The hydrochloride salt of a quinazoline derivative with antineoplastic properties. Competing with adenosine triphosphate, erlotinib reversibly binds to the intracellular catalytic domain of epidermal growth factor receptor (EGFR) tyrosine kinase, thereby reversibly inhibiting EGFR phosphorylation and blocking the signal transduction events and tumorigenic effects associated with EGFR activation. or A drug used to treat certain types of non-small cell lung cancer. It is also used with gemcitabine hydrochloride to treat pancreatic cancer that cannot be removed by surgery or has spread to other parts of the body. It is being studied in the treatment of other types of cancer. Erlotinib hydrochloride blocks a protein called epidermal growth factor receptor (EGFR), which may help keep cancer cells from growing. It is a type of EGFR tyrosine kinase inhibitor. Also called CP-358,774, OSI-774, and Tarceva.