpress EGFR at administered amounts that caused no decrease in body weight or normal tissue toxicity in non-tumor-bearing Balb/c mice., Advances in Knowledge and Implications for Patient Care: Our results suggest that Auger electron RIT with [ 111 In]In-Bn-DTPA-nimotuzumab may provide a novel therapeutic option for patients with TNBC or trastuzumab-resistant HER2-positive BC that overexpresses EGFR. The low normal tissue toxicity of this approach may allow combination with other targeted therapies such as antibody-drug conjugates (ADCs)., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Introduction: Molecular radiotherapy exploiting short-range Auger electron-emitting radionuclides has potential for targeted cancer treatment and, in particular, is an attractive option for managing micrometastatic disease. Here, an approach using chelator-trastuzumab conjugates to target radioactivity to breast cancer cells was evaluated as a proof-of-concept to assess the suitability of 67 Ga as a therapeutic radionuclide., Methods: THP-trastuzumab and DOTA-trastuzumab were synthesised and radiolabelled with Auger electron-emitters 67 Ga and 111 In, respectively. Radiopharmaceuticals were tested for HER2-specific binding and internalisation, and their effects on viability (dye exclusion) and clonogenicity of HER2-positive HCC1954 and HER2-negative MDA-MB-231 cell lines was measured. Labelled cell populations were studied by microautoradiography., Results: Labelling efficiencies for [ 67 Ga]Ga-THP-trastuzumab and [ 111 In]In-DOTA-trastuzumab were 90% and 98%, respectively, giving specific activities 0.52 ± 0.16 and 0.61 ± 0.11 MBq/μg (78-92 GBq/μmol). At 4 nM total antibody concentration and 200 × 10 3 cells/mL, [ 67 Ga]Ga-THP-trastuzumab showed higher percentage of cell association (10.7 ± 1.3%) than [ 111 In]In-DOTA-trastuzumab (6.2 ± 1.6%; p = 0.01). The proportion of bound activity that was internalised did not differ significantly for the two tracers (62.1 ± 1.4% and 60.8 ± 15.5%, respectively). At 100 nM, percentage cell binding of both radiopharmaceuticals was greatly reduced compared to 4 nM and did not differ significantly between the two (1.2 ± 1.0% [ 67 Ga]Ga-THP-trastuzumab and 0.8 ± 0.9% for [ 111 In]In-DOTA-trastuzumab). Viability and clonogenicity of HER2-positive cells decreased when each radionuclide was incorporated into cells by conjugation with trastuzumab, but not when the same level of radioactivity was confined to the medium by omitting the antibody conjugation, suggesting that 67 Ga needs to be cell-bound or internalised for a therapeutic effect. Microautoradiography showed that radioactivity bound to individual cells varied considerably within the population., Conclusions: [ 67 Ga]Ga-THP-trastuzumab reduced cell viability and clonogenicity only when cell-bound, suggesting 67 Ga holds promise as a therapeutic radionuclide as part of a targeted radiopharmaceutical. The causes and consequences of non-homogeneous uptake among the cell population should be explored., (Copyright © 2019 The Authors. Published by Elsevier Inc. All rights reserved.)

Introduction: Despite its desirable half-life and low energy Auger electrons that travel further than for other radionuclides, 67 Ga has been neglected as a therapeutic radionuclide. Here, 67 Ga is compared with Auger electron emitter 111 In as a potential therapeutic radionuclide., Methods: Plasmid pBR322 studies allowed direct comparison between 67 Ga and 111 In (1MBq) in causing DNA damage, including the effect of chelators (EDTA and DTPA) and the effects of a free radical scavenger (DMSO). The cytotoxicity of internalized (by means of delivery in the form of oxine complexes) and non-internalized 67 Ga and 111 In was measured in DU145 prostate cancer cells after a one-hour incubation using cell viability (trypan blue) and clonogenic studies. MDA-MB-231 and HCC1954 cells were also used., Results: Plasmid DNA damage was caused by 67 Ga and was comparable to that caused by 111 In; it was reduced in the presence of EDTA, DTPA and DMSO. The A 50 values (internalized activity of oxine complexes per cell required to kill 50% of cells) as determined by trypan blue staining was 1.0Bq/cell for both 67 Ga and 111 In; the A 50 values determined by clonogenic assay were 0.7Bq/cell and 0.3Bq/cell for 111 In and 67 Ga respectively. At the concentrations required to achieve these uptake levels, non-internalized 67 Ga and 111 In caused no cellular toxicity. Qualitatively similar results were found for MDA-MB-231 and HCC1954 cells., Conclusion: 67 Ga causes as much damage as 111 In to plasmid DNA in solution and shows similar toxicity as 111 In at equivalent internalized activity per cell. 67 Ga therefore deserves further evaluation for radionuclide therapy., Advances in Knowledge and Implications for Patient Care: The data presented here is at the basic level of science. If future in vivo and clinical studies are successful, 67 Ga could become a useful radionuclide with little healthy tissue toxicity in the arsenal of weapons for treating cancer., (Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.)

Introduction: Gold nanoparticles (AuNP; 30nm) were modified with polyethylene glycol (PEG) chains linked to trastuzumab for binding to HER2-positive breast cancer (BC) cells and diethylenetriaminepentaacetic acid (DTPA) for complexing the Auger electron-emitter, 111 In (trastuzumab-AuNP- 111 In). Our objective was to determine the cytotoxicity of trastuzumab-AuNP- 111 In on HER2-positive BC cells in vitro and evaluate its tumor growth inhibition properties and normal tissue toxicity in vivo following intratumoral (i.t.) injection in mice with s.c. HER2-overexpressing BC xenografts., Methods: Binding and internalization of trastuzumab-AuNP- 111 In or non-targeted AuNP- 111 In in SK-BR-3 (1-2×10 6 HER2/cell) and MDA-MB-361 (5×10 5 HER2/cell) human BC cells were studied. The surviving fraction (SF) of SK-BR-3 or MDA-MB-361 cells exposed to trastuzumab-AuNP- 111 In or AuNP- 111 In was determined. DNA double-strand breaks (DSBs) were assayed by probing for γ-H2AX. Tumor growth was monitored over 70days in CD1 athymic mice with s.c. MDA-MB-361 xenografts after i.t. injection of 10MBq (0.7mg; 2.6×10 12 AuNP) of trastuzumab-AuNP- 111 In and normal tissue toxicity was assessed by monitoring body weight, complete blood cell (CBC) counts and serum alanine aminotransferase (ALT) and creatinine (Cr)., Results: Trastuzumab-AuNP- 111 In was specifically bound by SK-BR-3 and MDA-MB-361 cells. Trastuzumab-AuNP- 111 In was more efficiently internalized than AuNP- 111 In and localized to a peri-nuclear region. The SF fraction of SK-BR-3 cells was reduced by 1.8-fold by treatment with 3nM (7MBq/mL) of trastuzumab-AuNP- 111 In. The SF of MDA-MB-361 cells was reduced by 3.7-fold at 14.4nM (33.6MBq/mL). In comparison, non-targeted AuNP- 111 In at these concentrations reduced the SF of SK-BR-3 or MDA-MB-361 cells by 1.2-fold (P=0.03) and 1.7-fold (P<0.0001), respectively. DNA DSBs were greater in SK-BR-3 and MDA-MB-361 cells exposed to trastuzumab-AuNP- 111 In compared to AuNP- 111 In, but unlabeled trastuzumab-AuNP did not increase DNA DSBs. Local i.t. injection of trastuzumab-AuNP- 111 In in CD1 athymic mice with s.c. MDA-MB-361 tumors arrested tumor growth for 70days. There was no apparent normal tissue toxicity. The radiation absorbed dose deposited in the tumor by trastuzumab-AuNP- 111 In was 60.5Gy, while normal organs received <0.9Gy., Conclusion: These results are promising for further development of trastuzumab-AuNP- 111 In as a novel Auger electron-emitting radiation nanomedicine for local treatment of HER2-positive BC., Advances in Knowledge and Implications for Patient Care: A local radiation treatment for HER2-positive BC based on AuNP modified with trastuzumab and labeled with the Auger electron-emitter, 111 In was developed and shown to arrest tumor growth with no normal tissue toxicity., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Introduction: (111)In-DTPA-NLS-CSL360 radioimmunoconjugates (RIC) recognize the overexpression of the interleukin-3 receptor α-subchain (CD123) relative to the β-subchain (CD131) on leukemia stem cells (LSC). Our aim was to study Auger electron radioimmunotherapy (RIT) of acute myeloid leukemia (AML) with (111)In-DTPA-NLS-CSL360 in non-obese diabetic severe combined immunodeficiency (NOD/SCID) mice or NOD-Rag1(null)IL2rγ(null) (NRG) mice engrafted with CD123(+) human AML-5 cells., Methods: The toxicity of three doses of (111)In-DTPA-NLS-CSL360 (3.3-4.8MBq; 11-15μg each) injected i.v. every two weeks was studied in non-engrafted NOD/SCID or NRG mice pre-treated with 200cGy of γ-radiation required for AML engraftment. Engraftment efficiency of (1-5)×10(6) cells AML-5 cells inoculated i.v. into NOD/SCID or NRG mice was assessed by flow cytometric analysis for human CD45(+) (hCD45(+)) cells in the bone marrow (BM) and spleen. AML-5 engrafted mice were treated with two or three doses (3.7MBq; 10μg each) every two weeks of (111)In-DTPA-NLS-CSL360, non-specific (111)In-DTPA-NLS-hIgG, unlabeled CSL360 (10μg) or normal saline. The percentage of hCD45(+) cells in the BM and spleen were measured at one week after completion of treatment., Results: (111)In-DTPA-NLS-CSL360 in combination with 200cGy of γ-radiation caused an initial transient decrease in body weight in NOD/SCID but not in NRG mice. There were no hematological, liver or kidney toxicities. The spleen exhibited 13-fold lower engraftment efficiency than the BM in NOD/SCID mice inoculated with 1×10(6) cells but both organs were highly (>85%) engrafted in NRG mice. Unexpectedly, (111)In-DTPA-NLS-CSL360 or non-specific (111)In-DTPA-NLS-hIgG caused a paradoxical 1.5-fold increase (P<0.0001) in the proportion of hCD45(+) cells in the BM of NOD/SCID mice compared to normal saline treated mice. (111)In-DTPA-NLS-CSL360 reduced hCD45(+) cells in the spleen by 3.0-fold compared to (111)In-DTPA-NLS-hIgG (P=0.0015) but the proportion of hCD45(+) cells was not significantly different than in normal saline treated mice. Unlabeled CSL360 decreased the percentage of hCD45(+) cells in the BM (P=0.004) or spleen (P=0.007) in NOD/SCID mice by 1.6-fold and 2.5-fold, respectively. (111)In-DTPA-NLS-CSL360 or unlabeled CSL360 did not decrease the proportion of hCD45(+) cells in the BM or spleen of NRG mice, due to a much higher leukemic burden., Conclusion: (111)In-DTPA-NLS-CSL360 and (111)In-DTPA-NLS-hIgG caused a paradoxical increase in the proportion of hCD45(+) cells in the BM of NOD/SCID mice. This may be due to a priming effect on the BM niche that promotes expansion of engrafted hCD45(+) cells, analogous to γ-radiation required for AML engraftment. There appears to be a competition between this effect and the cytotoxic effects of the Auger electrons on leukemia cells. The effectiveness of (111)In-DTPA-NLS-CSL360 on reducing hCD45(+) cells in the BM or spleen of NOD/SCID and NRG mice was dependent on the leukemic burden., Advances in Knowledge and Implications for Patient Care: This study demonstrates for the first time a paradoxical radiation priming effect of RIT on enhancing the hCD45(+) cell population in the BM and spleen of NOD/SCID or NRG mice. Our results have important implications for preclinical evaluation of radioimmunotherapies for patients with AML., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Gold nanoparticles (GNP) enhance the absorbance of photons thereby increasing emission of Auger-/photoelectrons in the nm-μm range. Yet, a major disadvantage is their diameter-dependent cellular uptake with an optimum of ~50 nm which may not offer optimal radiosensitization. A method was developed to enhance the uptake of small GNP. GNP (10nm) were linked to DNA and transferred into HeLa cells by transient transfection (GNP-DT). Treatment of cells with GNP-DT resulted in a strong perinuclear focal accumulation, whereas this was dimmer and sparser for GNP-T (lacking DNA) and close to background levels in GNP-treated cells. Only GNP-DT showed a significant radiosensitizing effect (p=0.005) on clonogenic survival using clinically relevant megavolt x-rays. Our novel method markedly increases the uptake/retention and alters the localization of small GNP in cells compared to unmodified GNP. This work finally enables studying the radiosensitizing effects of differentially sized GNP., From the Clinical Editor: In an effort to increase the radiosensitization of HeLa cells, his paper discusses a transient transfection-based method to enhance gold nanoparticle intracellular delivery., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Introduction: Leukemia stem cells (LSCs) are believed to be responsible for initiating and propagating acute myeloid leukemia (AML) and for causing relapse after treatment. Radioimmunotherapy (RIT) targeting these cells may improve the treatment of AML, but is limited by the low density of target epitopes. Our objective was to study a human polynucleotide kinase/phosphatase (hPNKP) inhibitor that interferes with DNA repair as a radiosensitizer for the Auger electron RIT agent, ¹¹¹In-NLS-7G3, which recognizes the CD123⁺/CD131⁻ phenotype uniquely displayed by LSCs., Methods: The surviving fraction (SF) of CD123⁺/CD131⁻ AML-5 cells exposed to ¹¹¹In-NLS-7G3 (33-266 nmols/L; 0.74MBq/μg) or to γ-radiation (0.25-5Gy) was determined by clonogenic assays. The effect of A12B4C3 (25 μmols/L) combined with ¹¹¹In-NLS-7G3 (16-66 nmols/L) or with γ-radiation (0.25-2Gy) on the SF of AML-5 cells was assessed. The density of DNA double-strand breaks (DSBs) in the nucleus was measured using the γ-H2AX assay. Cellular dosimetry was estimated based on the subcellular distribution of ¹¹¹In-NLS-7G3 measured by cell fractionation., Results: Binding of (111)In-NLS-7G3 to AML-5 cells was reduced by 2.2-fold in the presence of an excess (1μM) of unlabeled NLS-7G3, demonstrating specific binding to the CD123⁺/CD131⁻ epitope. ¹¹¹In-NLS-7G3 reduced the SF of AML-5 cells from 86.1 ± 11.0% at 33 nmols/L to 10.5 ± 3.6% at 266 nmols/L. Unlabeled NLS-7G3 had no significant effect on the SF. Treatment of AML-5 cells with γ-radiation reduced the SF from 98.9 ± 14.9% at 0.25Gy to 0.03 ± 0.1% at 5 Gy. A12B4C3 combined with ¹¹¹In-NLS-7G3 (16-66 nmols/L) enhanced the cytotoxicity up to 1.7-fold compared to treatment with radioimmunoconjugates alone and was associated with a 1.6-fold increase in DNA DSBs in the nucleus. A12B4C3 enhanced the cytotoxicity of γ-radiation (0.25-0.5Gy) on AML-5 cells by up to 1.5-fold, and DNA DSBs were increased by 1.7-fold. Exposure to ¹¹¹In-NLS-7G3 (66 nmols/L) delivered up to 0.6Gy to AML-5 cells., Conclusions: We conclude that A12B4C3 radiosensitized AML cells to the DNA damaging effects of ¹¹¹In-NLS-7G3. Combination treatment may increase the effectiveness for Auger electron RIT of AML targeting the LSC subpopulation., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Introduction: Low dose-rate radioimmunotherapy (RIT) using (125)I-labelled monoclonal antibodies ((125)I-mAbs) is associated with unexpected high cytotoxicity per Gy., Methods: We investigated whether this hypersensitivity was due to lack of detection of DNA damage by the targeted cells. DNA damage was measured with the alkaline comet assay, gamma-H2AX foci and the micronucleus test in p53(-/-) and p53(+/+) HCT116 cells exposed to increasing activities of internalizing anti-HER1 (125)I-mAbs or non-internalizing anti-CEA (125)I-mAbs. The expression of proteins involved in radiation response and progression of cells through the cycle were determined., Results: Cell hypersensitivity to low absorbed doses of anti-CEA (125)I-mAbs was not due to defect in DNA damage detection, since ATM (ataxia telangiectasia mutated gene), gamma-H2AX, p53 and p21 were activated in RIT-treated HCT116 cells and G2/M cell cycle arrest was observed. Moreover, the alkaline comet assay showed that DNA breaks accumulated when cells were placed at 4°C during exposure but were repaired under standard RIT conditions (37°C), suggesting that lesions detected under alkaline conditions (mostly DNA single strand breaks and alkali-labile sites) are efficiently repaired in treated cells. The level of gamma-H2AX protein corroborated by the level of foci measured in nuclei of treated cells was shown to accumulate with time thereby suggesting the continuous presence of DNA double strand breaks. This was accompanied by the formation of micronuclei., Conclusion: Hypersensitivity to non-internalizing (125)I-mAbs is not due to lack of detection of DNA damage after low absorbed dose-rates. However, DNA double strand breaks accumulate in cells exposed both to internalizing and non-internalizing (125)I-mAbs and lead to micronuclei formation. These results suggest impairment in DNA double strand breaks repair after low absorbed doses of (125)I-mAbs., (Copyright © 2014 Elsevier Inc. All rights reserved.)