Abstract: 5-Fluorouracil (5-FU) is a foundational antimetabolite chemotherapeutic agent widely used in the treatment of various malignancies, including colorectal, pancreatic, skin, and breast cancers. By acting as a false substrate for thymidylate synthase, 5-FU disrupts pyrimidine biosynthesis and induces fatal DNA and RNA damage in proliferating tumor cells. Despite its potent pharmacological activity, the clinical application of 5-FU is severely hindered by its narrow therapeutic index, extremely short half-life, rapid hepatic metabolism, and severe systemic toxicities such as cardiotoxicity and gastrointestinal mucositis. Furthermore, the emergence of drug resistance—mediated by microRNAs and gut microbiome interactions—poses a significant challenge. To overcome these barriers, recent research has pivoted toward nanoparticle-based drug delivery systems. Advanced formulations, including polymeric micelles, dendrimers, pH-responsive nanogels, microneedles, and targeted albumin-based nanoparticles, have demonstrated remarkable potential in enhancing the bioavailability, tumor-specific accumulation, and overall therapeutic efficacy of 5-FU while mitigating its adverse effects.
1. Introduction
5-Fluorouracil (5-FU) is a widely utilized antimetabolite chemotherapeutic agent that belongs to the fluoropyrimidine class [3]. Since its introduction, it has remained a cornerstone in the oncological management of various malignancies, including colorectal, head and neck, breast, gastric, skin, and pancreatic cancers [5]. While the pharmacological efficacy of 5-FU is well-established, its clinical utility is frequently compromised by a narrow therapeutic index, rapid metabolism, and significant inter-individual variability in pharmacokinetics [3]. To address these inherent limitations, modern oncological research has increasingly focused on the development of nanoparticle-based drug delivery systems. These micro- and nanostructures offer a promising alternative to conventional administration routes by enhancing drug bioavailability, prolonging circulation time, enabling controlled drug release, and facilitating precise tumor targeting [1].
2. Pharmacological Activity
5-FU primarily targets the various stages of tumor cell proliferation [8]. It is a mainstream option for neoadjuvant therapy in rectal cancer [2] and is extensively utilized in combination chemotherapy regimens. For example, in the treatment of metastatic pancreatic adenocarcinoma, 5-FU is frequently combined with leucovorin and liposomal irinotecan (nal-IRI) or oxaliplatin in regimens such as FOLFIRINOX and NALIRIFOX to significantly improve patient survival outcomes [6][7][12]. In the context of dermatology, topical 5-FU is an approved and effective treatment for superficial basal cell carcinoma (BCC) [1]. To further improve systemic administration and patient compliance, oral prodrugs of 5-FU, such as capecitabine and S-1, have been developed and established as standard treatments for patients with resected pancreatic cancer and other solid tumors [10].
3. Molecular Mechanism of Action
At the molecular level, 5-FU acts as a false, high-affinity substrate for the enzyme thymidylate synthase (TS) [3]. By competitively inhibiting TS, 5-FU effectively halts pyrimidine biosynthesis, a pathway that is critically required by cells exhibiting high proliferation rates [3][5]. In addition to TS inhibition, the active metabolites of 5-FU are erroneously incorporated into both ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) [5]. This dual mechanism of action—the biosynthetic depletion of endogenous thymidine combined with direct structural damage to DNA and RNA—triggers apoptosis, leading to potent cytotoxic activity and the suppression of tumor growth [3].
4. Structure-Activity Relationship (SAR)
5-FU is a synthetic uracil analog [5]. Its structural resemblance to endogenous pyrimidines is the fundamental basis for its ability to infiltrate nucleic acid synthesis pathways and act as a false substrate [3]. Because the unmodified 5-FU molecule suffers from poor pharmacokinetic properties, structural modifications in the form of prodrugs (e.g., capecitabine and S-1) have been successfully engineered to improve oral bioavailability and systemic delivery [3][10]. Furthermore, the encapsulation of 5-FU into nanocarriers—such as liposomes, polymeric micelles, and dendrimers—does not alter its core pharmacophore but drastically modifies its macroscopic activity profile. Nanoparticle encapsulation protects the molecule from rapid enzymatic degradation, alters its solubility, and facilitates targeted cellular uptake via receptor-mediated endocytosis [1][2].
5. Current Limitations
The clinical application of 5-FU is hindered by several formidable challenges:
- Pharmacokinetic Drawbacks: Intravenous 5-FU exhibits an extremely short half-life (approximately 10-15 minutes for a bolus dose) and low bioavailability [2][11]. It undergoes rapid hepatic metabolism by the enzyme dihydropyrimidine dehydrogenase into inactive metabolites, which severely hampers its accumulation within tumor tissues [2]. Additionally, gut microbiota (specifically Proteobacteria and Firmicutes) can metabolize 5-FU into its inactive metabolite, dihydrofluorouracil [11].
- Severe Toxicity: 5-FU possesses a narrow therapeutic index. Consequently, 20-30% of patients develop severe or life-threatening toxicities, requiring high doses that lead to cardiotoxicity, myelosuppression, hemorrhagic enteritis, and gastrointestinal mucositis [3][5][11].
- Drug Resistance: Tumor resistance to 5-FU is a major obstacle. In hepatocellular carcinoma (HCC), resistance is heavily regulated by various microRNAs (e.g., miR-141, miR-193a-3p, miR-195, miR-125b, let-7g, miR-133a, miR-326, and miR-503) that modulate apoptosis, glycolysis, and antioxidant pathways [9]. Furthermore, the gut microbiome can diminish 5-FU efficacy; for instance, Bacteroides vulgatus stimulates de novo nucleotide biosynthesis, facilitating tumor DNA repair and protecting cancer cells from 5-FU-induced damage [5].
6. Future Perspectives
To circumvent the limitations of free 5-FU, nanoparticle-based drug delivery systems represent the most promising frontier in future therapeutic strategies:
- Polymeric and Smart Nanocarriers: For skin cancer, amine-terminated dendrimers have been utilized as polymeric skin enhancers to increase the tissue internalization of hydrophilic 5-FU [1]. Additionally, pH-responsive double-walled PLGA-chitosan nanogels coated with eucalyptus oil have been developed to encapsulate 5-FU, demonstrating high skin penetration and cellular uptake in response to the acidic tumor microenvironment [1]. Nanoemulsions using Capyrol and PEG 400 have also shown a remarkable increase in 5-FU skin permeation compared to conventional gels [1].
- Microneedle (MN) Technology: The clinical effectiveness of topical 5-FU against basal cell carcinoma and melanoma is being significantly enhanced by combining it with polymeric MN arrays. Pretreatment with MNs has yielded a 4.5-fold increase in the permeation flux of 5-FU through the skin [1]. Furthermore, near-infrared (NIR) responsive 5-FU and indocyanine green-loaded MPEG-PCL nanoparticles integrated with hyaluronic acid dissolving MNs offer a highly promising single-dose chemo-photothermal therapy [1].
- Targeted Delivery in Rectal Cancer: To address the short half-life and toxicity of 5-FU in rectal cancer, pegylated albumin-based nanoparticles functionalized with a B5 ligand targeting LRP-1 have been engineered. This targeted approach significantly improves tumor accumulation and enhances the efficacy of neoadjuvant radiotherapy compared to the free drug [2].
- Microbiome-Modulating Nanoparticles: Novel 5-FU-loaded prebiotic-probiotic liposomes (utilizing a prebiotic xylan derivative, Sxy) are being developed to modulate the gut microbiota. This system enhances colorectal cancer chemotherapy outcomes by reshaping the microbiome toward a beneficial state while mitigating gastrointestinal toxicity [5].
- Carbon Nanotubes: Multi-walled carbon nanotubes (MWCNTs) have been shown to complement and significantly boost the antitumoral effect of 5-FU. 5-FU can be physically adsorbed to the nanotube surfaces via π-π stacking, leveraging the intrinsic microtubule-disrupting activity of MWCNTs to overcome standard resistance mechanisms [4].
