HER2-targeting Peptide Drug Conjugate with Better Penetrability for Effective Breast Cancer Therapy

Antibody-drug conjugates (ADCs) have the following advantages: target specificity; wide therapeutic index; and prolonged circulation half-life. A key limitation of ADCs, however, is the large size (∼150 kDa), which markedly slows diffusion through the interstitium of solid tumors and prevents efficient penetration. To address the size issue of ADCs in targeted drug delivery, we developed a HER2-targeting peptide-mertansine conjugate (HER2-TPMC) and conducted a head-to-head comparison with HER2-targeting antibody-mertansine conjugate (HER2-TAMC) as a possible alternative for high-penetration breast cancer therapeutics. As expected, a pharmacokinetic (PK) assay revealed that HER2-TP had lower levels persisting in the circulation after 1 h (∼75%) compared to 85% of HER2-targeting antibody (HER2-TA). The cellular cytotoxic effect of HER2-TPMC was similar to HER2-TAMC in the HER2+ BT474 breast cancer cell line, thus demonstrating similar bioactivity of both conjugates. HER2-TPMC not only revealed higher uptake and specificity in in vitro 3D spheroid cultures compared to the parental drug, mertansine, but HER2-TPMC also had a significant retention in the spheroids. This finding was in stark contrast to HER2-TAMC, a large-sized conjugate which was not able to penetrate the spheroid barrier, thus resulting minimal penetration. In vivo tumoral uptake in a BT474 orthotopic model indicated increased tumor uptake and penetration of HER2-TP compared to parental drug and HER2-TAMC. To summarize, we successfully developed a HER2-targeting peptide-mertansine conjugate with specific cellular uptake that resulted in longer retention times in vitro and in vivo. HER2-TPMC (∼5 kDa in size) exhibited rapid tissue penetration and enhanced tumoral uptake and retention in vitro and in vivo. Therefore, HER2-TPMC is a reasonable alternative for HER2-positive cancer chemotherapeutics.