Antibody-drug conjugates (ADC) represent a promising therapeutic modality for the clinical management of cancer. We sought to develop a novel ADC that targets 5T4, an oncofetal antigen expressed on tumorinitiating cells (TIC), which comprise the most aggressive cell population in the tumor. We optimized an anti-5T4 ADC (A1mcMMAF) by sulfydryl-based conjugation of the humanized A1 antibody to the tubulin inhibitor monomethylauristatin F (MMAF) via a maleimidocaproyl linker. A1mcMMAF exhibited potent in vivo antitumor activity in a variety of tumor models and induced long-term regressions for up to 100 days after the last dose. Strikingly, animals showed pathologic complete response in each model with doses as low as 3 mg antibody/kg dosed every 4 days. In a non-small cell lung cancer patient-derived xenograft model, in which 5T4 is preferentially expressed on the less differentiated tumor cells, A1mcMMAF treatment resulted in sustained tumor regressions and reduced TIC frequency. These results highlight the potential of ADCs that target the most aggressive cell populations within tumors, such as TICs. In exploratory safety studies, A1mcMMAF exhibited no overt toxicities when administered to cynomolgus monkeys at doses up to 10 mg antibody/kg/ cycle  2 and displayed a half-life of 5 days. The preclinical efficacy and safety data established a promising therapeutic index that supports clinical testing of A1mcMMAF. Mol Cancer Ther; 12(1); 38-47. Ó2012 AACR.
Poorly differentiated tumors in non-small cell lung cancer (NSCLC) have been associated with shorter patient survival and shorter time to recurrence following treatment. Here, we integrate multiple experimental models with clinicopathologic analysis of patient tumors to delineate a cellular hierarchy in NSCLC. We show that the oncofetal protein 5T4 is expressed on tumor-initiating cells and associated with worse clinical outcome in NSCLC. Coexpression of 5T4 and factors involved in the epithelial-to-mesenchymal transition were observed in undifferentiated but not in differentiated tumor cells. Despite heterogeneous expression of 5T4 in NSCLC patient-derived xenografts, treatment with an anti-5T4 antibody-drug conjugate resulted in complete and sustained tumor regression. Thus, the aggressive growth of heterogeneous solid tumors can be blocked by therapeutic agents that target a subpopulation of cells near the top of the cellular hierarchy. Cancer Res; 71(12); 4236-46. Ó2011 AACR.
As the antibody drug conjugate (ADC) community continues to shift towards site-specific conjugation technology, there is a growing need to understand how the site of conjugation impacts the biophysical and biological properties of an ADC. In order to address this need, we prepared a carefully selected series of engineered cysteine ADCs and proceeded to systematically evaluate their potency, stability, and PK exposure. The site of conjugation did not have a significant influence on the thermal stability and in vitro cytotoxicity of the ADCs. However, we demonstrate that the rate of cathepsin-mediated linker cleavage is heavily dependent upon site and is closely correlated with ADC hydrophobicity, thus confirming other recent reports of this phenomenon. Interestingly, conjugates with high rates of cathepsin-mediated linker cleavage did not exhibit decreased plasma stability. In fact, the major source of plasma instability was shown to be retro-Michael mediated deconjugation. This process is known to be impeded by succinimide hydrolysis, and thus, we undertook a series of mutational experiments demonstrating that basic residues located nearby the site of conjugation can be a significant driver of succinimide ring opening. Finally, we show that total antibody PK exposure in rat was loosely correlated with ADC hydrophobicity. It is our hope that these observations will help the ADC community to build "design rules" that will enable more efficient prosecution of next-generation ADC discovery programs.
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