Selpercatinib (LOXO-292) is a newly marketed oral selective receptor tyrosine kinase inhibitor targeting rearranged during transfection (RET), demonstrating precise therapeutic effects against RET-positive non-small cell lung cancer and thyroid cancer. In this study, an unknown acid forced degradation impurity of selpercatinib, designated sel-1, was isolated and purified using semi-preparative liquid chromatography (semi-Prep-LC). The purified sel-1 showed a chromatographic purity of 99.1 % as determined by high-performance liquid chromatography (HPLC). It appeared as a white amorphous powder, with a maximum absorption peak at 235 nm and a chemical formula of C