The development and in-vivo behavior of tin containing radiopharmaceuticals--I. Chemistry, preparation, and biodistribution in small animals

A single arm phase II study of bone-targeted Sn-117 m-DTPA in symptomatic castration-resistant prostate cancer with skeletal metastases

BACKGROUND

Several bone-seeking radionuclides have been developed for palliation of metastatic bone pain since 1956, however, so far radium-223 dichloride is the first and only Food and Drug Administration (FDA) approved targeted alpha therapy for metastatic castration-resistant prostate cancer (mCRPC) based on ALSYMPCA phase 3 study. While radium-223 does improve pain and overall survival outcomes, the improvement can come at the expense of side effects such as bone marrow toxicity. The development of new and better treatment with long-standing pain relief is clearly an unmet medical need.

METHODS

The study is a non-randomized phase II study. The study population consists of 25 patients with CRPC who had progressed on any lines of prior therapies and whose serum testosterone level is less than 50 ng/dl and have metastatic lesions to at least two bone sites, with at least one site that has clinically meaningful pain at baseline (≥ 4 on an 11-point intensity scale). Eligible patients will be given two cycles of Sn-117 m-DTPA every 8 weeks or 56 days. Treatment will be administered by slow IV injection over 5-10 min. Retreatment after two cycles is allowed if patients meet the following retreatment criteria. The primary objective is to evaluate the efficacy of Sn-117 m-DTPA on sustained pain response in patients with CRPC metastatic to at least two bone sites and at least one with clinically meaningful pain at baseline (≥ 4 on an 11-point pain intensity scale). Sustained pain response is defined as: 1) achieving pain index ≤ 3 within a 12-week period and 2) maintaining pain index ≤ 3 over a 16-week period. The secondary objectives are: safety and tolerability, measurement of Sn-117 m-DTPA activity by gamma-camera dosimetry scans, therapeutic efficacy, time to the first symptomatic skeletal event, duration of pain response, changes in PSA and ALP levels, patient-reported outcomes and progression free survival and overall survival.

DISCUSSION

Sn-117 m-DTPA is a unique bone-targeting theranostic radiopharmaceutical agent that selectively binds most heavily to bone metastases sites. This study will be the first prospective phase II trial to assess the pain efficacy and anti-tumor activity of Sn-117 m-DTPA in mCRPC with at least one clinically meaningful pain at baseline.

TRIAL REGISTRATION

ClincialTrials.gov Identifier: NCT04616547.

Radiopharmaceutical Validation for Clinical Use

Radiopharmaceuticals are reemerging as attractive anticancer agents, but there are no universally adopted guidelines or standardized procedures for evaluating agent validity before early-phase trial implementation. To validate a radiopharmaceutical, it is desirous for the radiopharmaceutical to be specific, selective, and deliverable against tumors of a given, molecularly defined cancer for which it is intended to treat. In this article, we discuss four levels of evidence—target antigen immunohistochemistry, in vitro and in vivo preclinical experiments, animal biodistribution and dosimetry studies, and first-in-human microdose biodistribution studies—that might be used to justify oncology therapeutic radiopharmaceuticals in a drug-development sequence involving early-phase trials. We discuss common practices for validating radiopharmaceuticals for clinical use, everyday pitfalls, and commonplace operationalizing steps for radiopharmaceutical early-phase trials. We anticipate in the near-term that radiopharmaceutical trials will become a larger proportion of the National Cancer Institute Cancer Therapy Evaluation Program (CTEP) portfolio.

Pharmaceutical and clinical development of phosphonate-based radiopharmaceuticals for the targeted treatment of bone metastases

Therapeutic phosphonate-based radiopharmaceuticals radiolabeled with beta, alpha and conversion electron emitting radioisotopes have been investigated for the targeted treatment of painful bone metastases for >35years. We performed a systematic literature search and focused on the pharmaceutical development, preclinical research and early human studies of these radiopharmaceuticals. The characteristics of an ideal bone-targeting therapeutic radiopharmaceutical are presented and compliance with these criteria by the compounds discussed is verified. The importance of both composition and preparation conditions for the stability and biodistribution of several agents is discussed. Very few studies have described the characterization of these products, although knowledge on the molecular structure is important with respect to in vivo behavior. This review discusses a total of 91 phosphonate-based therapeutic radiopharmaceuticals, of which only six agents have progressed to clinical use. Extensive clinical studies have only been described for (186)Re-HEDP, (188)Re-HEDP and (153)Sm-EDTMP. Of these, (153)Sm-EDTMP represents the only compound with worldwide marketing authorization. (177)Lu-EDTMP has recently received approval for clinical use in India. This review illustrates that a thorough understanding of the radiochemistry of these agents is required to design simple and robust preparation and quality control methods, which are needed to fully exploit the potential benefits of these theranostic radiopharmaceuticals. Extensive biodistribution and dosimetry studies are indispensable to provide the portfolios that are required for assessment before human administration is possible. Use of the existing knowledge collected in this review should guide future research efforts and may lead to the approval of new promising agents.

Tumour control probability derived from dose distribution in homogeneous and heterogeneous models: assuming similar pharmacokinetics, (125)Sn-(177)Lu is superior to (90)Y-(177)Lu in peptide receptor radiotherapy

Clinical trials on (177)Lu-(90)Y therapy used empirical activity ratios. Radionuclides (RN) with larger beta maximal range could favourably replace (90)Y. Our aim is to provide RN dose-deposition kernels and to compare the tumour control probability (TCP) of RN combinations. Dose kernels were derived by integration of the mono-energetic beta-ray dose distributions (computed using Monte Carlo) weighted by their respective beta spectrum. Nine homogeneous spherical tumours (1-25 mm in diameter) and four spherical tumours including a lattice of cold, but alive, spheres (1, 3, 5, 7 mm in diameter) were modelled. The TCP for (93)Y, (90)Y and (125)Sn in combination with (177)Lu in variable proportions (that kept constant the renal cortex biological effective dose) were derived by 3D dose kernel convolution. For a mean tumour-absorbed dose of 180 Gy, 2 mm homogeneous tumours and tumours including 3 mm diameter cold alive spheres were both well controlled (TCP > 0.9) using a 75-25% combination of (177)Lu and (90)Y activity. However, (125)Sn-(177)Lu achieved a significantly better result by controlling 1 mm-homogeneous tumour simultaneously with tumours including 5 mm diameter cold alive spheres. Clinical trials using RN combinations should use RN proportions tuned to the patient dosimetry. (125)Sn production and its coupling to somatostatin analogue appear feasible. Assuming similar pharmacokinetics (125)Sn is the best RN for combination with (177)Lu in peptide receptor radiotherapy justifying pharmacokinetics studies in rodent of (125)Sn-labelled somatostatin analogues.

Labeling conditions, in vitro properties and biodistributions of various Sn-labeled complexes

Conditions for preparing Sn-EDTMP, Sn-DTPMP, Sn-TTHMP, Sn-HEDTMP and Sn-DTPA in aqueous solution in open air environments, and their in vitro properties including adsorption on hydroxyapatite (HA) and collagen (I) and binding to bovine serum albumin (BSA) were studied using (117m)Sn and 113Sn as tracers. Biodistributions of SnO2.xH2O.yEDTMP, SnO2.xH2O.yDTPMP, SnO2.xH2O.yTTHMP, SnO2.xH2O.yHEDTMP, SnO2.xH2O.yDTPA in normal mice were also tested. Based on the above experiments, the relationship between in vitro biochemical properties and biodistributions of these SnO2.xH2O.yLigands was investigated. The results show that Sn(IV)-Ligands are prone to hydrolysis into SnO2.xH2O.yLigands in aqueous solutions in open air environments, especially when the ligand is DTPA, when the molar ratio of metal to ligand is higher than 1:200, or when the pH of the solution is higher than 10. The in vitro experiments show that all of the SnO2.xH2O.yLigands bind strongly to BSA, and the binding percentages of SnO2.xH2O.yLigands to BSA are much higher than those of the corresponding Sn(IV)-Ligands. The biodistribution data indicate that all of the SnO2.xH2O.yLigands locate mainly in bone with little uptake in liver. When the binding percentages of SnO2.xH2O.yLigands to BSA are similar, those SnO2.xH2O.yLigands with higher adsorption on HA and collagen (I) undergo lower liver uptake.

Comparison of the predicted in vivo behaviour of the Sn(II)–APDDMP complex and the results as studied in a rodent model

In a quest for more effective radiopharmaceuticals for pain palliation of metastatic bone cancer, this paper relates results obtained with ((117m)Sn labelled) Sn(II) complexed to the bone seeking bisphosphonate, N,N-dimethylenephosphonate-1-hydroxy-3-aminopropylidenediphosphonate (APDDMP). APDDMP is synthesised from the known bone cancer pain palliation agent 1-hydroxy-3-aminopropylidenediphosphonate (APD, Pamindronate). This work is performed to utilise the idea that the low bone marrow radio toxicity of (117m)Sn could afford a highly effective radiopharmaceutical in pain palliation but also in the curative treatment of bone metastasis. Complex-formation constants of APDDMP with the important blood plasma metal-ions, Ca(2+), Mg(2+), Zn(2+) as well as the added metal ion, Sn(2+) were measured by glass electrode potentiometry at 25 degrees C and I = 150 mM. Blood plasma models were constructed using the computer code ECCLES and the results compared with those gathered from tests on a rodent model. The ((117m)Sn-labelled) Sn(II)-APDDMP complex was found to have only some liver and bone uptake although a high trabecular to normal bone ratio was recorded. From the blood plasma model this was shown to be primarily due to the high affinity of APDDMP for Ca(II) causing some of the Sn(II)-APDDMP complex to dissociate. High kidney uptake and excretion as well as high bladder uptake was recorded which was shown to be due to the dissociation of the Sn(II)-APDDMP complex in blood plasma. Animal model observations could be explained by the blood plasma modelling.

Studies on the synthesis and biological properties of non-carrier-added [(125)I and (131)I]-labeled arylalkylidenebisphosphonates: potent bone-seekers for diagnosis and therapy of malignant osseous lesions

Arylalkylidenebisphosphonates labeled with nca [(125)I or (131)I] have been synthesized and their biological function investigated. The label was attached to the aromatic group in high yield and under mild conditions by means of iododesilylation. The bone affinities of the radioactive compounds were investigated in normal Balb/C mice. The compound 1-hydroxy(m-iodo[(125,131)I]-phenylethylidene)-1,1-bisphosphonate was found to possess superior bone affinity compared to others, and its in vivo deiodination was insignificant. The uptake in femur 24h after injection was 850 +/- 265% and 986 +/- 118% of injected dose per gram tissue times gram body weight in mice and rats, respectively. The therapeutic potential of the compound was investigated in two tumor models in athymic (nude) rats, one model for mixed lytic/sclerotic metastatic bone-lesions originating from breast cancer and the other model simulating osseous osteosarcoma. The effects in these models compare favorably to those observed for established treatment modalities. The experiments demonstrate that radioiodinated bisphosphonates may have a potential for diagnosis and therapy of malignant osseous lesions.

Use of radionuclides for the palliation of bone metastases

Pain palliation with bone-seeking radiopharmaceuticals is an effective and cost-effective management tool in patients with advanced cancer metastatic to bone. Strontium-89 ((89)Sr) (Metastron) and samarium-153 ((153)Sm) EDTMP (Lexidronam) are licensed for use in patients in the United States. Patients with a positive bone scan using technetium 99m methylene diphosphonate ((99m)Tc MDP) are eligible for treatment, and indications and contraindications for use are now well defined. The evidence in the literature now suggests that the radiopharmaceuticals can significantly reduce pain and analgesic requirements, can improve quality of life, can reduce lifetime radiotherapy requirements and management costs, and may slow the progression of painful metastatic lesions. Retreatment is safe and effective. Rhenium-186 ((186)Re) HEDP and Tin-117m diethylenetriaminepenta-acetic acid (DTPA) are in phase II/III trials to evaluate efficacy and compare efficacy with the licensed agents. Phosphorus-32 ((32)P) has been reassessed in two trials evaluating efficacy in comparison with (89)Sr and safety. Toxicity is reversible myelosuppression, which may be significant, and the treatments should not be given to patients with suspected disseminated intravascular coagulation.

Spectrophotometric determination of Sn(II) using palladium chloride in kits for 99mTc radiopharmaceuticals

A method of quantitating Sn(II) suitable for the analysis of kits for 99mTc radiopharmaceuticals, based on a spectrophotometric determination using the Pd(II)-Sn(II) complex (yellow species) is described. The absorbance of the complex is measured at 410 nm and Beer's law is obeyed up to 250 micrograms Sn(II) in the aqueous phase. Several radiopharmaceutical kits were analyzed for their Sn(II) content. The investigation indicates that the procedure is simple, rapid and accurate for quantitative estimation of Sn(II) in various 99mTc-labelling kits during development, manufacture and storage.

Overview of nuclides for bone pain palliation

A variety of radiopharmaceuticals has been used for systemic therapy to relieve pain from malignancies metastatic to bone, using electron emitting radionuclides either in ionic form or as labels for bone seeking compounds to irradiate locally at sites of metastases. The major complication comes from the absorbed dose to the bone marrow. Therefore, there has been a search for radionuclides with lower energy emissions and correspondingly shorter range in tissue. Some studies have shown that it is possible to delay the onset of new pain sites and perhaps increase life expectancy.

In-vivo tissue uptake and retention of Sn-117m(4+)DTPA in a human subject with metastatic bone pain and in normal mice

Organ and tissue uptake and retention of Sn-117m(4+)DTPA were studied in a human subject treated for metastatic bone pain, and the results were compared with the biodistribution studies in five normal mice. The explanted organs from a patient who received a therapy dose of 18.6 mCi (688.2 MBq) Sn-117m(4+)DTPA and who died 47 days later were imaged with a gamma-camera, and tissue samples were counted and also autoradiographed. Bone, muscle, liver, fat, lungs, kidneys, spleen, heart and pancreas tissue samples were assayed in a well counter for radioactivity. Regions of interest were drawn over bone and major organs to calculate and quantify clearance times using three in vivo Sn-117m(4+)DTPA whole-body scintigrams acquired at 1, 24 and 168 h after injection. Five normal mice injected with the same batch of Sn-117m(4+)DTPA as used for the human subject were sacrificed at 24 h, and tissue samples were collected and assayed for radioactivity for comparison with the human data. For the human subject, whole-body retention at 47 days postinjection was 81% of the injected dose, and the rest (19%) was excreted in urine. Of the whole-body retained activity at 47 days, 82.4% was in bone, 7.8% in the muscle and 1.5% in the liver, and the rest was distributed among other tissues. Gamma-ray scintigrams and electron autoradiographs of coronal slices of the thoracolumbar vertebral body showed heterogeneous metastatic involvement with normal bone between metastatic lesions. There was nonuniform distribution of radioactivity even within a single vertebral body, indicating normal bone between metastatic lesions. Lesion-to-nonlesion ratios ranged from 3 to 5. However, the osteoid-to-marrow cavity deposition ratio, from the microautoradiographs, was 11:1. The peak uptake in the human bone was seen at 137 h with no biological clearance. Soft tissues showed peak uptake at 1 h and exhibited three compartmental clearance components. Whole-body retention in normal mice was 38.7% of the injected dose at 24 h and the rest was excreted. At 24 h postinjection, bone in mice showed 84.2% of the whole-body retention, muscle 1.7% and liver 1.4%, and the rest was distributed in other soft tissues. Percent distribution of the retained dose among bone, muscle, liver and other soft tissues is very similar between mice and a human subject. To calculate precise radiation absorbed doses from bone pain palliation radionuclides, it is necessary to take into account soft-tissue uptake and retention that may not be readily evident from routine external gamma-scintigraphy.

Radionuclide development at BNL for nuclear medicine therapy

Radionuclides with medium energy beta emission and a several day half-life have often been viewed as attractive candidates for radioimmunotherapy. Among the most promising in this category are 47Sc, 67Cu, 153Sm, 188Re, and 199Au. The production of 67Cu, 153Sm, 199Au at BNL is summarized and the development of the latest candidate for this application, 47Sc, is described in detail. We also summarize the development of another important therapeutic radionuclide, 117mSn for bone pain palliation.

Cancer therapy using bone-seeking isotopes

Bone pain is a common symptom in disseminated malignancy and may be difficult to manage effectively. Radiation is of proven benefit for pain palliation and there is growing interest in the therapeutic potential of bone-seeking radiopharmaceuticals. Clinical data relating to the use of phosphorus-32, strontium-89, samarium-153 EDTMP, rhenium-186 HEDP and tin-117m DTPA are reviewed in the context of the pathophysiology of metastatic bone pain. Possible mechanisms of action of palliative radiotherapy and, in particular, the theoretical role of early response genes are discussed. The application of Monte Carlo simulation to targeted radiotherapy for bone metastases may provide the basis for a clearer understanding of the microdosimetry and radiobiology of bone pain palliation and for reliable prediction of clinical response and toxicity.

Is there life after technetium: what is the potential for developing new broad-based radionuclides?

The use of radionuclides for medical and for a multitude of other basic research applications has continued to grow at a very rapid pace. Procedures, based on their use as radiotracers for nuclear medicine imaging and for radiotherapy of cancer and other pathology, have become firmly established as important clinical modalities. It is estimated that on an annual basis in the United States alone, radionuclides are used medically in over 13 million imaging procedures, in over 100 million laboratory tests, and in an ever increasing number (> 100,000) for therapeutic administrations. One out of every four hospital patients undergoes a procedure that involves the use of radionuclides. Diagnostic imaging methods using planar/single-photon emission computed tomography and positron-emission tomography (PET) imaging, as well as the measurement of in vivo organ function, physiology, or biochemistry, have become indispensable tools in patient workup and management. More than 80% of all imaging studies (mostly anatomic) currently use technetium-99m (99mTc), because it has turned out to be the ideal isotope from various considerations. However, over the past few years, nuclear medicine has experienced a slow but steady evolution towards functional studies, quantitative PET imaging, and novel therapeutic approaches. New radionuclides are required for these applications, and their development has attracted considerable interest. This article reviews the current status and future prospects for the development of many new potential isotopes. Practical issues, such as the feasibility of large-scale production and wide-spread availability in a continuous reliable fashion, are addressed. To date, the data are not sufficient to answer the question as to whether any of these radionuclides (or their applications, for that matter) will eventually assume as broad-based a role as that of 99mTc. Nonetheless, there are a number of promising radionuclides that could assume an important place in the future practice of nuclear medicine.

Purposeful alteration of [99mTc]-pertechnetate biodistribution

A series of imaging studies were conducted in rats to assess the effect of stannous containing chemical species on the normal biodistribution of [99mTc]pertechnetate. The goal of the study was to determine if tissue activity could be altered by use of selected chemical agents and if such alteration could be used to clear non-target activity for enhanced image interpretation and/or to visualize two or more organ systems following a single injection of radioactivity. Two distinct patterns of tissue activity alteration could be induced. Tissue distribution studies in rats demonstrated statistically significant differences between tissue radioactivity in groups selectively studied for the type and magnitude of the induced alteration.

Rapid and precise micro-methods for quantitating active components in commercial bone scintigraphy kits--I. Stannous tin

Stannous tin-diphosphonate u.v. absorbing complexes have been quantitated at appropriate wavelengths in the presence of an excess of diphosphonate ligand and chloride ions, under a nitrogen atmosphere. Results in the range of 10-80 x 10(-9) At-g of Sn(II) per sample were obtained within 2 min, with a coefficient of variation of 0.7%.

The development and in-vivo behavior of tin containing radiopharmaceuticals--II. Autoradiographic and scintigraphic studies in normal animals and in animal models of bone disease

Various 117mSn (2+ and 4+) compounds in well defined oxidation states were studied in normal mice using whole body autoradiography (WBARG), tissue distribution and scintigraphy in animal models of vitamin A induced bone disease, fracture, infected fracture and ischemic muscle lesions. The 117mSn4+-DTPA showed high affinity to normal bone with low soft tissue concentration. Increased deposition of this compound in fractures and ischemic lesions in muscle was also demonstrated. In hypervitaminosis A, reduced bone uptake of 117mSn4+-DTPA was shown to occur. Nude mice bearing osteogenic sarcoma of human origin showed uptake in spiculated pattern. The similar distribution of 117mSn4+-DTPA which does not contain phosphate or phosphonate groups, and the 99mTc(Sn) skeletal imaging compounds may indicate that tin is important in binding to bone. 117mSn4+-DTPA may not be ideal for routine imaging except when long term follow up is required. It should however be considered for therapy of bone tumors because of the long physical half-life of 117mSn (t1/2 = 14.03 days), abundance of short-range conversion and Auger electrons and its preferential deposition in cortical bone as indicated by our results.