Stability Matters: Radiochemical Stability of Therapeutic Radiopharmaceutical ¹⁷⁷Lu-PSMA I&T

MATERIALS AND METHODS

The labeling of lutetium trichloride (177LuCl₃) and peptide (PSMA I&T; 40 mg of ascorbic acid and 10 mg of sodium hydroxide) was prepared with a fully automated radiopharmaceutical synthesis device using Modular-Lab Pharm Tracer (Eckert & Ziegler). PSMA I&T was labeled with ¹⁷⁷Lu using good manufacturing practice–grade disposable cassettes and reagent kits supplied by ABX Advanced Biochemical Compounds (Fig. 1). Labeling of ¹⁷⁷Lu-PSMA I&T was performed per the ABX and Eckert & Ziegler synthesis instructions, with the help of a pressure-based cassette (Eckert & Ziegler). The non–carrier-added 177Lucl₃ was supplied by ANSTO and ITG (Isotope Technologies Garching GmbH). Radiation quantity was procured on the basis of the number of patients treated per cycle. The required dose for 1 patient was usually 7 GBq, based on the estimated synthesis yield of 80%. ABX supplied the accessory chemical, including sodium hydroxide–ascorbic acid and 50 mL of sodium chloride (saline) along with 0.55 μmol filters, long needles, and vent needles. PSMA I&T was supplied by Huwai Chem in a 1-mg vial, which was fractionated into 200 μg and stored in the freezer. The required amount of PSMA I&T (200 μg/5 GBq) was reconstituted with 1.5 mL of sodium ascorbate (0.57 M) to adjust the pH to 4.5 ± 0.1. Labeling was performed using a computer-based automated system (Modular-Lab Eazy; Eckert & Ziegler). All production cassettes were supplied by vendors and were made for a single use only.

• Download figure
• Open in new tab
• Download powerpoint

FIGURE 1.

Modular-Lab Pharm cassettes supplied by Eckert and Ziegler.

The shelf life of prepared ¹⁷⁷Lu-PSMA I&T was established on the basis of the evaluation of radiochemical purity (RCP) by HPLC and ITLC-silica gel (ITLC-SG) (5).

HPLC was used for radiochemical analyses and purification of the ¹⁷⁷Lu-labeled PSMA I&T conjugates. A dual-pump HPLC unit with a C18 reversed-phase column (25 × 0.46 cm) (Knauer) purified the labeled conjugates. Mixtures of 1% trifluoroacetic acid, Ultrapure (Sigma-Aldrich) water (solvent A) and and 0.1% trifluoroacetic acid and acetonitrile (solvent B) were used as the mobile phase (6). The following gradient elution technique was adopted for the separation: 0–3 min—A, 100% and B, 0%; 3–10 min—A, 50% and B, 50%; 10–15 min—A, 0% and B, 100%. HPLC analysis showed that the fast eluting compound was hydrophilic ¹⁷⁷Lu cation (1.0 min), whereas ¹⁷⁷Lu-PSMA I&T with a high molecular weight was eluted after 8 min. The typical retention time of radiolabeled PSMA under the above conditions is approximately 500 s (8–10 min).

The ITLC-SG method was applied to check the RCP of the radiolabeled complex. The principle of the ITLC-SG analytic method is that a mobile phase moves along a stationary phase due to capillary forces. Therefore, depending on the distribution of components between the stationary and mobile phase, a radioactive sample spotted in the adsorbent will migrate with different velocities, and thus impurities are separated. The study was performed using a 10-cm-long Whatman 3MM chromatography paper’s stationary phase. For this study, 5 μL of the test solution was spotted at 1.5 cm from the lower end of the paper strips, developed in 10% ammonium acetate in methanol as mobile phases (30:70 vol/vol). After each synthesis, the given amount of radiolabeled complex (∼10 MBq) was kept at room temperature for 48 h while being checked by HPLC and ITLC-SG at specified time intervals of 0, 24, and 48 h after preparation (Tables 1 and 2).