Enantioselective pharmacokinetics of tramadol in CYP2D6 extensive and poor metabolizers

Dielectric relaxation study on tramadol monohydrate and its hydrochloride salt

Dielectric relaxation measurements as well as differential scanning calorimetry and X-ray diffraction investigations were performed on tramadol monohydrate and its hydrochloride salt. Examined samples do not crystallize during cooling and in consequence they reach the glassy state. In the case of the hydrochloride tramadol we are able to monitor alpha-relaxation process despite large contribution of dc conductivity to the loss spectra. It is the first such study on the salt of the drug. Up to now the dielectric spectroscopy has been regarded as useless in measuring such kind of API (active pharmaceutical ingredient). In this paper we also made some suggestions about the nature of the secondary relaxations in the amorphous tramadol monohydrate and its salt. The knowledge about the molecular mechanisms, which govern the observed secondary relaxations seems to be the key in predicting the stability of the amorphous form of the examined API. Finally additional dissolving measurements on the amorphous and crystal tramadol hydrochloride were performed. As a result we understood that dissolution properties of the amorphous form of the considered drug are comparable to those of crystalline one. However, we have found out that amorphous tramadol hydrochloride has greater ability to form tablets than its crystalline equivalent. This finding shows that amorphous drugs can be alternative even for the freely solved pharmaceuticals such as tramadol hydrochloride, because the former one has better ability to form tablets. It implies that during tabletting of the amorphous drugs there is no need to use any excipients and chemicals improving compaction properties of the API.

Personalized therapy in pain management: where do we stand?

Genomic variations influencing response to pharmacotherapy of pain are currently under investigation. Drug-metabolizing enzymes represent a major target of ongoing research in order to identify associations between an individual’s drug response and genetic profile. Polymorphisms of the cytochrome P450 enzymes (CYP2D6) influence metabolism of codeine, tramadol, hydrocodone, oxycodone and tricyclic antidepressants. Blood concentrations of some NSAIDs depend on CYP2C9 and/or CYP2C8 activity. Genomic variants of these genes associate well with NSAIDs’ side effect profile. Other candidate genes, such as those encoding (opioid) receptors, transporters and other molecules important for pharmacotherapy in pain management, are discussed; however, study results are often equivocal. Besides genetic variants, further variables, for example, age, disease, comorbidity, concomitant medication, organ function as well as patients’ compliance, may have an impact on pharmacotherapy and need to be addressed when pain therapists prescribe medication. Although pharmacogenetics as a diagnostic tool has the potential to improve patient therapy, well-designed studies are needed to demonstrate superiority to conventional dosing regimes.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Effects of the CYP2D6 gene duplication on the pharmacokinetics and pharmacodynamics of tramadol

The analgesic drug tramadol is bioactivated by CYP2D6 to the opioid receptor agonist O-desmethyltramadol. Case reports indicated that carriers of the CYP2D6 gene duplication may be at high risk for opioid adverse events. However, the effects of the CYP2D6 duplication on kinetics and dynamics of tramadol have not been systematically studied. Pharmacokinetics and effects were monitored after a single dose of 100 mg racemic tramadol in 11 carriers of a CYP2D6 gene duplication allele (ultrarapid metabolizer [UM]) and compared with 11 carriers of 2 active CYP2D6 genes (extensive metabolizer [EM]). Pharmacodynamics was measured by cold pressure test, pupillometry, and standardized adverse event recording. The maximum plasma concentrations of the active metabolite (+)R,R-O-desmethyltramadol were significantly higher in the UM group compared with the EM group (P = 0.005; t test) with a mean difference of 14 ng/mL (95% confidence limit of difference, 2-26 ng/mL). Median (+)R,R-tramadol area under the curve was 786 and 587 mug.h.L in EMs and UMs, and the corresponding median (+)R,R-O-desmethyltramadol area under the curve was 416 and 448 mug.h.L (P = 0.005, t test). There was an increased pain threshold and pain tolerance and a stronger miosis after tramadol in UMs compared with EMs. Almost 50% of the UM group experienced nausea compared with only 9% of the EM group. In conclusion, pharmacokinetic differences between EMs and UMs were smaller than expected; nevertheless, UMs were more sensitive to tramadol than EMs. Therefore, tramadol may frequently cause adverse effects in southern European and Northern African populations with a high proportion of UMs.

Influence of cytochrome P450 polymorphisms on drug therapies: pharmacogenetic, pharmacoepigenetic and clinical aspects

The polymorphic nature of the cytochrome P450 (CYP) genes affects individual drug response and adverse reactions to a great extent. This variation includes copy number variants (CNV), missense mutations, insertions and deletions, and mutations affecting gene expression and activity of mainly CYP2A6, CYP2B6, CYP2C9, CYP2C19 and CYP2D6, which have been extensively studied and well characterized. CYP1A2 and CYP3A4 expression varies significantly, and the cause has been suggested to be mainly of genetic origin but the exact molecular basis remains unknown. We present a review of the major polymorphic CYP alleles and conclude that this variability is of greatest importance for treatment with several antidepressants, antipsychotics, antiulcer drugs, anti-HIV drugs, anticoagulants, antidiabetics and the anticancer drug tamoxifen. We also present tables illustrating the relative importance of specific common CYP alleles for the extent of enzyme functionality. The field of pharmacoepigenetics has just opened, and we present recent examples wherein gene methylation influences the expression of CYP. In addition microRNA (miRNA) regulation of P450 has been described. Furthermore, this review updates the field with respect to regulatory initiatives and experience of predictive pharmacogenetic investigations in the clinics. It is concluded that the pharmacogenetic knowledge regarding CYP polymorphism now developed to a stage where it can be implemented in drug development and in clinical routine for specific drug treatments, thereby improving the drug response and reducing costs for drug treatment.

Stereospecific high-performance liquid chromatographic analysis of tramadol and its O-demethylated (M1) and N,O-demethylated (M5) metabolites in human plasma

A stereospecific method for simultaneous quantitation of the enantiomers of tramadol (T) and its active metabolites O-demethyl tramadol (M1) and O-demethyl-N-demethyl tramadol (M5) in human plasma is reported. After the addition of penbutolol (IS), plasma (0.5 ml) samples were extracted into methyl tert-butyl ether, followed by back extraction into an acidic solution. The separation was achieved using a Chiralpak AD column with a mobile phase of hexanes:ethanol:diethylamine (94:6:0.2) and a flow rate of 1 ml/min. The fluorescence of analytes was then detected at excitation and emission wavelengths of 275 and 300 nm, respectively. All the six enantiomeric peaks of interest plus three unknown metabolite peaks and IS peak (a total of 10 peaks) eluted within 23 min, free from endogenous interference. The assay was validated in the plasma concentration range of 2.5-250 ng/ml, with a lower limit of quantitation of 2.5 ng/ml, for all the six analytes. The extraction efficiency (n=5) was close to 100% for both T and M1 enantiomers and 85% for M5 and IS enantiomers. The application of the assay was demonstrated by simultaneous measurement of plasma concentrations of T, M1, and M5 enantiomers in a healthy volunteer after the administration of 50 mg oral doses of racemic T.

Concentrations of tramadol and O-desmethyltramadol enantiomers in different CYP2D6 genotypes

The influence of CYP2D6 genotype and CYP2D6 inhibitors on enantiomeric plasma levels of tramadol and O-desmethyltramadol as well as response to tramadol was investigated. One hundred and seventy-four patients received one hundred intravenous tramadol 3 mg/kg for postoperative analgesia. Blood samples drawn 30, 90, and 180 min after administration were analyzed for plasma concentrations of the enantiomers (+)-, (-)tramadol and (+)-, (-)O-desmethyltramadol by liquid chromatography-tandem mass spectrometry. Different CYP2D6 genotypes displaying zero (poor metabolizer (PM)), one (heterozygous individual (HZ)/intermediate metabolizer (IM)), two extensive metabolizer (EM), and three (ultra rapid metabolizer (UM)) active genes were compared. Concentrations of O-desmethyltramadol differed in the four genotype groups. Median (1/3 quartile) area under the concentration-time curves for (+)O-desmethyltramadol were 0 (0/11.4), 38.6 (15.9/75.3), 66.5 (17.1/118.4), and 149.7 (35.4/235.4) ng x h/ml for PMs, HZ/IMs, EMs, and UMs (P<0.001). Comedication with CYP2D6 inhibitors decreased (+) O-desmethyltramadol concentrations (P<0.01). In PMs, non-response rates to tramadol treatment increased fourfold compared with the other genotypes (P<0.001). In conclusion, CYP2D6 genotype determined concentrations of O-desmethyltramadol enantiomers and influenced efficacy of tramadol treatment.

Route-dependent stereoselective pharmacokinetics of tramadol and its activeO-demethylated metabolite in rats

The effects of route of administration on the stereoselective pharmacokinetics of tramadol (T) and its active metabolite (M1) were studied in rats. A single 20 mg/kg dose of racemic T was administered through intravenous, intraperitoneal, or oral route to different groups of rats, and blood and urine samples were collected. Samples were analyzed using chiral chromatography, and pharmacokinetic parameters (mean +/- SD) were estimated by noncompartmental methods. Following intravenous injection, there was no stereoselectivity in the pharmacokinetics of T. Both enantiomers showed clearance values (62.5 +/- 27.2 and 64.4 +/- 39.0 ml/min/kg for (+)- and (-)-T, respectively) that were equal or higher than the reported liver blood flow in rats. Similar to T, the area under the plasma concentration-time curves (AUCs) of M1 did not exhibit stereoselectivity after intravenous administration of the parent drug. However, the systemic availability of (+)-T was significantly (P < 0.05) higher than that of its antipode following intraperitoneal (0.527 +/- 0.240 vs. 0.373 +/- 0.189) and oral (0.307 +/- 0.136 vs. 0.159 +/- 0.115) administrations. The AUC of the M1 enantiomers, on the other hand, remained mostly nonstereoselective regardless of the route of administration. Pharmacokinetic analysis indicated that the stereoselectivity in the pharmacokinetics of oral T is due to stereoselective first pass metabolism in the liver and, possibly, in the gastrointestinal tract. The direction and extent of stereoselectivity in the pharmacokinetics of T and M1 in rats were in agreement with those previously reported in humans, suggesting that the rat may be a suitable model for enantioselective studies of T pharmacokinetics.