2.1. Decay Properties of ²²⁵Ac and ²¹³Bi

²²⁵Ac is the parent radionuclide of ²¹³Bi and is a relatively long-lived α-emitter with a half-life of 9.9 d. It decays via a cascade of six short-lived radionuclide daughters to stable ²⁰⁹Bi with four net alpha particles emitted per decay (Figure 2). ²¹³Bi has a half-life of 45.6 min and shows branching decay (β^– and α decay), and most of the α-particles emitted originate from the β^– branch (see Figure 2). Indeed, it mostly decays via β^– emission to the short-lived α-emitter ²¹³Po (T_1/2 = 4.2 µs, E_α = 8.375 MeV, 97.8%, Figure 2). The residual 2.2% of ²¹³Bi decays leads to ²⁰⁹Tl (E_α = 5.549 MeV, 0.16%, E_α = 5.869 MeV, 2.0%). Finally, ²¹³Po and ²⁰⁹Tl decay via ²⁰⁹Pb (T_1/2 = 3.25 h, β^–) to stable ²⁰⁹Bi [13].

Figure 2. Decay chain of thorium-229 to ²²⁵Ac and ²¹³Bi.

In human tissue, the α-particle released by ²¹³Po (E_α = 8.375 MeV) has a path length of 85 µm. It is this radionuclide that produces >98% of the α-particle energy released per disintegration of ²¹³Bi and could be considered as the radionuclide that provides ²¹³Bi cytotoxicity. The bulk of the total particle energy released per disintegration of ²¹³Bi comes from α-decay, accounting for 92.7%, while 7.3% comes from β-particle emission, which includes the decay of ²⁰⁹Pb [14]. The decay of ²¹³Bi is followed by the emission of a 435 keV-photon (98% abundance) that could potentially be used for SPECT with a gamma camera equipped with high energy collimators, permitting detailed evaluation of the biodistribution of ²¹³Bi in vivo [15].

It is noteworthy to mention that high activity ²²⁵Ac/²¹³Bi generators are required to allow production of clinical amounts of ²¹³Bi. Optimal clinical injected activities depend on the vector molecules; for prostate-specific membrane antigen (PSMA)-targeting radiopharmaceuticals, this has been determined to be 4–8 MBq of ²²⁵Ac ^[16][17][16,17]. In contrast, to have the same number of alpha-particles emitted, 5–10 GBq is needed for ²¹³Bi-labeled radiopharmaceuticals [18]. This was estimated based on the Equations (1) and (2) below and the fact that ²²⁵Ac delivers four α particles in its total decay scheme, while ²¹³Bi delivers net only one α particle. However, this is a rough estimation that does not consider the biological clearance of the radioprobes. Definitely, the much shorter half-life of ²¹³Bi compared to the half-life of ²²⁵Ac is the key reason for the high activity levels that are needed when using ²¹³Bi for targeted radionuclide therapy (Equation (2)).

where A = activity (Bq, disintegrations per second), N = number of atoms, λ = decay constant.

2.2. Current Strategies for ²²⁵Ac Production

Several approaches for ²²⁵Ac and ²¹³Bi production have been recently reviewed and discussed in detail (Figure 3) ^[7][13][7,13]. The most utilized strategy is the radiochemical extraction of ²²⁵Ac from ²²⁹Th (T_1/2 = 7317 years) sources originating from the decay of fissile ²³³U. This results in “carrier free” ²²⁵Ac using the generator method, but the available ²³³U stocks and thus ²²⁹Th stocks are limited because ²³³U management is restricted by the requirements concerning non-proliferation of fissile materials. Most of the ²¹³Bi and ²²⁵Ac used in clinical tests and research activities worldwide has so far been produced by this approach [19]. Two examples of ²²⁹Th sources that can produce clinically relevant activities of ²²⁵Ac are (1) the US Department of Energy, Oak Ridge National Laboratory (ORNL) in Oak Ridge, TN, United States of America and (2) the Directorate for Nuclear Safety and Security of the JRC of the European Commission in Karlsruhe, Germany. Additionally, at the Belgian Nuclear Research Centre (SCK CEN) in Mol, Belgium, very pure sources of ²²⁹Th were identified, processed, and used for pre-clinical studies [20]. The total global annual ²²⁵Ac production volume is approximately 55–65 GBq [21] and cannot meet the growing demand for ²²⁵Ac, but interestingly, it has been reported that stocks will increase by extraction of additional ²²⁹Th from US legacy wastes [7].

Figure 3. (A) Production routes for ²²⁵Ac and (B) the ²²⁵Ac/²¹³Bi generator.

Cyclotron production is an alternative method for ²²⁵Ac production. Medium-energy proton irradiation of ²²⁶Ra using the reaction ²²⁶Ra(p,2n)²²⁵Ac is considered to be a viable approach. The advantage of this method is the widespread availability of appropriate cyclotrons worldwide, especially in Europe ^[22][23][22,23], and high amounts of clinical doses of ²²⁵Ac could be produced in a reliable way. It is important to mention that medical cyclotrons used to produce radioisotopes (15–25 MeV) are considered to be feasible for basic and applied research [24]. The downside with this approach is that ²²⁶Ra is not an easy isotope to work with due to the presence of the highly radiotoxic noble gas ²²²Rn in its decay chain. Therefore, it is unlikely that this will ever enter a clinical cyclotron.

Further, high-energy proton irradiation (0.6–2 GeV) of uranium and thorium spallation targets via the reaction ^natU(p,x)²²⁵Ac is another approach for the production of ²²⁵Ac. The existence of several suitable high-energy proton facilities makes this an achievable prospect. However, such accelerator-produced ²²⁵Ac contains a low percentage (0.1–0.3%) ²²⁷Ac (T_1/2 = 21.77y) at the end of bombardment, which might be a serious limitation in terms of clinical translation and waste management ^[25][26][25,26]. Nevertheless, ²²⁵Ac obtained through high-energy accelerators could be entirely appropriate for ²²⁵Ac/²¹³Bi generator production (see 2.3), even with co-production of ²²⁷Ac, as all actinium species will be retained on the generator.

Another production path being explored is transmutation of ²²⁶Ra to ²²⁹Th by an intense neutron flux. This will lead to three successive neutron capture reactions: ²²⁶Ra(n,γ)²²⁷Ra, ²²⁷Ac(n,γ)²²⁸Ac, and ²²⁸Th(n,γ)²²⁹Th. However, production of many orders of magnitude of ²²⁸Th (T_1/2 = 1.9 y) intermediate, and handling of the radium target remains a challenge [18].

²²⁶Ra(γ,n)²²⁵Ra is another reaction path for ²²⁵Ac production that has been determined experimentally ^[27][28][27,28]. This strategy explored irradiation of old radium needles with high-energy X-rays from electron linear accelerators (linacs).

Finally, transmutation of ²²⁶Ra to ²²⁵Ra by fast neutrons via the reaction ²²⁶Ra(n,2n)²²⁵Ra is also under consideration. It is noteworthy to point out that the limitation for all the strategies discussed is the handling of the radium targets. However, earlier results with these methods have been promising [22].

To conclude, all clinical studies with ²²⁵Ac/²¹³Bi radiopharmaceuticals were performed to date with ²²⁵Ac originating from ²²⁹Th stocks, but other accelerator-based production routes were heavily investigated in the last decade. This progress will hopefully assure reliable production and delivery of ²²⁵Ac to radiopharmacy institutes in the near future, allowing more preclinical research and multicenter clinical trials with both ²²⁵Ac and ²¹³Bi-radiopharmaceuticals.

2.3. ²²⁵Ac/²¹³Bi Radionuclide Generators

²²⁵Ac can be loaded on ²²⁵Ac/²¹³Bi generators to deliver ²¹³Bi on site, but it can also be used directly as a therapeutic radionuclide. ²²⁵Ac/²¹³Bi generators are well explored and discussed ^[29][30][31][29,30,31]. The most established strategy is based on the direct generator method, in which the parent ²²⁵Ac in acidic solution (e.g., 0.05M HNO₃) is strongly retained by the sorbent (e.g., AG MP-50 cation exchange resin) and ²¹³Bi is eluted. Elution is performed generally with a mixture of 0.1M HCl/0.1M NaI to obtain ²¹³Bi in the form of ²¹³BiI₄^- and ²¹³BiI₅^2- that can be directly used for radiochemistry purposes. (Figure 3) [7]. These generators can potentially be used clinically due to the relatively long parent half-life, which allows shipment of the generator to radiopharmacy facilities at long distance. Additionally, the transient equilibrium of the ²²⁵Ac-²¹³Bi permits elution at approximately every 3 h [32]. These generators can provide weeks of reliable in-house generation of ²¹³Bi for radiolabeling purposes [33]. High activity (up to 4 GBq ²²⁵Ac) generator systems, developed at JRC Karlsruhe, have been reported with yields of ²¹³Bi elution exceeding 80% and low breakthrough of ²²⁵Ac of less than 0.2 ppm [32].