Practice question

Antibody-drug conjugates integrate precision of biologics with potency of small molecules to overcome poor therapeutic window of conventional chemotherapy that diffuses into all tissues causing systemic toxicity. Monoclonal antibody component provides tumor-selective homing through high affinity binding Kd 10^-9 to 10^-11 M to antigens preferentially expressed on malignant cells ten to hundred fold over normal tissues, such as HER2, CD30, CD22, and internalization via receptor mediated endocytosis clathrin dependent trafficking to early endosome then late endosome fusing with lysosome pH 4.5 containing cathepsins B and L. Chemical linker connecting payload determines release kinetics: cleavable valine-citrulline dipeptide sensitive to lysosomal cathepsin B with PABC self-immolative spacer, acid-labile hydrazone stable at neutral pH 7.4 but hydrolyzes at pH below 6.0, disulfide sensitive to cytosolic glutathione 1000 fold higher than plasma. Non-cleavable thioether linker SMCC requires complete proteolytic degradation of antibody backbone to release lysine-MCC-drug metabolite still active. This mechanistic insight supports diagnostic and therapeutic applications while reinforcing core immunological and cell biology principles taught in advanced curricula.