Anti-HIV Immunotoxin and Antibody-Drug Conjugate Targeted by the Same Anti-gp41 Antibody Display Distinct Modes of Cellular Killing

Background: We are developing cytotoxic immunoconjugates (CICs) to eradicate the persistent reservoir of HIV infection, the barrier to a cure. Having identified the most effective monoclonal antibodies (mAbs) for targeting CICs to HIV-infected cells, we investigated the efficacy and mode of killing of different forms of CICs targeted by the same mAb, an immunotoxin (IT) and antibody drug conjugate (ADC). Methods: We compared the in vitro effects of CICs made by conjugating anti-gp41 mAb 7B2 to either deglycosylated ricin A chain (7B2-dgA) or the anthracycline derivative PNU-159682 (7B2-PNU). Cytotoxicity was tested on cell lines stably expressing the HIV envelope. Metabolic and transcriptional analyses of treatment effects were performed on cells persistently infected with HIV. Results: 7B2-dgA was more potent and acted more rapidly to kill cells. 7B2-PNU induced bystander-cell killing but stimulated cell growth upon low dose or brief exposure. Six hr post treatment, 7B2-dgA elicited both metabolic and transcriptional alterations, whereas 7B2-PNU treated cells did not differ from untreated cells. At 24 hr, the profiles following both treatments differed from untreated cells and from each other. 7B2-dgA treated cells exhibited elevated intracellular levels of many AAs, and early activation of gene pathways for apoptosis and transcriptional control; later transcriptional changes included decreases in expression of enzymes involved in AA synthesis and carbon metabolism. Conclusions: An IT and ADC, each delivered by the same antibody and tested in the same cell lines showed substantial differences in their mode of killing, of potential clinical significance. The results of this side-by-side comparison of an ADC and IT have implications for the development of CICs to treat other conditions, including cancer.