Technical Aspects of Liquid Biopsy: Influence of Preanalytical Parameters on the Concentration of Circulating Tumor Dna in Plasma of Cancer Patients

Abstract

«Liquid biopsy» is gradually becoming a mandatory procedure in cancer diagnostics. The aim of this procedure is to detect and monitor tumor-specific markers in various body fluids (blood, urine, pleural fluid, etc.). Significant efforts have been made to convert the most common mutational tests (EGFR, KRAS, BRAF) into non-invasive procedures. Despite some advantages, “liquid biopsy” is still not equivalent to traditional tissue analysis due to limited sensitivity and specificity; it cannot be routinely used in cancer medicine until the standardization of pre-analytical procedures is agreed. We intend to improve the performance of liquid biopsy for detection of a number of clinically relevant mutations (EGFR: ex19del and L858R; KRAS: 12, 13, 61, 146 codon nucleotide substitutions; BRAF: V600E). 417 plasma samples obtained from 88 patients (KRAS/NRAS/BRAF-mutated colorectal cancer (CRC): n= 57; EGFR-mutated lung adenocarcinomas (LC): n = 14; BRAF-mutated melanoma: n = 17) were analyzed by ddPCR for the presence of corresponding mutations in the circulating tumor DNA (ctDNA). Presence of tumor-specific mutations in plasma was confirmed in 32/57 (56%) CRC, 7/14 (50%) LC, and 4/17 (24%) melanoma cases. The proportion of mutation-positive plasma cases was tended to be higher in the group of patients with distant metastases compared to subjects with localized disease [34/56 (61%) vs. 5/15 (33%), р = 0.058]. 86 patients provided their blood at 9.00 (morning) and at 16.00 (afternoon). In addition, blood-takes were performed before and 15 minutes after usual breakfast as well as before and 15 minutes after moderate physical exercise. The detection rate of cancer-specific mutations in plasma was not significantly correlated with described above circumstances of blood-take. Meanwhile, the noticeable intrapatient variability of circulating mutation success rate has been detected. Thus, depending on clinical circumstances, at least negative ctDNA tests could be advised to be repeated in some patients, in order to ensure the reliability of results.