Background: HAART has brought about a significant impact on the AIDS epidemic, however, eradication of HIV-1 is currently impossible. The emergence of drug-resistant HIV-1 variants and a number of inherent adverse effects exacerbates the limited efficacy of antiviral therapy of AIDS. Approximately 1% of Caucasians have a gene encoding a mutant form of CCR5 termed D32, known to contribute to their resistance to HIV-1 infection, which renders CCR5 an attractive target for possible intervention of HIV-1 infection.

Methods: We employed 2 methods in the search of lead compounds: a chemokine binding assay and assays for inhibition of cytosolic Ca²⁺ mobilization. Anti-HIV-1 activity of the compounds was determined using the MAGI assay employing CCR5+ MAGI cells and p24 assay using PBM.

Results: Low molecular weight spirodiketopiperazine derivatives which potently inhibit R5 HIV-1 were identified.  One such compound E913 (Mr. 484) specifically blocked the binding of MIP-1 to CCR5 (IC₅₀: 0.002 M) and MIP-1-elicited cellular Ca²⁺ mobilization (IC₅₀: ~0.02 M).  E913 potently inhibited the replication of laboratory and primary R5 HIV-1 strains as well as various multi-drug resistant monocyte/macrophage-tropic (R5) HIV-1 at IC₅₀ values of 0.03 - 0.06 M.  E913 was inactive against T cell-tropic (X4) HIV-1; however, when combined with a CXCR4 antagonist AMD-3100, E913 potently and synergistically inhibited the replication of dualtropic HIV-1 and a 50:50 mixture of R5 and X4 HIV-1.  E913 competed with the binding of antibodies to CCR5 that recognize the C-terminal half of the second extracellular loop (ECL2B) of CCR5.  E913 and its analogs proved to be acid-resistant and orally bioavailable in rodents.

Conclusions: These data warrant that spirodiketopiperazine derivatives are further developed as potential therapeutics for HIV-1 infection.