Comparison of the monoclonal antibodies 17-1A and 323/A3: The influence of the affinity on tumour uptake and efficacy of radioimmunotherapy in human ovarian cancer xenografts

The low-affinity monoclonal antibody (MAb) chimeric 17-1A (c-17-1A) and the high-affinity MAb mouse 323/A3 (m-323/A3) were used to study the effect of the MAb affinity on the tumour uptake and efficacy of radioimmunotherapy in nude mice bearing subcutaneously the human ovarian cancer xenografts FMa, OVCAR-3 and Ov.Pe. Both MAbs are directed against the same pancarcinoma glycoprotein. In vitro, the number of binding sites on tumour cells at 4°C was similar for both MAbs, but m-323/A3 had an approximately 5-fold higher affinity (1.3-3.0 x 10⁹ M^-1) than c-17-1A (3.0-5.4 x 10⁸ M^-1). This difference in affinity was more extreme at 37°C, when no binding of c-17-1A could be observed. MAb m-323/A3 completely blocked the binding of c-17-1A to tumour cells, whereas the reverse was not observed. Immunohistochemistry showed a similar but more intense staining pattern of m-323/A3 in human ovarian cancer xenografts than of c-17-1A. In vivo, the blood clearance in non-tumour-bearing nude mice was similar for both MAbs with terminal half-lives of 71.4 h for m-323/A3 and 62.7 h for c-17-1A. MAb m-323/A3 targeted better to tumour tissue, but was more heterogeneously distributed than c-17-1A. The cumulative absorbed radiation dose delivered by m-323/A3 to tumour tissue was 2.5- to 4.7-fold higher than that delivered by c-17-1A. When mice were treated with equivalent radiation doses of [¹³¹I]m-323/A3 and [¹³¹I]c-17-1A, based on a correction for the immunoreactivity of the radiolabelled MAbs, m-323/A3 induced a better growth inhibition in two of the three xenografts. When the radiation doses were adjusted to obtain a similar amount of radiation in the tumour c-17-1A was more effective in tumour growth inhibition in all three xenografts.