Circulating tumor DNA as a biomarker in metastatic melanoma patients treated with anti-PD1 antibodies

Background: Changes in circulating tumor DNA (ctDNA) levels correlate with response to MAPK inhibitors in patients with advanced melanoma, but the utility of ctDNA as a biomarker for immunotherapy remains unknown. In this study, ctDNA was monitored in melanoma patients treated with PD1 inhibitor alone or in combination with a CTLA4 inhibitor. Methods: Mutation-specific droplet digital PCR was used to measure ctDNA in patients with a known tumor-associated mutation. ctDNA levels were determined prior to therapy and during treatment, and correlated to RECIST 1.1 response at week 12. Results: In 85 patients with available week 12 response data, undetectable ctDNA at baseline (n = 40) was associated with a higher response rate (Table 1, P <0.05), and higher ORR was observed when brain progression was excluded (Table 1, P <0.01). ctDNA levels were monitored early during therapy in 80 patients (up to week 12, and in 75 patients ctDNA was measured prior to week 6). RECIST responses were more common in those with undetectable ctDNA early on treatment (Table 1, P <0.01), with ORR of 90% observed when intracranial progression only was excluded (Table 1, P <0.01). Conclusions: This is the single largest study to date demonstrating the utility of ctDNA in monitoring response to anti-PD1 therapy in melanoma. We note that ctDNA levels are not an accurate predictor of response in patients with predominant brain metastases. We confirm that undetectable ctDNA early during treatment is a strong indicator of response to anti-PD1 therapy. [table included in full text]