Influence of endosome-destabilizing peptides on efficacy of anti-HIV immunotoxins.

The effects on immunotoxin efficacy of fusogenic peptides derived from influenza virus hemagglutinin have been studied. These peptides have an amphipathic nature and change conformation from random at pH 7 to helical at pH 5. Fusogenic peptides are reported to destabilize endosomal membranes, resulting in the release of contents into the cytoplasm. The use of two related fusogenic peptides to enhance the efficacy of anti-HIV immunotoxins is described. The direct toxicity of the peptides was tested on HIV-infected H9/NL4-3 cells. Peptide HA24 was considerably more toxic than HA23. The peptides were mixed with two different immunotoxins. Immunotoxin action was enhanced by both peptides, with HA24 providing greater enhancement than HA23. Immunotoxins were then constructed by coupling HA23 or HA24 to the targeting antibody with disulfide-containing linkers. Peptide HA23 enhanced the activity of the immunotoxin 4-5-fold. Surprisingly, HA24 significantly inhibited immunotoxin activity. Coupling the peptides to the immunotoxin had no effect on antigen binding characteristics or the activity of the toxic moiety. Bafilomycin A1, an agent that inhibits vacuolar acidification, markedly potentiated the effects of all immunotoxins. These results demonstrate that amphipathic peptides can influence the efficacy of immunotoxins, but in sometimes unpredictable ways.