Although five ADCs have been approved and over sixty others are in development, the majority contain payloads belonging to two classes: tubulin inhibitors and DNA-interactive agents. Challenges in the development of ADCs include managing off-target toxicity and hydrophobicity. Some DNA-interactive payload classes [e.g. pyrolobenzodiazepine (PBD] dimers) are notably hydrophobic, leading to problems such as aggregation during conjugation, and systemic toxicities of the resultant ADCs are also beginning to emerge in clinical settings. Thus, there is interest in developing novel payloads which retain the potency of DNA cross-linking agents but have lower hydrophobicity and a wider therapeutic window when part of an ADC. The pyridinobenzodiazepines (PDDs) are a new class of sequence-selective, DNA mono-alkylating ADC payload, which contain a polyheterocyclic chain with sufficient molecular span and DNA base-pair recognition properties to guide them to specific DNA sequences (e.g. transcription factor binding sites). The favourable hydrophobicity profile of the PDDs and ease of conjugation, along with their novel mechanism of action, significant in vitro cytotoxicity and in vivo tolerability and efficacy when in an ADC format, indicate that they represent a promising new class of ADC payloads.