Enzyme induction in humans. Clinical aspects--an overview

Gastrointestinal and hepatotoxicity assessment of an anticancer extract from muricid molluscs

Marine molluscs from the family Muricidae are under development as a potential medicinal food for the prevention of colon cancer and treatment of gynaecological cancers. Here we report the outcome of the first in vivo toxicity assessment on an anticancer extract from a muricid mollusc containing brominated indole derivatives. Mice received the concentrated lipophilic extract by daily oral gavage over a two-week period. Mortality or clinical toxicity symptoms resulting from the extract were not detected during the trial, and there was no difference in the body weight of treated and control mice at the end of the trial. Histological analysis revealed some evidence for mild, idiosyncratic effects on the gastrointestinal tract and liver, including necrosis, fatty change, and inflammation in a small proportion (<40%) of mice. This is likely to result from first-pass hepatic metabolism of tyrindoxyl sulphate combined with second-pass metabolism of indoles. Overall however, oral administration of muricid extract containing brominated indoles does not result in severe clinical toxicity.

Hepatic Drug-Metabolizing Enzyme Induction and Implications for Preclinical and Clinical Risk Assessment

Hepatic drug metabolizing enzyme (DME) induction complicates the development of new drugs owing to altered efficacy of concomitant treatments, reduction in exposure resulting from autoinduction, and potential generation of toxic metabolites. Risk assessment of DME induction during clinical evaluation is confounded by several uncertainties pertaining to hazard identification and dose response analysis. Hepatic DME induction rarely leads to clinical evidence of altered metabolism and toxicity in the patient, which typically occur only if the DME induction is relatively severe. High drug doses are associated with a greater likelihood of hepatic DME induction and downstream effects; therefore, drugs of low potency requiring higher dosing tend to lead to a greater risk of drug–drug interactions. Vigilance in clinical trials for increased or diminished drug effect and, specifically, pharmacokinetic studies in the presence of other drugs and concomitant diseases are necessary for a drug risk assessment profile.

Efforts to remove hepatic DME-inducing drugs from development can be facilitated with current in vitro and in vivo assessments and will improve with the development of newer technologies. A carefully tailored case-by-case approach will lead to the development of efficacious drugs with an acceptable risk/benefit profile available to patients.

Digestive System 1

This chapter deals with the digestive system. The major and minor salivary glands and their secretions also represent and integral part of the protective mechanism of the oral cavity, and derangement of saliva production may lead to loss of integrity of the oral mucosa. Drug-induced abnormalities of taste sensation are also well-described phenomena occurring in man although human studies are necessary for the detection of these effects. Inflammation of the oral cavity may involve the buccal mucosa, the gingiva (gingivitis), the tongue (glossitis), and the peridontal tissues (peridontitis). Therapeutic agents can induce inflammatory lesions in the tongue. Moreover, a protective layer of mucus, a visco-elastic material containing high molecular weight glycoproteins produced by the major and minor salivary glands, covers the stratified squamous mucosa of the oral cavity. Salivary secretions also possess digestive enzyme activity although in herbivores and carnivores, it is usually low in contrast to high digestive enzyme activity in omnivorous species.

Extrapolation of in vitro enzyme induction data to humans in vivo

Enzyme induction generally increases the rate and extent of xenobiotic metabolism in vitro, but physiological constraints can dampen these effects in vivo. Biotransformation kinetics determined in hepatocytes in vitro can be extrapolated to whole animals based on the hepatocellularity of the liver, since the initial velocity of an enzyme-catalyzed reaction is directly proportional to the total enzyme present in the cell. The biotransformation kinetics of various xenobiotics determined with isolated hepatocytes in vitro have been shown to accurately predict pharmacokinetics in whole animals. Analysis of the kinetic data, using physiologically based pharmacokinetics, allows extrapolation of xenobiotic biotransformation across dose routes and species in a biologically realistic context. Several fold variations were observed in the bioactivation of the hepatotoxicant furan by isolated human hepatocytes, due to induction of cytochrome P450 2E1. Extrapolation of these data to humans in vivo showed that furan bioactivation was limited by hepatic blood flow delivery of the substrate. One important consequence of hepatic blood flow limitation is that the amount of metabolite formed in the liver is unaffected by increases in Vmax due to enzyme induction. Therefore, interindividual variations in cytochrome P450 2E1 among human populations would not affect the bioactivation of many rapidly metabolized hazardous chemical air pollutants. The hepatic blood flow limitation of biotransformation is also observed after oral bolus dosing of rapidly metabolized compounds. More slowly metabolized xenobiotics, such as therapeutic agents, are only partially limited by hepatic blood flow and other processes.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

Clinical significance of interactions with antifungal agents

Antifungal drugs act by a variety of mechanisms. Agents such as substituent imidazoles and triazoles, which act by inhibiting the fungal cytochrome P-450-dependent enzyme lanosterol N-demethylase, have the potential to inhibit host cytochrome P-450-dependent drug metabolism. This is discussed with respect to ketoconazole, fluconazole and itraconazole. In contrast, allylamines, which have a different mode of action and a weaker ability to bind to cytochrome P-450, are not expected to inhibit clinical drug oxidation. Inducers of drug metabolism, especially rifampicin, phenobarbitone and phenytoin, may lower plasma (and tissue) concentrations of those antifungals metabolized by mixed function oxidases, with therapeutic consequences.