Monitoring of secondary drug resistance mutations in circulating tumor DNA of patients with advanced ALK positive NSCLC

Background Disease progression in ALK positive NSCLC patients treated with crizotinib occurs after a median of 9–10 months of treatment. Several mechanisms of resistance were identified and include ALK gene mutations and amplification and activation of bypassing signaling pathways like EGFR, KRAS or c-KIT. Second-generation ALK-TKIs demonstrated an enhanced spectrum of activity in crizotinib-resistant patients. However, re-biopsy in NSCLC patients represents a critical issue and analysis of circulating cell-free DNA (cfDNA) has a promising role for the identification of mechanisms of resistance. Methods Sixteen patients progressed during ALK-TKI were enrolled. After progression, blood was collected and DNA was extracted from plasma using QIAamp circulating nucleic acid kit (Qiagen®) and tested for ALK secondary mutations and KRAS exon 12 mutations using the Digital Droplet PCR (ddPCR – BioRad®). Results All patients were stage IV adenocarcinoma; 11 female and 5 male. Nine were never-smokers and 7 former-smokers. Median age was 53 yrs (range 40–81). Fifteen patients received crizotinib and 1 ceritinib. ALK-TKIs was administered mainly as second-line, in 2 cases as first and in the remaining as third-line therapy. Twelve patients had partial response, 3 stable disease, one progressed. Median PFS was 8 months. In 12 cases brain was a site of progression and only 5 patients had a tumor site that could potentially undergo re-biospy. ALK secondary mutations were identified in 4 patients. One showed both p.L1196M and p.G1269A mutations which levels decreased after 2 months of therapy with second generation ALK-TKI, along with tumor response. The second and the third patient had p.L1196M and p.G1269A, respectively. The 4th patient showed p.F1174L after initiation of second generation ALK-TKI. A total of 9 patients KRAS mutations p.G12D or p.G12V appeared in cfDNA at the time of resistance to TKI, 3 of them presented both ALK and KRAS mutations. Conclusions ddPCR can detect resistance mutations in cfDNA of ALK+ NSCLC and is an effective alternative to re-biopsy. The assessment of mutant allele burden could be used for response monitoring during treatment. Moreover, KRAS mutations may play a role in resistance to ALK-TKIs.