Nanomedicine-Driven Therapeutic Strategies for Rheumatoid Arthritis-Associated Depression: Mechanisms and Pharmacological Progress

RA is a chronic systemic autoimmune disease characterized by persistent synovial inflammation and immune system hyperactivation. In RA patients, cytokines such as interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and interleukin-1 beta (IL-1β) are significantly upregulated in synovial tissue and peripheral circulation [11]. These cytokines facilitate synoviocyte proliferation, joint destruction, and propagate downstream immune cascades through multiple mechanistic pathways. Elevated levels of these pro-inflammatory mediators are also observed in the peripheral blood and cerebrospinal fluid of individuals with depression, indicating their potential role in central neuroinflammation and neuroimmune dysregulation within the CNS [12,13]. Peripheral cytokines act as a critical interface between RA and depression, gaining CNS access via humoral and neural routes. Cytokines can traverse the BBB at regions with natural permeability, such as circumventricular organs, or through BBB compromise induced by inflammation, which promotes transendothelial migration of immune cells and cytokines [14,15]. For example, TNF-α disrupts endothelial tight junction integrity by activating soluble guanylate cyclase (sGC) and protein tyrosine kinase (PTK), thereby increasing BBB permeability [16]. Via the neural pathway, afferent sensory nerve endings encircling the inflamed synovial tissue exhibit cytokine receptor expression, facilitating the transmission of peripheral inflammatory signals through the vagus nerve to central autonomic structures such as the nucleus tractus solitarius and the hypothalamus [13]. Neurons within the dorsal root ganglia (DRG) express interleukin-1 receptor (IL-1R), IL-6R, and tumor necrosis factor (TNF) receptors at their peripheral terminals, enabling direct detection of inflammatory mediators [1]. Once within the CNS, cytokines stimulate microglia—the primary producers of central pro-inflammatory mediators—which then secrete elevated levels of inflammatory cytokines and mediators [17]. Microglia-derived IL-1 and TNF-α further activate astrocytes [18]. Persistent chronic inflammation continuously activates the hypothalamic–pituitary–adrenal (HPA) axis and sympathetic nervous system, thereby elevating the risk of depressive disorders [19]. In RA, prolonged inflammatory stress elevates levels of CRH and cortisol [12]. Concurrently, calcium/calmodulin-dependent protein kinase II (CaMKII) activated by excitotoxic stress in the hippocampus enhances cyclooxygenase-2 (COX-2) expression and prostaglandin E2 synthesis, thereby intensifying synovial inflammatory responses [20].

Chronic systemic inflammation in RA facilitates central neuroinflammation through increased peripheral cytokine levels, modulation of BBB permeability, and neuroimmune signaling cascades. These inflammation-induced modifications in neurochemical balance and synaptic plasticity constitute a fundamental mechanistic framework for RA-associated depression. Notably, inflammatory pathways are intricately linked with oxidative stress and mitochondrial dysregulation. Persistent inflammation leads to excessive reactive oxygen and nitrogen species production, disrupts mitochondrial homeostasis, and hampers cellular energy metabolism, thereby exacerbating neuroinflammatory processes. Consequently, the interrelated roles of inflammation and oxidative stress-driven mitochondrial impairment form a pathogenic feedback loop that enhances vulnerability to depression in RA patients.

An increasing corpus of scientific evidence indicates that oxidative stress serves as a pivotal mediator connecting peripheral autoimmune inflammatory processes in RA with central neuroinflammatory mechanisms implicated in depression [20,21]. RA initiates mitochondrial oxidative stress, perturbs mitochondrial bioenergetic equilibrium, and promotes CNS neuroinflammation and bioenergetic dysfunction, thereby contributing to depressive symptomatology [21]. Oxidative stress is a quintessential feature of RA pathology, characterized by excessive generation of ROS that serve as pivotal pro-oxidant and pro-inflammatory mediators [22].

In the inflamed RA synovium, activated macrophages, neutrophils, and fibroblast-like synoviocytes (FLS) produce substantial amounts of ROS via mitochondrial electron transport and NADPH oxidase enzymatic activity [23,24,25]. In MDD, oxidative stress and mitochondrial dysregulation are pivotal pathogenic mechanisms underpinning neuroinflammation and neurodegeneration [26,27]. Within the CNS, activated microglia and astrocytes generate ROS and reactive nitrogen species (RNS) through NADPH oxidase 2 (NOX2) and inducible nitric oxide synthase (iNOS), inducing lipid peroxidation of cellular membranes and oxidative modification of proteins [28,29]. Oxidative stress is intricately associated with neurotransmitter homeostasis disruption. Specifically, it downregulates brain-derived neurotrophic factor (BDNF) expression and hampers synaptic plasticity within the hippocampus and prefrontal cortex, thereby heightening susceptibility to depressive disorders [30]. Research indicates that tilapia skin-derived peptides mitigate oxidative stress and depressive-like behaviors in murine models via modulation of the BDNF/TrkB/CREB signaling cascade [31]. Zhu et al. further elucidated that in C57BL/6 murine models displaying anxiety- and depression-like phenotypes, there is a significant increase in oxidative stress within the hippocampal and hypothalamic regions, concomitant with decreased serotonergic neurotransmitter levels. Restoration of redox homeostasis and 5-HT signaling was achieved through supplementation with tryptophan (Trp) oligopeptides [32]. Dysregulation of 5-HT, noradrenergic, dopaminergic, and glutamatergic neurotransmission—hallmark features of MDD—may be further intensified by oxidative neurotoxicity [32,33,34,35].

Excessive immune cell activation in RA leads to persistent secretion of pro-inflammatory cytokines. Due to the high vulnerability of mitochondria to oxidative stress, somatic mitochondrial DNA (mtDNA) damage readily ensues under these conditions [36]. RA patients demonstrate impaired mitochondrial biogenesis, dysregulated mitochondrial dynamics, and decreased mitochondrial membrane potential, collectively compromising neuronal oxidative phosphorylation, impairing synaptic plasticity, and destabilizing neurotransmitter homeostasis, thereby contributing to the manifestation of depressive symptoms [37,38]. Furthermore, persistent neuroinflammation promotes the generation of ROS within the CNS, intensifying mtDNA oxidative damage and perpetuating microglial activation. These interconnected processes establish a pathogenic feedback loop, ultimately resulting in neuronal impairment and neuropsychiatric disturbances [39,40]. RA-associated depression is intricately connected to dysfunctional mitophagy. Inhibition of the PINK1/Parkin pathway impairs mitochondrial quality control, resulting in the buildup of dysfunctional mitochondria that promote ROS accumulation, mtDNA release, and sustained activation of the NLRP3 inflammasome, thereby exacerbating neuroinflammatory processes [41,42,43]. Decreased mitophagy and suppressed Parkin gene expression have been documented in RA patients, contributing to sustained peripheral immune activation and exacerbated CNS inflammation through the BBB, thereby intensifying neuropsychiatric manifestations [44,45].

In conclusion, oxidative stress and mitochondrial impairment are pivotal in the comorbid pathogenesis of RA and depression, primarily through the escalation of inflammatory pathways and disruption of CNS homeostasis. This disequilibrium is not solely attributable to inflammatory processes; dysbiosis of the gut microbiota further compromises intestinal barrier integrity, facilitates translocation of microbial metabolites into systemic circulation, and intensifies systemic inflammation, thereby aggravating oxidative stress and neuroinflammatory mechanisms. Consequently, oxidative stress, mitochondrial dysfunction, and dysbiosis of gut microbiota collectively serve as critical peripheral mediators in RA-associated depression pathophysiology.

Gut microbiota dysbiosis serves as a key peripheral contributor to the pathophysiology of depression associated with RA. The persistent inflammatory response intrinsic to RA, alongside prolonged use of DMARDs, can disrupt intestinal microbial homeostasis. Microbial-derived metabolites such as lipopolysaccharides (LPS), short-chain fatty acids (SCFAs), and indole compounds can traverse the BBB and influence CNS functioning through multiple mechanisms, thereby heightening the risk of depression [46].

Patients with RA frequently demonstrate decreased microbiome diversity, proliferation of pathogenic microorganisms, and depletion of advantageous commensal bacteria [47]. Such dysbiosis compromises the integrity of the intestinal barrier, promotes chronic inflammation, and can trigger autoimmune responses [48]. For instance, the proliferation of Collinsella has been associated with the downregulation of tight junction proteins, increased intestinal permeability, and exacerbation of arthritic pathology [49]. Zonulin, a pivotal regulator of epithelial barrier integrity, is secreted by intestinal epithelial cells in response to microbial stimuli, promoting disassembly of tight junctions, enhancing paracellular permeability, and facilitating translocation of microbial metabolites into systemic circulation, thereby initiating systemic inflammatory responses [50]. Elevated serum zonulin concentrations have been observed in RA patients [51]. Similarly, individuals with depression exhibit increased serum levels of zonulin and LPS, implying that bacterial metabolites penetrate a compromised intestinal barrier and contribute to neuropsychiatric manifestations [52]. SCFAs, such as acetate, propionate, and butyrate, produced via microbial fermentation of dietary fibers, are crucial for maintaining intestinal epithelial integrity [53]. RA patients often demonstrate diminished SCFA levels and a reduction in SCFA-producing taxa within the phylum Firmicutes, with exogenous SCFA supplementation exerting protective effects in experimental models of arthritis [54]. In MDD, alterations in gut microbiota composition and metabolic function result in dysbiosis, epithelial barrier dysfunction, and increased intestinal permeability [55]. MDD is characterized by an elevated Bacteroidetes/Firmicutes ratio, with an enrichment of Bacteroides spp. and depletion of taxa such as Blautia, Faecalibacterium, and Eubacterium [56,57]. Consequently, microbial metabolites, structural components, and entire microbes may translocate via the compromised intestinal barrier into systemic circulation, potentially amplifying systemic inflammatory cascades [58]. Gut microbiota also modulate the synthesis of neurotransmitters and neuroactive compounds, including gamma-aminobutyric acid (GABA), 5-HT, and dopamine (DA) [59,60]. Dysbiosis can disrupt Trp metabolism, the precursor of 5-HT, thereby impairing neurochemical homeostasis within the CNS [61,62]. SCFAs support epithelial barrier function, enhance tight junction protein expression, and preserve mucosal homeostasis; however, levels are frequently markedly reduced in depressive states. Such depletion exacerbates intestinal permeability and attenuates the negative feedback regulation of the HPA axis, leading to HPA axis hyperactivation and associated pathophysiology [63,64,65].

Gut microbiota dysbiosis not only compromises the local intestinal mucosal barrier but also promotes systemic inflammation by reshaping host immune responses [66]. The gut microbiome can interact with innate immune pattern-recognition receptors, such as Toll-like receptors (TLRs), thereby participating in the pathogenesis of RA [67]. Activation of TLR4 triggers NF-κB signaling, which in turn upregulates IL-6, TNF-α, and COX-2 expression and contributes to RA immunopathology [68,69]. In addition, TLR engagement can drive synoviocyte proliferation and joint destruction through activation of MAPK pathways [70].

Within the intricate pathophysiological landscape of RA-associated depression, organic NPs afford multiple advantages, such as superior biocompatibility, customizable architectures, and elevated drug-loading efficiencies—facilitating concurrent modulation of peripheral joint inflammation and central neuroinflammatory processes. Surface functionalization further augments their capacity to traverse the BBB, rendering them particularly suitable for the targeted delivery of anti-inflammatory, antioxidative, or neuroprotective therapeutics. Consequently, organic NPs are uniquely positioned to enable dual-site (articular-brain) therapeutic interventions in RA-associated neuropsychiatric conditions.

Inorganic nanocarriers, as an emerging class of nanoplatforms utilized for targeted drug delivery, demonstrate significant potential in enhancing therapeutic efficacy through selective targeting, sustained-release profiles, and improved solubility and bioavailability of poorly soluble pharmacological agents, thereby optimizing clinical outcomes. The primary categories of inorganic NPs include metallic NPs (e.g., gold, silver, platinum, nickel), metal oxide NPs (e.g., titanium dioxide, manganese dioxide, magnetite, alumina), and semiconductor NPs (e.g., silicon, ceramic materials) [9].

Recent advances in bioinspired nanoplatforms leveraging cellular mimicry, targeted delivery mechanisms, and therapeutic paradigms have demonstrated capabilities surpassing conventional material classifications. Bionic NPs emulate natural cellular membranes and biofunctionalities, substantially improving nanocarrier stability, target specificity, and immunocompatibility within biological environments. Their primary advantage involves preserving native cell membrane proteins, receptors, and adhesion molecules, conferring extended systemic circulation, enhanced BBB penetration, and immune evasion [136,137]. These innovations inform novel nanodelivery strategies pertinent to complex neuroinflammatory and neurodegenerative co-pathologies.

Zhou et al. developed a macrophage membrane camouflage combined with MMP-responsive drug release mechanism, resulting in DSPE-GPLGVRGC-PEG (DGP) NPs with outer-layer coating. These vehicles facilitate macrophage membrane-mediated targeted delivery, enabling precise release of 2-aminoethoxydiphenyl borate (2-APB) at sites of inflammation. Additionally, encapsulating DEX within microglia membrane-coated PDA NPs enables crossing the BBB and targeting microglia involved in neuroinflammation. In stress-induced models, this approach demonstrates superior efficacy in alleviating depressive behaviors [89]. Previous studies have demonstrated that macrophage membrane-coated NPs can effectively cross the BBB and target intracerebral inflammatory lesions for the treatment of CNS inflammatory disorders, thus exhibiting potential therapeutic value for depression subtypes involving neuroinflammation—necessitating further direct validation [138]. Zhang et al. engineered neutrophil membrane-coated NPs that showed significant therapeutic efficacy in both CIA mouse models and human TNF-α transgenic arthritic mice for treatment and prophylaxis of CIA [139]. In post-glioblastoma therapy, Chen et al. developed a neutrophil-mimicking membrane (NM)-coated PLGA-PEG nanoparticle system loaded with DOX, termed NM-PD, for drug delivery [140]. The neutrophil membrane-coated nanocarrier exploits inflammation-induced homing properties to enhance BBB penetration and lesion targeting, demonstrating antioxidant, anti-inflammatory, and neuroprotective effects in models of neuroinflammation-associated neuropathology. These findings suggest a potential therapeutic approach for depression involving neuroinflammation, although direct validation within depression models remains imperative.

Under neuroinflammatory conditions, cell membrane homology and increased BBB permeability synergistically facilitate nanoparticle entry into the CNS, where they target activated microglia. Given the systemic comorbidity of RA-associated depression, biomimetic NPs theoretically offer the unique advantage of concurrently modulating peripheral joint inflammation and central neuroinflammation, potentially disrupting the pathogenic positive feedback loop of "peripheral inflammation-centrally mediated depressive-like changes." Although direct evidence in depression remains limited, considering the crucial role of neuroinflammation in the onset and progression of depression, this strategy holds promise for precision treatment of neuroinflammation-associated depressive subtypes. Nevertheless, further investigations are required to validate therapeutic efficacy and safety.

Gene and RNA-based therapeutics facilitate programmable, precise modulation of inflammatory and neuroplasticity pathways at the transcriptional or translational level, offering a novel intervention dimension for depression associated with RA. This systemic comorbidity is driven by persistent peripheral immune inflammation and propagated via the "joint-brain axis." Nanocarrier systems substantially enhance nucleic acid bioavailability within inflamed synovial tissue and central neural targets, overcoming intrinsic challenges such as nuclease-mediated degradation, limited tissue biodistribution and cellular internalization, inadequate endosomal escape, and activation of innate immune responses.

In RA, nanoplatform-mediated gene and RNA therapies have demonstrated promising potential for directly modulating key inflammatory mediators and immune cell phenotypes. For instance, lipid nanoparticle-based delivery of TNF-α siRNA has been utilized to downregulate this pivotal pro-inflammatory cytokine [141]. Beyond suppression of inflammatory factors, gene delivery can also augment anti-inflammatory mediators. Studies employing macrophage-targeting strategies to deliver IL-10 plasmid DNA have achieved joint-specific enrichment through macrophage recruitment at inflamed sites, inducing macrophage polarization, thereby modulating the RA microenvironment and providing insights into localized, sustained anti-inflammatory signaling within complex immune networks [142]. Furthermore, microRNAs (miRNAs), as critical post-transcriptional regulators of immune homeostasis, can be systemically modulated via exosomes or biomimetic nanovesicles. For example, exosomes derived from mesenchymal stem cells carrying miR-223 have been shown to downregulate macrophage NLRP3 inflammasome activation, alleviating RA-associated inflammation. This exemplifies the potential of inflammasome targeting to reprogram innate immune responses [77]. Exosome-mediated miRNA delivery also offers promising avenues for addressing depression-related neurodegenerative pathology, where microglia-derived exosomes containing miR-146a-5p regulate neurogenesis linked to depression, providing direct evidence for nucleic acid interventions aimed at restoring neuroplasticity [143]. Importantly, both peripheral synovial macrophages and central microglia amplify inflammatory signals via NLRP3 inflammasomes and NF-κB pathways, connecting these processes to depression-like behaviors, impaired neurogenesis, and synaptic dysfunction [144]. Recent advancements in crossing the BBB with nucleic acid delivery nanocarriers have yielded lipid NPs capable of intravenous administration, achieving high transfection efficiency in neurons and astrocytes across the CNS with favorable biocompatibility. This establishes a safe, effective platform for CNS RNA delivery [145]. Accordingly, the development of "joint-brain dual-targeting" RNA delivery strategies focusing on shared inflammatory nodes, such as NLRP3, holds promise for simultaneous attenuation of peripheral and central neuroinflammation.

Overall, emerging nanomedicine strategies are progressing from singular delivery platforms toward multifunctional, programmable, and systems-level regulatory modalities. For RA-associated depression, future nanotherapeutics should emphasize pathology-driven rational design over mere material complexity. The integration of biomimetic nanotechnologies with nanoRNA therapeutics could enable comprehensive intervention platforms capable of concurrently modulating immune and neuroinflammatory responses and metabolic dysregulation, thereby advancing precision medicine for depression linked to RA.

Beyond the aforementioned systems, emerging nanotechnologies such as dendrimers and carbon-based nanomaterials demonstrate unique potential in treating RA-associated depression.

Li et al. developed a multifunctional nanoplatform based on generation-5 poly(amidoamine) (PAMAM) dendrimers to target RA pathology driven by M1 macrophages. The dendrimers were sequentially modified with 1,3-propanesultone and PEGylated folic acid (FA) to achieve M1 macrophage targeting, while PEGylated α-tocopherol succinate (α-TOS) was incorporated to impart antioxidant activity. Gold NPs encapsulated within the dendrimer, along with a TNF-α inhibitor, facilitate targeted delivery to macrophages, effectively scavenging ROS and suppressing both mRNA and protein expression of TNF-α, thereby offering a promising strategy to alleviate RA-associated depression [79].

Carbon dots (CDs), a novel class of photoluminescent carbon nanomaterials, possess advantageous properties including ultrasmall size, facile synthesis, cost-effectiveness, tunable functionality, excellent biocompatibility, and photostability. He et al. engineered PEG-modified, multi-enzyme-mimicking carbon dot nanocomposites loaded with MTX, designated as CDs2-P@M. These CDs exhibit catalase- and superoxide dismutase-like activities, enabling broad-spectrum scavenging of H₂O₂, superoxide, and hydroxyl radicals. PEGylation enhances biocompatibility, diminishes systemic toxicity, and facilitates targeted accumulation within inflamed joints in RA models. Upon homing to arthritic tissues, the system releases MTX to suppress pro-inflammatory cytokine production while spontaneously leveraging the intrinsic catalytic properties of CDs to eliminate excess ROS, promote macrophage polarization toward the anti-inflammatory M2 phenotype, and inhibit osteoclast activation. Notably, Jia et al. proposed N-doped carbon-dot nanozymes (CDzymes) as innovative antidepressant agents, exploiting the mechanistic relationship between oxidative stress, gut microbiota dysbiosis, and gut–brain axis dysfunction in depression. CDzymes significantly ameliorated depressive-like behaviors, increased hippocampal levels of 5-HT and GABA, restored gut microbial diversity, and corrected amino acid metabolism disturbances [88]. Liu et al. developed a nano-delivery strategy for constructing EGCG-loaded tea superparticles (TSPs), using tea proteins as carriers to load epigallocatechin gallate (EGCG). This strategy can alleviate anxiety- and depression-like behaviors in mice by repairing the intestinal mucosal barrier and reshaping the balance of intestinal flora. Compared with synthetic nanoparticle platforms, plant-derived proteins exhibit superior biocompatibility, degradability, and functional adaptability [146]. Nanoparticle-based strategies targeting the gut microbiota represent a promising future paradigm for treating RA comorbid with depression, as they enable precise spatiotemporal regulation of microbial composition and metabolites to rebalance immune-inflammatory pathways and the microbiota–gut–brain axis.

These findings collectively indicate that carbon dots with multi-enzyme activity, low toxicity, and water solubility represent an emerging class of nanomaterials capable of modulating the gut–brain–joint axis, offering promising avenues for targeted intervention in RA-associated depression.

In RA, activated macrophages, FLS, and neovascular endothelial cells infiltrate the synovium, upregulating receptors such as CD44, FR-β, and SR, providing molecular targets for nanomedicine delivery [112,129]. Ligands such as hyaluronic acid, folic acid, polysaccharides, and joint-homing peptides have been utilized for selective recognition of inflamed synovium and targeted therapeutics. For example, hyaluronic acid-coated MTX-polyethyleneimine NPs facilitate selective uptake by activated macrophages via CD44 and accumulate in inflamed joints in arthritis models [151]. Folic acid-based targeting leverages FR-β overexpression, with nanomedicines employing folate modifications demonstrating specific engagement with RA synovium [152]. Dextran sulfate, a polyanionic polysaccharide, can bind SR-A on inflammatory macrophages, allowing for their selective internalization, as demonstrated by Kim et al.'s dextran sulfate-based self-assembled NPs [153]. Among peptide ligands, Meka et al. utilized the novel joint-homing peptide ART-1 to modify liposomes for targeted delivery of IL-27 to synovial endothelial cells, markedly enhancing anti-arthritic efficacy and improving safety [154]. Another joint-homing peptide, ART-2, guided DEX-loaded liposomes to arthritic joints in rat and mouse models, effectively suppressing disease progression [155]. For FLS, a key pathogenic cell type, the synovial-fibroblast-homing peptide HAP-1 enables nanocarriers to selectively recognize inflammatory FLS and simultaneously modulate their hypermetabolic state and inflammatory activity [156].

Collectively, nanodelivery systems based on hyaluronic acid, folic acid, dextran sulfate, and joint-homing peptides such as ART-1/ART-2 and HAP-1 have demonstrated robust active-targeting capabilities toward the RA synovium and inflammatory microenvironment in animal models, laying a solid foundation for future nanotherapeutic strategies addressing RA-associated depression.

Extensive clinical and translational studies indicate that BBB dysfunction, neuroinflammation, and altered synaptic plasticity are major pathological underpinnings of depression. Thus, the CNS and its pathological microenvironment constitute critical targets for nanodelivery strategies. Compromised tight junctions and increased BBB permeability are strongly associated with inflammation, stress susceptibility, and behavioral phenotypes in MDD, as supported by multiple reviews [157]. On this basis, NPs can be engineered with brain-targeting ligands, such as Tf, Angiopep-2, lactoferrin, and RVG peptides-or designed to exploit depression-related BBB and neuroinflammatory alterations to achieve active or pathology-dependent CNS accumulation. Tf-receptor-targeted NPs can markedly enhance BBB transcytosis while maintaining good safety profiles, providing a feasible route for delivering antidepressants and neuroprotective drugs [158]. He et al. constructed Tf-modified carboxymethyl-chitosan/chitosan NPs (NOCMS-CS-Tf) and demonstrated enhanced brain accumulation and excellent biocompatibility under depression-related BBB impairment, indicating their potential as a generalizable CNS-targeting nanocarrier for depression therapy [111]. An alternative approach is to bypass the BBB via the nose-to-brain pathway. Several antidepressants have been formulated into intranasal nanoformulations; for example, chitosan-functionalized lipid-polymer hybrid NPs encapsulating paroxetine hydrochloride exhibited optimal particle size, mucoadhesiveness, and BBB permeability, supporting their potential as a nasal-to-brain delivery platform for depression treatment [159]. Additionally, systematic reviews on Tf and cell-specific peptides or aptamers targeting neurons, microglia, and astrocytes have highlighted that surface modification of nanovesicles with such ligands can achieve highly precise cell-type-specific delivery, enabling fine regulation of neuroinflammation, neuroplasticity, and circuit activity-features highly relevant to designing CNS-targeted nanotherapies for depression [160,161].

The primary obstacle in developing dual-targeted nanopharmaceuticals is the co-encapsulation of anti-rheumatic and antidepressant agents within a single nanocarrier system. These pharmacological classes present significant disparities in physicochemical characteristics, targeting modalities, and release profiles. Agents used in RA, such as MTX and corticosteroids, are predominantly hydrophilic and designed for accumulation at peripheral inflammatory sites, often leveraging high affinity for inflamed microenvironments. Conversely, CNS-targeting antidepressants like FLX and Mem require traversing the BBB, typically exhibiting higher lipophilicity and requiring precise control over release timing. These inherent differences pose substantial challenges to synchronized delivery and compartmentalized release within a unified nanocarrier system. Furthermore, the targeting mechanisms for joint synovium and CNS tissues differ fundamentally. RA lesion targeting primarily depends on the EPR (enhanced permeability and retention) and ELVIS (extravasation through leaky vasculature and subsequent inflammatory cell-mediated sequestration) effects for passive accumulation, whereas CNS penetration necessitates receptor-mediated transcytosis mechanisms or exploitation of pathological BBB permeability alterations. Consequently, the critical design challenge in dual-targeted nanocarriers is to reconcile peripheral inflammatory targeting with efficient CNS delivery within a single platform.

Recent advancements have demonstrated partial feasibility of dual-targeted nanoparticle delivery systems by capitalizing on nanocarrier structural compartmentalization to enable spatial separation and functional coordination of diverse therapeutic agents. For example, Ail et al. engineered a thermosensitive nanoliposome-in situ gel system encapsulating hydrophilic FXT within the aqueous core and embedding lipophilic embelin (EMB) within the phospholipid bilayer, facilitating simultaneous delivery of hydrophilic and hydrophobic drugs within a unified lipid vesicle platform [162]. Future research may involve designing polymeric NPs with hydrophobic cores and hydrophilic shells or hybrid lipid-polymer systems to enhance compatibility, enabling encapsulation of antidepressants within hydrophobic regions and loading anti-inflammatory or immunosuppressive agents in hydrophilic outer layers.

Stimuli-responsive nanocarriers offer additional technological advancements for dual-targeted therapy. The microenvironment within RA lesions is characterized by elevated ROS, acidic pH, and hypoxia, which can be exploited to trigger localized drug release of anti-inflammatory agents. Concurrently, crossing the BBB can be facilitated via mechanisms such as sustained or secondary stimuli-triggered release of antidepressants, allowing for temporally and spatially tiered drug delivery. This hierarchical approach addresses delivery conflicts among target tissues, thereby enhancing overall therapeutic specificity.

The progression of biomimetic nanotechnology introduces innovative methodologies for dual-targeted delivery systems. Employing cell membrane cloaking strategies, such as macrophage and microglia membrane coatings, maintains the integrity of native membrane proteins and adhesion molecules, thereby conferring immune evasion properties and targeted homing capabilities to pathological tissues. Macrophage membrane-encapsulated NPs demonstrate preferential accumulation within inflamed synovial joints, while membrane structures derived from central immune cells facilitate recognition and retention in neuroinflammatory regions. Additionally, exosomes and their biomimetic nanovesicles—characterized by intrinsic BBB penetration and low immunogenicity—are considered optimal vectors for bidirectional modulation within the joint–brain axis. Modifications to exosomal surface proteins or internal nucleic acids are aimed at enabling synchronized regulation of immune-inflammatory signaling pathways and neuroplasticity mechanisms, offering a promising framework for systemic therapeutic interventions in RA-associated depression. RNA-based nanotherapies utilizing macrophage- or neutrophil-derived delivery systems for siRNA, antisense oligonucleotides (ASO), or mRNA target pathways such as TNF-α, IL-6/JAK-STAT, and NLRP3 inflammasome, thereby reducing synovitis and inflammation-associated depressive symptoms. Furthermore, these nanoparticle-based cellular carriers facilitate targeted delivery to modulate microglial inflammatory responses, kynurenine pathway activity, or glutamate excitotoxicity, enabling a synergistic therapeutic approach for RA-associated depression.

From a mechanistic standpoint, peripheral inflammatory processes and central neuroinflammation engage in reciprocal neuroimmune crosstalk, forming self-perpetuating feedback amplification loops. Advancements in multi-targeted nanodelivery platforms aim to concurrently intervene across various tissue compartments and pathogenic pathways, not merely through additive effects but via coordinated modulation of critical pathological nodes to optimize systemic therapeutic efficacy. In this context, dual-targeted nanodelivery systems afford the capacity to simultaneously disrupt these feedback circuits rather than rely on the cumulative effects of multiple drugs. Effective suppression of peripheral inflammation can restore the integrity of the BBB, reducing the ongoing infiltration of inflammatory mediators into the CNS [163,164]. Conversely, mitigating neuroinflammation and neurotransmitter imbalances-through modulation of neuroimmune axes—can exert feedback inhibition on peripheral immune activation, thereby attenuating inflammatory signaling across multiple levels. This bidirectional regulatory mechanism facilitates the restoration of HPA axis homeostasis, enhances mitochondrial function, and maintains redox balance, collectively yielding a synergistic amplification of therapeutic outcomes.

The therapeutic promise of nanomedicine for RA-associated depression primarily rests on extensive preclinical data. However, uncertainties regarding long-term biocompatibility and the extrapolation of existing evidence constitute fundamental obstacles to clinical translation. Current studies indicate that certain inorganic nanomaterials-such as metal oxides or carbon-based nanostructures-may pose risks of slow degradation or tissue accumulation, potentially resulting in organ toxicity and metabolic burden upon chronic exposure. Even organic nanocarriers, which are generally regarded as biocompatible, might elicit hepatic, splenic, or immune system perturbations under repeated dosing regimens. Evidence suggests that some nanomaterials can activate complement pathways or induce nonspecific inflammatory responses, which may accelerate clearance and diminish therapeutic efficacy [165]. Regarding biomimetic nanocarriers and exosome delivery platforms, although their immuno-compatibility tends to be higher, factors such as cellular origin, membrane protein heterogeneity, and manufacturing process stability can significantly influence immunological behaviors and batch-to-batch consistency. In a chronic comorbid setting requiring prolonged intervention-such as RA-associated depression-maintaining therapeutic efficacy while avoiding immune tolerance or aberrant immune activation remains an urgent challenge in nanotherapeutic design.

It is noteworthy that the aforementioned safety uncertainties are often magnified within existing evidentiary frameworks. Most current studies rely on single-disease animal models-for example, models of pure arthritis or depression that are inadequate to recapitulate the complex phenotype of patients experiencing long-term coexistence of peripheral chronic inflammation, neuroimmune imbalance, and behavioral changes. Consequently, the therapeutic effects observed in preclinical nanoplatform studies may be overestimated when extended to real-world clinical scenarios. Additionally, substantial heterogeneity exists across studies regarding model construction, dosing regimens, administration frequency, and endpoints related to behavior and inflammation, severely limiting cross-study comparability. Of particular concern is the prevalence of studies employing acute or short-term administration paradigms, which do not align with the clinical necessity for sustained, repeated therapy inherent in RA-associated depression. This discrepancy further exacerbates the translational uncertainty. Furthermore, systematic validation of long-term safety, immunogenicity, cumulative dosing effects, and BBB targeting stability across species and stages of disease remains lacking. Head-to-head comparisons with free drugs or standard combination therapies are limited, and very few studies simultaneously evaluate both joint inflammatory control and neuropsychiatric outcomes within the same comorbid model, obscuring the true advantage of nanoplatforms within the holistic "joint-brain axis" modulation.

Therefore, future investigations must undergo a paradigm shift at the level of computational modeling and translational validation: transitioning from isolated disease models to validated, standardized RA-depression comorbidity models; expanding from short-term pharmacodynamic assessments to systematic evaluations of long-term safety profiles, immunological compatibility, and biocompatibility under repeated dosing regimens; and incorporating critical parameters such as chronic toxicity, immune responses, and clinical feasibility early within the research pipeline. Only through the iterative refinement of safety datasets and disease-specific models can the therapeutic potential of nanomedicine in RA-associated depression be robustly substantiated and effectively translated into clinical intervention.

Despite the promising outlook of nanotherapeutics in targeting RA and depression-related comorbidities, substantial translational barriers hinder progress from preclinical validation to routine clinical implementation. These challenges are systemic, encompassing aspects such as disease modeling fidelity, efficacy assessment frameworks, scalable manufacturing processes, and regulatory pathways.

In particular, issues related to the scale-up of nanopharmaceutical manufacturing and quality control are pivotal during clinical translation. Compared to conventional small-molecule therapies, nanocarriers impose more rigorous requirements on parameters including particle size uniformity, surface charge stability, drug loading efficiency, and batch-to-batch reproducibility. For multi-component, biomimetic, or engineered nanostructures, production frequently involves complex physical and biological processes, thereby increasing technical difficulties associated with industrial-scale manufacturing. These complexities can elevate production costs and give rise to inter-batch variability in biodistribution and therapeutic efficacy, which in turn magnifies uncertainties in clinical trial data and translational success. It is noteworthy that the majority of nanomedicine approaches under investigation remain at the preclinical stage, predominantly validated in rodent models, with no nanoplatforms yet achieving regulatory approval specifically for the management of depression comorbid with RA. Currently, regulatory frameworks predominantly adhere to traditional pharmacological evaluation paradigms, lacking standardized or conclusive criteria for multifunctional drug co-delivery systems, biomimetic nanostructures, or gene delivery nanocarriers. These unresolved safety and regulatory issues constitute primary bottlenecks hindering clinical translation. Future research should utilize more clinically relevant animal models exhibiting comorbid conditions, incorporate long-term safety and pharmacokinetic assessments, and evaluate manufacturability and regulatory compliance of scalable nanotherapeutic production early in the development process. Enhancing integrated feedback mechanisms between fundamental research and clinical requirements is vital to bridging translational gaps in nanomedicine for RA-associated depression.

Advancing nanotherapeutic strategies for RA-associated depression necessitates a shift from material-centric design towards mechanism-based rational engineering. Deepening insights into the dynamic interactions within inflammation-neuro-metabolic pathways are critical for developing more targeted, multifunctional nanocarriers. Furthermore, integrating nanomedicine with precision medicine approaches-personalizing interventions based on individual inflammatory and depressive phenotypes-may improve therapeutic efficacy. Interdisciplinary collaboration among material scientists, immunologists, neuroscientists, and clinicians will be essential to expedite bench-to-bedside translation. As safety evaluation protocols for biomaterials are refined and regulatory pathways elucidated, nanomedicine is positioned to assume increasingly central role in the systemic management of the complex comorbidity of RA and depression.