ANT-043

Antibody–drug conjugates (ADC) are poised to embed themselves as pillars of cancer therapy after decades of development and fine-tuning. The vast majority of those in preclinical and clinical development are based on full-length immunoglobulins employing a variety of linker–payloads and conjugation strategies. Other, smaller formats are being considered to overcome some of the current limitations of ADCs, notably poor solid tumor penetration and prolonged systemic toxin exposure. By combining stable, high–lysine-containing single-chain variable antibody fragments, compact hydrophilic linkers, and a validated payload monomethyl auristatin E (MMAE), high–drug:antibody ratio (DAR) antibody fragment–drug conjugates (FDC) were made, which retained significant binding and developability properties. Against the established target HER2, an average DAR six FDC was reproducibly obtained (equivalent to a DAR 30 ADC by mass) with picomolar binding affinity, low aggregation, and translatable pharmacokinetics. Despite the faster elimination kinetics, rapid and intense tumor payload delivery was seen leading to tumor cure efficacy in multiple HER2 tumor xenografts at dosages as low as 0.6 mg/kg given weekly four times. Internalization and tumor uptake quantification data illustrate the benefits of the higher-penetrating format. Experience with more than a dozen linker–payload structures has provided an insight into the critical design features that could make FDCs a viable alternative to ADCs in the most challenging solid tumor indications.