Understanding Proteinuria in Patients with Diabetic Kidney Disease and Progress of Drug Therapy from a New Perspective

1. Introduction

Diabetic kidney disease (DKD) is one of the leading chronic microvascular complications in patients who have diabetes. The clinical manifestations include persistently increased urinary albumin levels and/or GFR progressively declining. DKD is the leading cause of end-stage kidney disease worldwide, and 30%–50% of end-stage renal disease (ESRD) cases globally are caused by DKD [1]; approximately 50% of patients with DKD in the United States undergo dialysis or kidney transplantation [2]. DKD also significantly increases the risk of cardiovascular disease in patients with diabetes and is an important risk factor for premature death [3].

Proteinuria in patients with DKD is primarily albuminuria, which has long been considered an important clinical feature of DKD. Notably, new therapeutic drugs for albuminuria in DKD are being developed and are widely used. Normoalbuminuric diabetic kidney disease (NADKD) is also gradually being recognized, and the timing of albuminuria control in patients with DKD has become more challenging; therefore, albuminuria in patients with DKD should be re-examined. This paper summarizes the pathophysiological mechanism of albuminuria in patients with DKD, the value of clinical diagnosis and prognosis, new progress and mechanisms of treatment, and multidrug therapy in patients with T2DKD, providing a new perspective on the clinical diagnosis and treatment of DKD.

2. Pathophysiological mechanism of albuminuria in DKD

Albuminuria is a prominent feature of DKD, which not only reflects damage to the glomerular filtration barrier but is also affected by glomerular hyperfiltration and renal tubular reabsorption. On the glomerular scale, hyperfiltration, endothelial dysfunction, basement membrane thickening, podocyte foot process fusion or detachment, glomerulosclerosis, and arteriolar hyalinization may lead to proteinuria [4]. Recent studies have demonstrated that both podocyte DNA double-strand breaks and glomerular DNA methylation are associated with the severity of albuminuria [5].

In the renal tubules, cubilin/megalin downregulation, tubular inflammation, atrophy, and reduced reabsorption of amino acids and proteins by the proximal tubules play a role in the development of DKD caused by proteinuria [4,6]. The mechanism underlying renal tubular proteinuria is primarily related to damage to the structure and function of the proximal tubules. Renal tubular epithelial cells (PCT) are located in chronic high-glucose or hypoxia-ischemic environments and exhibit abnormal metabolism and hemodynamics. Additionally, they express advanced glycation end-products (AGEs), reactive oxygen species, and other metabolites. Through a series of kinase cascades, they lead to extracellular matrix accumulation or macrophage infiltration to induce inflammation, eventually resulting in renal tubulointerstitial fibrosis and tubular injury, affecting renal tubular reabsorption function. This leads to renal tubular proteinuria development.

Ion channels and transporters play important roles in the pathogenesis of proteinuria [7]. Regarding ion channels, blocking the epithelial Na+ ion channels located in the epithelial cells of the distal convoluting tubules and collecting ducts can prevent Na+ reabsorption and decrease Na+ retention, thereby reducing proteinuria [8,9]. Blocking the P/Q-, N-, T-, and L-type voltage-gated Ca+ channels located in the arterioles of glomeruli can significantly restore the expression of podocyte-related proteins such as nephrin and podocin, protect podocytes, and reduce glomerular proteinuria [10]. Moreover, TRPC5 [11] and TRPC6 [12] ion channels located in podocytes protect the podocyte cytoskeletal hub structure, actin cytoskeleton, and glomerular proteinuria. Blocking Ca+ -activated K+ channels located in glomeruli and some renal tubules, as well as inhibiting Inflammatory factor-related signaling pathways such as growth factor-beta 1 (TGF-β1) signaling pathway and the expression of inflammatory markers, can reduce glomerular proteinuria and renal tubular proteins [13]. The activation of TRPV4 in proximal tubular epithelial cells can stimulate albumin endocytosis of tubular epithelial cells and reduce tubular proteinuria [14]. This activation also protects the expression of ClC-5, protects the proximal tubule from protein reabsorption, and reduces renal tubular proteinuria [15]. Furthermore, TRPV4 activation stimulates ATP-sensitive K+ channels located in the mitochondrial membrane of podocytes, inhibits the production of reactive oxygen species and podocyte injury, and reduces glomerular proteinuria [16]. Regarding ion transporters, the upregulation of Na+/K+/2Cl-cotransporters in the brush border membrane of the proximal tubule alleviates proteinuria [17]. The deeper the pathophysiological mechanism of DKD, the more precise the treatment for DKD.

Overall, this study summarizes the pathophysiological mechanisms underlying albuminuria in patients with DKD (Figure 1).

3. Role of albuminuria in the diagnosis and prognosis of DKD

According to the degree of proteinuria in patients with DKD, they are categorized into microalbuminuria (urinary albumin-to-creatinine ratio (UACR) 30–299 mg/g) and macroalbuminuria (UACR ≥300 mg/g). Microalbuminuria is used to indicate the possibility of chronic kidney disease (CKD) in patients [18]. Proteinuria is an important risk factor for renal prognosis and is not the only diagnostic criterion for DKD. An artificial intelligence system for detecting urinary microalbuminuria is expected to monitor the progression of diabetic nephropathy (DN) based on laboratory examinations [19]. Studies have confirmed that in patients with diabetes with any level of eGFR, the presence of proteinuria is associated with the risk of cardiovascular disease, CKD progression, and death [20,21]. Therefore, for patients with diabetes with macroalbuminuria, the treatment goal is to reduce urinary albumin by ≥30% to delay CKD progression. For patients with diabetes with microalbuminuria, the risk of progressin to macroalbuminuria should be reduced [22].

According to the concept of proteinuria defined by the National Kidney Foundation and the US Food and Drug Administration in 2009 [23], NADKD (UACR <30 mg/g) should be further refined to UACR <10 mg/g and UACR 10–29 mg/g groups. A UACR of <10 mg/g is the normal urine albumin content, and a UACR in the range of 10–29 mg/g exceeds the normal value [24]. However, a better understanding of NADKD may facilitate its early diagnosis.

4. Timing of control of albuminuria in patients with DKD

Some studies have found that some patients already have ESRD without proteinuria [25], and the specific underlying mechanism remains unclear, which may be related to hypertension, unrecovered acute kidney injury , and renal vascular disease. Data from an autopsy study of 105 community patients with normoalbuminuric diabetes in Japan, published in 2021, showed that advanced DN diagnosed using pathological analysis accounted for approximately 50% of cases. This suggests that the absence of detectable albuminuria does not rule out the presence of DKD. A Spanish autopsy study in 2022 with a small sample of 21 patients reached similar conclusions [26]. The time at which DKD develops in patients with type 2 diabetes remains unknown; however, these findings may lead to a breakthrough in future clinical practice regarding whether early treatment with renin-angiotensin-aldosterone system inhibitors (RAASI) or SGLT2i should be initiated before microalbuminuria develops in patients with diabetes. Future noninvasive diagnostic techniques may help identify patients with cryptic DKD who may benefit from early treatment [27,28]. Therefore, more randomized controlled trials (RCTs) and real-world studies are needed to help clinicians determine the appropriate time to control albuminuria in patients who suffer DKD.

5. Treatment progress and mechanism of albuminuria in patients with DKD

The RENAAL study, a previous large RCT, confirmed the renoprotective effect of reducing proteinuria in patients with type 2 diabetes [29]. Patients receiving losartan showed a 26% decrease in proteinuria within 3 months, a further decrease of more than 40% within 3 years, and a significant improvement in renal survival. The other RAASI were equally effective in reducing the progression of proteinuria and renal insufficiency. As the first landmark drug for DKD treatment, it has been used in clinical practice for more than 20 years. Both the 2023 American Diabetes Association (ADA) guidelines [30] and the 2022 Kidney Disease: Improving Global Outcome (KDIGO) guidelines [31] recommend the use of RAASI in treating microalbuminuria and macroalbuminuria in patients with DKD; however, insufficient evidence still exists to support the use of RAASI for the primary prevention of DKD in patients with diabetes with normal blood pressure and urinary protein.

5.1. SGLT2 i

Recently, the introduction of SGLT2i has revolutionized the clinical management of type 2 diabetes, not only effectively improving the cardiovascular outcomes of patients [32] but also having a renoprotective effect in patients with and without diabetes with moderate and severe renal insufficiency, significantly reducing proteinuria and becoming the second landmark drug for treating DKD [33,34,35,36,37,38]. Therefore, SGLT2i is recommended to delay CKD progression and reduce the risk of cardiovascular events in patients with CKD with type 2 diabetes and large albuminuria in recent guidelines. Compared with SGLT2i drugs, DPP4 inhibitors and GLP1 agonists have been shown to reduce proteinuria but have not been demonstrated to alleviate the deterioration of renal function [39,40]. Encouraging results were expected from the FLOW trial [41] using semaglutide.

The primary mechanism of SGLT2i in treating albuminuria in patients with DKD is to improve hemodynamics [33,42]. Since the mechanism of action of SGLT2i is independent of insulin, it is still functional in patients with T1DKD [43,44,45]. SGLT2i, which is located in the distal convoluted tubules in the macula densa and feels the higher concentration of sodium ions and signal transmission to the juxtaglomerular cell, plays a role in the adjustment of the “feedback” by blocking SGLT2 to sodium and reabsorption of glucose in PCT [46]. SGLT2i induces the production of adenosine, activates the adenosine A1 receptor, increases cytosolic Ca+, and restores glomerulotubular balance, thereby achieving the contraction of afferent arterioles [47]. In patients with type 2 diabetes receiving RAS blockers, SGLT2i causes dilation of the efferent arterioles upon RAS blockade by increasing adenosine and prostaglandin production [47]. SGLT2i has a renoprotective effect in reducing albuminuria by constricting and dilating afferent and efferent arterioles, respectively, during RAS blockade, lowering intraglomerular pressure and improving glomerular hyperfiltration. Because SGLT2 co-localizes with sodium-hydrogen exchanger 3 (NHE3) in PCT, SGLT2i blocks NHE3, which makes the natriuretic effect more significant [48]. In clinical and animal experiments, SGLT2i has been shown to increase podocyte autophagy, lower podocyte lipid content, protect podocytes, and reduce proteinuria [2,49,50]. Recent studies have demonstrated that dapagliflozin can delay renal tubulointerstitial fibrosis by inhibiting YAP/TAZ activation [51], and block CYP4A/20-HETE signaling to reduce reactive oxygen species and inflammation, thereby decreasing glomerulosclerosis and renal tubulointerstitial fibrosis [52]. SGLT2i provides novel perspectives for treating glomerular and tubular proteinuria.

5.2. MRAs

Finerenone, which is a nonsteroidal mineralocorticoid antagonist, is the only nonsteroidal MRA clinically proven to have renal and cardiovascular benefits [53]. It can rapidly reduce proteinuria and provide long-term renal protection in patients with diabetes treated with RAASI [54,55]. Additionally, the 2020 KDIGO guidelines [56] recommend a combination of ACEI/ARB and MRAs to reduce urinary protein levels in patients with T2DKD. The use of MRAs in treating patients with diabetes with CKD was further improved according to the 2022 KDIGO [31] and 2023 ADA guidelines [30].

The renal protective mechanism of finerenone in patients with DKD is mainly due to its anti-inflammatory and anti-fibrosis effects [57,58], and it may also achieve anti-albuminuria by improving hemodynamics [58]. In patients with DKD, aldosterone activity is increased in the kidney, and MRs are overactivated, leading to the overexpression of MR-mediated proinflammatory and pro-fibrotic genes in various somatic cells in the kidney, including distal tubular epithelial cells, macula densa, endothelial cells, mesangial cells, podocytes, macrophages, fibroblasts, and other cells, driving renal inflammation and fibrosis [59,60,61,62]. MR over-activation leads to a series of renal pathological changes, including glomerular basement membrane thickening, podocyte injury, mesangial cell proliferation and apoptosis, mesangial expansion, macrophage infiltration, collagen deposition, glomerulosclerosis, renal tubular epithelial cell atrophy and renal interstitial fibrosis, renal tubular inflammation, endothelial dysfunction, and arteriosclerosis [60,63,64]. These histopathological changes are important mechanisms that lead to albuminuria in patients with DKD. In the cytoplasm, finerenone binds to MR instead of aldosterone, resulting in a conformational change of MR. The MR-finerenone is translocated to the nucleus and binds to the hormone response element, where it cannot transcribe target genes due to the lack of transcription cofactors, leading to the down-regulation of proinflammatory and pro-fibrotic genes, such as NF-κB and AP-1, and ultimately reducing renal inflammation and fibrosis [63,65]. Furthermore, it alleviates the pathological changes in glomerular and renal tubular damage and improves albuminuria. Regarding hemodynamics, finerenone may improve glomerular hyperfiltration by preventing aldosterone-mediated contraction of efferent arterioles over afferent arterioles, reducing intraglomerular pressure, and achieving anti-albuminuria [58]. However, a multicenter, multiracial RCT involving 823 patients with type 2 diabetes and albuminuria treated with RAS blockade showed no significant association between dose-dependent reductions in UACR with finerenone and reductions in blood pressure or eGFR [66], which may indicate that the mechanism by which finerenone reduces albuminuria is hemokinesis-independent [67].

5.3. ERAs

Low-dose, short-term treatment with the selective endothelin A (ETA) receptor antagonist atrasentan has a positive therapeutic effect in reducing albuminuria and improving renal prognosis in some patients with T2DKD without causing significant sodium retention [68,69,70], and its clinical application still has broad development prospects. However, the results of the SONAR study showed that the use of atrasentan did not provide cardiovascular protection [71]; therefore, it is necessary to be alert to the occurrence of cardiovascular adverse events during its clinical application. Additionally, the SONAR trial was conducted to ensure safety and introduce strict inclusion and exclusion criteria, excluding patients who were unresponsive to atrasentan (proteinuria reduction ≤30%) or intolerant (history of heart failure or peripheral edema). Therefore, it generalizing these results for clinical applications is difficult. Furthermore, unresolved scientific questions remain, such as whether patients with T2DKD who do not respond to atrasentan benefit from SGLT2i-, finerenone-, or insulin-based therapies [33]. However, whether early albuminuria reduction with atrasentan treatment can predict long-term renoprotective effects remains unknown [72]. Therefore, the clinical value of selective ETA receptor antagonists in patients with T2DKD still needs substantial medical-based evidence.

The expression of endothelin-1 (ET-1) is increased in the kidney of patients with DKD [73,74,75]. ET-1 binds to the ETA receptor to produce pathological effects through various stimulation of kidney histopathological changes, such as damage to the endothelial cell glycocalyx and actin cytoskeleton of podocytes, causing dysfunction of the podocyte membrane, changing the basement membrane, causing mesangial matrix deposition, damaging renal blood vessels, enhancing vascular reactivity and coagulation, leading to renal tubulointerstitial fibrosis, and promoting inflammatory cell infiltration [69,75,76]. ERA antagonists increase endothelial glycocalyx, reduce glomerular heparanase, protect podocyte actin cytoskeleton and slit diaphragm function, and reduce inflammation, interstitial fibrosis, and extracellular matrix hyperplasia through various mechanisms [77,78]. Animal experiments have shown that blocking ETA can reverse the glomerular hyperosmolar state and reduce glomerular albuminuria caused by glomerular injury [79]. Therefore, ETA antagonists can be used to treat glomerular albuminuria by protecting the glomerular filtration barrier and reversing glomerular hyperfiltration. Additionally, they protect renal tubular epithelial cells, reduce renal tubulointerstitial fibrosis, and treat renal tubular proteinuria.

5.4. Chinese patent medicine

5.4.1. Keluoxin capsule

Keluoxin capsules are composed of Astragali radix, Pseudostellariae radix, Ligustri lucidi fructus, Lycii fructus, Rhei radix et rhizome, and Hirudo. It was the first Chinese patented medicine approved for the treatment of DKD. It can maintain kidney function, reduce the level of urinary protein, delay DKD development, and alleviate related symptoms [80,81,82]. Keluoxin capsules have been recommended to control proteinuria in patients with DKD according to several guidelines, such as the Guidelines for Diagnosis and Management of Diabetic Kidney Disease with Integrated Traditional Chinese and Western Medicine [83] and the Guidelines for the Prevention and Treatment of Type 2 Diabetes Mellitus in China [84]. A double-blind RCT suggested that keluoxin capsules can prominently reduce the ACR level in patients with early DKD and the ratio of those with deteriorated proteinuria, whose ACR levels continue to rise by 30% [85]. According to the study, Keluoxin capsules can also alleviate clinical symptoms, such as fatigue, dry mouth and throat, limb numbness, and pain, aggravated at night, to improve the life quality of patients. The latest meta-analysis, involving 20 studies and 1500 participants, has suggested that a combination of Keluoxin capsules and Western medicine presents better efficiency than a single use of Western medicine in treating DKD, including improving eGFR and reducing the level of microalbuminuria, urinary albumin excretion rate (UAER), 24-hour urine protein, serum creatinine (Scr), blood urea nitrogen, fasting blood glucose, total cholesterol, triglyceride (TG), and low-density lipoprotein [86]. Regarding safety, no statistically significant differences were found in adverse drug events between the two groups. Pharmacoeconomic research has shown that the combined use of keluoxin capsules and chemical medicine is more economical than chemical medicine alone, considering its current price level [87].

Traditional Chinese medicine involves multiple targets and pathways and can regulate the overall situation of disease treatment. Clinical studies and animal trials have suggested that keluoxin capsules can delay or treat DKD by improving glycolipid metabolism and microcirculation, enhancing histiocyte function, and alleviating renal damage [88]. Keluoxin capsules can improve glycolipid metabolism, regulate tubuloglomerular feedback to reduce kidney tubule injury and renal hypertrophy, and alleviate kidney hyperfiltration [89]. Additionally, Keluoxin capsules lower endothelin, thromboxane B2, atrial natriuretic peptide, and angiotensin [90]. Meanwhile, they can also enhance the level of 6-keto-prostaglandin1α, regulate the dynamic balance of the renin-prostacyclin system, and improve hemodynamics and hemorheology and renal microcirculation [90]. Simultaneously, Keluoxin capsules can decrease the level of oxidative stress and inflammatory cytokines, probably because they can regulate the JAK/STAT, MAPK, and NF-κB signaling pathways [91,92,93,94]. Moreover, Keluoxin capsules can regulate podocyte autophagy, alleviate renal damage, and protect the kidney [95]. Research on the mechanism of action of keluoxin capsules is ongoing. We expect that the mechanism of action will be interpreted more comprehensively.

5.4.2. Huangkui capsule

Evidence from the real world shows that Huangkui capsules are widely used to treat proteinuria in patients with DKD, with sufficient evidence of clinical efficacy and high frequency of use [96,97]. For example, a multicenter RCT study involving 413 patients with DKD demonstrated that after 24 weeks of treatment, Huangkui capsules combined with irbesartan had significantly greater reductions in UACR, 24-hour proteinuria, and UPCR (urinary protein creatinine ratio) than irbesartan alone (P<0.001, P=0.001, and P=0.001, respectively); the overall incidence of adverse events was low and largely similar between treatment groups, with no serious adverse events of reduced liver or renal function [98]. A recent meta-analysis of 13 RCTs has shown that Huangkui capsules can effectively reduce 24-hour urinary protein content in patients with CKD. The effectiveness of Huangkui capsules in DKD subgroups should be further clarified; therefore, larger sample sizes and high-quality RCTs in multiple countries are needed to confirm this [99].

A recent animal study has demonstrated that the Huangkui capsule can protect podocytes from doxorubicin (DOX)-induced proteinuria by inhibiting the JAK2/STAT3 and PI3K/Akt pathways and increasing the expression of Nephrin and Podocin in DOX-exposed podocytes [100]. Thus, proteinuria was alleviated. Another animal experiment confirmed the advantages of the Huangkui capsule in treating DN from a different perspective, such as regulating the intestinal microbiota and improving metabolite levels in DN [101]. In terms of reducing renal interstitial fibrosis, animal experiments have demonstrated that Huangkui capsule combined with metformin can effectively improve DN by inhibiting the expression of renal fibrosis-related proteins and blocking Klotho/TGF-β1/p38MAPK signaling pathway in DN rats [102]. Huangkui capsule inhibited NLRP3 inflammasome activation and TLR4/NF-κB signaling pathway and alleviated epithelial to mesenchymal transition of renal tubules [103].

5.4.3. Tripterygium glycosides

In China, T. wilfordii, combined with Western medicine, is widely used in patients with DKD. Many clinical RCTs have shown that T. wilfordii combined with Western medicine can effectively reduce 24-hour urinary protein levels and UAERs [104,105,106]. However, its clinical application is limited due to the presence of minor side effects that mainly focus on abnormal liver function [106]. Therefore, to explore its clinical safety, a meta-analysis involving 31 RCTs suggested that the duration of TG-mediated DN treatment should be less than 3 consecutive months [107]. Therefore, because of the generally low-quality evidence provided by systematic reviews and meta-analyses of Tripterygium for the adjuvant treatment of clinical DKD, it still needs to be treated cautiously [108]. Serious side effects should be carefully monitored, even in courses that are generally considered safe. Large-scale and long-term multicenter studies of T. wilfordii combined with Western medicine are expected. However, the mechanism of action of T. wilfordii in treating DN has not been fully elucidated. Untargeted metabolomic analysis using ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry revealed that Tripterygium improved impaired renal function mainly by promoting TG catabolism in DN mice [109].

5.4.4. Other Chinese patent medicine

Niaoduqing granules can be used to effectively treat proteinuria in patients with DKD. Through network pharmacology, it has been found that the mechanism of Niaoduqing granules in treating DN proteinuria is to inhibit the activity of the AGE/RAGE pathway and the overexpression of VEGF-A, ICAM-2, PTGS-5, and ACE in MPC1 cells to prevent podocyte injury in DN [110]. However, evidence of adverse drug reactions remains unclear [111]. An ESRD rat model confirmed that Niaoduqing granules improved renal fibrosis and urinary protein excretion by regulating the p38 MAPK/NF-κB signaling pathway [112].

Yishen Huashi granules are widely used to treat proteinuria in patients with DKD, and their therapeutic mechanism is related to the improvement in podocyte injury induced by macrophage-derived exosomes [113]. Recent animal experiments have shown that Yishenhuashi granules can improve the pathological mechanism of DKD by regulating the “gut-kidney axis” of intestinal flora and serum metabolites and improving the mRNA expression of the kidney [114].

Cordyceps sinensis preparations have been used in the adjuvant treatment of clinical DKD, and their combination with conventional treatment is better than conventional treatment alone in terms of protecting renal function and reducing the clinical efficacy of proteinuria, which has been clinically confirmed [115,116,117]. A meta-analysis of 38 RCT studies involving 3167 patients with DKD showed that Cordyceps sinensis preparations combined with ACEI/ARB significantly reduced 24-hour urinary protein compared with the ACEI/ARB alone group [standardized mean difference (SMD) = -1.99, 95% confidence interval [CI] (-2.68, P<0.05; -1.31, P<0.01], urinary microalbumin [mean difference (MD) = -37.41, 95% CI (-44.76, -30.06), P<0.01], UAER [MD= -24.11, 95% CI (-30.54, P<0.01), -17.68), P<0.01], and ACR [SMD =1.01, 95% CI (-1.73, -0.29), P<0.01], and no significant difference was found in adverse events between the two groups [118]. Cordyceps sinensis preparation can reduce renal TG accumulation in DN rats by regulating the PPARα pathway, thereby reducing glomerulosclerosis, renal tubulointerstitial injury, and renal fibrosis, as well as glomerular proteinuria and renal tubular proteinuria [119]. Based on the comprehensive metabolomics and pattern recognition technology of ultra-performance liquid chromatography-mass spectrometry, it was found that Cordyceps sinensis preparations may play a role in treating DN by searching abnormal metabolic pathways [120]. From the perspective of the Chinese healthcare system, Cordyceps sinensis preparations may be a cost-effective treatment option for DN in the Chinese population [121].

Therefore, Shenshuaining [122], Fufang Xueshuantong [123], Shenyan Kangfu tablets [124,125], and Liuwei Dihuang decoction [126], which are also combined with Western medicine for the treatment of DKD and effective reduction of proteinuria, should be further confirmed by real-world research and more high-quality, large-scale, and multicenter RCT studies. Chinese patent medicine has broad development potential and an upward space for treating DKD proteinuria [127,128,129].

Overall, this review summarizes the pharmacological mechanisms underlying albuminuria in patients with DKD (Table 1).

5.5. Targeted precision therapy

Although these drugs have shown encouraging efficacy in treating DKD proteinuria, some patients continue to experience uremia. Increasing evidence has shown that targeted drug delivery strategies, such as macromolecular carriers, nanoparticles, and liposomes, can improve drug efficacy and reduce adverse side effects, which may become a new milestone in treating DKD [130]. HDAC4 is involved in podocyte injury in DKD, and HDAC4 siRNA shows good therapeutic prospects for DKD. DTsiANp/HDAC4 can deliver HDAC4 siRNA to the podocytes of DKD rats, and a 4-week intervention can significantly reduce UAER and glomerulosclerosis [131]. Targeted renal delivery of gold nanoparticles with a diameter of 50 nm can reduce UAER, glomerular basement membrane thickness, and foot process width in DKD rats [132]. As an antioxidant, Coenzyme Q10 may be a promising treatment option for early-stage DKD. However, its low water solubility and nonspecific distribution limit its clinical application. Therefore, liposomes containing CoQ10 combined with ultrasound microbubbles were injected into DKD rats to target the kidneys and improve proteinuria and oxidative stress markers [133]. The above studies are still in the preclinical stage; however, as the research on the pathogenesis of DKD advances, targeted precision therapy will be explored for clinical applications.

6. Polydrug therapy for proteinuria in patients with T2DKD

A long history of care has discouraged the use of polypharmacy (more than five different drugs) in patients with diabetes because of the risk of adverse drug–drug interactions [134]. However, patients with diabetes mellitus frequently experience various complications and comorbidities. Multidrug treatment for lowering blood glucose, blood pressure, and lipids has been widely accepted because its efficacy has been confirmed, and the doses used are lower than a single drug dose, with fewer side effects.

Many clinical studies have shown that controlling proteinuria in patients with DKD can delay the progression of cardiovascular disease and CKD and reduce the risk of death. ACEI or ARB are recommended as the first treatment choice for patients with T2DKD, regardless of whether they are complicated with microalbuminuria or macroalbuminuria [53,135]. However, a combination of ACEI and ARB is not recommended, and ACEI/ARB is not recommended for the primary prevention of DKD in patients with diabetes with normal blood pressure and proteinuria [136]. Conversely, the Scr level at which RAS should be contraindicated remains controversial [137]. Non-dihydropyridine calcium channel blockers, such as verapamil, can stabilize or even reduce the level of urinary protein [138] and can be used as the first combination of ACEI/ARB drugs when blood pressure is not well controlled [139]. If a beta-blocker is combined, a vasodilator beta-blocker, including Carvedilol or Nebivolol, may be preferred because non-vasodilator beta-blockers accelerate albuminuria progression [140,141].

In patients with DKD, SGLT2i or mineralocorticoid-receptor antagonists can be added when the target urinary albumin levels are not achieved with RAS blockade [140]. SGLT2i can effectively reduce urinary protein levels in both micro-and macroalbuminuria [142]. Finerenone combined with an ACRI/ARB is effective in reducing urinary protein levels in patients with T2DKD [66], and no clear evidence of an increased risk of hyperkalemia or AKI exists. The FIDELIO-DKD and CREDENCE trials showed similar cardiorenal benefits [143]. Therefore, triple oral therapy with a RAS blocker, an SGLT2i, and an MRA should be considered if the urinary albumin level remains high. To date, no conclusive evidence exists that triple therapy is effective in treating DKD; however, a retrospective report from a prospective study showed that the addition of finerenone can further reduce proteinuria and delay the decline in renal function in patients with DKD who have already received RAS inhibitors, and a small number of individuals in this group also received SGLT2i [144]. Patients with advanced DKD should consider the risk of undertreatment. Therefore, whether triple therapy can be used in patients with diabetes with micro-and macroalbuminuria in the future requires further high-quality, medical-based evidence.

In clinical practice, there has been no experience with the combination of these drugs and ERAs. Theoretically, SGLT2i can reduce sodium and water retention in ERAs by constricting afferent arterioles. ERA dilates efferent arterioles and synergistically reduces glomerular hyperfiltration while complementing SGLT2i. However, an experiment in male db/db mice without kidney resection showed no significant differences in urinary albumin excretion, and histopathological changes were observed in the SGLT2i and ET-A antagonist groups compared with the SGLT2 inhibitor group alone [145].

7. Conclusion and prospect

DKD is a heterogeneous disease characterized by multiple factors and complex phenotypes. Pathophysiological differences among patients can lead to individual differences in drug responses. Therefore, large-scale prospective trials based on individual drug responses and phenotypic characteristics are required to guide precise treatments. Optimal combinations of new therapies can further delay DKD progression. Real-world studies can clarify which combination is beneficial for cardiac and renal outcomes in patients with T2DKD while being well tolerated, safe, easy to manage, and cost-effective. Simultaneously, to better understand the mechanism of DKD, particularly T2DKD, basic research should be intensified, especially based on the metabolic pathways and metabolites of renal cells in chronic high-glucose environments, and focus on the metabolic and hemodynamic pathways in the DKD environment. Furthermore, targeting the AGE-AGE receptor axis involves focusing on new targets for renoprotective intervention, such as AGE inhibitors pyridoxamine and glyoxalase-1, an enzyme that degrades the AGE precursor methylglyoxaldehyde [146]. Therefore, attention should be paid to the kinase cascade initiated by metabolite reactive oxygen species and the transcription factors involved in the reaction. Finally, the targeted drug delivery strategy for DKD is still in its early stages, and there are limitations regarding safety, stability, and clinical efficacy. The selection of the best kidney-targeting strategy is expected to provide a new treatment approach for DKD.