J. Med. Microbio1.--Vol. 43 (1993, 1-3 0 1995 The Pathological Society of Great Britain and Ireland

EDITORIAL

Quo vadis antiviral agents for herpes, and HIV?

The discovery of the nucleoside analogue acyclovir profile. SmithKline Beecham chemists synthesised and the establishment of its biochemical mode of famciclovir and penciclovir6-8 whilst Burroughs action as an anti-herpes agent were the scientific Wellcome have produced valaci~lovir.~Famciclovir is foundation stones for medical investigations which a pro-drug which is de-acetylated and oxidised to have proved its wide clinical usefulness over the last penciclovir giving high bioavailability of the latter two decades. I remember clearly the initial launch of molecule. The half-life is somewhat prolonged and so the drug in London and it was with considerable dosages may be wider apart. Similarly valaciclovir, the interest that I attended recent meetings to discuss the L-valyl ester of acyclovir, is hydrolysed to acyclovir antiviral activity of ‘‘ sisters ’* of acyclovir, namely, giving, after oral administration, plasma concen- famciclovir,penciclovir and, later vancyclovir.’*2It did trations equivalent to those obtained after intravenous not appear likely in the 1970s that a nucleoside acyclovir. At present all three appear to be hopeful analogue, or at least the triphosphate derivative, could antiviral agents with a useful future. have such a narrow specificity against a viral Can we draw from the herpes success some mean- -namely herpes DNA polymerase-without ingful conclusions to help us with two more intractable inhibiting cellular DNA polymerases. Countless viral candidates for extinction or, at least, contain- scientific investigations have since demonstrated the ment :influenza A and HIV- 1 ? The chemists’ favourite relative safety of the omp pound,^ its antiviral spec- molecule, the nucleoside analogue, still has much trum, its ability to prevent viral spread and recurrent potential, but the incredible mutation rates of RNA infection, and to alleviate the symptoms following viruses is an inherent problem. Ho et d1*have tissue destruction by the virus. Drug-resistant herpes described the emergence of drug-resistant mutants viruses have been slow to appear.4 within days of onset of antiviral chemotherapy of Given the success of acyclovir do we need alternative AIDS patients with a protease inhibitor, but the same anti-herpes compounds or could the efficacy or even study also described the milieu in which some antiviral the antiviral range of acyclovir be improved? The compounds exist; this has been described as a “mean

answer to the first question must be “yes”. New street ” with billions of virions being eradicated daily antiviral agents are always required, if for no other by millions of activated T ce1ls.l‘ At first sight, the reason that to treat persons infected with drug- situation may appear completely hopeless for any but resistant viruses, who include immunosuppressed the most powerful inhibitor. Although herpes viruses patients such as those receiving transplants or those also undergo massive replication in the body and can infected with HIV. The antiviral agent foscarnet has be stopped by antiviral agents, as demonstrated clearly a mode of action completely different from that of with acyclovir, DNA viruses do not have the pro- acyclovir and, as expected, cross-resistance is not a pensity for the high mutation rates of RNA viruses major problem. The drug is used to treat immuno- and so constitute easier targets. A trip back in time to suppressed patients suffering from serious infections the ice age would be required to discover as many caused by acyclovir-resistantherpes viruses and CMV. herpes mutants as one would discover with influenza A narrow antiviral spectrum has been a problem or HIV-1 in a journey lasting a single year. Never- with every antiviral agent. inhibits only theless there is no overwhelming evidence that a replication and not influenza B virus; patient would succumb to an infection with drug- zidovudine inhibits only HIV- 1 ;acyclovir may be very resistant HIV or, indeed, a drug-resistant influenza A potent against HSV- 1 but is much less so against VZV. virus. Rather, at least with HIV, mixed populations of Could this problem be circumvented by increased pro-viral genomes exist in a patient treated with dosages of acyclovir or, alternatively, by discovering zidovudine and even after long periods of drug closely related analogues which may produce hgh treatment,’* zidovudine-sensitive genome sequences plasma levels in the body because of increased half-life can still be detected.13 or different dosage scheduling? The task is to discover Amantadine has been shown to exert a strong and compounds which are similar enough to acyclovir in reproducible antiviral effect when used as a prophy- molecular structure to give confidence that they will lactic agent against human sub/types of influenza A have the same safety profile, but different enough to virus including H2N2, HlNl and H3N2.14 Drug- have new antiviral properties and to be patentable. resistant viruses can be selected in the lab~ratory’~and Ganciclovir has a different molecular structure, a in the patient16 and yet treated patients continue to different antiviral spectrum to include CMV but also, benefit from therapy. Amantadine- or rimantadine- unfortunately, a much less favourable toxicological resistant strains of influenza A were isolated even 2 EDITORIAL

before the drug was used'' whilst viruses resistant to ray-derived structures of energetically favourable zidovudine or non-nucleosideinhibitors of HIV can be substitutions to the unsaturated analogue isolated from a minority of patients before drug NeuSAc2en. The most interesting of these chemical therapy is commenced.13Pre-existing polymorphisms modifications was the replacement of the hydroxyl exist in the particular nucleotides of HIV reverse group at the four position on NeuSAc2en by an amino transcriptase or protease coding for amino-acid sub- group. Substitutions of the 4-hydroxyl group by an stitution which, in the corresponding , amino group should produce a significant increase in would result in a configuration change and drug the overall binding interaction due to a salt bridge resistance. formation with the side chain carboxylic acid group of Given these achievements, where will antiviral Glu 119 in the enzyme. Similarly, the replacement of agents proceed in the next few years? Inevitably, the the same hydroxyl groups by the significantly more solution of further X-ray structures of viral proteins basic guanidine group was predicted to produce an and computerised imaging will be keys to new develop- even tighter affinity of the substituted Neu5Ac2en for ments. Eleven years ago a group of scientists in the as a result of lateral binding through the Melbourne published the first data on the atomic X- terminal nitrogens of the guanidine group with both ray structure of influenza A virus . This Glu 119 and Glu 227. Direct measurements of viral enzyme is not unique to viruses; similar ones are neuraminidase enzyme inhibition showed that the 4- widespread in the animal kingdom. These are amino and 4-guanidino-substituted Neu5Ac2en were, glycohydrolases which cleave terminal sialic acid from indeed, high-affinity inhibitors for the influenza virus , glycolipids or oligosaccharides.In viro- enzyme. Moreover, the compounds inhibited influenza logical terms the enzyme functions at the stage of virus plaque formation in cell culture. Most excitingly, influenza virus release from an infected cell. The main the compounds had antiviral effects in viuo : Hayden et study of anti-neuraminidase drugs commenced a a1.,20 using intranasal prophylaxis, were unable to quarter of a century ago with the synthesis of the recover virus from 16 drug-treated volunteers, whereas NeuSAc2en, a derivative of sialic acid which inhibited virus was recovered from 65% of 17 untreated viral, bacterial and mammalian sialidases. A series of controls. The compound looks most interesting. these compounds was tested as anti-influenza virus But I have an important reservation about current drugs1* and although a number of analogues of searches for inhibitors of HIV. With influenza Neu5Ac2en inhibited influenza virus replication in cell infections of the respiratory tract there is clear evidence culture, no antiviral activity was detected in animal of viral replication in the trachea, bronchi and alveoli. models. In retrospect, this negative finding may be The disease is clearly related to known pathological attributed to the rapid metabolism of the early changes and to viral replication, and inhibitors of viral compounds tested ; newly synthesised analogues pro- replication have a logical scientific basis for clinical duce a specific anti-influenza effect in vi210'~ and action. This has been shown with amantadine and with clinically,20after topical application. the anti-neuraminidase compound. Similarly, herpes Influenza sialidase is a tetramer of identical sub- viruses cause cellular destruction in the skin, eye or units. There are six four-stranded anti-parallel P-sheets brain and these pathological changes are stopped by arranged as if in the blades of a propeller. The active antiviral compounds. But is the pathology of HIV so enzyme site is a deep cavity on the neuraminidase clear cut? Ho et a1.l' clearly believe it is. In their protein surface and is lined entirely by amino acids important study the antiviral effects of ABT 538, an that are invariant in sialidases of all strains of influenza inhibitor of HIV protease, were quantified by the A and B which have been characterised. In contrast, reduction in viral genomes and, in half the patients, by variable amino acids are found next to and encircling an increase in CD4 cell count. In approximately one- the active site and these are antigenic determinants third of the treated patients, no increase in CD4 cell reacting with post-infection anti-neuraminidase anti- count was noted, whilst the remaining patients formed bodies. The scientific trick is to avoid inhibitors an intermediate group. Thus uncertainty remains as to binding to these variable amino acids but rather to whether viral replication per se is destroying CD4 cells. select molecules to the invariant amino acids. What if the exquisitely hostile environment created by Sialic acid, the product of the enzyme-catalysed the immune system in AIDS patients" is also directed reaction, binds to the active site of the enzyme in a to self: ? Habeshaw et aL21have steadfastly maintained

" boat" configuration. The carboxylate interacts with that a strong auto-immune component can be three invariant arginine residues on the neuraminidase recognised in AIDS patients by virtue of stimulation enzyme. In this conformation it resembles the ge- of subsets of autoreactive T cells. Supporters of the ometry of the potent sialidase inhibitor (NeuSAc2en), hypothesis point to activation of CD4 lymphocytes as as noted above. Importantly, strain invariance extends a key feature of the pathogenesis. There is evidence to a number of other amino acids which do not that these T cell subsets kill uninfected CD4 cells as themselves make contact with sialic acid, but which well as virus-infected cells. These autoreactive T cells, provide a scaffold on which amino acids contacting the which are normally suppressed, recognise in an acti- bound sugar are supported. vated T cell a portion of the C terminus of gp120, The Melbourne group made predictions from X- which when presented on a class I molecule mimics the EDITORIAL 3 normal presentation of an allo-epitope HLA class I1 /? viral spike or, more controversially, the use of selective chain.22If auto-immunity contributes significantly to immunosuppressive drugs to counteract the auto- the pathogenesis of AIDS then new vistas appear for immune reaction. But perhaps most exciting would be chemotherapists. Certainly new inhibitors of viral the novel possibility of tolerising an AIDS patient with replication will still be the cornerstone of treatment high concentrations of the mimic peptide or by but would need to be used as soon as the patient is transfusing with antibodies to the mimic peptide. infected to reduce the opportunity of recognition of J. S. OXFORD class I1 /?chain presented on HLA class 1 by these pre- Department of Medical Microbiology, The London Hospital Medical College, existing autoreactive T cells. Later possibilities for Turner Street. London El 2AD chemotherapy would be drugs to mask the cross- reactive allo-epitope on the C terminus of the gp160

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