Kinetics of the reaction of ferrous ions with hydroxyl radicals in the temperature range 25-300 °C.

The kinetics and mechanism of the reaction between OH radicals and ferrous ions in the temperature range 25-300 °C were studied using pulse radiolysis. At temperatures <150 °C the rate of reaction is essentially independent of temperature, while at temperatures >150 °C the activation energy is 45.8 ± 3.0 kJ mol-1. The change in activation energy is attributed to a change in the dominant mechanism from hydrogen atom transfer (HAT) to dissociative ligand interchange. The kinetic isotope effect (KIE) was measured by repeating experiments in heavy water. A value of 2.9 was measured at room temperature where HAT is the dominant mechanism. The KIE decreases to zero at temperatures > 150 °C as ligand interchange becomes dominant and the O-H bond is no longer involved in the reaction.

Production and Supply of α-Particle-Emitting Radionuclides for Targeted α-Therapy.

Encouraging results from targeted α-therapy have received significant attention from academia and industry. However, the limited availability of suitable radionuclides has hampered widespread translation and application. In the present review, we discuss the most promising candidates for clinical application and the state of the art of their production and supply. In this review, along with 2 forthcoming reviews on chelation and clinical application of α-emitting radionuclides, The Journal of Nuclear Medicine will provide a comprehensive assessment of the field.

Nimotuzumab Site-Specifically Labeled with 89Zr and 225Ac Using SpyTag/SpyCatcher for PET Imaging and Alpha Particle Radioimmunotherapy of Epidermal Growth Factor Receptor Positive Cancers.

To develop imaging and therapeutic agents, antibodies are often conjugated randomly to a chelator/radioisotope or drug using a primary amine (NH2) of lysine or sulfhydryl (SH) of cysteine. Random conjugation to NH2 or SH groups can require extreme conditions and may affect target recognition/binding and must therefore be tested. In the present study, nimotuzumab was site-specifically labeled using ∆N-SpyCatcher/SpyTag with different chelators and radiometals. Nimotuzumab is a well-tolerated anti-EGFR antibody with low skin toxicities. First, ΔN-SpyCatcher was reduced using tris(2-carboxyethyl)phosphine (TCEP), which was followed by desferoxamine-maleimide (DFO-mal) conjugation to yield a reactive ΔN-SpyCatcher-DFO. The ΔN-SpyCatcher-DFO was reacted with nimotuzumab-SpyTag to obtain stable nimotuzumab-SpyTag-∆N-SpyCatcher-DFO. Radiolabeling was performed with 89Zr, and the conjugate was used for the in vivo microPET imaging of EGFR-positive MDA-MB-468 xenografts. Similarly, ∆N-SpyCatcher was conjugated to an eighteen-membered macrocyclic chelator macropa-maleimide and used to radiolabel nimotuzumab-SpyTag with actinium-225 (225Ac) for in vivo radiotherapy studies. All constructs were characterized using biolayer interferometry, flow cytometry, radioligand binding assays, HPLC, and bioanalyzer. MicroPET/CT imaging showed a good tumor uptake of 89Zr-nimotuzumab-SpyTag-∆N-SpyCatcher with 6.0 ± 0.6%IA/cc (n = 3) at 48 h post injection. The EC50 of 225Ac-nimotuzumab-SpyTag-∆N-SpyCatcher and 225Ac-control-IgG-SpyTag-∆N-SpyCatcher against an EGFR-positive cell-line (MDA-MB-468) was 3.7 ± 3.3 Bq/mL (0.04 ± 0.03 nM) and 18.5 ± 4.4 Bq/mL (0.2 ± 0.04 nM), respectively. In mice bearing MDA-MB-468 EGFR-positive xenografts, 225Ac-nimotuzumab-SpyTag-∆N-SpyCatcher significantly (p = 0.0017) prolonged the survival of mice (64 days) compared to 225Ac-control IgG (28.5 days), nimotuzumab (28.5 days), or PBS-treated mice (30 days). The results showed that the conjugation and labeling using SpyTag/∆N-SpyCatcher to nimotuzumab did not significantly (p > 0.05) alter the receptor binding of nimotuzumab compared with a non-specific conjugation approach. 225Ac-nimotuzumab-SpyTag-∆N-SpyCatcher was effective in vitro and in an EGFR-positive triple negative breast cancer xenograft model.

Construction of a thorium/actinium generator at the Canadian Nuclear Laboratories.

Targeted Alpha Therapy (TAT) has demonstrated considerable promise in the treatment of a range of cancers in both preclinical and, more recently clinical research. In particular, work with the alpha-emitting radionuclide 225Ac has been effectively employed due to the relatively rapid decay cascade that leads to 4 alpha and 2 beta emissions. One limitation for TAT has been caused by access to the vital radionuclide. Traditionally, 225Ac has been sourced from thorium/actinium generators based on the alpha decay of stockpiles of 229Th. 229Th is itself the alpha-decay product from 233U. Due to proliferation issues associated with 233U, only three thorium/actinium generators have been reported in the literature, capable of supporting clinical research. This paper describes the construction and operation of a thorium/actinium radionuclide generator at the Canadian Nuclear Laboratories, capable of supporting preclinical and limited clinical research in the area of TAT. Thorium was recovered and purified by a combination of anion exchange and extraction chromatography from aged 233U stockpiles. A separation scheme for 225Ra and 225Ac has been developed, based upon the chemical composition of the thorium material to allow for regular, routine milkings capable of supplying up to 3.7 GBq (100 mCi) of radiochemically pure 225Ac annually. This routine separation is accomplished using a combination of anion exchange chromatography to separate Ac and Ra isotopes from Th and extraction chromatography employing TEVA and DGA-N resins to separate actinium from radium and breakthrough thorium.

232Th-Spallation-Produced 225Ac with Reduced 227Ac Content.

Recent clinical results have demonstrated remarkable treatment responses of late-stage cancer patients when treated with alpha-emitting radionuclides such as actinium-225 (225Ac). The resulting intense global effort to produce greater quantities of 225Ac has triggered a number of emerging technologies to produce this rare, yet important, radionuclide. Accelerator-based methods for increasing global 225Ac production capacity have focused on the high energy (>100 MeV) proton irradiation of thorium, despite the coproduction of the undesirable 227Ac byproduct at 0.1-0.3% of the 225Ac activity. We at TRIUMF have developed a process for the production of a 225Ra/225Ac generator from irradiated thorium that results in an 225Ac product with reduced 227Ac content. 225Ac was separated from irradiated thorium and coproduced radioactive spallation and fission products using a thorium peroxide precipitation method followed by cation exchange and extraction chromatography. Stable and radioactive tracer studies demonstrated the ability of this method to separate Ac from most other elements, providing a directly produced Ac product with measured 227Ac content of (0.15 ± 0.04)%. A second, indirectly produced Ac product with 227Ac content of <7.5 × 10-5% is obtained by repeating the final extraction chromatography step with the 225Ra-containing fraction. The 225Ra-derived 225Ac showed similar or improved quality compared to the initial, directly produced 225Ac product in terms of chemical purity and radiolabeling capability, the latter of which was comparable with other 225Ac sources reported in the literature.

Synthesis and Evaluation of a Macrocyclic Actinium-225 Chelator, Quality Control and In Vivo Evaluation of 225 Ac-crown-αMSH Peptide.

Targeted alpha-therapy (TAT) has great potential for treating a broad range of late-stage cancers by delivering a focused and lethal radiation dose to tumors. Actinium-225 (225 Ac) is an emerging alpha emitter suitable for TAT; however, the availability of chelators for Ac remains limited to a small number of examples (DOTA and macropa). Herein, we report a new Ac macrocyclic chelator named 'crown', which binds quantitatively and rapidly (<10 min) to Ac at ambient temperature. We synthesized 225 Ac-crown-αMSH, a peptide targeting the melanocortin 1 receptor (MC1R), specifically expressed in primary and metastatic melanoma. Biodistribution of 225 Ac-crown-αMSH showed favorable tumor-to-background ratios at 2 h post injection in a preclinical model. In addition, we demonstrated dramatically different biodistrubution patterns of 225 Ac-crown-αMSH when subjected to different latency times before injection. A combined quality control methodology involving HPLC, gamma spectroscopy and radioTLC is recommended.

Comparison of various radioactive payloads for a human monoclonal antibody to glycoprotein 41 for elimination of HIV-infected cells.

BACKGROUND: cART has significantly improved the life expectancy of people living with HIV (PLWH). However, it fails to eliminate the long-lived reservoir of latent HIV-infected cells. Radioimmunotherapy (RIT) relies on antigen-specific monoclonal antibodies (mAbs) for targeted delivery of lethal doses of ionizing radiation to cells. Previously, we have demonstrated that human mAb 2556 against HIV gp41 conjugated with 213Bismuth radioisotope (t1/2 = 46 min, alpha-emitter) selectively killed HIV-infected cells. 225Actinium (t1/2 = 9.92 d, alpha-emitter) and 177Lutetium (t1/2 = 6.7 d, beta-emitter) are two long-lived clinically proven radioisotopes for cancer treatment which might be more effective in killing infected cells systemically and in CNS. METHODS: In this study we have conjugated 2556 mAb with 213Bi, 225Ac and 177Lu, and compared their ability to kill HIV-infected human peripheral blood mononuclear cells (PBMCs) and monocytes. PBMCs and monocytes from healthy donors were infected with HIVp49.5 and treated in vitro with increasing concentrations of 213Bi (4-20 µCi)-, 225Ac (20-100 nCi)- and 177Lu (4-50 µCi)-2556 mAb. RESULTS: After three days post-treatment of infected PBMCs and monocytes, 213Bi- and 177Lu-conjugated 2556 mAb reduced virus production measured by p24 level in a dose-dependent manner, whereas, 225Ac-2556 showed minimal effect. However, seven days post-treatment all three radioisotopes showed significantly more pronounced reduction of virus replication as compared to control labeled mAb with 225Ac-2556 showing the least non-specific killing. CONCLUSION: These results indicate that RIT holds promise as a novel treatment option for the eradication of HIV-infected cells that merits further study in combination with cART and reactivation drugs.