Nanomedicines for renal management: from imaging to treatment

Conspectus

Nanomedicine has benefited from recent advances in chemistry and biomedical engineering to produce nanoscale materials as theranostic agents. Well-designed nanomaterials may present optimal biological properties, influencing circulation, retention, and excretion for imaging and treatment of various diseases. As the understanding of nanomedicine pharmacokinetics expands continuously, efficient renal clearance of nanomedicines can significantly increase the signal-to-background ratio for precision diagnosis and lower potential toxicity for improved treatment. Studies on nano-kidney interactions have led to many novel findings on the underlying principles of nanomaterial renal clearance, targeting, and accumulation. In return, the optimized nanomedicines confer significant benefits to the detection and treatment of kidney dysfunction.

In this Account, we present an overview of recent progress in the development of nanomaterials for kidney theranostics, aiming to speed up translation and expand possible applications. We start by introducing biological structures of the kidney and their influence on renal targeting, retention, and clearance. Several key factors regarding renal accumulation and excretion, including nanomaterial types, sizes and shapes, surface charges, and chemical modifications, are identified and discussed. Next, we highlight our recent efforts investigating kidney-interacting nanomaterials and introduce representative nanomedicines for imaging and treatment of kidney diseases. Multiple renal-clearable and renal-accumulating nanomedicines were devised for kidney function imaging. By employing renal-clearable nanomedicines, including gold nanoparticles, porphyrin polymers, DNA frameworks and polyoxometalate clusters, we were able to non-invasively evaluate split renal function in healthy and diseased mice. Further engineering of renal-accumulating nanosystems has shifted attention from renal diagnosis to precision kidney protection. Many biocompatible nanomedicines, such as DNA origami, selenium-doped carbon quantum dots, melanin nanoparticles, and black phosphorus have all played essential roles in diminishing excessive reactive oxygen species for kidney treatment and protection. Finally, we discuss the challenges and perspectives of nanomaterials for renal care, their future clinical translation, and how they may affect the current landscape of clinical practices. We believe that this Account updates our current understanding of nano-kidney interaction for further design and control of nanomedicines for specific kidney diagnosis and treatment. This timely Account will generate broad interest in integrating nanotechnology and nano-bio interaction for state-of-the-art theranostics of renal diseases.

Graphical Abstract

1. Introduction

Nanomedicine is the engineering and application of nanomaterials for the diagnosis and treatment of diseases at the molecular level.⁵ Comparing with small-molecule drugs, scientists generally believe that well-designed nanomaterials present a multifunctional option for highly-controllable drug delivery, disease imaging, and treatment. Decades of research in nanomedicine has also dramatically expanded our understanding of nano-bio interactions regarding their circulation, distribution, and excretion in living organisms.⁶ While many nanomedicines are shown to be beneficial for imaging and treatment of cancer and other diseases, toxicity concerns linger. Nanomedicines that are renal-excretable can greatly reduce toxicity and also facilitate the diagnosis and treatment of kidney dysfunction.⁷ As such, renal-specific nanomaterials for kidney management have become a hot topic in biomedical research. A growing number of scientific discoveries on nano-kidney interaction has elucidated many essential design criteria. The implications of this work may broaden potential applications of nanomedicines for renal disease management, ranging from early diagnosis to precision treatment.⁸

In 2003, Tsutsumi and co-workers described a co-polymer system for selective renal drug delivery.⁹ In 2007, a pioneering report investigating the renal clearance of quantum dots (QDs), established an important principle for urinary excretion of nanomaterials: a final hydrodynamic diameter of ≤ 5.5 nm may lead to efficient renal elimination.¹⁰ This principle has led to many successful discoveries of nanomaterials that mitigate toxicity concerns. Subsequent preclinical and clinical investigations of nanomedicines have transcended our current understanding of “nanostructure-activity” relationships.

In this Account, we summarize recent advances of nanomedicines for kidney disease management, including strategies for imaging and treatment of renal dysfunction. Special focus is given to the design principles of kidney-interacting nanomedicines, such as nanomaterial types, sizes, shapes, surface charges, and chemical modifications, and how they can aid the management of kidney dysfunction (Figure 1).^{7–8, 11} Recent advances in tailoring nanomedicines for kidney theranostics highlights our ability to manipulate nanomaterials for improved kidney function diagnosis and protection. We intend for this Account to update the understanding of nano-kidney interactions that may further expand the nanomedicine paradigm for renal disease management.

Figure 1.

Nano-kidney interactions, including designable renal accumulation and renal clearance, can be harnessed for kidney imaging and disease treatment. GBM refers to glomerular basement membrane.

2. Design principles of nanomedicines for renal management

Once nanoparticles enter systemic circulation, typically by intravenous injection, they disseminate and are eventually excreted via hepatobiliary or renal routes. Nano-bio interactions dictate the biological fate of administrated nanomaterials at every phase, which determines the overall diagnostic and therapeutic efficacy for different diseases.^7–8 For example, an ideal nanomedicine for cancer treatment would circulate long enough to enable optimal tumor uptake but short enough to reduce hematological toxicity; it should only reside at tumor sites, and all off-target nanomedicine should be cleared by fast renal elimination, producing increased bioavailability and fewer side effects.^12–14 The search of perfect nanomedicines remains ongoing, for cancer and other diseases, but their renal clearance provide a nearly universal benefit by enhancing imaging contrast or reducing treatment toxicity.¹⁵

2.1. Kidney physiology and glomerular filtration barriers

Kidneys receive approximately 20-25% of all cardiac output (1-1.2 L/min of blood in humans). Each kidney is composed of more than 1 million nephrons. This basic functional unit of renal filtration is composed of a renal corpuscle and renal tubules. The glomerulus and Bowman’s capsule form the renal corpuscle, where the glomerulus is a capillary cluster that receives and filters blood flow to achieve renal filtration; Bowman’s capsule encloses the glomerulus to collect filtered fluid and solutes as the start of renal tubules. The filtered fluid runs through renal tubules, where water and lipophilic drugs are reabsorbed, ions and weak electrolyte drugs are secreted. Liquid finally runs into the collecting duct system and the bladder via the ureter (Figure 2a–c).⁸ To date, glomerular filtration proposes more challenges for nanomedicines and few were found to undergo tubular secretion or reabsorption.

Figure 2.

Kidney physiology and four constituents of the glomerular filtration barrier: the endothelial glycocalyx, glomerular endothelial cells, glomerular base membrane, and epithelium (podocytes). Penal a, b, and c adapted from Servier Medical Art (smart.servier.com) under Creative Commons Attribution 3.0 Unported License. Copyright (2005) LES LABORATOIRES SERVIER, SAS.

The glomerulus is composed of four filtration barriers, including the endothelial glycocalyx, glomerular endothelial cells, glomerular base membrane (GBM), and podocytes that are together referred to as the glomerular filtration barrier (GFB). The endothelial glycocalyx is located on the surface of endothelial cells and prevents protein leakage, acting as a size-selective filtration system and controls the speed of nanomedicine passage.¹⁶ The endothelial layer consists of endothelial cells with large pores of 70-90 nm. The negatively charged GBM has 2-8 nm pores and prevents the filtration of most solutes in the blood. Podocytes face the Bowman’s capsule and have a monolayer of cells with pores of 4-11 nm (Figure 2d).^{1, 16} The unique anatomical composition of the GFB introduces size restrictions and other physical properties that nanomedicines must circumvent to escape blood circulation through the kidneys.

In general, nanomaterials with a hydrodynamic diameter (HD) of less than 6 nm undergo glomerular filtration and pass into the bladder.¹⁰ The size cut-off threshold is influenced by the overall shape of nanomaterials, applying to the smallest size-dimension. For example, efficient urinary excretion was observed for carbon nanotubes with a diameter of 0.8-1.2 nm and lengths of 100-1000 nm.¹⁷ However, our study found that tubular DNA origami with a diameter of 7 nm and a length of 400 nm showed sustained renal accumulation (but not clearance) for as long as 12 h, highlighting the specificity of kidney filtration regarding sizes and shape.¹ Electrostatic forces are also involved, which are influenced by surface charges and chemical modifications.¹³ Since GBM and podocytes carry negative charges under physiological conditions, positively charged nanomaterials may pass through the GFB more efficiently than neutrally and anionic ones.⁸ The size, shape, and charge of nanomedicines fundamentally influence nano-kidney interactions and must be considered when designing nanomedicines for renal management.

2.2. Renal-clearable nanomedicines

The direct size dependence on renal clearance was first established using dextran and protein fragments, where renal excretion was found to be solely dependent on their size (molecular weight) without any tubular secretion or reabsorption. Results showed a size cut-off of 6-10 nm for biomacromolecules of different rigidity.⁸ In 2007, the first report of renal-clearable QDs has since made the HD size of nanomaterials the most prioritized aspect to induce urinary elimination. An HD less than 5.5 nm permitted enhanced renal clearance for cysteine-coated QDs.¹⁰ Ultrasmall nanomaterials of different origins have been developed, such as gold nanoparticles (AuNPs),^16,18 carbon nanotubes (CNTs),¹⁷ silica dots (Connell dots, or C dots),^19–20 DNA frameworks,^21–23 and many bio-responsive nanocomposites^{2, 24–28} also conform to this rule (Table S1).

While most studies have found that decreasing size accelerates excretion, this is only valid for particles on the nanoscale (Figure 3a–c). Recently, Zheng and colleagues further investigated the “size-clearance” relationship. The endothelial glycocalyx acted as a size-exclusion filter for AuNPs but diminished renal clearance at the sub-nanometre scale.¹⁶ Compared to Au₂₅ (~1 nm), a decrease in gold atoms (Au₁₈, Au₁₅, and Au_10-11) resulted in four-to-nine fold reduced renal clearance efficiencies (Figure 3c). This discovery highlights the precise filtration of the kidney and expands our understanding of nano-kidney interactions.

Figure 3.

Renal-clearable nanomaterials. (a) Whole-body X-ray images showed the delayed renal clearance of sub-nanoscale AuNPs as they decreased in size from Au₂₅ to Au_10-11. (b) Schematic of Au₂₅ and Au₁₈ passing through the endothelial glycocalyx, which serves as a size-selective filter. (c) Renal clearance efficiency of AuNPs, C dots, and QDs as a function of their HD sizes. (d) Dynamic positron emission tomography (PET) images show the efficient renal clearance of single-walled CNTs and mathematical modeling suggested that blood flow is strong enough to guide CNTs through renal filtration. (e) PET images showing fast renal clearance of tetrahedral DNA framework (~13 nm). (f) PET images showing fast renal clearance of POM clusters (1-2 nm). Panel a and b adapted with permission from Ref.16. Copyright (2017) Nature Publishing Group. Panel d adapted with permission from Ref.17. Copyright (2010) National Academy of Sciences. Panel e adapted with permission from Ref.22. Copyright (2019) Springer Nature. Panel f adapted with permission from Ref.27. Copyright (2017) American Chemistry Society.

When nanomaterial shape is changed from zero-dimensional dots to one-dimensional nanowires or twodimensional nanosheets, it complicates their renal clearance in profound ways. Lacerda et al. and Ruggiero et al. separately reported unexpected rapid urinary elimination of multi-walled and single-walled carbon nanotubes with lengths over 100 nm.^17,29 While the longitudinal dimension of CNTs excludes their direct transportation across the glomeruli, detailed theoretical modeling suggests that blood flow is strong enough to guide CNTs through GFB pores along the transverse dimension. Thus, the overall conformation of nanomaterials is a relevant factor that cannot be neglected (Figure 3d).¹⁷

Other than rigid and inert nanomaterials, many ‘soft’ and biodegradable nanosystems have been developed to facilitate renal clearance. It is well known that single-stranded DNA (ssDNA) can be excreted via the kidneys. Tetrahedral DNA frameworks assembled via the hybridization of multiple oligonucleotides showed an improved balance between clearance and circulation.³⁰ After functionalization with folic acid and siRNA, the DNA framework with sizes of 6-30 nm showed fast renal clearance (circulation half-lives between 5.3-24.2 min) and excellent tumor-to-background contrast.^21,31 A DNA tetrahedron (13 nm) was later devised for kidney function evaluation in a murine unilateral ureter obstruction (UUO) model (Figure 3e).²² Many small molecules have also been used as building blocks to achieve bio-responsive renal clearance. Our group has successfully engineered polyoxometalate clusters (POM)^26–27, coordination polymer nanodots,^28,32 and PEGylated porphyrin micelles using a similar concept.²

2.3. Renal-accumulating nanomedicines

As mentioned previously, electric charge and hydrophilicity are critical for nanomaterials to reach the kidneys. When developing nanomedicines with enhanced renal clearance to alleviate potential toxicity, our studies have highlighted methods to engineer nanomaterials with specific kidney accumulation by leveraging sizes, shapes as well as surface charges.

While tetrahedral DNA frameworks (size range 10-30 nm) were all rapidly excreted in urine and possessed negative charges from the unassembled DNA strands, a prolonged circulation was observed as the size increased. A reasonable hypothesis is that larger DNA frameworks and corresponding anionicity increase prolong kidney retention. Using positron emission tomography (PET) imaging, rectangular and triangular DNA origami nanostructures (DONs, both ~100 nm) showed exclusive kidney retention for as long as 12 h.¹ Moreover, a one-dimensional tubular DON resembling the shape of CNTs also enhanced profound renal uptake. The kidney accumulation of all three DONs reached 10-12 %ID at 12 h after injection, more than 20-fold higher than a free DNA strand (<0.5 %ID). The expanded sizes and increased negative charges of DONs prevented their direct glomerular filtration and intensified renal electrostatic repulsion, which collectively contributed to their primary kidney retention (Figure 4a).³³

Figure 4.

1D and 2D Nanomedicines with strong negative charges showed exclusive kidney accumulation. (a) Exclusive renal accumulation of 2D DNA origami nanostructures with different shapes. Adapted with permission from Ref.1. Copyright (2018) Nature Publishing Group. (b) Specific kidney localization of SeCQDs with sizes of ~40 nm. Adapted with permission from Ref.4 under the terms of the Creative Commons Attribution License.

By using graphene oxide as a model, Kostarelos and co-workers investigated in detail how thin twodimensional (2D) nanosheets with a thickness of 1-2 nm interacted with glomerular filtration in vivo and endothelial and podocyte cells in vitro.³⁴ Featuring an ultrathin 2 nm sheet, selenium-doped carbon quantum dots (SeCQDs) with similar sheet-like morphology and concentrated negative charges (−31.0 ± 1.4 mV) showed preferential kidney retention for more than 72 h (Figure 4b).⁴ Another similar case is the black phosphorus nanosheets that share the flat 2D shape and strong surface anionicity.³⁵ Fluorescence imaging revealed its specific kidney residence for more than 6 h and eventual urinary elimination.

It is important to note that nanomedicines that might readily pass through the kidneys can be trapped when kidney dysfunction exists. Molybdenum-coordinated POM nanoclusters are 1-2 nm zero-dimensional dots with stable renal elimination. When kidney injury manifests, the alternated in vivo pharmacokinetics can help with renal dysfunction diagnosis.²⁶ A similar phenomenon occurred for melanin nanodots with sizes of 4-5 nm.²⁴ Another interesting nanosystem is the PEGylated porphyrin nanoparticles that have adjustable blood circulation and renal elimination. As the PEG chains increased from 2, 5, 10, to 30 KDa, the resulted nanoparticles showed less clearance and prolonged circulation.² This nanosystem permits control over the balance between heart circulation and renal clearance for longer imaging time windows and lower imaging background, respectively. The responsiveness of these nanosystems and our ability to regulate their renal clearance and accumulation were essential for timely detection of kidney function and renal injury.

3. Imaging kidney disease

The understanding of nano-kidney interactions has been a barrier to overcome for recent advances in kidney function diagnosis. Traditional diagnostic methods, such as measuring creatinine (CRE) and blood urea nitrogen (BUN) levels, only provides information on the overall kidney filtration, which fails to detect early renal injury that might later cause severe kidney damage.³⁶ Molecular imaging, including optical imaging, nuclear imaging (imaging using radioactive isotopes), and magnetic resonance imaging (MRI), can help by using tracers to obtain real-time information concerning kidney metabolism.

3.1. Optical imaging

For kidney function diagnosis, AuNP-based fluorescent imaging successfully enabled renal function staging in murine UUO models and even revealed renal function compensation.^37–38 Within 7-9 days after UUO model establishment, traditional blood tests of BUN and CRE showed no abnormalities, indicating its lack of sensitivity for detecting early-stage kidney dysfunction (Figure 5a). In comparison, AuNPs identified the obstructed kidney, but the small molecule dye 800CW failed to do the same. Detailed data analysis suggested that AuNPs can not only differentiate mild and severe kidney damage after obstruction but visualized renal excretion compensation by the un-obstructed kidney (Figure 5b). These results suggest that optical imaging with nanomedicines provides more in-depth kidney filtration information and sensitively monitor split renal function.

Figure 5.

Optical imaging of kidney dysfunction. (a) Renal-clearable AuNPs evaluated kidney function in murine models of UUO via optical imaging. (b) AuNP-based imaging differentiated mild and severe kidney damage and identified renal function compensation. (c) Scheme principles of dual-functional molecular renal probes diagnosing cisplatin-induced acute kidney injury via responsive molecular renal probes (MRPs). (d) Optical images of mice after cisplatin treatment to induce kidney injury. The real-time on/off of chemiluminescence and NIRF signal from MRPs showed the process of cisplatin exposure and may help with kidney injury prediction. Panel a, and b adapted with permission from Ref.37, Copyright (2016) John Wiley and Sons. c and d adapted with permission from Ref.39. Copyright (2019) Nature Publishing Group.

Very recently, Pu and co-workers developed a series of molecular renal probes (MRPs) with fast kidney excretion for optical imaging of cisplatin-induced acute kidney injury.^39–43 MRP is composed of a renal-clearable cyclodextrin, a biomarker responsive moiety (responsive to different kidney injury markers), and a fluorescent signaling moiety. By combining multiple moieties, MRPs can monitor the earliest change of renal filtration and onset of kidney injury (Figure 5c–d). Results showed that kidney injury could be detected 36 h earlier than current medical imaging methods. In 2019, Stevens, Bhatia, and colleagues developed a renal-clearable AuNP nanozyme for colorimetric urinary readout of colorectal cancer.^44–45 Responsive proteins tethered to AuNPs may disassemble when encountering irregular biological proteases at diseased sites. While the complete urinary excretion and peroxidase-mimicking activity of AuNPs provide safety guarantee and necessary catalysis, this optical imaging strategy was highly sensitive and also versatile.

3.2. Nuclear imaging

Nuclear imaging, including PET and single-photon emission computed tomography (SPECT) imaging, employs trace amounts of radioisotopes to monitor biological processes non-invasively. Distinct from optical imaging, nuclear imaging offers unlimited penetration depth and extremely low biological background, rendering it highly popular in clinical practice. While most radiolabeled small molecules are eliminated too rapidly, nanomedicine-based tracers present excellent kidney specificity and signal-to-noise ratios for kidney function evaluation.

Many radiolabeled nanomaterials have been applied to image renal status. After incorporating radioactive ¹⁹⁸Au, SPECT imaging using renal-clearable AuNPs enhanced the imaging contrast and quantification accuracy for renal function assessment.⁴⁶ Later, the GSH-functionalized ultrasmall AuNPs were intrinsically radiolabeled with Cu-64 for dynamic and efficient evaluation of kidney function.⁴⁷ Our study developed a simple way of monitoring dynamic profiles of nanomedicines’ biodistribution and excretion in living organisms. In 2016, Lovell, Cai, and co-workers prepared a porphyrin-PEG mesh with intrinsic Cu-64 labeling for imaging of renal function.²⁵ Fluorescent imaging of porphyrin indicated systemic acute kidney injury after rhabdomyolysis (Figure 6a). Moreover, dynamic PET imaging established delayed excretion of the injured kidneys for potential clinical stratification. Later, our further study revealed that, when adjusting the length of PEG chains conjugated to a single porphyrin molecule, the resulted products have a well-regulated balance of heart circulation and renal clearance.² PET imaging showed that 10 kDa PEG chain showed the optimal balance and provided valuable insight as to how PEGylation may regulate pharmacokinetics of nanomedicines in healthy and diseased animals.

Figure 6.

Radioactive nanomedicines for renal function diagnosis. (a) Porphyrin-PEG mesh showed delayed excretion in mice with acute kidney injury (AKI). Adapted with permission from Ref.25. Copyright (2016) Elsevier. (b) PET imaging of POM clusters in healthy mice and mice with early or end stage of UUO. (c) Kidney kinetics of POM clusters guided the differentiation of UUO at the early and late stage. (d) UUO kidney at the early stage showed strong uptake of POM clusters, while at the end stage, no uptake was observed. Panel b-d adapted with permission from Ref.26. Copyright (2018) Elsevier.

Other than dual-modality nanosystems, our group developed Zr-89 radiolabeled POM nanoclusters and tetrahedral DNA nanostructures for detailed kidney dysfunction profiling. POM clusters sized at 2 nm showed prolonged heart circulation of 9.6 h and efficient kidney elimination in healthy mice.^26–27 In UUO models, ⁸⁹Zr-POM successfully distinguished early and end-stage kidney failure caused by the ureter obstruction (Figure 6b–6d). At the early stage, the obstructed kidney still showed tracer accumulation but no clearance, but no tracer uptake was observed at the end-stage. Separate dynamic PET imaging studies with Cu-64 labeled DNA tetrahedron demonstrated that the ureter obstruction affected tracer distribution as early as 12 h after obstruction, and the unobstructed kidney started to compensate, maintaining the overall body elimination as evidenced by the minimally altered circulation half-life and total excretion of DNA tetrahedron.²²

3.3. X-ray, CT, and Magnetic resonance imaging (MRI)

Renal-clearable AuNPs have also been used for X-ray evaluation of kidney nephropathy in UUO models.⁴⁸ By quantifying the AuNPs transportation in the renal cortex, medullar, and pelvis, a correlation of transportation kinetics with local pathological lesions was reached. Our lab further confirmed the feasibility of using this AuNP for CT imaging.⁴⁷ A comprehensive PET/CT/optical imaging confirmed the fast renal clearance of AuNPs but also suggested a 130-fold shorter circulation half-life comparing to previous optical findings. This study highlights the necessity of using multimodal imaging to understand transportation kinetics of nanomedicines in vivo.

MRI is widely used for anatomical imaging of abnormalities with higher resolution than traditional CT imaging. Brown, O’Brien and co-workers developed superparamagnetic iron oxide nanoparticles (SPIONs) for MRI and targeted renal drug delivery. SPIONs are composed of an iron oxide crystalline core, a monolayer of oleic acid alkyl chain, and an outer phospholipid monolayer that enables conjugation of virtually any type of biomolecules for targeted MRI. Kidney targeting efficiency was demonstrated with anti-MHC class II antibodies (RT1) that were chosen for renal medullary inflammation imaging.⁴⁹ Compared to bare SPIONs and non-specific SPIONs, the RT1-SPIONs showed the best contrast in terms of medulla-to-cortex ratios in T₂-weighted turbo spin-echo MRI images. This versatile iron oxide system showed great potential for targeted diagnosis of kidney injury.

To limit nephrogenic systemic fibrosis (NSF) risk associated with gadolinium-based MRI contrast agents, Ling and co-workers developed PEG-stabilized iron oxide nanoclusters (IONCs) for biocompatible T₂-weighted MRI.⁵⁰ Rats with renal failure were used to profile the distribution, excretion, toxicity, and fibrotic gene expressions after administrating IONCs. Results showed excellent biosafety of IONCs, which negligibly increased the risk of NSF in rats with renal failure.

4. Treatment of kidney diseases

Successful imaging of kidney function has yielded the foundation for the development of renal nanomedicines against kidney diseases of different origins. Current research has found close association between chronic kidney diseases (CKDs) and acute kidney injury. While CKDs are associated with diets, lifestyles, and personal health conditions, AKI causes more hospitalization and requires more immediate medical attention. AKI has a high incidence at >5000 cases per million people annually, and it is clinically characterized by abrupt loss of excretory function, accumulation of nitrogenous waste, and decreased urine output.³⁶ Traditional small molecule drugs showed suboptimal efficacies due to their fast clearance, low blood retention, and large systemic dose requirements. Lack of early intervention may lead to the acceleration of chronic kidney injury and eventual development of end-stage kidney failure. By downregulating excessive reactive oxygen species (ROS), neutralizing renal toxins, or targeted renal pro-drug delivery, researchers have taken advantage of renal-accumulating nanomedicines for improved kidney protection against kidney injury of different origins.

4.1. Rhabdomyolysis-induced acute kidney injury (RM-AKI)

RM-AKI may occur after severe dehydration, infection, sepsis, or exposure to certain toxins. The breakdown of muscle tissue releases myoglobin, among other substances, that becomes toxic after accumulating in renal tubules and generating massive amounts of ROS. Antioxidant-based therapy has been proposed to neutralize excessive ROS and restore redox balance. However, small-molecule antioxidants, such as N-acetyl cysteine (NAC) and amifostine, have limited renal bioavailability that hampers therapeutic effects. Our group has proposed several nanomedicines to overcome this limitation (Figure 7a).

Figure 7.

Nanomedicines for RM-AKI treatment. (a) Specific kidney accumulation of antioxidant nanomedicines facilitates RM-AKI treatment via local ROS scavenging. (b) Rectangular DNA origami nanostructure (Rec-DON) with high renal accumulation treated RM-AKI more efficiently than the clinically used drug NAC. Treatment dose of M13 and Rec-DON: 0.5 mg/kg; H-NAC: high-dose NAC (210 mg/kg); L-NAC: low-dose NAC (0.5 mg/kg) (c) Ultrasmall melanin nanoparticles outperformed clinical contrast agents for MRI imaging of kidney injury, and intrinsic antioxidant property enabled a theranostic approach. Panel a and b adapted with permission from Ref.1. Copyright (2018) Nature Publishing Group. Panel c adapted with permission from Ref.24. Copyright (2019) John Wiley and Sons.

We designed three different DNA origami nanostructures with enhanced renal accumulation for antioxidant protection against RM-AKI (Figure 7b).¹ Cellular DNA is known for its vulnerability against ROS, as DNA bases can be easily oxidized. For RM-AKI therapy, DNA bases served as potent kidney protectants to scavenge myoglobin-generated ROS in the vicinity of renal tubules. The ROS-neutralizing ability of DNA origami was tested with three major ROS in vivo: hydrogen peroxide, superoxide anions, and hydroxyl radicals. Results showed that G bases in DNA origami were most active toward all three types of ROS in aqueous solutions and on the cellular level. Compared to the small molecule drug NAC, the therapeutic dose of DNA origami was more than 400-fold lower in RM-AKI animal models. The high treatment efficacy of DNA origami was attributed to its elevated renal uptake and sustained tubular retention. Systematic toxicity evaluation revealed no adverse effects after intravenous injection of DNA origami in living organisms.

POM nanoclusters with a readily variable valence state of molybdenum ions also possess strong capacity of scavenging harmful ROS.³ Mo ions in the POM clusters may shift between 5+ and 6+ in reductive and oxidative environments. When challenged with H₂O₂, OH radicals, and superoxide anions, POM nanoclusters showed superior antioxidative properties at an extremely low concentration of 20 μg Mo/mL. In murine models of RM-AKI, POM clusters displayed extended kidney retention as evidenced by longitudinal PET imaging. Specific kidney tissue staining revealed that POM nanoclusters retained the levels of superoxide dismutase, lowered down the expression of kidney injury molecule-1 and heme oxygenase-1, and ultimately alleviated detrimental oxidative stress in the kidneys.

Melanin, an endogenous pigment and antioxidant, can be used for nanoparticle formation. After simple Mn²⁺ coordination and PEGylation, we obtained melanin nanoparticles with ultrasmall sizes and excellent biocompatibility for specific kidney protection.²⁴ T₁-weighted MRI and PET imaging of melanin nanoparticles showed their favorable renal accumulation in diseased mice (Figure 7c). Renal uptake doubled after RM-AKI, indicative of the responsiveness of this nanosystem for renal treatment. The ROS scavenging ability of melanin nanoparticles was extensively verified, and blood test and kidney staining results confirmed the robust antioxidant defense of melanin nanoparticles for RM-AKI.

4.2. Ischemia-reperfusion acute kidney injury (IR-AKI)

Ischemia-reperfusion injury poses a major clinical challenge to native and transplanted kidneys by inducing AKI. Self-generated ROS after the ischemia-reperfusion process is identified as the main culprit, which further leads to cell apoptosis, inflammation, and tissue necrosis. IR-AKI is especially detrimental for kidneys to be transplanted into patients.

Nagasaki and co-workers established a pH-responsive nitroxide radical-containing co-polymer (RNP^pH) for ROS scavenging to treat IR-AKI.⁵¹ ROS-neutralizing nitroxide radicals were conjugated to the hydrophobic part of amphiphilic block co-polymers and further compartmentalized in the core of the final structure. In an acidic environment such as the injured renal sites, RNP^pH can disintegrate to expose nitroxide radicals and catalytically remove detrimental ROS in the kidneys. The pH-regulated antioxidant release system showed improved treatment efficacy and lower toxicity comparing to nitroxide radicals in free form.

4.3. Drug-induced acute kidney injury and other nephropathies

Another form of AKI, which is also associated with elevated ROS levels, is cisplatin-induced acute kidney injury (CP-AKI). This condition occurs with the use of cisplatin for chemotherapy. Significant nephrotoxicity signs, including renal tubular damage and glomerular filtration decline, are dose-limiting. Hydration and diuresis are applied clinically to prevent kidney injury but prove supportive for detoxification.

In 2016, McDevitt and colleagues developed ammonium-functionalized carbon nanotube–mediated transport of siRNA (fCNT/siRNA) for CP-AKI treatment.⁵² The fCNT enhanced overall siRNA delivery to renal tubule cells and effectively knocked down the expression of several target genes, including Trp53, Mep1b, Ctr1, and EGFP. Moreover, combination therapy using fCNT/siMep1b and fCNT/siTrp53 cooperatively suppressed expression of Mep1b and Trp53, resulting in substantially improved progression-free survival (Figure 8a).

Figure 8.

Nanomedicines for other nephropathies. (a) fCNT delivered multiple siRNA to renal tubules for the treatment of cisplatin-induced AKI (CP-AKI). Adapted with permission from Ref.52. Copyright (2016) AAAS. (b) Selenium-doped carbon quantum dots showed specific kidney accumulation for the treatment of RM-AKI and the prevention of CP-AKI. Adapted with permission from Ref.4 under the terms of the Creative Commons Attribution License. Copyright (2020) John Wiley and Sons. (c) ZSJ-0228 prodrug micelles infiltrated renal inflammatory sites for suppression of lupus nephritis. Adapted with permission from Ref.53. Copyright (2018) American Chemical Society.

Our group applied kidney-accumulating SeCQDs for the prevention of CP-AKI, which were also used to treat RM-AKI.⁴ Due to its sustained kidney retention and strong antioxidant property, this biocompatible system prevented possible nephrological damage (arrows and asterisks in Figure 8b) from cisplatin at an extremely low dose of 50 μg per mouse. Pharmacokinetic and toxicological evaluation of this SeCQDs system in mice strongly encouraged further investigation toward clinical translation.

A major complication of lupus erythematosus, lupus nephritis, is caused by abnormal deposition of immune cells in the kidneys and often treated with immunosuppressive drug glucocorticoids that often cause severe adverse effects. In 2018, Wang and co-workers reported a glucocorticoids pro-drug nanomedicine, PEG-based micelle of dexamethasone (ZSJ0228), that can infiltrate inflammatory sites via leaky vasculatures and release the active drug locally (Figure 8c).⁵³ After confirming its kidney specificity, a monthly injection of ZSJ0288 offered sustainable alleviation of proteinuria and lupus nephritis that was therapeutically equivalent to daily administration of glucocorticoids.

5. Clinical translation and perspectives

In this Account, we summarized our recent work on nanomedicines for renal management, from imaging of kidney function to treatment of kidney diseases. While efficient renal clearance of nanomedicine has been the primary obstacle for clinical translation, we overcame this challenge and applied our findings for kidney diagnosis and treatment. We controlled nano-kidney interactions, including renal clearance and accumulation, by manipulating the sizes, shapes, surface charges, and composition of nanomaterials. The emerging “nanostructure-effect relationships” have yielded many successful nanomedicines for systematic management of kidney function for diagnosis and therapy. C dots have already been tested in patients with melanoma and glioma as a result of their fast clearance, active targeting capacity, and negligible toxicity. Renal-clearable AuNPs with optical/PET/SPECT/CT imaging abilities showed excellent imaging contrast for kidney function quantification, are thus highly promising to move from the bench to the bedside. Controllable renal accumulation can be achieved by refining nanomedicines using similar methods. With precise programmability, DNA framework with controllable kidney clearance and accumulation were used to image kidney function and efficiently treat acute kidney injury non-invasively. Many other biocompatible nanosystems that localize in the kidneys have also shown efficacious therapy of kidney diseases. Currently, many nanomedicines have entered clinical trials and most are approved for cancer diagnosis and treatment. Superparamagnetic iron oxide nanoparticles and ^99mTc-labeled sulfur colloid are approved for imaging of lymph node and liver matastasis. C-dots with efficient renal clearance are undergoing trials for PET/optical cancer imaging.¹⁴ The accumulated translational experiences formed a successful framework for the sequential development of novel nanomedicines for renal imaging and treatment.

Despite exciting progress, there are more questions than answers for nano-kidney interactions, which remains the bottleneck of developing translatable nanomedicines. Ongoing research investigating protein corona, nanoparticle density, and hydrophilicity/lipophilicity are other factors that will influence our understanding. Persistent and collective efforts across research disciplines are required to map the nano-kidney interactions. For the successful development of nanosystems to achieve renal rescue in clinical settings, there are several significant challenges to overcome. First, most research focuses solely on the kidneys, while the liver is equally or more important in determining the biological fate of nanomedicines. How the body balances nanoparticles between the hepatobiliary and the renal routes remains largely unknown. Second, although we have several basic principles to guide the design of kidney-interacting nanomaterials, many rule outliers were identified: PEG_5k-Au₅₀ at sizes of 75±25 nm targeted kidney mesangium;⁵⁴ some extremely large 400-nm mesoscale polymers specifically targeted the proximal tubule epithelium of the kidneys.⁵⁵ These reports strongly suggest that there is still much to explore into the nano-kidney interactions, such as transporter-mediated kidney accumulation, enzyme-based renal interactions, or potential tubular reabsorption. Third, near-infrared imaging in the second biological window (NIR-II) may significantly increase the penetration depth for probing deep tissue information in real-time. This emerging technique, when integrating with other imaging modalities, will be powerful for understanding nano-kidney interactions. Lastly, to facilitate clinical kidney management, nanomedicines need to be produced in bulk, which sets high standards for nanomaterial synthesis reproducibility and the influence on systematic preclinical toxicity evaluation and early human studies. We believe our recent advances have layed the groundwork for the establishment of a more comprehensive understanding of nano-kidney interactions, which may further expedite the clinical translation of nanomedicines for efficient kidney imaging and treatment.

Biographies

Biography

Dawei Jiang received his Ph.D. in inorganic chemistry from Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences in 2015. He then joined the University of Wisconsin-Madison for his postdoctoral training under the supervision of Prof. Weibo Cai. Dr. Jiang has been dedicated to the rational design and biomedical applications of biomolecule-based radiopharmaceuticals, including monoclonal antibodies and oligonucleotides, for imaging and treatment of cancer and kidney diseases.

Zachary T. Rosenkrans is a Ph.D. student at the University of Wisconsin-Madison under the supervision of Prof. Weibo Cai. He previously received a B.S. in Chemical Engineering from the University of Kansas in Lawrence, KS. His research focuses on image-guided drug delivery and theranostic platforms.

Dalong Ni received his Ph.D. degree in 2016 from the Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS). He joined the Department of Radiology at the University of Wisconsin-Madison as a postdoctoral fellow under the supervision of Prof. Weibo Cai. His research interests focus on biomedical applications of multifunctional nanoplatforms for imaging and therapy applications.

Jing Lin is a Distinguished Professor at the Department of Molecular Imaging, Shenzhen University, China. She received Ph.D. in Organic Chemistry from the Donghua University and Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences in 2010. Her research focuses on molecular imaging, nanomedicine, and theranostics.

Peng Huang is a Distinguished Professor, Chief of the Laboratory of Evolutionary Theranostics (LET), and Director of the Department of Molecular Imaging at the Shenzhen University, China. He received a Ph.D. degree in Biomedical Engineering from Shanghai Jiao Tong University in 2012. His research focuses on molecular imaging, nanomedicine, and theranostics.

Weibo Cai is a Vilas Distinguished Achievement Professor of Radiology and Medical Physics, Biomedical Engineering, Materials Science & Engineering, and Pharmaceutical Sciences at the University of Wisconsin – Madison, USA. He received a Ph.D. degree in Chemistry from UCSD in 2004. His research is focuses on molecular imaging and nanotechnology.