Drug-Drug Interactions: Basics

Basics of Drug-Drug Interactions

• Multiple drugs are frequently used in clinical practice because patients often have more than one chronic illness to manage.
• Multiple medications may be administered concurrently to improve drug adherence.
• Administering two or more drugs at the same time increases the possibility of drug-drug interactions.
• The risk of debilitating or fatal adverse drug events increases with the risk of drug-drug interactions.
• Drug-drug interactions occur when one drug (the perpetrator) alters the disposition of another co-administered drug (the victim).
• The most common mechanism causing drug-drug interactions is inhibition of cytochrome P450 (CYP450) enzymes.
• CYP450 enzymes are expressed mainly in the liver and gut mucosa.
• CYP450 enzymes play an important role in the clearance of various compounds from the bloodstream.
• CYP450 inhibition can be classified as reversible (competitive and non-competitive) or irreversible (mechanism-based).
• Each type of interaction necessitates a unique clinical management strategy.
• A thorough understanding of the mechanisms of CYP450-mediated inhibition is required to prevent or mitigate harmful drug interactions.

Key Concepts

• The active site is a physical space or pocket within an enzyme's protein structure where a molecule can bind and where a catalytic reaction occurs to convert a metabolite
• The allosteric site is a physical space or pocket within an enzyme's protein structure that is separated from the active site; this enables molecules to influence enzyme activity.
• Drugs can bind to the allosteric site and alter the three-dimensional structure of the enzyme.
• Allosteric inhibitors can prevent the active site from binding substrates or prevent it from catalyzing reactions.
• Almost all cases of non-competitive inhibition are known to be caused by allosteric regulation.
• Substrates are drugs that bind to an enzyme's active site and are transformed into metabolites while present in the active site
• A drug's biotransformation process may involve multiple enzymes that result in various metabolites; each metabolic route is dependent on specific characteristics
• The "binding affinity" of an enzyme is the strength of its attraction to a substrate.
• Substrates are classified as weak, intermediate, or strong based on their affinity for a specific enzyme.
• Binding affinity to an enzyme is measured by the Michaelis constant, Km, that is the concentration at which 50% of the maximum metabolic reaction known as Vmax occurs
• Vmax is defined as the maximum speed of product creation for a given amount of enzyme.
• Km can be thought of as the binding affinity of the substrate for the enzyme.
• A small Km indicates a strong binding, meaning Vmax will be reached at lower substrate concentration.
• A high Km indicates a weak binding, and Vmax will only be reached if the substrate concentration is high enough to saturate the enzyme.
• Intrinsic clearance measures the ability of the liver to clear unbound drug when there are no limitations to blood flow and binding considerations.
• The intrinsic clearance (CLint) of a substrate is defined by Vmax/Km.
• If a drug binds strongly to a metabolizing enzyme (small Km), the intrinsic clearance is high.
• If a drug binds weakly to a metabolizing enzyme (high Km), the intrinsic clearance is low.
• In many drug–drug interaction situations, the Km is increased for the victim drug which results in reduction of intrinsic clearance.

Classes of CYP450 Inhibition

• Mechanisms of CYP450 inhibition can be classified as reversible or irreversible.
• Reversible inhibition is a result of rapid association and dissociation between the substrate drug and the enzyme and can be classified as competitive or non-competitive.

Competitive Inhibition

• Drug interactions associated with reversible competitive inhibition are caused by the ability of a single CYP450 isoform to metabolize multiple substrates.
• Competitive inhibition occurs when two substrates compete for the same active site.
• The competition is determined by the binding affinities of the two substrates for the binding site and their concentrations near the enzyme.
• A substrate with strong affinity for an enzyme (Drug A) can displace a weaker substrate (Drug B) from the active site.
• If concentrations of the two substrates are comparable, Drug A (higher binding affinity) will be preferred at the active site, resulting in an accumulation of Drug B.
• This results in increased Km of the victim drug and reduced intrinsic clearance.
• For an active drug, a decrease in the intrinsic clearance of one of its metabolic pathways can lead to a decrease in the total clearance of the drug and can result in increased plasma concentrations, potentially precipitating adverse effects.
• For prodrugs, this interaction can instead result in a decrease in the formation of the active metabolite, thus reducing drug efficacy.
• Example: Theophylline (weak CYP1A2 substrate) is largely metabolized by CYP1A2 by binding to the active site; if that active site is occupied by a stronger affinity substrate such as Duloxetine, breakdown of Theophylline will be reduced, leading to increased plasma levels and possibly side effects such as nausea and vomiting.
• To minimize competitive inhibition, two competing substrates should be administered with as much time apart as possible.
• Competitive inhibition mechanism is sensitive to substrate concentrations, so it is considered reversible.
• If the concentrations of the weaker affinity substrate are substantially greater than those of the stronger affinity substrate, the weaker one can displace the stronger one and also prevent it from accessing the enzyme.
• The drug with a higher concentration preferentially binds to the enzyme and is metabolized, while the drug with a lower concentration is unable to bind and instead accumulates in the body.
• The concentration of the drug that preferentially occupies the enzyme’s active site, determines reversible inhibition.
• As its concentration decreases, the other drug is able to bind to the enzyme again, and the interaction ceases.
• The half-life of the drug that preferentially binds to the enzyme also influences the timing of the inhibition's onset and offset.
• Because many drugs have half-lives of less than 12 hours, the onset and offset will occur within 2 to 4 days of beginning or discontinuing the drug.

Non-Competitive Inhibition

• The non-competitive inhibitor binds to an allosteric site, it typically bears no structural similarities to the substrate.
• The non-competitive binding causes a conformational change in the active site's structure which prevents a substrate from binding to the active site.
• There is no direct competition at the active site between the inhibitor and the substrate
• This type of inhibition is typically long-lasting and cannot be overcome by increasing the substrate's concentration.
• Just like competitive inhibitors, non-competitive inhibitors also have an almost immediate effect.
• Unlike in competitive inhibition, separating the time of dosing will not alleviate non-competitive inhibition.
• Example: Fluconazole binds to an allosteric site of CYP2C9 causing the active site to lose its affinity for Carvedilol, concomitant administration will lead to an increase in serum concentrations of Carvedilol, leading to accumulation of the drug and an increase in the risk of hypotension and bradycardia.

Mixed Inhibition

• The inhibitor is capable of binding to either the enzyme's active site or to the enzyme's allosteric site thus preventing substrate from binding.
• Mixed inhibitors are usually more potent than competitive or non-competitive inhibitors.
• Example: Drug-drug interaction between Midazolam (CYP3A substrate) and Ketoconazole (mixed CYP3A inhibitor) leading to increased risk of side effects.
• If Ketoconazole occupies active site or allosteric site, Midazolam can’t be metabolized and accumulates in plasma.

Irreversible Inhibition

• Irreversible inhibition can be classified into two types: alternate substrate inhibition, and suicide inhibition.

Alternate Substrate Inhibition

• A stable intermediate metabolite formed during the normal metabolic cycle forms covalent bonds at the active site.
• This stable intermediate–enzyme complex is not easily broken by increasing substrate concentration.
• Since the enzyme structure remains otherwise unchanged, theoretically this reaction could be reversible with time. 
• Under in vivo conditions, with this metabolic intermediate complex being excessively stable, metabolic intermediate cannot be displaced and the enzyme remains inaccessible for metabolism so the reaction is technically irreversible.
• Example: Calrithromycin when the nitrosoalkene intermediate generated by N-demethylation forms covalent bonds with the active site of CYP3A4.

Suicide Inhibition

• A latent highly reactive intermediate is formed in the catalysis process.
• The reactive intermediate forms strong irreversible covalent bonds with the enzyme in a step that is not part of the normal metabolic pathway. 
• This process can significantly change the conformational structure of the enzyme and in some cases destroy the enzyme making it functionally unviable.
• Example: Inhibition of CYP2C19 by Esomeprazole mediates crosslinking the heme and apoprotein moieties in the enzyme, thereby changing its conformational structure.
• This is particularly problematic when Esomeprazole is co-administered with Clopidogrel. 
• Clopidogrel is a prodrug that requires CYP450 biotransformation, primarily by CYP2C19, to form an active metabolite that exerts anti-platelet activity. 
• When CYP2C19 is inhibited by Esomeprazole, plasma levels of the active metabolite of Clopidogrel can be significantly decreased and thus its anti-platelet activity reduced, resulting in higher risk of major cardiovascular events.