Relation between the location of elements in the periodic table and various organ-uptake rates

Biokinetic models for Group VB elements

This paper reviews biokinetic data for the Group VB elements vanadium, niobium, and tantalum, and presents biokinetic models describing their systemic behaviour. The model for systemic niobium in adults was developed earlier and described in Publication 134 of the International Commission on Radiological Protection. The model for niobium is used as a starting point for the development of models for vanadium and tantalum. Published biokinetic data for vanadium, including comparisons with niobium, indicate that the initial distribution of vanadium is broadly similar to that of niobium but that vanadium is less firmly fixed in most tissues and is excreted more rapidly than niobium. Biokinetic data for tantalum are more limited but suggest that its systemic behaviour closely resembles that of niobium at early times after administration. The model for niobium is proposed for application to tantalum in view of the suggested biological similarities of tantalum and niobium, their generally strong coherence in nature due to similar ionic radii and identical valence states, and the difficulties in developing parameter values directly from available data for tantalum. The proposed model for vanadium relies largely on vanadium-specific information and varies considerably from the model for niobium.

p-NO2-Bn-H4neunpa and H4neunpa-Trastuzumab: Bifunctional Chelator for Radiometalpharmaceuticals and 111In Immuno-Single Photon Emission Computed Tomography Imaging

Potentially nonadentate (N₅O₄) bifunctional chelator p-SCN-Bn-H₄neunpa and its immunoconjugate H₄neunpa-trastuzumab for ¹¹¹In radiolabeling are synthesized. The ability of p-SCN-Bn-H₄neunpa and H₄neunpa-trastuzumab to quantitatively radiolabel ¹¹¹InCl₃ at an ambient temperature within 15 or 30 min, respectively, is presented. Thermodynamic stability determination with In³⁺, Bi³⁺, and La³⁺ resulted in high conditional stability constant (pM) values. In vitro human serum stability assays have demonstrated both ¹¹¹In complexes to have high stability over 5 days. Mouse biodistribution of [¹¹¹In][In(p-NO₂-Bn-neunpa)]^-, compared to that of [¹¹¹In][In(p-NH₂-Bn-CHX-A″-diethylenetriamine pentaacetic acid (DTPA))]^2-, at 1, 4, and 24 h shows fast clearance of both complexes from the mice within 24 h. In a second mouse biodistribution study, the immunoconjugates ¹¹¹In-neunpa-trastuzumab and ¹¹¹In-CHX-A″-DTPA-trastuzumab demonstrate a similar distribution profile but with slightly lower tumor uptake of ¹¹¹In-neunpa-trastuzumab compared to that of ¹¹¹In-CHX-A″-DTPA-trastuzumab. These results were also confirmed by immuno-single photon emission computed tomography (immuno-SPECT) imaging in vivo. These initial investigations reveal the acyclic bifunctional chelator p-SCN-Bn-H₄neunpa to be a promising chelator for ¹¹¹In (and other radiometals) with high in vitro stability and also show H₄neunpa-trastuzumab to be an excellent ¹¹¹In chelator with promising biodistribution in mice.

Biokinetics of yttrium and comparison with its geochemical twin holmium

The transition metal yttrium (Y, atomic number 39) is chemically similar to elements in the lanthanide family (atomic numbers 57-71) and is found with the lanthanides in rare earth ores. Yttrium and the lanthanide holmium are referred to as geochemical twins because they generally show little fractionation from metamorphic or weathering processes, due to their closely similar chemical properties and nearly identical ionic radii. Extensive measurements on rocks, soils, and meteorites indicate that the Y/Ho mass concentration ratio rarely falls far from the so-called chondritic or solar system ratio of ∼26. This paper presents a new biokinetic model for yttrium in adult humans and examines whether yttrium and holmium may be biological as well as geochemical twins, considering model-based comparisons of their systemic behaviours in adult humans and model-free comparisons of their concentration ratios in human tissues and various types of vegetation. It appears that yttrium and holmium behave similarly in the human body and that their concentration ratios tend to cluster near the chondritic value in human tissues as well as plants, but the comparative information is too limited and imprecise to determine whether they are extremely close biological analogues.

Synthetic nanoparticles for delivery of radioisotopes and radiosensitizers in cancer therapy

Radiotherapy has been, and will continue to be, a critical modality to treat cancer. Since the discovery of radiation-induced cytotoxicity in the late 19th century, both external and internal radiation sources have provided tremendous benefits to extend the life of cancer patients. Despite the dramatic improvement of radiation techniques, however, one challenge persists to limit the anti-tumor efficacy of radiotherapy, which is to maximize the deposited dose in tumor while sparing the rest of the healthy vital organs. Nanomedicine has stepped into the spotlight of cancer diagnosis and therapy during the past decades. Nanoparticles can potentiate radiotherapy by specifically delivering radionuclides or radiosensitizers into tumors, therefore enhancing the efficacy while alleviating the toxicity of radiotherapy. This paper reviews recent advances in synthetic nanoparticles for radiotherapy and radiosensitization, with a focus on the enhancement of in vivo anti-tumor activities. We also provide a brief discussion on radiation-associated toxicities as this is an area that, up to date, has been largely missing in the literature and should be closely examined in future studies involving nanoparticle-mediated radiosensitization.

Toxic and transcriptional responses of PC12 cells to soluble tungsten alloy surrogates

There is increasing evidence that metals have a role in the etiology of diverse neurological diseases. This study used PC12 cells as an in vitro model to examine the toxicity of tungsten alloys that have important military applications. Initially, the relative concentrations of tungsten (W), nickel (Ni), and cobalt (Co) mobilized from pellets of a weapons-grade tungsten alloy incubated in physiologically relevant solutions were determined. Dosing solutions of soluble metal salts that were equivalent in ratio to those mobilized from these alloy pellets were used to treat nerve growth factor (NGF) differentiated PC12 cells. Treatments consisted of single (W, Ni or Co), paired (W/Ni, W/Co or Ni/Co) or complete (W/Ni/Co) metal exposures for 24 h followed by measurement of cytotoxicity, viability, and microarray analysis to examine their impact on survival and viability, global gene expression, and biological processes. Gene expression changed dramatically with addition of NGF. Addition of Ni or Co either singly or in combination further impacted gene expression. An observed additive effect of Ni and Co on gene expression was unaffected by the addition of W. The work showed that tungsten, as found in this tungsten alloy, had minimal relative toxicity as compared to the other alloy components when used either alone or in combination.

Matching chelators to radiometals for radiopharmaceuticals

Radiometals comprise many useful radioactive isotopes of various metallic elements. When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, e.g.(67)Ga, (99m)Tc, (111)In, (177)Lu) and positron emission tomography (PET, e.g.(68)Ga, (64)Cu, (44)Sc, (86)Y, (89)Zr), as well as therapeutic applications (e.g.(47)Sc, (114m)In, (177)Lu, (90)Y, (212/213)Bi, (212)Pb, (225)Ac, (186/188)Re). A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems. The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging. A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed. The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals. Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents.

H(2)azapa: a versatile acyclic multifunctional chelator for (67)Ga, (64)Cu, (111)In, and (177)Lu

Preliminary experiments with the novel acyclic triazole-containing bifunctional chelator H2azapa and the radiometals (64)Cu, (67)Ga, (111)In, and (177)Lu have established its significant versatile potential as an alternative to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) for metal-based radiopharmaceuticals. Unlike DOTA, H2azapa radiolabels quantitatively with (64)Cu, (67)Ga, (111)In, and (177)Lu in 10 min at room temperature. In vitro competition experiments with human blood serum show that (64)Cu remained predominantly chelate-bound, with only 2% transchelated to serum proteins after 20 h. Biodistribution experiments with [(64)Cu(azapa)] in mice reveal uptake in various organs, particularly in the liver, lungs, heart, intestines, and kidneys. When compared to [(64)Cu(DOTA)](2-), the lipophilic neutral [(64)Cu(azapa)] was cleared through the gastrointestinal tract and accumulated in the liver, which is common for lipophilic compounds or free (64)Cu. The chelator H2azapa is a model complex for a click-based bifunctional chelating agent, and the lipophilic benzyl "place-holders" will be replaced by hydrophilic peptides to modulate the pharmacokinetics and direct activity away from the liver and gut. The solid-state molecular structure of [In(azapa)(H2O)][ClO4] reveals a very rare eight-coordinate distorted square antiprismatic geometry with one triazole arm bound, and the structure of [(64)Cu(azapa)] shows a distorted octahedral geometry. The present study demonstrates significant potential for bioconjugates of H2azapa as alternatives to DOTA in copper-based radiopharmaceuticals, with the highly modular and "clickable" molecular scaffold of H2azapa easily modified into a variety of bioconjugates. H2azapa is a versatile addition to the "pa" family, joining the previously published H2dedpa ((67/68)Ga and (64)Cu), H4octapa ((111)In, (177)Lu, and (90)Y), and H5decapa ((225)Ac) to cover a wide range of important nuclides.

H4octapa: an acyclic chelator for 111In radiopharmaceuticals

This preliminary investigation of the octadentate acyclic chelator H(4)octapa (N(4)O(4)) with (111)In/(115)In(3+) has demonstrated it to be an improvement on the shortcomings of the current industry "gold standards" DOTA (N(4)O(4)) and DTPA (N(3)O(5)). The ability of H(4)octapa to radiolabel quantitatively (111)InCl(3) at ambient temperature in 10 min with specific activities as high as 2.3 mCi/nmol (97.5% radiochemical yield) is presented. In vitro mouse serum stability assays have demonstrated the (111)In complex of H(4)octapa to have improved stability when compared to DOTA and DTPA over 24 h. Mouse biodistribution studies have shown that the radiometal complex [(111)In(octapa)](-) has exceptionally high in vivo stability over 24 h with improved clearance and stability compared to [(111)In(DOTA)](-), demonstrated by lower uptake in the kidneys, liver, and spleen at 24 h. (1)H/(13)C NMR studies of the [In(octapa)](-) complex revealed a 7-coordinate solution structure, which forms a single isomer and exhibits no observable fluxional behavior at ambient temperature, an improvement to the multiple isomers formed by [In(DTPA)](2-) and [In(DOTA)](-) under the same conditions. Potentiometric titrations have determined the thermodynamic formation constant of the [In(octapa)](-) complex to be log K(ML) = 26.8(1). Through the same set of analyses, the [(111/115)In(decapa)](2-) complex was found to have nonoptimal stability, with H(5)decapa (N(5)O(5)) being more suitable for larger metal ions due to its higher potential denticity (e.g., lanthanides and actinides). Our initial investigations have revealed the acyclic chelator H(4)octapa to be a valuable alternative to the macrocycle DOTA for use with (111)In, and a significant improvement to the acyclic chelator DTPA.

Bifunctional coupling agents for radiolabeling of biomolecules and target-specific delivery of metallic radionuclides

Receptor-based radiopharmaceuticals are of great current interest in molecular imaging and radiotherapy of cancers, and provide a unique tool for target-specific delivery of radionuclides to the diseased tissues. In general, a target-specific radiopharmaceutical can be divided into four parts: targeting biomolecule (BM), pharmacokinetic modifying (PKM) linker, bifunctional coupling or chelating agent (BFC), and radionuclide. The targeting biomolecule serves as a "carrier" for specific delivery of the radionuclide. PKM linkers are used to modify radiotracer excretion kinetics. BFC is needed for radiolabeling of biomolecules with a metallic radionuclide. Different radiometals have significant difference in their coordination chemistry, and require BFCs with different donor atoms and chelator frameworks. Since the radiometal chelate can have a significant impact on physical and biological properties of the target-specific radiopharmaceutical, its excretion kinetics can be altered by modifying the coordination environment with various chelators or coligand, if needed. This review will focus on the design of BFCs and their coordination chemistry with technetium, copper, gallium, indium, yttrium and lanthanide radiometals.

High-dose gallium-67 therapy in patients with relapsed acute leukaemia: a feasibility study

Gallium-67 (67Ga) accumulates in malignant tissues via the transferrin receptor without need for a monoclonal antibody and emits cytotoxic low-energy electrons. In this study we investigated the feasibility, pharmacokinetics, toxicity and preliminary efficiency of high-dose 67Ga injected intravenously (i.v.) in patients with acute leukaemia not responding to conventional therapy. Twelve doses of 36-105 mCi of Gallium67 citrate were administered as a push injection to eight patients with resistant leukaemia in a pilot study. All five patients with acute myeloid leukaemia (AML) and three patients with acute lymphoblastic leukaemia (ALL) had resistant disease or resistant relapse. No (sub)acute toxicity was observed. Independent of the administered dose, whole-blood radioactivity levels 10 min after administration measured only 1.25 +/- 1.39 microCi ml-1, indicating a large volume of distribution. Urine excretion in the first 24 h ranged from 18% to 51.5% (median 29.5%) of the administered dose. Cellular uptake of 67Ga was less than in previous in vitro studies. Whole-body radiation dose was estimated to be 0.25 +/- 0.03 cGy mCi-1. Red marrow dose was estimated to be between 0.18 +/- 0.02 and 0.97 +/- 0.12 cGy mCi-1. One definite response was observed in an ALL patient with disappearance of skin lesions, normalisation of the enlarged spleen and profound leucopenia. Three other patients showed transient reductions in white blood cell counts without disappearance of blasts from the peripheral blood. We conclude that high-dose i.v. 67Ga can be safely administered but that the uptake of 67Ga in blast cells must increase to make 67Ga therapeutically useful in patients with relapsed leukaemia.