Influence of cellular trafficking on protein synthesis inhibition of immunotoxins directed against the transferrin receptor.

Previously, a quantitative analysis that related protein synthesis inhibition of transferrin-toxin conjugates to the cellular trafficking of transferrin was proposed (P.T. Yazdi and R. M. Murphy, Cancer Res., 54: 6387-6394, 1994). Here, this work is extended to evaluate cellular trafficking of anti-transferrin receptor antibodies and protein synthesis inhibition kinetics of immunotoxins constructed from the same antibodies and the toxin gelonin. Cellular trafficking models for two monoclonal anti-transferrin receptor antibodies (5E9 and OKT9) in HeLa cells were developed. The two mAbs had similar trafficking parameters, which differed significantly from those for transferrin. Protein synthesis inhibition kinetics for immunotoxins constructed from 5E9 or OKT9 and gelonin were measured. Analysis of the data using our previously proposed relationship between protein synthesis and cellular trafficking indicated that the relationship is also valid for these new systems. The protein synthesis inhibition constants for 5E9-gelonin and OKT9-gelonin conjugates were similar to those for the transferrin-gelonin conjugate. These results suggest that it may be possible to predict the efficacy of gelonin immunotoxins from knowledge of the trafficking of the corresponding targeting agent. A sensitivity analysis showed which cellular trafficking parameters have the greatest influence on immunotoxin efficacy and are, therefore, the most likely to be profitably manipulated.