Antibody-Mediated Delivery of Chimeric BRD4 Degraders. Part 1: Exploration of Antibody Linker, Payload Loading, and Payload Molecular Properties

The biological and medicinal impacts
of proteolysis-targeting chimeras
(PROTACs) and related chimeric molecules that effect intracellular
degradation of target proteins via ubiquitin ligase-mediated ubiquitination
continue to grow. However, these chimeric entities are relatively
large compounds that often possess molecular characteristics, which
may compromise oral bioavailability, solubility, and/or in vivo pharmacokinetic
properties. We therefore explored the conjugation of such molecules
to monoclonal antibodies using technologies originally developed for
cytotoxic payloads so as to provide alternate delivery options for
these novel agents. In this report, we describe the first phase of
our systematic development of antibody–drug conjugates (ADCs)
derived from bromodomain-containing protein 4 (BRD4)-targeting chimeric
degrader entities. We demonstrate the antigen-dependent delivery of
the degrader payloads to PC3-S1 prostate cancer cells along with related
impacts on MYC transcription and intracellular BRD4 levels. These
experiments culminate with the identification of one degrader conjugate,
which exhibits antigen-dependent antiproliferation effects in LNCaP
prostate cancer cells.