A trithiol bifunctional chelate for 72,77As: A matched pair theranostic complex with high in vivo stability

Radiochemistry: A Hot Field with Opportunities for Cool Chemistry

Recent Food and Drug Administration (FDA) approval of diagnostic and therapeutic radiopharmaceuticals and concurrent miniaturization of particle accelerators leading to improved access has fueled interest in the development of chemical transformations suitable for short-lived radioactive isotopes on the tracer scale. This recent renaissance of radiochemistry is paired with new opportunities to study fundamental chemical behavior and reactivity of elements to improve their production, separation, and incorporation into bioactive molecules to generate new radiopharmaceuticals. This outlook outlines pertinent challenges in the field of radiochemistry and indicates areas of opportunity for chemical discovery and development, including those of clinically established (C-11, F-18) and experimental radionuclides in preclinical development across the periodic table.

Prospects for the production of radioisotopes and radiobioconjugates for theranostics

The development of diagnostic methods in medicine as well as the progress in the synthesis of biologically active compounds allows the use of selected radioisotopes for the simultaneous diagnosis and treatment of diseases, especially cancerous ones, in patients. This approach is called theranostic. This review article includes chemical and physical characterization of chosen theranostic radioisotopes and their compounds that are or could be useful in nuclear medicine.

A Third Generation Potentially Bifunctional Trithiol Chelate, Its nat,1XXSb(III) Complex, and Selective Chelation of Radioantimony (119Sb) from Its Sn Target

The therapeutic potential of the Meitner-Auger- and conversion-electron emitting radionuclide ¹¹⁹Sb remains unexplored because of the difficulty of incorporating it into biologically targeted compounds. To address this challenge, we report the development of ¹¹⁹Sb production from electroplated tin cyclotron targets and its complexation by a novel trithiol chelate. The chelation reaction occurs in harsh solvent conditions even in the presence of large quantities of tin, which are necessary for production on small, low energy (16 MeV) cyclotrons. The ¹¹⁹Sb-trithiol complex has high stability and can be purified by HPLC. The third generation trithiol chelate and the analogous stable ^natSb-trithiol compound were synthesized and characterized, including by single-crystal X-ray diffraction analyses.

A New, Second Generation Trithiol Bifunctional Chelate for 72,77As: Trithiol(b)-(Ser)2-RM2

Trithiol chelates are suitable for labeling radioarsenic (72As: 2.49 MeV β+, 26 h; 77As: 0.683 MeV β-, 38.8 h) to form potential theranostic radiopharmaceuticals for positron emission tomography (PET) imaging and therapy. A trithiol(b)-(Ser)2-RM2 bioconjugate and its arsenic complex were synthesized and characterized. The trithiol(b)-(Ser)2-RM2 bioconjugate was radiolabeled with no-carrier-added 77As in over 95% radiochemical yield and was stable for over 48 h, and in vitro IC50 cell binding studies of [77As]As-trithiol(b)-(Ser)2-RM2 in PC-3 cells demonstrated high affinity for the gastrin-releasing peptide (GRP) receptor (low nanomolar range). Limited biodistribution studies in normal mice were performed with HPLC purified 77As-trithiol(b)-(Ser)2-RM2 demonstrating both pancreatic uptake and hepatobiliary clearance.

GRPr-mediated photothermal and thermodynamic dual-therapy for prostate cancer with synergistic anti-apoptosis mechanism

Conventional prostate cancer treatment strategies, including chemotherapy and radiotherapy, cannot effectively eradicate prostate cancer, especially castration resistance prostate cancer. Herein, we developed a novel nanotherapy platform that consists of synergic photothermal and photodynamic therapy via the unique properties of photothermal conversion by gold nanorods and free radicals generation by encapsulated initiators (AIPH). Mesoporous silica was employed as a coating material, and the bombesin peptide was conjugated onto the mesoporous silica coating layer as the targeting moiety to prostate cancer via its overexpressed gastrin-releasing peptide receptors. An in vitro study with the castration resistance prostate cancer cell exhibited a significant photothermal therapeutic effect as well as enhanced thermodynamic therapy via generating free radicals. P-p38 and p-JNK proteins, as key proteins involved in the cells' stress responses, were found to be upregulated by the synergetic treatment. The in vivo study demonstrated that a significant eradication of prostate tumour could be achieved by the nanoparticle therapeutic platform with a good biocompatibility profile. This work pioneers a novel approach for high-efficient castration resistance prostate cancer treatment by combining photothermal, thermodynamic, and site-specific drug delivery directed by an integrated nanoparticle system.

Radioarsenic: A promising theragnostic candidate for nuclear medicine

Molecular imaging is a non-invasive process that enables the visualization, characterization, and quantitation of biological processes at the molecular and cellular level. With the emergence of theragnostic agents to diagnose and treat disease for personalized medicine there is a growing need for matched pairs of isotopes. Matched pairs offer the unique opportunity to obtain patient specific information from SPECT or PET diagnostic studies to quantitate in vivo function or receptor density to inform and tailor therapeutic treatment. There are several isotopes of arsenic that have emissions suitable for either or both diagnostic imaging and radiotherapy. Their half-lives are long enough to pair them with peptides and antibodies which take longer to reach maximum uptake to facilitate improved patient pharmacokinetics and dosimetry then can be obtained with shorter lived radionuclides. Arsenic-72 even offers availability from a generator that can be shipped to remote sites and thus enhances availability. Arsenic has a long history as a diagnostic agent, but until recently has suffered from limited availability, lack of suitable chelators, and concerns about toxicity have inhibited its use in nuclear medicine. However, new production methods and novel chelators are coming online and the use of radioarsenic in the pico and nanomolar scale is well below the limits associated with toxicity. This manuscript will review the production routes, separation chemistry, radiolabeling techniques and in vitro/in vivo studies of three medically relevant isotopes of arsenic (arsenic-74, arsenic-72, and arsenic-77).

Evaluation of the nuclear reaction cross sections via proton induced reactions on 72Ge and 76Se to produce 72As: A potential entrant for the theranostic pairs

Theranostic applications of radiopharmaceuticals have revolutionized present era specially, dealing with cancer diseases. Increase in the uses of radionuclides in nuclear medicine has resulted in the demands of optimized new radionuclides to be produced focussing on the economy, simplicity and maximum yield. Two radionuclides of arsenic offer a well agreed theranostic systems namely ⁷⁷As and ⁷²As. Some arsenic radionuclides are capable of positron-emission, with range of hour to weeks half-lives and have potential to be used for nuclear medicine. Present work will elucidate the production of ⁷²As on Germanium and Selenium via proton induced nuclear reactions. The experimental results obtained by several nuclear reactions were analyzed. The results of nuclear model codes namely ALICE-IPPE, EMPIRE-3.2.3 and TALYS-1.9 are compared with the experimental cross sections to generate recommended cross section data. Recommended excitation functions are used to compute the thick target yields (TTY) of ⁷²As. Assessment of radionuclidic impurities are also studied and comparison of several radionuclidic impurities is done. To produce ⁷²As,⁷²Ge (p, n)⁷²As, ⁷³Ge (p, 2n)⁷²As, ⁷⁴Ge (p, 3n)⁷²As and ⁷⁶Se (p, x)⁷²As reactions in different energy ranges are discussed. We have identified ⁷²Ge (p, n)⁷²As reaction; gives pre-eminent yield with least impurities mark it as feasible entrant to be applied in Positron Emission Tomography (PET) and theranostic applications.

Chelators and metal complex stability for radiopharmaceutical applications

Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

Optimization of the reduction of 74As(V) to 74As(III) and of the labelling of dithiol dihydrolipoic acid

The radiochemical separation of n.c.a. arsenic on its own or for radio-labelling purposes usually involves the issue of reducing arsenic(V). Numerous approaches for reducing pentavalent arsenic have been examined. A novel HPLC method has also been presented for accessing the efficiency of the reduction in terms of *As(III)/*As(V). Labelling with trivalent radioarsenic seems to be a promising research field to access new radiopharmaceuticals, for example, using arsenic as a surrogate for phosphorus. Moreover, as a model system, the labelling reaction of *As(III) with dihydrolipoic acid has been systematically optimized.

Evaluation of 72Se/72As generator and production of 72Se for supplying 72As as a potential PET imaging radionuclide

Positron-emitting 72As is the PET imaging counterpart for beta-emitting 77As. Its parent, no carrier added (n.c.a.) 72Se, was produced for a 72Se/72As generator by irradiating an enriched 7°Ge metal-graphite target via the 70Ge(α, 2 n)72Se reaction. Target dissolution used a fast, environmentally friendly method with 93% radioactivity recovery. Chromatographic parameters of the 72Se/72As generator were evaluated, the eluted n.c.a. 72As was characterized with a phantom imaging study, and the previously reported trithiol and aryl-dithiol ligand systems were radiolabeled with the separated n.c.a. 72As in high yield.