Call to arms: need for radiobiology in molecular radionuclide therapy

Second Symposium of the European Working Group on the Radiobiology of Molecular Radionuclide Therapy

Molecular radionuclide therapy is a relatively novel anticancer treatment option using radiolabeled, tumor-specific vectors. On binding of these vectors to cancer cells, radioactive decay induces DNA damage and other effects, leading to cancer cell death. Treatments, such as with [177Lu]Lu-octreotate for neuroendocrine tumors and [177Lu]Lu-PSMA for prostate cancer, are now being implemented into routine clinical practice around the world. Nonetheless, research into the underlying radiobiologic effects of these treatments is essential to further improve them or formulate new ones. The purpose of the European Working Group on the Radiobiology of Molecular Radiotherapy is to promote knowledge, investment, and networking in this area. This report summarizes recent research and insights presented at the second International Workshop on Radiobiology of Molecular Radiotherapy, held in London, U.K., on March 13 and 14, 2023. The symposium was organized by members of the Cancer Research U.K. RadNet City of London and the European Working Group on the Radiobiology of Molecular Radiotherapy.

Faecalibacterium prausnitzii in Differentiated Thyroid Cancer Patients Treated with Radioiodine

BACKGROUND

Faecalibacterium prausnitzii, one of the most important bacteria of the human gut microbiota, produces butyrate (a short-chain fatty acid). Short-chain fatty acids are known to influence thyroid physiology and thyroid cancer's response to treatment. We aimed to analyze the relative abundance of Faecalibacterium prausnitzii on the gut microbiota of differentiated thyroid cancer patients compared to controls and its variation after radioiodine therapy (RAIT).

METHODS

Fecal samples were collected from 37 patients diagnosed with differentiated thyroid cancer before and after radioiodine therapy and from 10 volunteers. The abundance of F. prausnitzii was determined using shotgun metagenomics.

RESULTS

Our study found that the relative abundance of F. prausnitzii is significantly reduced in thyroid cancer patients compared to volunteers. We also found that there was a mixed response to RAIT, with an increase in the relative and absolute abundances of this bacterium in most patients.

CONCLUSIONS

Our study confirms that thyroid cancer patients present a dysbiotic gut microbiota, with a reduction in F. prausnitzii's relative abundance. In our study, radioiodine did not negatively affect F. prausnitzii, quite the opposite, suggesting that this bacterium might play a role in resolving radiation aggression issues.

Effect of Radium-223 on the Gut Microbiota of Prostate Cancer Patients: A Pilot Case Series Study

Radium-223 (Ra-223) is a targeted nuclear medicine therapy for castration-resistant prostate cancer with bone metastases. Its major route of elimination is the intestine. There is overwhelming evidence that the gut microbiota is altered by ionizing radiation (IR) from radiotherapy treatments. Nevertheless, it is known that extrapolation of outcomes from radiotherapy to nuclear medicine is not straightforward. The purpose of this study was to prospectively determine the effect of Ra-223 on selected important bacteria from the gut microbiota. Stool samples from three prostate cancer patients and two healthy individuals were obtained, processed, and analysed. We specifically measured the relative change of the abundance of important bacteria, determined by the 2^−ΔΔC method. We found that Ra-223 influenced the gut microbiota composition. The most relevant changes were increases of Proteobacteria and Atopobacter; and decreases of Bacteroidetes, Prevotella, Lactobacillus, Bifidobacterium, Clostridium coccoides, and Bacteroides fragilis. Additionally, our experiment confirms that the composition of gut microbiota from prostate cancer patients is altered. No significant correlation was found between each subject’s gut microbiome profile and their clinical indices. Despite its limited sample, the results of this pilot study suggest that ionizing radiation from Ra-223 alters the gut microbiota composition and that the gut microbiota of prostate cancer patients has an increase of the bacteria with known prejudicial effects and a decrease of the ones with favorable effects.

Understanding the Radiobiological Mechanisms Induced by 177Lu-DOTATATE in Comparison to External Beam Radiation Therapy

Radionuclide Therapy (RNT) with ¹⁷⁷Lu-DOTATATE targeting somatostatin receptors (SSTRs) in neuroendocrine tumours (NET) has been successfully used in routine clinical practice, mainly leading to stable disease. Radiobiology holds promise for RNT improvement but is often extrapolated from external beam radiation therapy (EBRT) studies despite differences in these two radiation-based treatment modalities. In a panel of six human cancer cell lines expressing SSTRs, common radiobiological endpoints (i.e., cell survival, cell cycle, cell death, oxidative stress and DNA damage) were evaluated over time in ¹⁷⁷Lu-DOTATATE- and EBRT-treated cells, as well as the radiosensitizing potential of poly (ADP-ribose) polymerase inhibition (PARPi). Our study showed that common radiobiological mechanisms were induced by both ¹⁷⁷Lu-DOTATATE and EBRT, but to a different extent and/or with variable kinetics, including in the DNA damage response. A higher radiosensitizing potential of PARPi was observed for EBRT compared to ¹⁷⁷Lu-DOTATATE. Our data reinforce the need for dedicated RNT radiobiology studies, in order to derive its maximum therapeutic benefit.

Ionizing Radiation from Radiopharmaceuticals and the Human Gut Microbiota: An Ex Vivo Approach

This study aimed to determine the effect of three widely used radiopharmaceuticals with intestinal excretion on selected relevant bacteria that are part of the human gut microbiota, using an ex vivo approach. Fecal samples obtained from healthy volunteers were analyzed. Each sample was divided into four smaller aliquots. One served as the non-irradiated control. The other three were homogenized with three radiopharmaceutical solutions ([¹³¹I]NaI, [^99mTc]NaTcO₄, and [²²³Ra]RaCl₂). Relative quantification of each taxa was determined by the 2^−ΔΔC method, using the ribosomal gene 16S as an internal control (primers 534/385). Twelve fecal samples were analysed: three controls and nine irradiated. Our experiment showed fold changes in all analyzed taxa with all radiopharmaceuticals, but results were more significant with I-131, ranging from 1.87–83.58; whereas no relevant differences were found with Tc-99m and Ra-223, ranging from 0.98–1.58 and 0.83–1.97, respectively. This study corroborates limited existing research on how ionizing radiation changes the gut microbiota composition, providing novel data regarding the ex vivo effect of radiopharmaceuticals. Our findings justify the need for future larger scale projects.

Know thy tumour: Biomarkers to improve treatment of molecular radionuclide therapy

Molecular radionuclide therapy (MRT) is an effective treatment for both localised and disseminated tumours. Biomarkers can be used to identify potential subtypes of tumours that are known to respond better to standard MRT protocols. These enrolment-based biomarkers can further be used to develop dose-response relationships using image-based dosimetry within these defined subtypes. However, the biological identity of the cancers treated with MRT are commonly not well-defined, particularly for neuroendocrine neoplasms. The biological heterogeneity of such cancers has hindered the establishment of dose-responses and minimum tumour dose thresholds. Biomarkers could also be used to determine normal tissue MRT dose limits and permit greater injected doses of MRT in patients. An alternative approach is to understand the repair capacity limits of tumours using radiobiology-based biomarkers within and outside patient cohorts currently treated with MRT. It is hoped that by knowing more about tumours and how they respond to MRT, biomarkers can provide needed dimensionality to image-based biodosimetry to improve MRT with optimized protocols and personalised therapies.

Rhenium Perrhenate (188ReO4) Induced Apoptosis and Reduced Cancerous Phenotype in Liver Cancer Cells

Recurrence in hepatocellular carcinoma (HCC) after conventional treatments is a crucial challenge. Despite the promising progress in advanced targeted therapies, HCC is the fourth leading cause of cancer death worldwide. Radionuclide therapy can potentially be a practical targeted approach to address this concern. Rhenium-188 (188Re) is a β-emitting radionuclide used in the clinic to induce apoptosis and inhibit cell proliferation. Although adherent cell cultures are efficient and reliable, appropriate cell-cell and cell-extracellular matrix (ECM) contact is still lacking. Thus, we herein aimed to assess 188Re as a potential therapeutic component for HCC in 2D and 3D models. The death rate in treated Huh7 and HepG2 lines was significantly higher than in untreated control groups using viability assay. After treatment with 188ReO4, Annexin/PI data indicated considerable apoptosis induction in HepG2 cells after 48 h but not Huh7 cells. Quantitative RT-PCR and western blotting data also showed increased apoptosis in response to 188ReO4 treatment. In Huh7 cells, exposure to an effective dose of 188ReO4 led to cell cycle arrest in the G2 phase. Moreover, colony formation assay confirmed post-exposure growth suppression in Huh7 and HepG2 cells. Then, the immunostaining displayed proliferation inhibition in the 188ReO4-treated cells on 3D scaffolds of liver ECM. The PI3-AKT signaling pathway was activated in 3D culture but not in 2D culture. In nude mice, Huh7 cells treated with an effective dose of 188ReO4 lost their tumor formation ability compared to the control group. These findings suggest that 188ReO4 can be a potential new therapeutic agent against HCC through induction of apoptosis and cell cycle arrest and inhibition of tumor formation. This approach can be effectively combined with antibodies and peptides for more selective and personalized therapy.

Combining External Beam Radiation and Radionuclide Therapies: Rationale, Radiobiology, Results and Roadblocks

The emergence of effective radionuclide therapeutics, such as radium-223 dichloride, [¹⁷⁷Lu]Lu-DOTA-TATE and [¹⁷⁷Lu]Lu-PSMA ligands, over the last 10 years is driving a rapid expansion in molecular radiotherapy (MRT) research. Clinical trials that are underway will help to define optimal dosing protocols and identify groups of patients who are likely to benefit from this form of treatment. Clinical investigations are also being conducted to combine new MRT agents with other anticancer drugs, with particular emphasis on DNA repair inhibitors and immunotherapeutics. In this review, the case is presented for combining MRT with external beam radiotherapy (EBRT). The technical and dosimetric challenges of combining two radiotherapeutic modalities have impeded progress in the past. However, the need for research into the specific radiobiological effects of radionuclide therapy, which has lagged behind that for EBRT, has been recognised. This, together with innovations in imaging technology, MRT dosimetry tools and EBRT hardware, will facilitate the future use of this important combination of treatments.

Methods and techniques for in vitro subcellular localization of radiopharmaceuticals and radionuclides

In oncology, the holy grail of radiotherapy is specific radiation dose deposition in tumours with minimal healthy tissue toxicity. If used appropriately, injectable, systemic radionuclide therapies could meet these criteria, even for treatment of micrometastases and single circulating tumour cells. The clinical use of α and β- particle-emitting molecular radionuclide therapies is rising, however clinical translation of Auger electron-emitting radionuclides is hampered by uncertainty around their exact subcellular localisation, which in turn affects the accuracy of dosimetry. This review aims to discuss and compare the advantages and disadvantages of various subcellular localisation methods available to localise radiopharmaceuticals and radionuclides for in vitro investigations.

Gold nanoparticles meet medical radionuclides

Thanks to their unique optical and physicochemical properties, gold nanoparticles have gained increased interest as radiosensitizing, photothermal therapy and optical imaging agents to enhance the effectiveness of cancer detection and therapy. Furthermore, their ability to carry multiple medically relevant radionuclides broadens their use to nuclear medicine SPECT and PET imaging as well as targeted radionuclide therapy. In this review, we discuss the radiolabeling process of gold nanoparticles and their use in (multimodal) nuclear medicine imaging to better understand their specific distribution, uptake and retention in different in vivo cancer models. In addition, radiolabeled gold nanoparticles enable image-guided therapy is reviewed as well as the enhancement of targeted radionuclide therapy and nanobrachytherapy through an increased dose deposition and radiosensitization, as demonstrated by multiple Monte Carlo studies and experimental in vitro and in vivo studies.

Combination radionuclide therapy: A new paradigm

Targeted molecular radionuclide therapy (MRT) has shown its potential for the treatment of cancers of multiple origins. A combination therapy strategy employing two or more distinct therapeutic approaches in cancer management is aimed at circumventing tumor resistance by simultaneously targeting compensatory signaling pathways or bypassing survival selection mutations acquired in response to individual monotherapies. Combination radionuclide therapy (CRT) is a newer application of the concept, utilizing a combination of radiolabeled molecular targeting agents with chemotherapy and beam radiation therapy for enhanced therapeutic index. Encouraging results are reported with chemotherapeutic agents in combination with radiolabeled targeting molecules for cancer therapy. With increasing awareness of the various survival and stress response pathways activated after radionuclide therapy, different holistic combinations of MRT agents with radiosensitizers targeting such pathways have also been explored. MRT has also been studied in combination with beam radiotherapy modalities such as external beam radiation therapy and carbon ion radiation therapy to enhance the anti-tumor response. Nanotechnology aids in CRT by bringing together multiple monotherapies on a single nanostructure platform for treating cancers in a more precise or personalized way. CRT will be a key player in managing cancers if correctly tailored to the individual patient profile. The success of CRT lies in an in-depth understanding of the radiobiological principles and pathways activated in response.

EANM position paper on the role of radiobiology in nuclear medicine

With an increasing variety of radiopharmaceuticals for diagnostic or therapeutic nuclear medicine as valuable diagnostic or treatment option, radiobiology plays an important role in supporting optimizations. This comprises particularly safety and efficacy of radionuclide therapies, specifically tailored to each patient. As absorbed dose rates and absorbed dose distributions in space and time are very different between external irradiation and systemic radionuclide exposure, distinct radiation-induced biological responses are expected in nuclear medicine, which need to be explored. This calls for a dedicated nuclear medicine radiobiology. Radiobiology findings and absorbed dose measurements will enable an improved estimation and prediction of efficacy and adverse effects. Moreover, a better understanding on the fundamental biological mechanisms underlying tumor and normal tissue responses will help to identify predictive and prognostic biomarkers as well as biomarkers for treatment follow-up. In addition, radiobiology can form the basis for the development of radiosensitizing strategies and radioprotectant agents. Thus, EANM believes that, beyond in vitro and preclinical evaluations, radiobiology will bring important added value to clinical studies and to clinical teams. Therefore, EANM strongly supports active collaboration between radiochemists, radiopharmacists, radiobiologists, medical physicists, and physicians to foster research toward precision nuclear medicine.

Investigating intrinsic radiosensitivity biomarkers to peptide receptor radionuclide therapy with [177Lu]Lu-DOTATATE in a panel of cancer cell lines

INTRODUCTION

[¹⁷⁷Lu]Lu-DOTATATE is an effective systemic targeted radionuclide therapy for somatostatin receptor (SSTR) positive metastatic or inoperable neuroendocrine tumours (NET). However, for a given injected activity, tumour responses are variable. Our aim was to investigate whether SSTR expression/functionality and known characteristics of intrinsic radiosensitivity, namely proliferation rate, glucose metabolism, cell cycle phase, DNA repair and antioxidant defences were predictors of sensitivity to [¹⁷⁷Lu]Lu-DOTATATE in SSTR expressing human cancer cell lines.

METHODS

In six human cancer cell lines and under basal condition, SSTR expression was assessed by qRT-PCR and immunocytochemistry. Its functionality was evaluated by binding/uptake assays with [⁶⁸Ga]Ga- and [¹⁷⁷Lu]Lu-DOTATATE. The radiosensitivity parameters were evaluated as follows: proliferation rate (cell counting), glucose metabolism ([¹⁸F]FDG uptake), antioxidant defences (qRT-PCR, colorimetric assay, flow cytometry), DNA repair (qRT-PCR) and cell cycle (flow cytometry). Effect of [¹⁷⁷Lu]Lu-DOTATATE on cell viability was assessed 3, 7 and 10 days after 4 h incubation with [¹⁷⁷Lu]Lu-DOTATATE using crystal violet.

RESULTS

Based on cell survival at day 10, cell lines were classified into two groups of sensitivity to [¹⁷⁷Lu]Lu-DOTATATE. One group with <20% of survival decrease (-14 to -1%) and one group with >20% of survival decrease (-22 to -33%) compared to the untreated control cell lines. The latter had significantly lower total antioxidant capacity, glutathione (GSH) levels and glucose metabolism (p < 0.05) compared to the first group. SSTR (p = 0.64), proliferation rate (p = 0.74), cell cycle phase (p = 0.55), DNA repair (p > 0.22), combined catalase and GSH peroxidase expression (p = 0.42) and superoxide dismutase (SOD) activity (p = 0.41) were not significantly different between the two groups.

CONCLUSION

Antioxidant defences may be major determinants in [¹⁷⁷Lu]Lu-DOTATATE radiosensitivity.

Inter and intra-tumor somatostatin receptor 2 heterogeneity influences peptide receptor radionuclide therapy response

Patients with neuroendocrine tumors (NETs) can be treated with peptide receptor radionuclide therapy (PRRT). Here, the somatostatin analogue octreotate radiolabeled with lutetium-177 is targeted to NET cells by binding to the somatostatin receptor subtype 2 (SST₂). During radioactive decay, DNA damage is induced, leading to NET cell death. Although the therapy proves to be effective, mortality rates remain high. To appropriately select more optimal treatment strategies, it is essential to first better understand the radiobiological responses of tumor cells to PRRT.

Methods: We analyzed PRRT induced radiobiological responses in SST₂ expressing cells and xenografted mice using SPECT/MRI scanning and histological and molecular analyses. We measured [¹⁷⁷Lu]Lu-DOTA-TATE uptake and performed analyses to visualize induction of DNA damage, cell death and other cellular characteristics.

Results: The highest accumulation of radioactivity was measured in the tumor and kidneys. PRRT induced DNA damage signaling and repair in a time-dependent manner. We observed intra-tumor heterogeneity of DNA damage and apoptosis, which was not attributed to proliferation or bioavailability. We found a strong correlation between high DNA damage levels and high SST₂ expression. PRRT elicited a different therapeutic response between models with different SST₂ expression levels. Heterogeneous SST₂ expression levels were also confirmed in patient NETs.

Conclusion: Heterogeneous SST₂ expression levels within NETs cause differentially induced DNA damage levels, influence recurrent tumor phenotypes and impact the therapeutic response in different models and potentially in patients. Our results contribute to a better understanding of PRRT effects, which might impact future therapeutic outcome of NET patients.

Combination Strategies to Improve Targeted Radionuclide Therapy

In recent years, targeted radionuclide therapy (TRT) has emerged as a promising strategy for cancer treatment. In contrast to conventional radiotherapy, TRT delivers ionizing radiation to tumors in a targeted manner, reducing the dose that healthy tissues are exposed to. Existing TRT strategies include the use of 177Lu-DOTATATE, 131I-metaiodobenzylguanidine, Bexxar, and Zevalin, clinically approved agents for the treatment of neuroendocrine tumors, neuroblastoma, and non-Hodgkin lymphoma, respectively. Although promising results have been obtained with these agents, clinical evidence acquired to date suggests that only a small percentage of patients achieves complete response. Consequently, there have been attempts to improve TRT outcomes through combinations with other therapeutic agents; such strategies include administering concurrent TRT and chemotherapy, and the use of TRT with known or putative radiosensitizers such as poly(adenosine diphosphate ribose) polymerase and mammalian-target-of-rapamycin inhibitors. In addition to potentially achieving greater therapeutic effects than the respective monotherapies, these strategies may lead to lower dosages or numbers of cycles required and, in turn, reduce unwanted toxicities. As of now, several clinical trials have been conducted to assess the benefits of TRT-based combination therapies, sometimes despite limited preclinical evidence being available in the public domain to support their use. Although some clinical trials have yielded promising results, others have shown no clear survival benefit from particular combination treatments. Here, we present a comprehensive review of combination strategies with TRT reported in the literature to date and evaluate their therapeutic potential.

Imaging DNA Damage Repair In Vivo After 177Lu-DOTATATE Therapy

Molecular radiotherapy using 177Lu-DOTATATE is a most effective treatment for somatostatin receptor-expressing neuroendocrine tumors. Despite its frequent and successful use in the clinic, little or no radiobiologic considerations are made at the time of treatment planning or delivery. On positive uptake on octreotide-based PET/SPECT imaging, treatment is usually administered as a standard dose and number of cycles without adjustment for peptide uptake, dosimetry, or radiobiologic and DNA damage effects in the tumor. Here, we visualized and quantified the extent of DNA damage response after 177Lu-DOTATATE therapy using SPECT imaging with 111In-anti-γH2AX-TAT. This work was a proof-of-principle study of this in vivo noninvasive biodosimeter with β-emitting therapeutic radiopharmaceuticals. Methods: Six cell lines were exposed to external-beam radiotherapy (EBRT) or 177Lu-DOTATATE, after which the number of γH2AX foci and the clonogenic survival were measured. Mice bearing CA20948 somatostatin receptor-positive tumor xenografts were treated with 177Lu-DOTATATE or sham-treated and coinjected with 111In-anti-γH2AX-TAT, 111In-IgG-TAT control, or vehicle. Results: Clonogenic survival after external-beam radiotherapy was cell-line-specific, indicating varying levels of intrinsic radiosensitivity. Regarding in vitro cell lines treated with 177Lu-DOTATATE, clonogenic survival decreased and γH2AX foci increased for cells expressing high levels of somatostatin receptor subtype 2. Ex vivo measurements revealed a partial correlation between 177Lu-DOTATATE uptake and γH2AX focus induction between different regions of CA20948 xenograft tumors, suggesting that different parts of the tumor may react differentially to 177Lu-DOTATATE irradiation. Conclusion:111In-anti-γH2AX-TAT allows monitoring of DNA damage after 177Lu-DOTATATE therapy and reveals heterogeneous damage responses.