1491P - PD-L1 expression in primary tumour vs metastatic samples in the phase III MYSTIC study in first-line metastatic (m) NSCLC

PD-L1 expression determined by immunohistochemistry (IHC) can be a useful biomarker to assess the likelihood of benefit with anti-PD-1/PD-L1 therapies in pts with mNSCLC. Understanding the impact of sample type (primary tumour or metastatic) on predictivity of benefit will inform the suitability of such samples for clinical testing. MYSTIC (NCT02453282) was an open-label, phase III study of durvalumab (D) ± tremelimumab vs chemotherapy (CT) as first-line treatment for mNSCLC; stratification factors for randomisation included tumour cell (TC) PD-L1 expression (≥25% vs