Current Drug Metabolism - Volume 5, Issue 2, 2004

Drug:drug interactions continue to be an obstacle for the pharmaceutical industry in the development of potential drug candidates. Considering the number of compounds that have been withdrawn from the market due to drug:drug interactions (e.g. cisapride, terfenadine and mibefradil), more pressure is placed on the pharmaceutical industry to investigate potential interactions prior to regulatory submission. In particular, induction and inhibition of drug metabolizing enzymes can profoundly alter the pharmacological and toxicological effects observed during monotherapy. However, due to differences in the expression and regulation of both metabolic enzymes and nuclear receptors responsible for induction, in vivo studies with pre-clinical species are not predictive of the human clinical situation. Although in vitro kinetic data also have limitations when extrapolating in vivo, in vitro testing has become more commonplace due to reduced cost and higher throughput. However, in the in vitro setting, complex enzyme kinetics can alter the estimation of kinetic parameters. Time-dependent or non-Michaelis-Menten kinetics can alter parameter estimates if experimental conditions are not optimal, and can therefore confound clinical predictions. Furthermore, mechanism-based inactivation (MBI) will reduce the active enzyme pool, both in vitro and in vivo, and thus complicate any parameter estimates. To further complicate matters, some compounds (e.g., ritonavir) inhibit, induce, as well as cause mechanism-based enzyme inactivation. For compounds such as ritonavir, the accurate estimation of kinetic parameters requires optimal experimental design at a minimum. This review will highlight the challenges in estimating enzyme kinetic parameters when both inhibition and induction are present, and will offer experimental viewpoints for the optimization of the experimental conditions.

Strategies used to screen new drug entities as potential inhibitors of CYP450 enzymes are now widely used to select candidates in the drug discovery process. However, the information obtained based on IC50 values are usually more of qualitative nature. The aim of this study was to find out whether a more quantitative assessment of interaction potential could be achieved on the basis of the ratio I / Ki (I corresponds to inhibitor concentration). Ki values, in vivo data, namely plasma exposures under control condition vs in presence of inhibitors, were obtained from literature for 36 compounds. For a quantitative assessment, the following inhibitor concentrations were considered: I max and I in,max (respectively, maximum I in systemic circulation and in portal vein), I max,u and I in,max,u (respectively, maximum unbound I in systemic circulation and in portal vein). The predicted interaction was calculated as AUCinhibitor / AUCcontrol = 1 + I / Ki, where AUCcontrol and AUCinhibitor represent, respectively, the area under curve of the plasma concentration vs time profile under control conditions (ie without inhibitor) and with inhibitor. The use of I / Ki allowed a more quantitative estimation of the interaction potential. In this context, protein binding appeared to be a key parameter to be considered to avoid overestimation of DDI potential. Thus, 60% successful predictions could be achieved based on the ratio I max,u / Ki. Yet, some major deviations between in vivo DDI were obtained with this approach and the observations on the relevance of the inhibitor concentrations and the impact of binding need to be interpreted very cautiously in the absence of information on additional parameters such as fm and fh for example.

Cirrhosis is the end stage of many forms of liver pathologies including hepatitis. The liver is known for its vital role in the processing of xenobiotics, including drugs and toxic compounds. Cirrhosis causes changes in the architecture of the liver leading to changes in blood flow, protein binding, and drug metabolizing enzymes. Drug metabolizing enzymes are primarily decreased due to loss of liver tissue. However, not all enzyme activities are reduced and some are only altered in specific cases. There is a great deal of discrepancy between various reports on cytochrome P450 alterations in liver cirrhosis, likely due to differences in disease severity and other underlying conditions. In general, however, CYP1A and CYP3A levels and related enzyme activities are usually reduced and CYP2C, CYP2A, and CYP2B are mostly unaltered. Both alcohol dehyrogenases and aldehyde dehydrogenases are altered in liver cirrhosis, although the etiology of the disease may determine the expression of alcohol dehydrogenases. Glucuronidation is mainly preserved, but there are a number of factors that determine whether glucuronidation is affected in patients with liver cirrhosis. Low sulphation rates are usually found in patients with liver disease but a decrease in sulfatase activity compensates for the decrease in sulphation rates. In all cases, a reduction in drug metabolizing enzyme activities in liver cirrhosis contributes to decreased clearance of drugs seen in patients with liver abnormalities. The reduction in drug metabolizing enzyme activity must be taken into consideration when adjusting doses, especially in patients with severe liver disease.

More than twenty heterocyclic aromatic amines (HAAs) have been identified in grilled meats, fish, poultry, and tobacco smoke condensate. HAAs are carcinogens and induce tumors at multiple sites in experimental laboratory animals. Because of the widespread occurrence of HAAs in foods, these chemicals may contribute to the etiology of several common human cancers that are associated with frequent consumption of grilled meats including colon, rectum, prostate, and breast. HAAs require metabolism in order to exert their genotoxic effects. Metabolic activation occurs by Nhydroxylation, a reaction catalyzed by cytochromes P450 (CYP). Some N-hydroxy-HAA metabolites may directly react with DNA, but further metabolism by N-acetyltransferases (NATs) or sulfotransferases (SULTs) may occur to form highly reactive N-acetoxy or N-sulfonyloxy esters that readily react with DNA bases. The N-acetoxy ester of the HAA 2- amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is detoxified by glutathione S-transferases (GSTs), which catalyze the reduction of the reactive intermediate back to the parent amine. Some HAAs also undergo detoxification through conjugation reactions with the phase II enzymes such as UDP-glucuronosyltransferases (UGTs) or SULTs to form stable, polar products that are readily eliminated. All of these xenobiotic metabolism enzyme systems (XMEs) display common genetic polymorphisms, which may affect protein expression, protein stability, catalytic activity, and thus, the biological potency of these procarcinogens. In this review, the roles of common genetic polymorphisms of XMEs involved in HAA metabolism and cancer risk are discussed.

Autoimmune diseases are a group of illnesses with multiple organ involvement. The prototype of this group of disorders is rheumatoid arthritis (RA) that aside from systemic organ involvement mainly presents with progressive destruction of many joints. Both activation and defective apoptosis of immune effector cells like T and B lymphocytes and macrophages play critical roles in the pathogenesis of autoimmune disorders. Current therapy for autoimmune diseases recommends a combination of several disease-modifying antirheumatic drugs (DMARDs) that preserve different immunomodulatory mechanisms. Because of limited success in prevention of RA joint destruction for currently available DMARDs, the development of more effective and less toxic DMARDs has been one of the major goals for pharmaceutical companies. The introduction of leflunomide and anti-tumor necrosis factor alpha therapies to the market recently serves as examples. In this context, the experience from ancient Chinese medicine gives an alternative consideration looking for potential DMARDs. Two commonly prescribed Chinese antirheumatic herbs are Tripterygium wilfordii hook f (TWHf) and tetrandrine (Tet) that preserve both anti-inflammatory and immunosuppressive effects. Importantly, the TWHf- or Tet-mediated immunomodulatory mechanisms are evidently different from the known DMARDs. The synergistic effects have also been demonstrated between these two Chinese antirheumatic herbs and DMARDs like FK506, cyclosporin and possibly chloroquine. Another potential Chinese herb for this consideration is Ginkgo biloba. This review summarizes evidence-based in vivo and in vitro studies on Chinese herbs as immunomodulators and potential DMARDs.

Development of effective preventive strategies for bladder cancer is of critical importance. Bladder cancer is among the most common human malignancies and typically recurs after treatment. Many plant-derived isothiocyanates (ITCs) have shown cancer-preventive activity in a variety of rodent organs. They are known to inhibit carcinogenactivating enzymes, and to induce carcinogen-detoxifying enzymes, apoptosis, cell cycle arrest, and differentiation of cancer cells. More importantly, orally ingested ITCs are efficiently absorbed, rapidly and almost exclusively excreted and concentrated in the urine as N-acetylcysteine conjugates (NAC-ITC). NAC-ITCs also possess anticarcinogenic activity, perhaps due largely to their facile dissociation to free ITCs. Storage of urine in the bladder makes the bladder epithelium, which lines the inner surface of the bladder and gives rise to the majority of bladder cancers, the most exposed tissue to ITCs / NAC-ITCs upon their ingestion. Storage of urine in the bladder also will increase the release of ITCs from NACITCs. Consequently, the amount of ITCs needed orally to achieve cancer prevention in the bladder may be much lower than that required for other organs. As a result, potential systemic adverse effects of ITCs may be minimized when using ITCs for chemoprevention of bladder carcinogenesis. Taken together, ITCs may be especially useful for the prevention of bladder cancer.

Cytochrome P450 (CYP450) enzymes have the capability of playing key metabolic roles in several aspects of cancer as a consequence of their unusually broad substrate specificities. CYP450 are also prominent players in the metabolism of anticancer therapeutic drugs, enhancing or diminishing the efficacy of the drugs depending on whether the drug or its metabolites are efficacious. As CYP450 enzymes are also found in lung tissue, lung metabolism can be of importance to the bioactivation of some anticancer agents. The presence of individual forms of CYP450 has been investigated in lung tumor to determine whether intra-tumor metabolism of anticancer agents by CYP450 could occur and thus influence the response of tumor to these agents. Differences in drug metabolism between normal and cancerous lung tissue have been shown to exist, therefore; the variable expression of CYP450 between tumor and normal tissue can provide a basis for selective sensitivity of tissue to anticancer drugs, thereby localizing drug actions to tumor. This review gives a detailed picture of the expression of CYP450 in lung tumor and the role of this enzyme in lung tumor in the fate of anticancer drugs.