Astatine-211 Radiopharmaceuticals; Status, Trends, and the Future

The low range of alpha particles provides an opportunity to better target cancer cells theoretically leading to the introduction of interesting alpha emitter radiopharmaceuticals including 225Ac, 212Pb, etc. The combination of high energy and short range of alpha emitters differentiates targeted radiotherapy from other methods and reduces unwanted cytotoxicity of the cells around the tumoral tissue. Among interesting alpha emitters candidates for targeted therapy, 211At, one of the radioisotopes with the best optimal decay properties, shows great promise for targeted radiotherapy in some animal prostate cancer xenograft studies and bone micro tumors with significant effects compared to other beta and alpha emitters and also demonstrates interesting properties for clinical applications. However, production and application of this alpha emitter in the development of actinium-based radiopharmaceuticals is hampered by many obstacles. This mini-review demonstrates 211At production methods, chemical separation, radiolabeling procedures, 211At-radiopharmaceuticals and their clinical trials, transport, logistics, and costs and future trends in the field for ultimate clinical applications. This review showed that there are limited clinical trials on 211Ac-based radiopharmaceuticals, which is due to the low accessibility of this radioisotope and other limitations. However, the development programs of major industries indicate the development of 211Ac-based radiopharmaceuticals in the future.

Production, isolation, and shipment of clinically relevant quantities of astatine-211: A simple and efficient approach to increasing supply

The alpha emitter astatine-211 (211At) is a promising candidate for cancer treatment based on Targeted Alpha (α) Therapy (TAT). A small number of facilities, distributed across the United States, are capable of accelerating α-particle beams to produce 211At. However, challenges remain regarding strategic methods for shipping 211At in a form adaptable to advanced radiochemistry reactions and other uses of the radioisotope.

PURPOSE

Our method allows shipment of 211At in various quantities in a form convenient for further radiochemistry.

PROCEDURES

For this study, a 3-octanone impregnated Amberchrom CG300M resin bed in a column cartridge was used to separate 211At from the bismuth matrix on site at the production accelerator (Texas A&M) in preparation for shipping. Aliquots of 6 M HNO3 containing up to ≈2.22 GBq of 211At from the dissolved target were successfully loaded and retained on columns. Exempt packages (<370 MBq) were shipped to a destination radiochemistry facility, University of Texas MD Anderson Cancer Center, in the form of a convenient air-dried column. Type A packages have been shipped overnight to University of Alabama at Birmingham.

MAIN FINDINGS

Air-dried column hold times of various lengths did not inhibit simple and efficient recovery of 211At. Solution eluted from the column was sufficiently high in specific activity to successfully radiolabel a model compound, 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (1), with 211At. The method to prepare and ship 211At described in this manuscript has also been used to ship larger quantities of 211At a greater distance to University of Alabama at Birmingham.

PRINCIPAL CONCLUSIONS

The successful proof of this method paves the way for the distribution of 211At from Texas A&M University to research institutions and clinical oncology centers in Texas and elsewhere. Use of this simple method at other facilities has the potential increase the overall availability of 211At for preclinical and clinical studies.

In vitro and in vivo evaluation of 211At-labeled fibroblast activation protein inhibitor for glioma treatment

Glioma is the most common primary intracranial tumor without effective treatment. Positron emission tomography tracers labeled with 68Ga targeting fibroblast activation protein (FAP) have shown favorable characteristics in the diagnosis of glioma. However, to the best of our knowledge, FAP-targeted endoradiotherapy has never been explored in glioma. Hence, in this study, we investigated the therapeutic effect of 211At-labeled fibroblast activation protein inhibitor (FAPI) for glioma in vitro and in vivo. By astatodestannylation reaction, we prepared 211At-FAPI-04 with a radiochemical yield of 45 ± 6.7% and radiochemical purity of 98%. With good stability in vitro, 211At-FAPI-04 showed fast and specific binding to FAP-positive U87MG cells, and could significantly reduce the cell viability, arrested cell cycle at G2/M phase and suppressed cell proliferative efficacy. Biodistribution studies revealed that 6-fold higher accumulation in tumor sites was achieved by intratumoral injection in comparison with intravenous injection. In U87MG xenografts, 211At-FAPI-04 obviously suppressed the tumor growth and prolonged the median survival in a dose-dependent manner without obvious toxicity to normal organs. In addition, reduced proliferation and increased apoptosis were also observed after 211At-FAPI-04 treatment. All these results suggest that targeted alpha-particle therapy (TAT) mediated by 211At-FAPI-04 can provide an effective and promising strategy for the treatment of glioma.

Gold Nanostars: A Novel Platform for Developing 211At-Labeled Agents for Targeted Alpha-Particle Therapy

Aim

To develop an innovative ²¹¹At nanoplatform with high radiolabeling efficiency and low in vivo deastatination for future targeted alpha-particle therapy (TAT) to treat cancer.

Methods

Star-shaped gold nanoparticles, gold nanostars (GNS), were used as the platform for ²¹¹At radiolabeling. Radiolabeling efficiency under different reaction conditions was tested. Uptake in the thyroid and stomach after systemic administration was used to evaluate the in vivo stability of ²¹¹At-labeled GNS. A subcutaneous U87MG human glioma xenograft murine model was used to preliminarily evaluate the therapeutic efficacy of ²¹¹At-labeled GNS after intratumoral administration.

Results

The efficiency of labeling GNS with ²¹¹At was almost 100% using a simple and rapid synthesis process that was completed in only 1 min. In vitro stability test in serum showed that more than 99% of the ²¹¹At activity remained on the GNS after 24 h incubation at 37°C. In vivo biodistribution results showed low uptake in the thyroid (0.44–0.64%ID) and stomach (0.21–0.49%ID) between 0.5 and 21 h after intravenous injection, thus indicating excellent in vivo stability of ²¹¹At-labeled GNS. The preliminary therapeutic efficacy study demonstrated that ²¹¹At labeled GNS substantially reduced tumor growth (P < 0.001; two-way ANOVA) after intratumoral administration.

Conclusion

The new ²¹¹At radiolabeling strategy based on GNS has the advantages of a simple process, high labeling efficiency, and minimal in vivo dissociation, making it an attractive potential platform for developing TAT agents that warrants further evaluation in future preclinical studies directed to evaluating prospects for clinical translation.

Synthesis and Preliminary Evaluation of 131I-Labeled FAPI Tracers for Cancer Theranostics

As an excellent target for cancer theranostics, fibroblast activation protein (FAP) has become an attractive focus in cancer research. A class of FAP inhibitors (FAPIs) with a N-(4-quinolinoyl)-Gly-(2-cyanopyrrolidine) scaffold were developed, which displayed nanomolar affinity and high selectivity. Compared with ⁹⁰Y, ¹⁷⁷Lu, ²²⁵Ac, and ¹⁸⁸Re, ²¹¹At seems to be more favored as a therapeutic candidate for FAPI tracers which have fast washout and short retention in tumor sites. Thus, the current study reported the synthesis of two FAPI precursors for ²¹¹At and ¹³¹I labeling and the preliminary evaluation of ¹³¹I-labeled FAPI analogues for cancer theranostics. FAPI variants with stannyl precursors were successfully synthesized and labeled with ¹³¹I using a radioiododestannylation reaction. Two radioactive tracers were obtained with high radiochemical purity over 99% and good radiochemical yields of 58.2 ± 1.78 and 59.5 ± 4.44% for ¹³¹I-FAPI-02 and ¹³¹I-FAPI-04, respectively. Both tracers showed high specific binding to U87MG cells in comparison with little binding to MCF-7 cells. Compared to ¹³¹I-FAPI-02, ¹³¹I-FAPI-04 exhibited higher affinity, more intracellular uptake, and longer retention time in vitro. Biodistribution studies revealed that both tracers were mainly excreted through the kidneys as well as the hepatobiliary pathway due to their high lipophilicity. In addition, higher accumulation, longer dwell time, and increased tumor-to-organ ratios were achieved by ¹³¹I-FAPI-04, which was clearly demonstrated by SPECT/CT imaging. Furthermore, intratumor injection of ¹³¹I-FAPI-04 significantly suppressed the tumor growth in U87MG xenograft mice without significant toxicity observed. The above results implied that FAP-targeted alpha endoradiotherapy (specific to ²¹¹At) should be used to treat tumors in the near future, considering the chemical similarity between iodine and astatine can ensure the labeling of the latter onto the designed FAPIs.