Investigating the Chemistry of Actinium, a Therapeutically Relevant Actinide

Actinium (Ac), element 89 on the periodic table, marks the beginning of the actinide series. Unlike its neighbors, its chemistry and chemical properties have scarcely been explored. This knowledge gap is mostly due to the lack of stable or long-lived isotopes of this element—the most abundant isotope of plutonium, ²³⁹Pu, for instance, has a half-life of over 24,000 years, whereas the longest-lived isotope of actinium, ²²⁷Ac, has a half-life of only 21.77 years. Therefore, capabilities to produce and handle macroscopic quantities of this highly radioactive element are correspondingly very limited. Most of our knowledge of the chemical properties of Ac arises from harrowing studies1 in the 1950s and 1960s, when researchers engaged in an operation to generate milligram quantities of this element for fundamental studies. Alas, due to the great hazards associated with this work and the scarce availability of Ac, those efforts were largely abandoned, and our understanding of this element has not substantially progressed in the last 50 years. In contrast, the chemistry of the other early actinide elements has been steadily developed throughout the last several decades, with thousands of research articles describing the chemistry of actinides such as plutonium and uranium. The motivations for these research efforts are fairly obvious—these actinides play crucial roles in the nuclear energy cycle and weapons. What uses and applications would suddenly prompt us to be concerned about the 50 year actinium chemistry hiatus?

In contrast to the other actinides, it has become clear within the last two decades that the most important application of Ac lies within the realm of medicine. Although seemingly contradictory to the known radiotoxicity of the actinides, the clinical use of ²²⁵Ac is a promising strategy for the treatment of cancer and other diseases, via targeted radionuclide therapy. ²²⁵Ac emits a total of four alpha particles throughout its decay chain with a short half-life (t_1/2 = 9.92 days), which renders it compatible for biological use (see Fig. 1 in subsequent article by Stein and Kerlin, p7). By chemically attaching this isotope via a chelating ligand to an antibody (a large biomolecule that selectively and strongly binds to cell surface receptors of cancer cells), the otherwise damaging alpha particles can be strategically harnessed to destroy malignant tumors (Fig. 1). Preclinical trials of ²²⁵Ac-antibody constructs have demonstrated the therapeutic utility of this isotope and general strategy. Highly selective antibodies are well developed, and efforts for large-scale production of ²²⁵Ac are also underway. An open question remains, however, regarding the most effective way to chemically attach the Ac ion to the cancer cell-targeting antibody. Displacement of Ac prior to its arrival at the target site will lead to toxic side-effects arising from unselective damage of healthy tissue from its alpha particles. The lack of progress in Ac chemistry has hindered the development of an appropriate chelating agent. A better appreciation and understanding of this element’s chemical properties will facilitate new ligand design, enabling the development of safer Ac-based radiopharmaceuticals.

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1 On a per mass basis, ²²⁷Ac is 2.2 × 10¹³ times more radioactive than ²³⁸U!

Ac³⁺ chemical properties

Several of the fundamental chemical properties of Ac are known. Only the +3 oxidation state, which has a closed-shell radon electron configuration, is stable in aqueous solution. As a result, the Ac³⁺ ion is diamagnetic with no valence electrons in the 6d or 5f orbitals, rendering this ion spectroscopically silent. A unique property of the Ac³⁺ ion is its large six-coordinate ionic radius of 1.13 Å; Ac³⁺ is the largest trivalent ion in the periodic table. The absolute chemical hardness of an ion, another fundamental property, is a measure of its polarizability. Metal-ligand interactions are dictated by the hard-soft acid-base (HSAB) principle, whereby “hard” ions interact most strongly with “hard” ligands and vice versa. This principle, therefore, dictates the choice of preferred donor atom for a given metal ion. Using density functional theory (DFT) calculations, we computed the chemical hardness of Ac³⁺ to be 14.5 eV. This value classifies Ac³⁺ as a moderately hard ion. Taken together, these data show that Ac³⁺ is the largest +3 ion in the periodic table with a stable noble gas electron configuration and indicate a preference for hard donor atoms.

Ligation studies

With this knowledge in hand, we began our investigation into Ac chemistry by evaluating the relative binding affinity of this ion with the nitrogen-rich macrocycles shown in Fig. 2 (nitrogen being classified as a moderately hard donor atom). The nature of each pendant N-heterocycle varies slightly with regard to chemical hardness. We reasoned that differences in binding affinity might shed light on an optimal value of ligand chemical hardness for forming stable bonds with the Ac ion.

Attempts to radiolabel these ligands with short-lived ²²⁵Ac, however, led to an unexpected result. Under all conditions screened, Ac³⁺ did not coordinate to the macrocyclic ligands. Instead, the third daughter isotope of ²²⁵Ac, ²¹³Bi, was incorporated rapidly and selectively (see Fig. 1 on p7). This result highlights the surprisingly low nitrogen atom affinity of the Ac³⁺ ion. Indeed, estimation of the binding constant (K) of Ac³⁺ for a quintessential nitrogen donor ligand, NH₃, gives a relatively small value of logK = 0.8. The higher corresponding value for Bi³⁺ (logK = 5.1) explains the observed selectivity. Additionally, preliminary work utilizing the expanded porphyrin-like macrocycle, texaphyrin (Fig. 2), again demonstrated selective labeling of ²¹³Bi over ²²⁵Ac. From these studies, it is clear that an ideal chelating agent for Ac³⁺ should bear some oxygen atom donors (which are less polarizable than nitrogen atoms), or other chemically hard donors, rather than nitrogen donors.

Fluorescence spectroscopy

Additional efforts to understand the chemical properties of Ac³⁺ employed the long-lived isotope, ²²⁷Ac (t_1/2 = 21.77 years). For this research, we were able to utilize a supply of 10 mCi of ²²⁷Ac, which corresponds to only 0.61 μmol, or 140 μg, of Ac. The small quantities available, and the challenges associated with handling this highly radioactive element, necessitate a method of chemical analysis that is highly sensitive to low concentrations. We reasoned that fluorescence spectroscopy, a technique that can be commonly used for the detection of sub-micromolar concentrations of analyte, would be an effective method for probing Ac³⁺ chemistry. Because Ac³⁺ is spectroscopically silent, an organic fluorophore with a metal ion-binding unit is needed to relay information. Furthermore, to extract useful data regarding the Ac³⁺ ion, a fluorescent sensor that responds in a metal ion-dependent manner, rather than a simple “on-off” response, is necessary.

These efforts led to the design and synthesis of a new ligand, Ds-DOTAM (Fig. 3). This ligand bears a macrocyclic metal ion-binding unit and a fluorescent dansyl (Ds) group. The hard oxygen atom of the sulfone group of dansyl can interact favorably with similarly hard metal ions, perturbing its photophysical properties. Indeed, addition of the non-radioactive lanthanide 4f⁰ ion La³⁺ (six-coordinate ionic radius 1.03 Å vs. 1.13 Å for Ac³⁺) to Ds-DOTAM led to a red shift in the absorbance maximum of the ligand (i.e., an increase in wavelength and quenching of the dansyl emission; see Fig. 3). DFT calculations indicate that this red shift arises from a stabilization of the lowest-unoccupied molecular orbital (LUMO), primarily due to an electrostatic interaction of the M³⁺ ion with the dansyl sulfone group. The addition of Ac(NO₃)₃ showed similar spectroscopic changes. The UV-vis. absorbance spectrum is, however, partially obscured by the presence of a large excess of NO₃ – ion, which absorbs at 290 nm and is carried over from radiochemical separation procedures. This data indicates that Ac³⁺ and La³⁺ interact with Ds-DOTAM in a similar manner. Nonetheless, strategies to purify the Ac³⁺ ion from excess NO₃ –ions remain an important goal for obtaining high quality spectra.

X-ray absorption spectroscopy

Although the fluorescence spectroscopy results revealed that both La³⁺ and Ac³⁺ interact with Ds-DOTAM, the precise nature of that interaction will certainly be different for the two ions, which possess substantially different ionic radii. To ascertain structural aspects of Ac chemistry, we directed efforts toward X-ray absorption spectroscopy (i.e., X-ray absorption near edge structure, XANES, and extended X-ray absorption fine structure, EXAFS), using high intensity synchrotron sources coupled with fluorescence detection methods.

Analyzing a small quantity of ²²⁷Ac with these methods gave a significant edge-rise at 15.87 keV (Fig. 4). This energy corresponds to the L_III edge of Ac, and is therefore diagnostic of this element. Although the XANES region is not expected to yield a large amount of information about the non-redox-active Ac³⁺ ion, the EXAFS region can inform us about Ac-ligand interatomic distances. For this particular sample, Ac³⁺ was dissolved in 0.1 M ammonium acetate pH 5 buffer. These conditions mimic those used to create therapeutically relevant ²²⁵Ac-labeled antibodies, and can therefore shed light on the speciation and nature of Ac³⁺ prior to conjugation.

In conjunction with DFT and molecular dynamics calculations, EXAFS data analysis is underway. These data represent the first X-ray absorption spectra of Ac, and will enable the determination of Ac-ligand interatomic distances and coordination numbers in the solution phase for the first time (see article by Stein & Kerlin).

Summary

For the first time in over 50 years, the chemistry of actinium is under active exploration by researchers at Los Alamos National Laboratory (LANL). This effort is motivated by the great therapeutic potential of ²²⁵Ac, the clinical development of which is currently limited by a lack of knowledge of the fundamental coordination chemistry of this element. Our research has thus far demonstrated that Ac³⁺ is generally averse to interacting with nitrogen donor ligands. We have also shown that fluorescence spectroscopy can be used to interrogate Ac-ligand binding, and that 2 mCi quantities of ²²⁷Ac provide sufficient signal to obtain EXAFS spectra from which interatomic distances and coordination numbers can be obtained. These results provide a strong foundation for the continued exploration of Ac chemistry. The data obtained from these efforts will have direct application in the design of better ligands for Ac³⁺, leading to safer, more effective ²²⁵Ac-based radiopharmaceutical agents.

Acknowledgments

This work is a collaborative effort with several others researchers: Beau J. Barker, Enrique R. Batista, John M. Berg, Eva R. Birnbaum, Jonathan W. Engle, Michael E. Fassbender, Maryline G. Kerlin, Kevin D. John, Stosh A. Kozimor, Joel R. Maassen, Richard L. Martin, F. Meiring Nortier, Valery Radchenko, and Marianne P. Wilkerson all contributed and played vital roles in initiating and contributing to this ongoing project. Financial support was provided by the U.S. Department of Energy through the LANL/LDRD program and the Seaborg Institute for the postdoctoral fellowship to Dr. Wilson.

Further reading:

1. S. Fried, F. Hagemann, and W. H. Zachariasen, “Te preparation and identifcation of some pure actinium compounds”, J. Am. Chem. Soc. 1950, 72, 771-75.
2. J. G. Stites, M. L. Salutsky, and B. D. Stone, “Preparation of actinium metal”, J. Am. Chem. Soc. 1955, 77, 237-40.
3. J. D. Farr, A. L. Giorgi, M. G. Bowman, and R. K. Money, “Te crystal structure of actinium metal and actinium hydride”, J. Inorg. Nucl. Chem. 1961, 18, 42-47.
4. F. Weigel, and H. Hauske, “Te lattice constants of actinium(III) oxalate deca-hydrate”, J. Less-Common Met. 1977, 55, 243-47.
5. K. A. Deal, I. A. Davis, S. Mirzadeh, S. J. Kennel, and M. W. Brechbiel, “Improved in vivo stability of actinium-225 macrocyclic complexes”, J. Med. Chem. 1999, 42, 2988-92.
6. M. R. McDevitt, D. Ma, L. T. Lai, J. Simon, P. Borchardt, R. K. Frank, K. Wu, V. Pelligrini, M. J. Curcio, M. Miederer, N. H. Bander, and D. A. Scheinberg, “Tumor therapy with targeted atomic nanogenerators”, Science 2001, 294, 1537-40.
7. M. R. McDevitt, D. Ma, J. Simon, R. K. Frank, and D. A. Scheinberg, “Design and synthesis of ²²⁵Ac radioimmuno-pharmaceuticals”, Appl. Radiat. Isot. 2002, 57, 841-47.
8. H. Kirby, and L. Morss, “Actinium”, in: L. Morss, N. Edelstein, J. Fuger (Eds.) Te Chemistry of the Actinide and Transactinide Elements, Springer Netherlands, 2006, pp. 18-51.
9. M. Miederer, D. A. Scheinberg, and M. R. McDevitt, “Realizing the potential of the Actinium-225 radionuclide generator in targeted alpha particle therapy applications”, Adv. Drug Delivery Rev. 2008, 60, 1371-82.
10. D. A. Scheinberg, and M. R. McDevitt, “Actinium-225 in targeted alpha-particle therapeutic applications”, Curr. Radiopharm. 2011, 4, 306-20.
11. J. J. Wilson, E. R. Birnbaum, E. R. Batista, R. L. Martin, and K. D. John, “Synthesis and characterization of nitrogen-rich macrocyclic ligands and an investigation of their coordination chemistry with lanthanum(III)”, Inorg. Chem. 2015, 54, 97-109.
12. J. J. Wilson, M. Ferrier, V. Radchenko, J. R. Maassen, J. W. Engle, E. R. Batista, R. L. Martin, F. M. Nortier, M. E. Fassbender, K. D. John, and E. R. Birnbaum, “Evaluation of nitrogen-rich macrocyclic ligands for the chelation of therapeutic bismuth radioisotopes”, Nucl. Med. Biol. 2015, 42, 428-38.
13. G. Tiabaud, V. Radchenko, J. J. Wilson, K. D. John, E. R. Birnbaum, and J. L. Sessler, “Rapid insertion of bismuth radioactive isotopes into texaphyrin in aqueous media”, J. Porphyr. Phthalocyanines 2017, 21, 12, 882-886.
14. N. A. Tiele, V. Brown, J. M. Kelly, A. Amor-Coarasa, U. Jermilova, S. N. MacMillan, A. Nikolopoulou, S. Ponnala, C. F. Ramogida, A. K. H. Robertson, C. Rodríguez-Rodríguez, P. Schafer, C. Williams Jr., J. W. Babich, V. Radchenko, and J. J. Wilson, “An Eighteen-Membered Macrocyclic Ligand for Actinium-225 Targeted Alpha Terapy”, Angew. Chem. Int. Ed. 2017, 56, 14712–14717.
15. N. A. Tiele, and J. J. Wilson, “Actinium-225 for Targeted α Terapy: Coordination Chemistry and Current Chelation Approaches”, Cancer Biother. Radiopharm. 2018, 33, 336-48.