Clinical Trials with Pseudomonas Exotoxin Immunotoxins

Abstract

Pseudomonas exotoxin (PE) has been used to make immunotoxins for cancer therapy for more than a decade (FitzGerald et al. 1983). The function of PE is described in detail elsewhere in this book. In summary, PE is a 613 amino acid (66kDa) single-chain protein secreted by Pseudomonas aeruginosa. X-ray crystallography (Allured et al. 1986) and mutational studies (Gray et al. 1984; Hwang et al. 1987) have shown that PE is composed of three major structural and functional domains: an NH₂-terminal cell binding domain (domain Ia, composed of amino acids 1–252), a central translocation domain (domain II, amino acids 253–364), and a COOH-terminal domain (III, amino acids 399–613). The latter catalyzes the ADP-ribosylation and inactivation of elongation factor 2 and thereby inhibits protein synthesis and leads to cell death. Domain III contain a COOH-terminal sequence (REDLK) that directs the endocytosed and processed toxin into the endoplasmic reticulum (see below). Substitution of REDLK with a KDEL sequence, which is known to retain newly synthesized proteins in the endoplasmic reticulum (Seetharam et al. 1991), results in a PE molecule that is more toxic to cells probably because it is more efficiently brought to the endoplasm reticulum where translocation seems to occur. Domain Ib is composed of amino acids 365–399 and has no known function; deletion of all of this domain results in no loss of activity (Fig. 1).