Therapeutic Advantage of High-Affinity Anticarcinoma Radioimmunoconjugates

Abstract The effect of the relative affinity ( K a ) on the antitumor efficacy of monoclonal antibodies (MAbs) has been questioned. It has previously been shown in experimental models that the use of MAbs with higher relative K a s manifests itself in a higher percentage of injected dose of MAb bound to tumor. On the other hand, mathematical models have proposed that the use of higher affinity MAbs may be disadvantageous for antitumor effects, since higher K a MAbs would bind more antigen and prevent penetration of MAb through tumor. To test this hypothesis, three MAbs reacting to the human pancarcinoma antigen TAG-72 were used as radioimmunoconjugates for therapeutic efficacy versus the LS-174T human colon carcinoma xenograft. MAbs B72.3, CC49, and CC83 have all been shown by depletion studies to react to the same molecule and to all react with overlapping epitopes. While the relative K a of B72.3 is 2.5 × 10 9 m −1 , the relative K a s of CC49 and CC83 are 16.2 and 27.7 × 10 9 m −1 , respectively. Each MAb was radiolabeled with 131 I, and each radioimmunoconjugate was assayed at five dose levels for therapeutic efficacy using the human xenograft model. The results of these studies demonstrate substantial therapeutic advantage of the higher affinity MAbs CC49 and CC83 versus B72.3 at every dose level. While 500 µCi of B72.3 were required to reduce tumor growth in only a minority of tumor-bearing animals, the use of the same amount or less of the radioimmunoconjugates of CC49 or CC83 resulted in strong antitumor effects in 80 to 100% of tumor-bearing animals. Thus, stronger antitumor effects were seen using as little as 2.5- to 3-fold less of the higher K a immunoconjugates CC49 and CC83 as compared with B72.3. While we acknowledge the potential disadvantages of higher K a MAbs in some situations, at least the experimental studies and model system described here show that a distinct therapeutic advantage exists with the use of higher affinity immunoconjugates.