LC/MS-MS quantification of myo-inositol: a novel biomarker for kidney disease

Abstract

Abstract Creatinine is the primary biomarker used for assessing kidney function in patients with suspected acute and chronic kidney injury. However, creatinine, a product of muscle creatine metabolism, is an imperfect proxy for renal function because it is subject to non-renal interferences including muscle mass, diet, and medications. Creatinine is particularly problematic as a biomarker of acute kidney injury (AKI) due to relatively high renal function reserve, requiring considerable kidney damage and time for a clinically significant change in creatinine to be detected. Given that AKI and chronic kidney injury (CKD) are associated with high mortality, morbidity, and contribute significantly to health care costs, there is an unmet need for highly sensitive and specific biomarkers for assessing renal function in patients. Myo-inositol oxygenase (MIOX) is a novel biomarker limitedly expressed in the proximal renal tubules. MIOX concentrations are elevated in mice with renal injury and in critically ill patients with AKI. Our previously published studies have demonstrated that elevation in MIOX concentrations precedes changes in creatinine concentrations by approximately 54 hours. Importantly, MIOX facilitates the conversion of myo-inositol to glucuronic acid. We developed a robust LC-MS/MS method to quantify myo-inositol in human plasma, hypothesizing that following renal injury (acute and chronic), plasma concentrations of myo-inositol will increase, and will correlate with increases in serum creatinine concentrations. A healthy outpatient cohort was identified from samples obtained from out-patients with normal plasma creatinine, eGFR>60, as well as normal urine protein: creatinine ratio, albumin: creatinine ratio, or urinalysis results that were pan-negative/pan-normal. Remnant samples from out-patients with clinically staged CKD who have not initiated dialyses were selected for the CKD group. In healthy adult out-patients with stable renal function, the central 95th% distribution of plasma myo-inositol concentrations was 16.61 - 44.2 uM (Median 27.7uM, 95% CI: 26.7-28-7). No sex-based differences in plasma myo-inositol were observed. We also observed excellent correlation between plasma myo-inositol and serum creatinine concentrations (r=0.84, 95% CI: 80-88), as well as excellent sensitivity (99%) and specificity (100%) at a myo-inositol concentration of 50.22uM for differentiating patients with stable kidney function from those with CKD. Patients with CKD exhibited higher plasma myo-inositol concentrations (p value <0.0001) with a central 95% percentile of 51.2-309.1 uM (Median 104.1 uM, 95% CI: 92.3- 113.6). These results imply that myo-inositol concentrations increase in accord with renal injury and MIOX buildup in patients with kidney dysfunction, making it a potential biomarker for renal impairment.