Long-Term Kinetics of Antibody-Toxin Conjugates Show Resistant Cells
A monocional anti-Thy1.1-ricin A chain conjugate (OX-7-A chain) inactivated protein synthesis in Thy1.1 positive cells with first order kinetics for 15 h. After this period, the surviving cells began growing again. The maximal rate of protein synthesis inactivation was one log inactivation each 10 h and was reached when the cell surface antigen was saturated with conjugate. Therefore, the maximal inactivation of one addition of conjugate over the 15 h lifetime would leave 3% survivors. If the ricin B chain was added to cells treated with OX-7-A chain, the maximal rate of protein synthesis inactivation was increased three-fold, but the length of time the conjugate was effective (15 h) was not changed. Extrapolation shows that the maximum cell killing with OX-7-A chain plus B chain would leave only 0.003% survivors.
We tested the effect of a second addition of conjugate after the first had reached its maximal inhibition of protein synthesis. OX-7-A was incubated at subsaturating concentrations with cells for 30 h and the survivors were then treated with a second addition of OX-7-A at the same concentration as the first. The survivors were inhibited by the second incubation only 5–20% as much as the initial cell population was inhibited by the first addition. Identical results were found for second additions of OX-7-A plus B and for OX-7-ricin plus lactose. Therefore, repeated additions of conjugate do not give repeated inhibition of cell growth. This result is consistent with a model that the target cell population was heterogenous in sensitivity to antibody-toxin conjugates and that the differences between cells were stable for 46 h.
KeywordsCell Surface Antigen Shiga Toxin Protein Synthesis Rate Repeated Inhibition Chain Conjugate
Unable to display preview. Download preview PDF.