Development of LAT1-Selective Nuclear Medicine Therapeutics Using Astatine-211

Abstract

We investigated nuclear medicine therapeutics targeting the L-type amino acid transporter 1 (LAT1). We previously reported that a nuclear medicine therapeutic drug using astatine 211 (²¹¹At), an alpha-emitting nuclide that can be produced in an accelerator and targets LAT1 as a molecular target, is effective. The seed compound was 3-[²¹¹At] Astato-α-methyl-L-tyrosine (²¹¹At-AAMT-OH-L). We used a unique labeling method. By changing the OH group of phenol to a methyl group, retention was successfully increased. It was also found that the amount of the L-isomer taken up by the D-isomer and L-isomer was clearly higher, and the L-isomer was superior as a therapeutic drug. Compounds in which the methyl group was replaced with an ethyl or propyl group were also examined, but their retention did not increase significantly. In fact, we observed increased non-specific accumulation and dynamics, suggesting that labeling may be off. In addition, ²¹¹At-AAMT-O-Me-L, which has a simple structure, was clearly superior in terms of uptake speed for several candidate compounds. As a result, we were able to develop a compound that can be easily labeled, has high specific radioactivity, is stable, and has a strong therapeutic effect.

Keywords: LAT1; amino acid; astatine-211; cancer therapy; nuclear medicine.

Figure 1

(A) HPLC analysis of ²¹¹At-AAMT-OMe-L. ²¹¹At-AAMT-OMe-L was purified by an HLB column before HPLC analysis. (B) TLC analyses of ²¹¹At-AAMT-OMe-L. Samples were collected 0, 24, and 48 h after labeling with ²¹¹At. (a) immediately after labeling, (b) 24 h after labeling, and (c) 48 h after labeling.

Figure 2

Comparison of uptake between the labeled compounds. (A) Relation between uptake ratio and LAT1 expression level. Uptake ratios were compared between LAT1-Tet/HEK293 cells. The Y-axis represents the uptake ratio, and the X-axis is concentration of Doxycycline (Dox) (TaKaRa Bio, Shiga, Japan). The collection time was 30 min after treatment. The expression of LAT1 increased with increasing amounts of Dox. (B) Comparison of uptake in PANC-1 cell line. The collection times were 0.5, 1, 5 and 10 min after treatment.

Figure 3

Comparison of intracellular uptake of ¹¹At-AAMT-O-Me L-isomer and ²¹¹At-AAMT-O-Me D-isomer of ²¹¹At-AAMT-O-Me. (A) Uptake in Mock/HEK293, LAT1/HEK293, and LAT1/HEK293 cell lines. (B) Uptake in PANC-1 cells. The y-axis represents %ID/mg protein. BCH: -Amino-2-norbornanecarboxylic acid. BCH was treated at 20 mM. ** p < 0.01, *** p < 0.001.

Figure 4

Anti-tumor effects in the PANC-1 xenograft model. (A) Tumor sizes. (B) Body weights. Each group consisted of three mice. All data are presented as means ± standard errors.