Preparation, Characterization, and Preliminary Imaging Study of [188Re]Re-Ibandronate as a Novel Theranostic Radiopharmaceutical for Bone Metastasis

Background

Bone is a common site of metastasis from a malignant tumor. Several radiopharmaceuticals are available to relieve bone pain in patients with cancer. However, every radiopharmaceutical has its own disadvantages, and there is still a need to investigate easily accessible and high bone affinity radiopharmaceuticals. Ibandronate (IBA) and ¹⁸⁸Re were used for radiolabeling to develop and evaluate a novel type of bone-seeking radiopharmaceutical.

Methods

The preparation conditions of [¹⁸⁸Re]Re-IBA were investigated, and thin-layer chromatography was used to analyze radiochemical purity. The stability, plasma protein binding rate, lipid-water distribution coefficient, safety and biodistribution in normal mice, and bone imaging of [¹⁸⁸Re]Re-IBA in New Zealand rabbits were studied. In addition, the nude mice model of bone metastasis was established, and biodistribution and imaging characteristics of [¹⁸⁸Re]Re-IBA in these nude mice were studied.

Results

[¹⁸⁸Re]Re-IBA was successfully prepared with radiochemical purity >95%. The optimum preparation conditions were as follows: IBA, 0.8–1.4 mg; ascorbic acid, 0.2–0.5 mg; stannous chloride, 0.14–0.18 mg; potassium perrhenate, 0.005 mg; and [¹⁸⁸Re]ReO₄⁻ activity, 18.5–296 MBq, reacted for 30 min at 95°C with pH = 2. [¹⁸⁸Re]Re-IBA demonstrated good stability, high plasma protein binding rate, good hydrophilicity, and low toxicity. The biodistribution and bone imaging in normal animals showed rapid blood clearance, high bone uptake, low uptake in the solid organs and soft tissue, and high contrast during imaging. The biodistribution and imaging of bone metastasis in nude mice showed that [¹⁸⁸Re]Re-IBA has higher uptake in bone metastasis lesions than normal bone.

Conclusions

Our study encompassed the successful preparation of [¹⁸⁸Re]Re-IBA, a novel bone-seeking radiopharmaceutical, and confirmed it has potential in the treatment of bone metastasis and monitoring through imaging.

Rhenium-188 Labeled Radiopharmaceuticals: Current Clinical Applications in Oncology and Promising Perspectives

Rhenium-188 (¹⁸⁸Re) is a high energy beta-emitting radioisotope with a short 16.9 h physical half-life, which has been shown to be a very attractive candidate for use in therapeutic nuclear medicine. The high beta emission has an average energy of 784 keV and a maximum energy of 2.12 MeV, sufficient to penetrate and destroy targeted abnormal tissues. In addition, the low-abundant gamma emission of 155 keV (15%) is efficient for imaging and for dosimetric calculations. These key characteristics identify ¹⁸⁸Re as an important therapeutic radioisotope for routine clinical use. Moreover, the highly reproducible on-demand availability of ¹⁸⁸Re from the ¹⁸⁸W/¹⁸⁸Re generator system is an important feature and permits installation in hospital-based or central radiopharmacies for cost-effective availability of no-carrier-added (NCA) ¹⁸⁸Re. Rhenium-188 and technetium-99 m exhibit similar chemical properties and represent a “theranostic pair.” Thus, preparation and targeting of ¹⁸⁸Re agents for therapy is similar to imaging agents prepared with ^99mTc, the most commonly used diagnostic radionuclide. Over the last three decades, radiopharmaceuticals based on ¹⁸⁸Re-labeled small molecules, including peptides, antibodies, Lipiodol and particulates have been reported. The successful application of these ¹⁸⁸Re-labeled therapeutic radiopharmaceuticals has been reported in multiple early phase clinical trials for the management of various primary tumors, bone metastasis, rheumatoid arthritis, and endocoronary interventions. This article reviews the use of ¹⁸⁸Re-radiopharmaceuticals which have been investigated in patients for cancer treatment, demonstrating that ¹⁸⁸Re represents a cost effective alternative for routine clinical use in comparison to more expensive and/or less readily available therapeutic radioisotopes.

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.

Applying quality by design principles to the small-scale preparation of the bone-targeting therapeutic radiopharmaceutical rhenium-188-HEDP

INTRODUCTION

Rhenium-188-HEDP ((188)Re-HEDP) is a therapeutic radiopharmaceutical for treatment of osteoblastic bone metastases. No standard procedure for the preparation of this radiopharmaceutical is available. Preparation conditions may influence the quality and in vivo behaviour of this product. In this study we investigate the effect of critical process parameters on product quality and stability of (188)Re-HEDP.

METHODS

A stepwise approach was used, based on the quality by design (QbD) concept of the ICH Q8 (Pharmaceutical Development) guideline. Potential critical process conditions were identified. Variables tested were the elution volume, the freshness of the eluate, the reaction temperature and time, and the stability of the product upon dilution and storage. The impact of each variable on radiochemical purity was investigated. The acceptable ranges were established by boundary testing.

RESULTS

With 2ml eluate, adequate radiochemical purity and stability were found. Nine ml eluate yielded a product that was less stable. Using eluate stored for 24h resulted in acceptable radiochemical purity. Complexation for 30min at room temperature, at 60°C and at 100°C generated appropriate and stable products. A complexation time of 10min at 90°C was too short, whereas heating 60min resulted in products that passed quality control and were stable. Diluting the end product and storage at 32.5°C resulted in notable decomposition.

CONCLUSION

Two boundary tests, an elution volume of 9ml and a heating time of 10min, yielded products of inadequate quality or stability. The product was found to be instable after dilution or when stored above room temperature. Our findings show that our previously developed preparation method falls well within the proven acceptable ranges. Applying QbD principles is feasible and worthwhile for the small-scale preparation of radiopharmaceuticals.