New insights into the mechanism underlying the synergistic action of ionizing radiation with platinum chemotherapeutic drugs: the role of low-energy electrons

Local delivery of carboplatin-loaded hydrogel and calcium carbonate enables two-stage drug release for limited-dose radiation to eliminate mouse malignant glioma

Postoperative radiotherapy remains the gold standard for malignant glioma treatment. Clinical limitations, including tumor growth between surgery and radiotherapy and the emergence of radioresistance, reduce treatment effectiveness and result in local disease progression. This study aimed to develop a local drug delivery system to inhibit tumor growth before radiotherapy and enhance the subsequent anticancer effects of limited-dose radiotherapy. We developed a compound of carboplatin-loaded hydrogel (CPH) incorporated with carboplatin-loaded calcium carbonate (CPCC) to enable two-stage (peritumoral and intracellular) release of carboplatin to initially inhibit tumor growth and to synergize with limited-dose radiation (10 Gy in a single fraction) to eliminate malignant glioma (ALTS1C1 cells) in a C57BL/6 mouse subcutaneous tumor model. The doses of carboplatin in CPH and CPCC treatments were 150 μL (carboplatin concentration of 5 mg/mL) and 15 mg (carboplatin concentration of 4.1 μg/mg), respectively. Mice receiving the combination of CPH-CPCC treatment and limited-dose radiation exhibited significantly reduced tumor growth volume compared to those receiving double-dose radiation alone. Furthermore, combining CPH-CPCC treatment with limited-dose radiation resulted in significantly longer progression-free survival than combining CPH treatment with limited-dose radiation. Local CPH-CPCC delivery synergized effectively with limited-dose radiation to eliminate mouse glioma, offering a promising solution for overcoming clinical limitations.

Interaction of low-energy electrons with radiosensitizers

We provide an experimentalist's perspective on the present state-of-the-art in the studies of low-energy electron interactions with common radiosensitizers, including compounds used in combined chemo-radiation therapy and their model systems. Low-energy electrons are important secondary species formed during the interaction of ionizing radiation with matter. Their role in the radiation chemistry of living organisms has become an important topic for more than 20 years. With the increasing number of works and reviews in the field, we would like to focus here on a very narrow area of compounds that have been shown to have radio-sensitizing properties on the one hand, and high reactivity towards low-energy electrons on the other hand. Gas phase experiments studying electron attachment to isolated molecules and environmental effects on reaction dynamics are reviewed for modified DNA components, nitroimidazoles, and organometallics. In the end, we provide a perspective on the future directions that may be important for transferring the fundamental knowledge about the processes induced by low-energy electrons into practice in the field of rational design of agents for concomitant chemo-radiation therapy.

Platinum-Based Nanoformulations for Glioblastoma Treatment: The Resurgence of Platinum Drugs?

Current therapies for treating Glioblastoma (GB), and brain tumours in general, are inefficient and represent numerous challenges. In addition to surgical resection, chemotherapy and radiotherapy are presently used as standards of care. However, treated patients still face a dismal prognosis with a median survival below 15-18 months. Temozolomide (TMZ) is the main chemotherapeutic agent administered; however, intrinsic or acquired resistance to TMZ contributes to the limited efficacy of this drug. To circumvent the current drawbacks in GB treatment, a large number of classical and non-classical platinum complexes have been prepared and tested for anticancer activity, especially platinum (IV)-based prodrugs. Platinum complexes, used as alkylating agents in the anticancer chemotherapy of some malignancies, are though often associated with severe systemic toxicity (i.e., neurotoxicity), especially after long-term treatments. The objective of the current developments is to produce novel nanoformulations with improved lipophilicity and passive diffusion, promoting intracellular accumulation, while reducing toxicity and optimizing the concomitant treatment of chemo-/radiotherapy. Moreover, the blood-brain barrier (BBB) prevents the access of the drugs to the brain and accumulation in tumour cells, so it represents a key challenge for GB management. The development of novel nanomedicines with the ability to (i) encapsulate Pt-based drugs and pro-drugs, (ii) cross the BBB, and (iii) specifically target cancer cells represents a promising approach to increase the therapeutic effect of the anticancer drugs and reduce undesired side effects. In this review, a critical discussion is presented concerning different families of nanoparticles able to encapsulate platinum anticancer drugs and their application for GB treatment, emphasizing their potential for increasing the effectiveness of platinum-based drugs.

DNA radiosensitization by terpyridine-platinum: damage induced by 5 and 10 eV transient anions

Chemoradiation therapy (CRT), which combines a chemotherapeutic drug with ionizing radiation (IR), is the most common cancer treatment. At the molecular level, the binding of Pt-drugs to DNA sensitizes cancer cells to IR, mostly by increasing the damage induced by secondary low-energy (0-20 eV) electrons (LEEs). We investigate such enhancements by binding terpyridine-platinum (Tpy-Pt) to supercoiled plasmid DNA. Fifteen nanometer thick films of Tpy-Pt-DNA complexes in a molar ratio of 5 : 1 were irradiated with monoenergetic electrons of 5 and 10 eV, which principally attach to the DNA bases to form transient anions (TAs) decaying into a multitude of bond-breaking channels. At both energies, the effective yields of crosslinks (CLs), base damage (BD) related CLs, single and double strand breaks (SSBs and DSBs), non-DSB-cluster lesions, loss of supercoiled configuration and base lesions are 6.5 ± 1.5, 8.8± 3.0, 88 ± 11, 5.3 ± 1.3, 9.6 ± 2.2, 106 ± 17, 189 ± 31 × 10^-15 per electron per molecule, and 11.9 ± 2.6, 19.9 ± 4.4, 128 ± 18, 7.7 ± 3.0, 13.4 ± 3.9, 144 ± 19, 229 ± 42 × 10^-15 per electron per molecule, respectively. DNA damage increased 1.2-4.2-fold due to Tpy-Pt, the highest being for BD-related CLs. These enhancements are slightly higher than those obtained by the conventional Pt-drugs cisplatin, carboplatin and oxaliplatin, apart from BD-related CLs, which are about 3 times higher. Enhancements are related to the strong perturbation of the DNA helix by Tpy-Pt, its high dipole moment and its favorable binding to guanine (G), all of which increase bond-breaking via TA formation. In CRT, Tpy-Pt could considerably enhance crosslinking within genomic DNA and between DNA and other components of the nucleus, causing roadblocks to replication and transcription, particularly within telomeres, where it binds preferentially within G-quadruplexes.

In vivo behavior of [64Cu]NOTA-terpyridine platinum, a novel chemo-radio-theranostic agent for imaging, and therapy of colorectal cancer

To overcome resistance to chemotherapy for colorectal cancer, we propose to validate in vivo a novel terpyridine-platinum (TP) compound radiolabeled with the radio-theranostic isotope ⁶⁴Cu. In vivo stability, biodistribution, PET imaging, tumor growth delay, toxicity and dosimetry of [⁶⁴Cu]NOTA-C3-TP were determined. The current experimental studies show that [⁶⁴Cu]NOTA-C3-TP is stable in vivo, rapidly eliminated by the kidneys and has a promising tumor uptake ranging from 1.8 ± 0.4 to 3.0 ± 0.2 %ID/g over 48 h. [⁶⁴Cu]NOTA-C3-TP retarded tumor growth by up to 6 ± 2.0 days and improved survival relative to vehicle and non-radioactive [^NatCu]NOTA-C3-TP over 17 days of tumor growth observation. This effect was obtained with only 0.4 nmol i.v. injection of [⁶⁴Cu]NOTA-C3-TP, which delivers 3.4 ± 0.3 Gy tumoral absorbed dose. No evidence of toxicity, by weight loss or mortality was revealed. These findings confirm the high potential of [⁶⁴Cu]NOTA-TP as a novel radio-theranostic agent.

Natural Baicalein-Rich Fraction as Radiosensitizer in Combination with Bismuth Oxide Nanoparticles and Cisplatin for Clinical Radiotherapy

Purpose

Methods

Results

Conclusion

Nitro rotation tuned dissociative electron attachment upon targeted radiosensitizer 4-substituted Z bases

In this work, a set of new potential radiation sensitizers (4-substituted Z-bases: 4XZ, X = F, Cl, Br, and I) are designed based on the artificial 6-amino-5-nitro-3-(1'-β-D-2'-deoxyribofuranosyl)-2(1H)-pyridone (Z), which can selectively bind to breast cancer cells. The calculated electron affinities in water solution show that the halogenated Z-bases are efficient electron acceptors which possess significant electron-withdrawing characters following the order of 4XZ > Z ≫ U. To ensure the effective electron attachment induced dissociation, we constructed the energy profiles related to the X-C bond cleavage of neutral and anionic bases. The results show that the X-C bond becomes relatively weak after the electron attachment. In particular, the electron induced dehalogenations of (4BrZ)^- and (4IZ)^- are low-barrier and exothermic, which support a high radiosensitivity. Furthermore, we characterized the vibrational excitation effect on the dissociative electron attachment, which demonstrates that the charge distribution can be regulated by the rotation-induced structural distortion accompanied by the electron localization on the nitro group. Also examined is the influence of base pairing on the dehalogenation, which is not only conducive to the electron-driven dissociation but is also beneficial to the stabilization of related products. The current study suggests 4BrZ and 4IZ can be regarded as potential targeted radiosensitizers with possible applications in reducing the side effects in radiotherapy.

Electron attachment to isolated and microhydrated favipiravir

Electron attachment and its equivalent in complex environments, single-electron reduction, are important in many biological processes. Here, we experimentally study the electron attachment to favipiravir, a well-known antiviral agent. Electron attachment spectroscopy is used to explore the energetics of associative (AEA) and dissociative (DEA) electron attachment to isolated favipiravir. AEA dominates the interaction and the yields of the fragment anions after DEA are an order of magnitude lower than that of the parent anion. DEA primary proceeds via decomposition of the CONH2 functional group, which is supported by reaction threshold calculations using ab initio methods. Mass spectrometry of small favipiravir-water clusters demonstrates that a lot of energy is transferred to the solvent upon electron attachment. The energy gained upon electron attachment, and the high stability of the parent anion were previously suggested as important properties for the action of several electron-affinic radiosensitizers. If any of these mechanisms cause synergism in chemo-radiation therapy, favipiravir could be repurposed as a radiosensitizer.

Low-Energy Electron Damage to Condensed-Phase DNA and Its Constituents

The complex physical and chemical reactions between the large number of low-energy (0-30 eV) electrons (LEEs) released by high energy radiation interacting with genetic material can lead to the formation of various DNA lesions such as crosslinks, single strand breaks, base modifications, and cleavage, as well as double strand breaks and other cluster damages. When crosslinks and cluster damages cannot be repaired by the cell, they can cause genetic loss of information, mutations, apoptosis, and promote genomic instability. Through the efforts of many research groups in the past two decades, the study of the interaction between LEEs and DNA under different experimental conditions has unveiled some of the main mechanisms responsible for these damages. In the present review, we focus on experimental investigations in the condensed phase that range from fundamental DNA constituents to oligonucleotides, synthetic duplex DNA, and bacterial (i.e., plasmid) DNA. These targets were irradiated either with LEEs from a monoenergetic-electron or photoelectron source, as sub-monolayer, monolayer, or multilayer films and within clusters or water solutions. Each type of experiment is briefly described, and the observed DNA damages are reported, along with the proposed mechanisms. Defining the role of LEEs within the sequence of events leading to radiobiological lesions contributes to our understanding of the action of radiation on living organisms, over a wide range of initial radiation energies. Applications of the interaction of LEEs with DNA to radiotherapy are briefly summarized.

High Cytotoxic Effect by Combining Copper-64 with a NOTA-Terpyridine Platinum Conjugate

Terpyridine platinum (TP)-based chemotherapeutic agents target three-dimensional structures on DNA known as G-quadruplexes. We report the rational design and synthesis of a TP conjugate combined with copper-64 (⁶⁴Cu), the decay characteristics of which include emission of β^- and Auger electrons for radiotherapy and β⁺ particles for positron emission tomography (PET) imaging. The present experimental studies show that the novel [⁶⁴Cu]Cu-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)-TP is stable, permitting selective killing of cancer cells. The antitumor activity of [⁶⁴Cu]Cu-NOTA-TP at high apparent molar activity is in the low nanomolar range and 27,800-fold greater than that of ^natCu-NOTA-TP at 24 h post treatment. These results suggest that this combination of a cytotoxic TP agent with ⁶⁴Cu has considerable potential for cancer treatment and PET imaging.

Application of New Radiosensitizer Based on Nano-Biotechnology in the Treatment of Glioma

Glioma is the most common intracranial malignant tumor, and its specific pathogenesis has been unclear, which has always been an unresolved clinical problem due to the limited therapeutic window of glioma. As we all know, surgical resection, chemotherapy, and radiotherapy are the main treatment methods for glioma. With the development of clinical trials and traditional treatment techniques, radiotherapy for glioma has increasingly exposed defects in the treatment effect. In order to improve the bottleneck of radiotherapy for glioma, people have done a lot of work; among this, nano-radiosensitizers have offered a novel and potential treatment method. Compared with conventional radiotherapy, nanotechnology can overcome the blood–brain barrier and improve the sensitivity of glioma to radiotherapy. This paper focuses on the research progress of nano-radiosensitizers in radiotherapy for glioma.

5-Nitro-2,4-Dichloropyrimidine as an Universal Model for Low-Energy Electron Processes Relevant for Radiosensitization

We report experimental results of low-energy electron interactions with.

Nano liposomal and cubosomal formulations with platinum-based anticancer agents: therapeutic advances and challenges

Nephrotoxicity, neurotoxicity and multidrug resistance in tumor cells can result from platinum-based anticancer (PBA) agents which can be reduced by nano formulations. Recently, novel formulations based on liposomes and cubosomes have been described as efficient strategies to overcome nephrotoxicity, ototoxicity, neurotoxicity, cardiotoxicity, hematological toxicities, hepatotoxicity and gastrointestinal toxicity as well as multidrug resistance. The co-delivery of anticancer agents concomitant with PBAs via biocompatible and biodegradable smart liposomes and cubosomes can augment therapeutic results of chemotherapy as well as radiotherapy owing to their high accessibility of surface and internal modification. For this purpose, surface, bilayer or core sections of these formulations can be functionalized by pure PBAs or modified PBAs. In this review, recent significant advances and challenges related to various liposomal and cubosomal formulations of PBA are presented in order to emphasize suitable formulations for anticancer applications with critical thoughts provided on how the field can progress.

Concomitant Chemoradiation Therapy with Gold Nanoparticles and Platinum Drugs Co-Encapsulated in Liposomes

A liposomal formulation of gold nanoparticles (GNPs) and carboplatin, named LipoGold, was produced with the staggered herringbone microfluidic method. The radiosensitizing potential of LipoGold and similar concentrations of non-liposomal GNPs, carboplatin and oxaliplatin was evaluated in vitro with the human colorectal cancer cell line HCT116 in a clonogenic assay. Progression of HCT116 tumor implanted subcutaneously in NU/NU mice was monitored after an irradiation of 10 Gy combined with either LipoGold, GNPs or carboplatin injected directly into the tumor by convection-enhanced delivery. Radiosensitization by GNPs alone or carboplatin alone was observed only at high concentrations of these compounds. Furthermore, low doses of carboplatin alone or a combination of carboplatin and GNPs did not engender radiosensitization. However, the same low doses of carboplatin and GNPs administered simultaneously by encapsulation in liposomal nanocarriers (LipoGold) led to radiosensitization and efficient control of cell proliferation. Our study shows that the radiosensitizing effect of a combination of carboplatin and GNPs is remarkably more efficient when both compounds are simultaneously delivered to the tumor cells using a liposomal carrier.

Clustered DNA Damages induced by 0.5 to 30 eV Electrons

Low-energy electrons (LEEs) of energies ≤30 eV are generated in large quantities by ionizing radiation. These electrons can damage DNA; particularly, they can induce the more detrimental clustered lesions in cells. This type of lesions, which are responsible for a large portion of the genotoxic stress generated by ionizing radiation, is described in the Introduction. The reactions initiated by the collisions of 0.5-30 eV electrons with oligonucleotides, duplex DNA, and DNA bound to chemotherapeutic platinum drugs are explained and reviewed in the subsequent sections. The experimental methods of LEE irradiation and DNA damage analysis are described with an emphasis on the detection of cluster lesions, which are considerably enhanced in DNA-Pt-drug complexes. Based on the energy dependence of damage yields and cross-sections, a mechanism responsible for the clustered lesions can be attributed to the capture of a single electron by the electron affinity of an excited state of a base, leading to the formation of transient anions at 6 and 10 eV. The initial capture is followed by electronic excitation of the base and dissociative attachment-at other DNA sites-of the electron reemitted from the temporary base anion. The mechanism is expected to be universal in the cellular environment and plays an important role in the formation of clustered lesions.

Convection-Enhanced Delivery in Malignant Gliomas: A Review of Toxicity and Efficacy

Malignant gliomas are undifferentiated or anaplastic gliomas. They remain incurable with a multitude of modalities, including surgery, radiation, chemotherapy, and alternating electric field therapy. Convection-enhanced delivery (CED) is a local treatment that can bypass the blood-brain barrier and increase the tumor uptake of therapeutic agents, while decreasing exposure to healthy tissues. Considering the multiple choices of drugs with different antitumor mechanisms, the supra-additive effect of concomitant radiation and chemotherapy, CED appears as a promising modality for the treatment of brain tumors. In this review, the CED-related toxicities are summarized and classified into immediate, early, and late side effects based on the time of onset, and local and systemic toxicities based on the location of toxicity. The efficacies of CED of various therapeutic agents including targeted antitumor agents, chemotherapeutic agents, radioisotopes, and immunomodulators are covered. The phase III trial PRECISE compares CED of IL13-PE38QQR, an interleukin-13 conjugated to Pseudomonas aeruginosa exotoxin A, to Gliadel® Wafer, a polymer loaded with carmustine. However, in this case, CED had no significant median survival improvement (11.3 months vs. 10 months) in patients with recurrent glioblastomas. In phase II studies, CED of recombinant poliovirus (PVSRIPO) had an overall survival of 21% vs. 14% for the control group at 24 months, and 21% vs. 4% at 36 months. CED of Tf-diphtheria toxin had a response rate of 35% in recurrent malignant gliomas patients. On the other hand, the TGF-β2 inhibitor Trabedersen, HSV-1-tk ganciclovir, and radioisotope 131I-chTNT-1/B mAb had a limited response rate. With this treatment, patients who received CED of the chemotherapeutic agent paclitaxel and immunomodulator, oligodeoxynucleotides containing CpG motifs (CpG-ODN), experienced intolerable toxicity. Toward the end of this article, an ideal CED treatment procedure is proposed and the methods for quality assurance of the CED procedure are discussed.

Clustered DNA Damage Induced by 2-20 eV Electrons and Transient Anions: General Mechanism and Correlation to Cell Death

The mechanisms of action of low-energy electrons (LEEs) generated in large quantities by ionizing radiation constitute an essential element of our understanding of early events in radiolysis and radiobiology. We present the 2-20 eV electron energy dependence of the yields of base damage (BD), BD-related cross-links (CLs), and non-double-strand break (NDSB) clustered damage induced in DNA. These new yield functions are generated by the impact of LEEs on plasmid DNA films. The damage is analyzed by gel electrophoresis with and without enzyme treatment. Maxima at 5 and 10 eV in BDs and BD-related CLs yield functions, and two others, at 6 and 10 eV, in those of NDSB clustered damage are ascribed to core-excited transient anions that decay into bond-breaking channels. The mechanism causing all types of DNA damages can be attributed to the capture of a single electron by a base followed by multiple different electron transfer pathways.

Absolute cross sections for chemoradiation therapy: Damages to cisplatin-DNA complexes induced by 10 eV electrons

In chemoradiation therapy, the synergy between the radiation and the chemotherapeutic agent (CA) can result in a super-additive treatment. A priori, this increased effectiveness could be estimated from model calculations, if absolute cross sections (ACSs) involved in cellular damage are substantially higher, when the CA binds to DNA. We measure ACSs for damages induced by 10 eV electrons, when DNA binds to the CA cisplatin as in chemotherapy. At this energy, DNA is damaged essentially by the decay of core-excited transient anions into bond-breaking channels. Films of cisplatin-DNA complexes of ratio 5:1 with thicknesses 10, 15, and 20 nm were irradiated in vacuum during 5-30 s. Conformation changes were quantified by electrophoresis and yields extrapolated from exposure-response curves. Base damages (BDs) were revealed and quantified by enzymatic treatment. The ACSs were generated from these yields by two mathematical models. For 3197 base-pair plasmid DNA, ACS for single strand breaks, double strand breaks (DSBs), crosslinks, non-DSB cluster damages, and total BDs is 71 ± 2, 9.3 ± 0.4, 10.1 ± 0.3, 8.2 ± 0.3, and 115 ± 2 ×10^-15 cm², respectively. These ACSs are higher than those of nonmodified DNA by factors of 1.6 ± 0.1, 2.2 ± 0.1, 1.3 ± 0.1, 1.3 ± 0.3, and 2.1 ± 0.4, respectively. Since LEEs are produced in large quantities by radiolysis and strongly interact with biomolecules, we expect such enhancements to produce substantial additional damages in the DNA of the nucleus of cancer cells during concomitant chemoradiation therapy. The increase damage appears sufficiently large to justify more elaborate simulations, which could provide a quantitative evaluation of molecular sensitization by Pt-CAs.

Drug Delivery Technology to the CNS in the Treatment of Brain Tumors: The Sherbrooke Experience

Drug delivery to the central nervous system (CNS) remains a challenge in neuro-oncology. Despite decades of research in this field, no consensus has emerged as to the best approach to tackle this physiological limitation. Moreover, the relevance of doing so is still sometimes questioned in the community. In this paper, we present our experience with CNS delivery strategies that have been developed in the laboratory and have made their way to the clinic in a continuum of translational research. Using the intra-arterial (IA) route as an avenue to deliver chemotherapeutics in the treatment of brain tumors, complemented by an osmotic breach of the blood-brain barrier (BBB) in specific situations, we have developed over the years a comprehensive research effort on this specialized topic. Looking at pre-clinical work supporting the rationale for this approach, and presenting results discussing the safety of the strategy, as well as results obtained in the treatment of malignant gliomas and primary CNS lymphomas, this paper intends to comprehensively summarize our work in this field.

Transition metal compounds as cancer radiosensitizers

Combining metallo-drugs with ionising radiation for synergistic cancer cell killing: chemical design principles, mechanisms of action and emerging applications.

Liposomal formulations of carboplatin injected by convection-enhanced delivery increases the median survival time of F98 glioma bearing rats

BACKGROUND

Effectiveness of chemotherapy for treating glioblastoma (GBM) brain tumors is hampered by the blood-brain barrier which limits the entry into the brain of most drugs from the blood. To bypass this barrier, convection-enhanced delivery (CED) was proposed to directly inject drugs in tumor. However, the benefit of CED may be hampered when drugs diffuse outside the tumor to then induce neurotoxicity. Encapsulation of drugs into liposome aims at increasing tumor cells specificity and reduces neurotoxicity. However, the most appropriate liposomal formulation to inject drugs into brain tumor by CED still remains to be determined. In this study, four liposomal carboplatin formulations were prepared and tested in vitro on F98 glioma cells and in Fischer rats carrying F98 tumor implanted in the brain. Impact of pegylation on liposomal surface and relevance of positive or negative charge were assessed.

RESULTS

The cationic non-pegylated (L1) and pegylated (L2) liposomes greatly improved the toxicity of carboplatin in vitro compared to free carboplatin, whereas only a modest improvement and even a reduction of efficiency were measured with the anionic non-pegylated (L3) and the pegylated (L4) liposomes. Conversely, only the L4 liposome significantly increased the median survival time of Fisher rats implanted with the F98 tumor, compared to free carboplatin. Neurotoxicity assays performed with the empty L4' liposome showed that the lipid components of L4 were not toxic. These results suggest that the positive charge on liposomes L1 and L2, which is known to promote binding to cell membrane, facilitates carboplatin accumulation in cancer cells explaining their higher efficacy in vitro. Conversely, negatively charged and pegylated liposome (L4) seems to diffuse over a larger distance in the tumor, and consequently significantly increased the median survival time of the animals.

CONCLUSIONS

Selection of the best liposomal formulation based on in vitro studies or animal model can result in contradictory conclusions. The negatively charged and pegylated liposome (L4) which was the less efficient formulation in vitro showed the best therapeutic effect in animal model of GBM. These results support that relevant animal model of GBM must be considered to determine the optimal physicochemical properties of liposomal formulations.

Electron impact ionisation cross sections of cis- and trans-diamminedichloridoplatinum(II) and its hydrolysis products

We report total electron-impact ionisation cross sections (EICSs) of cisplatin, its hydrolysis products and transplatin in the energy range from threshold to 10 keV using the binary-encounter-Bethe (BEB) and its relativistic variant (RBEB), and the Deutsch-Märk (DM) methods. We find reasonable agreement between all three methods, and we also note that the RBEB and the BEB methods yield very similar (almost identical) results in the considered energy range. For cisplatin, the resulting EICSs yield cross section maxima of 22.09 × 10-20 m2 at 55.4 eV for the DM method and 18.67 × 10-20 m2 at 79.2 eV for the (R)BEB method(s). The EICSs of monoaquated cisplatin yield maxima of 12.54 × 10-20 m2 at 82.8 eV for the DM method and of 9.74 × 10-20 m2 at 106 eV for the (R)BEB method(s), diaquated cisplatin yields maxima of 7.56 × 10-20 m2 at 118.5 eV for the DM method and of 5.77 × 10-20 m2 at 136 eV for the (R)BEB method(s). Molecular geometry does not affect the resulting EICS significantly, which is also reflected in very similar EICSs of the cis- and trans-isomer. Limitations of the work as well as desirable future directions in the research area are discussed.

Molecular efficacy of radio- and chemotherapy sequences from direct DNA damage measurements

PURPOSE

To investigate the molecular aspects of the synergy between ionizing radiation and platinum (Pt) chemotherapeutic agents in cancer treatment with chemoradiation therapy (CRT) by measuring damages induced by low-energy electrons (LEE) to DNA bound to cisplatin. LEE are produced abundantly by any type of ionizing radiation and cisplatin represents a typical Pt-chemotherapeutic agents.

MATERIALS AND METHODS

Our strategy involves two parallel administrations of cisplatin and irradiation with a 4.6 and 9.6 eV electron fluence of 1.1 × 10¹²: (1) LEE bombardment of supercoiled DNA and its subsequent reaction with cisplatin; (2) the reaction of DNA with cisplatin followed by LEE irradiation. The damage yields for the loss of supercoiled (LS), single-strand breaks (SSB) and double-strand breaks (DSB) were obtained from gel electrophoresis analysis. Base modifications were revealed by treating the samples with Escherichia coli base excision repair endonuclease (Nth and Fpg).

RESULTS

The yields were deduced from the respective time-response for the reaction of DNA with cisplatin. The results show that binding cisplatin to DNA followed by LEE irradiation, consistently yields more DNA damages than the reverse order. In comparison to non-treated DNA, administration (2) results in an increase of LS and SSB of 1.4-3.3 folds and of DSB by more than an order of magnitude. Furthermore, after enzyme treatment, the yields of DSB rise by factors of 5.3-15.4, indicating a large increase of clustered damages, which should at least partially translate into an increase of lethal damages in cancer cells during the CRT.

CONCLUSIONS

Our results demonstrate that a strong synergy between radiation and cisplatin can only be achieved at the molecular level, if the drug is present at the time of irradiation. Furthermore, this work confirms the LEE mechanism previously proposed to explain the synergy between radiation and Pt drugs in CRT. It involves chemical sensitization of DNA prior to irradiation, to facilitate strand breaks and clustered damages induced by the highly reactive LEE.

Stability of the Parent Anion of the Potential Radiosensitizer 8-Bromoadenine Formed by Low-Energy (<3 eV) Electron Attachment

8-Bromoadenine (8BrA) is a potential DNA radiosensitizer for cancer radiation therapy due to its efficient interaction with low-energy electrons (LEEs). LEEs are a short-living species generated during the radiation damage of DNA by high-energy radiation as it is applied in cancer radiation therapy. Electron attachment to 8BrA in the gas phase results in a stable parent anion below 3 eV electron energy in addition to fragmentation products formed by resonant exocyclic bond cleavages. Density functional theory (DFT) calculations of the 8BrA- anion reveal an exotic bond between the bromine and the C8 atom with a bond length of 2.6 Å, where the majority of the charge is located on bromine and the spin is mainly located on the C8 atom. The detailed understanding of such long-lived anionic states of nucleobase analogues supports the rational development of new therapeutic agents, in which the enhancement of dissociative electron transfer to the DNA backbone is critical to induce DNA strand breaks in cancerous tissue.

The Exploitation of Low-Energy Electrons in Cancer Treatment

Given the distinct characteristics of low-energy electrons (LEEs), particularly at energies less than 30 eV, they can be applied to a wide range of therapeutic modalities to improve cancer treatment. LEEs have been shown to efficiently produce complex molecular damage resulting in substantial cellular toxicities. Since LEEs are produced in copious amounts from high-energy radiation beam, including photons, protons and ions; the control of LEE distribution can potentially enhance the therapeutic radio of such beams. LEEs can play a substantial role in the synergistic effect between radiation and chemotherapy, particularly halogenated and platinum-based anticancer drugs. Radiosensitizing entities containing atoms of high atomic number such as gold nanoparticles can be a source of LEE production if high-energy radiation interacts with them. This can provide a high local density of LEEs in a cell and produce cellular toxicity. Auger-electron-emitting radionuclides also create a high number of LEEs in each decay, which can induce lethal damage in a cell. Exploitation of LEEs in cancer treatment, however, faces a few challenges, such as dosimetry of LEEs and selective delivery of radiosensitizing and chemotherapeutic molecules close to cellular targets. This review first discusses the rationale for utilizing LEEs in cancer treatment by explaining their mechanism of action, describes theoretical and experimental studies at the molecular and cellular levels, then discusses strategies for achieving modification of the distribution and effectiveness of LEEs in cancerous tissue and their associated clinical benefit.

Sensitizing DNA Towards Low-Energy Electrons with 2-Fluoroadenine

2-Fluoroadenine ((2F) A) is a therapeutic agent, which is suggested for application in cancer radiotherapy. The molecular mechanism of DNA radiation damage can be ascribed to a significant extent to the action of low-energy (<20 eV) electrons (LEEs), which damage DNA by dissociative electron attachment. LEE induced reactions in (2F) A are characterized both isolated in the gas phase and in the condensed phase when it is incorporated into DNA. Information about negative ion resonances and anion-mediated fragmentation reactions is combined with an absolute quantification of DNA strand breaks in (2F) A-containing oligonucleotides upon irradiation with LEEs. The incorporation of (2F) A into DNA results in an enhanced strand breakage. The strand-break cross sections are clearly energy dependent, whereas the strand-break enhancements by (2F) A at 5.5, 10, and 15 eV are very similar. Thus, (2F) A can be considered an effective radiosensitizer operative at a wide range of electron energies.

Convection-enhancement delivery of liposomal formulation of oxaliplatin shows less toxicity than oxaliplatin yet maintains a similar median survival time in F98 glioma-bearing rat model

Results of clinical trials with oxaliplatin in treating glioblastoma are dismal. Previous works showed that intravenous (i.v.) delivery of oxaliplatin did not increase the survival of F98 glioma-bearing Fisher rats. Low accumulation of the drug in tumor cells is presumed to be responsible for the lack of antitumor effect. In the present study, convection-enhanced delivery (CED) was used to directly inject oxaliplatin in brain tumor implanted in rats. Since CED can led to severe toxicity, the liposomal formulation of oxaliplatin (Lipoxal™) was also assessed. The maximum tolerated dose (MTD) of oxaliplatin was 10 μg, while that of Lipoxal™ was increased by 3-times reaching 30 μg. Median survival time (MeST) of F98 glioma-bearing rats injected with 10 μg oxaliplatin by CED was 31 days, 7.5 days longer than untreated control (p = 0.0002); while CED of 30 μg Lipoxal™ reached the same result. Compared to previous study on i.v. delivery of these drugs, their injection by CED significantly increased their tumoral accumulations as well as MeSTs in the F98 glioma bearing rat model. The addition of radiotherapy (15 Gy) to CED of oxaliplatin or Lipoxal™ increased the MeST by 4.0 and 3.0 days, respectively. The timing of radiotherapy (4 h or 24 h after CED) produced similar results. However, the treatment was better tolerated when radiotherapy was performed 24 h after CED. In conclusion, a better tumoral accumulation was achieved when oxaliplatin and Lipoxal™ were injected by CED. The liposomal encapsulation of oxaliplatin reduced its toxic, while maintaining its antitumor potential.

Alternative role of cisplatin in DNA damage – theoretical studies on the influence of excess electrons on the cisplatin–DNA complex

Interaction of excess electrons with cisplatin–DNA generates highly reactive Pt-containing species towards C–H abstraction, which strongly contributes to DNA damage.

Biomolecular damage induced by ionizing radiation: the direct and indirect effects of low-energy electrons on DNA

Many experimental and theoretical advances have recently allowed the study of direct and indirect effects of low-energy electrons (LEEs) on DNA damage. In an effort to explain how LEEs damage the human genome, researchers have focused efforts on LEE interactions with bacterial plasmids, DNA bases, sugar analogs, phosphate groups, and longer DNA moieties. Here, we summarize the current understanding of the fundamental mechanisms involved in LEE-induced damage of DNA and complex biomolecule films. Results obtained by several laboratories on films prepared and analyzed by different methods and irradiated with different electron-beam current densities and fluencies are presented. Despite varied conditions (e.g., film thicknesses and morphologies, intrinsic water content, substrate interactions, and extrinsic atmospheric compositions), comparisons show a striking resemblance in the types of damage produced and their yield functions. The potential of controlling this damage using molecular and nanoparticle targets with high LEE yields in targeted radiation-based cancer therapies is also discussed.

Decitabine rescues cisplatin resistance in head and neck squamous cell carcinoma

Cisplatin resistance in head and neck squamous cell carcinoma (HNSCC) reduces survival. In this study we hypothesized that methylation of key genes mediates cisplatin resistance. We determined whether a demethylating drug, decitabine, could augment the anti-proliferative and apoptotic effects of cisplatin on SCC-25/CP, a cisplatin-resistant tongue SCC cell line. We showed that decitabine treatment restored cisplatin sensitivity in SCC-25/CP and significantly reduced the cisplatin dose required to induce apoptosis. We then created a xenograft model with SCC-25/CP and determined that decitabine and cisplatin combination treatment resulted in significantly reduced tumor growth and mechanical allodynia compared to control. To establish a gene classifier we quantified methylation in cancer tissue of cisplatin-sensitive and cisplatin-resistant HNSCC patients. Cisplatin-sensitive and cisplatin-resistant patient tumors had distinct methylation profiles. When we quantified methylation and expression of genes in the classifier in HNSCC cells in vitro, we showed that decitabine treatment of cisplatin-resistant HNSCC cells reversed methylation and gene expression toward a cisplatin-sensitive profile. The study provides direct evidence that decitabine restores cisplatin sensitivity in in vitro and in vivo models of HNSCC. Combination treatment of cisplatin and decitabine significantly reduces HNSCC growth and HNSCC pain. Furthermore, gene methylation could be used as a biomarker of cisplatin-resistance.

Cisplatin Radiosensitization of DNA Irradiated with 2-20 eV Electrons: Role of Transient Anions

Platinum chemotherapeutic agents, such as cisplatin (cis-diamminedichloroplatinum(II)), can act as radiosensitizers when bound covalently to nuclear DNA in cancer cells. This radiosensitization is largely due to an increase in DNA damage induced by low-energy secondary electrons, produced in large quantities by high-energy radiation. We report the yields of single- and double-strand breaks (SSB and DSB) and interduplex cross-links (CL) induced by electrons of 1.6-19.6 eV (i.e., the yield functions) incident on 5 monolayer (ML) films of cisplatin-DNA complexes. These yield functions are compared with those previously recorded with 5 ML films of unmodified plasmid DNA. Binding of five cisplatin molecules to plasmid DNA (3197 base pairs) enhances SSB, DSB, and CL by factors varying, from 1.2 to 2.8, 1.4 to 3.5, and 1.2 to 2.7, respectively, depending on electron energy. All yield functions exhibit structures around 5 and 10 eV that can be attributed to enhancement of bond scission, via the initial formation of core-excited resonances associated with π → π* transitions of the bases. This increase in damage is interpreted as arising from a modification of the parameters of the corresponding transient anions already present in nonmodified DNA, particularly those influencing molecular dissociation. Two additional resonances, specific to cisplatin-modified DNA, are formed at 13.6 and 17.6 eV in the yield function of SSB. Furthermore, cisplatin binding causes the induction of DSB by electrons of 1.6-3.6 eV, i.e., in an energy region where a DSB cannot be produced by a single electron in pure DNA. Breaking two bonds with a subexcitation-energy electron is tentatively explained by a charge delocalization mechanism, where a single electron occupies simultaneously two σ* bonds linking the Pt atom to guanine bases on opposite strands.

DNA strand breaks and crosslinks induced by transient anions in the range 2-20 eV

The energy dependence of the yields of single and double strand breaks (SSB and DSB) and crosslinks induced by electron impact on plasmid DNA films is measured in the 2-20 eV range. The yield functions exhibit two strong maxima, which are interpreted to result from the formation of core-excited resonances (i.e., transient anions) of the bases, and their decay into the autoionization channel, resulting in π → π* electronic transitions of the bases followed by electron transfer to the C-O σ* bond in the phosphate group. Occupancy of the σ* orbital ruptures the C-O bond of the backbone via dissociative electron attachment, producing a SSB. From a comparison of our results with those of other works, including theoretical calculations and electron-energy-loss spectra of the bases, the 4.6 eV peak in the SSB yield function is attributed to the resonance decay into the lowest electronically excited states of the bases; in particular, those resulting from the transitions 13A'(π2 → π3*) and 13A″(n2 → π3*) of thymine and 13A'(π → π*) of cytosine. The strongest peak at 9.6 eV in the SSB yield function is also associated with electron captured by excited states of the bases, resulting mostly from a multitude of higher-energy π → π* transitions. The DSB yield function exhibits strong maxima at 6.1 and 9.6 eV. The peak at 9.6 eV is probably related to the same resonance manifold as that leading to SSB, but the other at 6.1 eV may be more restricted to decay into the electronic state 13A' (π → π*) of cytosine via autoionization. The yield function of crosslinks is dominated by a broad peak extending over the 3.6-11.6 eV range with a sharper one at 17.6 eV. The different line shape of the latter function, compared to that of SSB and DSB, appears to be due to the formation of reactive radical sites in the initial supercoiled configuration of the plasmid, which react with the circular form (i.e., DNA with a SSB) to produce a crosslink.

A single subexcitation-energy electron can induce a double-strand break in DNA modified by platinum chemotherapeutic drugs

The sensitization of malignant cells to ionizing radiation is the clinical rationale for the use of platinum-drug-based concurrent chemoradiotherapy (CCRT) for cancer treatment; however, the specific mechanisms of radiosensitization and their respective contributions still remain unknown. Biological mechanisms such as inhibition of DNA repair may contribute to the efficacy of CCRT; nevertheless, there is a dearth of information on the possible contribution of nanoscopic mechanisms to the generation of lethal DNA lesions, such as double-strand breaks (DSB). The present study demonstrates that the abundant near zero-eV (0.5 eV) electrons, created by ionizing radiation during radiotherapy, induce DSB in supercoiled plasmid DNA modified by platinum-containing anticancer drugs (Pt drugs), but not in unmodified DNA. They do so more efficiently than other types of radiation, including soft X-rays and 10 eV electrons. The formation of DSB by 0.5 eV electrons is found to be a single-hit process. These findings reveal insights into the radiosensitization mechanism of Pt drugs that can have implications for the development of optimal clinical protocols for platinum-based CCRT and the deployment of in situ sources of subexcitation-energy electrons (e.g., Auger electron-emitting radionuclides) to efficiently enhance DSB formation in DNA modified by Pt drugs in malignant cells.