Clinical Protocol
After 12 hours of fasting, all participants received a single dose of 300 mg GBP (Gabapentin, EMS, Hortolândia, Brazil) with 200 mL of water. Three hours after drug administration, non-diabetic participants received a standard meal and participants with diabetic neuropathic pain received a standard meal for diabetic patients. Serial blood samples were collected up to 24 hours after GBP administration and stored at -80 ºC until the analysis. Blood samples (10 mL) were also collected to evaluate the clinical biomarkers: urea, creatinine, aspartate aminotransferase (AST), alanine aminotransferase (ALT), gamma-glutamyl transferase (GGT), total bilirubin (TB), fasting glucose levels (FGL) and glycated haemoglobin (HbA1c). The estimated glomerular filtration rate (eGFR) was calculated from serum creatinine values using the CKD-EPI equation [47]. Whole blood was used for genotyping. All participants were genotyped for the SNPs 808G>T (rs316019) of SLC22A2 gene and 1507C>T (rs1050152) ofSLC22A4 , as previously reported [44,48]. The Hardy-Weinberg (HW) equilibrium was evaluated using the Chi-square test (χ2). The clinical history and the use of concomitant drugs were registered for all participants.