Radium-223 dichloride: a review of its use in patients with castration-resistant prostate cancer with symptomatic bone metastases

First measurements of radon-220 diffusion in mice tumors, towards treatment planning in diffusing alpha-emitters radiation therapy

BACKGROUND

Diffusing alpha-emitters radiation therapy ("Alpha-DaRT") is a new method for treating solid tumors with alpha particles, relying on the release of the short-lived alpha-emitting daughter atoms of radium-224 from interstitial sources inserted into the tumor. Alpha-DaRT tumor dosimetry is governed by the spread of radium's progeny around the source, as described by an approximate framework called the "diffusion-leakage model". The most important model parameters are the diffusion lengths of radon-220 and lead-212, and their estimation is therefore essential for treatment planning.

PURPOSE

Previous works have provided initial estimates for the dominant diffusion length, by measuring the activity spread inside mice-borne tumors several days after the insertion of an Alpha-DaRT source. The measurements, taken when lead-212 was in secular equilibrium with radium-224, were interpreted as representing the lead-212 diffusion length. The aim of this work is to provide first experimental estimates for the diffusion length of radon-220, using a new methodology.

METHODS

The diffusion length of radon-220 was estimated from autoradiography measurements of histological sections taken from 24 mice-borne subcutaneous tumors of five different types. Unlike previous studies, the source dwell time inside the tumor was limited to 30 min, to prevent the buildup of lead-212. To investigate the contribution of potential non-diffusive processes, experiments were done in two sets: fourteen in vivo tumors, where during the treatment the tumors were still carried by the mice with active blood supply, and 10 ex-vivo tumors, where the tumors were excised before source insertion and kept in a medium at37 ∘ C $37^\circ {\text{C}}$ with the source inside.

RESULTS

The measured diffusion lengths of radon-220, extracted by fitting the recorded activity pattern up to 1.5 mm from the source, lie in the range0.25 - 0.6 mm ${0.25-0.6}\nobreakspace {\text{mm}}$ , with no significant difference between the average values measured in in-vivo and ex-vivo tumors:L R n i n - v i v o = 0.40 ± 0.08 mm $L_{Rn}^{in-vivo}=0.40{\pm }0.08\nobreakspace {\text{mm}}$ versusL R n e x - v i v o = 0.39 ± 0.07 mm $L_{Rn}^{ex-vivo}=0.39{\pm }0.07\nobreakspace {\text{mm}}$ . However, in-vivo tumors display an enhanced spread of activity 2-3 mm away from the source. This effect is not explained by the current model and is much less pronounced in ex-vivo tumors.

CONCLUSIONS

The average measured radon-220 diffusion lengths in both in-vivo and ex-vivo tumors are consistent with published data on the diffusion length of radon in water and lie close to the upper limit of the previously estimated range of0.2 - 0.4 mm $0.2-0.4\nobreakspace {\text{mm}}$ . The observation that close to the source there is no apparent difference between in-vivo and ex-vivo tumors, and the good agreement with the theoretical model in this region suggest that the spread of radon-220 is predominantly diffusive in this region. The departure from the model prediction in in-vivo tumors at large radial distances may hint at potential vascular contribution, which will be the subject of future works.

The low-LET radiation contribution to the tumor dose in diffusing alpha-emitters radiation therapy

BACKGROUND

Diffusing alpha-emitters Radiation Therapy ("Alpha DaRT") is a new technique that enables the use of alpha particles for the treatment of solid tumors. Alpha DaRT employs interstitial sources carrying a few μ $\mu$ Ci of224 $^{224}$ Ra below their surface, designed to release a chain of short-lived atoms (progeny of224 $^{224}$ Ra) which emit alpha particles, along with beta, Auger, and conversion electrons, x- and gamma rays. These atoms diffuse around the source and create-primarily through their alpha decays-a lethal high-dose region measuring a few millimeters in diameter.

PURPOSE

While previous studies focused on the dose from the alpha emissions alone, this work addresses the electron and photon dose contributed by the diffusing atoms and by the atoms remaining on the source surface, for both a single Alpha DaRT source and multi-source lattices. This allows to evaluate the low-LET contribution to the tumor dose and tumor cell survival, and demonstrate the sparing of surrounding healthy tissue.

METHODS

The low-LET dose is calculated using the EGSnrc and FLUKA Monte Carlo (MC) codes. We compare the results of a simple line-source approximation with no diffusion to those of a full simulation, which implements a realistic source geometry and the spread of diffusing atoms. We consider two opposite scenarios: one with low diffusion and high212 $^{212}$ Pb leakage, and the other with high diffusion and low leakage. The low-LET dose in source lattices is calculated by superposition of single-source contributions. Its effect on cell survival is estimated with the linear quadratic model in the limit of low dose rate.

RESULTS

For sources carrying 3  μ $\umu$ Ci/cm224 $^{224}$ Ra arranged in a hexagonal lattice with 4 mm spacing, the minimal low-LET dose between sources is∼ 18 - 30 $\sim 18-30$  Gy for the two test cases and is dominated by the beta contribution. The low-LET dose drops below 5 Gy∼ 3 $\sim 3$  mm away from the outermost source in the lattice with an effective maximal dose rate of< 0.04 $<0.04$  Gy/h. The accuracy of the line-source/no-diffusion approximation is∼ 15 % $\sim 15\%$ for the total low-LET dose over clinically relevant distances (2-4 mm). The low-LET dose reduces tumor cell survival by a factor of∼ 2 - 200 $\sim 2-200$ .

CONCLUSIONS

The low-LET dose in Alpha DaRT can be modeled by conventional MC techniques with appropriate leakage corrections to the source activity. For 3  μ $\umu$ Ci/cm224 $^{224}$ Ra sources, the contribution of the low-LET dose can reduce cell survival inside the tumor by up to two orders of magnitude. The low-LET dose to surrounding healthy tissue is negligible. Increasing source activities by a factor of 5 can bring the low-LET dose itself to therapeutic levels, in addition to the high-LET dose contributed by alpha particles, leading to a "self-boosted" Alpha DaRT configuration, and potentially allowing to increase the lattice spacing.

Theranostics and Patient-Specific Dosimetry

Radiopharmaceutical therapy (RPT) is an invigorated form of cancer therapy that systemically delivers targeted radioactive drugs to cancer cells. Theranostics is a type of RPT that utilizes imaging, either of the RPT drug directly or a companion diagnostic, to inform whether a patient will benefit from the treatment. Given the ability to image the drug onboard theranostic treatments also lends itself readily to patient-specific dosimetry, which is a physics-based process that determines the overall absorbed dose burden to healthy organs and tissues and tumors in patients. While companion diagnostics identify who will benefit from RPT treatments, dosimetry determines how much activity these beneficiaries can receive to maximize therapeutic efficacy. Clinical data is starting to accrue suggesting tremendous benefits when dosimetry is performed for RPT patients. RPT dosimetry, which was once performed by florid and often inaccurate workflows, can now be performed more efficiently and accurately with FDA-cleared dosimetry software. Therefore, there is no better time for the field of oncology to adopt this form of personalize medicine to improve outcomes for cancer patients.

Small-molecule inhibitors, immune checkpoint inhibitors, and more: FDA-approved novel therapeutic drugs for solid tumors from 1991 to 2021

The United States Food and Drug Administration (US FDA) has always been a forerunner in drug evaluation and supervision. Over the past 31 years, 1050 drugs (excluding vaccines, cell-based therapies, and gene therapy products) have been approved as new molecular entities (NMEs) or biologics license applications (BLAs). A total of 228 of these 1050 drugs were identified as cancer therapeutics or cancer-related drugs, and 120 of them were classified as therapeutic drugs for solid tumors according to their initial indications. These drugs have evolved from small molecules with broad-spectrum antitumor properties in the early stage to monoclonal antibodies (mAbs) and antibody‒drug conjugates (ADCs) with a more precise targeting effect during the most recent decade. These drugs have extended indications for other malignancies, constituting a cancer treatment system for monotherapy or combined therapy. However, the available targets are still mainly limited to receptor tyrosine kinases (RTKs), restricting the development of antitumor drugs. In this review, these 120 drugs are summarized and classified according to the initial indications, characteristics, or functions. Additionally, RTK-targeted therapies and immune checkpoint-based immunotherapies are also discussed. Our analysis of existing challenges and potential opportunities in drug development may advance solid tumor treatment in the future.

FDA-Approved Small Molecule Compounds as Drugs for Solid Cancers from Early 2011 to the End of 2021

Solid cancers are the most common types of cancers diagnosed globally and comprise a large number of deaths each year. The main challenge currently in drug development for tumors raised from solid organs is to find more selective compounds, which exploit specific molecular targets. In this work, the small molecule drugs registered by the Food and Drug Administration (FDA) for solid cancers treatment between 2011 and 2022 were identified and analyzed by investigating a type of therapy they are used for, as well as their structures and mechanisms of action. On average, 4 new small molecule agents were introduced each year, with a few exceptions, for a total of 62 new drug approvals. A total of 50 of all FDA-approved drugs have also been authorized for use in the European Union by the European Medicines Agency (EMA). Our analysis indicates that many more anticancer molecules show a selective mode of action, i.e., 49 targeted agents, 5 hormone therapies and 3 radiopharmaceuticals, compared to less specific cytostatic action, i.e., 5 chemotherapeutic agents. It should be emphasized that new medications are indicated for use mainly for monotherapy and less for a combination or adjuvant therapies. The comprehensive data presented in this review can serve for further design and development of more specific targeted agents in clinical usage for solid tumors.

Radium-223 Dichloride in Peritoneal Dialysate Following Treatment of Metastatic Castration-resistant Prostate Cancer

ABSTRACT

Radium-223 dichloride was administrated to a patient with castration-resistant metastatic prostate cancer (mCRPC) undergoing in-home peritoneal dialysis. The peritoneal dialysate and tubing sets were collected following four treatment cycles and analyzed for removable contamination, dose rate, and radioactivity. The purpose of the study was to assess radiation safety concerns and compare data with patients having normal renal function. Sixty-two liters of dialysate in 22 samples were collected over 4 mo. The mean surface dose rate of the dialysate bags was 0.16 μSv h-1 (range 0.11-0.23 μSv h-1). The highest measured removable surface activity was 150 dpm per 100 cm2. The mean cumulative percent of administered activity was 1.7% (range 1.4-1.9%). The mean concentration of 223Ra in dialysate was 4.0 kBq L-1 (range 0.56-14.1 kBq L-1). There was no association between the measured 223Ra activity in dialysate with peritoneal dwell time or the number of exchanges following administration. The measurement of the 211Bi and 223Ra ratio in the dialysate compared with a standard showed an increase of 23% by 40 h post administration. The data presented suggest that 223Ra dialysate can be safely managed in the home without risk of radiation dose or contamination if minimal precautions are taken. Patients with normal renal function have been shown to excrete up to six times more 223Ra in urine compared to those undergoing hemodialysis or peritoneal dialysis. A potential consequence may be an increase in 223Ra activity transiting GI tract leading to intestinal effects.

Atomic Nanogenerators in Targeted Alpha Therapies: Curie's Legacy in Modern Cancer Management

Atomic in vivo nanogenerators such as actinium-225, thorium-227, and radium-223 are of increasing interest and importance in the treatment of patients with metastatic cancer diseases. This is due to their peculiar physical, chemical, and biological characteristics, leading to astonishing responses in otherwise resistant patients. Nevertheless, there are still a few obstacles and hurdles to be overcome that hamper the broader utilization in the clinical setting. Next to the limited supply and relatively high costs, the in vivo complex stability and the fate of the recoiling daughter radionuclides are substantial problems that need to be solved. In radiobiology, the mechanisms underlying treatment efficiency, possible resistance mechanisms, and late side effect occurrence are still far from being understood and need to be unraveled. In this review, the current knowledge on the scientific and clinical background of targeted alpha therapies is summarized. Furthermore, open issues and novel approaches with a focus on the future perspective are discussed. Once these are unraveled, targeted alpha therapies with atomic in vivo nanogenerators can be tailored to suit the needs of each patient when applying careful risk stratification and combination therapies. They have the potential to become one of the major treatment pillars in modern cancer management.

Actinium-225 for Targeted α Therapy: Coordination Chemistry and Current Chelation Approaches

The α-emitting radionuclide actinium-225 possesses nuclear properties that are highly promising for use in targeted α therapy (TAT), a therapeutic strategy that employs α particle emissions to destroy tumors. A key factor, however, that may hinder the clinical use of actinium-225 is the poor understanding of its coordination chemistry, which creates challenges for the development of suitable chelation strategies for this ion. In this article, we provide an overview of the known chemistry of actinium and a summary of the chelating agents that have been explored for use in actinium-225-based TAT. This overview provides a starting point for researchers in the field of TAT to gain an understanding of this valuable therapeutic radionuclide.

[A Comparison of Planar Sensitivity and Spatial Resolution among Different Collimators and Energy Windows on 223Ra Imaging]

OBJECTIVE

The present study aimed to reveal the influence of combination of different collimators and energy windows on the planar sensitivity and the spatial resolution during experimental ²²³Ra imaging, and to determine optimal imaging parameters.

METHODS

A vial type source containing ²²³Ra solution (4.55 MBq / 5.6 ml) was placed in the air at 100 mm away from the collimator surface. Planar images were acquired with LEHR, LMEGP, ELEGP and MEGP collimators on two dual-head gamma cameras (Symbia intevo (Siemens) and Infinia 3 (GE)). We compared three energy window combinations: 1) single window at 82 keV, 2) double window at 82+154 keV, 3) triple window at 82+154+270 keV. The energy spectrum, the sensitivity and the spatial resolution, such as full-width at half-maximum (FWHM) and full-width at tenth-maximum (FWTM), of each collimator were assessed.

RESULTS

Five energy spectra (at around 82, 154, 270, 351 and 405 keV) were essentially observed among four collimators. The sensitivity was high for LEHR collimator, then ELEGP and LMEGP collimator was 3-4 fold, which is greater than MEGP collimator. The 82 keV energy window of four collimators has best spatial resolution. Moreover, the spatial resolution of the 82 keV energy window with LMEGP and ELEGP collimator was almost equal to that of the triple window with MEGP collimator.

CONCLUSIONS

Optimal imaging parameters were single energy window using LMEGP or ELEGP, and then triple energy window using MEGP collimator.

Selection and monitoring of patients with metastatic castration-resistant prostate cancer for treatment with radium-223

Despite the improvement provided by androgenic suppression in the treatment of prostate cancer, most of tumors develop resistance to castration. However, new therapies have demonstrated an increase in patient survival such as radium-223 (Ra-223), an alpha emitter and calcium mimetic with the capability of targeting osteoblastic metastatic lesions. According to results of the ALSYMPCA phase III trial, Ra-223 has demonstrated its activity by improving symptoms and survival of patients with metastatic castration-resistant prostate cancer (mCRPC), symptomatic bone metastases, and no known visceral metastatic disease, without interfering with subsequent treatments. This review examines the key evidence to establish the best patient selection criteria to use Ra-223, how to assess the response to treatment, treatment-related toxicity, and follow-up, but also current research regarding imaging techniques and biomarkers to assess the efficacy of Ra-223. Finally, we briefly describe the clinical trials that are currently ongoing with Ra-223.

In Vitro and In Vivo Efficacy of a Novel CD33-Targeted Thorium-227 Conjugate for the Treatment of Acute Myeloid Leukemia

The clinical efficacy of the first approved alpha pharmaceutical, Xofigo (radium-223 dichloride, ²²³RaCl₂), has stimulated significant interest in the development of new alpha-particle emitting drugs in oncology. Unlike radium-223 (²²³Ra), the parent radionuclide thorium-227 (²²⁷Th) is able to form highly stable chelator complexes and is therefore amenable to targeted radioimmunotherapy. We describe the preparation and use of a CD33-targeted thorium-227 conjugate (CD33-TTC), which binds to the sialic acid receptor CD33 for the treatment of acute myeloid leukemia (AML). A chelator was conjugated to the CD33-targeting antibody lintuzumab via amide bonds, enabling radiolabeling with the alpha-emitter ²²⁷Th. The CD33-TTC induced in vitro cytotoxicity on CD33-positive cells, independent of multiple drug resistance (MDR) phenotype. After exposure to CD33-TTC, cells accumulated DNA double-strand breaks and were arrested in the G₂ phase of the cell cycle. In vivo, the CD33-TTC demonstrated antitumor activity in a subcutaneous xenograft mouse model using HL-60 cells at a single dose regimen. Dose-dependent significant survival benefit was further demonstrated in a disseminated mouse tumor model after single dose injection or administered as a fractionated dose. The data presented support the further development of the CD33-TTC as a novel alpha pharmaceutical for the treatment of AML. Mol Cancer Ther; 15(10); 2422-31. ©2016 AACR.

Pharmacokinetics of single dose radium-223 dichloride (BAY 88-8223) in Japanese patients with castration-resistant prostate cancer and bone metastases

OBJECTIVE

This open-label, non-randomized, phase I study examined the pharmacokinetics (PK) and radiation dosimetry of a single dose of radium-223 in Japanese patients with castration-resistant prostate cancer (CRPC) and bone metastases.

METHODS

Six male Japanese patients (mean age 72.5 years, range 65-79 years) with histologically or cytologically confirmed stage IV adenocarcinoma of the prostate were recruited. A single IV dose of radium-223 was delivered intravenously (IV) via slow bolus over a 2-5 min period: Cohort 1 received 50 kBq/kg and Cohort 2 received 100 kBq/kg.

RESULTS

Following IV injection, radium-223 was rapidly eliminated from the blood in a multi-phasic manner. The fraction of the injected activity of radium-223 retained in the whole body 24 h following injection was 85 %. Biodistribution results showed initial bone uptake was 52 % (range 41-57 %). The maximum activity of radium-223 in the bone was observed within 2 h of dosing. Activity of radium-223 passed through the small intestine within 24 h. No activity was detected in other organs. The major radiation dose from radium-223 was found in osteogenic cells; calculated absorbed doses in osteogenic cells and in the red marrow were 0.76 Gy/MBq and 0.09 Gy/MBq, respectively.

CONCLUSIONS

In Japanese patients with CRPC and bone metastases, radium-223 (IV) achieved maximum activity in the bone rapidly and passed through the intestine within 24 h, without signs of activity in other organs. The PK profile and absorbed radiation dose in organs and tissues in Japanese patients were similar to data from non-Japanese patients. Trial registration identification: NCT01565746.

[Bone metastases : New aspects of pathogenesis and systemic therapy]

The occurrence of bone metastases, in particular secondary to breast and prostate cancer, represents a complex medical condition that is debilitating for affected patients. In order to provide an efficient and personalized therapy, an interdisciplinary treatment approach is mandatory; therefore, systemic pharmacological therapy represents a core element of the overall treatment concept. In terms of pathophysiology, the cancer cells cause a massive disturbance of the local bone microenvironment, which as a rule leads to activation of bone resorbing osteoclasts. In addition to bisphosphonates, which can be considered classical antiresorptive agents, the monoclonal receptor activator of nuclear factor-kappa B ligand (RANKL) antibody denosumab has been in use in clinical practice since 2011. The alpha-emitting radioisotope Alpharadin was also recently approved for the treatment of metastatic prostate cancer. This article provides a summary of the most recent knowledge on the pathogenesis of how cancer cells alter the bone microenvironment as well as a review of established and future systemic treatment options.

The best of both worlds - managing the cancer, saving the bone

In the context of breast cancer, the importance of the skeleton in the regulation of primary tumour development and as a site for subsequent metastasis is well characterized. Our understanding of the contributions made by the host bone and bone marrow cells increasingly demonstrates the extent of the interaction between tumour cells and normal host cells. As a result, the need to develop and utilize therapies that can impede the growth and/or function of tumour cells while sparing normal host bone and bone marrow cells is immense and expanding. The need for these new treatments is, however, superimposed on the orthopaedic management of patients' quality of life, where pain control and continued locomotion are paramount. Indeed, the majority of the anticancer therapies used to date often result in direct or indirect damage to bone. Thus, although the bone microenvironment regulates tumour cell growth in bone, cells within the bone marrow niche also mediate many of the orthopaedic consequences of tumour progression as well as resistance to the antitumour effects of existing therapies. In this Review, we highlight the effects of existing cancer treatments on bone and the bone marrow microenvironment as well as the mechanisms mediating these effects and the current utility of modern orthopaedic interventions.

Cancer-targeted therapies and radiopharmaceuticals

The treatment of bone metastases remains a clinical challenge. Although a number of well-established agents, namely bisphosphonates and denosumab, are available to reduce the occurrence of skeletal-related events, additional cancer-targeted therapies are required to improve patients' prognosis and quality of life. This review focuses on novel targets and agents that are under clinical evaluation for the treatment of malignant bone diseases such as activin A, src and endothelin-1 inhibition or agents that are clinically approved and may positively influence bone, such as the mTOR inhibitor everolimus. In addition, the potential of alpharadin, a novel radiopharmaceutical approved for the treatment of prostatic bone disease, is discussed.

Radiolabeled nanobodies as theranostic tools in targeted radionuclide therapy of cancer

INTRODUCTION

The integration of diagnostic testing for the presence of a molecular target is of interest to predict successful targeted radionuclide therapy (TRNT). This so-called 'theranostic' approach aims to improve personalized treatment based on the molecular characteristics of cancer cells. Moreover, it offers new insights in predicting adverse effects and provides appropriate tools to monitor therapy responses. Recent findings using nanobodies emphasize their potential as theranostic tools in cancer treatment. Nanobodies are recombinant, small antigen-binding fragments that are derived from camelid heavy-chain-only antibodies.

AREAS COVERED

We review the current status of theranostic approaches in TRNT, with a focus on antibodies, peptides, scaffold proteins and emerging nanobodies. In recent years, nanobodies have been evaluated intensively for molecular imaging. In addition, novel data on TRNT using radiolabeled nanobodies for carcinomas and multiple myeloma highlight their promising opportunities in cancer treatment.

EXPERT OPINION

We trust that radiolabeled nanobodies will have a future potential as theranostic tools in cancer therapy, both for diagnosis as well as for TRNT.

Drug design strategies for the treatment of prostate cancer

INTRODUCTION

While metastatic prostate cancer remains an incurable tumor, remarkable progress has been made with novel drug design strategies for this incurable disease. Several new agents, including hormonal analogues, cytotoxic chemotherapy drugs, radionuclides and innovative targeted therapies, have recently been approved by the FDA for use in advanced and/or metastatic castrate-resistant prostate cancer. Furthermore, a growing number of new diagnostic or predictive genetic tests have also been incorporated into the management of this disease. Immunotherapy-based approaches have shown promise and have led to drug approvals. Other experimental approaches such as vascular targeting are in early translational clinical trials.

AREAS COVERED

Herein, the authors outline select state-of-the-art approaches in the field. They also discuss the current challenges and future opportunities in the medical care of prostate cancer patients.

EXPERT OPINION

An inherent challenge in the treatment of prostate cancer is to determine which patients need immediate aggressive treatment versus active surveillance. For patients needing aggressive treatment, integrating the sequence of therapeutic interventions, to provide the most benefit, remains a challenge that clinicians face. Recently, several genetic tests have been approved, facilitating early treatment decisions. Innovative targeted therapies are moving towards clinical applications, providing treatment options for tumors previously considered refractory to androgen ablation treatment.

From Traditional Drug Design to Catalytic Metallodrugs: A Brief History of the Use of Metals in Medicine

Traditional drug design has been effective in the development of therapies for a variety of disease states but there is a need for new approaches that will tackle new challenges and complement current paradigms. The use of metals in medicine has resulted in several successes and allows for the introduction of properties that cannot be achieved by use of organic compounds alone, but also introduces new challenges that can be addressed by a careful understanding of the principles of inorganic chemistry. Toward this end, the unique structural and coordination chemistry, as well as the reactivity of metals, has been used to design novel classes of therapeutic and diagnostic agents. This review briefly summarizes progress in the field of therapeutics, from the earliest use of metals to more recent efforts to design catalytic metallodrugs that promote the irreversible inactivation of therapeutically relevant targets.