HIV-Screening Strategies for the Discovery of Novel HIV-Inhibitors

Abstract

Since acquired immunodeficiency syndrome (AIDS) was recognized 27 years ago, 25 million people have died of human immunodeficiency virus (HIV)-related causes. On a global scale, although the HIV epidemic has stabilized since 2000, unacceptably high levels of new HIV infection and AIDS death still occur each year. In 2007, there were an estimated 33 million (30-36 million) people living with HIV, and 2 million (1.8-2.3 million) people died due to AIDS, compared with an estimated 1.7 million (1.5-2.3 million) in 2001. There are two main types of HIV: type 1 (HIV-1) and type 2 (HIV-2) (Buonaguro et al., 2007). HIV-1 is the most prevalent in the worldwide pandemic. HIV-2 is present mainly in West Africa, where it was discovered in 1986, and infects about one million people worldwide. HIV-2 is slowly but continuously spreading throughout Europe, Asia and the Americas, and has reached a significant prevalence in countries such as Portugal and India. After more than 20 years of research, HIV remains a difficult target for a vaccine; thus the treatment of AIDS continues to focus on the search for chemical anti-HIV agents. A working knowledge of the HIV replication cycle is essential for understanding the mechanism of action of antiviral drugs. The HIV is an enveloped virus that contains two copies of viral genomic RNA in its core. In addition to the copies of RNA, the viral core also contains the enzymes required for HIV replication. The first step in the HIV replication cycle is the interaction between the envelope proteins of the virus (gp120) and specific host-cell surface receptors (e.g. the T-cell receptor CD4 on the cellular membrane) of the host cell. In the second step, the virus binds to the chemokine coreceptors CXC-chemokine receptor 4 (CXCR4) and CC-chemokine receptor 5 (CCR5). This induces a conformational change in gp120 that opens up a high affinity binding site located within the third variable loop (V3) and surrounding surfaces for the chemokine coreceptors CXCR4 and CCR5. This gives rise to further conformational rearrangements of gp120 that expose the transmembrane glycoprotein gp41, and the heptad repeat (HR) regions of the three subunits of gp41, HR1 and HR2, fold into a six-helical bundle. This ultimately results in the “fusion” of the viral envelope and the cytoplasmic membrane. Fusion creates a pore through which the viral capsid enters the cells. HIV encodes three enzymes required for replication: HIV-1 reverse transcriptase (RT), HIVintegrase (IN) and HIV-protease (PT). Following entry into the cell, the viral RT enzyme catalyzes the conversion of viral RNA into DNA. This viral DNA enters the nucleus and