HIV protease inhibitors

How protease inhibitors work

In the natural life cycle of the human immunodeficiency virus, a specific enzyme — HIV aspartyl protease — plays a critically important role. Its main task is to cleave long precursor polypeptide chains (gag and pol polyproteins). These chains are synthesized based on viral messenger RNA.

Once the protease cleaves these long molecules, individual structural elements and key functional proteins are formed within the viral particle: reverse transcriptase, integrase, and the protease itself. Without this process, proper assembly and final maturation of a fully functional virion are impossible.

Protease inhibitor drugs act by competitively binding to the active site of this enzyme. As a result, the aspartyl protease is inactivated, the polypeptides remain uncleaved, and defective, "immature" viral particles are formed. Such virions completely lose the ability to infect new host cells.

Agents and pharmacokinetic features

This group of antiretroviral agents was introduced into clinical practice in the mid-1990s. Historically, the first representative was saquinavir. Subsequently, other drugs were synthesized: ritonavir, indinavir, nelfinavir, amprenavir, lopinavir, and atazanavir. For patient convenience, combination forms are available, for example, Kaletra, which is a fixed-dose combination of lopinavir and ritonavir.

Most protease inhibitors share common pharmacokinetic properties. They are characterized by a high degree of plasma protein binding and undergo metabolic inactivation in the liver. The key enzyme responsible for their metabolism is the CYP3A4 isoenzyme. At the same time, these drugs are capable of actively inhibiting this isoenzyme, which determines their high propensity for drug interactions.

The standard dosing regimen for such drugs requires frequent administration — three times a day or even more often. Absorption in the gastrointestinal tract is significantly affected by food. As a general rule, food intake improves the bioavailability of most protease inhibitors. In particular, nelfinavir is strongly recommended to be taken with fatty food. However, there is an important exception to this rule — indinavir. The presence of food, especially fatty food, catastrophically decreases its absorption (by 75%), so this drug must be taken strictly on an empty stomach or with a very light meal.

The "booster" effect of ritonavir

Among all drugs in the class, ritonavir plays a unique role. Its main feature is that it causes the most potent inhibition of the CYP3A4 isoenzyme in both the liver and the intestinal wall. Due to this property, ritonavir is used today in low doses not so much as an independent antiviral agent, but as a potent pharmacokinetic enhancer (the so-called "booster" effect) in combination with other protease inhibitors.

The mechanism of this enhancement lies in reducing the presystemic elimination of the primary drug. Ritonavir slows down the metabolism of the concomitant medication, leading to a significant increase in its bioavailability and a prolongation of its therapeutic effect. This is especially critical for drugs with a short half-life. The clinical benefit is obvious: combination with ritonavir allows reducing the doses of other antiretrovirals and decreasing the frequency of their administration to once or twice a day.

The only exception to this approach is nelfinavir. It is metabolized primarily by other isoenzymes (CYP3A4 contribution is small) and forms its own active metabolite during biotransformation. For this reason, nelfinavir is never prescribed together with ritonavir.

Rules of administration and adverse reactions

Protease inhibitors are widely used to treat HIV infection in both adult patients and children. A mandatory condition for their use is prescription as part of combination antiretroviral therapy — they are always used concomitantly with reverse transcriptase inhibitors.

The safety profile of these drugs includes a number of characteristic side effects. Patients most frequently encounter gastrointestinal issues: abdominal pain, diarrhea, nausea, and vomiting. Fortunately, these symptoms are usually transient and completely resolve about a month after therapy starts.

However, with long-term administration, serious metabolic and organ disorders come to the forefront:

• Lipid metabolism: an increase in blood levels of atherogenic lipoproteins is observed.
• Carbohydrate metabolism: hyperglycemia develops, which significantly increases the risk of diabetes mellitus.
• Fat tissue distribution: lipodystrophy develops (specific redistribution of fat in the body).
• Kidneys: the risk of nephrolithiasis (kidney stone formation) increases.
• Liver: hepatotoxicity manifests, requiring monitoring of organ function.

Class and exam questions

What else to study on this topic

• The role of gag and pol polyproteins in the HIV life cycle
• Mechanisms of lipodystrophy development during antiretroviral therapy
• The combination drug Kaletra: pharmacokinetic advantages
• Drug interactions at the CYP3A4 isoenzyme level
• Reverse transcriptase inhibitors (the backbone of combination therapy)

Other topics in the "Antivirals" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.