Effects of DPP-4 Inhibitor Linagliptin Versus Sulfonylurea Glimepiride as Add-on to Metformin on Renal Physiology in Overweight Patients With Type 2 Diabetes (RENALIS): A Randomized, Double-Blind Trial

Type 2 diabetes mellitus (T2DM) is the leading cause of chronic and end-stage kidney disease worldwide. Novel therapeutic strategies are urgently needed (1). Interestingly, analyses of cardiovascular outcome trials (CVOTs) in T2DM patients with high cardiovascular/renal risk suggest glucose-independent beneficial effects on secondary renal outcomes of new-generation glucose-lowering drug classes (i.e., incretin-based therapies and sodium–glucose cotransport 2 inhibitors) (1,2). This has recently changed clinical-recommendations.

In T2DM patients without cardiovascular disease/chronic kidney disease, clinicians have several treatment options, including dipeptidyl peptidase 4 (DPP-4) inhibitors (DPP-4i) and sulfonylureas, to intensify metformin monotherapy (3). However, very few head-to-head studies are available to guide clinicians, and secondary renal outcomes of the CARdiovascular Outcome study of LINAgliptin versus glimepiride in patients with type 2 diabetes (CAROLINA) (clinical trial reg. no. NCT01243424, ClinicalTrials.gov) are yet to be reported.

Preclinical studies, placebo-controlled trials, and CVOTs suggest that DPP-4i may prevent albuminuria onset/progression beyond glucose lowering (2,4). Underlying mechanisms may involve direct actions on the kidney, as membrane-bound DPP-4 and glucagon-like peptide 1 (GLP-1) receptors (GLP-1R) are putatively expressed in various nephron segments (2). We reported that sitagliptin modestly reduced estimated glomerular hydraulic pressure (P_GLO) and increased fractional excretion (FE) of sodium (FE_Na) in T2DM patients versus placebo (5). Although GLP-1R–mediated effects may underlie actions of DPP-4i on renal vasculature/tubules, GLP-1–independent effects of this drug class may also be implicated (4). Glucose-lowering per se influences renal physiology, underscoring the importance of attainment of glycemic equipoise.

A detailed description of material and methods is provided in the Supplementary Appendix 1. Briefly, this was a phase IV, randomized, double-blind, comparator-controlled, parallel-group, mechanistic intervention trial (clinical trial reg. no. NCT02106104). Eligible T2DM patients were Caucasian, men/postmenopausal women, aged 35–75 years, who received metformin alone and had HbA_1c 6.5–9.0%, BMI ≥25 kg/m², and estimated glomerular filtration rate (GFR) >60 mL/min/1.73 m². After a 6-week run-in, patients were randomly assigned to receive once-daily linagliptin 5 mg or glimepiride 1 mg added to ongoing metformin; study drugs were overencapsulated.

The protocol for determination of study end points is described in the Supplementary Appendix 1. The predefined coprimary end point was linagliptin-induced changes in GFR and effective renal plasma flow (ERPF) from baseline to week 8, compared with glimepiride, as derived from inulin and para-aminohippuric acid clearances based on timed urine sampling (Supplementary Appendix 1). Secondary end points included (intra)renal variables (i.e., P_GLO and afferent arteriolar resistance [R_A] and efferent arteriolar resistance [R_E] estimated according to the Gomez formulae), tubular functions (i.e., FE_Na, FE of endogenous lithium [FE_Li] [only assessed in linagliptin-treated patients], of potassium [FE_K], and of urea [FE_U]), urinary damage markers (i.e., urinary albumin-to-creatinine ratio [UACR], neutrophil gelatinase–associated lipocalin, and kidney injury molecule-1), and blood pressure (BP). Changes in body weight, hematocrit, body water percentage, HbA_1c, glucose, lipids, renin, insulin, glucagon, DPP-4 activity, DPP-4 substrates (i.e., total and intact GLP-1, substance P, active/pro neuropeptide Y, and stromal cell–derived factor-1α [SDF-1α]), and hypoglycemia were analyzed as safety/exploratory end points.

At the time of study design (2013), no data on effects of DPP-4 inhibition on renal physiology were available, and no formal sample size could be assessed. N = 21 per treatment arm should be sufficient to detect a GFR change ≥15%, assuming SD 10 mL/min, α = 0.05 (two-sided testing), and power (1 − β) of 80%. To allow for dropouts, we aimed at 24 patients/treatment arm. Multivariable linear regression models were used to examine linagliptin-induced effects compared with glimepiride. Corresponding baseline values were added as an independent variable to correct for potential between-group differences at baseline. Paired t tests or Wilcoxon signed rank tests were used appropriately for within-group comparisons. Spearman correlation analyses explored associations between changes in renal physiology and factors deemed relevant.

Demographic and clinical characteristics of the analyzed 46 patients were well balanced between treatment groups (Supplementary Appendixes 2 and 3). Reductions in HbA_1c were similar in the linagliptin (mean ± SEM –0.45 ± 0.09%) and glimepiride (–0.65 ± 0.10%) groups after 8 weeks of administration (Table 1 and Supplementary Appendix 4). At week 8, decreases in fasting plasma glucose were –1.17 ± 0.34 mmol/L with linagliptin and –1.54 ± 0.40 mmol/L with glimepiride (P = 0.82).

Eight-week linagliptin did not change GFR, ERPF, filtration fraction, or renal vascular resistance compared with glimepiride relative to baseline (Fig. 1 and Supplementary Appendix 5). Linagliptin was not associated with differences in P_GLO, R_A, or R_E compared with glimepiride (Supplementary Appendix 5). Linagliptin increased FE_Na (mean ± SEM increase of 17 ± 7%; P = 0.050) and FE_K, but this did not reach between-group significance (Table 1 and Fig. 1). Linagliptin did not affect FE_Li, FE_U, or urinary pH, whereas glimepiride increased urinary pH. Changes in plasma electrolytes are shown in the Supplementary Appendix 6. Linagliptin tended to reduce UACR by 26% from baseline, whereas glimepiride did not; no between-group differences were observed.

Glimepiride versus linagliptin increased body weight (increase of 0.8 kg) (Table 1). No treatment differences were observed in BP/heart rate (Table 1). Metabolic variables generally did not reveal relevant differences between groups (Table 1 and Supplementary Appendix 6). DPP-4 activity was reduced with linagliptin versus glimepiride. Linagliptin increased intact GLP-1 compared with glimepiride (P = 0.014). Linagliptin reduced SDF-1α by ∼50% (P < 0.001), while SDF-1α remained virtually unchanged with glimepiride (between-group mean difference −838 pg/mL [−970 to −705]; P < 0.001) (Table 1 and Supplementary Appendix 5C).

Correlation analyses between changes in FE_Na and selected factors are presented in the Supplementary Appendixes 7 and 8. In all patients, change in FE_Na was associated with change in urinary pH (R = 0.365; P = 0.015) yet was nonsignificant in separate treatment groups. In the linagliptin group, change in FE_Na correlated with change in SDF-1α (R = 0.660; P = 0.002) but not with changes in FE_Li, systolic BP, GFR, insulin, glucagon, intact GLP-1, active neuropeptide Y, or substance P.

Fewer patients experienced a probable symptomatic hypoglycemic event with linagliptin versus glimepiride (4% vs. 25%; P = 0.041). Reported adverse events were all mild or moderate in intensity (Supplementary Appendix 9).

As 8-week treatment with linagliptin and glimepiride reduced HbA_1c and fasting glucose to a similar extent, nonglycemic advantages and disadvantages of the two drugs could be explored in this trial.

We found that linagliptin affected neither fasting GFR and ERPF nor intrarenal hemodynamic functions compared with glimepiride. This neutral effect of DPP-4i on fasting inulin–measured GFR and para-aminohippuric acid–measured ERPF is in accordance with two placebo-controlled trials studying the renal effect of 4-week linagliptin (6) and 12-week sitagliptin (5) in T2DM patients without renal impairment. In the latter trial, sitagliptin was associated with a placebo-corrected P_GLO reduction of 2.8 mmHg (P = 0.043), possibly caused by glucose-lowering per se. Indeed, in the current study—which attained euglycemic between-group conditions—we did not observe such P_GLO decrease following DPP-4 inhibition, albeit identical methodologies in a comparable T2DM population. We assume that any renoprotective potential of DPP-4i does not involve changes in fasting renal hemodynamics.

In the current study, linagliptin modestly increased fasting FE_Na from baseline to week 8 in diuretic-naive patients, albeit not significantly compared with glimepiride. The observed linagliptin-induced natriuresis is consistent with two previous placebo-controlled studies in T2DM, in which sitagliptin enhanced fasting inulin–based FE_Na after 2 weeks (5) and creatinine-based FE_Na after 1 month (7) by up to 40%. DPP-4i–mediated natriuresis may involve inhibition of the Na-H exchanger (NHE)3—located at the brush border of the proximal tubule, bound to a complex that also contains DPP-4—either through direct membrane-bound pathways or mediated by active GLP-1 levels (2). Indeed, acute GLP-1 receptor agonist (GLP-1RA) administration confers natriuresis (8–10), perhaps by NHE3 inhibition (2). Moreover, GLP-1RA administration increases FE_Li (a marker of proximal tubular Na reabsorption) and urinary pH (8,10). In the current study, linagliptin augmented intact GLP-1 concentrations, but urinary pH and FE_Li remained unaffected, which is in disagreement with an inhibitory effect of linagliptin on NHE3. Also, we did not observe an association between linagliptin-induced changes in FE_Na and intact GLP-1. Rather, DPP-4i may (at least partly) promote natriuresis through pathways independent of GLP-1R signaling and NHE3 (7,11). Indeed, in mice lacking a functional GLP-1R, DPP-4 inhibition but not GLP-1RA demonstrated natriuresis (12). DPP-4 has numerous physiological substrates other than GLP-1 that are associated with natriuresis (e.g., neuropeptide Y, substance P, and SDF-1α) (4). While linagliptin did not affect circulating active neuropeptide Y or substance P in our trial, the drug did reduce a subfraction of SDF-1α, as was seen in other DPP-4i studies that used the identical assay for this DPP-4 substrate (13). SDF-1α is widely expressed in the kidney and localizes to glomerular podocytes and distal tubular cells (14). Also, SDF-1α/CXCR4 receptor signaling suppresses renal oxidative stress/fibrosis. Parallel with sitagliptin-induced natriuresis, DPP-4 inhibition robustly increased intact SDF-1α^1-67 (“active” form) and markedly decreases truncated SDF-1α^3-67 (“inactive” form) (7). Conversely, the SDF-1α/CXCR4 antagonist AMD3100 reversed the natriuretic effects of linagliptin (11). Our exploratory correlation analyses also link DPP-4 inhibition to enhanced FE_Na via SDF-1α.

As proximal NHE3 does not seem to be primarily involved, and FE_Li was unchanged in the current and a previous study (7), DPP-4i may induce natriuresis by blocking distal rather than proximal tubular Na reabsorption. Moreover, whereas agents that induce proximal tubular natriuresis—e.g., sodium–glucose cotransporter 2 inhibitors and carbonic anhydrase inhibitors—activate tubuloglomerular feedback and thereby affect renal hemodynamics, DPP-4i do not seem to exhibit a renal hemodynamic effect, suggesting that the drug class acts on a segment distal to the macula densa and its effect is consequently not coupled to this intrarenal autoregulatory mechanism. Potential distal tubular ion-transport channels that may link DPP-4i to distal natriuresis include the Na⁺/Cl^– thiazide-sensitive channel and the epithelial Na channel (7).

Our study has limitations. First, the sample size was relatively small, potentially leading to heterogeneity. Second, estimation of glomerular characteristics with Gomez formulae requires assumptions. Third, we did not measure 24‐h Na excretion or standardize/monitor Na intake; variability in FE_Na results may have occurred. Fourth, as most DPP-4 substrates are secreted postprandially, we cannot assess the net renal effect of DPP-4i over 24 h. Finally, our findings in T2DM patients with late-phase glomerular hyperfiltration and normal GFR (i.e., baseline filtration fraction ∼23% [15]) cannot be generalized to T2DM patients with either early-phase hyperfiltration or late-phase renal impairment.

We did not find any glucose-independent differences in fasting (intra)renal hemodynamics with linagliptin versus glimepiride in T2DM patients without overt nephropathy. The suggested renoprotective properties of DPP-4i may be produced by modest benefits in other renal risk factors (body weight, BP, or dyslipidemia) or preservation of DPP-4 substrates (notably, SDF-1α) that may have anti-inflammatory/antifibrotic properties. Linagliptin promotes modest natriuresis, possibly caused by SDF-1α at a tubular segment distal to the macula densa.

Acknowledgments. In memory of Prof. Michaela Diamant, whose experience and expertise were crucial for the design of this study. The authors extend gratitude to all study participants who took part in this study for their time and commitment to the demanding protocol. Furthermore, the authors thank the study nurses for their excellent practical support during the conduct of this study, with special thanks to Sandra Gassman and Jeannette Boerop (Diabetes Center, Department of Internal Medicine, VU University Medical Center). Finally, the authors thank Adele Dijk and Nel Willekes-Koolschijn (Department of Nephrology and Hypertension, University Medical Center), and Daniela Herzfeld de Wiza (Institute of Clinical Biochemistry and Pathobiochemistry) for much appreciated technical laboratory assistance.

Duality of Interest. Funding for the study was provided by Boehringer Ingelheim. M.H.A.M. is a speaker/consultant for AstraZeneca, Eli Lilly & Co., Novo Nordisk, and Sanofi. L.T. consulted for Eli Lilly & Co. and Novo Nordisk. Through M.H.H.K., the Amsterdam University Medical Centers, location VUmc, received research grants from Boehringer Ingelheim, Novo Nordisk, and Sanofi. D.H.v.R. serves on advisory boards of Boehringer Ingelheim, Eli Lilly Alliance, Novo Nordisk, Sanofi, and Merck Sharp & Dohme (MSD) and received research grants from AstraZeneca, Boehringer Ingelheim, Eli Lilly, Sanofi, and MSD. All authors from the Amsterdam University Medical Centers, location VUmc, declare that they did not receive personal fees in connection to these roles described above, and honoraria were paid to their employer. J.J.H. has been a member of advisory boards for Novo Nordisk. D.J.T. reports grants received from ZonMw, Chiesi Pharmaceuticals, and Astellas—all outside the scope of this study. No other potential conflicts of interest relevant to this article were reported.

This was an investigator-initiated study, planned and conducted under the scientific supervision of Michaela Diamant and, after her passing in April 2014, M.H.H.K. and D.H.v.R. The funder had no role in the study design, the analyses or interpretation of the data, drafting of the manuscript, or the decision to submit the manuscript for publication.

Author Contributions. M.H.A.M. participated in the design and planning of the study, coordinated the test visits and performed measurements, performed statistical analyses, produced the graphical representation of the data, interpreted the data, and wrote the manuscript. L.T. helped with data collection, performed statistical analyses, interpreted the data, and critically reviewed the manuscript. M.M.S., M.H.H.K., J.A.J., and D.H.v.R. contributed to the interpretation of the data, discussion of the intellectual content, and critical review of the manuscript. D.M.O., B.H., J.J.H., D.J.T., and A.H.J.D. generated data and/or contributed to the discussion of the intellectual content and critical review of the manuscript. All authors had full access to all the data in the study and had final responsibility for the decision to submit for publication. M.H.A.M. is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Prior Presentation. Parts of these data were presented in abstract form at the Annual Dutch Diabetes Research Meeting, Oosterbeek, the Netherlands, 1–2 December 2016, and European Diabetic Nephropathy Study Group Meeting, Helsinki, Finland, 19–20 May 2017.