Recent Patents on Anti-Infective Drug Discovery, 2006, 1, 67-73 67 1574-891X/06 $100.00+.00 © 2006 Bentham Science Publishers Ltd. An Insight on the Leading HIV Entry Inhibitors Ana Salomé Veiga 1 , Nuno C. Santos 2 and Miguel A. R. B. Castanho* ,1 1 Centro de Química e Bioquímica, Faculdade de Ciências da Universidade de Lisboa, Campo Grande Ed. C8, 1749-016 Lisboa, Portugal; 2 Instituto de Biopatologia Química / Faculdade de Medicina de Lisboa and Unidade de Biopatologia Vascular / Instituto de Medicina Molecular, Av. Prof. Egas Moniz, 1649-028 Lisboa, Portugal Received: July 03, 2005; Accepted: August 17, 2005; Revised: September 14, 2005 Abstract: The main strategies nowadays to fight AIDS rely on chemical therapy to inhibit the reverse transcriptase or protease of HIV. However, a synthetic 36 amino-acids peptide that blocks the entry of the virus in the target cells (enfuvirtide) has recently reached approval for clinical application. This molecule may probably be just the leader of a new generation of drugs that is about to emerge to interrupt the first step in the HIV life cycle, i.e. preventing the virus from actually entering cells. This paper reviews the enfuvirtide path from clinical trials to the attempts to detail its molecular-level mode of action. It is commonly accepted that this peptide would block the fusion between viral and cell plasma membrane through binding to the N-terminal heptad repeat (NHR) region of the viral protein gp41. However, there has been growing evidence that this model of action may be unrealistic, the action of enfuvirtide being more complex and diverse than initially thought. Membrane-assisted local concentration increase and interference with gp120/co-receptor docking may also contribute for the inhibitory action of the peptide. Selected HIV-entry inhibitors on clinical trials are presented to characterize the future drugs in the market in this class. Keywords: Entry, attachment, fusion, binding, entrance inhibitor, HIV, AIDS, virus, chemokine, peptide, receptor, co-receptor, synergism, clinical trial, enfuvirtide. 1. INTRODUCTION Combination therapy with reverse transcriptase and protease inhibitors is the most common current treatment of HIV-1 infection [1,2]. Despite the success of this therapy, namely reducing morbidity and mortality of HIV-1 infected patients [3-6], it has adverse effects and drug resistant HIV-1 strains emerged [7-11]. A new class of antiviral agents is in development, the entry inhibitors. These molecules target the first step in the HIV-1 replication cycle, the viral entry [12- 15]. Unlike reverse transcriptase and protease inhibitors, which target post-entry steps, entry or fusion inhibitors act extracellularly preventing viral entry into target cells. The most advanced and already approved by the Food and Drug Administration fusion inhibitor is enfuvirtide (T20; DP-178) from Trimeris/Roche [16]. HIV-1 Entry The HIV-1 envelope glycoproteins complex is expressed on the surface of the viral membrane as an oligomeric protein (trimer) and mediates the viral entry. The complex is composed of two subunits noncovalently associated: gp120, the surface glycoprotein, which interacts with cellular receptors, and gp41, the transmembrane glycoprotein, responsible for fusion of the viral and cellular membranes [e.g. 12,17,18]. HIV-1 entry into target cells is a multi-step process that is initiated by gp120 binding to the CD4 receptor, present in the target cells surface. This contact *Address correspondence to this author at the Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande C8, 1749-016 Lisboa, Portugal; Tel: + 351 21 7500931; Fax: +351 21 7500088; E-mail: castanho@fc.ul.pt induces a conformational change in gp120 enabling it to bind to a cellular co-receptor, usually CCR5 or CXCR4. gp120 binding to CD4 and co-receptor induces conformational changes in gp41 exposing the N-terminal fusion peptide and allowing its insertion into target cell membrane, thereby connecting both membranes. Subsequent changes within the gp41 ectodomain involve the interaction of two heptad repeat sequences (HR1, next the fusion peptide and HR2, preceding the transmembrane domain) and a six-helix bundle structure (also called hairpin structure) is formed. The hairpin formation brings the viral and cell membrane into closed proximity, allowing fusion of the membranes and then entry of the virus [12,13,17-19]. Each of the main steps in the HIV-1 entry process can be a target for entry inhibitors. The ones currently under development fall into three categories: gp120-CD4 binding inhibitors, gp120-co-receptors binding inhibitors, and membrane fusion inhibitors, which interfere with gp41 conformational changes [2,14,16,20]. Fusion inhibitors are the leading compounds, one of them, enfuvirtide, being already approved for clinical applications. 2. ENFUVIRTIDE CLINICAL TRIALS The publication in 1998 of the results of the TRI-001 clinical trial [21] is considered as the proof-of-concept that the entrance of the virus in the cell of the host can be blocked in vivo by enfuvirtide. This study evaluated the use of enfuvirtide subcutaneous infusion, as monotherapy, during 14 days. The TRI-001 allowed concluding that the administration, during a reduced period, is safe and induces a significant (1.14 log 10 ) viral charge decrease in the group that received a 90 mg deliverable dose of enfuvirtide (100 mg nominal dose) twice daily.