What Are Protease Inhibitors?

Medically Reviewed by Jonathan E. Kaplan, MD on May 22, 2023
3 min read

The term “protease inhibitor” refers to a class of medications rather than to a single medication. Specifically, they are antiviral drugs. But unlike the many, more common antiviral drugs that can be used to fight against viruses such as flus, protease inhibitors can also be used to help combat HIV (human immunodeficiency virus)

They were originally part of ART therapy, but are now used mainly when patients fail first-line therapies,. 

Currently,  these protease inhibtors are included in the current DHHS (NIH/CDC) HIV treatment guidelines:

  • Atazanavir
  • Darunavir 
  • Indinavir
  • Lopinavir
  • Ritonavir

They all have the same goal of blocking specific proteolytic enzymes (proteases) in the body, although each of these drugs targets a different compound. 

And what is a protease anyway? On their own, proteases aren’t bad. They are basically proteins that are used to break down other certain chemical structures of protein in your body — a process that can help with digestion or healing wounds.

However, proteases are also necessary for certain conditions — including HIV — to thrive.  ‌Protease inhibitors, which figure among the key drugs used to treat HIV, work by binding to proteolytic enzymes (proteases). That blocks their ability to function. 

Protease inhibitors don’t cure HIV. But by blocking proteases, they can stop HIV from reproducing itself. As such, they lower the body’s viral load — a term that refers to the amount of HIV in the body — and slow the progression of HIV.  

If you are HIV-positive, you should add treatment with protease inhibitor drugs — if your doctor agrees — to your plan of care as soon as you can. By doing so, you’ll increase your chances of keeping HIV from developing into AIDS, a potentially life-threatening condition. Treatment can greatly improve outcomes and may even allow you to enjoy a normal life expectancy. 

Protease inhibitors aren’t used on their own to treat HIV; they’re used together with other treatments. They are used mainly when patients fail first-line therapies.

A method called ART (antiretroviral therapy) is an effective and widely accepted treatment method for HIV and AIDS. The effectiveness of the ART method is based on the use of three or more medications simultaneously to treat HIV. The medications it relies on most heavily are protease inhibitors, along with other therapies. 

Although the ART method for treating HIV is aggressive in nature, it has a proven track record in the prevention of HIV-related deaths. It has even worked to reduce HIV viral loads to undetectable levels. 

What does this mean for HIV patients? If you had HIV, then having an undetectable viral load would not mean that you are cured. It would mean, however, that a standard blood test wouldn’t trace the HIV in your system.

That’s good news because having undetectable levels of HIV in your system is a sign that you cannot pass HIV on to sexual partners. You may even have a reduced risk of passing HIV on to a child during pregnancy or while breastfeeding. 

Fighting HIV has been the most common use of protease inhibitors. But in recent years, medical professionals have begun to look to protease inhibitors to combat other conditions. These conditions include fungal infections, other viral infections, parasites, and certain cancers.

But not many protease inhibitors currently work against a variety of conditions. Researchers lack some of the materials needed for making them, and some existing protease inhibitors are not very stable‌.

Confirmed potential side effects of protease inhibitors are:

  • Insulin resistance
  • Nausea and diarrhea
  • Development of gallstones or kidney stones
  • Changes in how things taste
  • Insomnia
  • Elevated numbers in liver function tests
  • Rash or dry skin
  • Elevated cholesterol
  • Negative interactions with other medications

You may also hear of other, as yet unproven, side effects of protease inhibitors. Patients have discussed them in chat forums or with doctors, but scientific studies have not directly connected them with the medication.  Among these potential side effects are: 

  • New or worsening cases of diabetes 
  • New or worsening cases of hyperglycemia, or too much sugar in the blood 
  • Spontaneous bleeding
  • Changes in body composition

Not all protease inhibitors cause the same potential side effects. Instead, different side effects are linked to different branches of protease inhibitors. Still, the intensity of protease inhibitor side effects is one major reason they are not used more often for other types of medical ailments.