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Relative Reactivity of DTPA, Immunoreactive Antibody-DTPA Conjugates, and Nonimmunoreactive Antibody-DTPA Conjugates Toward Indium-111

Radiopharmaceutical/Medicinal Chemistry Section, The George Washington University Medical Center, Washington, DC

Correspondence: For reprints contact: Chang H. Paik, PhD, 2300 I. Street NW, Washington, DC 20037.

ABSTRACT

Anti-human serum albumin antibody (Ab) was reacted with cyclic DTPA dianhydride (cDTPAA) at various cDTPAA/Ab molar ratios between 1 and 40. Using a carrier ln titration method for DTPA and DTPA-antibody conjugate (Ab-DTPA), we determined that the above reactions produced between 0.1 and 11 DTPA molecules per either immunoreactive antibody (sAb) or nonimmunoreactive antibody (nAb). The percentage of sAb remaining after the above reactions was between 88 and 62%. The reaction of no-carrier-added ¹¹¹ln with the reaction mixture from cDTPAA/Ab molar ratios of 1 to 40 gave radiochemical yields < 25% for the respective Ab-DTPA. The rest of the ¹¹¹ln activity was associated with free DTPA. Our results indicate that Ab-DTPA containing > 1 DTPA molecule per Ab is more reactive than that containing < 1 DTPA but is about as reactive as free DTPA. This allows us to label in the presence of free DTPA and consequently prevent colloid formation. The percentage of ¹¹¹ln activity incorporated into sAb-DTPA from the reactions at these molar ratios was similar to that found from the uv analysis. This indicates that the reactivity of sAb-DTPA and nAB-DTPA from the same conjugation reaction is similar. As a result, we were able to conjugate about one DTPA molecule to the Ab without causing deactivation of the Ab and label it with ¹¹¹ln in the presence of excess DTPA. We obtained a specific activity of 6 µCi ¹¹¹ln per µg of Ab using research grade ¹¹¹ln without further purification.

FOOTNOTES

^* Present address: Department of Nuclear Medicine, Clinical Center, National Institute of Health, Bethesda, MD 20205.

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