Nanoparticles in the Treatment of Renal Fibrosis

Introduction

CKD is defined as a persistent structural or functional abnormality of the kidneys, such as, renal glomerular filtration rate (GFR) reduction (< 60 mL/min/1.73 m²) or albuminuria > 30 mg per 24 h.¹ Arising from diverse etiologies, including diabetes, hypertension, and primary glomerular diseases, CKD represents a major global health concern, with a prevalence exceeding 10% and an even higher incidence in elderly populations.² Patients with CKD frequently develop multiple complications, imposing substantial clinical and economic burdens. Renal fibrosis constitutes a critical pathological hallmark in the progression of CKD, characterized by myofibroblast proliferation and extracellular matrix (ECM) accumulation, ultimately leading to a decline in renal function.³ The severity of renal fibrosis closely correlates with disease prognosis, making it a key therapeutic target in efforts to prevent CKD progression.

Traditional systemic therapies, including sodium-glucose cotransporter 2 inhibitors, non-steroidal mineralocorticoid receptor antagonists, and glucagon-like peptide-1 receptor agonists, are often limited by poor kidney-specific targeting, short circulation half-life, low renal drug utilization, and substantial off-target toxicities.^4,5 In addition, the unique anatomy of the kidney, characterized by specialized filtration barriers, pronounced cortical medullary compartmentalization, and highly selective permeability, further increases the difficulty for drugs to reach and act on specific pathological sites, resulting in suboptimal tissue exposure and therapeutic efficacy.^6,7 Collectively, these challenges underscore the need for advanced drug delivery systems capable of navigating renal physiological barriers and enabling more precise therapeutic intervention. In this context, nanotechnology-based delivery platforms offer unique advantages through their tunable physicochemical properties and targeting capacity.^5,8

NPs are materials engineered with atomic or molecular precision to exhibit unique physicochemical properties.⁹ Although classical nanoparticle definition restricts size to the nanoscale (1–100 nm), this criterion excludes many carrier systems used in biomedical applications. In the medical literature, carrier systems up to 1000 nm are frequently referred to as nanocarriers, and it has been proposed that nanocarriers be defined based on both function (ability to encapsulate or bind active ingredients) and size (at least one dimension <1000 nm).^10,11 Structurally, most NPs consist of an inner core or matrix surrounded by an outer shell, providing a versatile platform for encapsulating or conjugating a wide variety of therapeutic and diagnostic agents, including small molecules, proteins, peptides, nucleic acids, and imaging probes.¹² In recent years, NP-based therapeutic strategies for renal fibrosis have attracted increasing attention. A growing number of preclinical studies have explored NPs for the delivery of small-molecule drugs, nucleic acids, and biological agents aimed at modulating key fibrotic pathways, such as inflammation, myofibroblast activation, and extracellular matrix deposition.⁵ Although most renal nanotherapeutic approaches remain at the preclinical stage, these advances highlight the potential of nanotechnology to address long-standing challenges in the treatment of renal fibrosis.

In this review, we systematically reviewed the pathological mechanisms of renal fibrosis, the design principles of NPs for renal targeting, and presented representative examples of NP-based therapies in renal disease. In addition, we discussed the current challenges in clinical translation and provided perspectives on future directions for the development of nanomedicines targeting renal fibrosis.

Major Cell Types Participating in Renal Fibrosis

Renal fibrosis is increasingly recognized as a maladaptive extension of the normal wound-healing response following kidney injury.^13,14 In the early stages, transient ECM deposition may provide structural support and limit further damage. However, upon persistent or repetitive injury, this reparative response becomes dysregulated, leading to uncontrolled ECM protein accumulation, nephron loss and vascular rarefaction.^15,16 The resulting lesions, including glomerulosclerosis, tubular atrophy, interstitial fibrosis, and vascular remodeling, collectively compromise renal filtration and drive progressive renal failure.^17,18

The renal tubular epithelial cells (RTECs) and myofibroblasts are the main participants in renal fibrosis. Upon injury, resident mesenchymal cells are widely recognized to activate and differentiate into myofibroblasts,^19–21 which secrete pro-inflammatory mediators and ECM proteins. Instead of generating myofibroblasts,²² tubular epithelial cells promote the progression of fibrosis by acting as critical signaling hubs, mediating partial epithelial-to-mesenchymal transition (EMT),²³ inflammatory responses,²⁴ senescence,²⁵ and intercellular crosstalk.²⁶ In addition, other resident and infiltrating cell types, including mesangial cells, podocytes, and vascular smooth muscle cells, as well as macrophages, lymphocytes, and fibrocytes recruited from the circulation, also contribute to the renal fibrosis.^27–31

Myofibroblasts

In normal adult kidneys, type IV collagen is the predominant basement membrane component.^32,33 During renal interstitial fibrosis, fibrillar collagens, particularly type I and type III, are markedly upregulated, constituting the bulk of extracellular matrix deposition.^34,35 This pathological ECM accumulation is largely attributed to activated myofibroblasts, which are rare in normal kidneys but markedly increased in fibrotic kidneys.³⁶ Myofibroblasts possess characteristics of both fibroblasts and smooth muscle cells. The hallmark marker is α-smooth muscle actin (α-SMA), which forms stress fibers essential for matrix adhesion and cell contraction, facilitating matrix remodeling and wound closure.

However, the origin and heterogeneity of myofibroblasts remain incompletely understood. Resident mesenchymal cells, including fibroblasts, mesenchymal stem cells (MSCs), and pericytes, are considered common origins of myofibroblasts. Bone marrow-derived cells (macrophages and fibrocytes) and endothelial cells may also contribute to it.^37–39 But these studies are still limited and additional evidence is needed to substantiate their contribution. Recent single-cell sequencing of CKD human kidneys⁴⁰ identified three principal platelet-derived growth factor receptor (PDGFR)β⁺ myofibroblast sources: PDGFRα⁺ MEG3⁺ fibroblasts, COLEC11⁺ CXCL12⁺ fibroblasts, and PDGFRα⁻ RGS5⁺ NOTCH⁺ pericytes. Notably, increases in ECM genes in tubular epithelial cells are minimal, suggesting a minor role for EMT in renal fibrosis. The transition of pericytes or fibroblasts to myofibroblasts is regulated by activating protein-1 (AP-1) in early stage and transforming growth factor (TGF)-β1 in late stage, with transcription factor Runx1 driving transdifferentiation and upregulating several myofibroblast genes.⁴¹

Myofibroblasts are highly responsive to TGF-β1, which is the main regulator of myofibroblast differentiation.⁴² Experimental evidence strongly supports the significant role of activated myofibroblasts in renal fibrosis. Deletion of αV integrin, which is critical for TGF-β1 signaling, in mouse PDGFR-β⁺ cells reduce fibrosis in kidney unilateral ureteral obstruction (UUO), lung, and liver fibrosis models.⁴³ Consistently, a potent small-molecule inhibitor of αV integrin replicates this antifibrotic phenotype.⁴⁴ Furthermore, specific ablation of proliferating α-SMA+ myofibroblasts via a herpes simplex viral thymidine kinase system results in a 50–60% decrease in collagen deposition and interstitial fibrosis across multiple renal fibrosis models, thereby strongly indicating that proliferating myofibroblasts are principal cells driving fibrogenesis.³⁷

RTECs

RTECs are highly metabolically active and contain abundant mitochondria to support their reabsorptive and transport functions. Therefore, they are particularly vulnerable to various damages, such as ischemia, toxins, proteinuria, and metabolic disturbances. After injury, RTECs lose their polarity and cell-cell contacts, inhibiting normal tubular function and initiating repair processes.¹⁶ However, RTECs are both victims and active participants in renal fibrosis through complex mechanisms. When damage exceeds renal regenerative capacity, pathological changes drive RTECs to contribute to fibrosis through paracrine signaling and maladaptive cellular responses.

Inflammation is critical for tubular injury and fibrosis. Injured RTECs recruit and activate inflammatory cells, including monocytes/macrophages, lymphocytes, dendritic cells, and mast cells.⁴⁵ Proinflammatory M1 macrophages release cytokines such as interleukin (IL)-6, tumor necrosis factor α, IL-12, and nitric oxide, exacerbating tubular apoptosis and dysfunction.⁴⁶ Profibrotic macrophages secrete TGF-β1 to activates fibroblasts and remodels ECM.⁴⁷ TGF-β1 can also induce partial EMT in RTECs, which is characterized by dedifferentiation, loss of polarity, and acquisition of some mesenchymal traits, without a full transdifferentiation. Partial EMT enables RTECs to secrete profibrotic and proinflammatory mediators that contribute to fibroblast activation and ECM production, indirectly exacerbating fibrosis.⁴⁸

Apart from TGF-β1, several signaling pathways also regulate RTEC responses. Wnt/β-catenin signaling is transiently upregulated after injury to inhibit p53 and Bax expression and facilitate RTEC survival;⁴⁹ however, prolonged activation induces sustained epithelial dedifferentiation and interstitial fibrosis.⁵⁰ Hedgehog (Hh) ligands (Sonic and Indian Hh) show similar paracrine effects. They are increased in injured RTECs and accompanied with Hh pathway activation. Hh ligands activate interstitial pericytes and peritubular fibroblasts.⁵¹ The Notch signaling pathway involves Notch receptors (Notch1-Notch4) and their ligands, Delta and Jagged. Activation occurs through ligand–receptor interactions between neighboring cells, triggering intracellular signaling cascades that regulate gene expression critical for cell fate determination.^52,53 In chronic kidney disease, Notch signaling is reactivated, especially in epithelial cells, where it drives dedifferentiation and proliferation while inhibiting differentiation.⁵⁴ Notch activation is both necessary and sufficient to promote tubulointerstitial fibrosis, highlighting its key role in kidney fibrosis progression.

Design Criterion of NP Targeting Kidney

NPs can be produced from both organic and inorganic materials. Organic systems include liposomes, extracellular vesicles (EVs), biomimetic NPs, polymeric NPs, dendrimers, nanogels, and carbon-based nanostructures, while inorganic platforms encompass quantum dots, magnetic iron oxide particles, and metallic NPs.^55–57

In chronic renal fibrosis research, several classes of nanoparticle delivery systems have been investigated as platforms for antifibrotic therapy. Lipid-based nanoparticles represent a major class and include liposomes and solid lipid nanoparticles, which are structurally defined by lipid bilayer or lipid-core architectures that allow incorporation of drugs within aqueous or lipid compartments.⁴ EVs, naturally secreted lipid bilayer vesicles, have also emerged as promising nanocarriers due to their intrinsic biocompatibility, ability to transfer proteins and nucleic acids, and potential for cell-specific targeting.⁵⁸ Biomimetic organic nanoparticles, including biomimetic high-density lipoprotein (bHDL)-like nanoparticles and EV-mimetic assemblies, are distinguished by the incorporation of endogenous or biomimetic components that recapitulate structural features of natural lipoproteins or cellular vesicles.⁵⁹ Polymeric nanoparticles constitute one of the most widely studied categories and are typically fabricated from biodegradable polymers such as poly(lactic-co-glycolic acid) (PLGA) and related copolymers, in which therapeutic agents are encapsulated within a degradable polymer matrix.⁶⁰ Dendrimers are highly branched, monodisperse polymeric nanocarriers characterized by a well-defined architecture and multiple surface functional groups.⁶¹ In renal diseases, dendrimers have mainly been explored for acute kidney injury treatment and diagnostic detection.^62,63 However, their application in renal fibrosis therapy remains relatively limited. Beyond matrix nanoparticles, amphiphilic block copolymers can self-assemble into micelles that encapsulate therapeutic agents within nanoscale cores.⁶⁴ Nanogels constitute a further class of organic nanocarriers and are characterized by three-dimensional cross-linked hydrophilic polymer networks, commonly formed from materials such as poly(N-isopropylacrylamide) or chitosan derivatives.⁶⁵

A core strength of nanotechnology in medicine lies in the ability to control key physical parameters to optimize bioavailability, biodistribution, and target specificity. Such flexibility in design has led to two principal renal targeting strategies (Figure 1): passive targeting, which relies on physicochemical properties like size, surface charge and shape to navigate biological transport barriers,⁶⁶ and active targeting, which utilizes receptor-ligand interactions to achieve precise molecular recognition at desired tissue sites.⁶⁷ Active targeting can be achieved using ligands such as antibodies, antibody fragments, peptides, proteins, aptamers, or small molecules that bind to disease-specific biomarkers.

Figure 1 Nanoparticle properties to facilitate renal accumulation. Endothelial cells, glomerular basement membrane, and podocytes with slit diaphragms form the glomerular filtration barrier. Nanoparticle size, charge, shape, surface modification, and active targeting are the main factors in design to target kidney.

NP Size

The effectiveness and biodistribution of NPs in kidney disease are critically influenced by their size. Among the various physicochemical attributes of nanomaterials, particle diameter stands out as a major determinant of filtration, retention, clearance, and hence application potential in renal pathology.⁶⁸

The size of NPs is a critical determinant of their ability to traverse the glomerular filtration barrier (GFB), a key structural and functional interface governing renal clearance and kidney-specific drug delivery.^68,69 The GFB acts as a sophisticated multilayered filtration system separating the bloodstream from the urinary space within the renal glomerulus. It is composed of three major layers working in concert to regulate molecular passage based on size and charge. The innermost layer, the fenestrated glomerular endothelial cells, contains slit openings approximately 60–80 nm in diameter that facilitate plasma flow while retaining larger macromolecules. The middle layer, the glomerular basement membrane, is a dense, non-cellular extracellular matrix network with an average pore size of roughly 10 nm and a predominantly negative electrostatic charge that repels anionic macromolecules. The outermost layer consists of the interdigitating foot processes of podocytes, which are separated by slit diaphragms approximately 12 nm wide and reinforced by specialized proteins such as nephrin and podocin.⁷⁰ Together, these layers establish a precisely regulated permeability threshold that determines the renal filtration of NPs.

Physiological studies indicate that passive filtration across the intact GFB is governed by molecular size and charge selectivity. Small solutes traverse the barrier primarily via convective transport driven by the transcapillary hydrostatic pressure gradient, which outweighs the opposing oncotic pressure. In contrast, larger macromolecules are effectively excluded under physiological conditions due to steric hindrance and electrostatic repulsion.^71,72 Most NPs with size ranging from 1–10 nm, such as quantum dots or ultrasmall gold NPs, readily pass through the GFB and are efficiently cleared via urine, leading to low renal retention.^73–75 Those in the 10–20 nm range can still cross the GFB, especially if composed of soft organic materials.^76,77 It was previously thought that nanoparticles needed to be smaller than approximately 50–70 nm to pass through the fenestrations of the glomerular endothelium. However, larger particles can also accumulate in the kidney. For example, 90 nm Polyethylene glycol (PEG)-PLGA nanoparticles loaded with dexamethasone acetate and 79 nm PEGylated gold nanoparticles were reported to exhibit high renal accumulation because of their association with the mesangial cells, which directly connected to the fenestrated endothelium outside the glomerulus capillaries.^78,79

Theoretically, larger NPs (>100 nm) are typically excluded by the GFB. However, it has been reported that some large NPs can accumulate within proximal tubule epithelial cells. Wyss et al utilized transmission electron microscopy on kidney and observed the presence of intact spherical PLGA polymeric NPs with diameters ranging from 130 to 160 nm in glomerular vascular tuft, proximal convoluted tubule (PCT) cells and the tubule lumen.⁸⁰ They proposed that those NPs were transported from the vasculature of the glomerular tuft, taken up by PCT cells and then secreted into tubular lumen. However, it remains to be proven. Williams et al synthesized polymer-based mesoscale NPs of 400 nm diameter and intravenously injected in mice.⁸¹ The accumulation of NPs in the kidney was approximately 7-fold higher than in other organs, and they were primarily localized in the basal membrane of proximal tubule epithelial cells. They proposed that those NPs were initially endocytosed by endothelial cells of the peritubular capillaries, as demonstrated in vitro and supported by previous studies. Subsequently, they were transcytosed across the endothelial cells and released into the tubulointerstitial space between the capillaries and proximal tubule epithelial cells. From this interstitial space, they were taken up by proximal tubule epithelial cells via basolateral endocytosis, as confirmed in vitro. Similarly, Deng et al also reported that PLGA-b-mPEG mesoscale NPs approximately 400 nm in diameter selectively accumulated in proximal tubule cells, exhibiting the highest fluorescence intensity in the kidney compared to other organs.⁸² Although these studies provide convincing evidence that some mesoscale NPs selectively target proximal tubule epithelial cells and propose plausible transport mechanisms, in vivo validation of these pathways remains limited. The exact cellular and molecular processes governing nanoparticle uptake, transcytosis, and basolateral entry into tubular cells have yet to be rigorously confirmed.

During renal fibrosis, GFB structural remodeling, including basement membrane thickening with widened pores, and podocyte detachment resulting in the disruption of slit diaphragm integrity, leads to a marked reduction in size-selective filtration capacity.^8,83 Mesangial expansion and increased ECM deposition can also introduce additional barriers to diffusion.⁸⁴ However, the extent to which mesangial remodeling and ECM accumulation modulate renal nanoparticle uptake has not been systematically investigated, which represents an important gap for future studies aimed at optimizing kidney-targeted nanomedicine.

NP Charge

NP charge plays an important role in determining their interaction with the kidney, particularly with the GFB, which exhibits charge selectivity due to the abundance of negatively charged heparan sulfate in the endothelial glycocalyx and anionic proteoglycans on the glomerular capillary wall and podocyte glycocalyx.⁸⁵ These structural features create an electrostatic barrier that preferentially restricts the passage of negatively charged molecules, while allowing neutral and positively charged molecules to filter through.⁷⁴ Consequently, NPs of different surface charges, even when similar in size, display distinct renal biodistribution and clearance profiles.

Generally, positively charged NPs traverse the GFB more efficiently than negatively charged counterparts. Experimental studies using quantum dots⁷⁴ and micelles⁸⁶ have consistently demonstrated that cationic NPs exhibit higher renal accumulation and faster urinary excretion compared to anionic particles of comparable size. In terms of renal clearance, positively charged NPs show markedly enhanced kidney localization compared with their negatively charged analogs. Small cationic NPs, such as amine-functionalized copper sulfide or silicon nanodots, achieve nearly complete renal clearance within 24 hours after injection.^82,87 In contrast, negatively charged gold NPs of similar hydrodynamic diameters display significantly lower clearance efficiency,^88,89 illustrating that surface charge can modulate renal elimination kinetics.

Nevertheless, charge-dependent renal processing is not solely governed by simple electrostatic interactions. It has been reported that negatively charged Ficoll displays increased glomerular filtration compared to neutral Ficoll.^90,91 Among negatively charged nanomaterials, there is an inverse correlation between surface charge and renal clearance efficiency. Specifically, nanomaterials with more uniform and stronger negative surface charges exhibit higher renal clearance rates.^88,92

Indeed, NPs with high absolute zeta potential, whether positive or negative, tend to bind serum proteins more strongly, promoting opsonization and uptake by the mononuclear phagocyte system, thereby reducing renal filtration efficiency.^93,94 Conversely, NPs with near-neutral or zwitterionic surfaces had minimal macrophage uptake, which enables longer systemic circulation.^73,95,96 Neutral and zwitterionic coatings, such as sulfobetaine or organosiloxane, have been shown to preserve colloidal stability in serum and facilitate kidney entry.^97,98

Overall, the interplay among NP charge, surface chemistry, and protein interactions critically influences renal transport, clearance, and tissue retention. While cationic surfaces promote rapid renal filtration and excretion, neutral or zwitterionic surface modifications provide a more balanced approach, enhancing NP stability and prolonging circulation by reducing opsonization, thereby improving renal targeting and biodistribution. Importantly, the precise in vivo dynamics of NPs at the GFB remain poorly understood, underscoring the need for further investigation into renal NP transport.

NP Shape

NP geometry has been increasingly recognized as a critical factor influencing renal clearance and biodistribution. In the circulation, convective forces in blood flow dominate over Brownian motion, rendering shape a major determinant of hydrodynamic behavior and biological interactions.⁹⁹ Non-spherical NPs, including rods, disks, filamentous micelles, and sheets, exhibit distinct margination dynamics and altered interactions with immune cells compared to spherical particles.^100–102 NP with high aspect ratio, such as nanorods and nanoworms, often display prolonged circulation half-lives due to reduced macrophage uptake under shear flow, which in turn affects their organ distribution patterns.^{101,103–105}

Recent studies have shown that particle aspect ratio can modulate glomerular filtration efficiency independent of molecular weight. For instance, single-walled carbon nanotubes (1.2 nm in diameter, 100–1000 nm in length) can pass through the GFB despite molecular weights (300–500 kDa) exceeding the renal cutoff for globular proteins.^106–108 Their narrow cross-sectional dimension allows alignment with flow and insertion of their longitudinal axis into filtration slits, suggesting that directional orientation facilitates filtration. Comparable effects of geometry were found in polymeric dendrimers. 18 nm diameter Spherical polyamidoamine dendrimer with 2 kDa outer layer PEG showed negligible renal clearance (<10% ID/g), whereas the addition of two 20 kDa PEG chains transformed the NPs into elongated, cylindrical structures that achieved high kidney excretion (~80% ID/g).^109,110

Furthermore, temporal studies indicate that geometry not only determines the extent but also the kinetics of kidney accumulation. Shorter rods and smaller anisotropic particles tend to exhibit early renal uptake, whereas longer or flexible structures show delayed but sustained accumulation due to prolonged circulation and reduced reticuloendothelial recognition.¹¹¹

Beyond simple aspect ratio effects, three-dimensional geometries also modulate renal interactions and clearance behavior.^112,113 DNA origami nanostructures with triangular, rectangular and tubular geometries have been reported to exhibit preferential renal accumulation in both healthy mice and mice with acute kidney injury, as evidenced by noninvasive quantitative imaging after systemic administration.¹¹⁴ Radiolabelled intact DNA origami constructs localized prominently in the kidneys with comparatively low hepatic uptake, whereas partially folded DNA origami assemblies and the scaffold M13 single‑stranded DNA exhibited predominant liver sequestration, indicating a marked impact of structural integrity on biodistribution. PET imaging and ex vivo analyses demonstrated that all three fully folded DNA origami architectures accumulated in kidney tissue significantly more than shorter single‑stranded or partially folded analogues, underscoring the role of proper folding and compact architecture in modulating organ partitioning. DNA tetrahedron nanoparticles, a minimal wireframe DNA geometry, also displayed efficient renal clearance in mouse models, further illustrating that compact DNA nanostructures can be efficiently processed by the renal system.⁷⁶ Moreover, kidney-targeted DNA tetrahedral frameworks have been exploited for targeted cytosolic siRNA delivery, showing pronounced renal uptake and functional delivery to tubular cells.¹¹⁵

Taken together, these studies highlight that NP shape exerts a multifactorial influence on renal pharmacokinetics. Both simple anisotropy, such as high aspect ratio rods and cylinders, and more complex three-dimensional architectures, such as DNA nanostructures, can modulate the kinetics and tissue-specific distribution of nanoparticles.

NP Surface Modification

Various proteins in biological fluids can adsorb onto the NP surface forming protein corona. Protein corona not only changes the NP physicochemical properties, including charge, hydrophobicity, and functional ligands but also imparts a new biological identity that dictates recognition by the mononuclear phagocyte system (MPS).¹¹⁶ Surface engineering strategies have been widely employed to overcome the limitation. PEG conjugation remains a well-established approach to sterically hinder protein adsorption and reduce MPS uptake, thereby prolonging systemic circulation.¹¹⁷ However, PEGylation can also reduce cellular uptake by targeting cells,^118,119 highlighting the need to balance circulation longevity with effective delivery.

Beyond PEGylation, additional surface functionalization strategies, including short amino acid repeat peptides, single amino acid modifications, and prodrug-like display, have emerged to achieve more precise targeting. L-serine modified polyamidoamine (PAMAM) dendrimers have been reported to preferentially accumulate in the proximal tubules of the renal cortex after intravenous administration in mice.^120,121 In these constructs, multiple serine residues are covalently attached to the PAMAM surface. Because serine is an endogenous molecule with well-defined physicochemical characteristics, this modification has been suggested to provide advantages over conventional kidney-targeting moieties, including safety and synthetic simplicity.¹²² Chen et al engineered a multifunctional kidney-targeted nanoplatform based on a PAMAM dendrimer.¹²³ In this system, dendrimers were modified with multiple glutathione (GSH) moieties and covalently linked to triptolide (TP) to form a prodrug-type dendrimer–drug conjugate. The design exploited the high expression of γ-glutamyltransferase (GGT) in glomerular capillary endothelial cells of childhood nephrotic syndrome. GGT catalyzed γ-glutamyl transfer reactions of GSH, generating positively charged primary amine groups on the carrier surface. This charge conversion facilitated rapid caveolae-mediated endocytosis and subsequent transcytosis, thereby enhancing renal accumulation and cellular uptake. TP was progressively released through intracellular enzymatic hydrolysis, providing sustained drug exposure.

NP Active Targeting

Beyond the passive accumulation determined by NP physicochemical parameters such as size, charge, and shape, active targeting strategies have emerged as an essential approach to achieve precise renal drug delivery.

In the glomerular compartment, mesangial cells express a variety of receptors that can be leveraged for targeted nanotherapy, including mannose receptors, α8 integrin, Thy1.1 antigen, and α_vβ₃ integrin.^124–126 NPs functionalized with ligands recognizing these receptors have shown enhanced accumulation and therapeutic efficacy. For instance, mannose-modified polycationic cyclodextrin NPs have been successfully used to deliver siRNA selectively to mesangial cells.¹²⁷ α8 integrin-targeted liposome-PLGA NPs carrying dexamethasone and captopril significantly ameliorated glomerular inflammation and fibrosis.¹²⁸ And Thy1.1-directed liposomes and dual-ligand systems targeting both AT1r and α_vβ₃ integrin have demonstrated precise mesangial localization and suppression of extracellular matrix deposition.^129,130 In addition, peptide sequence CSAVPLC targeting glomerular endothelium successfully increases kidney accumulation of its conjugated NP by 2.6 times.¹³¹ Albumin-conjugated NPs target glucocorticoid receptors and the neonatal Fc receptor, which is highly expressed on podocytes, to enhance localization and therapeutic efficiency in glomeruli.^132,133

In renal tubules, active targeting primarily relies on the recognition of receptors expressed on RTECs, notably megalin and cubilin, which mediate endocytic uptake of various ligands.^134,135 Nanocarriers conjugated with aminoglycoside antibiotics gentamicin and neomycin accumulate predominantly in kidney through megalin receptor binding, resulting in a 13-fold enhancement in gene transfection efficiency compared with unmodified NPs.¹³⁶ Similarly, polymyxin B or low molecular weight chitosan (LMWC), which have also shown strong affinity for megalin, promotes NP uptake into proximal tubular cells and supports the delivery of therapeutic cargos such as metformin or fluorescent reporters.^137,138 Additionally, the kidney-targeting peptide (KKEEE)₃K has been identified as a specific binding ligand for megalin on proximal tubular cells.¹³⁹ When conjugated to 15 nm PEGylated micelles, this peptide enabled selective renal accumulation.¹⁴⁰ These designs highlight the potential of megalin-mediated endocytosis for treating tubular fibrosis and other chronic kidney pathologies.

Emerging active-targeting strategies also take advantage of pathological microenvironments. In fibrotic kidneys, ligand modification of NPs with kidney injury molecule-1 (KIM-1) antibodies allows selective binding to injured tubular epithelial cells, markedly increasing local drug concentration and therapeutic benefit.^141,142 Furthermore, environment-responsive systems, such as pH-sensitive NPs that disassemble under acidic conditions, enable controlled drug release while minimizing systemic exposure.¹⁴³

In addition to synthetic nanoparticle platforms, biologically derived vesicles such as exosomes have emerged as a distinct class of actively targeted nanocarriers for renal delivery.¹⁴⁴ They have more abundant entry routes than artificial nanoparticles, including receptor-mediated endocytosis, phagocytosis, macropinocytosis, lipid raft interactions, and direct fusion.^145–147 Exosomes can be engineered via surface modification or cargo loading to increase renal uptake and targeting efficiency.¹⁴⁸ Endogenous loading integrates therapeutic molecules into exosomes through genetic modification of parent cells,¹⁴⁹ while exogenous loading introduces small molecules or RNAs into pre-isolated exosomes using chemical or physical methods.¹⁵⁰

Taken together, these active targeting strategies underscore the transition from nonspecific passive accumulation to precision-guided renal nanomedicine. The integration of peptides, antibodies, and stimuli-responsive designs provides multiple options for achieving high-efficiency, site-selective NP delivery to distinct kidney compartments.

NPs in the Treatment of Renal Fibrosis

Targeting RTECs

There are many ongoing studies focusing on NPs targeting RTECs to treat renal fibrosis (Table 1). These studies include various strategies, such as assembling NPs with antibodies against KIM-1, which is expressed in injured RTECs, and utilizing chitosan-based materials to achieve effective targeting.

Table 1 NPs Targeting RTECs in the Treatment of Renal Fibrosis

In chronic kidney disease, persistent endoplasmic reticulum dysfunction and stress-driven apoptosis accelerate renal damage.¹⁶⁰ Thapsigargin (TG) induces moderate endoplasmic reticulum stress and activates autophagy to protect renal cells from oxidative injury.¹⁶¹ Building on this mechanism, KIM-1-targeted TG NPs (KIM-1-TG NPs) were developed to achieve kidney-specific delivery through targeting injured RTECs. These NPs were made from PLGA, and remained stable at neutral pH but release TG within acidic intracellular environments, ensuring precise, localized drug action. In an adenine diet-induced CKD mouse model, low doses (0.2 mg/kg) of KIM-1-TG NPs effectively reduced renal injury.¹⁵¹ Lin et al¹⁴² utilized similar carrier to synthesized KIM-1 targeting NPs loaded with resveratrol (Res). Res is a natural polyphenol exhibiting beneficial effects in several renal diseases, such as diabetic nephropathy, drug-induced injury, and unilateral ureteral obstruction.^162–164 In adenine -induced CKD mouse model, Res NPs increased AMPK and inhibited the Akt/mTOR signaling pathways to induce autophagy, therefore suppressing NLRP3 inflammasome and ameliorating CKD. It is reported that KIM-1-bHDL NPs loaded with both anti-inflammatory drug TP and anti-fibrotic drug nintedanib (BIBF) reduced renal fibrosis in UUO model, folic acid model and adenine model mice via remodeling the fibrosis niches.¹⁵² Exosomes could also be engineered as targeted delivery vehicles to injured renal tubular epithelial cells. For example, red blood cell–derived extracellular vesicles (REVs) have been modified with a KIM-1-binding peptide, enabling selective accumulation in injured tubules. Using this platform, siRNAs against the transcription factors P65 and Snai1 were efficiently delivered to ischemic kidneys, resulting in reduced inflammation and fibrosis.¹⁵³

LMWC can be selectively taken up by RTECs through megalin-mediated endocytosis.¹⁶⁵ Therefore, LMWC and its modified derivatives have been widely employed in the treatment of renal fibrosis. Wischnjow et al¹⁵⁴ synthesized GFP-loaded metformin-grafted chitosan NPs (CS-MET/GFP NPs). After intravenous injection, those NPs accumulated in kidney and released inside the cells, exerting anti-apoptotic, anti-inflammatory, and anti-fibrotic effects in UUO mouse model. Ghavimishamekh et al¹⁵⁵ also reported that oral intake of insulin-loaded trimethyl chitosan NP reduced TGF-β1 expression and amelibrated diabete-induced nephropathy in type 1 diabetic rat model. Qiao et al developed a kidney-specific drug nanocomplex using coordination bonding with catechol-modified low molecular weight chitosan (HCA-Chi) as the polymeric carrier.¹⁵⁶ The stable nanocomplex circulates at physiological pH, while upon lysosomal internalization, acidic pH triggers partial drug release by breaking coordination bonds, allowing subsequent transmembrane transport and paracrine effects in the tubulointerstitium. Emodin is a natural anthraquinone compound that exerts its anti-renal fibrosis effects via multiple mechanisms, including mediating mitochondrial homeostasis,¹⁶⁶ inhibiting TGF-β1/Smad signaling pathway,¹⁶⁷ and regulating autophagy¹⁶⁸ and apoptosis.¹⁶⁹ The HCA-Chi-Zn-emodin ternary nanocomplex effectively attenuates renal fibrosis by targeting and suppressing myofibroblast activation, as demonstrated by reduced α-SMA expression in UUO-injured kidneys. Histological analyses confirmed that this nanocomplex decreases tubular dilation, epithelial atrophy, and collagen I accumulation. In addition, HCA-Chi-Ca-salvianolic acid B (Sal B) NPs showed similar anti-fibrosis effects in UUO mouse model.¹⁵⁷

MicroRNAs (miRNAs) are negative regulators of gene expression, through repressing mRNA translation or promoting mRNA degradation. Recent research has revealed their significant role in the progression of kidney fibrosis.^170,171 However, miRNAs and siRNAs are inherently unstable and vulnerable to degradation by nucleases in biological environments. To overcome this limitation, NPs have been employed to encapsulate these nucleic acids, protecting them from nuclease-mediated degradation. miRNA-21 promoted renal fibrosis through TGF-β1/Smad3 and extracellular signal-regulated kinases/mitogen-activated protein (ERK/MAP) kinase signaling pathway.¹⁷² Geng et al¹³⁸ synthesized small-sized cationic miRNA-21 inhibitor (miRi)-PCNPs which utilized LMWC-modified PLGA to specifically deliver miRNA-21 inhibitor to RTECs. In UUO model, mice treated with miRi significantly reduced extracellular matrix accumulation, while fibrosis area after miRi-PCNPs treatment was 2.5-fold lower than miRi treatment. Khaja et al¹⁵⁸ reported that they utilized galactosamine-conjugated sterically stabilized phospholipid NPs (SSLNPs) to delivery small interfering RNA (siRNA) for connective tissue growth factor (CTGF), which was the important regulator of fibrosis in both hepatic and renal cells.^173,174 Bio-distribution and pharmacokinetic studies in mice confirmed their targeted delivery to liver and kidney tissues. However, their therapeutic effect on renal fibrosis still requires further investigation.

CD44 is expressed on both glomerular and tubular cells in the kidney, including renal epithelial cells, mesangial cells, and fibroblasts.^175–177 Chitosan/hyaluronan NPs carrying plasmid DNA (CS/HA^pDNA NPs) targets glomerular and tubular cells through HA binding CD44 receptors. CS/HA^pDNA NPs expressing bone morphogenetic protein 7 (BMP7) reversed renal fibrosis and regenerated tubules, whereas delivery of hepatocyte growth factor NK1 (HGF/NK1) eliminated collagen fiber deposition.¹⁵⁹

Targeting Glomeruli and Other Cells

Beyond renal tubular epithelial cells, NP therapies have been engineered to target other renal cell populations for precise and effective treatment (Table 2). Zhou et al¹²⁸ utilized PEG-modified liposome-PLGA, α8 integrin antibodies, dexamethasone (DXMS) and captopril (CAP) to synthesize DXMS/CAP@PLGA-ILs. The resulting NPs have a core-shell structure, suitable particle size (~119 nm), low cytotoxicity, and the ability to specifically accumulate in the glomerular mesangial cell region. DXMS/CAP@PLGA-ILs were intravenously injected in a mouse model of mesangial proliferative glomerulonephritis (MesPGN). In vivo pharmacodynamic studies demonstrated that DXMS/CAP@PLGA-ILs significantly decreased the levels of inflammatory factors, fibrotic markers, and reactive oxygen species (ROS), thereby alleviating renal inflammation and fibrosis. The design of 2i@DuaLR NPs mainly utilized passive targeting strategy: the size of 2i@DuaLR is 110 nm, which was larger than podocytes feet slits and smaller than glomerular endothelium pores; cationic octa-arginine (R8) peptide in 2i@DuaLR NPs was trapped in glomeruli with polyanionic surface glycoprotein.¹⁷⁸ 2i@DuaLR NPs, which were loaded with both p38α MAPK and p65 siRNA, successfully silenced p38α MAPK and p65 genes in mesangial cells and endothelial cells, and eventually alleviated proteinuria, inflammation and excessive extracellular matrix deposition.

Table 2 NPs Targeting Glomeruli and Other Cells in the Treatment of Renal Fibrosis

Rosiglitazone (RSG) is an exogenous agonist of peroxisome proliferator-activated receptor (PPAR)γ, which is widely expressed in the medullary collecting duct, glomeruli, and proximal tubular cells.¹⁸⁵ RSG-induced PPARγ activation inhibits inflammatory cytokine secretion and prevents ECM synthesis through inhibiting TGF-β1 signaling pathway.¹⁸⁶ PLNPs-RSG-MBs contain ultrasound-targeted microbubbles. When exposed to ultrasound, microbubbles mediate sonoporation, which results in temporary increase of cellular membrane or microvasculature permeabilization, and promotes the intracellular RSG uptake without inducing glomerular injury.¹⁸⁷ In the UUO rat model, PLNPs-RSG-MBs treatment with ultrasound exposure significantly reduced the expressions of TGF-β1, α-SMA and Collagen I.¹⁷⁹

Even though chitosan is considered a powerful RTEC-targeting strategy, it was also demonstrated that chitosan carrying cyclooxygenase type 2 (COX-2) siRNA accumulated in macrophages in the obstructed kidney of UUO mice.¹⁸⁰

Cationic bovine serum albumin (cBSA)/ miR-29b NPs loaded into shear-thinning F127-HA hydrogel (gel) could form a supramolecular hydrogel hybrid platform. A single injection of the hydrogel loaded with cBSA/miR-29b in UUO mice significantly downregulated proteins and genes related to fibrosis without affecting the normal liver or kidney functions.¹⁸¹

Wang et al¹⁸² reported that they engineered a delivery system for miR-29 by fusing the exosomal membrane protein Lamp2b with the rabies viral glycoprotein (RVG) peptide, which enables targeting to tissues expressing acetylcholine receptors. These engineered exosomes were administered intramuscularly in a UUO mouse model and partially attenuated renal fibrosis, which was associated with suppression of YY1 and TGF-β signaling pathways. EVs derived from genetically engineered MSCs were used as a delivery platform for erythropoietin (EPO) in the treatment of renal anemia.¹⁸³ Following intraperitoneal administration in a mouse model of chronic kidney disease, EPO⁽⁺⁾-EVs significantly increased hemoglobin levels and improved renal function. Although the precise mechanisms governing the distribution of EVs to injured kidneys were not fully defined, the authors suggested that surface adhesion molecules and integrins on EV membranes may contribute to their accumulation in damaged renal tissue. In another study, bioactive scaffolds incorporating multifunctional EVs (mEVs) were developed to promote kidney regeneration.¹⁸⁴ The mEVs consisted of a combination of EVs derived from SDF-1α overexpressing tonsil-derived MSCs and EVs from intermediate mesoderm cells differentiating toward kidney progenitor cells, aiming to enhance stem cell recruitment and renal lineage differentiation. These vesicles were embedded in porous PLGA-based scaffolds together with the antioxidant edaravone (EDV), which supported tubular regeneration and improved renal function through activation of the GDNF/RET signaling pathway. This work illustrates that EVs can also be integrated into scaffolds to provide sustained release and synergize with regenerative cues for kidney tissue engineering.

Challenges and Future Perspectives

Clinically, these NP-based strategies have the potential to shift renal fibrosis management from symptomatic treatment toward precise, mechanism-based interventions that can slow disease progression, improve kidney function, and reduce the need for dialysis or transplantation.

However, significant challenges remain before these approaches can be successfully translated into clinical practice. One of the primary barriers lies in the precise control of NP physicochemical properties, which critically determine renal accumulation and cellular uptake. Oversized particles may fail to penetrate the dense ECM of fibrotic kidneys, while smaller ones are prone to rapid renal clearance. Variability among patients in CKD stages introduces further complexity, as the structural and functional alterations in kidney, such as changes in filtration barrier integrity and pore size distribution, can significantly affect NP trafficking and retention.

In addition, ensuring NP stability and biocompatibility in vivo remains a formidable task. Synthetic nanocarriers, including polymeric and inorganic systems, may exhibit uncontrolled biodistribution or induce immune activation, compromising long-term safety.^188,189 Although biomimetic and cell membrane-coated NPs have shown improved circulation time and immune evasion, their production still faces challenges related to scalability, standardization, and compositional heterogeneity. The absence of unified manufacturing standards and good manufacturing practice (GMP)-grade synthesis for complex nanocarriers further limits reproducibility and comparability across studies.¹⁹⁰ Ongoing efforts toward modular, automated GMP-compatible synthesis and AI-assisted quality control are expected to bridge this gap and accelerate clinical readiness.

Another challenge arises from the limitations of current animal models, which fail to fully recapitulate human renal fibrosis. Animal models remain indispensable for evaluating the pharmacokinetics, biodistribution, and therapeutic efficacy of NP-based systems in renal fibrosis. Several studies have demonstrated cross-species correlations in NP pharmacokinetics, suggesting that, under certain conditions, preclinical models can partially predict human outcomes.^191–193 However, the anatomical and histological complexity of the human kidney, along with disease-specific heterogeneity such as glomerular sclerosis and interstitial remodeling, is difficult to fully recapitulate in commonly used animal models such as UUO rodents.¹⁹⁴ Recently, the emergence of human kidney organoids, precision-cut kidney slices, and humanized mouse models offers promising platforms to bridge this gap by providing physiologically relevant systems under human-like conditions.^195–197

Looking forward, several directions may accelerate progress in this field. The integration of artificial intelligence and machine learning will optimize the nanocarrier design by predicting the ideal physicochemical parameters for renal delivery, modeling NP-ECM interaction, and guiding ligand selection based on single-cell transcriptomic data.^198,199 Multi-functional and stimuli-responsive nanocarriers capable of co-delivering anti-fibrotic, anti-inflammatory, and ECM-degrading agents might be an option to overcome the multifactorial nature of renal fibrosis. Combining such systems with microfluidic synthesis and high-throughput screening technologies allow the controlled production of NPs with narrow size distributions and tunable surface chemistry suitable for clinical translation.^200,201 With continued innovation in material science, advanced modeling, and humanized disease systems, NP-based therapeutics hold great promise to transform the current therapeutic landscape of renal fibrosis, shifting it from symptomatic management toward precise, mechanism-based intervention and regeneration.