Advances in anticancer radiopharmaceuticals

Nanoradiopharmaceuticals: An Attractive Concept in Oncotherapy

Radiopharmaceuticals are of significant importance in the fields of tumor imaging and therapy. In recent decades, the increasing role of nanotechnology has led to the attractive concept of nanoradiopharmaceuticals. Consequently, it is imperative to provide a concise summary of the necessary guidelines to facilitate the translation of nanoradiopharmaceuticals. In this work, we have presented the contents of radiolabeling strategies and some applications of nanoradiopharmaceuticals. Such a framework can assist researchers in identifying more pertinent insights or making more informed decisions in the study of nanoradiopharmaceuticals.

Metallic radionuclide-labeled tetrameric 2,6-diisopropylphenyl azides for cancer treatment

This study proposes a new method for radionuclide therapy that involves the use of oligomeric 2,6-diisopropylphenyl azides and a chelator to form stable complexes with metallic radionuclides. The technique works by taking advantage of the endogenous acrolein produced by cancer cells. The azides react with the acrolein to give a diazo derivative that immediately attaches to the nearest organelle, effectively anchoring the radionuclide within the tumor. Preliminary in vivo experiments were conducted on a human lung carcinoma xenograft model, demonstrating the feasibility of this approach for cancer treatment.

Radionuclide-based theranostics - a promising strategy for lung cancer

PURPOSE

This review aims to provide a comprehensive overview of the latest literature on personalized lung cancer management using different ligands and radionuclide-based tumor-targeting agents.

BACKGROUND

Lung cancer is the leading cause of cancer-related deaths worldwide. Due to the heterogeneity of lung cancer, advances in precision medicine may enhance the disease management landscape. More recently, theranostics using the same molecule labeled with two different radionuclides for imaging and treatment has emerged as a promising strategy for systemic cancer management. In radionuclide-based theranostics, the target, ligand, and radionuclide should all be carefully considered to achieve an accurate diagnosis and optimal therapeutic effects for lung cancer.

METHODS

We summarize the latest radiotracers and radioligand therapeutic agents used in diagnosing and treating lung cancer. In addition, we discuss the potential clinical applications and limitations associated with target-dependent radiotracers as well as therapeutic radionuclides. Finally, we provide our views on the perspectives for future development in this field.

CONCLUSIONS

Radionuclide-based theranostics show great potential in tailored medical care. We expect that this review can provide an understanding of the latest advances in radionuclide therapy for lung cancer and promote the application of radioligand theranostics in personalized medicine.

A novel theranostic probe [111In]In-DO3A-NHS-nimotuzumab in glioma xenograft

Indium-111 (111In) has an appropriate half-life (T 1/2 = 67 h) and energy characteristics for cancer diagnosis via γ-ray imaging and cancer therapy with Auger electrons. The aim of our study is to evaluate the potential of [111In]In-DO3A-NHS-nimotuzumab as a theranostic agent for radioimmunoimaging (RII) and radioimmunotherapy (RIT) against human glioma xenografts in mice. We explored the chelators DO3A-NHS and DOTA-p-SCN-Bz to optimize 111In radiolabeling efficiency of nimotuzumab. The radiopharmaceuticals were purified by PD-10 mini-column and their in vitro stabilities were assessed. We investigated the biodistribution of [111In]In-DO3A-NHS-nimotuzumab as it had relatively superior labeling efficiency and stability in vitro. We conducted SPECT imaging on mice bearing glioma (U87MG) xenografts, which were injected with ∼3.7 MBq of [111In]In-DO3A-NHS-nimotuzumab. The in vivo radiotherapeutic effects of [111In]In-DO3A-NHS-nimotuzumab was analyzed via injecting a single 37 MBq dose, 2 × 18 MBq doses, or 2 × 37 MBq doses into mice bearing U87MG xenografts. The control groups were administered either 30 μg nimotuzumab or saline. The radiochemical yields of [111In]In-DO3A-NHS-nimotuzumab and [111In]In-DOTA-p-SCN-Bz-nimotuzumab were > 85% and > 75%, respectively. [111In]In-DO3A-NHS-nimotuzumab had > 95% radiochemical purity and was more stable in vitro than [111In]In-DOTA-p-SCN-Bz-nimotuzumab. Biodistribution study demonstrated that [111In]In-DO3A-NHS-nimotuzumab was highly stable in vivo. SPECT imaging disclosed that [111In]In-DO3A-NHS-nimotuzumab had excellent targeted tumor uptake and retained in tumors for 24 and 72 h. All [111In]In-DO3A-NHS-nimotuzumab treatments substantially inhibited tumor growth over the controls. The 2 × 37 MBq treatment was particularly efficacious, and presented with survival time prolonged by ≤66 days. In contrast, the survival time of the control group was only 30 days. In our study, we developed an optimized synthesis protocol for radiopharmaceutical 111In-DO3A-NHS-nimotuzumab and demonstrated that it is a promising theranostic agent. It could be highly efficacious in RII and RIT against EGFR-expressing glioma.

Theranostics – present and future

Theragnostics in nuclear medicine constitute an essential element of precision medicine. This notion integrates radionuclide diagnostics procedures and radionuclide therapies using appropriate radiopharmaceutics and treatment targeting specific biological pathways or receptors. The term theragnostics should also include another aspect of treatment: not only whether a given radioisotopic drug can be used, but also in what dose it ought to be used. Theragnostic procedures also allow predicting the effects of treatment based on the assessment of specific receptor density or the metabolic profile of neoplastic cells. The future of theragnostics depends not only on the use of new radiopharmaceuticals, but also on new gamma cameras. Modern theragnostics already require unambiguous pharmacokinetic and pharmacodynamic measurements based on absolute values. Only dynamic studies provide such a possibility. The introduction of the dynamic total-body PET-CT will enable this type of measurements characterizing metabolic processes and receptor expression on the basis of Patlak plot.

Oligonucleotide-Functionalized Gold Nanoparticles for Synchronous Telomerase Inhibition, Radiosensitization, and Delivery of Theranostic Radionuclides

Telomerase represents an attractive target in oncology as it is expressed in cancer but not in normal tissues. The oligonucleotide inhibitors of telomerase represent a promising anticancer strategy, although poor cellular uptake can restrict their efficacy. In this study, gold nanoparticles (AuNPs) were used to enhance oligonucleotide uptake. "match" oligonucleotides complementary to the telomerase RNA template subunit (hTR) and "scramble" (control) oligonucleotides were conjugated to diethylenetriamine pentaacetate (DTPA) for 111In-labeling. AuNPs (15.5 nm) were decorated with a monofunctional layer of oligonucleotides (ON-AuNP) or a multifunctional layer of oligonucleotides, PEG(polethylene glycol)800-SH (to reduce AuNP aggregation) and the cell-penetrating peptide Tat (ON-AuNP-Tat). Match-AuNP enhanced the cellular uptake of radiolabeled oligonucleotides while retaining the ability to inhibit telomerase activity. The addition of Tat to AuNPs increased nuclear localization. 111In-Match-AuNP-Tat induced DNA double-strand breaks and caused a dose-dependent reduction in clonogenic survival of telomerase-positive cells but not telomerase-negative cells. hTR inhibition has been reported to sensitize cancer cells to ionizing radiation, and 111In-Match-AuNP-Tat therefore holds promise as a vector for delivery of radionuclides into cancer cells while simultaneously sensitizing them to the effects of the emitted radiation.

Emerging trends of receptor-mediated tumor targeting peptides: A review with perspective from molecular imaging modalities

Natural peptides extracted from natural components such are known to have a relatively short in-vivo half-life and can readily metabolize by endo- and exo-peptidases. Fortunately, synthetic peptides can be easily manipulated to increase in-vivo stability, membrane permeability and target specificity with some well-known natural families. Many natural as well as synthetic peptides target to their endogenous receptors for diagnosis and therapeutic applications. In order to detect these peptides externally, they must be modified with radionuclides compatible with single photon emission computed tomography (SPECT) or positron emission tomography (PET). Although, these techniques mainly rely on physiological changes and have profound diagnostic strength over anatomical modalities such as MRI and CT. However, both SPECT and PET observed to possess lack of anatomical reference frame which is a key weakness of these techniques, and unfortunately, cannot be available freely in most clinical centres especially in under-developing countries. Hence, it is need of the time to design and develop economic, patient friendly and versatile strategies to grapple with existing problems without any hazardous side effects. Optical molecular imaging (OMI) has emerged as a novel technique in field of medical science using fluorescent probes as imaging modality and has ability to couple with organic drugs, small molecules, chemotherapeutics, DNA, RNA, anticancer peptide and protein without adding chelators as necessary for radionuclides. Furthermore, this review focuses on difference in imaging modalities and provides ample knowledge about reliable, economic and patient friendly optical imaging technique rather radionuclide-based imaging techniques.

Clinical Perspectives of Theranostics

Theranostics is a precision medicine which integrates diagnostic nuclear medicine and radionuclide therapy for various cancers throughout body using suitable tracers and treatment that target specific biological pathways or receptors. This review covers traditional theranostics for thyroid cancer and pheochromocytoma with radioiodine compounds. In addition, recent theranostics of radioimmunotherapy for non-Hodgkin lymphoma, and treatment of bone metastasis using bone seeking radiopharmaceuticals are described. Furthermore, new radiopharmaceuticals for prostatic cancer and pancreatic cancer have been added. Of particular, F-18 Fluoro-2-Deoxyglucose (FDG) Positron Emission Tomography (PET) is often used for treatment monitoring and estimating patient outcome. A recent clinical study highlighted the ability of alpha-radiotherapy with high linear energy transfer (LET) to overcome treatment resistance to beta--particle therapy. Theranostics will become an ever-increasing part of clinical nuclear medicine.

Effects of Repeated 131I-Meta-Iodobenzylguanidine Radiotherapy on Tumor Size and Tumor Metabolic Activity in Patients with Metastatic Neuroendocrine Tumors

¹³¹I-meta-iodobenzylguanidine (¹³¹I-MIBG) radiotherapy has shown some survival benefits in metastatic neuroendocrine tumors (NETs). European Association of Nuclear Medicine clinical guidelines for ¹³¹I-MIBG radiotherapy suggest a repeated treatment protocol, although none currently exists. The existing single-high-dose ¹³¹I-MIBG radiotherapy (444 MBq/kg) has been shown to have some benefits for patients with metastatic NETs. However, this protocol increases adverse effects and requires alternative therapeutic approaches. Therefore, the aim of this study was to evaluate the effects of repeated ¹³¹I-MIBG therapy on tumor size and tumor metabolic response in patients with metastatic NETs. Methods: Eleven patients with metastatic NETs (aged 49.2 ± 16.3 y) prospectively received repeated 5,550-MBq doses of ¹³¹I-MIBG therapy at 6-mo intervals. In total, 31 treatments were performed. The mean number of treatments was 2.8 ± 0.4, and the cumulative ¹³¹I-MIBG dose was 15,640.9 ± 2,245.1 MBq (286.01 MBq/kg). Tumor response was observed by CT and ¹⁸F-FDG PET or by ¹⁸F-FDG PET/CT before and 3-6 mo after the final ¹³¹I-MIBG treatment. Results: On the basis of the CT findings with RECIST, 3 patients showed a partial response and 6 patients showed stable disease. The remaining 2 patients showed progressive disease. Although there were 2 progressive-disease patients, analysis of all patients showed no increase in summed length diameter (median, 228.7 mm [interquartile range (IQR), 37.0-336.0 mm] to 171.0 mm [IQR, 38.0-270.0 mm]; P = 0.563). In tumor region-based analysis with partial-response and stable-disease patients (n = 9), ¹³¹I-MIBG therapy significantly reduced tumor diameter (79 lesions; median, 16 mm [IQR, 12-22 mm] to 11 mm [IQR, 6-16 mm]; P < 0.001). Among 5 patients with hypertension, there was a strong trend toward systolic blood pressure reduction (P = 0.058), and diastolic blood pressure was significantly reduced (P = 0.006). Conclusion: Eighty-two percent of metastatic NET patients effectively achieved inhibition of disease progression, with reduced tumor size and reduced metabolic activity, through repeated ¹³¹I-MIBG therapy. Therefore, this relatively short-term repeated ¹³¹I-MIBG treatment may have potential as one option in the therapeutic protocol for metastatic NETs. Larger prospective studies with control groups are warranted.

131I-IITM and 211At-AITM: Two Novel Small-Molecule Radiopharmaceuticals Targeting Oncoprotein Metabotropic Glutamate Receptor 1

Targeted radionuclide therapy (TRT) targeting oncoproteins facilitates the delivery of therapeutic radionuclides to tumor tissues with high precision. Herein, we developed 2 new radiopharmaceuticals, 4-¹³¹I-iodo- and 4-²¹¹At-astato-N-[4-(6-(isopropylamino)pyridine-4-yl)-1,3-thiazol-2-yl]-N-methylbenzamide (¹³¹I-IITM and ²¹¹At-AITM), targeting the ectopic metabotropic glutamate receptor 1 (mGluR1) in melanomas for TRT studies. Methods: ¹³¹I-IITM and ²¹¹At-AITM were synthesized by reacting a stannyl precursor with ¹³¹I-NaI and ²¹¹At in the presence of an oxidizing agent. The therapeutic efficacy and safety of the 2 radiopharmaceuticals were investigated using mGluR1-expressing B16F10 melanoma cells and melanoma-bearing mice. Results: ¹³¹I-IITM and ²¹¹At-AITM were obtained with a radiochemical purity of greater than 99% and radiochemical yields of 42.7% ± 10.4% and 45.7% ± 6.5%, respectively, based on the total radioactivity of used radionuclides. ¹³¹I-IITM and ²¹¹At-AITM exhibited a maximum uptake of 4.66% ± 0.70 and 7.68% ± 0.71 percentage injected dose per gram (%ID/g) in the targeted melanomas, respectively, and were rapidly cleared from nontarget organs after intravenous injection. Both agents markedly inhibited melanoma growth compared with the controls (61.00% and 95.68%, respectively). In the melanoma model, considerably greater therapeutic efficacy with negligible toxicity was observed using ²¹¹At-AITM. Conclusion: The nontoxic radiopharmaceuticals ¹³¹I-IITM and ²¹¹At-AITM are useful high-precision TRT agents that can be used to target the oncoprotein mGluR1 for melanoma therapy.

An assessment of bone-seeking radionuclides for palliation of metastatic bone pain in a vertebral model

OBJECTIVE

Bone-seeking radiopharmaceuticals have the main role in the treatment of painful bone metastases. The aim of this study was to dosimetrically compare radiopharmaceuticals in use for bone pain palliation therapy and bone scan.

METHODS

The MCNPX code was used to simulate the radiation transport in a vertebral phantom. Absorbed fractions were calculated for monoenergetic electrons, photons and alpha particles. S values were obtained for radionuclides ³²P, ³³P, ⁸⁹Sr, ⁹⁰Y, ^99mTc, ^117mSn, ¹⁵³Sm, ¹⁶⁶Ho, ¹⁶⁹Er, ¹⁷⁷Lu, ¹⁸⁶Re, ¹⁸⁸Re, ²²³Ra, ²²⁴Ra and their progenies for target regions including the active marrow and the bone endosteum.

RESULTS

The results demonstrated the dependence of dosimetric parameters on the source or target size, particle energy and location of the source. The electron emitters including ³³P, ^117mSn, ¹⁶⁹Er and ¹⁷⁷Lu and ²²³Ra as an α-emitter gave the lower absorbed dose to the active marrow. These radionuclides gave the highest values of the Relative Advantage Factor (RAF).

CONCLUSIONS

According to the results, ³³P, ^117mSn, ¹⁶⁹Er, ¹⁷⁷Lu and ²²³Ra have fewer side effects on the active marrow than other investigated radionuclides. Therefore, these radionuclides may be a better choice for use in palliative radiotherapy.

Radioiodination and biodistribution of newly synthesized 3-benzyl-2-([3-methoxybenzyl]thio)benzo[g]quinazolin-4-(3H)-one in tumor bearing mice

3-Benzyl-2-((3-methoxybenzyl)thio)benzo[g]quinazolin-4(3H)-one was previously synthesized and proved by physicochemical analyses (HRMS, 1H and 13C NMR). The target compound was examined for its radioactivity and the results showed that benzo[g]quinazoline was successfully labeled with radioactive iodine using NBS via an electrophilic substitution reaction. The reaction parameters that affected the labeling yield such as concentration, pH and time were studied to optimize the labeling conditions. The radiochemical yield was 91.2 ± 1.22% and the in vitro studies showed that the target compound was stable for up to 24 h. The thyroid was among the other organs in which the uptake of 125I-benzoquinazoline has increased significantly over the time up to 4.1%. The tumor uptake was 6.95%. Radiochemical and metabolic stability of the benzoquinazoline in vivo/in vitro and biodistribution studies provide some insights about the requirements for developing more potent radiopharmaceutical for targeting the tumor cells.

Subcellular Targeting of Theranostic Radionuclides

The last decade has seen rapid growth in the use of theranostic radionuclides for the treatment and imaging of a wide range of cancers. Radionuclide therapy and imaging rely on a radiolabeled vector to specifically target cancer cells. Radionuclides that emit β particles have thus far dominated the field of targeted radionuclide therapy (TRT), mainly because the longer range (μm-mm track length) of these particles offsets the heterogeneous expression of the molecular target. Shorter range (nm-μm track length) α- and Auger electron (AE)-emitting radionuclides on the other hand provide high ionization densities at the site of decay which could overcome much of the toxicity associated with β-emitters. Given that there is a growing body of evidence that other sensitive sites besides the DNA, such as the cell membrane and mitochondria, could be critical targets in TRT, improved techniques in detecting the subcellular distribution of these radionuclides are necessary, especially since many β-emitting radionuclides also emit AE. The successful development of TRT agents capable of homing to targets with subcellular precision demands the parallel development of quantitative assays for evaluation of spatial distribution of radionuclides in the nm-μm range. In this review, the status of research directed at subcellular targeting of radionuclide theranostics and the methods for imaging and quantification of radionuclide localization at the nanoscale are described.

Radioimmunotherapy for delivery of cytotoxic radioisotopes: current status and challenges

INTRODUCTION

Radioimmunotherapy (RIT) with monoclonal antibodies and their fragments labelled with radionuclides emitting α -particles, β-particles or Auger electrons have been used for many years in the development of anticancer strategies. While RIT has resulted in approved radiopharmaceuticals for the treatment of hematological malignancies, its use in solid tumors still remains challenging.

AREAS COVERED

In this review, we discuss the exciting progress towards elucidating the potential of current and novel radioimmunoconjugates and address the challenges for translation into clinical practice.

EXPERT OPINION

There are still technical and logistical challenges associated with the use of RIT in routine clinical practice, including development of novel and more specific targeting moieties, broader access α to α-emitters and better tailoring of pre-targeting approaches. Moreover, improved understanding of the heterogeneous nature of solid tumors and the critical role of tumor microenvironments will help to optimize clinical response to RIT by delivering sufficient radiation doses to even more radioresistant tumor cells.

Targeted radionuclide therapy in combined-modality regimens

Targeted radionuclide therapy (TRT) is a branch of cancer medicine concerned with the use of radioisotopes, radiolabelled molecules, nanoparticles, or microparticles that either naturally accumulate in or are designed to target tumours. TRT combines the specificity of molecular and sometimes physical targeting with the potent cytotoxicity of ionising radiation. Targeting vectors for TRT include antibodies, antibody fragments, proteins, peptides, and small molecules. The diversity of available carrier molecules, together with the large panel of suitable radioisotopes with unique physicochemical properties, allows vector-radionuclide pairings to be matched to the molecular, pathological, and physical characteristics of a tumour. Some pairings are designed for dual therapeutic and diagnostic applications. Use of TRT is increasing with the adoption into practice of radium-223 dichloride for the treatment of bone metastases and with the ongoing clinical development of, among others, 177Lu-dodecanetetraacetic acid tyrosine-3-octreotate (DOTATATE) for the treatment of neuroendocrine tumours and 90Y-microspheres for the treatment of hepatic tumours. The increasing use of TRT raises the question of how best to integrate TRT into multimodality protocols. Achievements in this area and the future prospects of TRT are evaluated in this Review.

Computational modeling of radiobiological effects in bone metastases for different radionuclides

PURPOSE

Computational simulation is a simple and practical way to study and to compare a variety of radioisotopes for different medical applications, including the palliative treatment of bone metastases. This study aimed to evaluate and compare cellular effects modelled for different radioisotopes currently in use or under research for treatment of bone metastases using computational methods.

METHODS

Computational models were used to estimate the radiation-induced cellular effects (Virtual Cell Radiobiology algorithm) post-irradiation with selected particles emitted by Strontium-89 (⁸⁹Sr), Samarium-153 (¹⁵³Sm), Lutetium-177 (¹⁷⁷Lu), and Radium-223 (²²³Ra).

RESULTS

Cellular kinetics post-irradiation using ⁸⁹Sr β^- particles, ¹⁵³Sm β^-particles, ¹⁷⁷Lu β^-particles and ²²³Ra α particles showed that the cell response was dose- and radionuclide-dependent. ¹⁷⁷Lu beta minus particles and, in particular, ²²³Ra alpha particles, yielded the lowest survival fraction of all investigated particles.

CONCLUSIONS

²²³Ra alpha particles induced the highest cell death of all investigated particles on metastatic prostate cells in comparison to irradiation with β^-radionuclides, two of the most frequently used radionuclides in the palliative treatment of bone metastases in clinical routine practice. Moreover, the data obtained suggest that the used computational methods might provide some perception about cellular effects following irradiation with different radionuclides.

Internalization of Auger electron-emitting isotopes into cancer cells: a method for spatial distribution determination of equivalent source terms

PURPOSE

A challenge for single-cell dosimetry of internalized Auger electron-emitting (AE) radiopharmaceuticals remains how best to elucidate their spatial distribution. To this end, a method, photoresist autoradiography (PAR), was previously developed to identify the lateral spatial distribution of AE-emitting radionuclides internalized in single cancer cells. In this paper, we present a simple mathematical model based on the radius and depth of radiation-induced patterns in photoresist material to identify the location in the z-plane of an 111In source capable of generating the pattern.

MATERIALS AND METHODS

SQ20B cells, derived from a head and neck squamous cell carcinoma, were exposed to 111In-labeled epidermal growth factor (EGF) (8 MBq/μg). The integrated electron fluence after four half-lives from the internalized radionuclide-containing construct was detected by a photoresist layer that was placed in close proximity to the cells. The resultant latent patterns were chemically developed and analyzed by atomic force microscopy (AFM). The features in the patterns were matched to locations of electrons emitted from simulated point sources, thereby determining the likely locations of internalized radionuclides.

RESULTS

The modeling procedure was validated using simple patterns. The model relates the depth and radius (in the x-y plane) of a pattern to the location and fluence of the source giving rise to the pattern. This point source modeling method provided a good fit to experimental data and can be expanded to analyze more complex patterns.

CONCLUSIONS

We have demonstrated the utility of the modelling technique to identify the location of internalized AE-emitting radionuclides. This methodology now needs to be extended to predict the source positions in more complex PAR patterns.

Interfaces with Tunable Mechanical and Radiosensitizing Properties

We report the fabrication of a composite containing nanostructured GaOOH and Matrigel with tunable radiosensitizing and stiffness properties. Composite characterization was done with microscopy and rheology. The utility of the interface was tested in vitro using fibroblasts. Cell viability and reactive oxygen species assays quantified the effects of radiation dosages and GaOOH concentrations. Fibroblasts' viability decreased with increasing concentration of GaOOH and composite stiffness. During ionizing radiation experiments the presence of the scintillating GaOOH triggered a different cellular response. Reactive oxygen species data demonstrated that one can reduce the amount of radiation needed to modulate the behavior of cells on interfaces with different stiffness containing a radiosensitizing material.

Targeted Radionuclide Therapy of Human Tumors

Targeted radionuclide therapy is one of the most intensively developing directions of nuclear medicine. Unlike conventional external beam therapy, the targeted radionuclide therapy causes less collateral damage to normal tissues and allows targeted drug delivery to a clinically diagnosed neoplastic malformations, as well as metastasized cells and cellular clusters, thus providing systemic therapy of cancer. The methods of targeted radionuclide therapy are based on the use of molecular carriers of radionuclides with high affinity to antigens on the surface of tumor cells. The potential of targeted radionuclide therapy has markedly grown nowadays due to the expanded knowledge base in cancer biology, bioengineering, and radiochemistry. In this review, progress in the radionuclide therapy of hematological malignancies and approaches for treatment of solid tumors is addressed.

Comparative analysis of 11 different radioisotopes for palliative treatment of bone metastases by computational methods

PURPOSE

Throughout the years, the palliative treatment of bone metastases using bone seeking radiotracers has been part of the therapeutic resources used in oncology, but the choice of which bone seeking agent to use is not consensual across sites and limited data are available comparing the characteristics of each radioisotope. Computational simulation is a simple and practical method to study and to compare a variety of radioisotopes for different medical applications, including the palliative treatment of bone metastases. This study aims to evaluate and compare 11 different radioisotopes currently in use or under research for the palliative treatment of bone metastases using computational methods.

METHODS

Computational models were used to estimate the percentage of deoxyribonucleic acid (DNA) damage (fast Monte Carlo damage algorithm), the probability of correct DNA repair (Monte Carlo excision repair algorithm), and the radiation-induced cellular effects (virtual cell radiobiology algorithm) post-irradiation with selected particles emitted by phosphorus-32 ((32)P), strontium-89 ((89)Sr), yttrium-90 ((90)Y ), tin-117 ((117m)Sn), samarium-153 ((153)Sm), holmium-166 ((166)Ho), thulium-170 ((170)Tm), lutetium-177 ((177)Lu), rhenium-186 ((186)Re), rhenium-188 ((188)Re), and radium-223 ((223)Ra).

RESULTS

(223)Ra alpha particles, (177)Lu beta minus particles, and (170)Tm beta minus particles induced the highest cell death of all investigated particles and radioisotopes. The cell survival fraction measured post-irradiation with beta minus particles emitted by (89)Sr and (153)Sm, two of the most frequently used radionuclides in the palliative treatment of bone metastases in clinical routine practice, was higher than (177)Lu beta minus particles and (223)Ra alpha particles.

CONCLUSIONS

(223)Ra and (177)Lu hold the highest potential for palliative treatment of bone metastases of all radioisotopes compared in this study. Data reported here may prompt future in vitro and in vivo experiments comparing different radionuclides for palliative treatment of bone metastases, raise the need for the careful rethinking of the current widespread clinical use of (89)Sr and (153)Sm, and perhaps strengthen the use of (223)Ra and (177)Lu in the palliative treatment of bone metastases.

Uptake, internalization and nuclear translocation of radioimmunotherapeutic agents

Radioimmunotherapy (RIT) agents that incorporate short-range particle-emitting radionuclides exploit the high linear energy transfer of α-particles and Auger electrons. Both are densely ionizing, generate complex DNA double-strand breaks and so are profoundly cytotoxic. Internalizing RIT agents enter tumor cells through receptor-mediated endocytosis and by incorporation of cell-penetrating peptides. Once internalized, some RIT agents mediate escape from endosomes and/or translocate to the nucleus. In the classical nuclear import pathway, α/β-importins recognize nuclear localization sequences in RIT agents. Translocation through nuclear pores enables RIT agents to bind to nuclear targets induced by, for example, cellular stress, growth factors or anticancer therapy, such as γH2AX or p27(KIP-1). This review discusses RIT agents designed to exploit the mechanisms underlying these complex processes and compares them with noninternalizing RIT agents.

The potential and hurdles of targeted alpha therapy - clinical trials and beyond

This article presents a general discussion on what has been achieved so far and on the possible future developments of targeted alpha (α)-particle therapy (TAT). Clinical applications and potential benefits of TAT are addressed as well as the drawbacks, such as the limited availability of relevant radionuclides. Alpha-particles have a particular advantage in targeted therapy because of their high potency and specificity. These features are due to their densely ionizing track structure and short path length. The most important consequence, and the major difference compared with the more widely used β⁻-particle emitters, is that single targeted cancer cells can be killed by self-irradiation with α-particles. Several clinical trials on TAT have been reported, completed, or are on-going: four using ²¹³Bi, two with ²¹¹At, two with ²²⁵Ac, and one with ²¹²Pb/²¹²Bi. Important and conceptual proof-of-principle of the therapeutic advantages of α-particle therapy has come from clinical studies with ²²³Ra-dichloride therapy, showing clear benefits in castration-resistant prostate cancer.

Effects and safety of ¹³¹I-metaiodobenzylguanidine (MIBG) radiotherapy in malignant neuroendocrine tumors: results from a multicenter observational registry

Effective treatments for malignant neuroendocrine tumors are under development. While iodine-131 metaiodobenzylguanidine (¹³¹I-MIBG) radiotherapy has been used in the treatment of malignant neuroendocrine tumors, there are few studies evaluating its therapeutic effects and safety in a multicenter cohort. In the current study, we sought to evaluate the effects and safety of ¹³¹I-MIBG therapy for conditions including malignant pheochromocytoma and paraganglioma within a multicenter cohort. Forty-eight malignant neuroendocrine tumors (37 pheochromocytoma and 11 paraganglioma) from four centers underwent clinical ¹³¹I-MIBG radiotherapy. The tumor responses were observed before and 3 to 6 months after the ¹³¹I-MIBG radiotherapy in accordance with RECIST criteria. We also evaluated the data for any adverse effects. The four centers performed a total of 87 ¹³¹I-MIBG treatments on 48 patients between January 2000 and March 2009. Of the treatments, 65 were evaluable using RECIST criteria. One partial response (PR), 40 stable disease (SD), and 9 progressive disease (PD) in malignant pheochromocytoma were observed after each treatment. Fourteen SD and one PD-were observed in paraganglioma. Patients with normal hypertension (systolic blood pressure (BP) > 130 mmHg) showed significantly reduced systolic BP after the initial follow-up (n=10, 138.1±8.2 to 129.5±13.5 mmHg, P=0.03). In adult neuroendocrine tumors with a treatment-basis analysis, there were side effects following 41 treatments (47.1%) and most of them (90.2%) were minor. In this multicenter registry, PR or SD was achieved in 84.6% of the treatment occasions in adult neuroendocrine tumors through ¹³¹I-MIBG radiotherapy. This indicated that most of the ¹³¹I-MIBG radiotherapy was performed safely without significant side effects.