[Frontiers in Bioscience 2, d147-159, March 1, 1997]

	[Frontiers in Bioscience 2, d147-159, March 1, 1997] Reprints PubMed CAVEAT LECTOR
	PATHOGENESIS AND TREATMENT OF HIV-1 INFECTION: RECENT DEVELOPMENTS Stephen Dewhurst, Ph.D. and Linda Whetter, D.V.M., Ph.D. Department of Microbiology and Immunology, and Cancer Center, University of Rochester Medical Center, Rochester NY Received 2/21/97; Accepted 2/25/97; On-line 3/1/97 4.1. Therapy for HIV-1 infection New combination drug therapies, and the use of protease inhibitors, have fueled a new mood of optimism in the treatment of HIV-1 infection and represent one of the most exciting breakthroughs in HIV-1 research since the epidemic began. Some of the more recent developments in therapy are summarized in this section. Gene therapy for HIV-1 infection is not discussed, for reasons of space, and readers are referred to recent reviews in this area (69-72). 4.1.1. Protease inhibitors and combination therapy Briefly, the HIV-1 Gag and Pol proteins are encoded in the form of large polypeptide precursors which must be proteolytically processed into mature proteins. The proteolytic cleavage of Gag and Pol precursor polyproteins is carried out by a virally-encoded aspartyl protease that is required for virus replication, and which has been structurally examined at the atomic level (reviewed in (73)). Using this information, enzyme inhibitors were designed (74), and their pharmocologic properties (e.g., oral bioavailability) modified so as to arrive at biologically effective antiviral drugs. The first to receive Food and Drug Administration (FDA) approval, in December of 1995, was invirase (saquinavir), followed swiftly by crixivan (indinavir) and norvir (ritonavir) (see Fig. 4). Other drugs, notably viracept, are likely to follow in 1997. FIGURE 4: Structures of selected HIV-1 protease inhibitors. Key mutations in protease, which are critical for virus drug resistance are noted (numbers refer to the amino acid residue affected). HIV-1 protease is a homodimeric protein composed of two identical subunits of 99 amino acids each, and the protease inhibitors are transition state analogs which bind the enzyme much more tightly than does the natural substrate (since the substrate must be distorted to assume its transition state configuration). Thus, the presently available protease inhibitors function as competitive enzyme inhibitors. The structure of HIV-1 protease, complexed with one of its inhibitors (VX-478) is presented in Figure 5. FIGURE 5: Structure of HIV-1 protease, complexed with the enzyme inhibitor VX-478. The individual protease monomers are colored orange and green. VX-478 has replaced the substrate, and is shown in red. The paired active site Asp residues (Asp25) are shown in blue, and the key residues involved in viral genetic resistance to VX-478 are shown in purple (Ile 50). This molecular representation was generated with RasMol , using the PDB database file 1HPV.PDB, and protease residues are shown in the "wireframe" format. A 3-D version of this figure, which can be manipulated by the reader, is also available. Protease inhibitors are typically used in combination with other antiviral drugs (reverse transcriptase inhibitors) in vivo, with results that can be quite impressive in many (but not all) patients. Triple drug combinations have received a widespread attention, and effective cocktails include saquinavir (invirase), zidovudine (AZT) plus zalcitabine (ddC) (75, 76) as well as norvir (indinavir), zidovudine (AZT) plus lamivudine (3TC) (77). In addition, multiple drugs which target a single viral enzyme can be combined effectively. For example, a combination of the reverse transcriptase inhibitors, nevirapine, zidovudine (AZT) and didanosine (ddI), has been shown to result in long-term immunologic and virologic improvements, relative to the treatment with zidovudine and didanosine alone (78). Future efforts will no doubt focus on the identification of optimal drug combinations for the treatment of HIV-1 infection, and one important consideration is that of cross-resistance. Specifically, viruses with a genetic resistance to one drug may also be resistant to other, similar, compounds that target the same viral enzyme. In the case of protease inhibitors, virus strains resistant to crixivan (indinavir) are also resistant to norvir (indinavir) -- but they remain susceptible to other enzyme inhibitors (79), suggesting that dual or even triple protease inhibitor therapy might be possible. A similar situation exists with respect to the nucleoside-based inhibitors of HIV-1 reverse transcriptase, most of which do not confer cross-resistance to one another. Given the enormous genetic diversity of HIV-1 variants present in each infected person, the emergence of HIV-1 strains with resistance to single or even multiple antiviral drugs can be expected and may even be inevitable. With this in mind, Coffin has noted that (1) therapy should be initiated early (before excessive diversity has accumulated), and (2) that the most effective antiviral drugs may be those which select for resistant strains that are genetically attenuated in some manner (11, 12). There is reason to think that this may practical. HIV-1 variants with a high-level resistance to certain protease inhibitors replicate more slowly than the wild-type viruses (80), and strains resistant to lamivudine (3TC) likewise exhibit an impaired fitness (81). 4.1.2. Future directions Improvements in therapies for HIV-1 infection may come both from improved or novel antiviral drugs and from a better understanding of the nature of the protective and harmful host immune responses to HIV-1. Several new classes of antiviral drugs are being developed, including inhibitors of the HIV-1 integrase (82), as well as compounds targeted against the highly conserved HIV-1 nucleocapsid protein zinc fingers involved in genome packaging and virus assembly (83). In addition, a synthetic chemokine antagonist, RANTES(9-68), has been shown to block the infection of T cells by M-tropic HIV-1 strains (84), as have the recently identified chemokine homologs encoded by human herpesvirus-8 (85). Other, virally-derived factors, might also emerge as useful anti-HIV-1 agents. For example, the putative CD4-binding protein encoded by human herpesvirus-7 can interfere with the HIV-1 infection of both T cells and macrophages (86, 87). An alternative, or adjunctive approach to therapy may be the use of immune modulators. Prevention of deleterious responses is one possibility, and this may be achievable through targeted blockade of specific inflammatory mediators, such as TNF-alpha. Specific inhibitors of this cytokine, notably thalidomide and pentoxifylline, are presented being investigated for their therapeutic potential -- particularly in HIV-1 infected persons with mycobacterial infections (88-90). In addition, it may be possible to facilitate or even to partially restore immune function in persons with HIV-1 infection. Examples of such approaches include the use of low-dose interleukin-2 treatment as a means of boosting CD4⁺ T cell levels (91, 92) and ex vivo proliferation of CD4⁺ cells using CD28 costimulation (93). The latter approach may also facilitate gene therapeutic approaches to HIV-1 infection, by allowing ex vivo transduction of CD4⁺ T cells with retrovirus vectors, followed by selection and expansion of transduced cells (93).

[Frontiers in Bioscience 2, d147-159, March 1, 1997]
Reprints
PubMed
CAVEAT LECTOR

PATHOGENESIS AND TREATMENT OF HIV-1 INFECTION: RECENT DEVELOPMENTS

Stephen Dewhurst, Ph.D. and Linda Whetter, D.V.M., Ph.D.

Department of Microbiology and Immunology, and Cancer Center, University of Rochester Medical Center, Rochester NY

Received 2/21/97; Accepted 2/25/97; On-line 3/1/97

4.1. Therapy for HIV-1 infection

New combination drug therapies, and the use of protease inhibitors, have fueled a new mood of optimism in the treatment of HIV-1 infection and represent one of the most exciting breakthroughs in HIV-1 research since the epidemic began. Some of the more recent developments in therapy are summarized in this section. Gene therapy for HIV-1 infection is not discussed, for reasons of space, and readers are referred to recent reviews in this area (69-72).

4.1.1. Protease inhibitors and combination therapy

Briefly, the HIV-1 Gag and Pol proteins are encoded in the form of large polypeptide precursors which must be proteolytically processed into mature proteins. The proteolytic cleavage of Gag and Pol precursor polyproteins is carried out by a virally-encoded aspartyl protease that is required for virus replication, and which has been structurally examined at the atomic level (reviewed in (73)). Using this information, enzyme inhibitors were designed (74), and their pharmocologic properties (e.g., oral bioavailability) modified so as to arrive at biologically effective antiviral drugs. The first to receive Food and Drug Administration (FDA) approval, in December of 1995, was invirase (saquinavir), followed swiftly by crixivan (indinavir) and norvir (ritonavir) (see Fig. 4). Other drugs, notably viracept, are likely to follow in 1997.

FIGURE 4: Structures of selected HIV-1 protease inhibitors. Key mutations in protease, which are critical for virus drug resistance are noted (numbers refer to the amino acid residue affected).

HIV-1 protease is a homodimeric protein composed of two identical subunits of 99 amino acids each, and the protease inhibitors are transition state analogs which bind the enzyme much more tightly than does the natural substrate (since the substrate must be distorted to assume its transition state configuration). Thus, the presently available protease inhibitors function as competitive enzyme inhibitors. The structure of HIV-1 protease, complexed with one of its inhibitors (VX-478) is presented in Figure 5.

FIGURE 5: Structure of HIV-1 protease, complexed with the enzyme inhibitor VX-478. The individual protease monomers are colored orange and green. VX-478 has replaced the substrate, and is shown in red. The paired active site Asp residues (Asp25) are shown in blue, and the key residues involved in viral genetic resistance to VX-478 are shown in purple (Ile 50). This molecular representation was generated with RasMol , using the PDB database file 1HPV.PDB, and protease residues are shown in the "wireframe" format. A 3-D version of this figure, which can be manipulated by the reader, is also available.

Protease inhibitors are typically used in combination with other antiviral drugs (reverse transcriptase inhibitors) in vivo, with results that can be quite impressive in many (but not all) patients. Triple drug combinations have received a widespread attention, and effective cocktails include saquinavir (invirase), zidovudine (AZT) plus zalcitabine (ddC) (75, 76) as well as norvir (indinavir), zidovudine (AZT) plus lamivudine (3TC) (77). In addition, multiple drugs which target a single viral enzyme can be combined effectively. For example, a combination of the reverse transcriptase inhibitors, nevirapine, zidovudine (AZT) and didanosine (ddI), has been shown to result in long-term immunologic and virologic improvements, relative to the treatment with zidovudine and didanosine alone (78).

Future efforts will no doubt focus on the identification of optimal drug combinations for the treatment of HIV-1 infection, and one important consideration is that of cross-resistance. Specifically, viruses with a genetic resistance to one drug may also be resistant to other, similar, compounds that target the same viral enzyme. In the case of protease inhibitors, virus strains resistant to crixivan (indinavir) are also resistant to norvir (indinavir) -- but they remain susceptible to other enzyme inhibitors (79), suggesting that dual or even triple protease inhibitor therapy might be possible. A similar situation exists with respect to the nucleoside-based inhibitors of HIV-1 reverse transcriptase, most of which do not confer cross-resistance to one another.

Given the enormous genetic diversity of HIV-1 variants present in each infected person, the emergence of HIV-1 strains with resistance to single or even multiple antiviral drugs can be expected and may even be inevitable. With this in mind, Coffin has noted that (1) therapy should be initiated early (before excessive diversity has accumulated), and (2) that the most effective antiviral drugs may be those which select for resistant strains that are genetically attenuated in some manner (11, 12). There is reason to think that this may practical. HIV-1 variants with a high-level resistance to certain protease inhibitors replicate more slowly than the wild-type viruses (80), and strains resistant to lamivudine (3TC) likewise exhibit an impaired fitness (81).

4.1.2. Future directions

Improvements in therapies for HIV-1 infection may come both from improved or novel antiviral drugs and from a better understanding of the nature of the protective and harmful host immune responses to HIV-1.

Several new classes of antiviral drugs are being developed, including inhibitors of the HIV-1 integrase (82), as well as compounds targeted against the highly conserved HIV-1 nucleocapsid protein zinc fingers involved in genome packaging and virus assembly (83). In addition, a synthetic chemokine antagonist, RANTES(9-68), has been shown to block the infection of T cells by M-tropic HIV-1 strains (84), as have the recently identified chemokine homologs encoded by human herpesvirus-8 (85). Other, virally-derived factors, might also emerge as useful anti-HIV-1 agents. For example, the putative CD4-binding protein encoded by human herpesvirus-7 can interfere with the HIV-1 infection of both T cells and macrophages (86, 87).

An alternative, or adjunctive approach to therapy may be the use of immune modulators. Prevention of deleterious responses is one possibility, and this may be achievable through targeted blockade of specific inflammatory mediators, such as TNF-alpha. Specific inhibitors of this cytokine, notably thalidomide and pentoxifylline, are presented being investigated for their therapeutic potential -- particularly in HIV-1 infected persons with mycobacterial infections (88-90). In addition, it may be possible to facilitate or even to partially restore immune function in persons with HIV-1 infection. Examples of such approaches include the use of low-dose interleukin-2 treatment as a means of boosting CD4⁺ T cell levels (91, 92) and ex vivo proliferation of CD4⁺ cells using CD28 costimulation (93). The latter approach may also facilitate gene therapeutic approaches to HIV-1 infection, by allowing ex vivo transduction of CD4⁺ T cells with retrovirus vectors, followed by selection and expansion of transduced cells (93).