The Journey of Osimertinib
Targeting T790M Mutant EGFR
Osimertinib is a third-generation EGFR tyrosine kinase inhibitor engineered to overcome resistance to first- and second-generation EGFR TKIs by forming a covalent bond with Cys797 in the EGFR ATP-binding pocket — potently inhibiting the T790M gatekeeper mutation that mediates acquired resistance, while sparing wild-type EGFR to reduce skin and GI toxicity and achieving remarkable CNS penetration to address brain metastases in EGFR-mutant NSCLC.
吸收
Osimertinib is orally administered with absolute bioavailability of
approximately 70%. Peak plasma concentrations are achieved within 3-8 hours (median approximately 6
hours) after oral dosing. Food has no clinically meaningful effect on osimertinib bioavailability —
the drug may be taken with or without food. Osimertinib is highly lipophilic (logP ~4.4), enabling
effective oral absorption despite its moderate molecular size (~500 Da). The drug is available as
40 mg and 80 mg tablets; the standard therapeutic dose is 80 mg once daily (40 mg for adjuvant
use or tolerability concerns). Tablets may be dispersed in 60 mL non-carbonated water and
administered via nasogastric tube for patients unable to swallow. P-glycoprotein (P-gp) does not
appear to significantly limit osimertinib absorption at therapeutic doses, unlike many other TKIs.
分布
Osimertinib distributes very extensively throughout body tissues
with a volume of distribution of approximately 986 L — reflecting extremely high tissue affinity.
Plasma protein binding is approximately 95%, primarily to albumin and alpha-1-acid glycoprotein.
The very large Vd combined with high protein binding reflects drug sequestration in tissue
compartments far beyond the vascular space. Crucially, osimertinib achieves excellent CNS
penetration — a key differentiator from first-generation EGFR TKIs (erlotinib, gefitinib) which
have limited brain penetration due to P-gp efflux at the blood-brain barrier. Osimertinib
accumulates in brain tissue at concentrations several-fold above plasma, making it the standard
treatment for NSCLC with brain metastases. Lung tissue concentrations substantially exceed
plasma, providing high drug concentrations at the primary tumor site.
作用机制
EGFR (epidermal growth factor receptor, HER1/ERBB1) mutations in
exon 19 (deletions, ~45% of EGFR-mutant NSCLC) and exon 21 (L858R point mutation, ~40%) create
constitutively active EGFR that drives tumor growth via PI3K/AKT/mTOR and RAS/MAPK pathways.
First-generation TKIs (erlotinib, gefitinib) initially control these mutations but after 9-13 months,
the T790M gatekeeper mutation emerges in approximately 60% of resistant patients — a methionine
substitution that bulks up the ATP binding pocket, reducing drug affinity. Osimertinib is a
mono-anilino-pyrimidine that forms an irreversible covalent bond with Cys797 in the EGFR ATP-binding
pocket — this covalent bonding mechanism overcomes the steric hindrance of T790M. Osimertinib's
unique pyrimidine scaffold confers very low affinity for wild-type EGFR (>200-fold selectivity
for mutant over wild-type), reducing the skin rash and GI toxicity of first-generation TKIs that
inhibit wild-type EGFR in normal skin and gut epithelium.
代谢
Osimertinib is primarily metabolized by CYP3A4 (main pathway)
and CYP3A5 to two active metabolites — AZ5104 and AZ7550 — that retain EGFR inhibitory activity
(particularly AZ5104 has similar potency to osimertinib against EGFR and is quantitatively significant
at approximately 10% of osimertinib plasma AUC at steady state). Further metabolism involves
oxidative N-dealkylation and glucuronidation. Strong CYP3A4 inducers (rifampicin) reduce
osimertinib AUC by approximately 78%, potentially leading to loss of efficacy — osimertinib should
be avoided or the dose increased to 160 mg with strong CYP3A4 inducers. Strong CYP3A4 inhibitors
modestly increase osimertinib AUC (approximately 24%), not requiring dose adjustment. Osimertinib
is a time-dependent inhibitor of CYP3A4 at higher concentrations. P-gp and BCRP may influence
CNS distribution of metabolites.
排泄
Osimertinib and its metabolites are excreted predominantly in feces
(approximately 68%) via biliary excretion, with approximately 14% in urine. Unchanged osimertinib
in feces accounts for approximately 2% of the dose; unchanged drug in urine is less than 2%.
The elimination half-life of osimertinib is approximately 48 hours, enabling once-daily dosing.
Steady-state concentrations are achieved after approximately 15 days of once-daily dosing. The long
half-life results from the large Vd and slow tissue release combined with the approximately 18-hour
half-life of AZ5104. No dose adjustment is required for mild-moderate renal impairment or mild-
moderate hepatic impairment. Severe renal or hepatic impairment has not been adequately studied.
No significant drug accumulation occurs at steady state with once-daily dosing.
临床意义
Osimertinib (Tagrisso) is the standard first-line therapy for EGFR
exon 19 deletion or L858R-mutant metastatic NSCLC (FLAURA trial: median PFS 18.9 months vs.
10.2 months with erlotinib/gefitinib; OS benefit), and for adjuvant therapy after resection of
stage IB-IIIA EGFR-mutant NSCLC (ADAURA: 80% reduction in disease recurrence). It is also the
standard treatment after disease progression on first/second-generation EGFR TKIs with confirmed
T790M mutation. Common adverse effects include rash (reduced vs. first-generation TKIs), paronychia
(nail inflammation), diarrhea, dry skin, and QTc prolongation. The major resistance mechanism
to osimertinib is the C797S tertiary mutation (which prevents covalent bonding at Cys797), along
with various bypass mechanisms (MET amplification, EGFR amplification, histological transformation).
Fourth-generation TKIs and bispecific approaches are in development.