Albumin binding domain derived affinity
proteins (ADAPTs) are a class of small and folded engineered scaffold
proteins that holds great promise for targeting
cancer tumors. Here, we have extended the in vivo half-life of an ADAPT, targeting the
human epidermal growth factor receptor 2 (HER2) by fusion with an
albumin binding domain (ABD), and armed it with the highly cytotoxic payload
mertansine (DM1) for an investigation of its properties in vitro and in vivo. The resulting drug conjugate, ADAPT6-ABD-mcDM1, retained binding to its intended targets, namely HER2 and serum
albumins. Further, it was able to specifically bind to cells with high HER2 expression, get internalized, and showed potent toxicity, with IC50 values ranging from 5 to 80 nM. Conversely, no toxic effect was found for cells with low HER2 expression. In vivo, ADAPT6-ABD-mcDM1, radiolabeled with 99mTc, was characterized by low uptake in most normal organs, and the main excretion route was shown to be through the kidneys. The
tumor uptake was 5.5% ID/g after 24 h, which was higher than the uptake in all normal organs at this time point except for the kidneys. The uptake in the
tumors was blockable by pre-injection of an excess of the
monoclonal antibody trastuzumab (having an overlapping
epitope on the HER2 receptor). In conclusion, half-life extended drug conjugates based on the ADAPT platform of affinity
proteins holds promise for further development towards targeted
cancer therapy.