e21510 Background: Most non-small lung cancer (NSCLC) with epidermal growth factor receptor (EGFR) mutations develop resistance when exposed to EGFR-tyrosine kinase inhibitors (TKIs). T790M develops in about half of patients treated with TKI and can be detected by tumor tissue and cfDNA hotspot tests. However, co-occurring mutations at other loci may impact efficacy. We conducted a prospective, multi-center, observational study to assess the detection rates and predictive values of plasma-based EGFR T790M detection methods for Japanese NSCLC patients treated with osimertinib. Methods: NSCLC patients with tumor EGFR mutations and disease progression after treatment with 1st- or 2nd-generation EGFR-TKI were enrolled. Plasma was collected at the time of clinical disease progression, before osimertinib treatment. The collected plasma was tested for EGFR T90M by in-house plasma MBP-QP and ddPCR assays and compared to clinically tested cobas (Roche) results (including tissue, plasma). The primary endpoint was to demonstrate comparability of our MBP-QP system to cobas using plasma-based EGFR T790M detection to predict the therapeuitic effect of osimertinib via objective response rate (ORR) and disease control rate (DCR). As an exploratory analysis, we used Guardant360 to retrospectively test available banked plasma samples collected describe time points. Results: From Feb 2017 to Jan 2019, 145 patients enrolled. T790M was detected by cobas in 57 cases (44 tissue, 16 plasma, 3 both). ORR and DCR in plasma cobas-positive cases were 62.5% and 81.3%, respectively. MBP-QP found T790M in 9 patients with ORR and DCR 66.7% and 77.8%. ddPCR found 17 cases with ORR and DCR 70.6% and 82.4%. ORR was not correlated to AF. In plasma samples from 54 patients, Guardant360 detected T790M in 57%. Co-occurring alterations such as amplification or minor mutations in EGFR or other genes such as TP53 did not impact ORR, but in the group with poor response to osimertinib, the number of detected gene alterations tended to be large. Two patients with small cell carcinoma transformation had RB1 mutations and MYC amplification. Conclusions: Regardless of the test system, the detection of T790M could predict a good therapeutic effect of osimertinib, but there was no difference in response to osimertinib depending on EGFR T790M AF. Compared to single-gene assessment of EGFR, NGS of cfDNA may be useful for guiding treatment decisions for patients with TKI-resistant NSCLC. Clinical trial information: UMIN000025930.
150P: EGFR mutations in circulating tumor DNA (ctDNA) and tissue rebiopsy at progression on treatment with EGFR tyrosine kinase inhibitors (TKI)
