Effect of nizatidine on paracetamol and its metabolites in human plasma

Assessment of cryopreserved human hepatocytes as a model system to investigate sulfation and glucuronidation and to evaluate inhibitors of drug conjugation

Cultured cryopreserved human hepatocytes are extensively used as a model system for studying drug metabolism, although they remain poorly characterized in respect of the major conjugation reactions glucuronidation and sulfation. Using paracetamol (acetaminophen), we assessed eleven samples of cryopreserved human hepatocytes for their suitability to investigate the simultaneous glucuronidation and sulfation of xenobiotics and evaluated inhibitors of conjugation. Kinetic characterization showed broadly similar values for paracetamol conjugation by hepatocytes (as reported in the literature for in vitro systems), with Km values of approximately 6 mM and 0.3 mM for glucuronidation and sulfation, respectively. Substantial interindividual differences were observed. The hepatocytes demonstrated a strong dose-dependent switch from a preponderance of sulfation at low concentrations of paracetamol to glucuronidation at higher doses, consistent with routes of clearance in vivo. A number of drugs, some of which such as probenecid and sulfinpyrazone are known to interact with paracetamol in vivo, were demonstrated to inhibit the sulfation and/or glucuronidation of paracetamol in hepatocytes, demonstrating the potential application of this model system for studying drug-drug interactions involving conjugation.

Drug Interactions with Paracetamol

Paracetamol (acetaminophen) is one of the most commonly used analgesic antipyretic drugs worldwide, and it is widely available by prescription and over the counter (OTC). Fortunately, few clinically significant drug interactions have been documented. There is probable potentiation of hepatotoxicity following an overdose from the paracetamol metabolite NAPQI by enzyme-inducing drugs. There is considerable controversy regarding the possible interaction with warfarin in its potential to increase its anticoagulant effects because of discrepancies between observational studies and those in healthy volunteers. Otherwise, no serious adverse drug interactions with therapeutic doses of paracetamol have been confirmed in humans. Because the absorption of paracetamol is so dependent on gastric emptying, other drugs that alter gastric emptying can change its pharmacokinetics; but this would not cause serious adverse effects. Although animal experiments have demonstrated that many compounds can modify paracetamol hepatotoxicity, these are unlikely to be important at therapeutic doses.

Extrapyramidal Symptoms Related to Adjunctive Nizatidine Therapy in an Adolescent Receiving Quetiapine and Paroxetine

Weight gain is a serious problem with recently introduced atypical antipsychotic agents. Nizatidine, a histamine2 (H2)-receptor antagonist, may help reduce this weight gain. To our knowledge, no adverse effects have been reported when nizatidine is given at recommended doses with atypical antipyschotic agents. We describe, however, an adolescent who was receiving quetiapine and paroxetine for schizophrenia and depression, and developed extrapyramidal symptoms (EPS; parkinsonism and akathisia) after taking nizatidine for weight loss. Based on a report of another patient who developed EPS after taking higher-than-recommended doses of nizatidine, we reviewed the literature on treatment with H2-receptor antagonists for weight gain and on central nervous system adverse effects of nizatidine. Nizatidine may be effective for reducing weight gain associated with both medical and psychiatric conditions. Its safety profile is usually benign, although some patients may develop serious adverse effects, such as EPS and delirium. Therefore, the drug is recommended for short-term management of weight gain associated with atypical antipsychotic agents. Patients receiving nizatidine therapy should be monitored closely for development of EPS, particularly when high doses are prescribed.