P1.13-18 Exploring the Resistance Mechanism of Osimertinib and Monitoring the Treatment Response Using Plasma ctDNA in Chinese NSCLC Patients

Abstract

Background: Osimertinib (AZD9291; Tagrisso) is a third generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) known to be effective for patients harboring the EGFR-T790M variant, which is accounts for more than half of the acquired resistance mechanisms to the first generation EGFR-TKIs. However, limited osimertinib resistance-mechanism was reported. Study on potential osimertinib-resistance mechanisms in advanced NSCLC is necessary. Method: This study enrolled eight T790M-positive (tissue validated) patients, treated with osimertinib after first generation EGFR-TKI (Erlotinib, Gefitinib, Icotinib) resistance and progressed rapidly. Serial plasma samples were collected until disease progressed. Plasma DNA was extracted and sequenced by target-capture deep sequencing of 1021 previously annotated genes related to solid tumors. Clonal EGFR T790M mutation was defined if mutation was in the cluster with the highest mean variated allele frequency with PyClone, and otherwise subclonal EGFR T790M mutation. Molecular tumor burden index (mTBI) was calculated with the mean variant allele frequency of mutations in trunk clonal population. Result: The median progression-free survival (PFS) of these eight rapidly-progressed patients was 3.82 months [95% CI 2.05-5.01] .Targeted capture sequencing of pretreatment ctDNA showed all of the eight patients (100%) were EGFRpositive (Exon19del [n¼6] and L858R [n¼2]), and seven patients (88%) harbored EGFR T790M mutation, except for the only one patient (P006) who showed an extremely low level of ctDNA. During the Osimertinib treatment, five patients (63%) had osimertinib resistancerelated mutations: EGFR C797S (in cis position), G724S, KRAS G12D, PIK3CA E542K, EGFR amplification, and ERBB2 amplification. Among them, two patients had more than one resistance mechanisms: patient P034 had EGFR G724S, KRAS G12D and EGFR amplification, simultaneously; patient P013 had amplification in both EGFR and ERBB2. Other potential resistance mechanisms were identified including EGFR T751I and K754E mutations in P002 and ERBB2 S603 in P013. Notably, the only one patient (P004) who had not been detected to have any known osimertinib resistance mechanism but progressed in 3 months, was demonstrated to harbor a subclonal EGFR T790M mutation by analysis of ctDNA clonal structure. Serial ctDNA monitoring showed mTBI increased when disease progressed in 88% (7/8) patients, except P006, whose mutation were negative at second (stable disease) and third (progressed disease) therapeutic evaluations due to the extremely low level of ctDNA. Conclusion: This study presented comprehensive the resistance mechanism of osimertinib progressed rapidly in ctDNA including multiple mechanisms co-occurred in same patient. Serial monitoring of plasma ctDNA may be a promising approach to explore resistance mechanism and monitored the treatment response of third generation EGFR-TKI.Background: First/second generation EGFR-tyrosine kinase inhibitors (EGFR-TKI) are eventually met with therapeutic fail...

Somatic Copy-Number Alterations in Plasma Circulating Tumor DNA from Advanced EGFR-Mutated Lung Adenocarcinoma Patients

“…We detected various rrSCNAs that were described in previous reports to mediate osimertinib resistance. In particular, it has previously been shown that amplifications of wildtype or mutant EGFR [ 36 , 37 , 38 ], ERBB2 amplifications [ 39 ], MET amplifications [ 40 ], and rrSCNAs of CDKN2A or CDK4/6 [ 21 , 40 ] are associated with rapid progression to osimertinib and acquired drug resistance. In our study, we observed no MET amplification in plasma samples prior to osimertinib and only one MET amplification at the time of osimertinib resistance, which is in contrast to published frequencies of MET amplification.…”

Resistance mechanisms to osimertinib in EGFR-mutated non-small cell lung cancer

“…Other KRAS mutations, such as G13D, Q61R and Q61K, have also been identified. 21,23,45 KRAS G12D has been reported after osimertinib failure both in first-line and in subsequent lines of therapy, 20,25,47,63 and pre-existing KRAS G12D mutations in association with PTEN loss were also detected in two patients who showed primary resistance to second-line osimertinib. 63 Within the RAS-MAPK pathway, the BRAF V600E mutation has been identified as being responsible for osimertinib resistance in 3% of cases, 20,25 both in firstand in second-line therapy, in association 22,64 or not 65 with the EGFR T790M mutation when osimertinib was administered after failure of previous EGFR-TKIs.…”

“…Other KRAS mutations, such as G13D, Q61R and Q61K, have also been identified. 21 , 23 , 45 KRAS G12D has been reported after osimertinib failure both in first-line and in subsequent lines of therapy, 20 , 25 , 47 , 63 and pre-existing KRAS G12D mutations in association with PTEN loss were also detected in two patients who showed primary resistance to second-line osimertinib. 63…”

“… 68 Among the known PIK3CA mutations, E545K, E542K, R88Q, N345K and E418K, which occur at a frequency of 4–11%, have been described to confer resistance to second-line osimertinib therapy. 20 , 21 , 23 , 47 , 63 The role of PIK3CA E545K in mediating osimertinib resistance has been confirmed in vitro. 23 Of note, in the next-generation sequencing analysis performed within the AURA3 study, PIK3CA amplifications co-occurred with HER2 amplifications in two out of three cases.…”

“…In the second-line setting, amplification of the wt EGFR allele in addition to the presence of the EGFR- ex19del allele constituted a novel mechanism of resistance. 45 – 47 A study by Kim and colleagues 48 also revealed increased EGF mRNA expression in tumour tissue samples of patients who progressed on osimertinib.…”