Abstract: 5-Fluorouracil (5-FU) is a cornerstone antimetabolite chemotherapeutic agent widely utilized in the treatment of various solid tumors, including colorectal, gastric, and hepatocellular carcinomas. Despite its established pharmacological efficacy in inhibiting pyrimidine biosynthesis and inducing DNA/RNA damage, the clinical utility of 5-FU is frequently compromised by a narrow therapeutic index, severe dose-limiting toxicities, and the emergence of chemoresistance. This comprehensive literature review synthesizes current research on the mechanisms underlying 5-FU resistance and explores innovative reversal strategies. Key resistance mechanisms include the dysregulation of noncoding RNAs (microRNAs and long noncoding RNAs) that modulate apoptosis, glycolysis, and drug efflux transporters, as well as gut microbiota-mediated metabolic alterations, such as the bacterial degradation of 5-FU and the upregulation of anti-apoptotic proteins. To overcome these challenges, emerging strategies focus on microbiota modulation (using probiotics, prebiotics, and postbiotics like urolithin A and butyrate), the development of multifunctional nanomedicines for targeted delivery, and the implementation of comprehensive pharmacogenetic profiling of DPYD variants to personalize dosing and mitigate life-threatening toxicities.
1. Introduction
Since its introduction in the 1950s, 5-Fluorouracil (5-FU) has remained one of the most commonly prescribed anticancer drugs for the treatment of various solid tumors, including colorectal, gastric, breast, and hepatocellular carcinomas [4][1]. As a cell-cycle specific antimetabolite agent, 5-FU primarily targets the proliferation stages of tumor cells [8][5]. However, the clinical management of 5-FU chemotherapy is complicated by a narrow therapeutic index and significant inter-individual variability in pharmacokinetics and pharmacodynamics [4]. While many patients are safely treated, a substantial proportion experiences severe, sometimes life-threatening, toxicities such as gastrointestinal mucositis, hemorrhagic enteritis, and hematological toxicity [4][5]. Furthermore, the development of intrinsic or acquired chemoresistance remains a major obstacle, often correlating with poor patient prognosis and treatment failure [6]. Recent advances in molecular biology and pharmacomicrobiomics have shed light on the complex orchestration of 5-FU resistance, paving the way for novel therapeutic interventions.
2. Pharmacological Activity
Belonging to the antimetabolite fluoropyrimidine class, 5-FU and its oral prodrug capecitabine act as false, high-affinity substrates for the enzyme thymidylate synthase (TS) [4]. By inhibiting TS, 5-FU disrupts pyrimidine biosynthesis in cells that display high proliferation rates. The active metabolites of 5-FU exhibit potent cytotoxic activity by causing the biosynthetic depletion of endogenous thymidine, which subsequently leads to direct damage to both DNA and RNA [4]. This disruption ultimately triggers cell cycle arrest and apoptosis, resulting in the suppression of tumor growth [3][4].
3. Molecular Mechanism of Action
The mechanisms dictating 5-FU efficacy and resistance are multifaceted, involving host genetic factors, epigenetic regulation via noncoding RNAs, and the gut microbiome.
Microbiota-Mediated Mechanisms: The gut microbiota significantly influences 5-FU metabolism and tumor sensitivity. Certain bacteria, such as Fusobacterium nucleatum, promote chemoresistance in colorectal cancer by upregulating the expression of BIRC3 (an inhibitor of apoptosis protein) via TLR-4 activation, and by modulating autophagy pathways [5][1]. Additionally, the bacterial preTA operon (found in Escherichia coli and other gut residents) encodes dihydropyrimidine dehydrogenase (DPYD), which metabolizes 5-FU into the inactive metabolite dihydrofluorouracil (DHFU), thereby reducing the drug's systemic bioavailability and efficacy [1]. Conversely, microbial uracil phosphoribosyltransferase (UPP) can convert 5-FU into 5-fluorouridine monophosphate (FUMP), enhancing its activation [1].
Noncoding RNA (ncRNA) Regulation: In gastric and hepatocellular carcinomas (HCC), dysregulated microRNAs (miRNAs) and long noncoding RNAs (lncRNAs) are pivotal in 5-FU resistance. Oncogenic miRNAs such as miR-200a-3p, miR-183, miR-141, miR-193a-3p, miR-BART20-5p, miR-147, and miR-17 promote resistance by suppressing target genes like PTEN, DUSP6, Keap1, SRSF2, and DEDD [2][3]. In contrast, tumor suppressor miRNAs—including miR-195, miR-125b, let-7g, miR-133a, miR-326, miR-503, miR-27b, miR-508-5p, miR-129-5p, miR-107, miR-181b, miR-429, miR-23b-3p, miR-31, miR-197, miR-BART15-3p, miR-204, miR-623, miR-939, and miR-124—are often downregulated in resistant cells. Restoring these miRNAs sensitizes cancer cells to 5-FU by targeting anti-apoptotic proteins (BCL-2, BCL-w, BCL-xl), glycolysis enzymes (HK2), translation factors (EIF4E), and ABC transporters [2][3]. Furthermore, lncRNAs such as XLOC_006753, MALAT1, and PVT-1 contribute to 5-FU resistance by modulating apoptosis and the PI3K/AKT/mTOR signaling pathways, whereas the lncRNA LEIGC can reverse resistance by inhibiting epithelial-mesenchymal transition (EMT) genes [3].
Drug Efflux Transporters: ATP-binding cassette (ABC) transporters, including ABCB1, ABCC1/2/3, and ABCG2, mediate the efflux of 5-FU, reducing intracellular drug concentrations and conferring multidrug resistance [1][3].
4. Structure-Activity Relationship (SAR)
While traditional SAR focuses on chemical modifications, the functional activity of 5-FU is heavily dependent on its structural mimicry of endogenous pyrimidines and its subsequent metabolic processing. 5-FU acts as a prodrug that must be converted into active nucleotides. Through the pyrimidine salvage pathway, enzymes such as UPP convert 5-FU into FUMP, a structural analog of uridine monophosphate (UMP). FUMP blocks de novo pyrimidine synthesis by inhibiting enzymes like carA/B, thereby exerting antitumor effects [1]. Conversely, the structural degradation of 5-FU by the detoxification enzyme DPD (encoded by the DPYD gene in both the host liver and gut bacteria) into DHFU renders the molecule inactive, directly impacting its pharmacokinetic profile and therapeutic efficacy [1][4].
5. Current Limitations
The clinical application of 5-FU is constrained by several significant limitations. Pharmacokinetically, intravenously administered 5-FU exhibits a very short half-life (10-15 minutes for a bolus dose), undergoes rapid hepatic metabolism, and has low bioavailability [5][6]. Consequently, high doses are required, which exacerbates toxicity. Approximately 20-30% of patients develop severe or life-threatening toxicities, leading to treatment delays or discontinuation [4].
A major cause of this toxicity is inefficient drug catabolism due to dihydropyrimidine dehydrogenase (DPD) deficiency. While current pharmacogenetic guidelines recommend testing for four common deleterious DPYD variants (DPYD*2A, DPYD*13, c.2846A>T, and c.1129-5923C>G), this targeted panel identifies less than 20% of patients at risk for severe toxicity [4]. Furthermore, 5-FU treatment frequently induces severe gastrointestinal mucositis and disrupts the gut microbiome (dysbiosis), which can further impair drug efficacy and exacerbate adverse effects [1][5].
6. Future Perspectives
To overcome the limitations and resistance associated with 5-FU, several innovative strategies are currently under investigation:
Microbiota Modulation: Manipulating the gut microbiome presents a promising avenue to optimize 5-FU therapy. Probiotics (e.g., Lactobacillus plantarum and Bifidobacterium) and prebiotics (e.g., xylan) can mitigate 5-FU-induced intestinal mucositis and restore microbial balance [1][5]. Postbiotics, such as microbiota-derived metabolites, also show chemosensitizing potential. For instance, urolithin A (UroA) downregulates the ABCG2 transporter by regulating the FOXO3-FOXM1 axis, thereby reducing 5-FU efflux [1]. Similarly, the short-chain fatty acid butyrate enhances 5-FU sensitivity by downregulating TS expression, inducing ROS-mediated mitophagy, and promoting apoptosis via the PINK1/Parkin and Akt/ERK pathways [1].
Nanomedicines: Multifunctional nanomedicines offer a strategy to improve the pharmacokinetic profile of 5-FU. Encapsulating 5-FU or its prodrug capecitabine into nanoparticles (such as xylan-stearic acid nanoparticles or liposomes) can increase intratumoral drug accumulation, prolong half-life, and simultaneously modulate antitumor immunity and the gut microbiome [1][6].
Advanced Pharmacogenetics: To better predict and prevent 5-FU toxicity, there is a critical need to implement comprehensive next-generation sequencing (NGS) of the DPYD gene. Identifying the burden of rare and novel DPYD variants, as well as variants in related genes like DPYS, will significantly enhance the pre-emptive identification of high-risk patients, allowing for precise, individualized dose adjustments [4].
Targeting Noncoding RNAs: The therapeutic targeting of specific miRNAs and lncRNAs involved in 5-FU resistance holds potential. Restoring the expression of tumor suppressor miRNAs or inhibiting oncogenic lncRNAs could reverse chemoresistance and re-sensitize gastrointestinal and hepatic tumors to 5-FU-based regimens [2][3].
