**1. Introduction**

In clinical practice, the need for the use of multiple drugs is common, as patients often present with numerous chronic diseases. To improve commodity and drug adherence, several medications are often administered concomitantly. Although this may represent a preferred clinical strategy, the administration of two or more drugs at overlapping times increases the likelihood of drug–drug interactions [1,2]. As the risk of drug–drug interactions increases, the risk of debilitating, even fatal, adverse drug events also increases [3]. From a pharmacokinetics standpoint, drug–drug interactions occur when one drug—the perpetrator drug—alters the disposition of another co-administered drug—the victim drug.

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Inhibition of cytochrome P450 (CYP450) enzymes is the most common mechanism leading to drug–drug interactions [4]. CYP450 inhibition can be categorized as reversible (including competitive and non-competitive inhibition) or irreversible (or quasi-irreversible), such as mechanism-based inhibition. Each type of interaction involves a distinct clinical managemen<sup>t</sup> strategy. This is why a comprehensive understanding of mechanisms of CYP450-mediated metabolism inhibition is needed to prevent or mitigate these harmful drug interactions. This review will focus on the CYP450 enzymatic system with a special look at one specific type of CYP450 inhibition, namely mechanism-based inhibition; clinical cases involving mechanism-based inhibitors will be discussed in this context.

#### **2. Basic Concepts of Enzyme, Substrate, and Inhibitor**

For clarity, as we address the various types of drug metabolism inhibition, the concepts of active (orthosteric) and allosteric sites, substrates, and inhibitors need to first be reviewed.

#### *2.1. Active Site or Orthosteric Site of the Enzyme*

The active or orthosteric site is a physical space or pocket within the protein structure of an enzyme where a molecule can bind and from where a catalytic reaction takes place to convert the molecule into a metabolite (addition of a hydroxyl moiety, removal of alkyl moieties, etc.). (Figure 1) For CYP450 isoforms, binding to the active site is independent of the NADPH-P450 oxidase reactions; however, the chemical reaction leading to the formation of the metabolite will employ electrons originated from NADPH.

**Figure 1.** Illustration of an enzyme with its active binding site for drug transformation and allosteric binding site (or regulatory site).

### *2.2. Allosteric Site*

The allosteric site is a physical space or pocket within the protein structure of an enzyme spatially separated from the active site. (Figure 1) The allosteric site allows molecules to modulate enzyme activity. These molecules can be allosteric activators or allosteric inhibitors, depending on how they influence enzyme activity. Drugs can bind to this site and change the three-dimensional structure of the enzyme (conformational change). Allosteric inhibitors may render the active site no longer accessible for substrate binding or make the site unable to catalyze reactions. It is widely known that almost all cases of non-competitive inhibition are caused by allosteric regulation (see discussion in Section 3).
