Radiochemical Stability of [18F]21 and [177Lu]21

[¹⁸F]21 or [¹⁷⁷Lu]21 (50 μCi, 10 μL) was added to PBS (90 μL) or murine serum (90 μL) and the mixture was incubated at 37 °C for a certain time ([¹⁸F]21: 2 h, [¹⁷⁷Lu]21: 8 d). The PBS mixture was injected directly into the radio-HPLC for analysis. The murine serum was precipitated with 0.1 mL of acetonitrile and centrifuged (10,000 rpm, 5 min). The supernatant was measured and analyzed by the radio-HPLC to determine the stability. For in vivo stability study, the radio tracers were injected into the tail vein of ICR mice and samples of blood were collected after a certain time post injection (pi) ([¹⁸F]21: 4 h, [¹⁷⁷Lu]21: 9 h). The plasma proteins were precipitated using an equal volume of acetonitrile and centrifuged (10,000 rpm, 5 min). The RCP of radio tracer in supernatants were analyzed and quantified by radio-HPLC. The radio-HPLC method was as follows: A: 0.1% TFA in H₂O, B: ACN, 0–10 min, 0–100% B. The flow rate was 1 mL/min, and the C18 column (4.6 × 150 mm, 5 μm, ZORBAX, Agilent) was used. When using HPLC to assess the stability of [¹⁷⁷Lu]21, the effluent was collected every 30 s due to its less radioactivity, and samples were measured by an automatic γ-counter (2480 Wizard2, Perkin Elmer). The counted samples were plotted as intensity (cpm) against fraction [19 (link)].