When several drugs are used simultaneously, there is always a risk of interaction between these drugs. A drug might become less effective, or the action of a drug might be significantly increased, thus causing side effects or even intoxication. It is well known today that some widely used drugs can interact with each other.
For example, antihistamines used together with tranquillisers could enhance drowsiness and prescribing both antidiabetics and beta blockers can increase the frequency of low blood sugar episodes.
The pharmacological action of a drug is determined by its interaction with specific binding sites at the cellular and sub-cellular level. The concentration of the drug in blood and tissues governs its therapeutic efficacy and also the toxicity. The systemic concentrations are the net result of the complex phenomena of absorption, distribution, metabolism and elimination. All of these processes can be affected by drug interaction.
Drug interactions have killed potential drugs during early development but have also resulted in refusal of marketing authorizations and withdrawal of approved drugs from the market. The risk of drug-drug interaction has an impact on drug development today. As these interactions may significantly change the therapeutic and safety profile of a compound, or even a medicinal herbal extract, this question has to be addressed before a drug candidate enters clinical development.
Regulatory aspects
A better understanding of human drug metabolizing enzymes has enabled the use of predictive drug interaction studies prior to clinical studies. Thus, subsequent clinical trials can be designed accordingly. The regulatory authorities of the USA and EU have issued guidance documents for in vitro and in vivo drug interaction studies to be conducted during drug development.
The FDA has recommended to perform detailed studies with the major cytochrome P450 (CYP) enzymes (CYP1A2, 2C9, 2C19, 2D6, 2E1 and 3A4) with determination of Ki values before the conduct of specific clinical trials. In spring 2004, the German BfArM (Bundesinstitut für Arzneimittel und Medizinprodukte) published an opinion paper concerning drug interactions of herbal preparations co-medicated with other drugs.
All regions emphasise that the in vitro investigation of metabolic pathways and of the inhibition or induction potential is a fundamental component of the drug safety assessment. If the in vitro experiments revealed a potential for drug-drug interaction, in vivo experiments have to be performed.
At present, the guidance is focused on the CYP-mediated drug interactions (additional information in the special text insert). The importance of other mechanisms like drug transport, has been recognised only recently and has not yet been incorporated into guidance documents.
Test systems
Consequently, the majority of proposals for in vitro tests are focused on the inhibition of CYP-mediated drug metabolism. The preferred test systems are microsomes and recombinant CYP enzymes. Although they do not represent the true physiological environment, the CYP kinetics can be quantified without interfering with other metabolic or transport processes.
To determine if a NCE (new chemical entity) compound inhibits a particular enzyme, microsomes are incubated with a substrate specific for the CYP enzyme in question. It can then be determined whether or not the formation rate or quantity of a specific metabolite of the substrate is influenced (decreased) by the presence of the NCE. For a first inhibition screening or ranking of a set of drug candidates, the value of several CYPs can be determined. This value describes the concentration of NCE that reduces the metabolism of the CYP standard substrate by 50%. To examine the type of inhibition (competitive or non-competitive) for a specific CYP, the dissociation constant for the enzyme inhibitor complex (Ki value) may be determined based on the values at different drug concentrations.
To investigate CYP induction, cultured hepatocytes are currently the most accepted test system. Typically, isolated hepatocytes are plated and treated with the test compounds. Afterwards the measurement of CYP activity will usually be used to reveal if enzyme induction has occurred or not. Changes in mRNA levels and protein concentrations (detected by specific antibodies) can also be used to demonstrate induction.
Experience in detecting drug transport interactions with in vitro tests is limited at present, and there are no generally accepted experimental approaches. Direct measurements in CaCo2 monolayer cell systems are often performed due to the high expression of the PGP transporter on their surface. However, transfected MDR-1 cell lines (kidney cells) and mouse fibroblasts are also used as test systems.
The CYP enzymes
The principal route of elimination of xenobiotics, including drugs, from the body is biotransformation, which is accomplished by enzymes. One large group of microsomal hemoprotein enzymes is called cytochromes P450 (CYPs) catalysing the oxidative, peroxidative and reductive metabolism of a wide variety of endogenous and exogenous compounds. The CYP superfamily is divided into families and subfamilies of isoenzymes.
The enzymes belonging to the families CYP1, CYP2 and CYP3 are mostly involved in the biotransformation of xenobiotics. About 70% of the CYP enzymes in the human liver belong to the families, which participate in drug metabolism. Among these, the isoenzyme CYP3A4 represents about 30% and the CYP2C about 20% of total amount.
Interactions can occur if one drug modulates the CYP-mediated metabolism of another compound metabolised by the same enzyme (competitive inhibition). A drug can also inhibit the metabolism of another drug by binding to the same enzyme without being metabolised itself (non-competitive inhibition).
Drug interactions mediated by induction of CYP enzymes are significantly less frequent than those mediated by CYP inhibition. It is also less likely that CYP induction would result in safety issues, but it may affect the efficacy of one or more medications.
Principle scheme of Cytochrom P-450 system
The PGP transporter
The modulation of transport processes has now been recognised as an important factor in absorption, tissue distribution and elimination of drugs and their metabolites. Drug transporters have been identified recently in the liver, gastrointestinal tract, kidney, blood-brain barrier and in other organs. They are responsible for the transport of endogenous compounds like sugars, amino acids, peptides and hormones, but are also involved in the uptake and excretion of exogenous compounds like drugs.
Transporters in the gastrointestinal tract influence the bioavailability of orally administered drugs, whereas those in the liver affect the hepatic uptake and biliary excretion.
The P-glycoprotein (PGP) transporter is currently the best understood drug efflux transporter, located in intestine, liver, brain and other epithelial tissues and involved in clinically significant drug interactions. The most prominent example probably is Digoxin which is poorly metabolized but eliminated mainly unchanged by the PGP transporter in the kidney. Elevated plasma concentrations of digoxin have been observed following co-medication with quinidine, verapamil, nifedipine, cyclosporin or St. John's wort. These compounds are able to inhibit the PGP transporter and thus reduce the clearance of digoxin.
The amount of PGP transporter protein is controlled by the MDR-1 gene (multi-drug resistance gene). Xenobiotics which affect the expression of this gene, in a positive or negative way, can also be responsible for PGP related drug interactions without interfering directly with the PGP transporter protein.
References
USA (FDA)
Guidance for Industry Drug Metabolism/Drug Interaction Studies in the Drug Development Process Studies in vitro.
April 1997
Guidance for Industry In vivo Drug Metabolism / Drug Interaction Studies - Study Design, Data Analysis and Recommendations for Dosing and Labelling.
November 1995
EU (EMEA)
Note for Guidance on the Investigation of Drug Interactions (CPMP/EWP/560/95).
June 1998
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