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All reagents in this study were commercial products used without further purification. Sodium [¹²⁵I]iodide ([¹²⁵I]NaI) and [¹²³I]NaI were purchased from PerkinElmer and FUJIFILM RI Pharma Co., Ltd., respectively. Smart Flash EPCLC W-Prep 2XY (Yamazen Corporation) was used for silica gel chromatography. ¹H NMR was recorded on a JNM-ECS400 (JEOL) with tetramethylsilane (TMS) as an internal standard. Coupling constants are reported in Hertz (Hz). Multiplicity was defined as singlet (s), doublet (d), triplet (t), multiplet (m), and quartet (q). ESI mass spectrometry was conducted with a Shimadzu LCMS-2020. High-resolution mass spectrometry (HRMS) was conducted with JMS-700V (JEOL). High-performance liquid chromatography (HPLC) was performed with Shimadzu system (an LC-20AD pump with an SPD-20A UV detector, λ = 254 nm) using a Cosmosil C₁₈ column (Nacalai Tesque, COSMOSIL 5C₁₈-AR-II 4.6 mm I.D. × 150 mm) and CH₃CN/H₂O (60: 40) as the mobile phase at a flow rate of 1.0 mL/min. HER2-expressing human gastric cancer cells (N87) were purchased from DS Pharma Biomedical (Osaka, Japan).

(S)-(-)-3-trimetylstannyl-2-hydroxy-6-methoxy-N-[(1-ethyl-2-pyrrolidinyl)methyl]-benzamide (TBZM; >95% purity) and S-(-)-IBZM ([¹²⁷I]IBZM, >95% purity) were purchased from the NARD Institute, Ltd. (Hyogo, Japan) and ABX (Radeberg, Germany), respectively. ¹²⁵I-NaI (3.3–4.1 GBq/mL) and ¹²³I-NaI (8.5–15.0 GBq/mL) were purchased from PerkinElmer, Inc. (Waltham, MA, USA) and FUJIFILM Toyama Chemical Co., Ltd. (Tokyo, Japan), respectively. Chloramine-T (>95% purity), sodium metabisulfite (>95% purity), and ammonium acetate (>97% purity) were purchased from Nacalai Tesque, Inc. (Kyoto, Japan); domperidone (>90% purity), ethanol (>99.5% purity), and hydrogen carbonate (>95% purity) were provided by FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan).