Auger electrons for cancer therapy - a review

Abstract

Background: Auger electrons (AEs) are very low energy electrons that are emitted by radionuclides that decay by electron capture (e.g. ¹¹¹In, ⁶⁷Ga, ^99mTc, ^195mPt, ¹²⁵I and ¹²³I). 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. ¹²⁵I-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 (¹²⁵I and ¹²³I) or radiometal chelation (¹¹¹In, ⁶⁷Ga, ^99mTc). 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 ¹¹¹In-DTPA-octreotide and ¹²⁵I-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 ¹²⁵I-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.

Keywords: 111In; Auger electrons; Cancer treatment; Clinical studies; Dosimetry; Monoclonal antibodies; Nanoparticles; Peptides; Preclinical studies; Radiolabelling.

Fig. 1

Energy contribution of γ-photons, X-rays, β-particles, internal conversion (IC) electrons, and Auger electrons (AEs) per decay event for several radionuclides. Energy estimates are based on MIRD Radionuclide Data and Decay Schemes (Eckerman and Endo 2008), and the National Nuclear Data Center for ¹⁶¹Tb (65-Terbium-161 2011). ^* Number of K- and L- shell Auger electrons only

Fig. 2

Auger electron emission can be initiated by electron capture (EC) or internal conversion (IC). In EC, protons capture an inner (K) orbital electron resulting in a primary electron vacancy. This vacancy Is filled by decay of a lower energy electron of a higher orbital (i.e. L-shell). The difference in the electronic binding energy of the two orbitals can either (a) result in the emission of a characteristic X-ray of energy equal to the electronic transition energy (E_L-E_K) or (b) be transferred to an electron of lower binding energy (E_b), imparting it with kinetic energy upon ejection from the atom as an Auger electron. Progressive higher shell vacancies occur in the electron shells due to these electron transitions (open circles). c IC occurs in the de-excitation of unstable nuclei that impart sufficient energy to an electron to result in its ejection as an IC electron with high energy, also resulting in an inner orbital vacancy

Fig. 3

Modes of cell death caused by Auger electron (AE) emission. AEs may cause DNA double-strand breaks (DSBs) by a direct effect or through an indirect effect mediated by hydroxyl free radicals (ROS) due to interaction with water molecules. AEs may also cause cell membrane damage leading to cell death. There is a localised short-range “cross-dose” effect of AEs on cancer cells which are directly adjacent to targeted cells, and a longer range “bystander” effect on more distant cells

Fig. 4

Outline of the steps to acquire time-integrated radioactivity (Ã) in vivo in tumours and normal organs, and in vitro in cancer cells for macro- or micro-dosimetry estimates, respectively

Fig. 5

Representative phantoms used in Monte Carlo Simulation for S-values of human organs, important kidney regions and subcellular compartments in a hexahedrally closely packed monolayer of cells: (a) a voxel-based realistic female phantom applied by OLINDA/EXM version 2.0 software (Stabin and Siegel 2018). b Kidney created using geometric shapes to represent renal pelvis, renal cortex and medullary pyramids (Bouchet 2003). c Cellular phantom with cell and cell nucleus represented by co-centric spheres with cell and nucleus radius of R_C and R_N, respectively (Cai et al. 2017)

Fig. 6

Examples of targeting vehicles for delivery of Auger electron (AE)-emitting radionuclides to cancer cells. Non-internalising monoclonal antibodies (mAbs) or peptides bind to cell-surface receptors and may kill cells through damaging the cell membrane. Internalising mAbs or peptides may be transported to the cell nucleus by appending nuclear localisation sequence (NLS) peptides or by an endogenous NLS present in some cell-surface receptors (e.g. EGFR or HER2) or combine internalising, endosomal escape and nuclear localizing sequences (modular nanotransporters). Nuclear localisation causes lethal DNA double-strand breaks (DSBs). Micelles or gold nanoparticles may be modified with mAbs or peptides to target cell surface receptors or are internalised non-specifically into cancer cells by endocytosis. ¹²⁵I-labelled anthracyclines diffuse into cancer cells and intercalate into DNA, while ¹²⁵I-2-iododeoxyuridine (¹²⁵I-IUdR) is taken up by nucleoside transporters and incorporated into DNA. These agents cause lethal DNA DSBs

Fig. 7

¹¹¹In-NLS-trastuzumab is an example of a targeted Auger electron (AE)-emitting radioimmunotherapeutic (RIT) agent composed of the anti-HER2 monoclonal antibody, trastuzumab modified with nuclear translocation sequence (NLS in red) peptides and benzylisothiocyanate DTPA (BzDTPA) to complex ¹¹¹In. ¹¹¹In-NLS-trastuzumab is internalised by HER2-overexpressing breast cancer cells and is transported to the cell nucleus, where the AEs cause lethal DNA double-strand breaks (DSBs) (Costantini et al. 2007)

Fig. 8

a Treatment of athymic mice with subcutaneous HER2-positive MDA-MB-361 human breast cancer xenografts with a single injection of ¹¹¹In-NLS-trastuzumab (9.25 MBq; 4 mg/kg) significantly slowed tumour growth compared to control mice receiving unlabelled trastuzumab (4 mg/kg) or normal saline. b Treatment of tumour-bearing mice with two injections of ¹¹¹In-NLS-trastuzumab (9.25 MBq; 4 mg/kg) significantly prolonged survival compared to control mice receiving unlabelled trastuzumab or normal saline (Costantini et al. 2010)

Fig. 9

a ¹¹¹In-trastuzumab-AuNPs are an example of an Auger electron (AE)-emitting radiation nanomedicine composed of gold nanoparticles (AuNPs; 30 nm) modified with 2 kDa polyethylene glycol (PEG) chains to stabilise the AuNPs and longer 5 kDa PEG chains conjugated to trastuzumab to bind HER2 or to DTPA to complex ¹¹¹In. b Dark-field and fluorescence microscopy demonstrating peri-nuclear localisation (nucleus is stained blue with DAPI) of ¹¹¹In-trastuzumab-AuNPs (yellow) in HER2-positive SK-BR-3 human breast cancer cells likely mediated by an endogenous nuclear localisation sequence (NLS) peptide in HER2. c DNA double-strand breaks (DSBs; bright foci) detected by immunofluorescence for γH2AX in the nucleus of SK-BR-3 cells exposed to ¹¹¹In-trastuzumab-AuNPs mediated by emission of AEs by ¹¹¹In. d Local intratumoural (i.t.) injection of ¹¹¹In-trastuzumab-AuNPs (10 MBq) in athymic mice with subcutaneous HER2-positive MDA-MB-361 human breast cancer xenografts arrested tumour growth compared to untreated mice (left panel) with no change in body weight (right panel) indicating no generalised normal tissue toxicity (Cai et al. 2016)

Fig. 10

a Treatment of athymic mice with subcutaneous EGFR-positive MDA-MB-468 human breast cancer xenografts with 5 weekly amounts of ¹¹¹In-DTPA-hEGF (cumulative dose, 92.5 MBq; 17 μg). Auger electron (AE) radiotherapy was effective compared to control mice treated with normal saline, but the growth of smaller, non-established tumours (right panel) was more strongly inhibited than larger tumours (left panel) (Chen et al. 2003). b SPECT and corresponding CT images of two patients at 24 h after injection of ¹¹¹In-DTPA-hEGF in a Phase 1 clinical trial, demonstrating uptake into a recurrent primary breast cancer (left panels) or a lung metastasis (right panels) (Vallis et al. 2014)