Optimization of the DTPA mixed-anhydride reaction with antibodies at low concentration

Diethylenetriaminepentaacetic acid (DTPA) was conjugated with antibody to human serum albumin (Ab) at low concentration (300 micrograms/ml, 2.0 microM) via the DTPA carboxycarbonyl mixed-anhydride method. To study parameters determining the balance between the degree of conjugation and the antibody-binding activity of Ab, a known concentration of the anhydride prepared at isobutylchloroformate (IBC)-to-DTPA ratios of 1, 2.1, or 4.2 was reacted with Ab. The percentage yields of the anhydride were determined by spectrophotometric and gravimetric titration. By the former method the percentage yields, based on DTPA concentration, were 18, 24, and 220, respectively, when the IBC-to-DTPA ratios were 1, 2.1, and 4.2. The corresponding percentage yields were 17, 39, and 262 when determined by the latter method. When the anhydride was prepared at an IBC-to-DTPA ratio of 2.1, an optimum conjugation giving three indium atoms per Ab was obtained, with 64% retention of antibody-binding activity. For an IBC-to-DTPA ratio of 1, the antibody retained almost 100% binding activity but the number of indium atoms incorporated (0.2) was too small. For an IBC-to-DTPA ratio of 4.2, up to 22 indium atoms were incorporated but antibody-binding activity was completely destroyed.