Background and Aims Kynurenine pathway of tryptophan metabolism is involved in the pathophysiology of chronic kidney disease (CKD) and diabetes mellitus (DM), mainly through the inflammation-induced activity of indoleamine 2,3-dioxygenase (IDO). Renin-angiotensin-aldosterone system inhibitors (RAASis) [angiotensin converting enzyme inhibitors (ACEis) and angiotensin II receptor antagonists (ARBs)] are recommended in these conditions to decrease proteinuria, slow CKD progression and reduce cardiovascular risk. The interactions between RAAS and the kynurenine pathway and the potential effects of RAASis have been reported in few experimental models but whether these drugs influence kynurenine levels in humans is unknown. We performed a single-centre cross-sectional observational study to evaluate tryptophan and kynurenine serum levels and IDO activity in CKD patients with and without type 2 DM, their correlations with markers of renal dysfunction, and their relationship with RAAS-inhibiting therapy. Method We enrolled 72 consecutive adult patients with CKD, of which 38 had DM, who were admitted to the Unit of Nephrology and Dialysis of our Hospital. Of them, 55 were receiving RAASis whereas 17 were not. Tryptophan was assessed by HPLC (high-performance liquid chromatography) analysis with an UltiMateTM 3000 chromatograph (Thermo Fisher Scientific, 168 Third Avenue Waltham, MA, USA); kynurenine was measured using an enzyme-linked immunosorbent assay (ELISA) kit (Catalogue n. K 7728; Immundiagnostik AG, Bensheim, Germany); IDO activity was calculated with the formula (kynurenine/tryptophan) x 100. Results Patients receiving RAASis and patients not under therapy only differed for frequency of arterial hypertension (100% vs 76.47%; P=0.002) and kynurenine levels, the latter being significantly lower in the treated group compared to the untreated one (1.56 ± 0.79 vs 2.16 ± 1.51 µmol/l; P=0.0378). Kynurenine did not correlate with estimated glomerular filtration rate (eGFR), proteinuria or albuminuria in the whole study cohort. Conversely, in patients not receiving RAASis it was inversely related to eGFR (r=-0.4862; P=0.0478) and directly related to proteinuria (ρ=0.493; P=0.0444) and albuminuria (ρ=0.542; P=0.0247); moreover, it varied across classes of albuminuria, being lower in patients with normoalbuminuria (0.98 ± 0.55 µmol/l) and higher in those with micro- (2.63 ± 1.53 µmol/l) and macroalbuminuria (3.09 ± 1.61 µmol/l) at the ANOVA test (F=4.008; P=0.042). In the same group, the relationship between IDO activity and eGFR was significant (ρ=-0.554; P=0.0210) and IDO activity differed across classes of albuminuria (F=3.702; P=0.05). Moreover, IDO activity was higher in patients with history of cardiovascular disease compared to those with no such history [10.22 (8.39 to 22.92) vs 7.57 (4.76 to 8.09) %; P=0.0343] whereas tryptophan had an opposite behaviour (17.00 ± 2.06 vs 22.00 ± 3.78 µmol/l; P=0.0036). Conclusion We observed lower kynurenine levels in CKD patients treated with RAASis compared to untreated patients, independently of the presence of DM, and a significant association between kynurenine and markers of renal damage only in the group not receiving RAASis. We could hypothesize that kynurenine may play a role in the pathophysiology of renal damage, as already suggested, and that drugs interfering with RAAS activation may act also by reducing kynurenine levels, in addition to their already known effects at the renal level. Indeed, kynurenine synthesis results to be higher in states of inflammation and RAAS has pro-inflammatory and pro-fibrotic actions; accordingly, RAASis could reduce the enzymatic activity responsible for kynurenine increase and this could be one of the mechanisms mediating the beneficial effects of RAAS inhibition on CKD progression. This may be relevant because kynurenine has shown pro-thrombotic effects and is emerging as a potential new biomarker of CKD.