Clinical Application Of Plasma DdPCR Detection Of EGFR 20Exon T790M Mutation In NSCLC

Epidermal growth factor receptor（EGFR）mutation is one of the main driver genes of non-small cell lung cancer.EGFR Exon20 T790M mutation is the major mechanism of acquired resistance to 1^st/2^ndgeneration EGFR-TKI.3^rdgeneration of EGFR-TKI can bring significant clinical benefits to patients with EGFR Exon20T790M mutation.Liquid biopsy is easier to be operated and less traumatic than tissue biopsy which provides a new option for patients who cannot provide sufficient tissue specimen.Plasma droplet digital PCR（ddPCR）detection is more sensitive than other detection methods.Circulating Tumor Cell（CTC）counts have also been confirmed to have certain application prospects in the monitoring of EGFR-TKI efficacy.Ourresearch intends to discuss 1)the relationship between the plasma ddPCR detection results such as droplets and mutation rates of EGFR 20exon T790M mutation and the progression pattern of 1^stgeneration EGFR-TKI;the relationship between the plasma ddPCR detection results of EGFR 20exon T790M mutation and the clinical efficacy of the third generation EGFR-TKI;2)consistency of plasma ddPCR detection ofEGFR Exon20 T790M mutation using the same platform,the relationship between multiple EGFR Exon20 T790M mutation detection and clinical benefits;3)Relationship between plasma ddPCR detection results and circulating tumor cell counts.Methods:The EGFR Exon20 T790M mutation was detected by ddPCR.Three kinds of immunomagnetic liposomes were used to dynamically monitor the changes of CTC in patients treated with 1^stEGFR-TKI.Result:1)Median droplets of ddPCR detection of EGFR Exon20 T790M mutation in patients with a new lesion as the EGFR-TKI progression mode werehigher than the median droplets in patients with target lesion enlargement among 159patients detected by ddPCR（median droplets 16 vs 6;95%CI 8-65 vs 2-18,P=0.077）.Similarly,the mutation rate in patients with new lesions was higher than that in patients with increased target lesion（0.45%vs0.19%,p=0.023）.There were nosignificant statistic differences in ORR,DCR,and PFS in patients stratified by droplet number and mutation rate.2)ddPCR detection results of EGFR 20exon T790Mmutation ddPCR（n=81）from two testing centers on the same platform weremoderate consistent,with Kappa value of 0.42.The different detection results mainly existed in low mutation rates among 0.00%-1.00%.The ORR and DCR were similar in EGFR Exon20 T790M mutation-single positive patients who were detected only positive in one testing center but negative in another testing center and doublepositive patients who were detected both positive in two centers,and there was no significant statistic difference in PFS（265 days vs 289 days,P=0.098）.3)CTC_totalcounts at EGFR-TKI RECIST-PD increased more in patients with high EGFR Exon20T790M mutation rate than patients with low mutation rate compared to CTC_totalatBEST response（8.5 vs 4,P=0.020）.This result is similar in CTC_EGFRcounts（2.5 vs1,P=0.063）.Conclusion:1)Plasma ddPCR detection results such as droplets and mutation rates of EGFR Exon20 T790M mutation are related to EGFR-TKI PD mode,but its value is not yet used as a predictor of clinical efficacy of 3^rdgeneration EGFR-TKI.2)Plasma ddPCR detection results of EGFR Exon20 T790M mutation show difference in low mutation rates.Thus,repeated detection for EGFR Exon20 T790M mutation can help to select patients with low mutation rate from initially T790M-negativepatients and they have similar clinical benefit.3)There is a certain correlation between CTC counts and plasma 20T790M test results in patients resistant to 1^stgeneration EGFR-TKI.