Design and evaluation of mRNA translation inhibitors for use as antibody drug conjugate payloads

Introduction: Eukaryotic initiation factor 4A (eIF4A) mediates recruitment of ribosomes to mRNA and catalyzes the unwinding of mRNA secondary structure, thereby facilitating ribosome scanning and translation initiation. Inhibitors of eIF4A (e.g., silvestrol, zotatifin, and other rocaglates) block translation of select mRNAs that contain a high degree of secondary structure (e.g. mRNAs encoding MYC, KRAS, and CDK4/6, among others), resulting in inhibition of cell proliferation and induction of apoptosis. Despite limited efficacy observed for zotatifin in a phase I clinical trial, antibody conjugation of a novel eIF4A payload offers the possibility of enhanced exposure and improved tolerability, which could lead to more robust responses. Furthermore, eIF4A antibody-drug conjugates (ADCs) would constitute a novel mechanism with the potential to address payload-specific resistance mechanisms that have been reported following progression on ADCs with camptothecin payloads. Methods: A panel of novel rocaglamide-derived eIF4A inhibitors containing linkable functional groups were synthesized and evaluated for potency, hydrophilicity, and DMPK properties. Select analogs were elaborated into drug-linkers containing a hydrophilic polyethylene glycol (PEG) side chain to minimize hydrophobicity. Conjugation to endogenous cysteines using maleimide chemistry provided ADCs targeting diverse tumor associated antigens that were evaluated for in vitro potency, in vivo efficacy, and tolerability. Results: ADCs carrying eIF4A inhibitors with a drug-to-antibody ratio of 8 required use of hydrophilic PEG groups to minimize aggregation. In vitro, trastuzumab ADCs showed potent antigen-dependent activity in HER2-expressing cell lines and bystander activity in mixed co-cultures of HER2+ and HER2- cells. Selection of lead drug-linkers was based on robust in vivo anti-tumor activity of HER2 targeted ADCs in NCI-N87 and SKOV-3 CDX models as well as tolerability in non-tumor bearing mice and Sprague Dawley rats. The breadth of efficacy of the lead drug-linkers was evaluated in vivo using Ly6E, PTK7, TROP2, cMET, and EGFR targeted ADCs. Conclusions: Novel eIF4A ADCs constructed using hydrophilic drug linkers demonstrated potent and target-specific in vitro cytotoxicity. Promising in vivo efficacy was observed across multiple tumor associated targets, however the observed toxicity level in both mice and rats suggests a potentially lower than anticipated preclinical therapeutic window.