This is phase IIIb, randomised (study medication is assigned by chance), open-label (both the patient and the study physician will know to which treatment group the patient is assigned) trial to compare the efficacy, safety and tolerability of darunavir/ritonavir (DRV/rtv) 800/100 mg once daily monotherapy with a triple combination therapy containing DRV/rtv 800/100 mg once daily and an investigator-selected background of 2 other anti-HIV drugs of the class nucleoside/nucleotide reverse transcriptase inhibitors (N\[t\]RTIs). The investigator-selected N\[t\]RTIs is a dual combination of either be abacavir (ABC), lamivudine (3TC), zidovudine (AZT), tenofovir disoproxil fumarate (TDF) or emtricitabine (FTC). Approximately 260 HIV-1 infected patients, who have received HAART for at least 48 weeks, have not changed their treatment within the last 8 weeks, and who have documented evidence of plasma viral load (plasma HIV-1 RNA) below 50 copies/mL for at least 48 weeks prior to being screened, participate in the study. The study period includes a screening period of maximum 6 weeks, a 4-week run-in period, a 96 week treatment period, followed by a 4 weeks follow-up period. According to the original protocol, at the start of the 4-week run-in period all patients replaced their 3rd agent (non-nucleoside reverse transcriptase (NNRTI), protease inhibitor (PI) or integrase inhibitor) of the HAART medication with DRV/rtv and continued with the 2 N\[t\]RTIs. After 4 weeks the patient was randomly assigned (like flipping a coin) to either the monotherapy group or the triple therapy group. If assigned to the monotherapy group, the 2 N\[t\]RTIs were stopped and only DRV/rtv was continued. If assigned to the triple therapy group, DRV/rtv were continued together with 2 N\[t\]RTIs, which can be the same as already taken or are switched to new N\[t\]RTIs. Based on the primary efficacy analysis after Week 48, the protocol was amended such that subjects in the monotherapy arm who entered the study with a nadir CD4+ count of \<200 cells/μL will also receive 2 N\[t\]RTIs (ie, triple therapy) as soon as possible.

The main purpose of this study is to demonstrate that DRV/rtv monotherapy is as effective as a triple combination therapy containing DRV/rtv and 2N\[t\]RTIs. In addition, the study looks at overall safety and tolerability between the two treatment groups. During the study, patients' health are monitored by physical examination, checking of vital signs (blood pressure / pulse), and laboratory testing on blood and urine samples. Also blood samples are drawn to measure the antiviral effectiveness (i.e., decrease of the plasma viral load to a level \<50 HIV-1 RNA copies/mL) and immunology assessments (to assess the body's immune system). A battery of neurocognitive function tests is performed during the study visits. A sub-study takes place in selected hospitals. Approximately 100 patients have 2 additional tests done, at the start of the run-in phase and after 48-weeks of randomisation. For this substudy a lumbar puncture (extraction of cerebrospinal fluid \[CSF\] from the spinal canal) for laboratory testing (antiviral effectiveness, pharmacokinetic analysis, biochemistry and immune markers) and an additional blood sample for pharmacokinetic analysis (to measure the drug level in blood) is taken. The study hypothesis is that, after 48 weeks of randomised treatment, DRV/rtv monotherapy is as effective as the triple therapy containing DRV/rtv plus 2 N\[t\]RTIs. Two 400 mg tablets of darunavir and one 100 mg tablet ritonavir are taken together once daily orally within 30 minutes after completion of a meal, for 100 weeks. The intake of the investigator-selected N\[t\]RTIs as according the local prescribing information.