1. Introduction
Chronic Kidney Disease (CKD) is a persistent and irreversible kidney damage characterized by abnormalities in kidney structure or function, which endure for more than three months to have significant impacts on individuals’ health [
1]. Chronic kidney disease (CKD) is a worldwide public health issue. Epidemiological studies have indicated a CKD prevalence of approximately 13.1% of the population in the United States, 12.9% in Japan, and 10.8% in China [
1]. In Taiwan, the national prevalence of CKD is 11.9%, and a marked increase in CKD prevalence is apparent in people older than 60 years. In the annual report of the United States Renal Database System (USRDS) in 2020, Taiwan had the highest ranking for the incidence of dialysis worldwide [
2,
3]. Therefore, a clinical approach is needed to alleviate the progression of CKD worldwide.
Previous studies have demonstrated that chronic renal failure is associated with oxidative stress [
4,
5]. An antioxidant-deficient diet increased the progression of renal diseases in animals with nephrectomy [
6]. Levels of malondialdehyde (MDA), a product of lipid peroxidation, have been shown to increase after 5/6 nephrectomy in rats [
7]. We are therefore conceivable that CKD progression may be mediated through oxidative stress-induced chronic renal failure may associate with oxidative stress. In addition, elevated levels of inflammatory markers, interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and C-reactive protein (CRP) in patients with chronic kidney disease (CKD) may induce oxidative stress, thereby accelerating the progression of renal damage. However, the detailed mechanism by which oxidative stress affects CKD pathogenesis remains unclear [
8,
9].
The antioxidant N-acetylcysteine (NAC) is a source of sulfhydryl groups in cells and, due to its interaction with ROS, is a scavenger of free radicals [
10]. However, inconsistent effects of NAC on CKD have been reported previously. For example, NAC had no effect on proteinuria, surrogate markers of tubular injury, or renal fibrosis in non-diabetic patients with CKD [
11]. In contrast, the administration of NAC 1200 mg twice daily for two weeks resulted in a significant improvement in residual renal function in a small number of hemodialysis (HD) patients (n = 20) [
8]. A retrospective study indicated that NAC use was associated with a reduced risk of progression to end-stage renal disease (ESRD) [
12]. In the present study, we enrolled 554 patients with CKD stages 3–5 who had used or not used NAC for three years, and we collected data for renal function—serum creatinine (SCr) and the estimated glomerular filtration rate (eGFR) every 6 months for three years to explore whether NAC use could improve renal function and reduce the risks associated with hemodialysis (HD).
2. Materials and Methods
2.1. Study Participants
The study participants with stage 3 to stage 5 CKD (n = 7668) were collected from the National Health Insurance (NHI) program in Taiwan (1 January 2014 to 31 December 2020). The study participants were older than 20 years, and NAC was taken (NAC users, n = 265) or not taken (NAC non-users, n = 289) for three years. The dosage of NAC was 600 mg orally twice per day for three years. All patients received chronic kidney disease health education during follow-up appointments, which encompassed topics such as nutritional diet, disease-specific medications, and lifestyle guidance.
The levels of SCr and eGFR in all participants were evaluated every six months for three years. Exclusion criteria for NAC users (n = 265) were: (I) NAC was taken before 1 January 2014 (n = 29), (II) acute renal damage (n = 9), (III) malignancy (n = 28), (IV) incomplete cases (n = 8), (V) kidney transplantation (n = 3), and (VI) dialysis before 1 January 2014 (n = 23). Finally, 165 NAC users were enrolled in the study group. Exclusion criteria for NAC non-users (n = 289) were: (I) NAC was taken before 1 January 2014 (n = 32), (II) acute renal damage (n = 13), (III) malignancy (n = 35), (IV) kidney transplantation (n = 5), (V) dialysis before 1 January 2014 (n = 29), and (VI) incomplete cases (n = 10). Therefore, 165 non-users were enrolled into the control group. The study flow diagram for participant selection is shown in
Figure 1.
2.2. Ethics Statement
The National Health Research Institutes maintain the NHI Research Database (NHIRD), which contains all claims data. The information about the participants included sex, date of birth, medical services received, comorbidities, history of drug use, etc. The study was approved by the Institutional Review Board, Kung Tien General Hospital (KTGH No: 11005).
2.3. Determination of SCr and eGFR Levels in Enrolled CKD Patients
We collected venous blood from the enrolled CKD patients to determine the levels of SCr and eGFR at 0, 6, 12, 18, 24, 30, and 36 months during the three years. Blood serum collected from CKD patients to evaluate the levels of SCr. SCr levels were measured using a Beckman Coulter DxC 800 (Kung-Tien General Hospital, Taichung, Taiwan, ROC). eGFR was calculated using serum creatinine and other factors, such as age and gender. The level of eGFR was calculated using the four-variable MDRD formula: eGFR = 186 × [serum creatinine (mg/dL)] − 1.154 × (age) − 0.203 × (0.742 if female).
2.4. Statistical Analysis
Statistical analysis was performed using the SPSS statistical software for Windows (Version 22.0). The chi-square test and independent t-sample test were used to assess the difference between the study and control group. p < 0.05 was defined as statistically significant.
4. Discussion
Most NAC studies have focused on protection against contrast-induced renal damage, but previous reports show inconsistent findings [
13,
14,
15,
16,
17]. Few studies have explored whether NAC use could reduce the risk of progression of CKD to ESRD. A meta-analysis was conducted to analyze the efficacy and safety of NAC in the treatment of CKD. The results showed that NAC did reduce cardiovascular events among people with CKD. More interestingly, eGFR and SCr were found to be statistically significantly better in the NAC group compared with the control group [
18]. This finding may support our present study, showing that NAC use for three years may improve the renal function of CKD patients by modulating the levels of SCr and eGFR, thereby reducing the risks faced by patients with CKD when undergoing hemodialysis.
Previously, short-term treatment with N-acetylcysteine (NAC), whether administered orally, intravenously, or at high doses, did not improve renal function [
19,
20,
21]. However, a retrospective case-control study indicated that continuous use of NAC at a daily dose of 1200 mg for 90 days reduced the risk of progression of chronic kidney disease [
16]. The levels of SCr and eGFR were significantly changed by NAC use from 12 to 36 months (
Table 2). The changes in SCr and eGFR levels by NAC use in patients with stage 3a CKD were also observed from 12 months to 36 months (
Table 3), and the levels of both SCr and eGFR were changed by NAC use in patients with stage 3b, stage 4, and stage 5 CKD from 24 to 36 months (
Table 4,
Table 5,
Table 6 and
Table 7). These results clearly indicate that NAC use may improve renal function in patients with CKD who consume NAC for one or two years. Therefore, NAC use should continue for 2 years for protection of renal function in patients with CKD.
NAC used had no effect on renal function in the present study (
Table 8). Over a three-year period, DKD patients using NAC experienced a gradual increase in SCr every six months (F = 3.537,
p = 0.002) and a significant decrease in eGFR (F = 2.682,
p = 0.014). In contrast, NAC use had no significant impact on SCr and eGFR in non-DKD patients. When comparing NAC users and non-users, DKD patients showed significant differences in SCr and eGFR at the 24th, 30th, and 36th months. Notably, NAC use in DKD patients resulted in a significant increase in SCr every six months (F = 3.537,
p = 0.002) and a significant decrease in eGFR (F = 2.682,
p = 0.014). However, NAC had no significant effect on renal function in non-DKD patients. Overall, while NAC did not improve renal function in either DKD or non-DKD patients, it did not lead to a significant deterioration in non-DKD patients. A previous retrospective study indicated that NAC use was associated with a reduced risk of progression to ESRD, but this was shown in non-DKD, not in DKD patients [
12]. This conflicting finding could reflect the small number of non-DKD patients (n = 50) enrolled in the present study compared to the previous report (n = 1354) [
16].
NAC use might be a beneficial clinical approach to prevent the progression of CKD to ESRD. However, this study was subject to potential uncontrollable biases as it relied on clinical data from a retrospective analysis of cases over a three-year period, following N-acetylcysteine (NAC) drug protocol conformity in patients with CKD and concurrent pulmonary diseases. Furthermore, the cases lacked comprehensive proteinuria data, which could not be included in the analysis and therefore remains an unaccounted-for factor. Therefore, it is necessary to plan prospective, randomized, double-blind, placebo-controlled trials and longer-term follow-up studies in the future to confirm the observations of the present study.