Cell-targeted serine protease constructs are provided. Such constructs can be used in methods for targeted cell killing such as for treatment cell of proliferative diseases (e.g., cancer). In some aspects, recombinant serine proteases, such as Granzyme B polypeptides, are provided that exhibit improved stability and cell toxicity. Methods and compositions for treating lapatinib or trastuzumab-resistant cancers are also provided.