Entry and Fusion Inhibitors: an Update Joseph J. Eron, md Associate Professor of Medicine, University of North Carolina at Chapel Hill Director, Clinical Core, unc Center for aids Research Christine M. Hogan, md Research Associate, The Aaron Diamond aids Research Center Reprinted from The PRN Notebook,™ march 2002. Rockefeller University Dr. James F. Braun, Editor-in-Chief. Tim Horn, Executive Editor. Published in New York City by the Physicians’ Research Network, Inc.,® Summary by Tim Horn John Graham Brown, Executive Director. For further information and other articles available online, visit http://www.PRN.org All rights reserved. © march 2002. Edited by Calvin Cohen, md, and John Moore, phd radication of hiv has not been pos- complete, and understanding the details lab, enough has been learned about the sible with currently available reverse involved has led to the development of in- three major steps involved in hiv fusion transcriptase inhibitors and protease hibitors for each step of hiv entry. and entry—the binding of gp120 to CD4, inhibitors; and multiclass drug-re- The hiv fusion and entry process begins the binding of gp120 to either CCR5 or sistant mutants of hiv—along with a with interactions between the trimeric en- CXCR4, and the formation of gp41 fusion multitude of disabling and some- velope complex—a cluster of proteins on determinants that allows the virus to mesh times life-threatening side effects—are a hiv’s outer coat, sometimes referred to as with its cellular target—to develop thera- growing threat. Thus, there is a great need the gp160 spike—and both CD4 and a peutic strategies targeting each of these for compounds that target non-protease chemokine coreceptor (either CCR5 or steps. Indeed, a number of compounds and reverse transcriptase elements of the CXCR4) on the cell surface. This complex is are in various stages of development. For hiv lifecycle. Not only might such novel made up of three transmembrane glyco- individuals with hiv resistant to any or therapies increase the potency of initial proteins (gp41), which anchor the cluster all of the current inhibitors of reverse hiv treatment, but they may also provide to the virus, and three extracellular gly- transcriptase and protease, these fusion hope for patients who have exhausted coproteins (gp120), which contain the and entry inhibitors represent truly unique current treatment options. binding domains for both CD4 and the classes of drugs. This article, based on prn lectures de- chemokine receptors. livered by Drs. Joseph Eron and Chris- As illustrated in Figure 1, the first step tine Hogan in January 2002, reviews one in fusion involves the high-affinity attach- Attachment Inhibitors of the most promising areas of hiv drug ment of the CD4 binding domains of gp120 development: inhibitors of hiv fusion and to the N-terminal membrane-distal do- (Agents Targeting CD4) entry. As reviewed by both investigators, main of CD4. Once bound, the envelope the initial interaction of hiv GP120 with two fusion inhibitors targeting hiv’s gp41 complex undergoes a structural change, CD4 on the cell surface offers the highly envelope protein (T-20 and T-1249) continue bringing the chemokine binding domains of conserved CD4 binding site on gp120 as a to advance through clinical trials and have gp120 into proximity with the chemokine therapeutic target. Unfortunately, this ap- both received fast-track designations by receptor on the CD4+ cell, allowing for a proach is not without caveats. As ex- the U.S. Food and Drug Administration. As more stable two-pronged attachment. plained by Dr. John Moore in the March for other entry inhibitors targeting cellular With the virus now latched on to both 1999 Notebook article referenced above, proteins in the earlier stages of develop- CD4 and the chemokine receptor, addi- the conserved CD4 binding sites of gp120 ment (e.g., inhibitors of CD4, CCR5, and tional conformational changes take place are protected by glycosylation—a flexible CXCR4), Drs. Eron and Hogan noted some in the envelope complex, whereby gp41, layer of sugars that can occlude the pep- of the advances and setbacks that have originally tucked away within the trimer- tide backbone of gp120, alter its anti- occurred over the past few years. ic complex, is exposed and the N-terminal genicity, and reduce its immunogenicity. fusion peptide of gp41 enters the CD4+ This has been one of the main concerns cell’s membrane. From there, two heptad regarding the potential effectiveness of hiv Fusion and Entry repeat sequences (HR1 and HR2) of gp41 the gp120 vaccine (Vaxgen) for the pre- since 1996, when the elusive cd4+ cell interact, resulting in collapse of the ex- vention and possibly immune-based treat- coreceptors CCR5 and CXCR4 were discov- tracellular portion of gp41 to form a hair- ment of hiv infection. Similarly, studies ered, the process by which hiv gains entry pin, which is sometimes referred to as a employing a soluble form of CD4 (rsCD4) into these cells has been a major focus of coiled-coil bundle. The hairpin structure as a therapeutic compound showed research (see “The hiv Envelope Glyco- brings the virus and cell membrane close promise in vitro but had little noticeable proteins: Their Roles in Virus Entry and as together, allowing fusion of the membranes activity against primary hiv isolates and a Vaccine Target,” published in the March and subsequent entry of the virus core. was rapidly cleared from the body (Daar, 1999 issue of The PRN Notebook). What we While additional details of this multistep 1990; Schooley, 1990; Schacker, 1995). now know about this process is much more process are still being worked out in the Fortunately, newer compounds continue 16 THE PRN NOTEBOOK™ • VOLUME 7, NUMBER 1 • MARCH 2002 • WWW.PRN.ORG A B rna HIV gp41 gp120 cd4 Receptor Chemokine Receptor CD4+ CELL C D figure 1. hiv Fusion and Entry. panel a: The process begins with interactions between the trimeric envelope complex—a cluster of proteins on hiv’s outer coat, some- times referred to as the gp160 spike—and both cd4 and a chemokine receptor (either ccr5 or ccr4) on the cell surface. This complex is made up of three transmembrane glycoproteins (gp41), which anchor the cluster to the virus, and three extracellular glycoproteins (gp120), which contain the binding domains for both cd4 and the chemokine receptors. panel b: The first step in fusion involves the high-affinity attachment of the cd4 binding domains of gp120 to the N-terminal membrane- distal domains of cd4. cd4 attachment inhibitors (e.g., pro 542) act here. panel c: Once gp120 is bound with the cd4 protein, the envelope complex undergoes a structural change, bringing the chemokine bind- ing domains of gp120 into proximity with the chemokine receptor, allowing for a more stable two-pronged attachment. Antagonists of ccr5 (e.g., sch-c) and cxcr4 act here. If the virus latches on to both cd4 and the chemokine receptor, additional conformational changes allow for the N-terminal fusion peptide of gp41 to enter the cd4+ cell membrane. panel d: Two heptad repeat sequences—hr1 (blue) and hr2 (orange)—of gp41 interact, resulting in collapse of the extracellular por- tion of gp41 to form a hairpin, which is sometimes referred to as a coiled-coil bundle. The fusion inhibitors t-20 and t-1249 act here by mimicking hr2, resulting in a botched formation of the hairpin. In the absence of an inhibitor, the hairpin structure brings the virus and cell membrane close together, allowing fusion of the membranes and subsequent entry of viral rna. THE PRN NOTEBOOK™ • VOLUME 7, NUMBER 1 • MARCH 2002 • WWW.PRN.ORG 17 to hold promise as attachment inhibitors. Of had sustained reductions in serum hiv- emergence of CXCR4-tropic variants is wide- particular interest are two vaginal micro- rna; the other children had rebounded to ly considered to be associated with accel- bicides: PRO 2000 and Cyanovirin-N. PRO baseline levels. erated loss of CD4+ cells and progression to 2000, a naphthalene sulfonate polymer that A second set of phase II clinical trials, aids (Koot, 1993). Although the evolution is active against a wide range of hiv isolates, which are also being conducted in adults from CCR5- to CXCR4-tropic viruses is slow in is currently in phase II clinical trials being and children, were kicked off in 2000. hiv-infected individuals, there has been conducted around the world. Cyanovirin-N, These studies will include patients with some concern that suppression of hiv in- a mannose binding protein derived from hiv resistant to current antiretroviral op- fection with a CCR5 antagonist might accel- the blue-green algae Nostoc ellipsospo- tions. The drug is currently being evalu- erate the selection for CXCR4- or dual-trop- rum, is active against a diverse range of ated in an improved formulation for sub- ic (CCR5/CXCR4) strains. “It still remains un- viruses with mannose-rich glycoproteins, cutaneous administration. clear whether or not this phenotypic switch including HIV-1, HIV-2, and human herpes is a consequence or a cause of disease pro- virus-6 (HHV-6). Both compounds are ap- gression,” added Dr. Hogan. “Is it that the plied topically and are being followed Chemokine Receptors (CCR5 Antagonists) virus changes tropism and induces disease closely by advocates for women’s health progression? Or is it that disease progres- and hiv prevention. another site in the hiv entry process sion is already happening, resulting in loss that has been the focus of therapeutic re- of immune control of CXCR4-tropic virus? pro 542 search is the CD4-dependent interaction We are not really sure yet.” there is also progenics pharmaceuticals’ between hiv gp120 and the CCR5 chemo- At least one report, published in the PRO 542, also known as CD4-IgG2.