Ann. Oncol.
Severe immune-related adverse events are common with sequential PD-(L)1 blockade and osimertinib.   
Concurrent programmed death-ligand-1 (PD-(L)1) plus osimertinib is associated with severe immune related adverse events (irAE) in epidermal growth factor receptor (EGFR)-mutant non-small-cell lung cancer (NSCLC). Now that PD-(L)1 inhibitors are routinely used as adjuvant and first-line treatments, sequential PD-(L)1 inhibition followed by osimertinib use may become more frequent and have unforeseen serious toxicity.
We identified patients with EGFR-mutant NSCLC who were treated with PD-(L)1 blockade and EGFR- tyrosine kinase inhibitors (TKIs), irrespective of drug or sequence of administration (total n = 126). Patient records were reviewed to identify severe (NCI-CTCAE v5.0 grades 3-4) toxicity.
Fifteen percent [6 of 41, 95% confidence interval (CI) 7% to 29%] of all patients treated with sequential PD-(L)1 blockade followed later by osimertinib developed a severe irAE. Severe irAEs were most common among those who began osimertinib within 3 months of prior PD-(L)1 blockade (5 of 21, 24%, 95% CI 10% to 45%), as compared with >3-12 months (1 of 8, 13%, 95% CI 0% to 50%), >12 months (0 of 12, 0%, 95% CI 0% to 28%). By contrast, no severe irAEs were identified among patients treated with osimertinib followed by PD-(L)1 (0 of 29, 95% CI 0% to 14%) or PD-(L)1 followed by other EGFR-TKIs (afatinib or erlotinib, 0 of 27, 95% CI 0% to 15%). IrAEs occurred at a median onset of 20 days after osimertinib (range 14-167 days). All patients with irAEs required steroids and most required hospitalization.
PD-(L)1 blockade followed by osimertinib is associated with severe irAE and is most frequent among patients who recently received PD-(L)1 blockade. No irAEs were observed when osimertinib preceded PD-(L)1 blockade or when PD-(L)1 was followed by other EGFR-TKIs. This association appears to be specific to osimertinib, as no severe irAEs occurred with administration of other EGFR-TKIs.

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