Kinetic analysis of cytotoxicity

Abstract

The action of immunotoxins (ITs) on target cells can be divided into three steps: 1) binding to the cell-surface target antigen, 2) translocation from the cell surface membrane to the cytosol compartment, and 3) inactivation of intracellular substrate by the toxin moiety [1]. IT binding to target cell is typically very fast. In most cases the binding of radiolabeled monoclonal antibody to target cells reaches saturation within 30 minutes [2]. It has also been shown that removal of unbound IT 10 minutes after treatment yielded the same degree of target cell intoxication in vitro as that obtained without removing the unbound IT [3]. The inactivation of ribosomes by toxin in a cell-free system was completed in 30 minutes [4]. With few exceptions [5, 6, 17], however, the kinetics of protein synthesis inhibition in intact cells by an IT is generally much slower; often from several hours to 60 hours of incubation is necessary to reach 50% protein synthesis inhibition (see Table 1). This suggests that the translocation of IT from the cell surface to the cytosol is the limiting step for intoxication.