Efficacy of Atazanavir/Ritonavir Monotherapy as Maintenance in Patients With Viral Suppression

This is a randomised (1:1), multicentre, comparative, parallel-group, prospective, open label, non-inferiority controlled clinical trial. Enrolled patients, taking an ATV/r based HAART and with stable HIV-RNA \< 50c/ml (24 weeks), will be randomized to: * continue the same regimen ATV/RTV 300mg/100mg OD plus 2 NRTIs (according to the specific dosing schedule) as backbone (HAART arm) with ATV/r * or simplify therapy to ATV/RTV 300mg/100mg OD as monotherapy (Monotherapy arm) with ATV/r The study follow up will be 96 weeks after randomization and primary objective will be evaluated at week 48. Patients will be followed every 4 weeks for the first 16 weeks, and then every 8 weeks until week 48, then every 12 weeks until week 96 or discontinuation ; at each visit the following evaluations will be performed: * clinical assessment. * routine laboratory tests (hematological tests and hematochemistry) including creatinine, phosphorus, calcium, alkaline phosphatase, gammaGT; urine analysis, lipid profile, level of HIV-RNA and CD4 cell counts. During follow-up, at randomization, week 48, week 96 or discontinuation, patients will additionally undergo: * Fat redistribution evaluation by DEXA (dual-energy X-ray absorptiometry * Vertebral and femoral bone mineral density evaluation by DEXA. * ECG; * Glicate haemoglobin. * Adherence assessment (questionnaire and/or pills counts). * Neurocognitive evaluation \[HIV-associated neurocognitive disorders (HANDs) evaluated by validated neuropsychological tests\]. In case of viral rebound (defined as 2 consecutive measurement of HIV-RNA \> 50 c/ml) patients will be immediately contacted in order to perform genotypic tests. Furthermore a plasma PK analysis will also be performed. Any patients with virological rebound will be selected for a reintensification therapy with NRTIs and if not suppressed after 12 weeks they will be discontinued.