Bulk Scale Formulation of Therapeutic Doses of Clinical Grade Ready-to-Use 177Lu-DOTA-TATE: The Intricate Radiochemistry Aspects

Abstract

Introduction: ¹⁷⁷Lu-DOTA-TATE is a clinically useful and promising therapeutic radiopharmaceutical for peptide receptor radionuclide therapy of neuroendocrine tumors (NETs) overexpressing somatostatin receptors. Currently, the radiopharmaceutical is prepared in-house at nuclear medicine centers, thereby restricting its use to limited centers only. In this article, the authors describe systematic studies toward bulk scale formulation of "ready-to-use" ¹⁷⁷Lu-DOTA-TATE using medium specific activity ¹⁷⁷Lu (740-1110 GBq/mg) at a centralized radiopharmacy facility.

Methods: In an optimized protocol, ¹⁷⁷Lu-DOTA-TATE synthesis was carried out by direct heating of ¹⁷⁷LuCl₃ (Sp. act. 740-1110 GBq/mg) with DOTA-TATE peptide (1.5-3.0 equivalents) in ammonium acetate buffer (0.2 M) containing 2,5-dihydroxy benzoic acid (gentisic acid). Thereafter, the crude labeled product was purified using a Sep-Pak^® C18 column and diluted with acetate buffer-gentisic acid (1.5% w/v) solution to final radioactive concentration of 740 MBq/mL. This was further sterilized and dispensed as 7.4 GBq patient dose/vial with 2 days postformulation calibration.

Results: A peptide/metal ratio of 1.5-3.0 is essential for complexation wherein radiolabeling yields >90% are obtained minimizing free ¹⁷⁷Lu waste. For formulation of 7.4 GBq patient dose (2 days postproduction), even specific activity of about 555 GBq/mg was found to be adequate for the radiometal. The ready-to-use 740 MBq/mL ¹⁷⁷Lu-DOTA-TATE formulation with gentisic acid (1.5% w/v) is observed to be safe for human use for more than 1 week (radiochemical purity >98%) from the day of production when stored at -70°C. However, the target specificity may get affected beyond 2 days as the total peptide content for 7.4 GBq dose may exceed the critical peptide limit of 300 μg. Patient treatment carried with several batches of present formulation in diseased NET patients exhibited desired distribution at the tumor and its metastatic site.

Conclusions: A ready-to-use formulation of ¹⁷⁷Lu-DOTA-TATE was successfully prepared and optimized for regular bulk scale production and supply to distant nuclear medicine centers.

Keywords: 177Lu; DOTA-TATE; neuroendocrine tumor; therapeutic radiopharmaceutical.