Inclusion Criteria:

Adult patients (over 18 years of age) of any ethnic group without restricted legal competence and who are capable of following the study instructions HIV-1 infection documented by confirmed positive HIV-1 antibody test and/or positive PCR for HIV-1 RNA Stable (no interruption or change of regimen in last 3 months) AZT-and 3TC-containing regimen > 3 months No documented prior virologic failure (virological failure defined as 2 consecutive measurements 4 weeks apart with viral load of HIV-RNA > 400 copies/mL while on ART) CD4+ counts > 50 cells/µL Viral load < 50 copies/mL Karnofsky performance status ≥ 80% For women with childbearing potential, negative urine pregnancy test at Screening visit

Exclusion Criteria:

Serum phosphate level < 0.65 mmol/L Documented active opportunistic infections Subjects with previously documented K65R, 69S mutations or 3 or more thymidine analogue mutations Documented active malignant disease (excluding Kaposi sarcoma limited to the skin) Female of childbearing potential not willing to use a barrier method of contraception during heterosexual intercourse during the duration of study Women who are pregnant or breast feeding Known history of drug, medication or alcohol abuse within the last 12 months preceding the study Simultaneous participation in another study with an investigational drug or within less than one month prior to this study Inability or not willing to meet the requirements of the protocol History of seizures, central nervous system disorders or psychiatric disability thought to be clinically significant in the opinion of the investigator Limited mental capacity to the extent that he/she cannot provide informed consent or information regarding adverse events of the study medication Renal disease (creatinine clearance < 50 ml/min according to Cockroft_Gault formula) or severe hepatic, respiratory or cardiovascular disease Patients who have previously been enrolled into this study Contraindication for one of the study substances.