Practice question

HER2 positive metastatic breast cancers progressing after trastuzumab plus chemotherapy develop resistance mechanisms including truncated p95HER2 lacking extracellular binding epitope, upregulation of MUC4 masking epitope, activation of downstream PIK3CA H1047R mutation, and alternative receptor tyrosine kinase signaling AXL. Ado-trastuzumab emtansine also called T-DM1 extends HER2 targeting beyond signaling inhibition delivering cytotoxic maytansine chemically. Trastuzumab backbone retains anti-signaling activity via inhibition of HER2 heterodimerization and metalloprotease cleavage, suppression of PI3K-AKT, plus Fc mediated antibody dependent cellular cytotoxicity through FcγRIIIa. Chemical linker SMCC succinimidyl trans-4-(maleimidylmethyl) cyclohexane-1-carboxylate forms stable non-reducible thioether bond between cysteine sulfhydryl of antibody and primary amine of DM1 derivative maytansine that binds tubulin near vinca site with binding affinity 100 times vincristine inhibiting microtubule polymerization, causing aberrant mitosis with multipolar spindles and subsequent mitochondrial apoptosis. After HER2 mediated internalization to lysosome acidic proteases cathepsin B and L degrade antibody polypeptide releasing active metabolite lysine-MCC-DM1 retaining microtubule inhibitory activity while sparing systemic exposure because stable thioether prevents premature release in plasma, mitigating myelosuppression and neuropathy.