Influence of genetic polymorphism on tramadol pharmacokinetics and pharmacodynamics

• Purpose: There is a significant interindividual variation in the response to tramadol (TRA), which can partly be explained by genetic variation. The main purpose of this study was to determine if there is a correlation between the metabolic ratio of O-desmethyltramadol (ODT) to TRA (MR) and time after drug administration. We also studied the association between genetic polymorphisms in CYP2D6, OPRM1 and ABCB1 and pharmacokinetic and pharmacodynamic properties of TRA.Methods: Nineteen healthy volunteers were randomized into two groups receiving a single dose of either 50 or 100 mg of orally administrated TRA. Blood samples were collected prior to dosing and up to 72 h after drug intake. The subjects were asked to report drug related symptoms (DRS) during the experimental day.Results: We found a positive correlation between MR and the time after drug intake for both intermediate metabolizers (IMs) and extensive metabolizers (EMs). For the only poor metabolizer (PM) with detectable ODT levels the MR was almost constant. The AUC MR and Cmax MR were associated with CYP2D6 genotype, showing the highest mean values for EMs. Multiple regression analysis showed that 56% of the  variation in AUC MR could be explained by CYP2D6 alone and 78% by investigated SNPs altogether. There was great interindividual variation in DRS, but no associations could be found between DRS and investigated polymorphisms.Conclusions: MR can be used for estimation of the time of drug intake when the CYP2D6 genotype is known and taken into consideration. The influence of genetic polymorphisms in ABCB1 and OPRM1 requires further study. We propose that pharmacogenetics should be taken into consideration when interpreting clinical pharmacology and forensic toxicology results, more specifically CYP2D6 genotypes when interpreting the pharmacokinetics of TRA.

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