In summary, DLV has been well-tolerated in > 1,000 HIV-1 infected patients. Skin rash is the most prevalent medical event associated with DLV therapy. The rash can be successfully dosed through or rechallenged in > 85% of patients. The pharmacokinetics are non-linear as DLV is metabolized primarily by cytochrome P4503A in the liver. Serum levels of DLV +/- 10 microM can easily be achieved in most HIV-1 patients, which are 100 fold above the in vitro IC90 activity. In clinical trials, DLV inhibits viral replication as demonstrated by positive surrogate marker responses (CD4 counts, P24 antigen concentration, PMBC and plasma virus titers, and plasma HIV RNA concentration). Susceptibility of HIV-1 strains to DLV decrease over time in a majority of subjects in which virus can be cultured. However, HIV strains from about 80% of subjects had a DLV IC50 < 10 microM (the trough DLV concentration in plasma) throughout the Upjohn trial. In HIV strains from about 20% of subjects, susceptibility to DLV remained unchanged in the first 8 months of DLV combination therapy, or HIV-1 was not recovered at all or most timepoints. In contrast, development of resistance to nevirapine or L-697,661 monotherapy or combination therapy with ZDV has occurred in the first eight weeks of therapy. The most common genotypic mutations seen to date are K103N and P236L. The clinical significance of the phenotypic, genotypic and surrogate marker changes associated with DLV remain to be elucidated. The surrogate marker responses in clinical trials suggest that DLV has clinical synergy with ZDV +/- ddI as evidenced by a better and more sustained surrogate marker response when a subject is sensitive to or naive to the nucleoside RTI combined with DLV. Future therapy with DLV will likely be in combination with one or more nucleoside or non-nucleoside RTIs, protease inhibitors and/or immunomodulatory agents.