Background: Auger electrons (AEs) are very low energy electrons that are emitted by radionuclides that decay by electron capture (e.g. 111 In, 67 Ga, 99m Tc, 195m Pt, 125 I and 123 I). This energy is deposited over nanometre-micrometre distances, resulting in high linear energy transfer (LET) that is potent for causing lethal damage in cancer cells. Thus, AE-emitting radiotherapeutic agents have great potential for treatment of cancer. In this review, we describe the radiobiological properties of AEs, their radiation dosimetry, radiolabelling methods, and preclinical and clinical studies that have been performed to investigate AEs for cancer treatment. Results: AEs are most lethal to cancer cells when emitted near the cell nucleus and especially when incorporated into DNA (e.g. 125 I-IUdR). AEs cause DNA damage both directly and indirectly via water radiolysis. AEs can also kill targeted cancer cells by damaging the cell membrane, and kill non-targeted cells through a cross-dose or bystander effect. The radiation dosimetry of AEs considers both organ doses and cellular doses. The Medical Internal Radiation Dose (MIRD) schema may be applied. Radiolabelling methods for complexing AE-emitters to biomolecules (antibodies and peptides) and nanoparticles include radioiodination (125 I and 123 I) or radiometal chelation (111 In, 67 Ga, 99m Tc). Cancer cells exposed in vitro to AE-emitting radiotherapeutic agents exhibit decreased clonogenic survival correlated at least in part with unrepaired DNA double-strand breaks (DSBs) detected by immunofluorescence for γH2AX, and chromosomal aberrations. Preclinical studies of AE-emitting radiotherapeutic agents have shown strong tumour growth inhibition in vivo in tumour xenograft mouse models. Minimal normal tissue toxicity was found due to the restricted toxicity of AEs mostly on tumour cells targeted by the radiotherapeutic agents. Clinical studies of AEs for cancer treatment have been limited but some encouraging results were obtained in early studies using 111 In-DTPAoctreotide and 125 I-IUdR, in which tumour remissions were achieved in several patients at administered amounts that caused low normal tissue toxicity, as well as promising improvements in the survival of glioblastoma patients with 125 I-mAb 425, with minimal normal tissue toxicity. Conclusions: Proof-of-principle for AE radiotherapy of cancer has been shown preclinically, and clinically in a limited number of studies. The recent introduction of many biologically-targeted therapies for cancer creates new opportunities to design novel AE-emitting agents for cancer treatment. Pierre Auger did not conceive of the application of AEs for targeted cancer treatment, but this is a tremendously exciting future that we and many other scientists in this field envision.

In this work, we employ electron spin resonance (ESR) spectroscopy to investigate the effects of temperature on excess electron and hole transfer through DNA. The competitive processes of tunneling, protonation at carbon, and hopping are investigated in hydrated DNA solids (hydrated to 14 waters/nucleotide) and frozen glassy aqueous (D 2 O) solutions of DNA intercalated with mitoxantrone (MX) at temperatures from 4 to 195 K. Monitoring the changes in the ESR signals of MX radicals, one-electron oxidized guanines (G •+ ), oneelectron reduced cytosines [C(N3)D • (CD • )], thymine anion radicals (T •-), and irreversibly deuterated thymine radicals [T(C6)D • (TD • )] with time at different temperatures allows for distinguishing the thermal barriers of each process. The tunneling of electrons from DNA radicals to MX is found to be the dominant process at temperatures less than or equal to 77 K. The value of the average tunneling distance decay constant, β avg , is found to be the same at 4 and 77 K. Working with hydrated DNA allows the distinction between electron adducts to cytosine and those to thymine, a distinction not possible in glassy systems. In the solid hydrated DNA, we find that CD • does not undergo significant electron loss in the time of our experiments below 170K and that electron tunneling in DNA is mainly from T •-. Irreversible deuteration of T •at carbon position 6, which results in TD • , begins at 130 K and increases in relative fractions of the radicals as temperature increases. Hole and electron hopping resulting in the recombination of G •+ and CD • are not substantial until temperatures near 195 K are reached. Above 130 K, the tunneling processes are not competitive with deuteration of T •-, and above 170 K, they are not competitive with recombination, which presumably results via activated excess electron hopping.

No abstract

Our aim was to evaluate the effectiveness and normal tissue toxicity of radioimmunotherapy (RIT) of s.c. PANC-1 human pancreatic cancer (PnCa) xenografts in NRG mice using anti-EGFR panitumumab linked to metal-chelating polymers (MCPs) that present 13 DOTA chelators to complex the β-emitter, 177 Lu. The clonogenic survival (CS) of PANC-1 cells treated in vitro with panitumumab-MCP-177 Lu (0.3−1.2 MBq) and DNA double-strand breaks (DSBs) in the nucleus of these cells were measured by confocal immunofluorescence microscopy for γ-H2AX. Subcellular distribution of radioactivity for panitumumab-MCP-177 Lu was measured, and absorbed doses to the cell nucleus were calculated. Normal tissue toxicity was assessed in non tumor-bearing NRG mice by monitoring body weight, complete blood cell counts (CBC), serum alanine aminotransferase (ALT), and creatinine (Cr) after i.v. injection of 6 MBq (10 μg) of panitumumab-MCP-177 Lu. RIT was performed in NRG mice with s.c. PANC-1 tumors injected i.v. with 6 MBq (10 μg) of panitumumab-MCP-177 Lu. Control mice received nonspecific human IgG-MCP-177 Lu (6 MBq; 10 μg), unlabeled panitumumab (10 μg), or normal saline. The tumor growth index (TGI) was compared. Tumor and normal organ doses were estimated based on biodistribution studies. Panitumumab-MCP-177 Lu reduced the CS of PANC-1 cells in vitro by 7.7-fold at the highest amount tested (1.2 MBq). Unlabeled panitumumab had no effect on the CS of PANC-1 cells. γ-H2AX foci were increased by 3.8-fold by panitumumab-MCP-177 Lu. Panitumumab-MCP-177 Lu deposited 3.84 Gy in the nucleus of PANC-1 cells. Administration of panitumumab-MCP-177 Lu (6 MBq; 10 μg) to NRG mice caused no change in body weight, CBC, or ALT and only a slight increase in Cr compared to NRG mice treated with normal saline. Panitumumab-MCP-177 Lu strongly inhibited tumor growth in NRG mice (TGI = 2.3 ± 0.2) compared to normal saline-treated mice (TGI = 5.8 ± 0.5; P < 0.01). Unlabeled panitumumab had no effect on tumor growth (TGI = 6.0 ± 1.6; P > 0.05). The absorbed dose of PANC-1 tumors was 12.3 Gy. The highest normal organ doses were absorbed by the pancreas, liver, spleen, and kidneys. We conclude that EGFR-targeted RIT with panitumumab-MCP-177 Lu was able to overcome resistance to panitumumab in KRAS mutant PANC-1 tumors in NRG mice and may be a promising approach to treatment of PnCa in humans.

Background Epidermal growth factor receptors (EGFR) are overexpressed on > 90% of pancreatic cancers (PnCa) and represent an attractive target for the development of novel therapies, including radioimmunotherapy (RIT). Our aim was to study RIT of subcutaneous (s.c.) PANC-1 human PnCa xenografts in mice using the anti-EGFR monoclonal antibody, panitumumab labeled with Auger electron (AE)-emitting, 111In or β-particle emitting, 177Lu at amounts that were non-toxic to normal tissues. Results Panitumumab was conjugated to DOTA chelators for complexing 111In or 177Lu (panitumumab-DOTA-[111In]In and panitumumab-DOTA-[177Lu]Lu) or to a metal-chelating polymer (MCP) with multiple DOTA to bind 111In (panitumumab-MCP-[111In]In). Panitumumab-DOTA-[177Lu]Lu was more effective per MBq exposure at reducing the clonogenic survival in vitro of PANC-1 cells than panitumumab-DOTA-[111In]In or panitumumab-MCP-[111In]In. Panitumumab-DOTA-[177Lu]Lu caused the greatest density of DNA double-strand breaks (DSBs) in the nucleus measured by immunofluorescence for γ-H2AX. The absorbed dose in the nucleus was 3.9-fold higher for panitumumab-DOTA-[177Lu]Lu than panitumumab-DOTA-[111In]In and 7.7-fold greater than panitumumab-MCP-[111In]In. No normal tissue toxicity was observed in NOD/SCID mice injected intravenously (i.v.) with 10.0 MBq (10 μg; ~ 0.07 nmoles) of panitumumab-DOTA-[111In]In or panitumumab-MCP-[111In]In or in NRG mice injected i.v. with 6.0 MBq (10 μg; ~ 0.07 nmoles) of panitumumab-DOTA-[177Lu]Lu. There was no decrease in complete blood cell counts (CBC) or increased serum alanine aminotransferase (ALT) or creatinine (Cr) or decreased body weight. RIT inhibited the growth of PANC-1 tumours but a 5-fold greater total amount of panitumumab-DOTA-[111In]In or panitumumab-MCP-[111In]In (30 MBq; 30 μg; ~ 0.21 nmoles) administered in three fractionated amounts every three weeks was required to achieve greater or equivalent tumour growth inhibition, respectively, compared to a single amount of panitumumab-DOTA-[177Lu]Lu (6 MBq; 10 μg; ~ 0.07 nmoles). The tumour doubling time (TDT) for NOD/SCID mice with s.c. PANC-1 tumours treated with panitumumab-DOTA-[111In]In or panitumumab-MCP-[111In]In was 51.8 days and 28.1 days, respectively. Panitumumab was ineffective yielding a TDT of 15.3 days vs. 15.6 days for normal saline treated mice. RIT of NRG mice with s.c. PANC-1 tumours with 6.0 MBq (10 μg; ~ 0.07 nmoles) of panitumumab-DOTA-[177Lu]Lu increased the TDT to 20.9 days vs. 11.5 days for panitumumab and 9.1 days for normal saline. The absorbed doses in PANC-1 tumours were 8.8 ± 3.0 Gy and 2.6 ± 0.3 Gy for panitumumab-DOTA-[111In]In and panitumumab-MCP-[111In]In, respectively, and 11.6 ± 4.9 Gy for panitumumab-DOTA-[177Lu]Lu. Conclusion RIT with panitumumab labeled with Auger electron-emitting, 111In or β-particle-emitting, 177Lu inhibited the growth of s.c. PANC-1 tumours in NOD/SCID or NRG mice, at administered amounts that caused no normal tissue toxicity. We conclude that EGFR-targeted RIT is a promising approach to treatment of PnCa.