Poster Presentation 27th Lorne Cancer Conference 2015

Analysis of Circulating Tumour DNA to Monitor Systemic Therapy Response and Escape Mutations in Metastatic Melanoma (#170)

Elin S Gray 1 , Anna L Reid 1 , Kelvin Siew 2 , Matteo Carlino 3 , Georgina Long 4 , Michael Millward 2 , Helen Rizos 5 , Mel Ziman 1
  1. Edith Cowan University, Joondalup, WA, Australia
  2. Department of Medical Oncology, Sir Charles Gairdner Hospital, Nedlands, WA, Australia
  3. Westmead Institute of Cancer Research, Sydney, NSW, Australia
  4. Melanoma Institute Australia, Sydney, NSW, Australia
  5. Australian School of Advanced Medicine, Macquarie University, Sydney, NSW, Australia

Cancers acquire resistance to systemic treatment as a result of clonal evolution and selection. Repeat biopsies to study genomic changes after therapy are difficult, invasive and may be confounded by intra-tumour heterogeneity. The analysis of circulating tumour DNA (ctDNA) has been shown to provide information on the genetic evolution of the tumour in response to therapy as well as prognostic information.
In this study we used mutation specific droplet digital PCR to measure plasma concentrations of BRAFV600 mutant DNA copies in melanoma patients. The amount of mutant plasma ctDNA was quantified in 21 patients with advanced metastatic melanoma prior to commencing systemic therapies (baseline). A subgroup of 17 patients was also sampled at 6-12 weeks after therapy initiation (1st follow up) and 6 patients were followed longitudinally until treatment relapse. The presence of NRAS Q61 mutations in patients progressing after BRAF inhibitor therapy was also evaluated.
We were able to detect ctDNA in 15/21 (68%) of patients with confirmed BRAF mutation in their tumours at baseline. Of these, 5 out 5 (100%) had a BRAFV600K mutation and 10/16 (45%) had a BRAFV600E mutation. The concentration of ctDNA decreased significantly after therapy initiation (p=0.0007, paired t-test, N=17). This decrease was more apparent for patients treated with BRAF inhibitors (p=0.0002, N=11, 8 dabrafenib/trametinib and 3 vemurafenib), than patients receiving immunotherapies (p=0.59, N=6, 4 ipilimumab and 2 pembrolizumab).
In six cases with tumour relapse, after previously responding to BRAF inhibitors, a significant increase of ctDNA copies was detected in the plasma at the time of clinically identifiable progressive disease compared to the plasma obtained before progression was clinically apparent (p=0.02, N=6). Moreover, we demonstrated the acquisition of resistance mutations in 3/7 (43%) patients. The NRASQ61K (N=2) and NRASQ61R (N=1) were found in ctDNA at progressive disease but not at baseline.
Overall, our results demonstrate that the quantity of ctDNA in plasma tracks the patient’s response to treatment. Moreover ctDNA provides an easily accessible sample to assess genomic evolution of the tumour and to evaluate the development of escape mutations.