Atazanavir

BRAND NAMES

Reyataz – This is the most widely known brand name for atazanavir. 

Evotaz – This is a combination of atazanavir and cobicistat (a pharmacokinetic enhancer).

MECHANISM OF ACTION 

Atazanavir is an antiretroviral drug belonging to the class of protease inhibitors. It works by selectively inhibiting the HIV-1 protease enzyme, which is crucial for processing viral polyproteins into functional proteins needed for the assembly of mature, infectious virus particles. By binding to the active site of the protease, atazanavir blocks the cleavage of the Gag and Gag-Pol polyproteins, resulting in the production of immature, non-infectious viral particles. This interference with viral maturation effectively reduces HIV replication and lowers the viral load in patients. To enhance its effectiveness, atazanavir is often administered with pharmacokinetic boosters such as ritonavir or cobicistat, which increase its plasma concentration.

PHARMACOKINETICS

Absorption 

Atazanavir is orally administered and is absorbed through the gastrointestinal tract. Its absorption is pH-dependent, meaning that stomach acidity plays a critical role in how much of the drug enters the bloodstream. Peak plasma concentrations are usually reached 2–3 hours after oral administration.

Distribution 

Atazanavir is highly protein-bound (≈86%) and distributes mainly into body fluids and tissues, including lymphoid tissues where HIV replicates. Its penetration into the cerebrospinal fluid is limited, and it has a moderate volume of distribution (0.3–0.5 L/kg), allowing effective systemic and tissue concentrations.

Metabolism 

Atazanavir is mainly metabolized in the liver by CYP3A4 into inactive metabolites. It is often boosted withritonavir or cobicistat to increase plasma levels and effectiveness.

Elimination 

Atazanavir is primarily eliminated through feces (≈79%), with a smaller portion excreted in urine (≈13%). Its elimination is mainly via hepatic metabolism, and the drug has a half-life of about 7 hours when unboosted, which can be prolonged with ritonavir or cobicistat.

PHARMACODYNAMICS 

Atazanavir is a protease inhibitor that acts by selectively inhibiting the HIV-1 protease enzyme, preventing the cleavage of viral polyproteins Gag and Gag-Pol into functional proteins. This inhibition blocks the maturation of viral particles, resulting in the production of immature, non-infectious HIV virions. By reducing the number of infectious viral particles, atazanavir effectively lowers viral load and helps restore CD4+ T-cell counts, improving immune function in patients with HIV infection. Its antiviral activity is dose-dependent and enhanced when combined with pharmacokinetic boosters like ritonavir or cobicistat.

ADMINISTRATION

Atazanavir is administered orally in the form of capsules or tablets and is usually taken once daily with food to enhance absorption. It is often co-administered with ritonavir or cobicistat to boost plasma levels and improve antiviral effectiveness. Drugs that increase stomach pH, such as proton pump inhibitors or antacids, can reduce its absorption, so care should be taken with timing or alternative medications. The capsules or tablets should be swallowed whole and not crushed or chewed to ensure proper bioavailability.

DOSAGE AND STRENGTH 

Atazanavir is available in capsule strengths of 100 mg, 150 mg, 200 mg, and 300 mg, and in 300 mg tablets. For adults, the usual dose is 400 mg once daily with food when given unboosted, or 300 mg once daily with 100 mg ritonavir or 150 mg cobicistat to enhance its effectiveness. Dose adjustments may be necessary in patients with hepatic impairment, and pediatric dosing is determined by weight and age under specialist guidance.

DRUG INTERACTIONS 

Atazanavir interacts with drugs that affect CYP3A4 or stomach acidity. CYP3A4 inhibitors can increase its levels, while inducers can reduce effectiveness. Acid-reducing agents like proton pump inhibitors and antacids may lower absorption. It can also interact with certain antiretrovirals, statins, and QT-prolonging drugs, requiring monitoring or dose adjustments.

FOOD INTERACTIONS 

Atazanavir absorption is improved when taken with food, so it should always be taken with a meal. High-fat meals do not significantly affect absorption, but taking it on an empty stomach may reduce drug levels. Additionally, foods or supplements that raise stomach pH, such as antacids, should be spaced apart from dosing to avoid reduced absorption.

CONTRAINDICATIONS 

Atazanavir is contraindicated in patients with a known hypersensitivity to the drug or any of its components and in those with moderate to severe hepatic impairment, as liver metabolism is essential for its clearance. It should not be used with certain drugs that are highly dependent on CYP3A4 metabolism, such as ergot derivatives, some sedatives, or cisapride, due to the risk of serious toxicity. Caution is also advised in patients with pre-existing heart conduction abnormalities or a history of hyperbilirubinemia, as atazanavir can prolong the PR interval and increase bilirubin levels.

SIDE EFFECTS 

Common:

• Jaundice / elevated bilirubin

• Nausea and vomiting

• Diarrhea

• Abdominal pain

• Rash

• Headache

• Fatigue

• Insomnia

Less common / serious:

• PR interval prolongation (heart conduction issues)

• Hepatotoxicity

• Kidney stones (nephrolithiasis)

• Hyperlipidemia

TOXICITY 

Atazanavir toxicity can occur due to overdose, prolonged use, or interactions with other drugs. The most common toxic effect is hyperbilirubinemia, which may cause jaundice but is usually reversible and not harmful. It can also cause hepatotoxicity, especially in patients with pre-existing liver disease, and cardiac effects such as PR interval prolongation, which may lead to heart block in susceptible individuals. Rarely, it can cause kidney stones due to drug crystallization in urine. Severe gastrointestinal symptoms, including nausea, vomiting, and diarrhea, may also occur. Supportive care is the primary treatment, as hemodialysis is ineffective due to the drug’s high protein binding.