Recent advances in radionuclide therapy

Internal dosimetry studies of 177Lu-BBN-GABA-DOTA, as a cancer therapy agent, in human tissues based on animal data

The goal of using radiopharmaceuticals for therapeutic purposes is twofold: first, the most damage to cancer cells and, second, the most negligible dose transfers to healthy tissues. As ¹⁷⁷Lu has the potential to cure a wide range of malignancies due to its varied range of beta energies, ¹⁷⁷Lu-BBN-GABA-DOTA has been developed for therapeutic applications. In addition, ¹⁷⁷Lu-BBN-GABA-DOTA can be over-expressed on gastrin-releasing peptide (GRP) receptors of the prostate, breast, small cell lung cancer, gastric, and colon tumors. The purpose of this study was to calculate the amount of dose absorption in human body organs using medical internal radiation dose (MIRD) and GATE code methods, after animal injection. In this study, the amount of absorbed dose in different organs (spleen, kidney, Lung, Pancreas, Heart, Adrenal, Intestine, Stomach, and Liver) were calculated for 1-MBq accumulation of ¹⁷⁷Lu-BBN-GABA-DOTA in source organs (spleen, kidney, Lung, Pancreas, Heart, Adrenal, Intestine, Stomach, and Liver) using Monte Carlo Simulation (GATE code) with Zubal phantom. Moreover, compared with MIRD method, the results of the simulation showed considerable consistency. It was estimated that a 1-MBq administration of ¹⁷⁷Lu-BBN-GABA-DOTA to the human body would result in an absorbed dose of 1.07E-02 mGy and 4.97E-02 (MIRD method) and 1.26E-02 mGy and 5.19E-02 (Gate code) in the Pancreas and adrenal 120 h after injection, respectively. The highest and lowest percentage differences between MIRD and Gate results are related to the Pancreas and spleen, respectively. Finally, the results showed that there is a good agreement between MIRD method and Gate code simulation for absorbed dose estimation.

Comparison of Y-90 and Ho-166 Dosimetry Using Liver Phantom: A Monte Carlo Study

BACKGROUND

It is estimated that more than 1 million people are diagnosed with liver malignancy each year and one of the treatments is radioembolization with Y-90 and Ho-166. <P> Objective: The aim of this study is to calculate the absorbed doses caused by Y-90 and Ho-166 in tumor and liver parenchyma using a phantom via Monte Carlo method. <P> Methods: A liver model phantom including a tumor imitation of sphere (r =1.5cm) was defined in GATE. The total activity of 40 mCi Y-90 and Ho-166 was prescribed into tumor imitation as source and 2x2x2 mm3 voxel-sized DoseActors were identified at 30 locations. The simulation, performed to calculate the absorbed doses left by particles during 1 second for Y-90 and Ho-166, was run for a total of 10 days and 11 days, respectively. Total doses were calculated by taking the doses occurring in 1 second as a reference. <P> Results: The maximum absorbed doses were found to be 2.334E+03±1.576E+01 Gy for Y-90 and 7.006E+02±6.013E-01 Gy for Ho-166 at the center of tumor imitation. The minimum absorbed doses were found to be 2.133E-03±1.883E-01 Gy for Y-90 and 1.152E-02±1.036E-03 Gy for Ho-166 at the farthest location from source. The mean absorbed doses in tumor imitation were found to be 1.50E+03±1.36E+00 Gy and 4.58E+02±4.75E-01 Gy for Y-90 and Ho-166, respectively. And, the mean absorbed doses in normal parenchymal tissue were found to be2.07E+01±9.58E-02 Gy and 3.79E+00±2.63E-02 Gy for Y-90 and Ho-166, respectively. <P> Conclusion: Based on the results, Ho-166 is a good alternative to Y-90 according to dosimetric evaluation.

Production of scandium radionuclides for theranostic applications: towards standardization of quality requirements

In the frame of "precision medicine", the scandium radionuclides have recently received considerable interest, providing personalised adjustment of radiation characteristics to optimize the efficiency of medical care or therapeutic benefit for particular groups of patients. Radionuclides of scandium, namely scandium-43 and scandium-44 (43/44Sc) as positron emitters and scandium-47 (47Sc), beta-radiation emitter, seem to fit ideally into the concept of theranostic pair. This paper aims to review the work on scandium isotopes production, coordination chemistry, radiolabeling, preclinical studies and the very first clinical studies. Finally, standardized procedures for scandium-based radiopharmaceuticals have been proposed as a basis to pave the way for elaboration of the Ph.Eur. monographs for perspective scandium radionuclides.

Development of 186/188Re-Chitosan as an Effective Therapeutic Agent for Rheumatoid Arthritis

BACKGROUND

Rheumatoid arthritis (RA) is an inflammatory chronic disease characterized by inflammation, pain, swelling and disability, and radiosynovectomy is one of the disease treatment lines. In this study, the possibility of providing rhenium-186/rhenium-188 chitosan radiopharmaceuticals, optimization of conditions for their production and bio-distribution are reported.

OBJECTIVE

In order to build perrhenic acid for labeling, natural rhenium was exposed to radiation. Radionuclidic and radiochemical purities of (186/188Re)-NaReO4 were examined by gamma spectroscopy and paper chromatography methods, respectively.

METHODS

Labeling of chitosan with rhenium was done in different acidic situations. The radiochemical purity 186/188Re-chitosan was applied by radio thin layer chromatography (RTLC). Lastly, the bio-distribution of the radiolabeled chitosan was studied in various organs after intra articular injection of the complex to lab rats. Gamma spectrometry confirmed the high rhenium radionuclidic purity. Chromatography results showed that perrhenic acid was produced with purity greater than 97% and rhenium chitosan labeling was done over 98% in pH = 3. Dissection results showed a high bio-distribution of 186/188Re-chitosan after injection into the joint with no leakage to surrounding organs.

CONCLUSION

According to the results, there is a possibility of labeling rhenium with chitosan in very high radiochemical purity. Regarding the high retention of these radiopharmaceuticals in joints with no leakage to surrounding organs, 186/188Re-chitosan can be applied as new radiosynovectomy drugs for rheumatoid arthritis treatment.

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.

Potential of MRI in Radiotherapy Mediated by Small Conjugates and Nanosystems

Radiation therapy has made tremendous progress in oncology over the last decades due to advances in engineering and physical sciences in combination with better biochemical, genetic and molecular understanding of this disease. Local delivery of optimal radiation dose to a tumor, while sparing healthy surrounding tissues, remains a great challenge, especially in the proximity of vital organs. Therefore, imaging plays a key role in tumor staging, accurate target volume delineation, assessment of individual radiation resistance and even personalized dose prescription. From this point of view, radiotherapy might be one of the few therapeutic modalities that relies entirely on high-resolution imaging. Magnetic resonance imaging (MRI) with its superior soft-tissue resolution is already used in radiotherapy treatment planning complementing conventional computed tomography (CT). Development of systems integrating MRI and linear accelerators opens possibilities for simultaneous imaging and therapy, which in turn, generates the need for imaging probes with therapeutic components. In this review, we discuss the role of MRI in both external and internal radiotherapy focusing on the most important examples of contrast agents with combined therapeutic potential.

177Lu/153Sm-Ethylenediamine Tetramethylene Phosphonic Acid Cocktail: A Novel Palliative Treatment for Patients with Bone Metastases

Background: Production of effective, low-cost, and efficient radiopharmaceuticals is an important task and requires further research and clinical studies. In this clinical trial, safety and efficacy of ¹⁷⁷Lu/¹⁵³Sm-ethylenediamine tetramethylene phosphonic acid (EDTMP) cocktail has been evaluated for pain relief of bone metastases. Materials and Methods: Twenty-five patients with the mean age of 55.5 ± 15.8 years participated in this study. Patients received a total dose of 37 MBq/kg. Pain and performance assessments were followed using a Brief Pain Inventory form. Complete blood count and renal and liver function tests were also performed up to 12 weeks postadministration. Results: Eighteen patients (72%) demonstrated complete pain relief (relief = 100%) and approximately all patients (96%) experienced significant improvement in their quality of life. No grade IV hematological toxicity was observed during the 12-week follow-up period, and grade III toxicity was seen in 1 patient only. In addition, no abnormalities were seen in renal and liver function during the follow-up period. Conclusions: There were no considerable complications after administration of ¹⁷⁷Lu/¹⁵³Sm EDTMP; this cocktail seems to be a safe and effective treatment for bone pain palliation in patients with skeletal metastases and improves the quality of life.

Ce-141-labeled DOTMP: A theranostic option in management of pain due to skeletal metastases

Owing to its favorable radioactive decay characteristics (T_1/2  = 32.51 d, E_β [max] = 434.6 keV [70.5%] and 580.0 keV [29.5%], E_γ  = 145.4 keV [48.5%]), ¹⁴¹ Ce could be envisaged as a theranostic radionuclide for use in nuclear medicine. The present article reports synthesis and evaluation of ¹⁴¹ Ce complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylenephosphonic acid (DOTMP) as a potent theranostic agent targeting metastatic skeletal lesions. Ce-141 was produced with 314 ± 29 MBq/mg (n = 6) specific activity and >99.9% radionuclidic purity (n = 6). Around 185 MBq dose of [¹⁴¹ Ce]Ce-DOTMP was synthesized with 98.6 ± 0.5% (n = 4) radiochemical yield under optimized conditions of reaction, and the preparation showed adequately high in vitro stability. Biodistribution studies in normal Wistar rats demonstrated significant skeletal localization and retention of injected activity (2.73 ± 0.28% and 2.63 ± 0.22% of injected activity per gram in femur at 3 hours and 14 days post-injection, respectively) with rapid clearance from non-target organs. The results of biodistribution studies were corroborated by serial scintigraphic imaging studies. These results demonstrate the potential utility of ¹⁴¹ Ce-DOTMP as a theranostic molecule for personalized patient care of cancer patients suffering from painful metastatic skeletal lesions.

Cancer radiotheranostics targeting carbonic anhydrase-IX with 111In- and 90Y-labeled ureidosulfonamide scaffold for SPECT imaging and radionuclide-based therapy

Hypoxic cells dynamically translocate during tumor growth and after radiotherapy. The most desirable direction for therapy targeting hypoxic cells is combining imaging and therapy (theranostics), which may help realize personalized medicine. Here, we conducted cancer radiotheranostics targeting carbonic anhydrase-IX (CA-IX), which is overexpressed in many kinds of hypoxic cancer cells, using low-molecular-weight 111In and 90Y complexes with a bivalent ureidosulfonamide scaffold as the CA-IX-binding moiety ([111In/90Y]US2). Methods: The targeting ability of [111In]US2 was evaluated by in vivo biodistribution study in CA-IX high-expressing (HT-29) tumor-bearing mice. In vivo imaging of HT-29 tumors was carried out using single photon emission computed tomography (SPECT). [90Y]US2 was administered to HT-29 tumor-bearing mice to evaluate cancer therapeutic effects. Results: [111In]US2 highly and selectively accumulated within HT-29 tumors (4.57% injected dose/g tumor at 1 h postinjection), was rapidly cleared from the blood pool and muscle after 4 h based on a biodistribution study, and visualized HT-29 tumor xenografts in mice at 4 h postinjection with SPECT. Radionuclide-based therapy with [90Y]US2 significantly delayed HT-29 tumor growth compared with that of untreated mice (P = 0.02 on day 28, Student's t-test), without any critical hematological toxicity due to its rapid pharmacokinetics. Conclusion: These results indicate that cancer radiotheranostics with [111In/90Y]US2 provides a novel strategy of theranostics for cancer hypoxia.

Electron linear accelerator production and purification of scandium-47 from titanium dioxide targets

The photonuclear production of no-carrier-added (NCA) ⁴⁷Sc from solid ^NatTiO₂ and the subsequent chemical processing and purification have been developed. Scandium-47 was produced by the ⁴⁸Ti(γ,p)⁴⁷Sc reaction with Bremsstrahlung photons produced from the braking of electrons in a high-Z (W or Ta) convertor. Production yields were simulated with the PHITS code (Particle and Heavy Ion Transport-code System) and compared to experimental results. Irradiated TiO₂ targets were dissolved in fuming H₂SO₄ in the presence of Na₂SO₄ and ⁴⁷Sc was purified using the commercially available Eichrom DGA resin. Typical ⁴⁷Sc recovery yields were >90% with excellent specific activity for small batches (<185 MBq batches).

Preparation & in vitro evaluation of ⁹⁰Y-DOTA-rituximab

Background & objectives:

Radioimmunotherapy is extensively being used for the treatment of non-Hodgkin's lymphoma (NHL). Use of rituximab, a chimeric anti-CD20 antibody directed against the CD20 antigen in combination with suitable beta emitters is expected to result in good treatment response by its cross-fire and bystander effects. The present work involves the conjugation of p-isothiocyanatobenzyl DOTA (p-SCN-Bn-DOTA) to rituximab, its radiolabelling with ⁹⁰Y and in vitro and in vivo evaluation to determine its potential as a radioimmunotherapeutic agent.

Methods:

Rituximab was conjugated with p-SCN-Bn-DOTA at 1:1 antibody: DOTA molar ratio. The number of DOTA molecules linked to one molecule of rituximab was determined by radioassay and spectroscopic assay. Radiolabelling of rituximab with ⁹⁰Y was carried out and its in vitro stability was evaluated. In vitro cell binding studies were carried out in Raji cells expressing CD20 antigen. Biodistribution studies were carried out in normal Swiss mice.

Results:

Using both radioassay and spectroscopic method, it was determined that about five molecules of DOTA were linked to rituximab. Radiolabelling of the rituximab conjugate with ⁹⁰Y and subsequent purification on PD-10 column gave a product with radiochemical purity (RCP) > 98 per cent which was retained at > 90 per cent up to 72 h when stored at 37°C. In vitro cell binding experiments of ⁹⁰Y-DOTA-rituximab with Raji cells exhibited specific binding of 20.7 ± 0.1 per cent with ⁹⁰Y-DOTA-rituximab which reduced to 15.5 ± 0.2 per cent when incubated with cold rituximab. The equilibrium constant K_d for ⁹⁰Y-DOTA-Rituximab was determined to be 3.38 nM. Radiolabelled antibody showed clearance via hepatobiliary and renal routes and activity in tibia was found to be quite low indicating in vivo stability of ⁹⁰Y-DOTA-rituximab.

Interpretation & conclusions:

p-SCN-Bn-DOTA was conjugated with rituximab and radiolabelling with ⁹⁰Y was carried out. In vitro studies carried out in Raji cells showed the specificity of the radiolabelled conjugate suggesting the potential uitability of the formulation as a radiopharmaceutical for therapy of NHL.

Sc-47 production from titanium targets using electron linacs

In this work we have studied the feasibility of photonuclear production of (47)Sc from (48)Ti via (48)Ti(γ,p)(47)Sc reaction. Photon flux distribution for electron beams of different energies incident on tungsten converter was calculated using MCNPX radiation transport code. (47)Sc production rate dependence on electron beam energy was found and (47)Sc yields were estimated. It was shown that irradiating a natural Ti target results in numerous scandium isotopes which can reduce the specific activity of (47)Sc. Irradiating enriched (48)Ti targets with a 22MeV 1mA beam will result in hundreds of MBq/g activity of (47)Sc and no other isotopes of scandium. Decreasing the size of the target will result in much higher average photon flux through the target and tens of GBq/g levels of specific activity of (47)Sc. Increasing the beam energy will also result in higher yields, but as soon as the electron energy exceeds the (48)Ti(γ,np)(46)Sc reaction threshold, (46)Sc starts being produced and its fraction in total scandium atoms grows as beam energy increases. The results of the simulations were benchmarked by irradiating natural titanium foil with 22MeV electron beam incident on the tungsten converter. Measured (47)Sc activities were found to be in very good agreement with the predictions.

Improving external beam radiotherapy by combination with internal irradiation

The efficacy of external beam radiotherapy (EBRT) is dose dependent, but the dose that can be applied to solid tumour lesions is limited by the sensitivity of the surrounding tissue. The combination of EBRT with systemically applied radioimmunotherapy (RIT) is a promising approach to increase efficacy of radiotherapy. Toxicities of both treatment modalities of this combination of internal and external radiotherapy (CIERT) are not additive, as different organs at risk are in target. However, advantages of both single treatments are combined, for example, precise high dose delivery to the bulk tumour via standard EBRT, which can be increased by addition of RIT, and potential targeting of micrometastases by RIT. Eventually, theragnostic radionuclide pairs can be used to predict uptake of the radiotherapeutic drug prior to and during therapy and find individual patients who may benefit from this treatment. This review aims to highlight the outcome of pre-clinical studies on CIERT and resultant questions for translation into the clinic. Few clinical data are available until now and reasons as well as challenges for clinical implementation are discussed.

(47)Ca production for (47)Ca/(47)Sc generator system using electron linacs

In this work we have studied the feasibility of photonuclear production of (47)Ca from (48)Ca for (47)Ca/(47)Sc generators. Photon flux distribution for electron beams of different energies incident on a tungsten converter was calculated using the MCNPX radiation transport code. The (47)Ca production rate dependence on electron beam energy was found and (47)Ca/(47)Sc yields were estimated for a 40MeV electron beam. It was shown that irradiating enriched targets with a 40MeV, 1mA beam will result in tens of MBq g(-1) (few mCi g(-1)) activity of (47)Sc. The results of the simulations were benchmarked by irradiating 22.5g of CaCl2 powder with a 39MeV electron beam incident on a tungsten converter. Measured (47)Ca/(47)Sc activities were found to be in very good agreement with the predictions.

Synthesis and comparative biological evaluation of bifunctional ligands for radiotherapy applications of (90)Y and (177)Lu

Zevalin® is an antibody-drug conjugate radiolabeled with a cytotoxic radioisotope ((90)Y) that was approved for radioimmunotherapy (RIT) of B-cell non-Hodgkin's lymphoma. A bifunctional ligand that displays favorable complexation kinetics and in vivo stability is required for effective RIT. New bifunctional ligands 3p-C-DE4TA and 3p-C-NE3TA for potential use in RIT were efficiently prepared by the synthetic route based on regiospecific ring opening of aziridinium ions with prealkylated triaza- or tetraaza-backboned macrocycles. The new bifunctional ligands 3p-C-DE4TA and 3p-C-NE3TA along with the known bimodal ligands 3p-C-NETA and 3p-C-DEPA were comparatively evaluated for potential use in targeted radiotherapy using β-emitting radionuclides (90)Y and (177)Lu. The bifunctional ligands were evaluated for radiolabeling kinetics with (90)Y and (177)Lu, and the corresponding (90)Y or (177)Lu-radiolabeled complexes were studied for in vitro stability in human serum and in vivo biodistribution in mice. The results of the comparative complexation kinetic and stability studies indicate that size of macrocyclic cavity, ligand denticity, and bimodality of donor groups have a substantial impact on complexation of the bifunctional ligands with the radiolanthanides. The new promising bifunctional chelates in the DE4TA and NE3TA series were rapid in binding (90)Y and (177)Lu, and the corresponding (90)Y- and (177)Lu-radiolabeled complexes remained inert in human serum or in mice. The in vitro and in vivo data show that 3p-C-DE4TA and 3p-C-NE3TA are promising bifunctional ligands for targeted radiotherapy applications of (90)Y and (177)Lu.

Radiolabeled Cetuximab Conjugates for EGFR Targeted Cancer Diagnostics and Therapy

The epidermal growth factor receptor (EGFR) has evolved over years into a main molecular target for the treatment of different cancer entities. In this regard, the anti-EGFR antibody cetuximab has been approved alone or in combination with: (a) chemotherapy for treatment of colorectal and head and neck squamous cell carcinoma and (b) with external radiotherapy for treatment of head and neck squamous cell carcinoma. The conjugation of radionuclides to cetuximab in combination with the specific targeting properties of this antibody might increase its therapeutic efficiency. This review article gives an overview of the preclinical studies that have been performed with radiolabeled cetuximab for imaging and/or treatment of different tumor models. A particularly promising approach seems to be the treatment with therapeutic radionuclide-labeled cetuximab in combination with external radiotherapy. Present data support an important impact of the tumor micromilieu on treatment response that needs to be further validated in patients. Another important challenge is the reduction of nonspecific uptake of the radioactive substance in metabolic organs like liver and radiosensitive organs like bone marrow and kidneys. Overall, the integration of diagnosis, treatment and monitoring as a theranostic approach appears to be a promising strategy for improvement of individualized cancer treatment.

General overview of radioimmunotherapy of solid tumors

Radioimmunotherapy (RIT) represents an attractive tool for the treatment of local and/or diffuse tumors with radiation. In RIT, cytotoxic radionuclides are delivered by monoclonal antibodies that specifically target tumor-associated antigens or the tumor microenvironment. While RIT has been successfully employed for the treatment of lymphoma, mostly with radiolabeled antibodies against CD20 (Bexxar(®); Corixa Corp., WA, USA and Zevalin(®); Biogen Idec Inc., CA, USA and Schering AG, Berlin, Germany), its use in solid tumors is more challenging and, so far, few trials have progressed beyond Phase II. This review provides an update on antibody-radionuclide conjugates and their use in RIT. It also discusses possible optimization strategies to improve the clinical response by considering biological, radiobiological and physical features.

Reactor production and electrochemical purification of (169)Er: a potential step forward for its utilization in in vivo therapeutic applications

INTRODUCTION

The aim of the present study was to develop and demonstrate a viable method for the reactor production of (169)Er with acceptable specific activity using moderate flux reactor and its purification from (169)Yb following electrochemical pathway based on mercury-pool cathode to avail (169)Er in radionuclidically pure form essential for its therapeutic use.

METHODS

Erbium-169 was produced in reactor by neutron bombardment of isotopically enriched (98.2% in (168)Er) erbium target at a thermal neutron flux of ~8×10(13) n.cm(-2).s(-1) for 21 d. A thorough optimization of irradiation parameters including neutron flux, irradiation time and target cooling time was carried out. The influence of different experimental parameters for the quantitative removal (169)Yb from (169)Er was investigated, optimized and based on the results; a two-cycle electrochemical separation procedure was adopted. The suitablility of purified (169)Er for application in radiation synovectomy and bone pain palliation was ascertained by carrying out radiolabeling studies with hydroxypaptite (HA) particles and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaminomethylene phosphonic acid (DOTMP), respectively.

RESULTS

Thermal neutron irradiation of 10mg of isotopically enriched (98.2% in (168)Er) erbium target at a flux of ~8×10(13) n.cm(-2).s(-1) for 21 d followed by a two-step electrochemical separation of (169)Yb impurity yielded ~3.7GBq (100mCi) of (169)Er with a specific activity of ~370MBq/mg (10mCi/mg) and radionuclidic purity of >99.99%. The reliability of this approach was amply demonstrated by performing several production batches, where the performance of each batch remained consistent. The utility of the purified (169)Er was demonstrated in the radiolabeling studies with HA particles and DOTMP, wherein both the radiolabeled products were obtained with high radiolabeling yield (>99%).

CONCLUSIONS

A viable strategy for the batch production and purification of (169)Er, suitable for therapeutic applications, has been developed and demonstrated.

Production, quality control and biodistribution studies of thulium-170-labeled ethylenediamine (tetramethylene phosphonic acid)

Thulium-170 (T 1/2 = 128.4 d, E β (max) = 968 keV, E γ = 84 keV (3.26%)) has radionuclidic properties suitable to be used in palliative therapy of bone metastases as an alternative to 89SrCl2. 170Tm can be produced by a relatively easy route involving thermal neutron bombardment on natural Tm2O3 (100% 169Tm) in medium flux research reactors. The requirement for an enriched target does not arise and radionuclidic impurities are not formed by radiative capture during neutron activation. In this study 170Tm was produced using Tm(NO3)3, prepared by neutron activation (3–4E13) of a natural sample. Ethylenediamine (tetramethylene phosphonic acid) (EDTMP) was synthesized and radiolabeled with 170Tm. Complexation parameters were optimized to achieve maximum yields (>99%). The radiochemical purity of 170Tm-EDTMP was checked by RTLC. It was found to retain its stability at room temperature even after 2 months of preparation (> 95%). Biodistribution studies of the complexes carried out in wild-type rats showed significant bone uptake with rapid clearance from blood. The properties of produced 170Tm-EDTMP suggest applying a new efficient bone pain palliative therapeutic agent in the country instead of some other in-use radiopharmaceuticals, such as 89SrCl2 and 32P, in order to overcome metastatic bone pains.

A Bridge not too Far: Personalized Medicine with the use of Theragnostic Radiopharmaceuticals

This article deals primarily with the selection criteria, production, and the nuclear, physical, and chemical properties of certain dual-purpose radionuclides, including those that are currently being used, or studied and evaluated, and those that warrant future investigations. Various scientific and practical issues related to the production and availability of these radionuclides is briefly addressed. At brookhaven national laboratory (BNL), we have developed a paradigm that involves specific individual ‘dual-purpose’ radionuclides or radionuclide pairs with emissions suitable for both imaging and therapy, and which when molecularly (selectively) targeted using appropriate carriers, would allow pre-therapy low-dose imaging plus higher-dose therapy in the same patient. We have made an attempt to sort out and organize a number of such theragnostic radionuclides and radionuclide pairs that may thus potentially bring us closer to the age-long dream of personalized medicine for performing tailored low-dose molecular imaging (SPECT/CT or PET/CT) to provide the necessary pretherapy information on biodistribution, dosimetry, the limiting or critical organ or tissue, and the maximum tolerated dose (MTD), etc., followed by performing higher-dose targeted molecular therapy in the same patient with the same radiopharmaceutical. As an example, our preclinical and clinical studies with the theragnostic radionuclide Sn-117m are covered in somewhat greater detail.

A troublesome problem that remains yet to be fully resolved is the lack of availability, in sufficient quantities and at reasonable cost, of a majority of the best candidate theragnostic radionuclides in a no-carrier-added (NCA) form. In this regard, a summary description of recently developed new or modified methods at BNL for the production of five theragnostic radionuclide/radionuclide pair items, whose nuclear, physical, and chemical characteristics seem to show promise for therapeutic oncology and for treating other disorders that respond to radionuclide therapy, is provided.

How to cite this article

Srivastava SC. A Bridge not too Far: Personalized Medicine with the use of Theragnostic Radiopharmaceuticals. J Postgrad Med Edu Res 2013;47(1):31-46.

Synthesis and preclinical evaluation of bifunctional ligands for improved chelation chemistry of 90Y and 177Lu for targeted radioimmunotherapy

We report a practical and high-yield synthesis of a bimodal bifunctional ligand 3p-C-NETA-NCS containing the isothiocyanate group for conjugation to a tumor targeting antibody. 3p-C-NETA-NCS was conjugated to a tumor-targeting antibody, trastuzumab, and the corresponding 3p-C-NETA-trastuzumab conjugate was evaluated and compared to trastuzumab conjugates of the known bifunctional ligands C-DOTA, C-DTPA, and 3p-C-DEPA for radiolabeling kinetics with (90)Y and (177)Lu. 3p-C-NETA-trastuzumab conjugate exhibited extremely rapid complexation kinetics with (90)Y and (177)Lu. (90)Y-3p-C-NETA-trastuzumab and (177)Lu-3p-C-NETA-trastuzumab conjugates were stable in human serum for 2 weeks. A pilot biodistribution study was conducted to evaluate in vivo stability and tumor targeting of (177)Lu-radiolabeled trastuzumab conjugate using nude mice bearing ZR-75-1 human breast cancer. (177)Lu-3p-C-NETA-trastuzumab conjugate displayed low radioactivity level at blood (1.6%), low organ uptake (<2.2%), and high tumor-to-blood ratio (6.4) at 120 h. 3p-C-NETA possesses favorable in vitro and in vivo profiles and is an excellent bifunctional chelator that can be used for targeted RIT applications using (90)Y and (177)Lu and has the potential to replace DOTA and DTPA analogues in current clinical use.

Chemoradionuclide therapy with 186re-labeled liposomal doxorubicin: toxicity, dosimetry, and therapeutic response

This study was performed to determine the maximum tolerated dose (MTD) and therapeutic effects of rhenium-186 ((186)Re)-labeled liposomal doxorubicin (Doxil), investigate associated toxicities, and calculate radiation absorbed dose in head and neck tumor xenografts and normal organs. Doxil and control polyethylene glycol (PEG)-liposomes were labeled using (186)Re-N,N-bis(2-mercaptoethyl)-N',N'-diethylethylenediamine (BMEDA) method. Tumor-bearing rats received either no therapy (n=6), intravenous Doxil (n=4), or escalating radioactivity of (186)Re-Doxil (185-925 MBq/kg) or (186)Re-PEG-liposomes (1110-1665 MBq/kg) and were monitored for 28 days. Based on body weight loss and systemic toxicity, MTD for (186)Re-Doxil and (186)Re-PEG-liposomes were established at injected radioactivity/body weight of 740 and 1480 MBq/kg, respectively. (186)Re-injected radioactivity/body weight for therapy studies was determined to be 555 MBq/kg for (186)Re-Doxil and 1295 MBq/kg for (186)Re-PEG-liposomes. All groups recovered from their body weight loss, leucopenia, and thrombocytopenia by 28 days postinjection. Normalized radiation absorbed dose to tumor was significantly higher for (186)Re-Doxil (0.299±0.109 Gy/MBq) compared with (186)Re-PEG-liposomes (0.096±0.120 Gy/MBq) (p<0.05). In a separate therapy study, tumor volumes were significantly smaller for (186)Re-Doxil (555 MBq/kg) compared with (186)Re-PEG-liposomes (1295 MBq/kg) (p<0.01) at 42 days postinjection. In conclusion, combination chemoradionuclide therapy with (186)Re-Doxil has promising potential, because good tumor control was achieved with limited associated toxicity.

Paving the way to personalized medicine: production of some theragnostic radionuclides at Brookhaven National Laboratory

This paper introduces a relatively novel paradigm that involves specific individual radionuclides or radionuclide pairs that have emissions that allow pre-therapy low-dose imaging plus higher-dose therapy in the same patient. We have made an attempt to sort out and organize a number of such theragnostic radionuclides and radionuclide pairs that may potentially bring us closer to the age-long dream of personalized medicine for performing tailored low-dose molecular imaging (SPECT/CT or PET/CT) to provide the necessary pre-therapy information on biodistribution, dosimetry, the limiting or critical organ or tissue, and the maximum tolerated dose (MTD), etc. If the imaging results then warrant it, it would be possible to perform higher-dose targeted molecular therapy in the same patient with the same radiopharmaceutical. A major problem that remains yet to be fully resolved is the lack of availability, in sufficient quantities, of a majority of the best candidate theragnostic radionuclides in a no-carrier-added (NCA) form. A brief description of the recently developed new or modified methods at BNL for the production of four theragnostic radionuclides, whose nuclear, physical, and chemical characteristics seem to show great promise for personalized cancer therapy are described.

Emergence and present status of Lu-177 in targeted radiotherapy: the Indian scenario

177Lu is presently considered to be a potential radionuclide for the development of agents for radionuclide therapy owing to its favorable nuclear decay characteristics [T 1/2 = 6.65 d, E β(max) = 0.497 MeV, E γ = 113 KeV (6.4%) and 208 KeV (11%)]. While the long half-life of this promising radioisotope offers distinct logistic advantage, particularly, in countries having limited reactor facilities, the feasibility of its large-scale production with adequate specific activity and excellent radionuclidic purity in medium flux research reactors constitute yet another desirable feature. Extensive studies have been carried out to optimize the production of this isotope, with high specific activity and radionuclidic purity by the (n,γ) route using the highest available flux and the optimum irradiation time. The gradual evolution of clin ical grade 177LuCl3 as a new radiochemical, ready for commercial deployment by Radiopharmaceuticals Division, Bhabha Atomic Research Centre, to nuclear medicine centers all over India was accomplished in 2010 in a stepwise manner with the commencement of the production of high specific activity 177Lu from enriched target in 2001. Research on 177Lu has demonstrated its immense potential in radiotherapeutic applications, a direct outcome of which has resulted in indigenous development of two agents viz. 177Lu-EDTMP and 177Lu-DOTA-TATE presently being evaluated in human patients for palliative care of bone pain due to skeletal metastases and treatment of malignancies of neuroendocrine origin, respectively. Using locally produced 177Lu, the radiolabeling of a plethora of other molecules with potential applicability in radiation synovectomy and targeted therapy of malignant tumors have been successfully demonstrated. A few of these agent such as a novel 177Lu-labeled porphyrin has shown considerable promise in initial studies and is presently evaluated. In the present article, our research efforts toward standardization of production methodology of 177Lu in high specific activity and its utilization in the devel opment of agents for targeted radiotherapy are being reported.

A highly effective bifunctional ligand for radioimmunotherapy applications

A novel bifunctional ligand (3p-C-NETA) for antibody-targeted radioimmunotherapy (RIT) of β-emitting radioisotopes (90)Y and (177)Lu was efficiently synthesized via an unexpected regiospecific ring opening of an aziridinium ion. 3p-C-NETA instantly formed a very stable complex with (90)Y or (177)Lu. 3p-C-NETA is an excellent bifunctional ligand for RIT.

Anti-EGFRvIII monoclonal antibody armed with 177Lu: in vivo comparison of macrocyclic and acyclic ligands

INTRODUCTION

Monoclonal antibody (mAb) L8A4 binds specifically to the epidermal growth factor receptor variant III (EGFRvIII) that is present on gliomas but not on normal tissues, and is internalized rapidly after receptor binding. Because of the short range of its β-emissions, labeling this mAb with (177)Lu would be an attractive approach for the treatment of residual tumor margins remaining after surgical debulking of brain tumors.

MATERIALS AND METHODS

L8A4 mAb was labeled with (177)Lu using the acyclic ligands [(R)-2-amino-3-(4-isothiocyanatophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-pentaacetic acid (CHX-A″-DTPA) and 2-(4-isothiocyanatobenzyl)-6-methyldiethylene-triaminepentaacetic acid (1B4M-DTPA), and the macrocyclic ligands S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-tetraacetic acid (C-DOTA) and α-(5-isothiocyanato-2-methoxyphenyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (MeO-DOTA). Paired-label tissue distribution experiments were performed in athymic mice bearing subcutaneous EGFRvIII-expressing U87.ΔEGFR glioma xenografts over a period of 1 to 8 days to directly compare (177)Lu-labeled L8A4 to L8A4 labeled with (125)I using N-succinimidyl 4-guanidinomethyl-3-[(125)I]iodobenzoate ([(125)I]SGMIB).

RESULTS

Except with C-DOTA, tumor uptake for the (177)Lu-labeled mAb was significantly higher than the co-administered radioiodinated preparation; however, this was also the case for spleen, liver, bone and kidneys. Tumor/normal tissue ratios for (177)Lu-1B4M-DTPA-L8A4 and, to an even greater extent, (177)Lu-MeO-DOTA-L8A4 were higher than those for [(125)I]SGMIB-L8A4 in most other tissues.

CONCLUSIONS

Tumor and normal tissue distribution patterns for this anti-EGFRvIII mAb were dependent on the nature of the bifunctional chelate used for (177)Lu labeling. Optimal results were obtained with 1B4M-DTPA and MeO-DOTA, suggesting no clear advantage for acyclic vs. macrocyclic ligands for this application.

Multispecies animal investigation on biodistribution, pharmacokinetics and toxicity of 177Lu-EDTMP, a potential bone pain palliation agent

INTRODUCTION

Radionuclide therapy (RNT) is an effective method for bone pain palliation in patients suffering from bone metastasis. Due to the long half-life, easy production and relatively low beta- energy, (177)Lu [T(1/2)=6.73 days, E(beta max)=497 keV, E(gamma)=113 keV (6.4%), 208 keV (11%)]-based radiopharmaceuticals offer logistical advantage for wider use. This paper reports the results of a multispecies biodistribution and toxicity studies of (177)Lu-EDTMP to collect preclinical data for starting human clinical trials.

METHODS

(177)Lu-EDTMP with radiochemical purity greater than 99% was formulated by using a lyophilized kit of EDTMP (35 mg of EDTMP, 5.72 g of CaO and 14.1 mg of NaOH). Biodistribution studies were conducted in mice and rabbits. Small animal imaging was performed using NanoSPECT/CT (Mediso, Ltd., Hungary) and digital autoradiography. Gamma camera imaging was done in rabbits and dogs. Four levels of activity (9.25 through 37 MBq/kg body weight) of (177)Lu-EDTMP were injected in four groups of three dogs each to study the toxicological effects.

RESULTS

(177)Lu-EDTMP accumulated almost exclusively in the skeletal system (peak ca. 41% of the injected activity in bone with terminal elimination half-life of 2130 and 1870 h in mice and rabbits, respectively) with a peak uptake during 1-3 h. Excretion of the radiopharmaceutical was through the urinary system. Imaging studies showed that all species (mouse, rat, rabbit and dog) take up the compound in regions of remodeling bone, while kidney retention is not visible after 1 day postinjection (pi). In dogs, the highest applied activity (37 MBq/kg body weight) led to a moderate decrease in platelet concentration (mean, 160 g/L) at 1 week pi with no toxicity.

CONCLUSION

The protracted effective half-life of (177)Lu-EDTMP in bone supports that modifying the EDTMP molecule by introducing (177)Lu does not alter its biological behaviour as a specific bone-seeking tracer. Species-specific pharmacokinetic behavior differences were observed. Toxicity studies in dogs did not show any biological adverse effects. The studies demonstrate that (177)Lu-EDTMP is a promising radiopharmaceutical that can be further evaluated for establishing as a radiopharmaceutical for human use.

Imaging in targeted delivery of therapy to cancer

We review the current status of imaging as applied to targeted therapy with particular focus on antibody-based therapeutics. Antibodies have high tumor specificity and can be engineered to optimize delivery to, and retention within, the tumor. Whole antibodies can activate natural immune effector mechanisms and can be conjugated to beta- and alpha-emitting radionuclides, toxins, enzymes, and nanoparticles for enhanced therapeutic effect. Imaging is central to the development of these agents and is used for patient selection, performing dosimetry and assessment of response. gamma- and positron-emitting radionuclides may be used to image the distribution of antibody-targeted therapeutics While some radionuclides such as iodine-131 emit both beta and gamma radiation and are therefore suitable for both imaging and therapy, others are more suited to imaging or therapy alone. Hence for radionuclide therapy of neuroendocrine tumors, patients can be selected for therapy on the basis of gamma-emitting indium-111-octreotide imaging and treated with beta-emitting yttrium-90-octreotate. Positron-emitting radionuclides can give greater sensitivity that gamma-emitters but only a single radionuclide can be imaged at one time and the range of radionuclides is more limited. The multiple options for antibody-based therapeutic molecules, imaging technologies and therapeutic scenarios mean that very large amounts of diverse data are being acquired. This can be most effectively shared and progress accelerated by use of common data standards for imaging, biological, and clinical data.

177Lu-DOTMP: A viable agent for palliative radiotherapy of painful bone metastasis

The suitable nuclear decay characteristics [T 1/2=6.73 d, E β (max)=497 keV, E γ=113 keV (6.4%), 208 keV (11%)] as well as the feasibility of large-scale production with adequate specific activity and radionuclidic purity using a moderate flux reactor are important attributes towards 177Lu to be considered as a promising radionuclide for palliative care in painful bone metastasis. The present study describes the preparation of 177Lu complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylene phosphonic acid (DOTMP) and its preliminary biological evaluation in animal models with an aim to proposing it as a viable radiopharmaceutical for bone pain palliation. The choice DOTMP as the polyaminophosphonic acid carrier ligand is based on the enhanced thermodynamic stability and kinetic inertness of the metal-ligand complexes with macrocyclic chelators. 177Lu was produced with a specific activity of ∼12 GBq/mg (∼324 mCi/mg) and radionuclidic purity of 99.98% by irradiation of natural Lu2O3 target at a thermal neutron flux of ∼6×1013 n/cm2s for 21 d. 177Lu-DOTMP complex was prepared in high yield and excellent radiochemical purity (>99%) using DOTMP synthesized and characterized in-house. The complex exhibited excellent in-vitro stability at room temperature. Biodistribution studies in Wistar rats showed rapid skeletal accumulation of the injected activity [(1.60±0.19) per gram in femur at 3 h post-injection] with fast clearance from blood and minimal uptake in any of the major organs. Scintigraphic studies carried out in normal Wistar rats and New Zealand white rabbits also demonstrated significant accumulation of the agent in skeleton and almost no retention in any other vital organs.

Radioimmunotherapy of experimental human metastatic melanoma with melanin-binding antibodies and in combination with dacarbazine

PURPOSE

Melanin has emerged as an attractive target for radioimmunotherapy (RIT) of melanoma, and a radiolabeled monoclonal antibody (mAb) 6D2 to melanin is currently in clinical evaluation. We investigated two approaches to improve the targeting of radiation to tumors using melanin-binding mAbs: (a) the use of an additional mAb to melanin could provide information on whether using antibodies to melanin can serve as a general approach to development of therapeutics for melanoma, and (b) as melanin targeting involves the antibody binding to extracellular melanin released from necrotic melanoma cells, we hypothesized that the administration of a chemotherapeutic agent followed by RIT would facilitate the delivery of radiation to the tumors due to the increased presence of free melanin.

EXPERIMENTAL DESIGN

We evaluated the therapeutic efficacy of two melanin-binding IgM mAbs labeled with (188)Re (6D2 and 11B11). We compared the efficacy of RIT with (188)Re-6D2 to chemotherapy with dacarbazine (DTIC) and to combined chemotherapy and RIT in human metastatic melanoma-bearing nude mice.

RESULTS

Therapeutic efficacy of (188)Re-labeled 6D2 and 11B11 was comparable despite differences in their affinity and binding site numbers. Comparison of chemotherapy with DTIC and RIT revealed that RIT was more effective in slowing tumor growth in mice. Administration of DTIC followed by RIT was more effective than either modality alone.

CONCLUSIONS

These results provide encouragement for the development of RIT for melanoma with melanin-binding mAbs and suggest that combining chemotherapy and RIT may be a promising approach for the treatment of metastatic melanoma.

Synthesis and biological evaluation of a novel decadentate ligand DEPA

An efficient and short synthetic route to a novel decadentate ligand 7-[2-(bis-carboxymethyl-amino)-ethyl]-4,10-bis-carboxymethyl-1,4,7,10-tetraaza-cyclododec-1-yl-acetic acid (DEPA) with both macrocyclic and acyclic binding moieties is reported. A reproducible and scalable synthetic method to a precursor molecule of DEPA, 1,4,7-tris(tert-butoxycarbonylmethyl)tetraazacyclododecane was developed. DEPA was evaluated as a chelator of (177)Lu, (212)Bi, and (213)Bi for potential use in an antibody-targeted cancer therapy, radioimmunotherapy (RIT) using Arsenazo III based spectroscopic complexation kinetics, in vitro serum stability, and in vivo biodistribution studies.

177Lu-EDTMP: a viable bone pain palliative in skeletal metastasis

Designing ideal radiopharmaceuticals for use as bone pain palliatives require the use of a moderate energy beta() emitter as a radionuclide and a suitable polyaminophosphonic acid as a carrier molecule. Owing to its suitable decay characteristics [T(1/2) = 6.73 d, E((max)) = 497 keV, E() = 113 keV (6.4%), 208 keV (11%)] as well as the feasibility of large-scale production in adequate specific activity and radionuclidic purity using a moderate flux reactor, 177Lu could be considered as a promising radionuclide for palliative care in painful bone metastasis. The present study was therefore, oriented toward the preparation and biologic evaluation of 177Lu complex of ethylenediaminetetramethylene phosphonic acid (EDTMP) in various animal models, with an aim to prepare a viable radiopharmaceutical for bone pain palliation. 177Lu was produced with a specific activity of approximately 12 GBq/mg (approximately 324 mCi/mg) and radionuclidic purity of 99.98% by irradiation of natural Lu2O3 targeted at a thermal neutron flux of approximately 6 x 10(13) n/cm(2).s for 21 days. 177Lu-EDTMP complex was prepared in high-yield and excellent radiochemical purity (>99%), using EDTMP synthesized and characterized in-house. The complex exhibited excellent in vitro stability at room temperature. Biodistribution studies in Wistar rats showed a rapid skeletal accumulation of injected activity [(1.74 +/- 0.30)% per gram in femur at 3 hours postinjection] with a fast clearance from blood and minimal uptake in any of the major organs. Scintigraphic imaging studies carried out in normal Wistar rats, New Zealand white rabbits, as well as in Beagle dogs also demonstrated significant accumulation of the agent in the skeleton and almost no retention of activity in any other vital organs.

Novel bimodal bifunctional ligands for radioimmunotherapy and targeted MRI

The structurally novel bifunctional ligands C-NETA and C-NE3TA, each possessing both acyclic and macrocyclic moieties, were prepared and evaluated as potential chelates for radioimmunotherapy (RIT) and targeted magnetic resonance imaging (MRI). Heptadentate C-NE3TA was fortuitously discovered during the preparation of C-NETA. An optimized synthetic method to C-NETA and C-NE3TA including purification of the polar and tailing reaction intermediates, tert-butyl C-NETA (2) and tert-butyl C-NE3TA (3) using semiprep HPLC was developed. The new Gd(III) complexes of C-NETA and C-NE3TA were prepared as contrast enhancement agents for use in targeted MRI. The T 1 relaxivity data indicate that Gd(C-NETA) and Gd(C-NE3TA) possess higher relaxivity than Gd(C-DOTA), a bifunctional version of a commercially available MRI contrast agent; Gd(DOTA). C-NETA and C-NE3TA were radiolabeled with (177)Lu, (90)Y, (203)Pb, (205/6)Bi, and (153)Gd; and in vitro stability of the radiolabeled corresponding complexes was assessed in human serum. The in vitro studies indicate that the evaluated radiolabeled complexes were stable in serum for 11 days with the exception being the (203)Pb complexes of C-NETA and C-NE3TA, which dissociated in serum. C-NETA and C-NE3TA radiolabeled (177)Lu, (90)Y, or (153)Gd complexes were further evaluated for in vivo stability in athymic mice and possess excellent or acceptable in vivo biodistribution profile. (205/6)Bi- C-NE3TA exhibited extremely rapid blood clearance and low radioactivity level at the normal organs, while (205/6)Bi- C-NETA displayed low radioactivity level in the blood and all of the organs except for the kidney where relatively high renal uptake of radioactivity is observed. C-NETA and C-NE3TA were further modified for conjugation to the monoclonal antibody Trastuzumab.

Rational design and generation of a bimodal bifunctional ligand for antibody-targeted radiation cancer therapy

An antibody-targeted radiation therapy (radioimmunotherapy, RIT) employs a bifunctional ligand that can effectively hold a cytotoxic metal with clinically acceptable complexation kinetics and stability while being attached to a tumor-specific antibody. Clinical exploration of the therapeutic potential of RIT has been challenged by the absence of adequate ligand, a critical component for enhancing the efficacy of the cancer therapy. To address this deficiency, the bifunctional ligand C-NETA in a unique structural class possessing both a macrocyclic cavity and a flexible acyclic moiety was designed. The practical, reproducible, and readily scalable synthetic route to C-NETA was developed, and its potential as the chelator of (212)Bi, (213)Bi, and (177)Lu for RIT was evaluated in vitro and in vivo. C-NETA rapidly binds both Lu(III) and Bi(III), and the respective metal complexes remain extremely stable in serum for 14 days. (177)Lu -C-NETA and (205/6)Bi -C-NETA possess an excellent or acceptable in vivo biodistribution profile.

Targeted alpha-therapy: past, present, future?

Monoclonal antibodies have become a viable strategy for the delivery of therapeutic, particle emitting radionuclides specifically to tumor cells to either augment anti-tumor action of the native antibodies or to solely take advantage of their action as targeting vectors. Proper and rational selection of radionuclide and antibody combinations is critical to making radioimmunotherapy (RIT) a standard therapeutic modality due to the fundamental and significant differences in the emission of either alpha- and beta-particles. The alpha-particle has a short path length (50-80 microm) that is characterized by high linear energy transfer (100 keV microm(-1)). Actively targeted alpha-therapy potentially offers a more specific tumor cell killing action with less collateral damage to the surrounding normal tissues than beta-emitters. These properties make targeted alpha-therapy an appropriate therapy to eliminate minimal residual or micrometastatic disease. RIT using alpha-emitters such as (213)Bi, (211)At, (225)Ac, and others has demonstrated significant activity in both in vitro and in vivo model systems. Limited numbers of clinical trials have progressed to demonstrate safety, feasibility, and therapeutic activity of targeted alpha-therapy, despite having to traverse complex obstacles. Further advances may require more potent isotopes, additional sources and more efficient means of isotope production. Refinements in chelation and/or radiolabeling chemistry combined with rational improvements of isotope delivery, targeting vectors, molecular targets, and identification of appropriate clinical applications remain as active areas of research. Ultimately, randomized trials comparing targeted alpha-therapy combined with integration into existing standards of care treatment regimens will determine the clinical utility of this modality.

Role of radiosynovectomy in the treatment of rheumatoid arthritis and hemophilic arthropathies

Radiosynovectomy is a novel method of treatment for several acute and chronic inflammatory joint disorders. A small amount of a beta-emitting radionuclide is injected into the affected joint delivering a radiation dose of 70 to 100 Gy to the synovia. The proliferative tissue is destroyed, secretion of fluid and accumulation of inflammation causing cellular compounds stops and the joint surfaces become fibrosed, providing long term symptom relief. The radionuclides are injected in colloidal form so that they remain in the synovium and are not transported by lymphatic vessels causing radiation exposure to other organs. Complete reduction of knee joint swelling has been seen in above 40% and pain relief in 88% of patients. Wrist, elbow, shoulder, ankle and hip joints showed significant improvement in 50-60% and restoration of normal function and long term pain relief has been achieved in about 70% of small finger joints. In hemophilic arthropathies complete cessation of bleeding in about 60% and improved mobility in 75% of patients has been reported.

Melanin as a potential target for radionuclide therapy of metastatic melanoma

Melanoma is diagnosed in approximately 100,000 patients worldwide and for those with metastatic disease, the 5-year survival is extremely poor at just 6%, because there are no satisfactory treatments. Targeted radionuclide therapy is currently gaining momentum and has evolved into an efficient modality for the treatment of patients with malignancies such as non-Hodgkins lymphoma in whom standard antineoplastic therapies are not effective. Melanoma is named after the pigment melanin, which in turn is derived from the Greek word for black. Most melanomas are pigmented by the presence of melanin, some of which is extracellular as a result of cellular turnover. Thus, melanin presents a promising target for the drugs carrying a cytotoxic payload of radiation provided such therapies spare other melanotic tissues. There are a variety of substances that could potentially serve as delivery vehicles of radionuclides for the treatment of melanoma. These substances can be divided into melanin binders, melanin precursors and binders to melanogenesis-related proteins. The authors are optimistic that therapeutic agents targeting melanin to deliver radionuclide therapy could appear in the clinic within a decade.

Preclinical Investigations of Drug and Radionuclide Conjugates of Bisphosphonates for the Treatment of Metastatic Bone Cancer

The potential targeting of therapeutic bisphosphonate conjugates to bone metastatic lesions was evaluated in vivo in mice. A bisphosphonate conjugate with 5-fluorouracil was synthesized as a potential chemotherapy agent, and a bisphosphonate conjugate with diethylenetriaminepentaacetic acid (DTPA) was prepared as a potential carrier of cytotoxic radionuclides. The compounds are hypothesized to be able to deliver either high doses of radiation or a high concentration of chemotherapy agents at sites of increased osteoclastic activity in patients with bony metastases while exhibiting minimal toxicity to normal tissues. Tissue distribution studies with the 99mTc-labeled bisphosphonate conjugates with DTPA and 5-fluorouracil showed rapid blood clearance and excretion of unbound activity, clearance from most tissues, and substantial retention of the bisphosphonates in bone. For the DTPA conjugate, activity in the bone represents 13.6% of the total injected dose at 8 hours following injection, representing 54.3% of the total whole-body activity at this time period. Under the same conditions, the 5-fluorouracil conjugate showed a 17.1% bone uptake at 60.2% of the whole-body activity. This normal bone uptake predicts that high concentrations of conjugates are expected to be achieved at sites of bone metastatic disease. Chemotherapy and radiotherapy studies with these compounds in animal models of metastatic bone cancer are underway.