Presentations

Strategic development of novel antiretroviral therapies

Abstract

Currently available antiretroviral agents can be classed according to the target viral enzyme that they inhibit. Nucleoside analogues (zidovudine, didanosine, zalcitabine, stavudine, lamivudine and abacavir) and non-nucleoside analogues (efavirenz, delavirdine, nevirapine) inhibit the enzyme responsible for viral RNA transcription, reverse transcriptase. Protease inhibitors (nelfinavir, saquinavir, indinavir, ritonavir, amprenavir and lopinavir) prevent the action of the protease enzyme, which is responsible for the cleavage of polyproteins into constituent proteins of a mature virion. Drugs within these two classes comprise the constituent elements of combination therapy regimens that are widely used today. The fact that they work at different stages in the viral life cycle has led many to believe that divergent therapy (using drugs effective against different viral targets) provides the optimal approach to combat the virus. However, there is, in contrast, a theoretical benefit to convergent therapy (using drugs from the same class that target a single viral protein). Such an aggressive selection pressure against one target in the viral life cycle might force the virus to mutate into a non-viable strain. However, there is little evidence to suggest that this is what happens in the clinical setting.

In therapy-experienced patients, virus may be resistant to one or more agents in a particular drug class as well as being resistant to more than one drug class. Cross-resistance limits therapeutic benefit from subsequent therapy, even when multiple agents are used from the various available drug classes. It is clear, therefore, that there is a pressing need to develop inhibitors of new viral targets, particularly those which are active against strains resistant to conventional targets.

Potential new targets identified for such new drug classes include:

• Viral integrase enzyme (responsible for the integration of viral DNA into the cellular DNA)
• Viral regulatory enzymes (responsible for the transcription of proviral DNA into RNA)
• Viral zinc finger nucleocapsid proteins (responsible for the formation of the nucleocapsid)
• Viral entry into host cell (initial attachment of virus into cell, chemokine receptor interactions and subsequent fusion events) (See figure 1).

Figure 1

Neuraminidase inhibitors for treatment of influenza have provided a strong precedent to support the consideration of the development of agents targeted at viral/cell associations. Agents developed in this group include zanamivir (Relenza�), oseltamivir (Tamiflu�) and an investigational compound, RWJ-270201. Looking at the replication cycle of the influenza virus, we can see that it follows the typical pattern of viral replication: fusion; loss of outer membrane; replication of genetic material; translation into constituent proteins; and finally, assembly and budding. Like protease inhibitors, neuraminidase agents inhibit the final stages of the viral life cycle, but they differ in that neuraminidase inhibitors block viral shedding from the host cell while protease inhibitors block the construction of the constituent proteins required for a mature virion. The successful development of agents which block viral/cell associations has been an important advance and provides encouraging results for those involved in the search for new viral targets.

Looking in more detail at viral/cell associations in the HIV field, it is clear that in the last decade a considerable amount has been learned about the process of HIV attachment and fusion to host cells. The process is characterised by a number of distinct stages. Firstly, the external viral envelope glycoprotein, gp120, interacts with a domain on the primary cellular receptor, CD4, on the surface of T-helper cells or macrophages.

After this, the conformation of gp120 is altered, revealing a hitherto concealed area that is able to bind to the receptors known as CXCR-4 or CCR-5. These are chemokine co-receptors. After gp120 interacts with CD4 and these co-receptors, these molecules fall away allowing the �spring-loaded� gp41 molecule to spring open and inject into the fusion domain of the target cell, revealing the intermediate structure known as a pre-hairpin intermediate.

The N- and C-terminal domains of this intermediate fold back onto each other in an anti-parallel fashion, bringing the viral and cellular membranes together. Fusion then occurs, a process which requires further elucidation (see figure 2).

Potential inhibitors of viral entry can be divided into three mechanistically distinct classes: attachment inhibitors, co-receptor inhibitors, and fusion inhibitors.

Figure 2

Looking first at attachment inhibitors, over the past 15 years attempts to block viral attachment to target cells have not been clinically successful, including the use of soluble CD4, which did however show promising in vitro activity [1]. More recently a novel protein, PRO 542, has been associated with more successful results. PRO 542 (CD4-IgG2) consists of the N-terminal domains of human CD4 fused to the constant heavy and light chain regions of human IgG2. In vitro, PRO 542 has successfully neutralized a broad range of HIV variants and shown activity in SCID-Hu models with primary isolates. In a small, single-injection, dose-ranging trial, in subjects with HIV RNA >3000 copies/ml and CD4 counts >50 cells/mm 3, PRO 542 was well tolerated, non-immunogenic and displayed linear pharmacokinetics. In the highest dose group (10 mg/kg) the maximum mean decrease in viral load (0.36 log 10 decrease) occurred 4 hours after the single dose [2]. In a phase I/II paediatric trial, four of six subjects treated with PRO 542 at a dose of 10 mg/kg experienced a >0.7 log 10 decrease in HIV RNA, which at day 14 was sustained in three of the four subjects. Again, the trial showed that PRO 542 was well tolerated [3].

Moving on to consider chemokine receptor inhibitors, these can be divided into two classes according to the chemokine receptor that they inhibit (CXCR-4 or CCR-5). There are currently at least two inhibitors in early development against CCR-5. PRO 140 is a murine anti-CCR-5 monoclonal antibody, whereas SCH-C is a small molecule with a pharmacokinetic profile that may support oral administration. AMD-3100 is an inhibitor of CXCR-4 that has been studied when administered by continuous IV infusion.

Focusing on fusion inhibitors, the synthetic C peptides, T-20 and T-1249, have been developed to mimic the C-terminal region of gp41 and prevent the fold-back process through binding to the N-terminal region. T-20 is a 36 amino-acid peptide, which has shown activity against NSI and SI viruses, as well as synergy with other entry inhibitors and also with reverse transcriptase and protease inhibitors. T-1249 is a 39 amino-acid peptide in earlier clinical development with more potent in vitro activity and the potential for once daily administration.

Despite the need for parenteral administration, the potential advantages of T-20 and T-1249 fusion inhibitors are significant and include:

• Potency
• Safety
• Lack of drug interactions
• Lack of cross resistance to conventional agents (should work against resistant strains).

It is promising that in vitro synergy has already been demonstrated between T-20 and an attachment inhibitor (PRO-542), a CXCR-4 inhibitor (AMD 3100) and a CCR-5 inhibitor (TAK779).

In summary, the current situation is promising and it is anticipated that a new class of inhibitors will be developed to block viral entry. It is also anticipated that these drugs, which would work outside of the cell, will make a significant contribution to the existing therapeutic arsenal.

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References

1. Schooley RT, Merigan TC, Gaut P et al. Recombinant soluble CD4 therapy in patients with the acquired immunodeficiency syndrome (AIDS) and AIDS-related complex. A phase I-II escalating dosage trial. Annals of Internal Medicine 1990; 112:247-253

2. Jacobson JM, Lowy I, Fletcher CV et al. Single-dose safety, pharmacology, and antiviral activity of the human immunodeficiency virus (HIV) type 1 entry inhibitor PRO 542 in HIV-infected adults. Journal of Infectious Diseases 2000; 182:326-329

3. Shearer W and Israel R. rCD4-IgG2 in HIV-1-infected children: phase I/II study. 7th Conference on Retroviruses and Opportunistic Infections, San Francisco, California, USA, 30 January - 2 February 2000. Abstract 701

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Biography

Dr Joep Lange began his career at the University of Amsterdam, specialising in Internal Medicine with a special interest in infectious diseases. In 1984, he became involved in the Amsterdam Cohort Study on HIV-infection and AIDS and in laboratory research on HIV infection at the University of Amsterdam, which led to the first full description of the serological antibody response pattern to HIV infection (thesis �Serological markers in HIV infection�, 1987).

Dr Lange was appointed Director of the newly founded National AIDS Therapy Evaluation Centre (NATEC), in 1990. NATEC is a government-sponsored body, responsible for the initiation and co-ordination of clinical trials in the field of HIV infection and its secondary complications, in the Netherlands.

Prior to his return to the University of Amsterdam as Professor of Internal Medicine in 1995, he was Chief, Clinical Research and Drug Development (later Clinical Research and Product Development), Global Programme on AIDS, World Health Organization, Geneva, Switzerland.

Dr Lange has been principal investigator of more than 10 trials on antiretroviral therapy, the first of which was a pilot dose-efficacy study of zidovudine + acyclovir in asymptomatic HIV-infected subjects at high risk for disease progression, which started in April 1987 (the first trial in which zidovudine was administered to asymptomatic HIV-infected subjects).

He serves or has served on the Editorial Boards of 'Genitourinary Medicine', 'AIDS', and 'Clinical Trials and Meta-Analysis' and has been Section Editor for the Clinical Treatment Section of the �AIDS� 1991 and 1992/1993 Annual Supplements. He is also Editor-in-Chief of the newly founded journal �Antiviral Therapy�. He was also co-chairman of the Clinical Care track of the VIII International Conference on AIDS, which was held in Amsterdam in July 1992.

Dr Lange serves or has served on several advisory groups on antivirals to pharmaceutical companies, on several Data and Safety Monitoring Boards of international antiviral drug trials, on several NIH review panels, is a member of the ACTG International Virology Committee, and was a member of the Executive Committee that drafted the US Public Health Service Task Force Recommendations on the Use of Zidovudine to Reduce Perinatal Transmission of Human Immunodeficiency Virus.

Dr Lange has published more than 100 papers on the serology, natural history and treatment of HIV infection.

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