3.1.1. Competitive Inhibition

The ability of a single CYP450 isoform to metabolize multiple substrates is responsible for several drug interactions associated with reversible competitive inhibition. Competitive inhibition occurs when two substrates compete for the same active site—such as the prosthetic heme iron or substrate-binding region—of CYP450s. The competition is a function of the respective a ffinities of the two substrates for the binding site and their concentrations in the proximity of the enzyme. First, the most clinically relevant situation will be discussed.

#### Two Substrates with Di fferent A ffinities Administered Concomitantly

This situation is often encountered in clinical practice. Under a competitive inhibition condition, a substrate with strong a ffinity (acting as a perpetrator) can displace a weaker substrate (behaving as a victim) from the active site (Figure 2), thus increasing the *Km* of the victim drug (decreased a ffinity) and reducing the extent of its breakdown (decrease in its *CLint*) over a period of time.

**Figure 2.** Illustration of reversible competitive inhibition where ligand A (**orange**) is a substrate with strong a ffinity and ligand B (**yellow**) is a substrate with a weaker a ffinity for a specific enzyme (**purple**). As long as the concentrations of the two substrates are comparable, the stronger a ffinity substrate with higher binding a ffinity will be preferred at the active site of the enzyme resulting in an accumulation of ligand B.

For an active drug, the decrease in the *CLint* of one of its metabolic pathways can lead to a decrease in the total clearance of the drug (capacity for eliminating the drug) and can result in increased plasma concentrations, potentially precipitating adverse e ffects. However, for prodrugs, this interaction can instead result in a decrease in the formation of the active metabolite, reducing drug e fficacy. The magnitude of changes observed in the overall disposition of the victim drug (increase in its plasma levels) will be a function of the relative contribution of the inhibited metabolic pathway to the clearance of this drug. For example, if 15% of a drug is excreted unchanged in urine—35% by enzyme 1 and 50% by enzyme 2—a 50% decrease in the total *CL* of the victim drug is expected if enzyme 2 is inhibited:

$$CL = CL\_{real} + CL\_{untabic} = 0.15 + 0.85^{\circ}$$

and,

> *CLmetabolic* = *CLenzyme* 1 + *CLenzyme* 2

or,

> 0.85 = 0.35 + 0.5

under conditions of enzyme 2 inhibition (whether it is reversible or irreversible),

$$\text{CL}\_{\text{metabolic}} = \text{CL}\_{\text{enzzyme 1}} + \text{CL}\_{\text{enzzyme 2}}$$

or,

$$0.35 = 0.35 + 0.0$$

and,

> *CL* = *CLrenal* + *CLmetabolic* = 0.15 + 0.35 = 0.5

Since,

$$CL = Dose/ALIC\_{0\text{-}\infty}$$

Under steady-state conditions, the area under the drug concentration curve (*AUC*) measured over a dosing interval (τ) is equal to *AUC*0–<sup>∞</sup>. Since the average concentration over a dosing interval (*Cav*) at steady state can be estimated by *AUC*0-τ/<sup>τ</sup>, the equation could be rearranged in a simpler manner to yield:

$$\mathbb{C}L = Dose/(\mathbb{C}\_{\text{uv}} \times \tau)$$

A 50% decrease in *CL* will be associated with a doubling in the average plasma concentrations of the victim drug.

According to a competitive inhibition mechanism, every substrate of an enzyme is a potential perpetrator drug (inhibitor) towards another substrate metabolized by the same enzyme. Competitive inhibition is almost immediate and the degree of inhibition of the enzyme does not change with time if the concentration of the two substrates remains the same.

In an example illustrating this scenario, theophylline (weak CYP1A2 substrate) is largely metabolized by CYP1A2 by binding to its active site. (Figure 3) If that active site is occupied by a stronger substrate like duloxetine (moderate CYP1A2 affinity substrate), breakdown of theophylline will be reduced (↓*CLint*), leading to increased plasma levels of theophylline and possibly side effects (e.g., headache, nausea, vomiting). To minimize competitive inhibition, two competing substrates should be administered with as much time apart as possible.


**Figure 3.** CYP450 metabolic pathways involved in the metabolism of duloxetine and theophylline and their respective affinities are depicted. Competitive inhibition between duloxetine and theophylline will be expected at CYP1A2. Duloxetine acts as the perpetrator drug over theophylline, the victim drug.

#### Two Substrates with Largely Di fferent Concentrations

As mentioned previously, the competitive inhibition process is sensitive to substrate concentrations. If concentrations of the weaker a ffinity substrate are much higher than concentrations of the stronger affinity substrate, the weaker a ffinity substrate can displace the stronger a ffinity substrate and overcome the enzyme inhibition, which is why this type of inhibition is deemed reversible. (Figure 4) The greater the di fference there is between the a ffinity of the weaker a ffinity substrate and the stronger a ffinity substrate, the more the concentration of the weaker a ffinity substrate needs to be increased to displace the stronger a ffinity substrate. This situation can be observed clinically when very high concentrations of a weak a ffinity substrate are present in the intestine or liver (high micromolar concentrations), following its oral administration, while concentrations of another higher a ffinity substrate have long been absorbed and distributed to various tissues leading to plasma concentrations in the low nanomolar range. In this case, the extent of victim drug inhibition would be minimal. A direct application of this principle is to alleviate the degree of inhibition by separating the time of administration of the two competing drugs.

**Figure 4.** Illustration of reversible competitive inhibition where ligand A (**orange**) is a substrate with strong a ffinity and ligand B (**yellow**) is a substrate with weaker a ffinity for a specific enzyme (**purple**). When the concentrations of the weaker a ffinity substrate are su fficiently high, it can outcompete the stronger a ffinity substrate for the active site of the enzyme.
