This study will compare the efficacy and safety of ATV/r at either 200/100 mg or 300/100mg given daily in Thai patients in combination with 2NRTIs.

To demonstrate non-inferiority of treatment with atazanavir/ritonavir (ATV/r) 200/100 mg once daily (OD) compared to the control group (ATV/r 300/100 mg OD) in regards to the proportion of virologic responders (plasma HIV RNA < 200 copies/mL) at 48 weeks in ARV-experienced HIV-1 infected subjects.

Inclusion Criteria:

1. HIV infected adults aged more than or equal to 18 years
2. Received ritonavir boosted PI-based HAART for >3 months prior screening visit
3. History of HIV RNA < 50 copies/ml within 12 months prior to screening visit
4. HIV-RNA < 50 copies/ml at screening visit
5. Signed written informed consent

Exclusion Criteria:

1. Active AIDS-defining disease or active opportunistic infection
2. History of virological failure (plasma HIV-RNA ≥1,000 copies/ml) while using any ritonavir boosted PI-based HAART
3. Pregnancy or lactation at screening visit
4. Relevant history or current conditions or illnesses that might interfere with drug absorption, distribution, metabolism or excretion e.g. chronic diarrhea, malabsorption
5. Use of concomitant medication that may interfere with the pharmacokinetics of the study drugs e.g. rifampicin, proton pump inhibitor
6. History of sensitivity/idiosyncrasy to the drug or chemically related compounds which may be employed in the study
7. ALT ≥200 IU/L at screening visit
8. Creatinine clearance < 60 c.c. per min by Cockroft-Gault formula at screening visit

study protocol available at www.hivnat.org