Toxicity of ligand and antibody‐directed ricin A‐chain conjugates recognizing the epidermal growth factor receptor

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Abstract
Approximately equal amounts of 125I‐mAb 225 (a monoclonal antibody recognizing the human epidermal growth factor receptor) and 125I‐labeled epidermal growth factor (125I‐EGF) were bound by HeLa cells. However, these two EGF receptor binding moieties had different fates after binding. Sixty percent of cell‐associated 125I‐EGF was internalized. The majority of internalized 125I was released from the cell within 2 hr. In contrast, whereas only 30% of bound 125I‐mAb 225 was internalized by HeLa cells, the internalized radioactivity remained cell‐associated. EGF and mAb 225 were used to construct ricin A‐chain (RTA) conjugates. The two chimeric molecules, EGF‐RTA and mAb 225‐RTA, were equally toxic to human HeLa cells. EGF‐RTA was also toxic to murine 3T3 cells. In contrast, mAb 225‐RTA was not toxic to 3T3 cells, consistent with the human EGF‐receptor specificity of mAb 225. Neither conjugate was cytotoxic to EGF‐receptor‐deficient 3T3‐NR6 cells. Rapidity and potency of protein synthesis inhibition of HeLa cells were equivalent for the two chimeric conjugates, as was the degree to which colony‐forming ability was reduced. However, ammonium chloride enhanced the toxicity of EGF‐RTA but not mAb 225‐RTA, suggesting that the two toxic chimeric toxins‐‐like the unconjugated receptor‐binding moieties‐‐are processed differently by HeLa cells.

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