Role of kinurenic acid in the systemic sclerosis renal involvement

Systemic sclerosis (SSc) subclinical renal vasculopathy is characterized by progressive increase of intrarenal stiffness and reduction of parenchymal thickness due to post ischemic fibrosis secondary to the renal Raynaud phenomenon. Aims of this study were to evaluate kinurenic acid (KYNA) serum level in SSc patients and healthy controls (HC) and to assess the role of KYNA in SSc subclinical nephropathy. Serum level of KYNA was evaluated in 52 SSc patients and 20 HC, matched for sex and age. Renal function was calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation to estimate glomerular filtration rate (eGFR) and renal doppler ultrasound was performed to evaluate kidneys' morphology and indices of intrarenal stiffness. The parameters registered were renal longitudinal length, atrophy index (AI), renal sinus, parenchymal thickness, renal resistive index (RRI), pulsatile index (PI) and systolic/diastolic ratio (S/D). SSc patients had lower median value of KYNA than HC [54.43 ng/ml (IQR 44.44-63.64) vs 61.94 ng/ml (IQR 55.23-88.75), p < 0.001]. SSc patients with AI ≥ 0.70 had lower KYNA than SSc patients with AI < 0.70 [47.85 ng/ml (IQR 41.16-59.91) vs 55.5 ng/ml (IQR 49.99-67.33), p < 0.05] and a slightly significant negative linear correlation was found between KYNA and AI (r =  - 0.249, p < 0.05). SSc patient with RRI ≥ 0.70 had higher KYNA than SSc patients with RRI < 0.70 [58.25 ng/ml (IQR 50.49-69.68) vs 50.07 ng/ml (IQR 42.70-56.31), p < 0.05] and a significant positive correlation was found between KYNA and RRI (r = 0.318, p < 0.05). KYNA may be used as a marker to evaluate the renal involvement in SSc patients.