GD2-Targeted Minibody–Drug Conjugates Match the Potency of IgG-Based ADCs in a Mouse Cancer Model

Despite the clinical success of antibody–drug conjugates (ADCs), their efficacy in solid tumors remains constrained by limited tumor penetration of the IgG format. Smaller antibody fragment–drug conjugates (FDCs) present a compelling alternative, potentially offering superior intratumoral distribution and a wider therapeutic window driven by rapid systemic clearance. This study compares therapeutic activity of ganglioside GD2-specific minibody–drug conjugates against full-length ch14.18 antibody–drug conjugates, and biodistribution of the respective minibody (scFv-CH3 homodimer) and IgG formats in the GD2-positive B78-D14 melanoma syngeneic mouse model. We conjugated the minibody and antibody with MMAE or MMAF via a cathepsin-cleavable linker, generating FDCs with drug–antibody ratio (DAR) of 2 and ADCs with DAR of 2 or 4. The biodistribution analysis showed no significant difference in tumor uptake for both formats early in the analysis (2–4 h) and a higher tumor uptake for the IgG at 24 h post-injection. However, the minibody achieved a superior tumor-to-blood ratio (TBR) at all timepoints, reaching a TBR > 1 compared to ~0.2 for the antibody by 24 h. In vitro studies demonstrated higher cytotoxicity for the ADCs regardless of drug load (DAR 2 or 4) compared to the FDCs, although the difference between conjugates with equal DAR was modest in B78-D14 cells. Critically, superior in vitro ADC potency did not translate in vivo. Minibody–MMAF and minibody–MMAE achieved 74% and 55% tumor growth inhibition, respectively, by the study endpoint—demonstrating comparable efficacy to ADCs with twice the drug load when administered to mice at equimass dosing. Stron/g in vivo efficacy of anti-GD2 FDCs, combined with the superior TBR for the minibody format, underscores the potential of minibody–drug conjugates for treating GD2-positive tumors, particularly when ADC-associated toxicity precludes high-dose regimens.