Development of a novel xCT transporter imaging using a radiopharmaceutical with inhibitor-type compounds

The cystine/glutamate antiporter xCT has attracted attention due to its role in maintaining redox homeostasis. Noninvasive techniques for visualizing xCT expression in tumors provide valuable diagnostic information for evaluating tumor biology. Therefore, we developed and evaluated a novel SPECT radiopharmaceutical to visualize xCT expression in cancer cells based on inhibitors.

We designed and synthesized a radioiodinated SSZ derivative ([¹²⁵I]I-SSZd), based on the structure of sulfasalazine (SSZ)-an xCT inhibitor. We radioiodinated 4-hydroxy-L-phenylglycine (L-HPG) and 4-hydroxy-D-phenylglycine (D-HPG) to synthesize [¹²⁵I]I-L-HPG and [¹²⁵I]I-D-HPG, respectively. The affinities of [¹²⁵I]I-SSZd, [¹²⁵I]I-L-HPG, and [¹²⁵I]I-D-HPG to xCT were evaluated using SSZ and cystine inhibition assays in two human-derived colon cancer cell lines (LS180 and DLD-1). xCT expression was quantified using real-time polymerase chain reaction. In vivo, the biodistribution of [¹²⁵I]I-L-HPG was examined in tumor-bearing mice. Stability analysis was performed in mice liver and kidney homogenates.

[¹²⁵I]I-L-HPG showed high affinity for xCT. [¹²⁵I]I-D-HPG exhibited some affinity for xCT. xCT was not involved in [¹²⁵I]I-SSZd accumulation. The gene expression levels of xCT were higher in LS180 than in DLD-1 cells. [¹²⁵I]I-L-HPG and [¹²⁵I]I-D-HPG showed high accumulation in LS180 cells. [¹²⁵I]I-L-HPG showed higher accumulation than [¹²⁵I]I-D-HPG in both cell lines. In the kidneys, [¹²⁵I]I-L-HPG accumulation decreased over time. In the thyroid, [¹²⁵I]I-L-HPG accumulation increased over time; however, deiodination did not occur in mice. Tumor-to-muscle ratio was >2.0 at almost all timepoints for both types of tumor-bearing mice, and the maximum tumor-to-large intestine ratio reached 4.0 in LS180 tumors with high xCT expression.

In tumors, [¹²⁵I]I-L-HPG shows promise as a radiopharmaceutical by exhibiting high affinity for xCT.