Transferrin toxin but not transferrin receptor immunotoxin is influenced by free transferrin and iron saturation

Background Cytotoxic agents can be targeted successfully to cancer cells. The efficacy of such novel and potent anticancer strategies may be influenced by variables of iron metabolism. Methods The in vitro cytotoxicity against glioma cells of transferrin (Tf)‐based targeted toxins was compared with that of α‐transferrin receptor (TfR)‐immunotoxin. Results Of four Tf‐based targeted toxins, Tf‐gelonin, Tf‐pokeweed antiviral protein, Tf‐momordin and Tf‐saporin, inhibitory concentration 50% values against glioma‐derived cell lines HS683 and U251, ranged from [4·8 ± 1·5] × 10−10 m for Tf‐saporin to [26·9 ± 15·3] × 10−10 m for Tf‐gelonin in [3H]‐leucine incorporation assays. Tf‐saporin and α‐TfR‐saporin‐immunotoxin had similar efficacy, even in the more quantitative clonogenic assay (4–5 log kill with 1 × 10−9 m) using the myeloma cell line RPMI 8226 and glioma cell line U251. However, on RPMI 8226, the efficacy of Tf‐saporin 1 × 10−9 m was reduced by 90% in the presence of 150 µg mL−1 (=20% of normal plasma value) competing diferric transferrin, whereas the efficacy of the corresponding immunotoxin was affected only marginally. In addition, the efficacy of Tf‐based conjugates will depend on their iron saturation state. Iron desaturation of Tf‐saporin was demonstrated by [59Fe]‐labelling, subsequent CM‐Sepharose chromatography and SDS‐PAGE. Desaturation led to virtually complete loss of affinity for the transferrin receptor, as determined by flow cytometry, which could be largely restored upon resaturation. Conclusion Transferrin‐based toxin conjugates are strongly influenced by the presence of free transferrin and the iron saturation state. The corresponding α‐transferrin receptor‐immunotoxin does not show these disadvantages, has similar efficacy and should be preferred for further experiments.