Targeted alpha therapy (TAT) is an emerging and powerful tool for treating late-stage cancers for which therapeutic options are limited. At the core of TAT is the development of targeted radiopharmaceuticals, where isotopes are paired with biological targeting vectors that enable tissue- or cell-specific delivery of alpha-emitters. 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and diethylenetriamine pentaacetic acid (DTPA) are commonly used to chelate metallic radionuclides, but have several limitations. Significant efforts are underway to develop effective and stable chelators for alpha-emitters and are at various stages of development and community adoption. Isotopes such as ¹⁴⁹Tb, ^212/213Bi, ²¹²Pb (for ²¹²Bi), ²²⁵Ac, ^226/227Th have identified suitable chelators, although further studies, especially in vivo studies, are required; while for others, including ²²³Ra, ²³⁰U, and arguably ²¹¹At an ideal chelate remains elusive. In this review, we will summarize the chelation chemistry reported to date for the incorporation of ¹⁴⁹Tb, ²¹¹At, ^212/213Bi, ²¹²Pb (for ²¹²Bi), ²²³Ra, ²²⁵Ac, ^226/227Th and ²³⁰U into radiopharmaceuticals, with a focus on new discoveries and remaining challenges.

Actinium-225 (225Ac, t1/2 = 9.9 d) is a promising candidate radionuclide for use in targeted alpha therapy (TAT), though the currently limited global supply has hindered the development of a suitable Ac-chelating ligand and 225Ac-radiopharmaceuticals towards the clinic. We at TRIUMF have leveraged our Isotope Separation On-Line (ISOL) facility to produce 225Ac and use the resulting radioactivity to screen a number of potential 225Ac-radiopharmaceutical compounds. MBq quantities of 225Ac and parent radium-225 (225Ra, t1/2 = 14.8 d) were produced and separated using solid phase extraction DGA resin, resulting in a radiochemically pure 225Ac product in > 98% yield and in an amenable form for radiolabeling of ligands and bioconjugates. Of the many polydentate picolinic acid (“pa”) containing ligands evaluated (H4octapa [N4O4], H4CHXoctapa [N4O4], p-NO2-Bn-H4neunpa [N5O4], and H6phospa [N4O4]), all out-performed the current gold standard, DOTA for 225Ac radiolabeling ability at ambient temperature. Moreover, a melanocortin 1 receptor-targeting peptide conjugate, DOTA-modified cyclized α-melanocyte-stimulating hormone (DOTA-CycMSH), was radiolabeled with 225Ac and proof-of-principle biodistribution studies using B16F10 tumour-bearing mice were conducted. At 2 h post-injection, tumour-to-blood ratios of 20.4 ± 3.4 and 4.8 ± 2.4 were obtained for the non-blocking (molar activity [M.A.] > 200 kBq/nmol) and blocking (M.A. = 1.6 kBq/nmol) experiment, respectively. TRIUMF’s ISOL facility is able to provide 225Ac suitable for preclinical screening of radiopharmaceutical compounds; [225Ac(octapa)]−, [225Ac(CHXoctapa)]−, and [225Ac(DOTA-CycMSH)] may be good candidates for further targeted alpha therapy studies.

The cystine transporter (system xC-) is an antiporter of cystine and glutamate. It has relatively low basal expression in most tissues and becomes upregulated in cells under oxidative stress (OS) as one of the genes expressed in response to the antioxidant response element (ARE) promoter. We have developed ¹⁸F-5-fluoro-aminosuberic acid (FASu), a Positron Emission Tomography (PET) tracer that targets system xC-. The goal of this study was to evaluate the suitability of ¹⁸F-FASu as a specific gauge for system xC- activity in vivo and its potential usefulness for breast cancer imaging. Methods: ¹⁸F-FASu specificity towards system xC- was studied by cell inhibition assay, cellular uptake following OS induction with diethyl maleate (DEM), with and without anti-xCT siRNA knockdown, in vitro uptake studies and in vivo uptake in a system xC- transduced xenograft model. In addition, radiotracer uptake was evaluated in three separate breast cancer models MDA-MB-231, MCF-7 and ZR-75-1. Results: Reactive oxygen species (ROS)-inducing DEM increased glutathione level and ¹⁸F-FASu uptake, while gene knocking down with anti-xCT siRNA led to decreased tracer uptake. ¹⁸F-FASu uptake was robustly inhibited by system xC- inhibitors or substrates, while the uptake was significantly higher in transduced cells and tumors expressing xCT compared to the wild type HEK293T cells and tumors (p<0.0001 for cells, P = 0.0086 for tumors). The ¹⁸F-FASu demonstrated tumor uptake in all three breast cancer cell lines studied. Among them, triple negative breast cancer MDA-MB-231 had the highest tracer uptake (P = 0.0058 when compared with MCF-7; p<0.0001 when compared with ZR-75-1), which also has the highest xCT mRNA level. Conclusion: ¹⁸F-FASu as a system xC- substrate is a specific PET tracer for functional monitoring of system xC- and OS imaging. By enabling non-invasive analysis of xC- responses in vivo, this biomarker may serve as a valuable target for the diagnosis and treatment monitoring of certain breast cancers.