Towards Effective Targeted Alpha Therapy for Neuroendocrine Tumours: A Review

This review article explores the evolving landscape of Molecular Radiotherapy (MRT), emphasizing Peptide Receptor Radionuclide Therapy (PRRT) for neuroendocrine tumours (NETs). The primary focus is on the transition from β-emitting radiopharmaceuticals to α-emitting agents in PRRT, offering a critical analysis of the radiobiological basis, clinical applications, and ongoing developments in Targeted Alpha Therapy (TAT). Through an extensive literature review, the article delves into the mechanisms and effectiveness of PRRT in targeting somatostatin subtype 2 receptors, highlighting both its successes and limitations. The discussion extends to the emerging paradigm of TAT, underlining its higher potency and specificity with α-particle emissions, which promise enhanced therapeutic efficacy and reduced toxicity. The review critically evaluates preclinical and clinical data, emphasizing the need for standardised dosimetry and a deeper understanding of the dose-response relationship in TAT. The review concludes by underscoring the significant potential of TAT in treating SSTR2-overexpressing cancers, especially in patients refractory to β-PRRT, while also acknowledging the current challenges and the necessity for further research to optimize treatment protocols.

Experimental validation of an innovative approach in biokinetics study for personalised dosimetry of molecular radiation therapy treatments

One of today's main challenges in molecular radiation therapy is to assess an individual dosimetry that allows treatment to be tailored to the specific patient, in accordance with the current paradigm of 'personalized medicine'. The evaluation of the absorbed doses for tumor and organs at risk in molecular radiotherapy is typically based on MIRD schema acquiring few experimental points for the assessement of biokinetic parameters. WIDMApp, the wearable individual dose monitoring apparatus, is an innovative approach for internal dosimetry based on a wearable radiation detecting system for individual biokinetics sampling, a Monte Carlo simulation for particle interaction, and an unfolding algorithm for data analysis and integrated activity determination at organ level. A prototype of a WIDMApp detector element was used to record the photon emissions in a body phantom containing 3 spheres with liquid sources (18F,64Cu and99mTc) to simulate organs having different washout. Modelling the phantom geometry on the basis of a CT scan imaging, the Monte Carlo simulation computed the contribution of each emitting sphere to the signal detected in 3 positions on the phantoms surface. Combining the simulated results with the data acquired for 120 h, the unfolding algorithm deconvolved the detected signal and assessed the decay half-life (T1/2) and initial activity values (A(0)) that best reproduces the observed exponential decays. A 3%-18% level of agreement is found between the actualA(0) andT1/2values and those obtained by means of the minimization procedure based on the Monte Carlo simulation. That resulted in an estimation of the cumulated activity <15%. Moreover, WIDMApp data redundancy has been used to mitigate some experimental occurrences that happened during data taking. A first experimental test of the WIDMApp approach to internal radiation dosimetry is presented. Studies with patients are foreseen to validate the technique in a real environment.

Radiation safety of current European practices of therapeutic nuclear medicine: survey results from 20 HERCA countries

The purpose of this study was to acquire up-to-date information on nuclear medicine treatments in Europe and on the implementation of the requirements of the Basic Safety Standards Directive in HERCA Heads of the European Radiological Protection Competent Authorities (HERCAs) member states. An electronic survey was distributed to competent authorities of 32 HERCA member states. The questionnaire addressed 33 explicitly considered treatments using 13 different radionuclides, and for each treatment, a similar set of questions was included. Questions covered the use of treatments, hospitalisation of patients and radioactive waste management related to therapeutic nuclear medicine involving other radionuclides than the well-known¹³¹I. The survey also covered justification of treatments, individual treatment planning, involvement of a medical physics expert (MPE) and radiation protection instructions given to the patient at the time of release. Responses were obtained from 20 HERCA countries. All of these countries used Na[¹³¹I]I for benign thyroid diseases and thyroid ablation of adults^{. 223}RaCl₂(Xofigo®) for bone metastases,¹⁷⁷Lu-somatostatin analogues for neuroendocrine tumours and¹⁷⁷Lu-labelled PSMA for castration resistant prostate cancer (PC) and PC-metastases were used in 90%, 65% and 55% of countries, respectively. Only a few countries had specific criteria for hospitalisation and waste management for new therapeutic nuclear medicine. Regulatory requirements for justification of new therapeutic nuclear medicine were in place in almost all countries. Individual treatment planning was required for all therapies in 55% and for some therapies in 28% of the responding countries. Implementation of the requirement for MPEs to be closely involved in nuclear medicine practices varied to a great extend among countries. Almost all responding countries answered that some radiation protection instructions existed for patients released after treatment with radionuclides other than¹³¹I treatment, however only few countries had developed specific guidelines in the field.

In Vitro Characterization of 177Lu-DOTA-M5A Anti-Carcinoembryonic Antigen Humanized Antibody and HSP90 Inhibition for Potentiated Radioimmunotherapy of Colorectal Cancer

Carcinoembryonic antigen (CEA) is an antigen that is highly expressed in colorectal cancers and widely used as a tumor marker. ¹³¹I and ⁹⁰Y-radiolabeled anti-CEA monoclonal antibodies (mAbs) have previously been assessed for radioimmunotherapy in early clinical trials with promising results. Moreover, the heat shock protein 90 inhibitor onalespib has previously demonstrated radiotherapy potentiation effects in vivo. In the present study, a ¹⁷⁷Lu-radiolabeled anti-CEA hT84.66-M5A mAb (M5A) conjugate was developed and the potential therapeutic effects of ¹⁷⁷Lu-DOTA-M5A and/or onalespib were investigated. The ¹⁷⁷Lu radiolabeling of M5A was first optimized and characterized. Binding specificity and affinity of the conjugate were then evaluated in a panel of gastrointestinal cancer cell lines. The effects on spheroid growth and cell viability, as well as molecular effects from treatments, were then assessed in several three-dimensional (3D) multicellular colorectal cancer spheroid models. Stable and reproducible radiolabeling was obtained, with labeling yields above 92%, and stability was retained at least 48 h post-radiolabeling. Antigen-specific binding of the radiolabeled conjugate was demonstrated on all CEA-positive cell lines. Dose-dependent therapeutic effects of both ¹⁷⁷Lu-DOTA-M5A and onalespib were demonstrated in the spheroid models. Moreover, effects were potentiated in several dose combinations, where spheroid sizes and viabilities were significantly decreased compared to the corresponding monotherapies. For example, the combination treatment with 350 nM onalespib and 20 kBq ¹⁷⁷Lu-DOTA-M5A resulted in 2.5 and 2.3 times smaller spheroids at the experimental endpoint than the corresponding monotreatments in the SNU1544 spheroid model. Synergistic effects were demonstrated in several of the more effective combinations. Molecular assessments validated the therapy results and displayed increased apoptosis in several combination treatments. In conclusion, the combination therapy of anti-CEA ¹⁷⁷Lu-DOTA-M5A and onalespib showed enhanced therapeutic effects over the individual monotherapies for the potential treatment of colorectal cancer. Further in vitro and in vivo studies are warranted to confirm the current study findings.

Molecular Radiotherapy with 177Lu-Immunoliposomes Induces Cytotoxicity in Mesothelioma Cancer Stem Cells In Vitro

Malignant mesothelioma (MM) is a lethal tumor originating in the mesothelium with high chemotherapeutic resistance. Cancer stem cells (CSCs) persist in tumors and are critical targets responsible for tumor resistance and recurrence. The identification and characterization of CSCs may help develop effective treatment for MM. The objective of this study was to evaluate the therapeutic effect of molecular targeted radiotherapy by ¹⁷⁷Lu-labeled immunoliposomes (¹⁷⁷Lu-ILs) on CSCs of mesothelioma. MM CSCs were sorted based on CD26/CD24 expression level and their functional significances were established by small interference RNA. CSC potential of MM was evaluated for drug resistance, cell invasion, and cell growth rate in vitro. CSC metabolism was evaluated with the uptake of ¹⁸F-FDG. Therapeutic effects of ¹⁷⁷Lu-labeled immunoliposomes targeting CD26 and CD24 were evaluated in vitro through proliferation and apoptotic assays. CSCs sorted from H28 cells exhibited significant drug resistance and enhanced proliferative activity as well as increased metabolism indicated by higher ¹⁸F-FDG uptake. Treatment with ¹⁷⁷Lu-ILs, compared with ¹⁷⁷Lu-CL and ILs, showed enhanced therapeutic effects on inhibition of proliferation, up-regulation of apoptosis, and suppression of CD26 and CD24 expression. Thus, our results suggest that molecular radiotherapy targeting both CD26 and CD24 could be a promising approach for CSC-targeting therapy for MM.

FZD10-targeted α-radioimmunotherapy with 225 Ac-labeled OTSA101 achieves complete remission in a synovial sarcoma model

Synovial sarcomas are rare tumors arising in adolescents and young adults. The prognosis for advanced disease is poor, with an overall survival of 12‐18 months. Frizzled homolog 10 (FZD10) is overexpressed in most synovial sarcomas, making it a promising therapeutic target. The results of a phase 1 trial of β‐radioimmunotherapy (RIT) with the ⁹⁰Y‐labeled anti‐FZD10 antibody OTSA101 revealed a need for improved efficacy. The present study evaluated the potential of α‐RIT with OTSA101 labeled with the α‐emitter ²²⁵Ac. Competitive inhibition and cell binding assays showed that specific binding of ²²⁵Ac‐labeled OTSA101 to SYO‐1 synovial sarcoma cells was comparable to that of the imaging agent ¹¹¹In‐labeled OTSA101. Biodistribution studies showed high uptake in SYO‐1 tumors and low uptake in normal organs, except for blood. Dosimetric studies showed that the biologically effective dose (BED) of ²²⁵Ac‐labeled OTSA101 for tumors was 7.8 Bd higher than that of ⁹⁰Y‐labeled OTSA101. ⁹⁰Y‐ and ²²⁵Ac‐labeled OTSA101 decreased tumor volume and prolonged survival. ²²⁵Ac‐labeled OTSA101 achieved a complete response in 60% of mice, and no recurrence was observed. ²²⁵Ac‐labeled OTSA101 induced a larger amount of necrosis and apoptosis than ⁹⁰Y‐labeled OTSA101, although the cell proliferation decrease was comparable. The BED for normal organs and tissues was tolerable; no treatment‐related mortality or obvious toxicity, except for temporary body weight loss, was observed. ²²⁵Ac‐labeled OTSA101 provided a high BED for tumors and achieved a 60% complete response in the synovial sarcoma mouse model SYO‐1. RIT with ²²⁵Ac‐labeled OTSA101 is a promising therapeutic option for synovial sarcoma.

The History of Prostate-Specific Membrane Antigen as a Theranostic Target in Prostate Cancer: The Foundational Role of the Prostate Cancer Foundation

Prostate-Specific Membrane Antigen (PSMA) is a credentialed imaging and therapy (theranostic) target for the detection and treatment of prostate cancer. PSMA-targeted positron emission tomography (PET) imaging and molecular radiotherapy (MRT) are promising evolving technologies that will improve the outcomes of prostate cancer patients. In anticipation of this new era in prostate cancer theranostics, this article will review the history of PSMA from discovery, through early and late stage clinical trials. Since 1993, the Prostate Cancer Foundation (PCF) has funded critical and foundational PSMA research that established this theranostic revolution. The history and role of PCF funding in this field will be discussed.

Prospective phase 2 trial of PSMA-targeted molecular RadiothErapy with 177Lu-PSMA-617 for metastatic castration-reSISTant Prostate Cancer (RESIST-PC): efficacy results of the UCLA cohort

The objective of this study was to determine prospectively the efficacy profile of 2 activity regimens of ¹⁷⁷Lu-PSMA therapy in patients with progressive metastatic castrate-resistant prostate cancer (mCRPC): 6.0 vs. 7.4 GBq. Methods: RESIST-PC (NCT03042312) was a prospective multicenter phase 2 trial. Patients with progressive mCRPC after ≥ 1 novel androgen-axis drug, either chemotherapy naïve or postchemotherapy, with sufficient bone marrow reserve, normal kidney function, and sufficient PSMA expression by PSMA PET were eligible. Patients were randomized (1:1) into 2 activity groups (6.0 or 7.4 GBq) and received up to 4 cycles every 8 wk. The primary endpoint was the efficacy of ¹⁷⁷Lu-PSMA measured by the prostate-specific antigen (PSA) response rate (RR) after 2 cycles (≥50% decline from baseline). Secondary endpoints included the PSA RR (≥50% decline) at any time (best response), and overall survival (OS). Results: The study was closed at enrollment of 71/200 planned patients because of sponsorship transfer. We report here the efficacy of the University of California Los Angeles cohort results only (n = 43). The PSA RRs after 2 cycles and at any time were 11/40 (28%, 95% CI 15-44), 6/13 (46%, 95% CI 19-75), and 5/27 (19%, 95% CI 6-38), and 16/43 (37%, 95% CI 23-53), 7/14 (50%, 95% CI 23-77), and 9/29 (31%, 95% CI 15-51) in the whole cohort, the 6.0-GBq group, and the 7.4-GBq group, respectively (P = 0.12 and P = 0.31). The median OS was 14.0 mo (95% CI 10.1-17.9), 15.8 (95% CI 11.8-19.4), and 13.5 (95% CI 10.0-17.0) in the whole cohort, the 6.0-GBq group, and the 7.4 GBq group, respectively (P = 0.87). OS was longer in patients who experienced a PSA decline ≥ 50% at any time than in those who did not: median, 20.8 versus 10.8 mo (P = 0.005). Conclusion: In this prospective phase 2 trial of ¹⁷⁷Lu-PSMA for mCRPC, the median OS was 14 mo. Despite the heterogeneous study population and the premature study termination, the efficacy profile of ¹⁷⁷Lu-PSMA appeared to be favorable and comparable with both activity regimens (6.0 vs. 7.4 GBq). Results justify confirmation with real-world data matched-pair analysis and further clinical trials to refine and optimize the ¹⁷⁷Lu-PSMA therapy administration scheme to improve tumor radiation dose delivery and efficacy.

Preclinical Evaluation of Podoplanin-Targeted Alpha-Radioimmunotherapy with the Novel Antibody NZ-16 for Malignant Mesothelioma

The prognosis of advanced mesothelioma is poor. Podoplanin (PDPN) is highly expressed in most malignant mesothelioma. This study aimed to evaluate the potential alpha-radioimmunotherapy (RIT) with a newly developed anti-PDPN antibody, NZ-16, compared with a previous antibody, NZ-12.

METHODS

The in vitro properties of radiolabeled antibodies were evaluated by cell binding and competitive inhibition assays using PDPN-expressing H226 mesothelioma cells. The biodistribution of ¹¹¹In-labeled antibodies was studied in tumor-bearing mice. The absorbed doses were estimated based on biodistribution data. Tumor volumes and body weights of mice treated with ⁹⁰Y- and ²²⁵Ac-labeled NZ-16 were measured for 56 days. Histologic analysis was conducted.

RESULTS

The radiolabeled NZ-16 specifically bound to H226 cells with higher affinity than NZ-12. The biodistribution studies showed higher tumor uptake of radiolabeled NZ-16 compared with NZ-12, providing higher absorbed doses to tumors. RIT with ²²⁵Ac- and ⁹⁰Y-labeled NZ-16 had a significantly higher antitumor effect than RIT with ⁹⁰Y-labeled NZ-12. ²²⁵Ac-labeled NZ-16 induced a larger amount of necrotic change and showed a tendency to suppress tumor volumes and prolonged survival than ⁹⁰Y-labeled NZ-16. There is no obvious adverse effect.

CONCLUSIONS

Alpha-RIT with the newly developed NZ-16 is a promising therapeutic option for malignant mesothelioma.

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.

Development of a Compartmental Pharmacokinetic Model for Molecular Radiotherapy with 131I-CLR1404

Pharmacokinetic modeling of the radiopharmaceuticals used in molecular radiotherapy is an important step towards accurate radiation dosimetry of such therapies. In this paper, we present a pharmacokinetic model for CLR1404, a phospholipid ether analog that, labeled with ¹²⁴I/¹³¹I, has emerged as a promising theranostic agent. We follow a systematic approach for the model construction based on a decoupling process applied to previously published experimental data, and using the goodness-of-fit, Sobol’s sensitivity analysis, and the Akaike Information Criterion to construct the optimal form of the model, investigate potential simplifications, and study factor prioritization. This methodology was applied to previously published experimental human time-activity curves for 9 organs. The resulting model consists of 17 compartments involved in the CLR1404 metabolism. Activity dynamics in most tissues are well described by a blood contribution plus a two-compartment system, describing fast and slow uptakes. The model can fit both clinical and pre-clinical kinetic data of ¹²⁴I/¹³¹I. In addition, we have investigated how simple fits (exponential and biexponential) differ from the complete model. Such fits, despite providing a less accurate description of time-activity curves, may be a viable alternative when limited data is available in a practical case.

Recommendations for Multicentre Clinical Trials Involving Dosimetry for Molecular Radiotherapy

Multicentre clinical trials involving a dosimetry component are becoming more prevalent in molecular radiotherapy and are essential to generate the evidence to support individualised approaches to treatment planning and to ensure that sufficient patients are recruited to achieve the statistical significance required. Quality assurance programmes should be considered to support the standardisation required to achieve meaningful results. Trials should be designed to ensure that dosimetry results from image acquisition systems across centres are comparable by incorporating steps to standardise the methodologies used for the quantification of images and dosimetry. Furthermore, it is essential to assess the expertise and resources available at each participating site prior to trial commencement. A quality assurance plan should be drawn up and training provided if necessary. Standardisation of quantification and dosimetry methodologies used in a trial are essential to ensure that results from different centres may be collated. In addition, appropriate uncertainty analysis should be carried out to correct for differences in methodologies between centres. Recommendations are provided to support dosimetry studies based on the experience of several previous and ongoing multicentre trials.

Scientific Developments in Imaging and Dosimetry for Molecular Radiotherapy

Molecular radiotherapy is a rapidly developing field with new vector and isotope combinations continually added to market. As with any radiotherapy treatment, it is vital that the absorbed dose and toxicity profile are adequately characterised. Methodologies for absorbed dose calculations for radiopharmaceuticals were generally developed to characterise stochastic effects and not suited to calculations on a patient-specific basis. There has been substantial scientific and technological development within the field of molecular radiotherapy dosimetry to answer this challenge. The development of imaging systems and advanced processing techniques enable the acquisition of accurate measurements of radioactivity within the body. Activity assessment combined with dosimetric models and radiation transport algorithms make individualised absorbed dose calculations not only feasible, but commonplace in a variety of commercially available software packages. The development of dosimetric parameters beyond the absorbed dose has also allowed the possibility to characterise the effect of irradiation by including biological parameters that account for radiation absorbed dose rates, gradients and spatial and temporal energy distribution heterogeneities. Molecular radiotherapy is in an exciting time of its development and the application of dosimetry in this field can only have a positive influence on its continued progression.

Personalisation of Molecular Radiotherapy through Optimisation of Theragnostics

Molecular radiotherapy, or targeted radionuclide therapy, uses systemically administered drugs bearing a suitable radioactive isotope, typically a beta emitter. These are delivered via metabolic or other physiological pathways to cancer cells in greater concentrations than to normal tissues. The absorbed radiation dose in tumour deposits causes chromosomal damage and cell death. A partner radiopharmaceutical, most commonly the same vector labelled with a different radioactive atom, with emissions suitable for gamma camera or positron emission tomography imaging, is used to select patients for treatment and to assess response. The use of these pairs of radio-labelled drugs, one optimised for therapy, the other for diagnostic purposes, is referred to as theragnostics. Theragnostics is increasingly moving away from a fixed number of defined activity administrations, to a much more individualised or personalised approach, with the aim of improving treatment outcomes, and minimising toxicity. There is, however, still significant scope for further progress in that direction. The main tools for personalisation are the following: imaging biomarkers for better patient selection; predictive and post-therapy dosimetry to maximise the radiation dose to the tumour while keeping organs at risk within tolerance limits; imaging for assessment of treatment response; individualised decision making and communication about radiation protection, adjustments for toxicity, inpatient and outpatient care.

Preclinical Pharmacokinetics and Dosimetry Studies of 124I/131I-CLR1404 for Treatment of Pediatric Solid Tumors in Murine Xenograft Models

Cancer is the second leading cause of death for children between the ages of 5 and 14 y. For children diagnosed with metastatic or recurrent solid tumors, for which the utility of external-beam radiotherapy is limited, the prognosis is particularly poor. The availability of tumor-targeting radiopharmaceuticals for molecular radiotherapy (MRT) has demonstrated improved outcomes in these patient populations, but options are nonexistent or limited for most pediatric solid tumors. 18-(p-iodophenyl)octadecylphosphocholine (CLR1404) is a novel antitumor alkyl phospholipid ether analog that broadly targets cancer cells. In this study, we evaluated the in vivo pharmacokinetics of ¹²⁴I-CLR1404 (CLR 124) and estimated theranostic dosimetry for ¹³¹I-CLR1404 (CLR 131) MRT in murine xenograft models of the pediatric solid tumors neuroblastoma, rhabdomyosarcoma, and Ewing sarcoma. Methods: Tumor-bearing mice were imaged with small-animal PET/CT to evaluate the whole-body distribution of CLR 124 and, correcting for differences in radioactive decay, predict that of CLR 131. Image volumes representing CLR 131 provided input for Geant4 Monte Carlo simulations to calculate subject-specific tumor dosimetry for CLR 131 MRT. Pharmacokinetics for CLR 131 were extrapolated to adult and pediatric humans to estimate normal-tissue dosimetry. In neuroblastoma, a direct comparison of CLR 124 with ¹²⁴I-metaiodobenzylguanidine (¹²⁴I-MIBG) in an MIBG-avid model was performed. Results: In vivo pharmacokinetics of CLR 124 showed selective uptake and prolonged retention across all pediatric solid tumor models investigated. Subject-specific tumor dosimetry for CLR 131 MRT presents a correlative relationship with tumor-growth delay after CLR 131 MRT. Peak uptake of CLR 124 was, on average, 22% higher than that of ¹²⁴I-MIBG in an MIBG-avid neuroblastoma model. Conclusion: CLR1404 is a suitable theranostic scaffold for dosimetry and therapy with potentially broad applicability in pediatric oncology. Given the ongoing clinical trials for CLR 131 in adults, these data support the development of pediatric clinical trials and provide detailed dosimetry that may lead to improved MRT treatment planning.

Analysis of a National Programme for Selective Internal Radiation Therapy for Colorectal Cancer Liver Metastases

AIMS

Patients with chemotherapy-refractory colorectal cancer liver metastases have limited therapeutic options. Selective internal radiation therapy (SIRT) delivers yttrium 90 microspheres as a minimally invasive procedure. This prospective, single-arm, observational, service-evaluation study was part of National Health Service England Commissioning through Evaluation.

METHODS

Patients eligible for treatment had histologically confirmed carcinoma with liver-only/liver-dominant metastases with clinical progression during or following oxaliplatin-based and irinotecan-based chemotherapy. All patients received SIRT plus standard of care. The primary outcome was overall survival; secondary outcomes included safety, progression-free survival (PFS) and liver-specific PFS (LPFS).

RESULTS

Between December 2013 and March 2017, 399 patients were treated in 10 centres with a median follow-up of 14.3 months (95% confidence interval 9.2-19.4). The median overall survival was 7.6 months (95% confidence interval 6.9-8.3). The median PFS and LPFS were 3.0 months (95% confidence interval 2.8-3.1) and 3.7 months (95% confidence interval 3.2-4.3), respectively. During the follow-up period, 143 patients experienced an adverse event and 8% of the events were grade 3.

CONCLUSION

Survival estimates from this pragmatic study show clinical outcomes attainable in the National Health Service comparable with previously published data. This study shows the value of a registry-based commissioning model to aid national commissioning decisions for highly specialist cancer treatments.

Radiosynthesis of microtubule-targeted theranostic methyl N-[5-(3'-radiohalobenzoyl)-1H-benzimidazol-2-yl]carbamates

Microtubules are a target for a broad spectrum of drugs used as chemotherapeutics to treat hematological malignancies and solid tumors. Most of these drugs have significant dose-limiting toxicities including peripheral neuropathies that can be debilitating and permanent. In an ongoing effort to develop safer and more effective drugs, benzimidazole-based compounds are being developed as replacement for vincristine and similar agents. In this report, we describe radiosyntheses of novel microtubule-targeting methyl N-[5-(3'-radiohalobenzoyl)-1H-benzimidazol-2-yl]carbamates 4 that are intended as potential imaging agents and molecular radiotherapeutics. 125 I- and 131 I-radiolabeled derivatives were prepared either by direct radioiodination of methyl N-(6-benzoyl-1H-benzimidazol-2-yl)carbamate 1 or radioiododestannylation of the corresponding stannane precursor 3. The direct radioiodination was conducted in a solution of 1 in triflic acid and produced after ~1 hour at elevated temperatures and HPLC purification on average 62% of the no-carrier added products 125 I-4 and 131 I-4. Radioiododestannylation of 3'-trimethylstannane 3 proceeded with ease at room temperature in the presence of H2 O2 as the oxidant and produced no-carrier-added 125 I-4 and 131 I-4 in high isolated yields, on average 85%. The radiohalodestannylation protocol is universal and can be applied to other radiohalides including 124 I to produce 124 I-4, a positron emission tomography agent, and 211 At to produce 211 At-4, an α-particle emitting radiotherapeutic.

Dual-Nuclide Radiopharmaceuticals for Positron Emission Tomography Based Dosimetry in Radiotherapy

Improvement of the accuracy of dosimetry in radionuclide therapy has the potential to increase patient safety and therapeutic outcomes. Although positron emission tomography (PET) is ideally suited for acquisition of dosimetric data because PET is inherently quantitative and offers high sensitivity and spatial resolution, it is not directly applicable for this purpose because common therapeutic radionuclides lack the necessary positron emission. This work reports on the synthesis of dual-nuclide labeled radiopharmaceuticals with therapeutic and PET functionality, which are based on common and widely available metal radionuclides. Dual-chelator conjugates, featuring interlinked cyclen- and triazacyclononane-based polyphosphinates DOTPI and TRAP, allow for strictly regioselective complexation of therapeutic (e.g., 177 Lu, 90 Y, or 213 Bi) and PET (e.g., 68 Ga) radiometals in the same molecular framework by exploiting the orthogonal metal ion selectivity of these chelators (DOTPI: large cations, such as lanthanide(III) ions; TRAP: small trivalent ions, such as GaIII ). Such DOTPI-TRAP conjugates were decorated with 3 Gly-urea-Lys (KuE) motifs for targeting prostate-specific membrane antigen (PSMA), employing Cu-catalyzed (CuAAC) as well as strain-promoted (SPAAC) click chemistry. These were labeled with 177 Lu or 213 Bi and 68 Ga and used for in vivo imaging of LNCaP (human prostate carcinoma) tumor xenografts in SCID mice by PET, thus proving practical applicability of the concept.

Re-thinking the role of radiometal isotopes: Towards a future concept for theranostic radiopharmaceuticals

The potential and future role of certain metal radionuclides, for example, ⁴⁴ Sc, ⁸⁹ Zr, ⁸⁶ Y, ⁶⁴ Cu, ⁶⁸ Ga, ¹⁷⁷ Lu, ²²⁵ Ac, and ²¹³ Bi, and several terbium isotopes has been controversially discussed in the past decades. Furthermore, the possible benefits of "matched pairs" of isotopes for tandem applications of diagnostics and therapeutics (theranostics) have been emphasized, while such approaches still have not made their way into routine clinical practice. Analysis of bibliographical data illustrates how popularity of certain nuclides has been promoted by cycles of availability and applications. We furthermore discuss the different practical requirements for diagnostic and therapeutic radiopharmaceuticals and the resulting consequences for efficient development of clinically useful pairs of radionuclide theranostics, with particular emphasis on the underlying economical factors. Based on an exemplary assessment of overall production costs for ⁶⁸ Ga and ¹⁸ F radiopharmaceuticals, we venture a look into the future of theranostics and predict that high-throughput PET applications, that is, diagnosis of frequent conditions, will ultimately rely on ¹⁸ F tracers. PET radiometals will occupy a niche in the clinical low-throughput sector (diagnosis of rare diseases), but above all, dominate preclinical research and clinical translation. Matched isotope pairs will be of lesser relevance for theranostics but may become important for future PET-based therapeutic dosimetry.

A radiobiological model of metastatic burden reduction for molecular radiotherapy: application to patients with bone metastases

Skeletal tumour burden is a biomarker of prognosis and survival in cancer patients. This study proposes a novel method based on the linear quadratic model to predict the reduction in metastatic tumour burden as a function of the absorbed doses delivered from molecular radiotherapy treatments. The range of absorbed doses necessary to eradicate all the bone lesions and to reduce the metastatic burden was investigated in a cohort of 22 patients with bone metastases from castration-resistant prostate cancer. A metastatic burden reduction curve was generated for each patient, which predicts the reduction in metastatic burden as a function of the patient mean absorbed dose, defined as the mean of all the lesion absorbed doses in any given patient. In the patient cohort studied, the median of the patient mean absorbed dose predicted to reduce the metastatic burden by 50% was 89 Gy (interquartile range: 83-105 Gy), whilst a median of 183 Gy (interquartile range: 107-247 Gy) was found necessary to eradicate all metastases in a given patient. The absorbed dose required to eradicate all the lesions was strongly correlated with the variability of the absorbed doses delivered to multiple lesions in a given patient (r  =  0.98, P  <  0.0001). The metastatic burden reduction curves showed a potential large reduction in metastatic burden for a small increase in absorbed dose in 91% of patients. The results indicate the range of absorbed doses required to potentially obtain a significant survival benefit. The metastatic burden reduction method provides a simple tool that could be used in routine clinical practice for patient selection and to indicate the required administered activity to achieve a predicted patient mean absorbed dose and reduction in metastatic tumour burden.

Depth-dependent concentrations of hematopoietic stem cells in the adult skeleton: Implications for active marrow dosimetry

PURPOSE

The hematopoietically active (or red) bone marrow is the target tissue assigned in skeletal dosimetry models for assessment of stochastic effects (leukemia induction) as well as tissue reactions (marrow toxicity). Active marrow, however, is in reality a surrogate tissue region for specific cell populations, namely the hematopoietic stem and progenitor cells. Present models of active marrow dosimetry implicitly assume that these cells are uniformly localized throughout the marrow spaces of trabecular spongiosa. Data from Watchman et al. and Bourke et al., however, clearly indicate that there is a substantial spatial concentration gradient of these cells with the highest concentrations localized near the bone trabeculae surfaces. The purpose of the present study was thus to explore the dosimetric implications of these spatial gradients on active marrow dosimetry.

METHODS

Images of several bone sites from a 45-yr female were retagged to group active marrow voxels into 50 μm increments of marrow depth, after which electron and alpha-particle depth-dependent specific absorbed fractions were computed for four source tissues - active marrow, inactive marrow, bone trabeculae volumes, and bone trabeculae surfaces. Corresponding depth-dependent S values (dose to a target tissue per decay in a source tissue) were computed and further weighted by the relative target cell concentration. These depth-weighted radionuclide S values were systematically compared to the more traditional volume-averaged radionuclide S values of the MIRD schema for both individual bones of the skeleton and their skeletal-averaged quantities.

RESULTS

For both beta-emitters and alpha-emitters localized in the active and inactive marrow, depth-weighted S values were shown to differ from volume-averaged S values by only a few percent, as dose gradients across the marrow tissues are nonexistent. For bone volume and bone surface sources of alpha-emitters and lower energy beta-emitters, when marrow dose gradients are expected, explicit consideration of target cell spatial concentration gradients are shown to significantly impact marrow dosimetry.

CONCLUSIONS

For medical isotopes currently utilized for treatment of skeletal metastases, namely ¹⁵³ Sm and ²²³ Ra, accounting for hematopoietic stem and progenitor cell concentration gradients resulted in maximum percent differences to reference skeletal-averaged S values of ~21% and 55%, respectively.

A Phase 1, Open-Label Study of the Biodistribution, Pharmacokinetics, and Dosimetry of 223Ra-Dichloride in Patients with Hormone-Refractory Prostate Cancer and Skeletal Metastases

The aim of this single-site, open-label clinical trial was to determine the biodistribution, pharmacokinetics, absorbed doses, and safety from 2 sequential weight-based administrations of (223)Ra-dichloride in patients with bone metastases due to castration-refractory prostate cancer.

METHODS

Six patients received 2 intravenous injections of (223)Ra-dichloride, 6 wk apart, at 100 kBq/kg of whole-body weight. The pharmacokinetics and biodistribution as a function of time were determined, and dosimetry was performed for a range of organs including bone surfaces, red marrow, kidneys, gut, and whole body using scintigraphic imaging; external counting; and blood, fecal, and urine collection. Safety was assessed from adverse events.

RESULTS

The injected activity cleared rapidly from blood, with 1.1% remaining at 24 h. The main route of excretion was via the gut, although no significant toxicity was reported. Most of the administered activity was taken up rapidly into bone (61% at 4 h). The range of absorbed doses delivered to the bone surfaces from α emissions was 2,331-13,118 mGy/MBq. The ranges of absorbed doses delivered to the red marrow were 177-994 and 1-5 mGy/MBq from activity on the bone surfaces and from activity in the blood, respectively. No activity-limiting toxicity was observed at these levels of administration. The absorbed doses from the second treatment were correlated significantly with the first for a combination of the whole body, bone surfaces, kidneys, and liver.

CONCLUSION

A wide range of interpatient absorbed doses was delivered to normal organs. Intrapatient absorbed doses were significantly correlated between the 2 administrations for any given patient. The lack of gastrointestinal toxicity is likely due to the low absorbed doses delivered to the gut wall from the gut contents. The lack of adverse myelotoxicity implies that the absorbed dose delivered from the circulating activity may be a more relevant guide to the potential for marrow toxicity than that due to activity on the bone surfaces.

A review of 3D image-based dosimetry, technical considerations and emerging perspectives in 90Y microsphere therapy

Yttrium-90 radioembolization (⁹⁰Y-RE) is a well-established therapy for the treatment of hepatocellular carcinoma (HCC) and also of metastatic liver deposits from other malignancies. Nuclear Medicine and Cath Lab diagnostic imaging takes a pivotal role in the success of the treatment, and in order to fully exploit the efficacy of the technique and provide reliable quantitative dosimetry that are related to clinical endpoints in the era of personalized medicine, technical challenges in imaging need to be overcome. In this paper, the extensive literature of current ⁹⁰Y-RE techniques and challenges facing it in terms of quantification and dosimetry are reviewed, with a focus on the current generation of 3D dosimetry techniques. Finally, new emerging techniques are reviewed which seek to overcome these challenges, such as high-resolution imaging, novel surgical procedures and the use of other radiopharmaceuticals for therapy and pre-therapeutic planning.

Radiation exposure to comforters and carers during paediatric molecular radiotherapy

BACKGROUND

To show whether the incidental radiation exposure received by comforters and carers of children undergoing molecular radiotherapy was kept as low as reasonably achievable and was within English national dose constraints.

PROCEDURE

The radiation exposure of adult comforters and carers was routinely monitored with a whole body personal dose meter while the child was in hospital. Data were collected on iodine-131 meta-iodobenzylguanidine (¹³¹ I-mIBG), lutetium-177 DOTATATE (¹⁷⁷ Lu-DOTATATE), and iodine-131 sodium iodide (¹³¹ I-NaI) treatments.

RESULTS

Data were available for 50 treatments with high-administered activity double-infusion ¹³¹ I-mIBG and 12 single administrations; 15 ¹⁷⁷ Lu-DOTATATE treatments and 28 ¹³¹ I-NaI administrations. The median age was 7 years (1-18). The median administered activity of: ¹³¹ I-mIBG was 16.2 GBq (6.8-59 GBq) for double infusion patients and 8.1 GBq (5.26-16.25 GBq) for single administrations; ¹⁷⁷ Lu-DOTATATE was 7.2 GBq (2.5-7.5 GBq); and ¹³¹ I-NaI was 3 GBq for thyroid remnant ablation and 5.5 GBq for cancer therapy. The median number of comforters and carers for all administrations was 2 (range 1-9). The median exposure values for comforters and carers for high-administered activity ¹³¹ I-mIBG administrations was 302 µSv (0-5282 µSv); for single fraction ¹³¹ I-mIBG 163 µSv (3-3104 µSv); ¹⁷⁷ Lu-DOTATATE 6 µSv (1-79 µSv); and ¹³¹ I-NaI 37 µSv (0-274 µSv). Only one of the comforters and carers exceeded the dose constraint of 5 mSv.

CONCLUSIONS

Doses to comforters and carers were in all but one case within the dose constraint nationally recommended by the Health Protection Agency, now part of Public Health England. New evidence is presented which show that comforter and carer radiation exposure levels from paediatric molecular radiotherapy in routine clinical practice are acceptably low. Pediatr Blood Cancer 2015;62:235-239. © 2014 Wiley Periodicals, Inc.

Preclinical evaluation of investigational radiopharmaceutical RISAD-P intended for use as a diagnostic and molecular radiotherapy agent for prostate cancer

BACKGROUND

The androgen receptor (AR) plays a dominant role in the pathogenesis of prostate cancer. 5-Radioiodo-3'-O-(17β-succinyl-5α-androstan-3-one)-2'-deoxyuridin-5'-yl phosphate (RISAD-P) is an AR-targeting reagent developed for noninvasive assessment of AR and proliferative status of the AR-expressing tumors, and for molecular radiotherapy with Auger electron-emitting radionuclides. In this study, the preclinical toxicity and targeting potential of RISAD-P was evaluated.

METHODS

Effects of nonradioactive ISAD-P and RISAD-P labeled with (123) I, (124) I, and (125) I were evaluated in male mice. Expanded-acute single dose toxicity studies, hematologic toxicity, liver and kidney function, pharmacokinetics, biodistribution, and imaging studies were conducted. Imaging and pilot therapy studies were conducted in transgenic mice.

RESULTS

RISAD-P is not toxic at doses projected for clinical use. Its tissue distribution compares favorably with the distribution reported for (18) F-dihydrotestosterone derivatives. RISAD-P has excellent prostate cancer targeting properties. One hour after (125) IRISAD-P administration, nearly 10% of the injected dose is associated with prostate tumor. The tumor clearance is biphasic and plateaus between 24 and 48 hr post-injection. The estimated radiation doses calculated for 1 g tumor using the MIRD convention are well within the therapeutic range with values of 170, 250, 1,240 Gy × MBq(-1)  × g(-1) for (125) I-, (123) I-, and (124) I-labeled RISAD-P, respectively. The transient uptake of radioactivity is observed in the genitourinary tract and stomach. Without the potassium iodide blockade, thyroid uptake is also observed.

CONCLUSIONS

Biodistribution, toxicity, and radiation dosimetry studies suggest that RISAD-P holds characteristics of a promising candidate for imaging of AR expression and tumor proliferation, as well as molecular radiotherapy for metastatic or locally, regionally advanced prostate cancer.

Co-targeting androgen receptor and DNA for imaging and molecular radiotherapy of prostate cancer: in vitro studies

BACKGROUND

The androgen receptor (AR) axis, the key growth and survival pathway in prostate cancer, remains a prime target for drug development. 5-Radioiodo-3'-O-(17β-succinyl-5α-androstan-3-one)-2'-deoxyuridin-5'-yl phosphate (RISAD-P) is the AR-seeking reagent developed for noninvasive assessment of AR and proliferative status, and for molecular radiotherapy of prostate cancer with Auger electron-emitting radionuclides.

METHODS

RISAD-P radiolabeled with 123I, 124I, and 125I were synthesized using a common stannylated precursor. The cellular uptake, subcellular distribution, and radiotoxicity of 123I-, 124I-, and (125) IRISAD-P were measured in LNCaP, DU145, and PC-3 cell lines expressing various levels of AR.

RESULTS

The uptake of RISAD-P by prostate cancer cells is proportional to AR levels and independent of the radionuclide. The intracellular accumulation of radioactivity is directly proportional to the extracellular concentration of RISAD-P and the duration of exposure. Initially, RISAD-P is trapped in the cytoplasm. Within 24 hr, radioactivity is associated exclusively with DNA. The RISAD-P radiotoxicity is determined by the radionuclide; however, the cellular responses are directly proportional to the AR expression levels. LNCaP cells expressing high levels of AR are killed at the rate of up to 60% per day after a brief 1 hr RISAD-P treatment. For the first time, the AR expression in PC-3 and DU 145 cells, generally reported as AR-negative, was quantitated by the ultra sensitive RISAD-P-based method.

CONCLUSIONS

RISAD-P is a theranostic drug, which targets AR. Its subcellular metabolite participates in DNA synthesis. RISAD-P is a promising candidate for imaging of the AR expression and tumor proliferation as well as molecular radiotherapy of prostate cancer.

Molecular targeted α-particle therapy for oncologic applications

OBJECTIVE

A significant challenge facing traditional cancer therapies is their propensity to significantly harm normal tissue. The recent clinical success of targeting therapies by attaching them to antibodies that are specific to tumor-restricted biomarkers marks a new era of cancer treatments.

CONCLUSION

In this article, we highlight the recent developments in α-particle therapy that have enabled investigators to exploit this highly potent form of therapy by targeting tumor-restricted molecular biomarkers.

Planning combined treatments of external beam radiation therapy and molecular radiotherapy

Molecular radiotherapy (MRT) with radiolabeled molecules has being constantly evolving, leading to notable results in cancer treatment. In some cases, the absorbed doses delivered to tumors by MRT are sufficient to obtain complete responses; in other cases, instead, to be effective, MRT needs to be combined with other therapeutic approaches. Recently, several studies proposed the combination of MRT with external beam radiation therapy (EBRT). Some describe the theoretical basis within radiobiological models, others report the results of clinical phase I-II studies aimed to assess the feasibility and tolerability. The latter includes the treatment of various tumors, such as meningiomas, paragangliomas, non-Hodgkin's lymphomas, bone, brain, hepatic, and breast lesions. The underlying principle of combined MRT and EBRT is the possibility of exploiting the full potential of each modality, given the different organs at risk. Target tissues can indeed receive a higher irradiation, while respecting the threshold limits of more than one critical tissue. Nevertheless, clinical trials are empirical and optimization is still a theoretical issue. This article describes the state of the art of combined MRT and EBRT regarding the rationale and the results of clinical studies, with special focus on the possibility of treatment improvement.

Treatment planning in molecular radiotherapy

In molecular radiotherapy a radionuclide or a radioactively labelled pharmaceutical is administered to the patient. Treatment planning therefore comprises the determination of activity to administer. This administered activity should maximize tumour cell sterilization while minimizing normal tissue damage. In this work we present different approaches that are frequently used for determining the suitable activity. These approaches may be cohort- based as in chemotherapy, or patient-specific using dosimetry based on individual biokinetics. The approaches are different with respect to the input complexity, the corresponding costs and - in consequence - the quality of the therapy. In addition, a general scheme for data collection and analysis is proposed. To develop an effective and safe treatment, elaborate data need to be obtained. The main challenges, however, are collecting these complex data and analyse them properly.