Mechanism of Action

Atazanavir is an azapeptide HIV-1 protease inhibitor (PI). The compound selectively inhibits the virus-specific processing of viral Gag-Pol proteins in HIV-1 infected cells, thus preventing formation of mature virions and infection of other cells.

Antiviral activity in vitro: Atazanavir exhibits anti-HIV-1 (including all clades tested) and anti-HIV-2 activity in cell culture.

Pharmacodynamic Properties

Resistance

Antiretroviral treatment naive adult patients

In clinical trials of antiretroviral treatment naive patients treated with unboosted atazanavir, the I50L substitution, sometimes in combination with an A71V change, is the signature resistance substitution for atazanavir. Resistance levels to atazanavir ranged from 3.5- to 29-fold without evidence of phenotypic cross resistance to other PIs. In clinical trials of antiretroviral treatment naive patients treated with boosted atazanavir, the I50L substitution did not emerge in any patient without baseline PI substitutions. The N88S substitution has been rarely observed in patients with virologic failure on atazanavir (with or without ritonavir). While it may contribute to decreased susceptibility to atazanavir when it occurs with other protease substitutions, in clinical studies N88S by itself does not always lead to phenotypic resistance to atazanavir or have a consistent impact on clinical efficacy.

Table 1. De novo substitutions in treatment naive patients failing therapy with atazanavir + ritonavir (Study 138, 96 weeks):

Frequencyde novo PI substitution (n=26)a
>20% none
10-20% none

a Number of patients with paired genotypes classified as virological failures (HIV RNA ≥400 copies/ml).

The M184I/V substitution emerged in 5/26 atazanavir/ritonavir and 7/26 lopinavir/ritonavir virologic failure patients, respectively.

Antiretroviral treatment experienced adult patients

In antiretroviral treatment experienced patients from Studies 009, 043, and 045, 100 isolates from patients designated as virological failures on therapy that included either atazanavir, atazanavir + ritonavir, or atazanavir + saquinavir were determined to have developed resistance to atazanavir. Of the 60 isolates from patients treated with either atazanavir or atazanavir + ritonavir, 18 (30%) displayed the I50L phenotype previously described in naive patients.

Table 2. De novo substitutions in treatment experienced patients failing therapy with atazanavir + ritonavir (Study 045, 48 weeks):

Frequency de novo PI substitution (n=35)a,b
>20% M36, M46, I54, A71, V82
10-20% L10, I15, K20, V32, E35, S37, F53, I62, G73, I84, L90

a Number of patients with paired genotypes classified as virological failures (HIV RNA ≥400 copies/ml).
b Ten patients had baseline phenotypic resistance to atazanavir + ritonavir (fold change [FC]>5.2). FC susceptibility in cell culture relative to the wild-type reference was assayed using PhenoSenseTM (Monogram Biosciences, South San Francisco, California, USA)

None of the de novo substitutions (see Table 2) are specific to atazanavir and may reflect re-emergence of archived resistance on atazanavir + ritonavir in Study 045 treatment-experienced population.

The resistance in antiretroviral treatment experienced patients mainly occurs by accumulation of the major and minor resistance substitutions described previously to be involved in protease inhibitor resistance.

Pharmacokinetic Properties

The pharmacokinetics of atazanavir were evaluated in healthy adult volunteers and in HIV-infected patients; significant differences were observed between the two groups. The pharmacokinetics of atazanavir exhibit a non-linear disposition.

Absorption

In HIV-infected patients (n=33, combined studies), multiple dosing of atazanavir 300 mg once daily with ritonavir 100 mg once daily with food produced a geometric mean (CV%) for atazanavir, Cmax of 4466 (42%) ng/ml, with time to Cmax of approximately 2.5 hours. The geometric mean (CV%) for atazanavir Cmin and AUC was 654 (76%) ng/ml and 44185 (51%) ng•h/ml, respectively.

In HIV-infected patients (n=13), multiple dosing of atazanavir 400 mg (without ritonavir) once daily with food produced a geometric mean (CV%) for atazanavir Cmax of 2298 (71) ng/ml, with time to Cmax of approximately 2.0 hours. The geometric mean (CV%) for atazanavir Cmin and AUC were 120 (109) ng/ml and 14874 (91) ng•h/ml, respectively.

Food effect

Co-administration of atazanavir and ritonavir with food optimises the bioavailability of atazanavir. Co-administration of a single 300 mg dose of atazanavir and 100 mg dose of ritonavir with a light meal resulted in a 33% increase in the AUC and a 40% increase in both the Cmax and the 24 hour concentration of atazanavir relative to the fasting state. Co-administration with a high-fat meal did not affect the AUC of atazanavir relative to fasting conditions and the Cmax was within 11% of fasting values. The 24 hour concentration following a high fat meal was increased by approximately 33% due to delayed absorption; the median Tmax increased from 2.0 to 5.0 hours. Administration of atazanavir with ritonavir with either a light or a high-fat meal decreased the coefficient of variation of AUC and Cmax by approximately 25% compared to the fasting state. To enhance bioavailability and minimise variability, atazanavir is to be taken with food.

Distribution

Atazanavir was approximately 86% bound to human serum proteins over a concentration range of 100 to 10,000 ng/ml. Atazanavir binds to both alpha-1-acid glycoprotein (AAG) and albumin to a similar extent (89% and 86%, respectively, at 1,000 ng/ml). In a multiple-dose study in HIVinfected patients dosed with 400 mg of atazanavir once daily with a light meal for 12 weeks, atazanavir was detected in the cerebrospinal fluid and semen.

Metabolism

Studies in humans and in vitro studies using human liver microsomes have demonstrated that atazanavir is principally metabolised by CYP3A4 isozyme to oxygenated metabolites. Metabolites are then excreted in the bile as either free or glucuronidated metabolites. Additional minor metabolic pathways consist of N-dealkylation and hydrolysis. Two minor metabolites of atazanavir in plasma have been characterised. Neither metabolite demonstrated in vitro antiviral activity.

Elimination

Following a single 400 mg dose of 14C-atazanavir, 79% and 13% of the total radioactivity was recovered in the faeces and urine, respectively. Unchanged drug accounted for approximately 20% and 7% of the administered dose in the faeces and urine, respectively. Mean urinary excretion of unchanged drug was 7% following 2 weeks of dosing at 800 mg once daily. In HIV-infected adult patients (n=33, combined studies) the mean half-life within a dosing interval for atazanavir was 12 hours at steady state following a dose of 300 mg daily with ritonavir 100 mg once daily with a light meal.

Special populations

Renal impairment

In healthy subjects, the renal elimination of unchanged atazanavir was approximately 7% of the administered dose. There are no pharmacokinetic data available for atazanavir with ritonavir in patients with renal insufficiency. Atazanavir (without ritonavir) has been studied in adult patients with severe renal impairment (n=20), including those on haemodialysis, at multiple doses of 400 mg once daily. Although this study presented some limitations (i.e. unbound drug concentrations not studied), results suggested that the atazanavir pharmacokinetic parameters were decreased by 30% to 50% in patients undergoing haemodialysis compared to patients with normal renal function. The mechanism of this decrease is unknown.

Hepatic impairment

Atazanavir is metabolised and eliminated primarily by the liver. Atazanavir (without ritonavir) has been studied in adult subjects with moderate-to-severe hepatic impairment (14 Child-Pugh Class B and 2 Child-Pugh Class C subjects) after a single 400 mg dose. The mean AUC(0-∞) was 42% greater in subjects with impaired hepatic function than in healthy subjects. The mean half-life of atazanavir in hepatically impaired subjects was 12.1 hours compared to 6.4 hours in healthy subjects. The effects of hepatic impairment on the pharmacokinetics of atazanavir after a 300 mg dose with ritonavir have not been studied. Concentrations of atazanavir with or without ritonavir are expected to be increased in patients with moderately or severely impaired hepatic function.

Age/Gender

A study of the pharmacokinetics of atazanavir was performed in 59 healthy male and female subjects (29 young, 30 elderly). There were no clinically important pharmacokinetic differences based on age or gender.

Race

A population pharmacokinetic analysis of samples from Phase II clinical trials indicated no effect of race on the pharmacokinetics of atazanavir.

Pregnancy

The pharmacokinetic data from HIV-infected pregnant women receiving atazanavir capsules with ritonavir are presented in Table 8.

Table 8. Steady-State Pharmacokinetics of Atazanavir with ritonavir in HIV-Infected Pregnant Women in the Fed State:

 atazanavir 300 mg with ritonavir 100 mg
Pharmacokinetic Parameter2nd Trimester (n=9)3rd Trimester (n=20)postpartuma (n=36)
Cmax ng/mL3729.09 3291.46 5649.10
Geometric mean (CV%) (39) (48) (31)
AUC ng•h/mL34399.1 34251.5 60532.7
Geometric mean (CV%) (37) (43) (33)
Cmin ng/mLb663.78 668.48 1420.64
Geometric mean (CV%) (36) (50) (47)

a Atazanavir peak concentrations and AUCs were found to be approximately 26-40% higher during the postpartum period (4-12 weeks) than those observed historically in HIV infected, non-pregnant patients. Atazanavir plasma trough concentrations were approximately 2-fold higher during the postpartum period when compared to those observed historically in HIV infected non-pregnant patients.
b Cmin is concentration 24 hours post-dose.

Paediatric population

There is a trend toward a higher clearance in younger children when normalised for body weight. As a result, greater peak to trough ratios are observed, however at recommended doses, geometric mean atazanavir exposures (Cmin, Cmax and AUC) in paediatric patients are expected to be similar to those observed in adults.

Preclinical Safety Data

In repeat-dose toxicity studies, conducted in mice, rats, and dogs, atazanavir-related findings were generally confined to the liver and included generally minimal to mild increases in serum bilirubin and liver enzymes, hepatocellular vacuolation and hypertrophy, and, in female mice only, hepatic singlecell necrosis. Systemic exposures of atazanavir in mice (males), rats, and dogs at doses associated with hepatic changes were at least equal to that observed in humans given 400 mg once daily. In female mice, atazanavir exposure at a dose that produced single-cell necrosis was 12 times the exposure in humans given 400 mg once daily. Serum cholesterol and glucose were minimally to mildly increased in rats but not in mice or dogs.

During in vitro studies, cloned human cardiac potassium channel (hERG), was inhibited by 15% at a concentration (30 μM) of atazanavir corresponding to 30 fold the free drug concentration at Cmax in humans. Similar concentrations of atazanavir increased by 13% the action potential duration (APD90) in rabbit Purkinje fibres study. Electrocardiographic changes (sinus bradycardia, prolongation of PR interval, prolongation of QT interval, and prolongation of QRS complex) were observed only in an initial 2 week oral toxicity study performed in dogs. Subsequent 9 month oral toxicity studies in dogs showed no drug-related electrocardiographic changes. The clinical relevance of these non-clinical data is unknown. Potential cardiac effects of this product in humans cannot be ruled out. The potential for PR prolongation should be considered in cases of overdose.

In a fertility and early embryonic development study in rats, atazanavir altered oestrus cycling with no effects on mating or fertility. No teratogenic effects were observed in rats or rabbits at maternally toxic doses. In pregnant rabbits, gross lesions of the stomach and intestines were observed in dead or moribund does at maternal doses 2 and 4 times the highest dose administered in the definitive embryodevelopment study. In the pre- and postnatal development assessment in rats, atazanavir produced a transient reduction in body weight in the offspring at a maternally toxic dose. Systemic exposure to atazanavir at doses that resulted in maternal toxicity was at least equal to or slightly greater than that observed in humans given 400 mg once daily.

Atazanavir was negative in an Ames reverse-mutation assay but did induce chromosomal aberrations in vitro in both the absence and presence of metabolic activation. In in vivo studies in rats, atazanavir did not induce micronuclei in bone marrow, DNA damage in duodenum (comet assay), or unscheduled DNA repair in liver at plasma and tissue concentrations exceeding those that were clastogenic in vitro.

In long-term carcinogenicity studies of atazanavir in mice and rats, an increased incidence of benign hepatic adenomas was seen in female mice only. The increased incidence of benign hepatic adenomas in female mice was likely secondary to cytotoxic liver changes manifested by single-cell necrosis and is considered to have no relevance for humans at intended therapeutic exposures. There were no tumorigenic findings in male mice or in rats.

Atazanavir increased opacity of bovine corneas in an in vitro ocular irritation study, indicating it may be an ocular irritant upon direct contact with the eye.

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