TAK-220 is a CCR5 antagonist, part of the new class of anti-human immunodeficiency virus type 1 (anti-HIV-1) entry inhibitors. We evaluated the anti-HIV-1 interactions between TAK-220 and various antiretrovirals in vitro. Synergy was observed with all drugs at the 90 and 95% inhibitory concentrations. The favorable drug interactions observed suggest that further clinical evaluation is warranted.New antiretroviral drug regimens are needed, especially for subjects who have failed two or three previous regimens (17). Among potential sites in the human immunodeficiency virus type 1 (HIV-1) replicative cycle, HIV-1 attachment and entry is a particularly promising target (3, 10).TAK-220 is a small molecule that targets the binding of gp120 to the CCR5 coreceptor, is orally bioavailable, and has potent anti-HIV-1 activity in vitro at nanomolar concentrations (9). As with other classes of antiretroviral drugs, attachment-entry inhibitors are likely to be used in combination with other antiretrovirals. Combinations have many potential advantages over monotherapy, including a delay in the emergence of resistant viral variants, increased potency, and broadened coverage against variants that exist in the population (15). Previous studies have shown that the use of combinations in vitro may lead to various outcomes, ranging from synergy to antagonism, and that these outcomes may be predictive of subsequent clinical results (8,(11)(12)(13)(14). Therefore, we studied TAK-220 in combination with various other antiviral compounds in vitro.Peripheral blood mononuclear cells (PBMCs) from HIV-1-seronegative donors were obtained by Ficoll-Hypaque density gradient centrifugation of heparinized venous blood. After 3-day photohemagglutinin assay stimulation, PBMCs were resuspended at a concentration of 1 ϫ 10 6 cells/ml in RPMI 1640 culture medium (Sigma, St Louis, MO) supplemented with 20% heat-inactivated fetal calf serum (Sigma), penicillin (50 U/ml), streptomycin (50 g/ml), L-glutamine (2 mM), HEPES buffer (10 mM), and 10% interleukin-2 in 24-well tissue culture plates (Becton Dickinson, San Jose, CA). Single drugs or combinations of two or three drugs were added to each well by using a fixed ratio among the drugs and the serial dilutions. The drugs were dissolved by using dimethyl sulfoxide for TAK-220 and efavirenz and phosphate-buffered saline for the other drugs. They were added simultaneously with the HIV-1 inoculum (800 to 1,000 50% tissue culture infective doses/10 6 cells), and the plates were incubated at 37°C in a humidified 5% CO 2 atmosphere. Each condition was tested in duplicate, and each experiment was repeated at least twice. Cell-free culture supernatants were harvested and analyzed by an enzyme-linked immunosorbent assay (Du Pont, Wilmington, DE) for HIV-1 p24 antigen production on day 7 of culture. In addition, uninfected drug-treated cytotoxicity controls were maintained at the highest concentration of each agent tested, singly or in combination. No toxicity was observed at concentrations up to 10 M. Cell prolif...