A number of cytotoxic pyrrolobenzodiazepine (PBD) monomers containing various disulfide-based prodrugs were evaluated for their ability to undergo activation (disulfide cleavage) in vitro in the presence of either glutathione (GSH) or cysteine (Cys).
Pyrrolobenzodiazepines, a class of natural products produced by various actinomycetes (a broad group of bacteria that form thread-like filaments in the soil and are responsible for the distinctive scent of freshly exposed, moist soil), are sequence selective DNA alkylating compounds with significant antitumor properties. As a class of DNA-crosslinking agents they are significantly more potent than systemic chemotherapeutic drugs. Some PBDs have the ability to recognize and bond to specific sequences of DNA.
In the study, researchers observed a good correlation between in vitro GSH stability and in vitro cytotoxicity toward tumor cell lines. The prodrug-containing compounds were typically more potent against cells with relatively high intracellular GSH levels (e.g., KPL-4 cells). Several antibody-drug conjugates (ADCs) were subsequently constructed from pyrrolobenzodiazepine (PBD) dimers that incorporated selected disulfide-based prodrugs.
Such HER2 conjugates exhibited potent antiproliferation activity against KPL-4 cells in vitro in an antigen-dependent manner.
However, the disulfide prodrugs contained in the majority of such entities were surprisingly unstable toward whole blood from various species. One HER2-targeting conjugate that contained a thiophenol-derived disulfide prodrug was an exception to this stability trend.
It exhibited potent activity in a KPL-4 in vivo efficacy model that was approximately 3-fold weaker than that displayed by the corresponding parent ADC. The same prodrug-containing conjugate demonstrated a 3-fold improvement in mouse tolerability properties in vivo relative to the parent ADC which did not contain the prodrug.