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  • br It is important to note the following weaknesses


    It is important to note the following weaknesses of the current study. Although the strategy of using mutant KRAS molecules as a tumor marker may be theoretically optimal in a disease like PDAC, where KRAS mutation rates exceed 90%, the stochastic nature of circulating nucleic acids released in circulation may lead to underestimation of the true circulating tumor burden if detection is limited to a single mutation. This may likely be a contributing factor to the poor predictive potential of ctDNA in the context of metastatic disease and those patients receiving neoadjuvant therapy. We found notable differences in our previously published exoKRAS and ctDNA detection rates in patients with metastatic (85% and 57.9%, respectively) and local-ized disease (67% and 45.5%, respectively) obtained from a retrospective bio-banked cohort.11 The differences in detection rates are largely due to the fact that exoDNA and ctDNA in the previous study underwent whole-genome  Gastroenterology Vol. 156, No. 1
    amplification to increase sensitivity of KRAS detection in the context of early detection efforts. Although this was a possibility in the current series, we opted against amplifi-cation, because this would have distorted the MAFs found through ddPCR and thus affected our clinical endpoints. The use of a tumor gene panel (eg, KRAS, TP53, CDKN2A, and SMAD4) may achieve greater sensitivity for detection and monitoring.2,6 Additionally, the fact that our multigene panel does not cover KRAS hotspot mutations in Vaborbactam 61 may lead to underestimation of our true sensitivity, because the current panel has a theoretical detection rate of up to 80% of known KRAS mutations in PDAC.30 Although our detec-tion rates of KRAS mutant molecules are relatively modest at 32%–41% in baseline treatment-naïve metastatic patients based on the liquid biopsy compartment, a fact that may limit the number of patients who may benefit from such an assay, when we look at general detection in both compart-ments at once, detection rates increase to 73.1%, which is near the theoretical limit of our assay. This underlines the complementary nature of these biomarkers, especially in the setting of low-volume disease (such as after treatment or when monitoring for recurrence), whereby the absence of mutant detection in one does not preclude the ability to gain valuable genomic information in the other. Additionally, although exoDNA mutant KRAS detection levels compared with ctDNA detection levels are not significantly better in the current cohort, exosomes provide the added ability to perform specific enrichment of cancer-derived material, allowing for capture of DNA, RNA, and proteins derived from tumors for mutation, gene expression, and possibly even neoantigen detection.14 The need for a criterion stan-dard validation is also important when pursuing liquid bi-opsy assays such as the one described in this study. As such, recent work has attempted to validate concordance between mutations found in liquid biopsy and tissue biopsy samples.31–36 In the context of PDAC, acquiring tissue bi-opsy samples for molecular profiling is particularly difficult in the metastatic setting, where fine needle aspirates are typically reserved for diagnostic purposes. We thus selected a subset of 34 patients with localized disease with available matched tissues, where concordance rates ranged from 66.7% to 95.5% depending on the liquid biopsy platform and tissue source. Unsurprisingly, surgical tissue specimens showed greater rates of concordance, particularly in exoDNA, which is likely associated with the greater sensi-tivity of mutation detection in exosomes. Overall, KRAS mutation detection rates were high in liquid biopsy samples as a whole, although it remains to be seen if profiling of additional mutations can achieve this sensitivity and specificity. r>