How Tagrisso (Osimertinib) Works: Mechanism of Action Explained

Tagrisso (osimertinib) is an oral EGFR tyrosine kinase inhibitor, usually shortened to “EGFR TKI.” It is a tablet taken once daily, not an antibody or an infusion, and it is used in lung cancers driven by specific changes in the EGFR gene. This guide explains in plain language what the Tagrisso mechanism of action is, what the FDA label says about how it binds, why selectivity matters, and what is known about brain penetration. It does not cover chemical structure diagrams, and nothing here is advice about treatment.

Updated November 4, 2026.

What does an EGFR tyrosine kinase inhibitor do?

EGFR (epidermal growth factor receptor) is a protein on the surface of cells. Part of it, the kinase domain, acts as an on switch for growth signals. In some non-small cell lung cancers (NSCLC), changes in the EGFR gene keep that switch on. The FDA label for afatinib, an older EGFR TKI, describes these “non-resistant” kinase-domain mutations as increasing the receptor’s autophosphorylation, with exon 21 L858R substitutions and exon 19 deletions the most common (DailyMed, Gilotrif label). A tyrosine kinase inhibitor is a drug that blocks that kinase activity so the growth signal is reduced. For the mutations themselves, see our explainer on EGFR exon 19 deletions and L858R.

How does Tagrisso (osimertinib) bind EGFR?

The FDA prescribing information (revised 09/2024) states that osimertinib “binds irreversibly to certain mutant forms of EGFR (T790M, L858R, and exon 19 deletions).” Irreversible, or covalent, means the drug forms a permanent chemical bond rather than attaching and detaching repeatedly (FDA, Tagrisso label).

Published reviews identify the anchor point as a cysteine residue at position 797 of EGFR, written C797, in the ATP-binding site; the label itself does not name C797. One review summarized in literature searches states that osimertinib covalently bonds with C797 regardless of T790M status. The same biology explains one resistance route: a 2016 review in the Journal of Hematology & Oncology reported that a C797S mutation, which swaps out that cysteine, conferred resistance to AZD9291 (osimertinib’s development name). In the 15 patients it analyzed who had become resistant, only 6 carried C797S, so other mechanisms are also involved (Wang et al., J Hematol Oncol 2016). What follows resistance is covered in what comes after Tagrisso.

Why does selectivity for mutant EGFR matter?

Normal (wild-type) EGFR is present on healthy tissues such as skin and the gut. A drug that blocks it broadly tends to cause rash and diarrhea. The Tagrisso label says osimertinib binds the mutant forms at roughly 9-fold lower concentrations than wild-type EGFR, which is a relative preference, not an absolute one. The label also reports that higher exposure raised the probability of adverse reactions, specifically rash, diarrhea and interstitial lung disease, while no clear exposure relationship was seen for response or progression-free survival over the dose range studied.

Two active metabolites, AZ7550 and AZ5104, circulate at about 10% of parent drug levels. AZ5104 was more potent against exon 19 deletion and T790M mutants (about 8-fold) and wild-type EGFR (about 15-fold). In laboratory studies osimertinib also inhibited HER2, HER3, HER4, ACK1 and BLK at clinically relevant concentrations (FDA label, section 12).

How is this different from earlier EGFR TKIs?

The erlotinib label says it “reversibly inhibits the kinase activity of EGFR” (DailyMed, erlotinib label). The afatinib label says it “covalently binds to the kinase domains of EGFR (ErbB1), HER2 (ErbB2), and HER4 (ErbB4)” (DailyMed, Gilotrif label). Osimertinib binds irreversibly and lists T790M among its labeled targets. The 2016 Wang review describes T790M as the most common mechanism of resistance to first- and second-generation EGFR TKIs, which is why a drug active against it was clinically important. Osimertinib’s original 2015 FDA approval was for T790M-positive disease after prior EGFR TKI therapy, and it was later tested head-to-head against gefitinib or erlotinib in FLAURA. See Tagrisso vs Tarceva and Iressa for that comparison.

Does Tagrisso reach the brain?

Brain metastases are common in EGFR-mutated NSCLC, so central nervous system (CNS) activity is a frequent question. The label reports that osimertinib distributed to the brain in monkeys, rats and mice, with brain-to-plasma exposure ratios of approximately 2, and tumor regression and longer survival in a mouse brain-metastasis model. Those are animal findings.

In people, the label reports exploratory FLAURA analyses in the small subset of patients with measurable CNS lesions on baseline brain scans: CNS response rate was 77% with osimertinib (n=22) versus 63% with gefitinib or erlotinib (n=19). Because these were pre-specified exploratory analyses in 41 patients, they are suggestive rather than definitive (FDA label, section 14).

What the mechanism does not tell you

Several points are easy to over-read. A 9-fold preference for mutant EGFR is a laboratory measurement, and it does not mean wild-type EGFR is spared; the label’s adverse reaction profile, including rash and diarrhea, reflects that. A brain-to-plasma ratio of about 2 in animals is not a measure of tumor control in people. And the mechanism says nothing about which patients benefit, which is determined by the mutation tests and trial evidence described elsewhere.

Mechanism explains why a drug is expected to work, not how well it works. Outcomes come from randomized trials, summarized in the Tagrisso trial guide. For the wider commercial and clinical context, see our osimertinib (Tagrisso) landscape article.

Next step

For teams that need to track how mechanism-driven competition in EGFR-mutated lung cancer is moving, Oncology Resource Group offers competitive-intelligence support. See ONCRG Pipeline Strategies for how pipeline analysis is approached.

This article is for informational and competitive-intelligence purposes only and is not medical advice. Approvals, labels and trial results change; verify details against current prescribing information and primary sources.