In this lecture we’re gonna cover the basics of drug-drug
interactions, so let’s get right into it. Multiple drugs are often used in clinical
practice because patients frequently present with multiple chronic diseases. Several medications
are frequently administered concurrently to improve drug adherence. Although this may
be a preferred clinical strategy, administering two or more drugs at the same time increases the
possibility of drug-drug interactions. As the risk of drug-drug interactions rises, so does the risk
of debilitating, even fatal, adverse drug events. Drug-drug interactions occur when one drug,
the perpetrator drug, alters the disposition of another co-administered drug, the victim drug.
The most common mechanism causing drug-drug interactions is inhibition of cytochrome
P450 (CYP450) enzymes, which are expressed mainly in the liver and gut mucosa, where
they play an important role in the clearance of various compounds from the bloodstream.
Now, CYP450 inhibition can be classified as reversible including competitive and
non-competitive inhibition or irreversible such as mechanism-based inhibition.
Each type of interaction necessitates a unique clinical management strategy.
This is why, in order to prevent or mitigate harmful drug interactions,
a thorough understanding of the mechanisms of CYP450-mediated inhibition is required.
To understand the various types of drug metabolism inhibition,
first we must review the concepts of active and allosteric sites, substrates, and inhibitors.
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. The allosteric site enables molecules to influence
enzyme activity. Depending on how they influence enzyme activity, these molecules can be allosteric
activators or allosteric inhibitors. Drugs can bind to this 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.
Depending on its chemical structure and physical properties, a substrate's binding
affinity for an active site can vary. 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. More precisely, Vmax is defined as the maximum speed of product
creation for a given amount of enzyme. This can be better understood by looking
at Michaelis-Menten plot, which relates the reaction velocity,
or how fast the reaction is going as a function of the increasing substrate concentration.
As the concentration of substrate increases along the x-axis, the curve begins to flatten
out as it approaches Vmax. In other words, at some point, there is so much substrate that the
enzymes are already working as fast as they can. Once this saturation point is reached,
the addition of any new substrate has no effect on how fast the enzyme can work.
Km on the other hand can be thought of as the binding affinity of the substrate for the enzyme.
Naturally, the substrate is more likely to be "caught" by the enzyme and transformed into
the product if it can bind the enzyme with a higher affinity. So a small Km indicates
a strong binding, meaning Vmax will be reached at lower substrate concentration. On the other hand,
a high Km indicates a weak binding, and Vmax will only be reached if the substrate concentration
is high enough to saturate the enzyme. Now, liver metabolism is the major route
for elimination for a wide variety of drugs. So when it comes to drug interactions, another
important concept to understand is the intrinsic clearance, which measures the ability of 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.
So let’s think about what this means. If a drug binds strongly to a metabolizing enzyme,
meaning Km is small, the ability of liver to clear the drug, the intrinsic clearance is high.
On the other hand if a drug binds weakly to a metabolizing enzyme, meaning Km is high,
the ability of liver to clear the drug is low. In many drug–drug interaction situations,
the Km is increased for the victim drug which results in reduction of intrinsic clearance.
Now, drug interactions resulting from drug metabolism inhibition are common because
many drugs are eliminated primarily through CYP450-mediated metabolism and
they might compete for the same CYP450 active site. As previously mentioned,
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.
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. This competition is determined by the
affinities of the two substrates for the binding site and their concentrations near the enzyme.
This situation is often encountered in clinical practice. Under a competitive inhibition
condition, a substrate with strong affinity for an enzyme, Drug A in this example, acting
as a perpetrator can displace a weaker substrate form the active site, Drug B, acting as a victim.
As long as the concentrations of the two substrates are comparable, Drug A with higher
binding affinity will be preferred at the active site resulting in an accumulation of Drug B. This
will result in increased Km of the victim drug and reduced intrinsic clearance. For an active drug,
the 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. However, for prodrugs, this interaction can instead result
in a decrease in the formation of the active metabolite, thus reducing drug efficacy.
In an example illustrating this scenario, Theophylline, which is a weak CYP1A2 substrate,
is largely metabolized by CYP1A2 by binding to its 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. Now, because the competitive inhibition mechanism is sensitive to substrate
concentrations, it is considered reversible. So, 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.
Clinically, this situation can be observed when the plasma concentration of one drug is
much greater than that of the second drug. In this case, 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
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.
All right, moving on to the non-competitive inhibition. Because 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. As a result, 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.
However, unlike in competitive inhibition, separating the time of dosing will not
alleviate non-competitive inhibition. In an example illustrating drug-drug
interaction due to non-competitive inhibition, Fluconazole binds to an allosteric site
of CYP2C9 causing the active site to lose its affinity for Carvedilol. Therefore, 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.
Now, in addition to competitive and non-competitive inhibition, we can have something
called a mixed inhibition. In the case of 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.
An example of this type of inhibition is drug-drug interaction between Midazolam,
which is a CYP3A substrate and Ketoconazole, which is a mixed CYP3A inhibitor.
If Ketoconazole occupies active site or allosteric site, Midazolam can’t
be metabolized and accumulates in plasma leading to increased risk of side effects.
Now, lastly lets move on to irreversible inhibition also known as mechanism-based
inhibition. So, irreversible inhibition can be classified into two types:
The first type, called alternate substrate inhibition occurs when 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. However, in in vivo conditions, with this metabolic
intermediate complex being excessively stable, the metabolic intermediate cannot be displaced and the
enzyme remains inaccessible for metabolism so the reaction is technically irreversible. An example
of this type of inhibition can be observed with Calrithromycin when the nitrosoalkene
intermediate generated by N-demethylation forms covalent bonds with the active site of CYP3A4.
Moving on to the second type of irreversible inhibition that is suicide inhibition.
This situation takes place when 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. For example, inhibition of CYP2C19 by Esomeprazole was found to be mediated
by 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. And with that I wanted to thank you for watching,
I hope you enjoyed this video and as always, stay tuned for more.