Flupirtine is metabolized in-vitro via carbamate cleavage and N acetylation to glucuronides and mercapturic acid derivatives. The formation of reactive, toxic intermediate products may be influenced by genetic polymorphisms of the involved conjugative metabolic pathways. So the purpose of this study is to measure pharmacokinetics, metabolism and analgesic effects of flupirtine in dependence on the function of NAT2, UGT1A1 and GSTP1.

Flupirtine is a centrally acting analgesic drug. Its mechanism of action differs obviously from opiates because flupirtine antagonizes the morphine-induced tail phenomenon in mice in relatively low doses. The drug exerts no relevant anesthetic activity and it has only slight inhibitory effect on prostaglandin formation in animals. As major action mechanism, activation of descending noradrenergic neuronal pathways is discussed, thereby inhibiting afferent nociceptive stimuli on spinal, subcortical and cortical brain areas. Recently has been shown, that flupirtine exhibits functional NMDA receptor antagonistic properties by selective opening the neuronal potassium channel.

The anticonvulsant drug retigabine, which chemical structure only slightly differs from flupirtine (aromatic ring instead of the pyrimidine ring), is subjected to intensive glucuronidation and N-acetylation in man. In-vitro, retigabine is a substrate of the recombinant uridine glucoronosyl transferase (UGT) 1A1, 1A3, 1A4 and 1A9. Disposition of retigabine, however, was not influenced by the frequent UGT1A1*28 polymorphism (Gilbert-Meulengracht syndrome) whereas the polymorphism of the N-acetyltransferase 2 (NAT2) resulted in significant changes in retigabine disposition.

Similar to retigabine, the carbamate group of flupirtine is hydrolyzed by carboxyl esterases. The decarbamylated product undergoes N-acetylation to form the major metabolite D13223. This acetylation is catalyzed in-vitro both by recombinant human NAT2 and NAT1. Furthermore, relatively stable quinone diimines for flupirtine and D13223 as catalyzed by peroxidases were detectable in in-vitro experiments. After repeated oral administration of flupirtine in man, quinone diimines and glutathione adducts of them in the form of mercapturic acid conjugates were detected.

Therefore, we hypothesize, that highly reactive diimine radicals may appear as intermediates which are detoxified by conjugation with glutathione via a glutathione S-transferase (GST). Diimine intermediates are known to have high cell toxicity, genotoxicity and carcinogenicity as shown for the intermediates of acetaminophen. By analogy to acetaminophen, the GSTP1 might be the isoform that is involved also in detoxification of flupirtine intermediates.

According to our hypothesis, the net appearance of toxic intermediates with diimine structure in hepatocytes is dependent on the activity of NAT1/NAT2, UGTs and GSTP1. Because NAT2, UGT1A1 and GSTP1 are highly polymorphic enzymes, the risk of flupirtine hepatotoxicity may be dependent on the genotype of the subjects that are treated with the drug.

Our clinical study was initiated to confirm, whether polymorphisms of NAT2, UGT1A1 and GSTP1 may significantly influence disposition and analgesic effect of flupirtine and whether the genetic background is of risk for the appearance of toxic intermediates and its stable terminal conjugates.

Inclusion Criteria:

• age: 18 - 45 years
• sex and genetics: male and female genotyped for NAT2, UGT1A1 and GSTP1
• ethnic origin: Whites
• body weight: 19 - 27 kg/m²
• good health as evidenced by the results of the clinical examination, ECG, and the laboratory check-up, which are judged by the clinical investigator not to differ in a clinical relevant way from the normal state
• written informed consent

Exclusion Criteria:

• existing cardiac or hematological diseases and/or pathological findings which might interfere with safety, pharmacodynamic effect and/or pharmacokinetics of the study drug
• existing or further obstructive lung disease (e.g. bronchial asthma)
• myasthenia gravis
• existing hepatic and renal diseases and/or pathological findings which might interfere with safety, pharmacodynamic effect and/or pharmacokinetics of the study drug
• existing gastrointestinal diseases and/or pathological findings which might interfere with safety, pharmacodynamic effect and/or pharmacokinetics of the study drug
• acute or chronic diseases which could affect drug absorption or metabolism
• history of any serious psychological disorder
• drug or alcohol dependence
• positive drug screening or -only in suspicious case- positive alcohol test
• smokers of 10 or more cigarettes per day
• positive screening results for HIV, HBV and HCV
• volunteers who are on a diet which could affect the pharmacokinetics of the drug (vegetarian)
• heavy tea or coffee drinkers (more than 1l per day)
• lactation and pregnancy test positive or not performed
• volunteers suspected or known not to follow instructions
• volunteers who are unable to understand the written and verbal instructions, in particular regarding the risks and inconveniences they will be exposed to as a result of their participation in the study
• volunteers liable to orthostatic dysregulation, fainting, or blackouts
• blood donation or other blood loss of more than 400 ml within the last 12 weeks prior to the start of the study
• participation in a clinical trial during the last 3 months prior to the start of the study
• less than 14 days after last acute disease
• any systemically available medication within 4 weeks prior to the intended first administration unless, because of the terminal elimination half-life, complete elimination from the body can be assumed for the drug and/or its primary metabolites (except oral contraceptives)
• repeated use of drugs during the last 4 weeks prior to the intended first administration, which can influence hepatic biotransformation (e.g. barbiturates, cimetidine, phenytoin, rifampicin)
• repeated use of drugs during the last 2 weeks prior to the intended first administration which affect absorption (e.g. laxatives, metoclopramide, loperamide, antacids, H2-receptor antagonists)
• intake of grapefruit containing food or beverages within 7 days prior to administration
• known allergic reactions to the active ingredients used or to constituents of the pharmaceutical preparation
• subjects with severe allergies or multiple drug allergies