Optimized conditions for chelation of Yttrium-90-DOTA immunoconjugates

Radioimmunotherapy (RIT) with Y-90-labeled immunoconjugates has shown promi se in clinical trials. The macrocyclic chelating agent 1,4,7,10-tetraazacyc lododecane-N,N',N ",N"'-tetraacetic acid (DOTA) binds Y-90 with extraordina ry stability, minimizing the toxicity of Y-90-DOTA immunoconjugates arising from loss of 90Y to bone. However, reported Y-90-DOTA immunoconjugate prod uct yields have been typically only less than or equal to 50%. Improved yie lds are needed for RIT with Y-90-DOTA immunoconjugates to be practical. Met hods: (S) 2-[p-(bromoacetamido)benzyl]-DOTA (BAD) was conjugated to the mon oclonal antibody Lym-1 via 2-iminothiolane (2lT). The immunoconjugate produ ct, 2lT-BAD-Lym-1, was labeled in excess yttrium in various buffers over a range of concentrations and pH. Kinetic studies were performed in selected buffers to estimate radiolabeling reaction times under prospective radiopha rmacy labeling conditions. The effect of temperature on reaction kinetics w as examined. Optimal radiolabeling conditions were identified and used in e ight radiolabeling experiments with 2lT-BAD-Lym-1 and a second immunoconjug ate, DOTA-peptide-chimeric L6, with 248-492 MBq (6.7-13.3 mCi) of Y-90. Res ults: Ammonium acetate buffer (0.5 M) was associated with the highest uptak e of yttrium. On the basis of kinetic data, the time required to chelate 94 % of 90Y (four half-times) under prospective radiopharmacy labeling conditi ons in 0.5 M ammonium acetate was 17-148 min at pH 6.5, but it was only 1-1 0 min at pH 7.5. Raising the reaction temperature from 25 degrees C to 37 d egrees C markedly increased the chelation rate. Optimal radiolabeling condi tions were identified as: 30-min reaction time, 0.5 NI ammonium acetate buf fer, pH 7-7.5 and 37 degrees C. In eight labeling experiments under optimal conditions, a mean product yield (+/- s.d.) of 91% +/- 8% was achieved, co mparable to iodination yields. The specific activity of final products was 74-130 MBq (2.0-3.5 mCi) of 90Y per mg of monoclonal antibody. The immunore activity of Y-90-labeled immunoconjugates was 100% +/- 11%. Conclusion: The optimization of Y-90-DOTA chelation conditions represents an important adv ance in Y-90 RIT because it facilitates the dependable and cost-effective p reparation of Y-90-DOTA pharmaceuticals.