Comparison of Pyrrolobenzodiazepine Dimer Bis-imine versus Mono-imine: DNA Interstrand Cross-linking, Cytotoxicity, Antibody–Drug Conjugate Efficacy and Toxicity

Antibody-drug conjugates (ADCs) delivering pyrrolobenzodiazepine (PBD) DNA crosslinkers are currently being evaluated in clinical trials with encouraging results in Hodgkin and non-Hodgkin lymphomas. The first example of an ADC delivering a PBD DNA crosslinker (loncastuximab tesirine) has been recently approved by the U.S. Food and Drug Administration for the treatment of relapsed and refractory diffuse large B-cell lymphoma. There has also been considerable interest in mono-alkylating PBD analogues. We conducted a head-to-head comparison of a conventional PBD bis-imine and a novel PBD mono-imine. Key Mitsunobu chemistry allowed clean and convenient access to the mono-imine class. Extensive DNA binding studies revealed that the mono-imine mediated a type of DNA interaction that is described as "pseudo-crosslinking," as well as alkylation. The PBD mono-imine ADC demonstrated robust antitumor activity in mice bearing human tumor xenografts at doses threefold higher than those that were efficacious for the PBD bis-imine ADC. A single-dose toxicology study in rats demonstrated that the maximum tolerated dose of the PBD mono-alkylator ADC was approximately threefold higher than that of the ADC bearing a bis-imine payload, suggesting a comparable therapeutic index for this molecule. However, although both ADCs caused myelosuppression, renal toxicity was observed only for the bis-imine after repeated dosing, indicating possible differences in toxicological profiles that could influence tolerability and therapeutic index. These data show that mono-amine PBDs have physicochemical and pharmaco-toxicological properties distinct from their crosslinking analogues and support their potential utility as a novel class of ADC payload.