Optimisation of cyclotron production parameters for the 209Bi(alpha, 2n) 211At reaction related to biomedical use of 211At

Production of [211At]NaAt solution under GMP compliance for investigator-initiated clinical trial

BACKGROUND

The alpha emitter astatine-211 (211At) is garnering attention as a novel targeted alpha therapy for patients with refractory thyroid cancer resistant to conventional therapy using beta emitter radioiodine (131I). Herein, we aimed to establish a robust method for the manufacturing and quality control of [211At]NaAt solution for intravenous administration under the good manufacturing practice guidelines for investigational products to conduct an investigator-initiated clinical trial.

RESULTS

211At was separated and purified via dry distillation using irradiated Bi plates containing 211At obtained by the nuclear reaction of 209Bi(4He, 2n)211At. After purification, the 211At trapped in the cold trap was collected in a reaction vessel using 15 mL recovery solution (1% ascorbic acid and 2.3% sodium hydrogen carbonate). After stirring the 211At solution for 1 h inside a closed system, the reaction solution was passed through a sterile 0.22 μm filter placed in a Grade A controlled area and collected in a product vial to prepare the [211At]NaAt solution. According to the 3-lot tests, decay collected radioactivity and radiochemical yield of [211At]NaAt were 78.8 ± 6.0 MBq and 40 ± 3%, respectively. The radiochemical purity of [211At]At- obtained via ion-pair chromatography at the end of synthesis (EOS) was 97 ± 1%, and remained > 96% 6 h after EOS; it was detected at a retention time (RT) 3.2-3.3 min + RT of I-. LC-MS analysis indicated that this principal peak corresponded with an astatide ion (m/z = 210.988046). In gamma-ray spectrometry, the 211At-related peaks were identified (X-ray: 76.9, 79.3, 89.3, 89.8, and 92.3 keV; γ-ray: 569.7 and 687.0 keV), whereas the peak at 245.31 keV derived from 210At was not detected during the 22 h continuous measurement. The target material, Bi, was below the 9 ng/mL detection limit in all lots of the finished product. The pH of the [211At]NaAt solution was 7.9-8.6; the concentration of ascorbic acid was 9-10 mg/mL. Other quality control tests, including endotoxin and sterility tests, confirmed that the [211At]NaAt solution met all quality standards.

CONCLUSIONS

We successfully established a stable method of [211At]NaAt solution that can be administered to humans intravenously as an investigational product.

The effects of nuclear level density model and alpha optical model potential to the excitation functions of novel therapeutic radionuclides

In this study, the theoretical cross sections of 209Bi(α,2n)211At, 65Cu(α,n)68Ga, 100Ru(α,n)103Pd, and 121Sb(α,n)124I are calculated using TALYS 1.96, incorporating the effects of the alpha optical model potential and nuclear level density models. The validation process involves comparing the calculated cross sections with experimental data and utilizing statistical deviation factors. This comparison allows us to determine the optimal combination of nuclear model parameters for each reaction. The result shows that theoretical calculations which utilized semi microscopic level density models and alpha OMP managed to describe the excitation functions close to the experimental data. The comparison of nuclear model calculations with experimental data plays a crucial role in ensuring the reliability of the data, making it an essential aspect of modern evaluation procedures.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

Nuclear data for light charged particle induced production of emerging medical radionuclides

Whatever the radionuclide to be used in nuclear medicine, it is essential to know the expected yield during the production process, but also of all the possible radionuclidic impurities coproduced, that can have an impact on the product final quality, as well as in the related waste management. The availability of the majority of emerging radioisotopes, including the theranostic ones or pairs, is mainly limited by the fact that, for most of them, the optimal production route still needs to be strengthened if not defined in some cases. The aim of this work is to present a review on the charged particle induced nuclear cross sections to produce some emerging radionuclides for medical applications to show that all types of projectiles should be considered in the quest of producing medical radionuclides. An accurate analysis of the production routes is presented for some radionuclides (67Cu, 47Sc, 89Zr, 103Pd, 186gRe, 97Ru, 211At) chosen as examples to highlight (i) how the quality of the final product strongly depends on the chosen target/projectile/energy parameters set, (ii) how deuteron production routes may sometimes be more effective than the proton ones or lead to a different impurity profile and (iii) how α-particle beams may allow to bypass the limitations occurring when using Z = 1 beams. An overview of possible advantages and drawbacks of the cited production routes and of potential cross sections that still need to be measured, is also reported.

The role of chemistry in accelerator-based production and separation of radionuclides as basis for radiolabelled compounds for medical applications

Radiochemical separations used in large scale routine production of diagnostic and therapeutic radionuclides at a particle accelerator for patient care are briefly outlined. The role of chemistry at various stages of development of a production route of a novel radionuclide, namely nuclear data measurement, high-current targetry, chemical processing and quality control of the product, is discussed in detail. Special attention is paid to production of non-standard positron emitters (e.g. 44gSc, 64Cu, 68Ga, etc.) at a cyclotron and novel therapeutic radionuclides (e.g. 67Cu, 225Ac, etc.) at an accelerator. Some typical examples of radiochemical methods involved are presented.

Production, purification and availability of 211At: Near term steps towards global access

The promising characteristics of the 7.2-h radiohalogen 211At have long been recognized; including having chemical properties suitable for labeling targeting vectors ranging from small organic molecules to proteins, and the emission of only one α-particle per decay, providing greater control over off-target effects. Unfortunately, the impact of 211At within the targeted α-particle therapy domain has been constrained by its limited availability. Paradoxically, the most commonly used production method - via the 209Bi(α,2n)211At reaction - utilizes a widely available natural material (bismuth) as the target and straightforward cyclotron irradiation methodology. On the other hand, the most significant impediment to widespread 211At availability is the need for an accelerator capable of generating ≥28 MeV α-particles with sufficient beam intensities to make clinically relevant levels of 211At. In this review, current methodologies for the production and purification of 211At - both by the direct production route noted above and via a 211Rn generator system - will be discussed. The capabilities of cyclotrons that currently produce 211At will be summarized and the characteristics of other accelerators that could be utilized for this purpose will be described. Finally, the logistics of networks, both academic and commercial, for facilitating 211At distribution to locations remote from production sites will be addressed.

Excitation functions for 209Bi reactions induced by threshold to 50 MeV energy alpha particles

Excitation functions for ²⁰⁹Bi(α,2n)²¹¹At, ²⁰⁹Bi(α,3n)²¹⁰At and ²⁰⁹Bi(α,4n)²⁰⁹At reactions were calculated using TALYS-1.95 nuclear code from threshold to 50 MeV by invoking suitable options for level densities, nucleon-nucleus optical model potentials and alpha optical model potentials. Statistical factors were used to verify the quality of matching between theoretical model calculations and the experimental data from the EXFOR database. The TTY of ²¹¹At calculated using the excitation function of ²⁰⁹Bi(a,2n)²¹¹At reaction is compared with existing experimental studies from literature The results of the present study are important for the validation of nuclear model approaches with increased predictive power for ²⁰⁹Bi(α,xn) reactions for the production of ²¹¹At.

Uses of alpha particles, especially in nuclear reaction studies and medical radionuclide production

Alpha particles exhibit three important characteristics: scattering, ionisation and activation. This article briefly discusses those properties and outlines their major applications. Among others, α-particles are used in elemental analysis, investigation and improvement of materials properties, nuclear reaction studies and medical radionuclide production. The latter two topics, dealing with activation of target materials, are treated in some detail in this paper. Measurements of excitation functions of α-particle induced reactions shed some light on their reaction mechanisms, and studies of isomeric cross sections reveal the probability of population of high-spin nuclear levels. Regarding medical radionuclides, an overview is presented of the isotopes commonly produced using α-particle beams. Consideration is also given to some routes which could be potentially useful for production of a few other radionuclides. The significance of α-particle induced reactions to produce a few high-spin isomeric states, decaying by emission of low-energy conversion or Auger electrons, which are of interest in localized internal radiotherapy, is outlined. The α-particle beam, thus broadens the scope of nuclear chemistry research related to development of non-standard positron emitters and therapeutic radionuclides.

Preliminary production of 211At at the Texas A&M University Cyclotron Institute

A feasibility study for the production of the alpha particle-emitting radionuclide At was performed at the Texas A&M University Cyclotron Institute as part of the Interdisciplinary Radioisotope Production and Radiochemistry Program. The mission of this program centers upon the production of radionuclides for use in diagnostic and therapeutic nuclear medicine with the primary focus on development of novel therapeutic strategies. As a first step in establishing this program, two goals were outlined: (i) verify production of At and compare results to published data, and (ii) evaluate shielding and radiological safety issues for large-scale implementation using an external target. The radionuclide At was produced via the Bi (α, 2n) At reaction using the K500 cyclotron. Two experiments were conducted, using beam energies of 27.8 MeV and 25.3 MeV, respectively. The resulting yields for At were found to be 36.0 MBq μA h and 12.4 MBq μA h, respectively, which fall within the range of published yield data. Strategies for increasing absolute yield and production efficiency were also evaluated, which focused chiefly on using a new target designed for use with the K150 cyclotron, which will enable the use of a higher beam current. Finally, neutron and gamma dose rates during production were evaluated by using the Monte Carlo code MCNPX. It was determined that a simple structure consisting of 4-in thick borated polyethylene will reduce the neutron dose rate within the cyclotron production vault by approximately a factor of 2, thereby decreasing activation of equipment.

Production of α-particle emitting ²¹¹At using 45 MeV α-beam

Among the α-particle emitting radionuclides, (211)At is considered to be a promising radionuclide for targeted cancer therapy due to its decay properties. The range of alpha particles produced by the decay of (211)At are less than 70 µm in water with a linear energy transfer between 100 and 130 keV µm(-1), which are about the maximum relative biological effectiveness for heavy ions. It is important to note that at the present time, only a few of cyclotrons routinely produce (211)At. The direct production method is based on the nuclear reactions (209)Bi(α,2n)(211)At. Production of the radionuclide (211)At was carried out using the MC-50 cyclotron at the Korea Institute of Radiological and Medical Sciences (KIRAMS). To ensure high beam current, the α-beam was extracted with an initial energy of 45 MeV, which was degraded to obtain the appropriate α-beam energy. The calculations of beam energy degradation were performed utilizing the MCNPX. Alumina-baked targets were prepared by heating the bismuth metal powder onto a circular cavity in a furnace. When using an E(α, av) of 29.17 MeV, the very small contribution of (210)At confirms the right choice of the irradiation energy to obtain a pure production of (211)At isotope.

Recent developments in nuclear data measurements and chemical separation methods in accelerator production of astatine and technetium radionuclides

The cyclotron produced neutron deficient technetium radionuclides (93Tc,94(m+g)Tc,95Tc,96Tc) have gained renewed interest in various fields, including nuclear imaging, provided they can be obtained in a pure form. Similarly,211At due to its moderate half-life and high intensityα-particle energy (both from211At as well as its transient decay product211Po) is of prime interest in targeted therapy. Another interest is to study the astatine chemistry, which is least studied compared to other halogens due to its non-occurrence in natural systems. For maximum production of these radionuclides various parameters need to be standardized. A chemical separation is required to achieve high radiochemical purity beforein-vivoapplication. This review describes various production routes of neutron deficient astatine and technetium radionuclides that have been reported after the year 2000. The analytical chemistry developed for separation of no-carrier-added (nca) Tc and At radionuclides in the same period is also discussed in detail.

Astatine-211: production and availability

The 7.2-h half life radiohalogen (211)At offers many potential advantages for targeted α-particle therapy; however, its use for this purpose is constrained by its limited availability. Astatine-211 can be produced in reasonable yield from natural bismuth targets via the (209)Bi(α,2n)(211)At nuclear reaction utilizing straightforward methods. There is some debate as to the best incident α-particle energy for maximizing 211At production while minimizing production of (210)At, which is problematic because of its 138.4-day half life α-particle emitting daughter, (210)Po. The intrinsic cost for producing (211)At is reasonably modest and comparable to that of commercially available (123)I. The major impediment to (211)At availability is attributed to the need for a medium energy α-particle beam for its production. On the other hand, there are about 30 cyclotrons in the world that have the beam characteristics required for (211)At production.

Astatine Radiopharmaceuticals: Prospects and Problems

For the treatment of minimum residual diseases such micrometastases and residual tumor margins that remain after debulking of the primary tumor, targeted radiotherapy using radiopharmaceuticals tagged with alpha-particle-emitting radionuclides is very attractive. In addition to the their short range in tissue, which helps minimize harmful effects on adjacent normal tissues, alpha-particles, being high LET radiation, have several radiobiological advantages. The heavy halogen, astatine-211 is one of the prominent alpha-particle-emitting radionuclides in practice. Being a halogen, it can often be incorporated into biomolecules of interest by adapting radioiodination chemistry. A wide spectrum of compounds from the simple [(211)At]astatide ion to small organic molecules, peptides, and large proteins labeled with (211)At have been investigated with at least two reaching the stage of clinical evaluation. The chemistry, cytotoxic advantages, biodistribution studies, and microdosimetry/pharmacokinetic modeling of some of these agents will be reviewed. In addition, potential problems such as the harmful effect of radiolysis on the synthesis, lack of sufficient in vivo stability of astatinated compounds, and possible adverse effects when they are systemically administered will be discussed.

Production and separation of Astatine Radionuclides: some new addition to Astatine Chemistry

For the first time no-carrier-added (nca) (209,210)At radionuclides were produced by heavy ion ((7)Li) activation from 4 mg/cm(2) lead nitrate target. Astatine was separated from the bulk target by three different approaches. Nca astatine radionuclide was selectively partitioned in the (i) polymer rich phase of an aqueous biphasic system consisting of polyethylene glycol (PEG) 4000 (50% w/w) and 2M Na(2)SO(4) solution (ii) aqueous phase of a liquid liquid extraction system being comprised of a liquid cation exchanger, HDEHP (di-2-ethylhexyl phosphoric acid) (0.5%) and liquor ammonia (iii) liquid phase of a solid liquid extraction system of cation exchange resin Dowex-50 and 10(-6)M HCl. Very high separation factors have been achieved in all the three methods.

The bioinorganic and medicinal chemistry of carboranes: from new drug discovery to molecular imaging and therapy

The role of carboranes in medicinal chemistry has diversified in recent years and now extends into areas of drug discovery, molecular imaging, and targeted radionuclide therapy. An introduction to carborane chemistry is provided to familiarize the non-expert with some key properties of these molecules, followed by an overview of current medicinally-orientated research involving carboranes. The broad-ranging nature of this research is illustrated, with emphasis placed on recent highlights and advances in this field.

Assessing the 210At impurity in the production of 211At for radiotherapy by 210Po analysis via isotope dilution alpha spectrometry

A method for assessing the impurity 210At in cyclotron-produced 211At via isotope dilution alpha spectrometry is presented. The activity of 210At is quantified by measuring the activity of daughter nuclide 210Po. Counting sources are prepared by spontaneous deposition of Po on a silver disc. Activity of 210At (at the time of 210Po maximum activity) is found to be 83.5+/-9.0 Bq, corresponding to an atom ratio (210At:211At at the time of distillation) of 0.010+/-0.007% (k=2). The method produces high-quality alpha spectra, with baseline alpha-peak resolution and chemical yields of greater than 85%.

Preparation and in vivo evaluation of novel linkers for 211At labeling of proteins

The syntheses, radiolabeling, antibody conjugation and in vivo evaluation of new linkers for (211)At labeling of monoclonal antibodies are described. Syntheses of the N-succinimidyl esters and labeling with (211)At to form succinimidyl 4-methoxymethyl-3-[(211)At]astatobenzoate (9) and succinimidyl 4-methylthiomethyl-3-[(211)At]astatobenzoate (11) from the corresponding bromo-aryl esters is reported. Previously reported succinimidyl N-{4-[(211)At]astatophenethyl}succinamate (SAPS) is employed as a standard of in vivo stability. Each agent is conjugated with Herceptin in parallel with their respective (125)I analogue, succinimidyl 4-methoxymethyl-3-[(125)I]iodobenzoate (10), succinimidyl 4-methylthiomethyl-3-[(125)I]iodobenzoate (12) and succinimidyl N-{4-[(125)I]iodophenethyl}succinamate (SIPS), respectively, for comparative assessment in LS-174T xenograft-bearing mice. With 9 and 11, inclusion of an electron pair donor in the ortho position does not appear to provide in vivo stability comparable to SAPS. Variables in radiolabeling chemistry of these three agents with (211)At are notable. Sequential elimination of acetic acid and oxidizing agent, N-chlorosuccinimide (NCS), from the (211)At radiolabeling protocol for forming SAPS improves yield, product purity and consistency. NCS appears to be critical for the radiolabeling of 6 with (211)At. Formation of 11, however, is found to require the absence of NCS. Elimination of acetic acid is found to have no effect on radiolabeling efficiency or yield for either of these reactions.

A new internal target system for production of (211)At on the cyclotron U-120M

The alpha emitter (211)At is a radionuclide with good potential for use in the therapy of smaller tumours and metastases. However, limited availability of this radionuclide hinders development of this application and the research of astatine chemistry in general. In this general context we have designed and tested a new internal target system. A thin bismuth layer (3-5 microm) was evaporated onto a light target backing (7.5 g) and irradiated at 0.5-1.5 degrees angles with 29.5 MeV alpha particles beam of intensity up to 30 microA. The backing was then released from the target holder and used directly for astatine separation via dry distillation. Astatine condensed on the Teflon capillary walls was then eluted into 150-250 microl of methanol. The saturation yield was found to be ca. 400 MBq/microA, and the radionuclidic purity of (211)At acceptable for medical applications (activity ratio (210)At/(211)At<10(-3) at EOB). The overall separation yield was 65-75%.

Experimental study of the cross-sections of alpha-particle induced reactions on 209Bi

alpha-particle-induced nuclear reactions for generation of (211)At used in therapeutic nuclear medicine and possible contaminants were investigated with the stacked foil activation technique on natural bismuth targets up to E(alpha)=39 MeV. Excitation functions are reported for the reactions (209)Bi(alpha,2n)(211)At, (209)Bi(alpha,3n)(210)At and (209)Bi(alpha,x)(210)Po. Results obtained from direct alpha-emission measurements and gamma-spectra from decay products are compared and correspond well with earlier literature values. Thick target yields have been deduced from the experimental cross-sections and optimised production pathways for minimal contamination are presented. A comparison with the results of the theoretical model code ALICE-IPPE is discussed.

External targets for the production of 211At--review and status of the target development at the Hannover cyclotron

211At is an alpha emitter which can be produced with cyclotrons capable of accelerating helium-4 ions to at least 28 MeV in case of external targets. Different targets were used and improved to increase the yield of 211At to amounts considered to be suitable for alpha therapy of tumours.

Incorporation of thiosemicarbazide in Amberlite IRC-50 for separation of astatine from alpha-irradiated bismuth oxide

A chelating resin was synthesized by incorporating thiosemicarbazide into Amberlite IRC-50, a weakly acidic polymer. Astatine radionuclides produced by alpha-irradiating bismuth oxide were separated using the newly synthesized chelating resin. The resin showed high selectivity for astatine. The adsorbed astatine was recovered using 0.1M EDTA at pH approximately 10.