P450 enzyme inhibition

Several higher throughput in vitro assays may be used to assess various DMPK properties of NCEs. One common parameter is that HPLC/MS/MS is the method of choice for the analytical step.11 17 26 These higher throughput assays include the Caco-2 assay, p450 enzyme inhibition assay, and in vitro stability assay. Each assay has different requirements and solutions and they will be described individually. 

In-vitro Microsomal Stability Screen Caco-2 Absorption Screen P450 Enzyme Inhibition Screen Rapid Rat" (CARRS) PO/PK Screen... 

Higher throughput screening with human cytochrome P450 to study P450-medi-ated metabolism is now available [38-40]. Similarly, rapid microhtre plate assays to conduct for example, P450 enzyme inhibition studies have been developed, using individually expressed CYP enzymes (Supersomes) [41]. 

Several studies have been carried out using different substrate probes to determine the inhibitory potency of various members of this class against CYP2D6 (102-105). The potential implications of CYP2D6 (and other P450 enzymes) inhibition by this class of dmgs has been exhaustively reviewed (106-116) and is not considered further here. 

P450 enzyme inhibition by SSRIs may also augment effects of alcohol, tramadol, methadone, terfenadine (danger of cardiac arrhythmia), -caine anaesthetics and theophylline. 

Ticlopidine inhibits the P2Yj2 platelet ADP receptor, thus inhibiting ADP-dependent activation of the GP Ilb/IIIa receptor. It has a slow onset of action and takes 3-7 days to reach its maximal antiplatelet effect. It is inactive in vitro and must undergo activation by the hepatic cytochrome p450 enzyme system. Secondary prevention trials have found that ticlopidine-treated patients have an estimated RRR of 33% for the composite endpoint of stroke, myocardial infarction, or vascular death after ischemic stroke. Significant adverse effects include bone marrow depression, rash, diarrhea, and thrombotic thrombocytopenic purpura. No clinical trials have studied ticlopidine for the treatment of stroke in the acute phase. 

This interceptor theory does not seem to be the only protective mechanism in operation. Inhibition of cytochrome P450 enzymes related to the bioactivation of mutagens and toxic radical scavenger activities have been proposed to integrate the different modes of action. Other investigations have reported the involvement of chlorophyUin in inducing apoptosis in human colon cells, which may be important in limiting cancer cell invasion and metastasis. ... 

The experiment was conducted in the presence of the fungus and the compound ABT at sufficiently high concentration, 5 mM, to cause the inhibition of Cyp P450 enzyme system expression by the fungus. 

Ketoconazole inhibits a variety of cytochrome P450 enzymes, including 11-hydroxylase and 17-hydroxylase. It is highly effective in lowering cortisol in Cushing s disease, and patients can be maintained successfully on therapy for months to years. The most common adverse effects are reversible elevation of hepatic transaminases and GI upset. It can cause gynecomastia and lower plasma testosterone values. 

Human liver microsomes (HLMs) are the most common in vitro sources of enzymes for inhibition studies, and selective probe substrates are required. Recombinant human P450 enzymes have become commercially available. They are widely used for screening, and less selective probe substrate can be used. Hepatocytes and liver slices48 have also been used for P450 inhibition screening to a lesser extent. 

Testino, A.S., Jr. and Patonay, G. 2003. High-throughput inhibition screening of major human cytochrome P450 enzymes using an in vitro cocktail and liquid chromatography/tandem mass spectrometry. J. Pharm. Biomed. Anal. 30 1459. 

Metofluthrin (I) The committee determined that the new data were sufficient to support a mitogenic mode of action for the development of liver tumors in rats exposed to metofluthrin in the carcinogenicity study. The report summarized mode of action study data that characterized effects such as increased P450 enzyme levels, increased smooth endoplasmic reticulum, hepatocellular hypertrophy, hepatocellular proliferation, and inhibition of intracellular communication, which were described as steps leading to tumor development via a nongenotoxic mechanism (i.e., mitogenicity). Some of these studies used sodium phenobarbital as a positive control,... 

Correia MA, Ortiz de Montellao PR. Inhibition of cytochrome P450 enzymes. In Ortiz de Montellano PR, ed. Cytochrome P450 Structure, Mechanism, and Biochemistry. 3rd ed. New York Kluwer/Plenum 2005 247-322. 

Lin, J.H. and Lu, A.Y. (2001) Interindividual variability in inhibition and induction of cytochrome P450 enzymes. Annual Review of Pharmacology and Toxicology, 41, 535—567. 

Atkinson, A., Kenny, J.R. and Grime, K. (2005) Automated assessment of time-dependent inhibition of human cytochrome P450 enzymes using liquid chromatography-tandem mass spectrometry analysis. Drug Metabolism and Disposition, 33 (11), 1637—1647. 

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