ALX 40-4C is a small peptide inhibitor of the chemokine receptor CXCR4, inhibits SDF-1 from binding CXCR4 with a Ki of 1 μM, and suppresses the replication of X4 strains of HIV-1; ALX 40-4C Trifluoroacetate also acts as an antagonist of the APJ receptor, with an IC50 of 2.9 μM. ALX 40-4C is a small peptide inhibitor of the chemokine receptor CXCR4, interacts with the second extracellular loop of CXCR4 and inhibits infection exclusively by blocking direct virus-CXCR4 interactions. ALX 40-4C shows potent anti HIV-1 effect, with EC50s of 0.34 ± 0.04 μg/mL, 0.37 ± 0.01 μg/mL for HIV-1 NL4-3, NC10, and 0.18 ± 0.11 μg/mL, 0.06 ± 0.02 μg/mL for HIV-1 HXB2, HC43, respectively, and with a CC50 (50% cytotoxic concentration) of 21 μg/mL. ALX 40-4C also exhibits potent activity against env-recombinant HIV, with EC50s of 0.38 ± 0.01 μg/mL, 0.40 ± 0.0 μg/mL for HIV-1 NL4-3 env, NC10, and 1.34 ± 0.06 μg/mL, 1.02 ± 0.29 μg/mL for HIV-1 HXB2 env, HC43, and a CC50 of 21 μg/mL. ALX 40-4C binds to APJ with an IC50 of 2.9 μM. ALX 40-4C inhibits HIV-1 gp120/APJ-mediated cell membrane fusion, with an IC50s of 3.41 μM and 3.1 μM for IIIB isolate and 89.6 isolate, respectively.