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Page 4 of 20 Singh et al. Cancer Drug Resist. 2025;8:56

Figure 1. This schematic illustrates the formation of circRNAs through back-splicing of pre-mRNA, where exons are circularized via RBP or
Alu repeat elements. The resulting circRNA is exported from the nucleus into the cytoplasm. CircRNAs can perform multiple regulatory
functions, including (1) miRNA sponging (e.g., ciRS-7 sequestering miR-7); (2) stabilization of protein complexes (e.g., circACC1
supporting AMP kinase activity); and (3) other cellular roles, such as promoting proliferation and modulating gene expression. Though
rare, circRNAs may also influence transcriptional regulation in the nucleus. These diverse functions contribute to circRNAs’ roles in cancer
progression and drug resistance [Created in BioRender. Singh DD (2025)]. circRNAs: Circular RNAs; RBP: RNA-binding proteins; miRNA:
microRNA; AMP: adenosine monophosphate.

LIQUID BIOPSY-BASED DETECTION OF CIRCULATING CIRCRNAS
Cancer treatment has evolved considerably with the introduction of targeted therapies, immunotherapies,
and combination regimens . Unfortunately, drug resistance (whether intrinsic or acquired) remains a
[29]
significant challenge that can culminate in therapeutic failure, progression of disease, and suboptimal
survival . Due to the dynamic and heterogeneous nature of tumors, it is wise to continually assess
[44]
molecular changes during treatment . Although tissue biopsies can be educational, they are invasive,
[6]
typically only provide a limited evaluation of tumor biology, and cannot be used for longitudinal
monitoring . As a result, liquid biopsy has emerged as a novel, non-invasive application to assess real-time
[45]
cancer progression and treatment resistance . Table 2 presents a comparative analysis of the components
[46]
involved in liquid biopsy-based detection of circulating circRNAs for monitoring drug resistance in
cancer . Liquid biopsy is the characterization of tumor-derived constituents, such as CTCs, ctDNA,
[55]
extracellular vesicles, and different species of RNA, including circRNAs, in biofluids (e.g., blood) . Due to
[56]
their closed-loop structure that lacks free 5′ and 3′ ends, circRNAs resist exonuclease degradation, enabling
them to freely circulate and serve as stable and strong biomarkers . In the cancer context, circRNAs can
[6]
exhibit multiple roles, e.g., serving as a miRNA sponge, interacting with RBP, regulating transcription, and
even translated into functional peptides [16,57] . Importantly, dysregulated circRNA expression could result in
mechanisms of drug resistance across multiple malignancies, such as lung, breast, liver, and colon cancer .
[44]
CircRNAs detected through liquid biopsy will have a multitude of advantages for the clinic, many based on
what we discussed in the previous sections . First, circRNAs in liquid biopsies are exceptionally stable,
[24]
allowing their detection in biofluids often without immediate specimen processing . Second, liquid biopsies
[58]
enable clinicians to collect real-time samples from patients multiple times, providing insights into system or
tumor responses and facilitating early detection of resistance . Third, liquid biopsies are non-invasive and
[2]

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