[en] Radioimmunotherapy (RIT) using ¹⁷⁷Lu-labeled monoclonal antibodies is increasingly gaining prominence in therapeutic nuclear medicine practices. Towards the use of these agents, the major challenges involved are preparation of the clinical dose and optimization of radiolabeling parameters of differently conjugated ¹⁷⁷Lu- labeled monoclonal antibodies (MoAbs) viz. ¹⁷⁷Lu-DOTA-Rituximab, ¹⁷⁷Lu-DOTA-Trastuzumab and ¹⁷⁷Lu-DOTA-Pertuzumab. Efficient radiolabeling of these MoAbs with varied molecular weight (143-148 KDa) depends on optimum pre-concentration, incubation-temperature, time and pH of conjugation and suitable purification of immunoconjugates. The present study merits the development of a single protocol which has been optimized for conjugation of rituximab, trastuzumab and pertuzumab with p-NCS-benzyl-DOTA and radiolabeling of these immunoconjugates using low specific activity (LSA) ¹⁷⁷LuCl₃. Commercial rituximab, trastuzumab, pertuzumab (5.0 mg/500 µL) preconcentrated to 80-100 µL using 30kDa MW cut-off ultra-centrifugal filtration-device at 3000 g for 17 minutes. Coupling of rituximab, trastuzumab, pertuzumab with p-NCS-benzyl-DOTA carried out at 1:10 molar ratio under incubation at 24℃ (2 h), followed by 4℃ (18 h) at pH~8.0. Reaction-mixtures purified using pre-conditioned PD-10 columns. All the three immunoconjugates eluted from PD-10 using 0.2M CH₃COONa-buffer (pH~5.5) and concentrations estimated by Bradford's-assay. The number of DOTA molecules per antibody determined by spectrophotometric-method using Pb (II)-Arsenazo (III) complex. Prior to radiolabeling, pH of ¹⁷⁷LuCl₃ (250-300 mCi, SA: 15-20 mCi/µg) adjusted to 6.5, using 0.2M CH₃COONa solution. ¹⁷⁷Lu-acetate incubated with all the three purified immunoconjugates at 37℃ for 90 minutes. Radiolabeled reaction-mixture purified using pre-conditioned PD-10 column. In-vitro stability of radioconjugates ascertained by adding ascorbic acid (60 mg). The RCP evaluated using TLC and HPLC both. Using 300 mCi of LSA ¹⁷⁷LuCl₃ (15 mCi/µg), all the three radioconjugates (80-85 mCi) could be prepared. All the three radioconjugates were clear and colorless, pH (5.5-6.0) and RAC (8-10 mCi/mL). The RCP of all the three radioconjugates estimated by TLC was >98% (R_f: 0.0-0.1), while RCP derived by HPLC was >98% (Rt: 11-13 minutes). The RLY of all the three radioconjugates were between 32-35%. The EL was <6 EU/mL and product was sterile with in-vitro stability upto 96 h (storage at -20℃) ℃with stabilizer. In all the three immunoconjugates, number of DOTA attached per antibody were ~3 and MoAbs concentration were ~2.8 mg/mL. A single, protocol could be successfully optimized for coupling of rituximab, trastuzumab, pertuzumab with p-NCS-benzyl-DOTA. The subsequent radiolabeling with 15-20 mCi/µg of ¹⁷⁷LuCl₃ yielded patient doses of the radioconjugates i.e ¹⁷⁷Lu-DOTA-Rituximab, ¹⁷⁷Lu-DOTA-Trastuzumab and ¹⁷⁷Lu-DOTA-Pertuzumab. Further studies towards clinical translation of all the three radiopharmaceuticals in patient are underway. (author)