Comparison of different phosphorus-containing ligands complexing 68Ga for PET-imaging of bone metabolism

Comparison of 99m Tc-methylenediphosphonate and 68 Ga-BPAMD PET/computed tomography imaging in bone metastasis

OBJECTIVE

Bone is considered as the third most common site of metastases, besides lung and liver. Early detection of skeletal metastases aids in better management of skeletal-related events. In the present study cold kit-based 2,2 ' ,2 '' -(10-(2-((diphosphonomethyl)amino)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl) triacetic acid (BPAMD) was labeled with 68 Ga. The radiolabeling parameters and clinical evaluation in patients with suspected bone metastases were compared with routinely used 99m Tc-methylenediphosphonate ( 99m Tc-MDP).

METHODOLOGY

The kit components of MDP were incubated with at room temperature for 10 min, followed by radiochemical purity testing using thin-layer chromatography. For radiolabeling of BPAMD, the cold kit components reconstituted in 400 μL of HPLC grade water were transferred and incubated with 68 GaCl 3 in the reactor vessel of the fluidic module at 95°C for 20 min. Radiochemical yield and purity were determined with instant thin-layer chromatography using 0.5 M sodium citrate as mobile phase. For clinical evaluation, patients ( n  = 10) with suspected bone metastases were enrolled. 99m Tc-MDP and 68 Ga-BPAMD scans were performed on two different days in random order. Imaging outcomes were noted and compared.

RESULTS

Radiolabeling of both tracers is facile using cold kit, although BPAMD requires heating. The radiochemical purity was observed to be greater than 99% for all preparations. Both MDP and BPAMD detected skeletal lesions; however, additional lesions were detected in total of seven patients which were not visualized clearly on 99m Tc-MDP scan.

CONCLUSION

BPAMD can be easily tagged with 68 Ga using cold kits. The radiotracer is suitable and efficient for detection of bone metastases using PET/computed tomography.

[177Lu]Lu-DOTA-ZOL bone pain palliation in patients with skeletal metastases from various cancers: efficacy and safety results

Background

[¹⁷⁷Lu]Lu-DOTA-ZOL has shown promising results from the dosimetry and preclinical aspects, but data on its role in the clinical efficacy are limited. The objective of this study is to evaluate the efficacy and safety of [¹⁷⁷Lu]Lu-DOTA-ZOL as a bone pain palliation agent in patients experiencing pain due to skeletal metastases from various cancers.

Methods

In total, 40 patients experiencing bone pain due to skeletal metastases were enrolled in this study. The patients were treated with a mean cumulative dose of 2.1 ± 0.6 GBq (1.3–2.7 GBq) [¹⁷⁷Lu]Lu-DOTA-ZOL in a median follow-up duration of 10 months (IQR 8–14 months). The primary outcome endpoint was response assessment according to the visual analogue score (VAS). Secondary endpoints included analgesic score (AS), global pain assessment score, Eastern Cooperative Oncology Group Assessment performance status (ECOG), Karnofsky performance status, overall survival, and safety assessment by the National Cancer Institute’s Common Toxicity Criteria V5.0.

Results

In total, 40 patients (15 males and 25 females) with a mean age of 46.6 ± 15.08 years (range 24–78 years) were treated with either 1 (N = 15) or 2 (N = 25) cycles of [¹⁷⁷Lu]Lu-DOTA-ZOL. According to the VAS response assessment criteria, complete, partial, and minimal responses were observed in 11 (27.5%), 20 (50%), and 5 patients (12.5%), respectively with an overall response rate of 90%. Global pain assessment criteria revealed complete, partial, minimal, and no response in 2 (5%), 25 (62.5%), 9 (22.5%), and 4 (10%) patients, respectively. Twenty-eight patients died and the estimated median overall survival was 13 months (95% CI 10–14 months). A significant improvement was observed in the VAS, AS, and ECOG status when compared to baseline. None of the patients experienced grade III/IV haematological, kidney, or hepatotoxicity due to [¹⁷⁷Lu]Lu-DOTA-ZOL therapy.

Conclusion

[¹⁷⁷Lu]Lu-DOTA-ZOL shows promising results and is an effective radiopharmaceutical in the treatment of bone pain due to skeletal metastases from various cancers.

68Ga-Labeled bismacrocyclic methylene phosphonate as potential bone seeking PET radiopharmaceutical

Phosphonates-based agents are well-known bone-seeking radiopharmaceuticals with application in detection and therapy. With higher sensitivity and resolution offered by Positron Emission Tomography (PET), tracers based on this technique are gaining huge attention. ⁶⁸Ga-based generator and radiotracers render independence from the on-site cyclotron. We report the development of ⁶⁸Ga-labeled DOTA-based bismacrocyclic phosphonate derivative, for bone PET imaging. The synthesis and characterization of ⁶⁸Ga- DO3P-AME-DO3P was carried out in > 95% purity. The radiotracer displayed high stability and low binding affinity (<3%) to blood serum. High in vitro binding affinity were observed for synthetic hydroxyapatite, SAOS-2, osteoclast and osteoblast cells. In vivo pharmacokinetics revealed fast washout with biphasic release pattern. The deposition of radiotracer in osseous tissues was high (Bone/Muscle ratio:18), as studied from the biodistribution studies. In vivo PET/CT and biodistribution analyses revealed the ability of ⁶⁸Ga-DO3P-AME-DO3P to target and accumulate in bone, thus displaying its potential as a PET bone imaging agent.

Preliminary results of biodistribution and dosimetric analysis of [68Ga]Ga-DOTAZOL: a new zoledronate-based bisphosphonate for PET/CT diagnosis of bone diseases

OBJECTIVE

Pre-clinical studies with gallium-68 zoledronate ([⁶⁸Ga]Ga-DOTA^ZOL) have proposed it to be a potent bisphosphonate for PET/CT diagnosis of bone diseases and diagnostic counterpart to [¹⁷⁷Lu]Lu-DOTA^ZOL and [²²⁵Ac]Ac-DOTA^ZOL. This study aims to be the first human biodistribution and dosimetric analysis of [⁶⁸Ga]Ga-DOTA^ZOL.

METHODS

Five metastatic skeletal disease patients (mean age: 72 years, M: F; 4:1) were injected with 150-190 MBq (4.05-5.14 mCi) of [⁶⁸Ga]Ga-DOTA^ZOL i.v. Biodistribution of [⁶⁸Ga]Ga-DOTA^ZOL was studied with PET/CT initial dynamic imaging for 30 min; list mode over abdomen (reconstructed as six images of 300 s) followed by static (skull to mid-thigh) imaging at 45 min and 2.5 h with Siemens Biograph 2 PET/CT camera. Also, blood samples (8 time points) and urine samples (2 time points) were collected over a period of 2.5 h. Total activity (MBq) in source organs was determined using interview fusion software (MEDISO Medical Imaging Systems, Budapest, Hungary). A blood-based method for bone marrow self-dose determination and a trapezoidal method for urinary bladder contents residence time calculation were used. OLINDA/EXM version 2.0 software (Hermes Medical Solutions, Stockholm, Sweden) was used to generate residence times for source organs, organ absorbed doses and effective doses.

RESULTS

High uptake in skeleton as target organ, kidneys and urinary bladder as organs of excretion and faint uptake in liver, spleen and salivary glands were seen. Qualitative and quantitative analysis supported fast blood clearance, high bone to soft tissue and lesion to normal bone uptake with [⁶⁸Ga]Ga-DOTA^ZOL. Urinary bladder with the highest absorbed dose of 0.368 mSv/MBq presented the critical organ, followed by osteogenic cells, kidneys and red marrow receiving doses of 0.040, 0.031 and 0.027 mSv/MBq, respectively. The mean effective dose was found to be 0.0174 mSv/MBq which results in an effective dose of 2.61 mSv from 150 MBq.

CONCLUSIONS

Biodistribution of [⁶⁸Ga]Ga-DOTA^ZOL was comparable to [¹⁸F]NaF, [^99mTc]Tc-MDP and [⁶⁸Ga]Ga-PSMA-617. With proper hydration and diuresis to reduce urinary bladder and kidney absorbed doses, it has clear advantages over [¹⁸F]NaF owing to its onsite, low-cost production and theranostic potential of personalized dosimetry for treatment with [¹⁷⁷Lu]Lu-DOTA^ZOL and [²²⁵Ac]Ac-DOTA^ZOL.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

Comparison of macrocyclic and acyclic chelators for gallium-68 radiolabelling

Gallium-68 (68Ga) is a positron-emitting isotope used for clinical PET imaging of peptide receptor expression. 68Ga radiopharmaceuticals used in molecular PET imaging consist of disease-targeting biomolecules tethered to chelators that complex 68Ga3+. Ideally, the chelator will rapidly, quantitatively and stably coordinate 68Ga3+ at room temperature, near neutral pH and low chelator concentration, allowing for simple routine radiopharmaceutical formulation. Identification of chelators that fulfil these requirements will facilitate development of kit-based 68Ga radiopharmaceuticals. Herein the reaction of a range of widely used macrocyclic and acyclic chelators with 68Ga3+ is reported. Radiochemical yields have been measured under conditions of varying chelator concentrations, pH (3.5 and 6.5) and temperature (25 and 90 °C). These chelators are: 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), 1,4,7-triazacyclononane macrocycles substituted with phosphonic (NOTP) and phosphinic (TRAP) groups at the amine, bis(2-hydroxybenzyl)ethylenediaminediacetic acid (HBED), a tris(hydroxypyridinone) containing three 1,6-dimethyl-3-hydroxypyridin-4-one groups (THP) and the hexadentate tris(hydroxamate) siderophore desferrioxamine-B (DFO). Competition studies have also been undertaken to assess relative complexation efficiencies of each chelator for 68Ga3+ under different pH and temperature conditions. Performing radiolabelling reactions at pH 6.5, 25 °C and 5-50 μM chelator concentration resulted in near quantitative radiochemical yields for all chelators, except DOTA. Radiochemical yields either decreased or were not substantially improved when the reactions were undertaken at lower pH or at higher temperature, except in the case of DOTA. THP and DFO were the most effective 68Ga3+ chelators at near-neutral pH and 25 °C, rapidly providing near-quantitative radiochemical yields at very low chelator concentrations. NOTP and HBED were only slightly less effective under these conditions. In competition studies with all other chelators, THP demonstrated highest reactivity for 68Ga3+ complexation under all conditions. These data point to THP possessing ideal properties for rapid, one-step kit-based syntheses of 68Ga-biomolecules for molecular PET imaging. LC-MS and 1H, 13C{1H} and 71Ga NMR studies of HBED complexes of Ga3+ showed that under the analytical conditions employed in this study, multiple HBED-bound Ga complexes exist. X-ray diffraction data indicated that crystals isolated from these solutions contained octahedral [Ga(HBED)(H2O)], with HBED coordinated in a pentadentate N2O3 mode, with only one phenolic group coordinated to Ga3+, and the remaining coordination site occupied by a water molecule.

Gallium(III) chelates of mixed phosphonate-carboxylate triazamacrocyclic ligands relevant to nuclear medicine: Structural, stability and in vivo studies

Three triaza macrocyclic ligands, H₆NOTP (1,4,7-triazacyclononane-N,N',N″-trimethylene phosphonic acid), H₄NO2AP (1,4,7-triazacyclononane-N-methylenephosphonic acid-N',N″-dimethylenecarboxylic acid), and H₅NOA2P (1,4,7-triazacyclononane-N,N'-bis(methylenephosphonic acid)-N″-methylene carboxylic acid), and their gallium(III) chelates were studied in view of their potential interest as scintigraphic and PET (Positron Emission Tomography) imaging agents. A ¹H, ³¹P and ⁷¹Ga multinuclear NMR study gave an insight on the structure, internal dynamics and stability of the chelates in aqueous solution. In particular, the analysis of ⁷¹Ga NMR spectra gave information on the symmetry of the Ga³⁺ coordination sphere and the stability of the chelates towards hydrolysis. The ³¹P NMR spectra afforded information on the protonation of the non-coordinated oxygen atoms from the pendant phosphonate groups and on the number of species in solution. The ¹H NMR spectra allowed the analysis of the structure and the number of species in solution. ³¹P and ¹H NMR titrations combined with potentiometry afforded the measurement of the protonation constants (log K_Hi) and the microscopic protonation scheme of the triaza macrocyclic ligands. The remarkably high thermodynamic stability constant (log K_GaL=34.44 (0.04) and stepwise protonation constants of Ga(NOA2P)^2- were determined by potentiometry and ⁶⁹Ga and ³¹P NMR titrations. Biodistribution and gamma imaging studies have been performed on Wistar rats using the radiolabeled ⁶⁷Ga(NO2AP)^- and ⁶⁷Ga(NOA2P)^2-chelates, having both demonstrated to have renal excretion. The correlation of the molecular properties of the chelates with their pharmacokinetic properties has been analysed.

[68Ga]/[188Re] Complexed [CDTMP] Trans-1,2-Cyclohexyldinitrilotetraphosphonic Acid As a Theranostic Agent for Skeletal Metastases

Objective

Metastasis of the osseous tissue is one of the frequent and severe aggravations as a result of several neoplastic conditions, such as metabolic disorders, infections, and cancer. The objective of this study was to evaluate the pertinence of [⁶⁸Ga]-trans-1,2-cyclohexyldinitrilo tetramethylene phosphonic acid (CDTMP) as a potential bone imaging agent for positron emission tomography (PET) applications as well as to assess [¹⁸⁸Re]-CDTMP for bone pain palliation in metastatic skeletal disorders.

Methods

⁶⁸Ga complex of CDTMP was prepared at 80°C at pH 3.5, and ¹⁸⁸Re complex of CDTMP was prepared at room temperature. [⁶⁸Ga]-CDTMP complex was investigated as PET tracer while the therapeutic efficacy was assessed for [¹⁸⁸Re]-CDTMP. Labeling efficiency, biodistribution, myelotoxicity, and imaging studies were carried out for the complexes synthesized. Both PET and MicroPET imaging studies were performed for [⁶⁸Ga]-CDTMP whereas SPECT acquisitions were acquired for [¹⁸⁸Re]-CDTMP. Data were analyzed semiquantitatively for all the scintigraphic scans obtained.

Results

The radiolabeling efficiency was observed to be >70% for [⁶⁸Ga]-CDTMP. High bone uptake of [⁶⁸Ga]-CDTMP as compared to contralateral tissue was found in PET imaging in Balb/C mice and New Zealand rabbit; the similar result for bone uptake was correlated in the biodistribution study of the compound in BALB/c mice at different time intervals. Biodistribution experiments carried out in mice showed maximum uptake of 6.12 ± 1.22%ID/g at 45 min postinjection. For [¹⁸⁸Re]-CDTMP, total skeletal uptake was 8.12 ± 1.11%ID/g observed at 1 h postinjection from biodistribution data. High renal uptake confirms renal route of excretion. A good hydroxyapatite binding too was seen for both the complexes. No evidence of destruction or adverse functioning of vital organs was observed for the ¹⁸⁸Re complex.

Conclusion

[⁶⁸Ga]-CDTMP complex can be used as a promising PET bone imaging agent and [¹⁸⁸Re]-CDTMP as a surrogate moiety for therapeutic application. Owing to the short half-life of ⁶⁸Ga (68 min), cyclotron-independent radiopharmacy, fast clearance, and rapid renal excretion as evidenced in preclinical animal models. Very low myelotoxicity and highly selective bone uptake prove the potential of [¹⁸⁸Re]-CDTMP for therapeutic application.

Novel Radiolabeled Bisphosphonates for PET Diagnosis and Endoradiotherapy of Bone Metastases

Bone metastases, often a consequence of breast, prostate, and lung carcinomas, are characterized by an increased bone turnover, which can be visualized by positron emission tomography (PET), as well as single-photon emission computed tomography (SPECT). Bisphosphonate complexes of ^99mTc are predominantly used as SPECT tracers. In contrast to SPECT, PET offers a higher spatial resolution and, owing to the ⁶⁸Ge/⁶⁸Ga generator, an analog to the established ^99mTc generator exists. Complexation of Ga(III) requires the use of chelators. Therefore, DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), NOTA (1,4,7-triazacyclododecane-1,4,7-triacetic acid), and their derivatives, are often used. The combination of these macrocyclic chelators and bisphosphonates is currently studied worldwide. The use of DOTA offers the possibility of a therapeutic application by complexing the β-emitter ¹⁷⁷Lu. This overview describes the possibility of diagnosing bone metastases using [⁶⁸Ga]Ga-BPAMD (⁶⁸Ga-labeled (4-{[bis-(phosphonomethyl))carbamoyl]methyl}-7,10-bis(carboxymethyl)-1,4,7,10-tetraazacyclododec-1-yl)acetic acid) as well as the successful application of [¹⁷⁷Lu]Lu-BPAMD for therapy and the development of new diagnostic and therapeutic tools based on this structure. Improvements concerning both the chelator and the bisphosphonate structure are illustrated providing new ⁶⁸Ga- and ¹⁷⁷Lu-labeled bisphosphonates offering improved pharmacological properties.

Development and biological evaluation of 90Y-BPAMD as a novel bone seeking therapeutic Agent

Nowadays, the bone-seeking radiopharmaceuticals play an important role in the treatment of the bone-related pathologies. Whereas various phosphonate ligands have already been identified, a DOTA-based bisphosphonate, 4-{[(bis(phosphonomethyl))carbamoyl]methyl}- 7,10-bis(carboxymethyl)-1,4,7,10-tetraazacyclododec- 1-yl (BPAMD) with better characteristics has recently been synthesized. In this study, 90Y-BPAMD was developed with radiochemical purity >98% and the specific activity of 3.52 TBq/mmol in the optimized conditions as a new bone-seeking therapeutic agent. The complex demonstrated significant stability at room temperature and in human serum even after 48 h. At even low amount of hydroxyapatite (5 mg), more than 90% binding to hydroxyapatite was observed. Biodistribution studies after injection of the complex into the Syrian rats showed major accumulation of the labelled compound in the bone tissue and an insignificant uptake in the other organs all the times after injection. Generally, 90Y-BPAMD demonstrated interesting characteristics compared to the other 90Y bone-seeking agents and even 166Ho-BPAMD, and can be considered as a new bone-seeking candidate for therapeutic applications.

Bone targeting compounds for radiotherapy and imaging: *Me(III)-DOTA conjugates of bisphosphonic acid, pamidronic acid and zoledronic acid

Background

Bisphosphonates have a high adsorption on calcified tissues and are commonly used in the treatment of bone disorder diseases. Conjugates of bisphosphonates with macrocyclic chelators open new possibilities in bone targeted radionuclide imaging and therapy. Subsequent to positron emission tomography (PET) examinations utilizing ⁶⁸Ga-labelled analogues, endoradiotheraphy with ¹⁷⁷Lu-labelled macrocyclic bisphosphonates may have a great potential in the treatment of painful skeletal metastases.

Methods

Based on the established pharmaceuticals pamidronate and zoledronate two new DOTA-α-OH-bisphosphonates, DOTA^PAM and DOTA^ZOL(MM1.MZ) were successfully synthesized. The ligands were labelled with the positron emitting nuclide ⁶⁸Ga and the β^- emitting nuclide ¹⁷⁷Lu and compared in in vitro studies and in ex vivo biodistribution studies together with small animal PET and single photon emission computed tomography (SPECT) studies against [¹⁸F]NaF and a known DOTA-α-H-bisphosphonate conjugate (BPAPD) in healthy Wistar rats.

Results

The new DOTA-bisphosphonates can be labelled in high yield of 80 to 95 % in 15 min with post-processed ⁶⁸Ga and >98 % with ¹⁷⁷Lu. The tracers showed very low uptake in soft tissue, a fast renal clearance and a high accumulation on bone. The best compound was [⁶⁸Ga]DOTA^ZOL (SUV _Femur = 5.4 ± 0.6) followed by [¹⁸F]NaF (SUV _Femur = 4.8 ± 0.2), [⁶⁸Ga]DOTA^PAM (SUV _Femur = 4.5 ± 0.2) and [⁶⁸Ga]BPAPD (SUV _Femur = 3.2 ± 0.3). [¹⁷⁷Lu]DOTA^ZOL showed a similar distribution as the diagnostic ⁶⁸Ga complex.

Conclusion

The ⁶⁸Ga labelled compounds showed a promising pharmacokinetics, with similar uptake profile and distribution kinetics. Bone accumulation was highest for [⁶⁸Ga]DOTA^ZOL, which makes this compound probably an interesting bone targeting agent for a therapeutic approach with ¹⁷⁷Lu. The therapeutic compound [¹⁷⁷Lu]DOTA^ZOL showed a high target-to-background ratio. SPECT experiments showed concordance to the PET scans in healthy rats. [⁶⁸Ga/¹⁷⁷Lu]DOTA^ZOL appears to be a potential theranostic combination in the management of disseminated bone metastases.

Electronic supplementary material

The online version of this article (doi:10.1186/s41181-016-0017-1) contains supplementary material, which is available to authorized users.

A DOTA based bisphosphonate with an albumin binding moiety for delayed body clearance for bone targeting

Radiolabeled bisphosphonates are commonly used in the diagnosis and therapy of bone metastases. Blood clearance of bisphosphonates is usually fast and only 30%-50% of the injected activity is retained in the skeleton, while most of the activity is excreted by the urinary tract. A longer blood circulation may enhance accumulation of bisphosphonate compounds in bone metastases. Therefore, a chemically modified macrocyclic bisphosphonate derivative with an additional human albumin binding entity was synthesized and pharmacokinetics of its complex was evaluated. The DOTA-bisphosphonate conjugate BPAMD was compared against the novel DOTAGA-derived albumin-binding bisphosphonate DOTAGA(428-d-Lys)M^BP (L1). The ligands were labeled with ⁶⁸Ga(III) and were evaluated in in vitro binding studies to hydroxyapatite (HA) as well as to human serum albumin. The compounds were finally compared in in vivo PET and ex vivo organ distribution studies in small animals over 6h. Binding studies revealed a consistent affinity of both bisphosphonate tracers to HA. Small animal PET and ex vivo organ distribution studies showed longer blood retention of [⁶⁸Ga]L1. [⁶⁸Ga]BPAMD is initially more efficiently bound to the bone but skeletal accumulation of the modified compound and [⁶⁸Ga]BPAMD equalized at 6h p.i. Ratios of femur epiphyseal plate to ordinary bone showed to be more favorable for [⁶⁸Ga]L1 than for [⁶⁸Ga]BPAMD due to the longer circulation time of the new tracer. Thus, the chemical modification of BPAMD toward an albumin-binding bisphosphonate, L1, resulted in a novel PET tracer which conserves advantages of both functional groups within one and the same molecule. The properties of this new diagnostic tracer are expected to be preserved in ¹⁷⁷Lu therapeutic agent with the same ligand (a theranostic pair).

(177)Lu-labelled macrocyclic bisphosphonates for targeting bone metastasis in cancer treatment

Background

Metastatic bone lesion is a common syndrome of many cancer diseases in an advanced state. The major symptom is severe pain, spinal cord compression, and pathological fracture, associated with an obvious morbidity. Common treatments including systemic application of bisphosphonate drugs aim on pain reduction and on improving the quality of life of the patient. Particularly, patients with multiple metastatic lesions benefit from bone-targeting therapeutic radiopharmaceuticals. Agents utilizing beta-emitting radionuclides in routine clinical praxis are, for example, [⁸⁹Sr]SrCl₂ and [¹⁵³Sm]Sm-EDTMP. No-carrier-added (n.c.a.) ¹⁷⁷Lu is remarkably suitable for an application in this scope.

Methods

Five 1,4,7,10-tetraazacyclododecane N,N′,N′′,N′′-tetra-acetic acid (DOTA)- and DO2A-based bisphosphonates, including monomeric and dimeric structures and one 1,4,7-triazacyclononane-1,4-diacetic acid (NO2A) derivative, were synthesized and labelled with n.c.a. ¹⁷⁷Lu. Radio-TLC and high-performance liquid chromatography (HPLC) methods were successfully established for determining radiochemical yields and for quality control. Their binding to hydroxyapatite was measured in vitro. Ex vivo biodistribution experiments and dynamic in vivo single photon computed tomography (SPECT)/CT measurements were performed in healthy rats for 5 min and 1 h periods. Data on %ID/g or standard uptake value (SUV) for femur, blood, and soft-tissue organs were analyzed and compared with [¹⁷⁷Lu]citrate.

Results

Radiolabelling yields for [¹⁷⁷Lu]Lu-DOTA and [¹⁷⁷Lu]Lu-NO2A monomeric bisphosphonate complexes were >98 % within 15 min. The dimeric macrocyclic bisphosphonates showed a decelerated labelling kinetics, reaching a plateau after 30 min of 60 to 90 % radiolabelling yields. All ¹⁷⁷Lu-bisphosphonate complexes showed exclusive accumulation in the skeleton. Blood clearance and renal elimination were fast. SUV data (all for 1 h p.i.) in the femur ranged from 3.34 to 5.67. The bone/blood ratios were between 3.6 and 135.6, correspondingly. ¹⁷⁷Lu-bisphosphonate dimers showed a slightly higher bone accumulation (SUV_femur = 4.48 ± 0.38 for [¹⁷⁷Lu]Lu-DO2A(P^BP)₂; SUV_femur = 5.41 ± 0.46 for [¹⁷⁷Lu]Lu-DOTA(M^BP)₂) but a slower blood clearance (SUV_blood = 1.25 ± 0.09 for [¹⁷⁷Lu]Lu-DO2A(P^BP)₂; SUV_blood = 1.43 ± 0.32 for [¹⁷⁷Lu]Lu-DOTA(M^BP)₂).

Conclusions

Lu-complexes of macrocyclic bisphosphonates might become options for the therapy of skeletal metastases in the near future, since they show high uptake in bone together with a very low soft-tissue accumulation.

Estimated human absorbed dose of ¹⁷⁷Lu-BPAMD based on mice data: Comparison with ¹⁷⁷Lu-EDTMP

In this work, the absorbed dose of human organs for (177)Lu-BPAMD was evaluated based on biodistribution studies into the Syrian mice by RADAR method and was compared with (177)Lu-EDTMP as the only clinically used Lu-177 bone-seeking agent. The highest absorbed dose for both (177)Lu-BPAMD and (177)Lu-EDTMP is observed on the bone surface with 8.007 and 4.802 mSv/MBq. Generally, (177)Lu-BPAMD has considerable characteristics compared with (177)Lu-EDTMP and can be considered as a promising agent for the bone pain palliation therapy.

Synthesis and preliminary in vivo evaluation of well-dispersed biomimetic nanocrystalline apatites labeled with positron emission tomographic imaging agents

In recent years, biomimetic synthetic apatite nanoparticles (AP-NPs), having chemical similarity with the mineral phase of bone, have attracted a great interest in nanomedicine as potential drug carriers. To evaluate the therapeutic perspectives of AP-NPs through the mechanisms of action and organs they interact with, the noninvasive monitoring of their in vivo behavior is of paramount importance. To this aim, here the feasibility to radiolabel AP-NPs ("naked" and surface-modified with citrate to reduce their aggregation) with two positron emission tomographic (PET) imaging agents ([(18)F]NaF and (68)Ga-NO2AP(BP)) was investigated. [(18)F]NaF was used for the direct incorporation of the radioisotope into the crystal lattice, while the labeling by surface functionalization was accomplished by using (68)Ga-NO2AP(BP) (a new radio-metal chelating agent). The labeling results with both tracers were fast, straightforward, and reproducible. AP-NPs demonstrated excellent ability to bind relevant quantities of both radiotracers and good in vitro stability in clinically relevant media after the labeling. In vivo PET studies in healthy Wistar rats established that the radiolabeled AP-NPs gave significant PET signals and they were stable over the investigated time (90 min) since any tracer desorption was detected. These preliminary in vivo studies furthermore showed a clear ability of citrated versus naked AP-NPs to accumulate in different organs. Interestingly, contrary to naked AP-NPs, citrated ones, which unveiled higher colloidal stability in aqueous suspensions, were able to escape the first physiological filter, i.e., the lungs, being then accumulated in the liver and, to a lesser extent, in the spleen. The results of this work, along with the fact that AP-NPs can be also functionalized with targeting ligands, with therapeutic agents, and also with metals for a combination of different imaging modalities, make AP-NPs very encouraging materials for further investigations as theranostic agents in nanomedicine.

A cyclen-based tetraphosphinate chelator for the preparation of radiolabeled tetrameric bioconjugates

The cyclen-based tetraphosphinate chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis[methylene(2-carboxyethyl)phosphinic acid] (DOTPI) comprises four additional carboxylic acid moieties for bioconjugation. The thermodynamic stability constants (logK(ML)) of metal complexes, as determined by potentiometry, were 23.11 for Cu(II), 20.0 for Lu(III), 19.6 for Y(III), and 21.0 for Gd(III). DOTPI was functionalized with four cyclo(Arg-Gly-Asp-D-Phe-Lys) (RGD) peptides through polyethylene glycol (PEG4) linkers. The resulting tetrameric conjugate DOTPI(RGD)4 was radiolabeled with (177)Lu and (64)Cu and showed improved labeling efficiency compared with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). The labeled compounds were fully stable in transchelation challenges against trisodium diethylenetriaminepentaacetate (DTPA) and disodium ethylenediaminetetraacetic acid (ETDA), in phosphate buffered saline (PBS), and human plasma. Integrin αvβ3 affinities of the non-radioactive Lu(III) and Cu(II) complexes of DOTPI(RGD)4 were 18 times higher (both IC50 about 70 picomolar) than that of the c(RGDfK) peptide (IC50 = 1.3 nanomolar). Facile access to tetrameric conjugates and the possibility of radiolabeling with therapeutic and diagnostic radionuclides render DOTPI suitable for application in peptide receptor radionuclide imaging (PRRI) and therapy (PRRT).

(68)Ga-BPAMD: PET-imaging of bone metastases with a generator based positron emitter

PURPOSE

Bone metastases are a serious aggravation for patients suffering from cancer. Therefore, early recognition of bone metastases is of great interest for further treatment of patients. Bisphosphonates are widely used for scintigraphy of bone lesions with (99m)Tc. Using the (68)Ge/(68)Ga generator together with a macroyclic bisphosphonate a comparable PET-tracer comes into focus.

PROCEDURES

The bisphosphonate DOTA-conjugated ligand BPAMD was labelled with (68)Ga. [(68)Ga]BPAMD was evaluated in vitro concerning binding to hydroxyapatite and stability. The tracer's in vivo accumulation was determined on healthy rats and bone metastases bearing animals by μ-PET.

RESULTS

BPAMD was labelled efficiently with (68)Ga after 10 min at 100°C. [(68)Ga]BPAMD showed high in vitro stability within 3h and high binding to hydroxyapatite. Consequently, μ-PET experiments revealed high accumulation of [(68)Ga]BPAMD in regions of pronounced remodelling activity like bone metastases.

CONCLUSIONS

(68)Ga BPAMD reveals great potential for diagnosis of bone metastases via PET/CT. The straight forward (68)Ga-labelling could be transferred to a kit-preparation of a cyclotron-independent PET tracer instantaneously available in many clinical sites using the (68)Ge/(68)Ga generator.