Precision Nanomedicine for Renal Tubular Injury: From Passive Accumulation to Subcellular Targeting

Abstract

Renal tubular epithelial cell injury is a central pathogenic feature of kidney diseases, yet precise therapeutic interventions remain elusive. Conventional nanomedicines relying on passive accumulation lack the specificity required to treat complex tubular pathologies. This review elucidates the evolution of renal nanotherapeutics from non-specific organ accumulation to active, biologically informed targeting. We examine strategies utilizing specific surface receptors and injury-associated microenvironmental signals to engineer ligand-directed and stimuli-responsive nanocarriers. Furthermore, we highlight biomimetic platforms, such as engineered extracellular vesicles. Crucially, to achieve the ultimate goal of precision intervention, we emphasize the necessity of subcellular targeting. With a primary focus on mitochondria, we delineate hierarchical delivery strategies designed to restore cellular bioenergetics and metabolic homeostasis. Finally, we provide a forward-looking perspective on overcoming translational barriers by integrating DNA barcoding and artificial intelligence to accelerate the discovery of clinically translatable, precision nanomedicines.

Introduction

Kidney diseases, encompassing acute kidney injury (AKI) and chronic kidney disease (CKD), impose a substantial and escalating global health burden. Epidemiological data indicate that AKI affects approximately 10–15% of hospitalized patients and exceeds 50% of intensive care unit admissions, whereas CKD prevails in over 10% of the general population.^1–3 Beyond high short-term mortality, AKI episodes significantly precipitate the risk of subsequent CKD, end-stage kidney disease (ESKD), and cardiovascular events, thereby amplifying the long-term clinical and socioeconomic impact.² Despite these severe outcomes, therapeutic options remain predominantly supportive, focusing on hemodynamic optimization, nephrotoxin avoidance, and renal replacement therapy. Crucially, no approved pharmacological agents currently exist to specifically prevent, repair, or regenerate injured renal tubules.^1,2 This therapeutic gap is particularly critical because renal tubular injury constitutes the core pathology driving the progression of a broad spectrum of kidney diseases. As the most abundant and metabolically active cells in the renal parenchyma, tubular epithelial cells act as a critical pathogenic nexus. Following acute ischemic or toxic insults, the maladaptive repair of damaged tubules and their aberrant crosstalk with immune cells orchestrate the detrimental transition to chronic interstitial fibrosis and eventual organ failure. Consequently, directly alleviating tubular injury represents a fundamental therapeutic requirement to halt disease progression, thereby underscoring the urgent clinical need for the precision nanomedicine platforms discussed in this review.^1–3

However, the limited efficacy of current therapies is partially attributable to the pharmacokinetic constraints inherent to the systemic administration of small-molecule drugs. Such systemic approaches frequently engender suboptimal drug concentrations within the renal parenchyma while promoting substantial off-target accumulation in reticuloendothelial organs, particularly the liver and spleen, which precipitates dose-limiting systemic toxicities. These biodistribution challenges are particularly problematic as renal tubules occupy a pivotal position in disease pathophysiology. Tubular epithelial cells (TECs), specifically proximal tubular epithelial cells (PTECs), constitute the majority of the renal parenchyma and exhibit intrinsic vulnerability to metabolic and ischemic insults owing to their high energy demands and reliance on mitochondrial oxidative phosphorylation. Moreover, PTECs act as key drivers of disease progression, serving as hubs for metabolic reprogramming, inflammatory signalling, and the activation of profibrotic pathways that culminate in ESKD.^4–6 Consequently, the development of renal tubule-targeted nanomedicine platforms capable of traversing biological barriers to selectively deliver therapeutics to these vulnerable cells has become an urgent priority.

Nanomaterial-based delivery systems have emerged as a promising frontier to enhance renal drug retention while mitigating systemic toxicity.⁷ However, the majority of existing renal nanomedicines continue to rely on passive accumulation, a process dictated strictly by physicochemical properties rather than specific molecular recognition. This passive targeting operates via two primary mechanisms: filtration-dependent accumulation, where small nanostructures traverse the glomerular filtration barrier (GFB) into the tubular lumen, and filtration-independent accumulation, where larger nanoparticles or extracellular vesicles (EVs) extravasate from peritubular capillaries to access the basolateral membrane.^8–10 In both scenarios, delivery is fundamentally constrained by particle size, charge, and geometry, parameters that rarely permit cell-type-specific uptake. Such non-specific distribution often results in suboptimal therapeutic efficacy and persistent off-target exposure, underscoring the inability of passive strategies to precisely address the complex pathology of tubular injury.⁷ To surmount these constraints, a paradigm shift from passive accumulation toward active tubular-targeting strategies is imperative.¹¹ Active targeting exploits distinct molecular signatures of injured tubules by functionalizing nanocarriers with specific moieties such as antibodies, peptides, or small-molecule ligands. By functionalizing nanocarriers with specific moieties—such as antibodies, peptides, or small-molecule ligands—active targeting exploits the distinct molecular signatures of injured tubules. This approach enables nanomedicines to bypass reliance on stochastic filtration or extravasation, thereby maximizing therapeutic retention in target epithelial cells while minimizing accumulation in healthy tissues.^12–14 Consequently, the development of biologically informed active-targeting platforms represents a critical evolution in renal nanomedicine, offering the spatiotemporal control necessary to intervene effectively in the progression of tubular injury.^7,15

This review establishes a comprehensive framework for developing next-generation nanomedicines tailored to renal tubular injury. We first examine the pathophysiology of tubular epithelial cells, elucidating how their distinct structural and metabolic specializations define rational entry points for therapeutic intervention. Subsequently, we systematically categorize emerging active targeting strategies, spanning ligand–receptor interactions, microenvironment-responsive platforms, and biomimetic vesicle-based carriers. A central focus is directed toward mitochondria as a pivotal subcellular frontier, where we explore hierarchical targeting strategies designed to restore metabolic homeostasis. Finally, we discuss critical translational barriers and future opportunities, advocating for the integration of advanced biotechnologies to accelerate the clinical realization of these precise therapeutic systems.

Renal Tubules: From Physiological Architecture to Pathobiological Vulnerability

Architectural and Functional Fundamentals

The renal tubular system constitutes the predominant structural component of the renal parenchyma, comprising the proximal tubule, the loop of Henle, the distal convoluted tubule, and the collecting duct system. Within this complex network, the proximal tubule is consistently identified as the primary susceptibility locus in both ischemic and toxic AKI, positioning PTECs as the focal point for nanomedicine design.^1,2,6 As illustrated in Figure 1, a comprehensive understanding of the distinct physiological specializations of PTECs, particularly their metabolic reliance and mitochondrial density, provides the biological rationale for targeted therapeutic interventions.

Figure 1.

Physiological Specialization and Pathological Reprogramming of PTECs. (A) Segmental heterogeneity under physiological conditions. The cortical S1 segment features tall epithelial cells characterized by dense apical brush borders and abundant basolateral Na⁺/K⁺-ATPase pumps to support robust active transport. Transitioning through the S2 segment, epithelial height and brush border density progressively decline. The S3 segment, located in the outer medulla, resides in a physiologically hypoxic microenvironment, rendering it intrinsically vulnerable. Metabolically, healthy PTECs across these segments rely on FAO driven by PPARα and ERRα for ATP production. (B) Metabolic and structural reprogramming of injured PTECs. Upon ischemic or toxic insults, injured PTECs undergo profound maladaptation. The suppression of FAO enzymes, such as CPT1A, precipitates a metabolic shift to glycolysis, leading to lipotoxicity (intracellular lipid accumulation) and ATP depletion. Mitochondrial dysfunction acts as a central pathogenic hub, characterized by Drp1-mediated fragmentation, defective mitophagy, and the leakage of danger signals including mtROS and mtDNA. These signals activate innate immune sensors, such as the cGAS-STING pathway and the NLRP3 inflammasome, within epithelial cells to drive sterile inflammation. This inflammatory cascade promotes the release of DAMPs, chemokines, and cytokines that orchestrate the recruitment and activation of inflammatory cells within the interstitium. Consequently, injured PTECs adopt a secretory phenotype, releasing profibrotic mediators (eg, TGF-β, PDGF, Wnt ligands, and Shh) that promote myofibroblast activation and microvascular rarefaction. The red lightning bolts indicate ischemic or toxic insults, and the red cross mark denotes the blockade or suppression of the corresponding metabolic pathway. Bold text in the schematic diagram denotes panel headings and major biological or structural categories.

Abbreviations: ATP, adenosine triphosphate; cGAS-STING, cyclic GMP-AMP synthase-stimulator of interferon genes; CPT1A, carnitine palmitoyltransferase 1A; DAMPs, damage-associated molecular patterns; Drp1, dynamin-related protein 1; ERRα, estrogen-related receptor alpha; FAO, fatty acid β-oxidation; mtDNA, mitochondrial DNA; mtROS, mitochondrial reactive oxygen species; Na⁺/K⁺-ATPase, sodium-potassium adenosine triphosphatase; NLRP3, NOD-like receptor family pyrin domain-containing 3; PDGF, platelet-derived growth factor; PPARα, peroxisome proliferator-activated receptor alpha; Shh, sonic hedgehog; PTECs, proximal tubular epithelial cells; TGF-β, transforming growth factor beta.

The proximal tubule exhibits profound segmental heterogeneity, partitioned sequentially into the S1, S2, and S3 segments, each presenting unique biological barriers, metabolic demands, and distinct injury susceptibilities. The cortical S1 segment is characterized by prominent cuboidal epithelial cells with a dense apical brush border and extensive basolateral infoldings enriched in sodium-potassium adenosine triphosphatase (Na⁺/K⁺-ATPase), features that maximize the surface area for active solute transport. This epithelial height and brush border density progressively diminish through the S2 transition zone. To sustain this massive energy demand, the highly perfused S1 and S2 segments possess an exceptionally high mitochondrial density and rely almost exclusively on oxidative phosphorylation. In contrast, the S3 segment, located in the outer medulla, operates within a physiologically hypoxic microenvironment characterized by lower perfusion pressures (Figure 1A). Crucially, although S3 cells retain significant mitochondrial density, they exhibit distinct metabolic adaptations. Operating near its metabolic limit, the S3 segment utilizes glycolysis to compensate for lower oxygen tension (pO₂).^16,17 Furthermore, the proximal tubule is uniquely capable of gluconeogenesis, synthesizing glucose from substrates such as lactate or glutamine during stress—a function vital for systemic homeostasis.¹

A defining hallmark of PTECs is their immense energy demand, which is sustained by an extensive mitochondrial network. To generate the adenosine triphosphate (ATP) supply required for active transport, these cells preferentially oxidize fatty acids via mitochondrial β-oxidation rather than utilizing glucose.^1,6 This metabolic program is tightly governed by transcriptional regulators, including peroxisome proliferator-activated receptor alpha (PPARα) and estrogen-related receptor alpha (ERRα).^18,19 Maintaining this high-energy state necessitates continuous mitochondrial fusion/fission dynamics and the rigorous clearance of damaged organelles via mitophagy.¹⁷ Crucially, this direct correlation between regional oxygen supply, mitochondrial abundance, and metabolic preference fundamentally dictates segment-specific vulnerabilities. The strict metabolic inflexibility of the S1 and S2 segments renders them highly susceptible to abrupt anoxia and nephrotoxins. Upon insult, the precipitous drop in oxygen precipitates massive mitochondrial fragmentation, ATP depletion, and catastrophic oxidative stress, characterized by the explosive generation of reactive oxygen species (ROS). Conversely, because S3 cells operate at the threshold of physiological hypoxia, even minor hemodynamic fluctuations can plunge this segment into severe and persistent hypoxia. Consequently, the S3 segment represents the most profoundly injured region following ischemia-reperfusion injury. Understanding this precise interplay provides a rigorous pathophysiological blueprint for precision nanomedicine. The high density of malfunctioning mitochondria and excessive ROS generation in injured S1/S2 segments provide a robust rationale for deploying ROS-responsive and mitochondria-targeted therapeutic nanocarriers. Concurrently, the profound and persistent hypoxia characteristic of the injured S3 segment robustly upregulates the expression of vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1). This extreme hypoxic and inflammatory signature serves as a potent endogenous navigational cue for biomimetic delivery systems, particularly engineered EVs expressing corresponding integrins for ischemia-targeted homing.^16–19

Tubules as the Nexus of Disease Pathogenesis

The profound reliance of PTECs on oxidative phosphorylation renders them intrinsically susceptible to insults that compromise mitochondrial function or oxygen supply. Upon ischemic or toxic challenge, this specialized metabolic architecture undergoes rapid dysregulation, initiating a well-defined pathological cascade (Figure 1B). A defining upstream event is the transcriptional suppression of fatty acid β-oxidation (FAO) enzymes, particularly carnitine palmitoyltransferase 1A (CPT1A), which precipitates a metabolic shift from oxidative phosphorylation to aerobic glycolysis.¹ This reprogramming, analogous to the Warburg effect, initially supports cell survival but rapidly becomes maladaptive, resulting in ATP depletion and the loss of gluconeogenic capacity. Consequently, the intracellular accumulation of unoxidized fatty acids drives lipotoxicity, a key pathogenic process that exacerbates cellular stress, promotes dedifferentiation, and underpins the clinical manifestation of AKI.^20,21

This metabolic failure directly triggers downstream mitochondrial quality control defects and structural fragmentation. Disrupted mitochondrial dynamics lead to excessive organelle fission, mediated by dynamin-related protein 1 (Drp1), accompanied by impaired fusion governed by mitofusin 1/2 (Mfn1/2) and OPA1.^17,22–24

Concurrently, defective mitophagy precludes the effective clearance of these damaged organelles. The cumulative failure of these quality control mechanisms permits the accrual of dysfunctional mitochondria, sustaining profound intracellular oxidative stress and precipitating the explosive generation of mitochondrial reactive oxygen species (mtROS) alongside the cytosolic leakage of mitochondrial DNA (mtDNA). Crucially, the accumulation of mtROS and cytosolic translocation of mtDNA act as potent danger signals that activate innate immune sensors within the epithelial cells. Specifically, these mitochondrial components trigger the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway and the NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome. Activation of these inflammatory axes drives cellular pyroptosis and orchestrates a state of sterile inflammation, amplifying the intracellular and microenvironmental stress.^17,22–24

Driven by these innate immune cascades, injured PTECs transition into a prominent, maladaptive secretory phenotype. They release damage-associated molecular patterns (DAMPs), chemokines, and pro-inflammatory cytokines into the surrounding tissue. This tubule-derived inflammatory cascade acts as a powerful chemotactic signal, orchestrating the recruitment and persistent activation of inflammatory cells within the renal interstitium, which further propagates local tissue damage.^20,21

Ultimately, the persistent inflammatory microenvironment and the sustained secretory profile of injured PTECs directly drive tubulointerstitial fibrosis and microvascular damage. To facilitate matrix remodeling, these damaged cells secrete a broad spectrum of profibrotic mediators, including transforming growth factor beta (TGF-β), platelet-derived growth factor (PDGF), Wnt ligands, and Sonic Hedgehog (Shh).^24,25 These factors activate resident fibroblasts and pericytes, forcing their differentiation into myofibroblasts. Although initially aimed at tissue repair, chronic myofibroblast activation results in excessive extracellular matrix deposition, progressive tubular atrophy, and severe peritubular capillary rarefaction.²¹ This pathogenic feed-forward circuit, progressing sequentially from metabolic shutdown and mitochondrial collapse to innate immunity and fibrotic scarring, permanently alters renal architecture, culminating in irreversible ESKD. Thus, tubulointerstitial damage serves as a robust predictor of long-term clinical outcomes, firmly establishing the renal tubules as the central hub in kidney disease pathogenesis.²¹

Implications for Therapeutic Targeting

The distinctive apicobasal polarity, heavy mitochondrial reliance, and secretory capacity of PTECs not only define their physiological centrality but also underpin their inherent susceptibility to injury. Crucially, these specific biological attributes present rational, mechanistically grounded avenues for therapeutic intervention. Nanoscale delivery systems designed to preserve mitochondrial integrity, facilitate intracellular and organelle-specific trafficking within PTECs, or modulate tubular drivers of inflammation and fibrosis represent promising candidates for next-generation PTEC-directed nanomedicine.¹⁹

Active Tubular Targeting Strategies Beyond Passive Renal Accumulation

Historically, renal nanomedicine has relied predominantly on passive accumulation strategies, which are strictly governed by the intrinsic permselectivity of the GFB. Although nanoparticles smaller than 10 nm undergo rapid filtration and those exceeding 200 nm are sequestered by the reticuloendothelial system, retention within the intermediate size range depends on a delicate balance of physicochemical properties, including size, charge, and geometry.^8,9 However, this physicochemical-driven paradigm is inherently stochastic and lacks the precision necessary to address complex tubular pathologies. The requisite narrow size window excludes many clinically relevant carriers, while reliance on ligand-independent uptake frequently results in rapid urinary excretion rather than effective cellular internalization.⁷ Furthermore, while renal injury may transiently compromise GFB integrity, the heterogeneity and unpredictability of these pathological changes render them an unreliable mechanism for controlled drug delivery. Consequently, overcoming these physiological barriers and bioavailability bottlenecks mandates a paradigm shift from passive accumulation toward active tubular targeting, a strategy that exploits specific molecular cues to achieve precise spatiotemporal delivery.

Ligand–Receptor Mediated Tubular Targeting

Ligand-receptor functionalization represents the cornerstone of active renal nanomedicine, designed to bypass the physicochemical limitations of passive filtration through precise molecular recognition. Unlike passive strategies that depend solely on favorable hemodynamics and GFB permeability, this approach exploits the specific upregulation or high expression of surface receptors on injured PTECs to facilitate receptor-mediated endocytosis (Figure 2A). By engineering nanocarriers with high-affinity moieties—such as antibodies, peptides, or small molecules—this strategy not only enhances retention against the shear stress of urinary flow but also significantly minimizes off-target accumulation in healthy tissues. In this section, we elucidate representative receptor systems that have been successfully leveraged to achieve disease-selective tubular targeting.^7,26

Figure 2.

Engineering Precision: Active Targeting Strategies and Responsive Mechanisms for Renal Tubular Injury. (A) Ligand-receptor mediated targeting. The megalin/cubilin complex facilitates the physiological endocytosis of protein- or chitosan-conjugated therapeutics. KIM-1, upregulated specifically on injured PTECs, serves as a pathology-specific docking site for peptide-functionalized nanocarriers. Inflammation-induced VCAM-1/ICAM-1 molecules enable biomimetic carriers, such as membrane-coated nanoparticles, to mimic leukocyte recruitment. (B) Microenvironment-responsive activation. Smart nanocarriers exploit pathological gradients: ROS-responsive systems utilize oxidation-sensitive linkers or nanozymes to trigger payload release; pH-responsive platforms undergo charge reversal or disassembly in acidic extracellular (fibrotic) or endolysosomal compartments; Enzyme-responsive carriers (eg, DNA frameworks) release payloads upon cleavage by aberrant enzymatic activity. (C) Biomimetic and hybrid engineering. Strategies to enhance EVs performance include surface functionalization with high-affinity ligands and the construction of hybrid nanocarriers by fusing biological membranes with synthetic liposomes, thereby combining immune evasion with optimized loading capacity. Bold text in the schematic diagram denotes panel headings and major biological or structural categories.

Abbreviations: EVs, extracellular vesicles; ICAM-1, intercellular adhesion molecule-1; KIM-1, kidney injury molecule-1; PTECs, proximal tubular epithelial cells; ROS, reactive oxygen species; VCAM-1, vascular cell adhesion molecule-1.

Megalin/Cubilin-Mediated Endocytosis

The megalin and cubilin receptor complex forms a high-capacity endocytic system localized principally to the apical brush border of proximal tubular epithelial cells. Physiologically, this multiligand receptor orchestrates the scavenger reabsorption of low-molecular-weight proteins from the glomerular filtrate to prevent urinary protein loss.²⁷ Researchers have effectively repurposed this physiological salvage pathway for renal drug delivery by conjugating small-molecule therapeutics to endogenous protein ligands. For instance, lysozyme, a low-molecular-weight protein, has been extensively utilized as a macromolecular vector to transport therapeutic agents, such as imatinib, baicalin, and methylprednisolone specifically into proximal tubular cells.^28–30 Mechanistically, owing to its small size, lysozyme readily traverses the glomerular filtration barrier into the tubular lumen, where it acts as a specific ligand for the abundant megalin receptors expressed on the apical membrane. This precise molecular recognition triggers receptor-mediated endocytosis, facilitating the rapid internalization of lysozyme-drug conjugates into intracellular endosomal vesicles. Subsequently, these endosomes mature and traffic to lysosomes, where the lysozyme protein backbone undergoes enzymatic degradation. Concurrently, the specific chemical linkers connecting the drug to the carrier are cleaved, ultimately releasing the active pharmacological agents into the cytosol to exert their therapeutic efficacy. Consequently, these protein-drug conjugates exhibit significantly enhanced renal retention, prolonged drug exposure within the kidneys, and reduced systemic toxicity compared to their free small-molecule counterparts, firmly validating the utility of the megalin receptor as a robust portal for targeted tubular entry.

Beyond protein vectors, polysaccharide-based platforms have emerged as versatile alternatives for receptor-mediated targeting. Notably, chitosan oligosaccharide serves as a compelling carrier owing to its favorable biocompatibility and intrinsic affinity for megalin.³¹ Wang et al demonstrated that identifying the optimal molecular weight of chitosan oligosaccharide is critical for maximizing renal tropism. They developed a stepwise targeting conjugate wherein chitosan oligosaccharide directs the payload to the kidney via megalin interaction, followed by an esterase-responsive release of prednisolone within the tubular cells. This design significantly attenuated renal inflammation and improved functional recovery in models of acute kidney injury.³¹

Furthermore, synthetic peptide engineering offers a precise strategy to exploit this pathway without the immunogenic risks associated with protein carriers. The synthetic peptide (KKEEE)3K was identified to bind megalin with high specificity.³² Pharmacokinetic studies revealed that radiolabeled (KKEEE)3K accumulates in the kidneys at levels approximately 75-fold higher than in other organs, a tropism that is abolished in megalin-deficient models. Building on this finding, He et al incorporated a similar tubular-targeting peptide into a poly(lactic-co-glycolic acid)-polyethylene glycol (PLGA-PEG) nanosystem to deliver the antifibrotic agent asiatic acid. This modification facilitated the efficient internalization of nanoparticles by tubular epithelial cells and significantly enhanced therapeutic efficacy against renal fibrosis compared to non-targeted formulations.³³ Collectively, these studies underscore that targeting the megalin/cubilin axis, whether through proteins, polysaccharides, or synthetic peptides, provides a foundational strategy for achieving high-concentration drug delivery to the proximal tubule.

KIM-1 Targeting

Kidney Injury Molecule-1 (KIM-1) represents a quintessential target for precision nephrology owing to its highly distinctive expression profile. While basal expression is negligible in healthy renal tissue, KIM-1 undergoes robust and rapid upregulation on the apical membrane of proximal tubular epithelial cells following ischemic or toxic insults.^34–37 This pathology-restricted distribution provides a unique molecular address to direct nanomedicines exclusively to damaged nephrons, effectively minimizing off-target accumulation in healthy organs (Figure 2A).

Pioneering strategies to exploit this receptor utilized specific binding sequences, such as the KIT peptide (CNWMINKEC), to facilitate targeted delivery. Song et al demonstrated that fusing basic fibroblast growth factor with the KIT peptide significantly enhanced the retention of this growth factor within ischemic kidneys. This targeted approach amplified the cytoprotective efficacy of the therapeutic cargo and accelerated the repair of damaged tubules in rat models of ischemia-reperfusion injury.³⁸ Building on the concept of peptide-mediated targeting, our group utilized phage display technology to identify a novel heptapeptide ligand, LTHVVWL, which exhibits high binding affinity for the extracellular domain of KIM-1. We subsequently engineered red blood cell-derived EV surface-functionalized with this peptide to construct a biomimetic delivery platform termed REV_LTH. This system demonstrated efficient homing capabilities to KIM-1-positive tubules in both ischemia-reperfusion and unilateral ureteral obstruction models. The enhanced accumulation of these engineered vesicles in injured segments translated into superior structural preservation and functional recovery, establishing a proof of concept for using KIM-1 as an inducible entry point for vesicle-based nanomedicine.³⁹

Complementing these biological carriers, natural polymeric systems have been developed to integrate receptor targeting with microenvironment-responsive drug release. Liu and co-workers designed an L-serine-modified chitosan carrier that leverages L-serine moieties to facilitate KIM-1-dependent endocytosis. To achieve subcellular precision, they conjugated the mitochondria-targeted antioxidant peptide SS31 to this carrier via a reactive oxygen species-sensitive thioketal linker. Upon internalization by oxidative stress-rich tubular cells, high levels of reactive oxygen species trigger the cleavage of the linker and release the SS31 payload. This sophisticated design combines receptor-mediated uptake with logic-gated drug release, thereby maximizing mitochondrial protection while limiting systemic exposure.⁴⁰

VCAM-1/ICAM-1 Targeting

VCAM-1 and ICAM-1 undergo robust upregulation on both activated endothelial cells and injured renal tubular epithelial cells during inflammatory states, particularly ischemic acute kidney injury. These surface proteins serve as high-affinity docking sites for leukocyte integrins, specifically Very Late Antigen-4 (VLA-4, integrin α₄β₁) and Lymphocyte Function-Associated Antigen-1 (LFA-1, integrin α_Lβ₂). Nanomedicine strategies have effectively repurposed this endogenous leukocyte recruitment mechanism to direct therapeutic carriers specifically to inflamed renal tissues via molecular mimicry (Figure 2A).^41–43

Biomimetic carriers derived from stem cells have demonstrated intrinsic targeting capabilities through this axis. In our previous work, we identified that EVs derived from mesenchymal stem cells naturally retain functional integrins on their membrane surface. These vesicles preferentially accumulate in ischemic kidneys and colocalize extensively with VCAM-1-positive and ICAM-1-positive proximal tubules. Furthermore, antibody-mediated blockade of VLA-4 and LFA-1 significantly abrogated this renal accumulation, confirming that integrin-adhesion molecule interactions are the primary drivers of this tubular tropism.⁴³

In parallel, immune cell-derived vesicles represent a robust platform for inflammation-guided delivery. We engineered macrophage-derived microvesicles to encapsulate the corticosteroid dexamethasone for the treatment of renal inflammation. These microvesicles retained the integrin profile of their parent cells, including high levels of integrin α₄ and α_L, which facilitated their specific binding to VCAM-1 and ICAM-1 expressed on injured renal cells.⁴¹ Similarly, our group utilized the inflammation-homing properties of macrophage membranes to construct IL-10-loaded EVs enriched in integrins α₄β₁ and α_Lβ₂. This engineered system achieved targeted accumulation in ischemia-reperfusion injury models, thereby enhancing the therapeutic efficacy of the anti-inflammatory cytokine payload.⁴² Collectively, these findings highlight that exploiting the VCAM-1/ICAM-1 axis effectively transforms the inflammatory microenvironment from a pathological barrier into a navigational beacon for precise tubular delivery.

Other Ligand-Receptor Systems for Tubular Targeting

While the megalin, KIM-1, and VCAM-1/ICAM-1 axes constitute the primary targets for renal nanomedicine, the expanding repertoire of ligand-receptor pairs offers complementary avenues for addressing diverse tubular pathologies. For instance, the folate receptor alpha (FRα), which is typically restricted in healthy tissue, undergoes significant upregulation in the cyst-lining tubular epithelial cells of polycystic kidneys. Shi et al exploited this pathological signature to engineer folate-conjugated liposomes loaded with the dual phosphoinositide 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) inhibitor dactolisib. This receptor-mediated strategy facilitated the preferential accumulation of the inhibitor within renal cysts, resulting in the potent suppression of cyst proliferation and the preservation of renal function in murine models of polycystic kidney disease.⁴⁴

In parallel, peptide-functionalized biopolymers represent another frontier in renal targeting. Mahdi et al utilized a specific kidney-targeting peptide conjugated to thermally responsive elastin-like polypeptides to modulate the renal deposition of therapeutic proteins. By constructing a chimeric fusion of this targeting moiety with vascular endothelial growth factor (VEGF), they achieved enhanced retention of the growth factor within the renal parenchyma. Although the precise molecular docking site of this kidney-targeting peptide requires further elucidation, such systems demonstrate that combining peptide navigation with biopolymer scaffolds can effectively promote microvascular stabilization and tubular recovery.⁴⁵ These alternative targeting moieties broaden the available ligand toolbox, providing essential options for multi-target strategies designed to navigate the heterogeneous landscape of chronic and genetic kidney diseases.

Comparative Evaluation of Ligand-Receptor Strategies

Despite significant advances in receptor-mediated tubular delivery, translating these nanomedicines into clinical practice necessitates a critical evaluation of target receptor specificity and ligand-binding capabilities (Table 1). An optimal target must balance high endocytic capacity with strict disease-state specificity to maximize therapeutic efficacy and minimize off-target toxicity. Physiological entry portals, such as the megalin/cubilin complex, offer massive endocytic capacity but lack disease-specific upregulation, increasing the risk of collateral drug accumulation in healthy nephrons^28–33. Conversely, targeting the VCAM-1/ICAM-1 axis exploits robust inflammatory homing mechanisms; however, during systemic inflammatory states (eg, sepsis), this approach risks severe extra-renal sequestration in the reticuloendothelial system.^41–43 To achieve strict pathological specificity, KIM-1 serves as an exceptional target due to its dramatic upregulation exclusively on injured tubules. Nevertheless, its clinical utility is constrained by a strict spatiotemporal expression window and the potential risk of inadvertently exacerbating tubular apoptosis or fibrosis upon ligand binding.^34–40 Finally, while receptors like FRα effectively target specific structural anomalies such as cyst-lining cells in polycystic kidney disease, their constitutive extra-renal expression and lack of injury-driven upregulation limit their broad applicability in conventional ischemic or toxic AKI.⁴⁴ Ultimately, the rational design of targeted nanomedicines must meticulously align receptor selection with the spatiotemporal heterogeneity of the underlying pathology.

Table 1.

Systematic Evaluation of Representative Target Receptors for Renal Nanomedicine

Targeting Axis	Expression Profile	Representative Ligands	Therapeutic Advantages	Translational Limitations
Megalin/Cubilin28–33	Constitutively high in healthy PTECs; low disease specificity.	Low-molecular-weight proteins (eg, lysozyme), chitosan, synthetic peptides.	Massive endocytic capacity; efficient intracellular trafficking; versatile ligand compatibility.	Off-target accumulation in healthy nephrons; endogenous protein competition; strict glomerular filtration limits.
KIM-134–40	Dramatically upregulated strictly on injured PTECs.	Targeting peptides (eg, LTHVVWL, CNWMINKEC), functionalized biomaterials.	Exceptional disease-state specificity; highly efficient active phagocytic internalization.	Strict spatiotemporal expression window; potential to inadvertently trigger pro-apoptotic/fibrotic signaling.
VCAM-1/ICAM-141–43	Upregulated on inflamed PTECs and activated endothelial cells.	Integrins (α4β1, αLβ2) natively expressed on immune cells or EVs.	Leverages robust endogenous inflammatory homing; highly compatible with biomimetic EV platforms.	High risk of extra-renal sequestration (eg, liver, spleen) during systemic inflammation (eg, sepsis).
FRα44	Upregulated in cyst-lining cells of polycystic kidneys; expressed in specific normal tissues.	Folic acid, folate-conjugates.	High affinity; precise delivery to renal cysts in polycystic kidney disease models.	Lacks specific upregulation in conventional AKI; risk of off-target accumulation in healthy FRα expressing tissues.

Abbreviations: AKI, acute kidney injury; EVs, extracellular vesicles; FRα, folate receptor alpha; ICAM-1, intercellular adhesion molecule-1; KIM-1, kidney injury molecule-1; PTECs, proximal tubular epithelial cells; VCAM-1, vascular cell adhesion molecule-1.

Microenvironment-Responsive Nanocarriers for Tubular Targeting

Renal tubular injury profoundly alters the local biochemical landscape, generating characteristic cues that can be exploited for targeted drug delivery. Injured proximal tubules typically exhibit exacerbated oxidative stress, localized acidification, and dysregulated enzymatic activity.^1,2 Nanocarriers programmed to respond to these pathological signals (Figure 2B) can execute the release of therapeutic payloads with high spatial and temporal precision, thereby enhancing efficacy while limiting systemic toxicity.

ROS-Responsive Nanocarriers

The renal microenvironment in pathologies such as AKI, ischemia-reperfusion injury, and crystal-induced nephropathy is defined by substantial oxidative stress, wherein excessive ROS trigger a deleterious cascade of tubular apoptosis, mitochondrial dysfunction, and proinflammatory signaling.^1,2,46 Capitalizing on this pathological hallmark, ROS-responsive nanomedicines have been engineered to couple site-selective activation with intrinsic antioxidant capabilities.⁴⁶

A paradigmatic strategy involves the deployment of ROS-sensitive organic scaffolds that undergo degradation to release therapeutic payloads. For instance, Wang et al developed a hyaluronic acid-bilirubin conjugate (HA-Br) encapsulating the calcium chelator BAPTA-AM.⁴⁷ In this design, hyaluronic acid facilitates preferential accumulation in injured proximal tubules, while the bilirubin backbone functions as a sacrificial antioxidant. Upon exposure to ROS, bilirubin oxidizes to biliverdin, triggering nanoparticle disassembly and the on-demand release of BAPTA-AM to mitigate calcium overload and endoplasmic reticulum (ER) stress.^47,48

Complementing organic systems, inorganic nanozymes offer robust catalytic activity for ROS scavenging. A notable example is the inflammation-sensing PTC-M platform, wherein thioketal-linked micelles degrade in oxidative milieus to expose manganese oxide (Mn₃O₄) cores, thereby mimicking catalase activity to decompose hydrogen peroxide.⁴⁹ Furthermore, recent advances have integrated biomimetic strategies with ROS-scavenging nanozymes to enhance therapeutic outcomes. He et al and Deng et al reported erythrocyte- and macrophage-membrane-coated nanoplatforms utilizing cerium oxide and platinum nanozymes, respectively. These biomimetic systems not only harness the intrinsic superoxide dismutase- and catalase-mimicking activities of noble metal or metal oxide clusters to restore redox homeostasis but also effectively suppress M1 macrophage polarization and crystal deposition in complex AKI models.^50–52 Collectively, these designs demonstrate the versatility of ROS-responsive nanoplatforms in converting local oxidative cues into potent therapeutic actions, ranging from controlled drug release to immunomodulatory redox regulation.

pH-Responsive Delivery

Pathological acidification constitutes a distinguishing microenvironmental feature of injured renal tissue, establishing a biological gradient that can be exploited for targeted drug delivery.^53,54 Ischemic injury, inflammation, and fibrotic remodeling induce localized extracellular acidosis, while the endolysosomal compartments within TECs maintain a significantly lower pH environment. Consequently, pH-responsive nanomedicines have been engineered to remain inert under physiological conditions (pH 7.4) but to undergo physicochemical transitions, such as protonation, swelling, or structural disassembly, upon encountering these acidic thresholds, thereby enabling site-specific payload release.^54,55

Poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) serves as a paradigm for targeting the mildly acidic microenvironment of fibrotic kidneys. PDPA-based nanoparticles, designed with a pKa of approximately 6.68, exploit the pH transition in fibrotic tissue to trigger protonation-induced disassembly. In murine models of unilateral ureteral obstruction, this pH-selective activation facilitates the localized release of dinitrosyl iron complexes (DNIC) as nitric oxide donors, effectively suppressing collagen deposition and myofibroblast activation while minimizing systemic hypotension risks.⁵⁶

Beyond polymer disassembly, pH gradients can regulate the catalytic activity of inorganic nanomaterials. For instance, ultra-small platinum-sulfur clusters have been developed as pre-nanozymes that remain catalytically dormant at neutral pH. Upon exposure to the mildly acidic inflammatory microenvironment (approximately pH 6.5) characteristic of AKI, these clusters are activated to release hydrogen sulfide and transform into platinum nanozymes with potent ROS and reactive nitrogen species (RNS) scavenging capabilities, thereby providing dual-mechanism cytoprotection.⁵⁷

Furthermore, the deeper acidification of the endolysosomal pathway offers a trigger for organelle-targeted therapies. Polymeric nanopolyplexes constructed from hyaluronic acid and chitosan exemplify this strategy. These carriers are stable in the bloodstream but rapidly disassemble within the lysosomal pH range (pH 4.5–5.0) owing to charge reversal. This intracellular burst release mechanism has been successfully applied to deliver cardiolipin-binding, mitochondria-targeted peptide SS-31 in models of LPS-induced AKI, ensuring that the peptide escapes lysosomal sequestration to restore mitochondrial integrity.⁵⁸ Collectively, these strategies demonstrate that exploiting the pathological pH gradient, extending from the extracellular space to intracellular organelles, provides a robust physicochemical rationale for enhancing the precision and efficacy of renal nanotherapeutics.

Enzyme-Responsive Nanoplatforms

Enzyme-responsive nanoplatforms harness the aberrant enzymatic milieu characteristic of kidney diseases to orchestrate site-specific drug release, thereby mitigating off-target systemic toxicity. These systems are engineered to undergo programmable structural or chemical transformations in response to enzymes that are overexpressed or selectively activated within the renal microenvironment, such as matrix metalloproteinases (MMPs), esterases, hyaluronidases, or cathepsins.⁵⁹ For instance, the incorporation of MMP-cleavable peptide motifs into nanocarriers enables precise cargo liberation solely within regions undergoing active extracellular matrix remodeling, a hallmark of tubular injury and fibrosis.^60,61

While protease-responsive strategies primarily target the extracellular space, nucleic acid-based platforms offer an additional dimension of precision by responding to intracellular enzymatic triggers. A notable example is the nanoparachute system developed by Li et al, which utilizes a tetrahedral framework nucleic acid scaffold loaded with therapeutic miR-125.⁵⁹ This nanostructure is ingeniously designed with ribonucleotide segments that form DNA-RNA hybrid regions, serving as specific substrates for RNase H, an endonuclease abundant in the mammalian cytosol.⁶² Upon internalization by renal tubular epithelial cells, RNase H-mediated cleavage triggers the conformational disassembly of the scaffold, resulting in the controlled release of the miRNA cargo. In models of ischemia-reperfusion injury, this bio-responsive mechanism effectively facilitates renal accumulation and attenuates tubular apoptosis and mitochondrial dysfunction.⁵⁹ Collectively, these enzyme-responsive strategies, spanning from extracellular matrix modulation to intracellular gene regulation, represent a promising frontier for precision nephrology. Future efforts should focus on refining enzymatic selectivity within the heterogeneous renal pathology and integrating these triggers into multi-stimuli-responsive designs to further optimize therapeutic outcomes.

Biomimetic Strategies for Tubule-Targeted Nanomedicines

While synthetic nanocarriers offer versatile control over physicochemical parameters, their clinical translation is frequently impeded by rapid immune clearance, limited circulatory half-life, and suboptimal biocompatibility.^63,64 To circumvent these limitations, biomimetic strategies have emerged as a superior alternative, leveraging biological membranes or naturally derived vesicles to interface seamlessly with the immune system and target tissues. Among these bio-inspired approaches, EV-based systems have garnered the most extensive evidence for renal application. This section focuses on EVs as a paradigmatic class of biomimetic carriers, highlighting their intrinsic properties and the expanding landscape of engineering techniques designed to achieve precise tubular targeting.

EV-Based Delivery Systems

EVs, particularly exosomes, have gained prominence as robust nanocarriers for renal therapy. Distinguished by a phospholipid bilayer structure, EVs naturally shield diverse biomolecular cargoes, ranging from nucleic acids and proteins to metabolic substrates, from enzymatic degradation while exhibiting low immunogenicity compared to synthetic counterparts.^63–66 The therapeutic efficacy of EVs is intrinsically governed by their cellular origin, which dictates their surface protein composition and homing capabilities.

Systemically administered EVs derived from mesenchymal stromal cells (MSC-EVs) and renal tubular epithelial cells (TEC-EVs) exhibit a natural propensity to accumulate in injured kidneys. This passive tropism is primarily driven by the enhanced vascular permeability of the inflamed kidney and the upregulation of adhesion molecules, such as ICAM-1, on the injured microvasculature.^43,66,67 Upon internalization by TECs, these EVs orchestrate tissue repair by transferring bioactive cargoes. For instance, our group previously demonstrated that MSC-EVs enriched with miR-125b-5p effectively attenuated tubular cell apoptosis and promoted repair in ischemic AKI models by directly suppressing p53 signaling.⁴³ Beyond structural cells, immune cell-derived EVs have also attracted significant attention. Macrophage-derived EVs play a pivotal role in modulating the renal microenvironment; depending on the polarization state of the parent macrophage, these vesicles can either propagate inflammation or, more desirably, deliver anti-inflammatory mediators such as IL-10 to resolve tubular injury and suppress fibrosis. Collectively, these findings establish EVs as versatile, inherently bioactive carriers that leverage biological recognition mechanisms to intervene in kidney disease progression.^68,69

Engineered and Hybrid Vesicle Platforms for Precision Targeting

Despite the advantages of native EVs, their clinical utility can be constrained by insufficient targeting specificity and variable cargo loading. To address these challenges, advanced engineering strategies have been developed to transform EVs into precision delivery platforms (Figure 2C).⁷⁰

Surface Functionalization for Active Targeting: A primary strategy involves decorating the EVs surface with ligands that recognize receptors overexpressed on injured TECs. In our recent investigations, we successfully engineered red blood cell-derived EVs (REVs) modified with a KIM-1-binding peptide (LTH). Our results indicated that this modification significantly amplified EVs accumulation specifically within KIM-1-positive injured tubules, thereby converting the passive renal tropism of native EVs into an active, disease-specific targeting mechanism.³⁹ Similarly, other groups have employed albumin-binding domains to extend renal retention, further validating surface engineering as a robust approach for kidney-targeted delivery.⁷¹

Cargo Loading and Parent Cell Engineering: The interior of EVs can be optimized to carry exogenous therapeutics, including siRNAs and small molecules, via techniques such as electroporation. Alternatively, genetic engineering of parent cells offers a method to produce pre-loaded EVs. For example, we engineered macrophages to overexpress IL-10, generating EVs with sustained, high-potency anti-inflammatory activity that significantly outperformed free cytokines in mitigating ischemia-reperfusion injury.⁴²

Hybrid and Stimuli-Responsive Systems: To synergize the advantages of biological and synthetic systems, hybrid vesicle platforms have been developed. Fusing EV membranes with synthetic liposomes or membranes from other cell types, such as neutrophils, yields hybrid vesicles that combine the immune-evasion capabilities of cells with the customizable loading capacity of lipid nanoparticles.⁷² Furthermore, stimuli-responsive elements are being integrated to couple biomimetic targeting with microenvironment sensitivity. Recent work by Cheng et al exemplified this by combining MSC-EVs with hypoxia-responsive amphiphiles, enabling payload release specifically within the hypoxic milieu of the injured kidney.⁷³

Beyond EV-based systems, other emerging biomimetic platforms have also demonstrated profound potential for targeted renal delivery, significantly broadening the biomimetic repertoire. For instance, cell membrane coating technology enables synthetic nanocarriers to inherit the natural tropism of their source cells. A recent cutting-edge study by Zhang et al developed a bioengineered platelet-mimicking nanoplatform.⁷⁴ By cloaking dexamethasone-loaded nanoparticles with native platelet membranes, this system hijacked the natural affinity of platelets for the inflamed renal microenvironment, successfully exerting dual anti-inflammatory and anti-fibrotic effects while minimizing systemic toxicity in chronic nephritis models. Furthermore, biomimetic high-density lipoprotein (bHDL) nanoparticles represent another highly innovative vector.⁷⁵ He et al recently demonstrated that bHDL nanoparticles inherently target injured renal tubular epithelial cells through KIM-1-mediated internalization. Utilizing this platform to co-deliver triptolide and nintedanib effectively remodeled the fibrotic niches and alleviated renal fibrosis. These alternative biomimetic strategies perfectly complement EV-based platforms, offering robust and customizable solutions to navigate complex renal biological barriers.

Translational Barriers: Despite these advances, the translation of EV-based nanomedicines faces hurdles regarding standardization. Current isolation methods and dose definitions vary significantly, complicating cross-study comparisons. Moreover, scalable production under Good Manufacturing Practice conditions and a comprehensive understanding of human pharmacokinetics remain critical areas requiring rigorous optimization to realize the full potential of these biomimetic carriers.^70,76

Subcellular Frontiers in Tubule-Targeted Nanomedicine: Mitochondria and Beyond

The trajectory of renal nanomedicine is progressively shifting from tissue-level retention toward the precise interrogation of subcellular organelles. While organelles such as lysosomes and the ER represent viable therapeutic entry points, mitochondria occupy a preeminent position within the context of renal tubular physiology. Proximal tubular epithelial cells, driven by the immense metabolic cost of active solute reabsorption, possess an exceptionally high mitochondrial density, a specialization that paradoxically renders them intrinsically susceptible to bioenergetic collapse. Consequently, mitochondria function not merely as passive victims of injury but as active determinants of cell fate.^77–79 This section focuses on mitochondria as the prototypical subcellular target, delineating how nanomedicine can bridge the divide between mitochondrial pharmacology and effective tubular access.

Mitochondrial Control of Tubular Fate

Mitochondrial dysfunction constitutes a convergent node in the pathogenesis of both AKI and CKD. The renal proximal tubule exhibits one of the highest mitochondrial densities in the human body, a physiological adaptation that transforms into a critical vulnerability during pathological stress.^79,80 Primary pathological features include a collapse in electron transport chain activity, ATP depletion, and the explosive generation of mtROS. Crucially, these bioenergetic defects are inextricably linked to dysregulated mitochondrial dynamics. An imbalance characterized by excessive fission, mediated by Drp1, and impaired fusion, governed by Mfn1/2 and OPA1, precipitates mitochondrial fragmentation.^81,82 Concomitant with defective mitophagy, this fragmentation prevents the clearance of damaged organelles, leading to the accumulation of dysfunctional mitochondria that perpetuate oxidative stress and trigger distinct cell death pathways, including apoptosis, necroptosis, and ferroptosis. Furthermore, persistent mitochondrial injury drives the maladaptive repair of tubular cells, promoting the secretion of proinflammatory cytokines and profibrotic factors that fuel the transition from AKI to fibrosis.⁸³ Thus, restoring mitochondrial integrity represents a fundamental strategy to arrest disease progression.

Pharmacological Axes for Mitochondrial Modulation

The recognition of mitochondria as a core pathogenic hub has catalyzed the identification of several druggable axes (Figure 3A). The first major axis involves the direct scavenging of mtROS to preserve redox homeostasis. Mitochondria-targeted antioxidants, such as MitoQ and peptide SS-31 (elamipretide), have demonstrated efficacy in stabilizing cristae architecture and improving oxidative phosphorylation in preclinical models.^84–86

Figure 3.

Pharmacological Mechanisms and Hierarchical Targeting Strategies for Mitochondrial Nanomedicine. (A) Multidimensional axes for mitochondrial modulation. Targeted nanotherapeutics restore bioenergetics via four distinct mechanisms: Redox Homeostasis: Scavenging mtROS using nanozymes or antioxidants; Dynamics Regulation: Inhibiting Drp1-mediated fission to preserve network integrity; Mitophagy Restoration: Facilitating PINK1/Parkin-mediated clearance of damaged organelles; and Biogenesis: Stimulating the AMPK/PGC-1α axis to replenish the functional mitochondrial pool. (B) Hierarchical delivery across biological barriers. To enable subcellular intervention, nanocarriers follow a stepwise trajectory: Tissue Accumulation: Ligand-directed retention in the renal interstitium; Intracellular Trafficking: Endosomal escape to avoid lysosomal degradation; and Mitochondrial Entry: Surface modification with lipophilic cations (eg, TPP⁺) or penetrating peptides (eg, SS-31) drives translocation across the mitochondrial double membrane via membrane potential. The red cross mark denotes the blockade or suppression of the corresponding metabolic pathway. Bold text in the schematic diagram denotes panel headings and major biological or structural categories.

Abbreviations: AMPK, AMP-activated protein kinase; Drp1, dynamin-related protein 1; mtROS, mitochondrial reactive oxygen species; PGC-1α, peroxisome proliferator-activated receptor gamma coactivator 1-alpha; PINK1, PTEN-induced kinase 1; TPP⁺, triphenylphosphonium.

A second therapeutic axis focuses on rebalancing mitochondrial dynamics and quality control. However, successful clinical translation mandates that these pharmacological interventions be carefully aligned with specific, context-dependent pathological windows and intervention timings. For instance, targeting mitochondrial dynamics by inhibiting Drp1 with agents such as Mdivi-1 successfully prevents pathological mitochondrial fragmentation and apoptosis in diabetic nephropathy.⁸⁷ Nevertheless, because basal Drp1 activity is physiologically essential, recent evidence emphasizes that the effects of Drp1 modulation are highly tissue-specific and context-dependent. This suggests that the intervention timing must be carefully controlled to prevent massive fragmentation during acute injury while avoiding the disruption of normal cellular homeostasis during subsequent tissue regeneration.⁸³

This critical requirement for stage- and context-adaptive intervention is further exemplified by mitophagy regulation. The optimal therapeutic direction, whether enhancing or suppressing mitophagy, depends entirely on the specific disease model and pathological stage. In contexts such as diabetic kidney disease where autophagic clearance is impaired, enhancing mitophagy through the PINK1/Parkin pathway protects against renal injury by clearing damaged mitochondria and reducing oxidative stress.^88,89 Conversely, in other distinct pathological models like adenine-induced chronic kidney disease, suppressing excessive mitophagy with compounds such as honokiol is required to improve renal function and halt apoptosis.⁹⁰ Ultimately, recognizing these fascinating paradoxes dictates that next-generation nanomedicines must be engineered not only for spatial subcellular precision but also for programmable, stage-adaptive release to match the evolving therapeutic windows of kidney repair.

Additionally, stimulating mitochondrial biogenesis via the AMP-activated protein kinase (AMPK)/peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) pathway offers a powerful means to replenish the functional mitochondrial pool.^91,92 Notably, existing pharmacotherapies such as metformin and sodium-glucose cotransporter 2 inhibitors (eg, dapagliflozin) have been shown to exert renoprotective effects partially through this mechanism, repurposing metabolic regulation for structural protection.⁹³ These axes provide a robust mechanistic framework for the design of next-generation nanotherapeutics (Figure 3A).

Engineering Hierarchical Targeting for Mitochondrial Nanomedicine

Despite the availability of potent mitochondrial modulators, their clinical translation is severely impeded by poor bioavailability and the formidable challenge of traversing multiple biological barriers: the systemic circulation, the renal vascular endothelium, the tubular cell membrane, and finally, the mitochondrial double membrane.^80,94 Nanomedicine offers a versatile platform to overcome these hurdles through hierarchical, multi-stage targeting designs.

For the successful implementation of the aforementioned therapeutic strategies, nanocarriers are required to traverse a hierarchical pathway. This entails the sequential crossing of the capillary wall, the cellular membrane, and finally the double membrane of mitochondria, a stepwise process that progressively targets deeper biological compartments (Figure 3B). The first level involves ensuring preferential accumulation in renal tubules using ligand-receptor strategies or microenvironment-responsive carriers. The second level achieves subcellular precision, often by functionalizing nanocarriers with lipophilic cations, such as triphenylphosphonium (TPP⁺), or mitochondria-penetrating peptides that exploit the negative membrane potential of mitochondria for internalization.⁹⁵

Recent advances illustrate the potential of integrating these targeting domains with stimuli-responsive payloads. For instance, Yu et al engineered a sophisticated ROS-responsive system comprising mitochondria-targeting ceria nanoparticles loaded with atorvastatin.⁵⁰ In this design, the TPP⁺ moiety drives mitochondrial accumulation, while the ceria core acts as a nanozyme to scavenge excessive ROS, creating a synergistic effect that protects tubular cells from oxidative stress and inflammation. Similarly, biomimetic strategies utilizing extracellular vesicles or cell membrane coatings are emerging as tools to enhance tubular compatibility and facilitate intracellular trafficking. By combining active tubular targeting with subcellular localization, these “mito-nano” systems promise to deliver therapeutics directly to the metabolic engine of the cell, offering a precise method to halt kidney disease at its origin (Figure 3B).^50,83

Despite the robust mitochondrial-targeting efficacy of TPP⁺, its intrinsically strong cationic nature introduces critical translational hurdles. From a safety perspective, elevated intracellular accumulation of TPP⁺ exceeding 10 μM can precipitate unexpected mitochondrial membrane depolarization and proton leakage, culminating in cytotoxicity.⁹⁶ Furthermore, this high charge density profoundly compromises in vivo pharmacokinetics. Specifically, the strong positive charge exacerbates non-specific serum protein opsonization and triggers rapid sequestration by the reticuloendothelial system.⁹⁷ Such circulatory instability severely restricts the bioavailable dose reaching the renal tubules, thereby diminishing the overall renal targeting efficiency. This necessitates the integration of charge-shielding strategies, such as surface modification with polyethylene glycol, to balance organelle penetrability with systemic safety.⁹⁸

Emerging Subcellular Targets: Endoplasmic Reticulum and Lysosomes

While mitochondria represent a primary pathogenic hub, true subcellular precision necessitates acknowledging the highly interconnected organelle network governing tubular cell fate. Specifically, the ER and lysosomes are functionally coupled with mitochondria and present highly promising, emerging targets for tubule-directed nanomedicine.

During acute kidney injury, severe intracellular calcium overload and oxidative stress rapidly disrupt ER homeostasis, triggering the unfolded protein response and ER stress-induced apoptosis. Because the ER and mitochondria are tethered via mitochondria-associated membranes, this ER calcium leakage directly precipitates mitochondrial bioenergetic collapse. Consequently, delivering agents that buffer intracellular calcium offers a potent strategy to halt this cascade. Indeed, the HA-bilirubin/BAPTA-AM nanosystem (Wang et al) previously highlighted in ROS-Responsive Nanocarriers exemplifies this organelle-level intervention. By specifically buffering intracellular calcium spikes within the cytoplasm, this targeted delivery rapidly halts the aberrant ER stress-associated apoptotic signaling axes and prevents the subsequent downstream mitochondrial collapse.⁴⁷

Downstream of these events, lysosomes act as the terminal executioners of cellular quality control via autophagy. The successful clearance of dysfunctional mitochondria relies entirely on lysosomal integrity. However, nephrotoxins such as cisplatin frequently provoke lysosomal membrane permeabilization (LMP), which not only stalls autophagic flux but also leaks harmful proteases, such as cathepsin B (CTSB), into the cytosol to drive tubular cell apoptosis. Highlighting the criticality of resolving LMP, a recent seminal study by Tian et al demonstrated that Endosomal Sorting Complex Required for Transport III (ESCRT-III)-mediated lysosomal repair is pivotal for mitigating renal tubular cell injury.⁹⁹ Enhancing the function of ESCRT-III subunits, specifically CHMP4A, efficiently restores lysosomal membrane integrity, reactivates autophagosome-lysosome fusion, and curtails CTSB-mediated apoptotic cascades.

Importantly, while robust organelle-anchoring moieties (such as TPP⁺) exist for mitochondria, nanocarriers physically anchored to the ER or lysosomes via specific ligands remain in their infancy for renal applications. Thus, engineering nanocarriers designed to directly deliver specific ESCRT-III-modulating agents, lysosomal lipid mimetics, or ER-stabilizing drugs represents a highly promising, yet underexplored, frontier. By expanding subcellular strategies beyond mitochondria to encompass the ER and lysosomes, future nanotherapeutics can orchestrate a comprehensive restoration of the entire renal organelle network.

Translational Considerations and Future Directions

Despite the remarkable preclinical efficacy of tubule- and mitochondria-targeted nanomedicines, a formidable gap persists between laboratory successes and clinical implementation. A primary obstacle lies in the limitations of current evaluation methodologies, as most studies rely on whole-organ imaging that fails to deconvolute accumulation in specific nephron segments or subcellular compartments. This lack of spatial resolution is further compounded by disease-induced physiological alterations, including compromised filtration, proteinuria, and hemodynamic instability, which complicate pharmacokinetic interpretation and potentially mask off-target toxicity.^100–104 Furthermore, the inherent complexity of advanced nanocarriers, which often integrate multiple targeting ligands, microenvironment-responsive linkers, and organelle-specific motifs, raises significant translational concerns regarding large-scale manufacturing, batch-to-batch consistency, and long-term immunogenicity.^105–109

Additionally, the pathological transition from acute kidney injury to chronic kidney disease involves profound spatiotemporal heterogeneity in oxidative stress, inflammation, and metabolic reprogramming. Consequently, static formulations designed for a single stage are unlikely to maintain optimal efficacy across the evolving disease continuum involving fluctuating receptor profiles and microenvironmental parameters.^1,2

Bridging this translational divide necessitates the convergence of high-throughput screening technologies and computational intelligence. Emerging platforms utilizing DNA barcoding have revolutionized the discovery pipeline by enabling the simultaneous in vivo evaluation of hundreds of chemically diverse nanoparticles. As exemplified by recent work from Wang et al, this high-throughput approach demonstrated that intrinsic renal tropism is predominantly dictated by specific polymer compositions rather than conventional parameters such as global size or surface charge. Specifically, their screening of 143 in situ polymerized nanocapsules identified “Z40”, a highly tubule-avid nanocarrier featuring a precise copolymer composition of an anionic monomer, 2-carboxyethyl acrylate, and a cationic monomer, (3-methacrylamidopropyl) trimethylammonium chloride. In vivo validation in a murine model of cisplatin-induced acute kidney injury confirmed its translational potential; when utilized to encapsulate the antioxidant enzyme catalase, the Z40-based nanomedicine successfully navigated to the injured interstitium to scavenge reactive oxygen species, thereby alleviating proximal tubular necrosis and preserving brush border integrity. Crucially, the comprehensive in vivo biodistribution datasets generated from such barcoded libraries establish the essential, high-quality training data required for artificial intelligence (AI) and machine learning models. By analyzing these physicochemical descriptors, advanced algorithms can accurately predict bio-nano interactions and generate novel tubule-avid chemical structures de novo, facilitating the rational design of targeted carriers prior to extensive animal testing.^110–116

Parallel to these discovery tools, the development of carrier systems must pivot toward modular and scalable architectures. Biomimetic platforms, including extracellular vesicles and hybrid systems, offer a compelling balance of high biological compatibility and low immunogenicity; however, their production must adhere to rigorous quality control standards to ensure reproducibility.^63–66 Ultimately, the next generation of renal nanomedicines should aim for stage-adaptive precision by exploiting complementary uptake pathways and responding to dynamic pathological cues.^117–119 To further de-risk clinical translation, a pragmatic strategy involves encapsulating FDA-approved agents with demonstrated pleiotropic metabolic benefits or integrating advanced clinical candidates. Leveraging the established safety profiles and pharmacological mechanisms of these molecules, rather than developing new chemical entities de novo, offers a streamlined regulatory trajectory for these novel mitochondria- and tubule-targeting platforms.^86,93

Conclusion

The field of renal nanomedicine is currently undergoing a pivotal transformation, transitioning from reliance on passive accumulation toward the development of sophisticated systems designed for active tubular and subcellular interrogation (Table 2). As elucidated in this review, this evolution is underpinned by three strategic pillars. First, active targeting strategies leverage injury-specific receptors and microenvironmental gradients to achieve precise local retention within the damaged nephron. Second, biomimetic platforms, particularly engineered extracellular vesicles, utilize endogenous transport mechanisms to navigate biological barriers and enhance biocompatibility. Third, subcellular interventions position mitochondria as a critical therapeutic frontier, aiming to restore the bioenergetics required for tubular cell survival and repair.

Table 2.

The Evolutionary Pathway of Precision Nanomedicines for Renal Tubular Injury

Evolutionary Stage	Mechanistic Strategy	Representative Platforms	Therapeutic Benefits	Translational Barriers
1. Passive Accumulation	Size/charge-dependent filtration and peritubular extravasation.	Polymeric nanoparticles, unmodified EVs, bare liposomes.	Formulation simplicity; highly scalable manufacturing.	Stochastic distribution; rapid excretion; reticuloendothelial sequestration.
2. Active Tubular Targeting	Receptor-mediated endocytosis via tissue-specific or injury-induced receptors (eg, Megalin/Cubilin, KIM-1, VCAM-1/ICAM-1).	Megalin-targeted conjugates (eg, lysozyme), KIM-1-binding peptide-functionalized carriers, and VCAM-1/ICAM-1-targeted biomimetic vesicles.	Overcomes filtration limits; maximizes specific tubular retention.	Strict receptor expression windows; endogenous ligand competition.
3. Stimuli-Responsive Activation	Microenvironment-triggered physicochemical transitions (ROS, pH, enzymes).	ROS-cleavable capsules, pH/enzyme-sensitive polyplexes.	Spatiotemporally controlled, localized payload liberation.	In vivo activation heterogeneity due to dynamic microenvironments.
4. Engineered and Biomimetic Delivery	Biological membrane cloaking for immune evasion and specific tropism.	MSC/macrophage EVs, peptide-engineered EVs, hybrid vesicles.	Exceptional biocompatibility; customizable active tropism.	Variable endogenous loading; rigorous GMP scaling challenges.
5. Subcellular Targeting	Organelle-specific navigation to resolve organelle stress.	TPP+-modified antioxidants, SS-31-conjugated carriers.	Direct interception of bioenergetic collapse at the source.	Cation-induced cytotoxicity (eg, depolarization); rapid systemic clearance.

Abbreviations: EVs, extracellular vesicles; GMP, Good Manufacturing Practice; ICAM-1, intercellular adhesion molecule-1; KIM-1, kidney injury molecule-1; MSC, mesenchymal stem cell; ROS, reactive oxygen species; TPP⁺, triphenylphosphonium; VCAM-1, vascular cell adhesion molecule-1.

While the potential of these technologies is significant, their clinical realization necessitates a concerted effort to bridge the gap between preclinical innovation and practical application. Future advancements will depend on the integration of high-throughput screening and AI to accelerate carrier optimization, alongside a focus on developing scalable, biocompatible formulations that encapsulate validated therapeutics. Ultimately, mastering these hierarchical targeting capabilities represents the key to fundamentally altering the therapeutic trajectory of tubular kidney injuries.

Funding Statement

This study was supported by the grants from the National Natural Science Foundation of China (82572522, 82100721).

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure

The authors declare that they have no conflicts of interest.