Combining intracellular antibodies to restore function of mutated p53 in cancer

TP53 is a tumor suppressor gene that is mutated in 50% of cancers, and its function is tightly regulated by the E3 ligase, Mdm2. Both p53 and Mdm2 are localized in the cell nucleus, a site that is impervious to therapeutic regulation by most antibodies. We identified a cell-penetrating lupus monoclonal anti-DNA antibody, mAb 3E10, that targets the nucleus, and we engineered mAb 3E10 to function as an intranuclear transport system to deliver therapeutic antibodies into the nucleus as bispecific single chain Fv (scFv) fragments. Bispecific scFvs composed of 3E10 include PAb421 (3E10-PAb421) that binds p53 and restores the function of mutated p53, and 3G5 (3E10-3G5) that binds Mdm2 and prevents destruction of p53 by Mdm2. We documented the therapeutic efficacy of these bispecific scFvs separately in previous studies. In this study, we show that combination therapy with 3E10-PAb421 and 3E10-3G5 augments growth inhibition of cells with p53 mutations compared to the effect of either antibody alone. By contrast, no enhanced response was observed in cells with wild-type p53 or in cells homozygous null for p53. What's new? About half of all cancers carry mutations in p53, and some cancers also overexpress the oncogenic E3 ligase Mdm2, which normally regulates p53 nuclear export and degradation. Nuclear localization of p53 and Mdm2 have kept the molecules largely beyond the reach of therapeutics, but as this study shows, both can be rendered susceptible to therapies via a combination of bispecific single-chain antibody Fv fragments (scFvs). Bispecific scFvs constructed from mAb 3E10 bound to p53 and restored its function. They also bound to Mdm2, blocking p53 degradation. Combined intracellular antibodies could augment cell-killing effects when used alongside conventional cancer treatments.