Antimicrobial Chemotherapy, 5th Edition
Part 4 - Therapeutic Use of Antimicrobial Agents
Diseases caused by viruses are extremely common and range from trivial coughs and colds to life-threatening infections. Most viral infections are self-limiting, although some that normally cause relatively minor problems can be serious in immunosuppressed patients or neonates. Some viruses nearly always cause severe disease regardless of the initial status of the host. Rabies, human immunodeficiency virus (HIV), and several of the viruses causing haemorrhagic fevers fall into this category. Furthermore, long-term complications may arise from viral encephalitides (Chapter 26) and poliovirus.
Most success in the control of serious virus infections has been achieved by immunoprophylaxis, and to a lesser extent immunotherapy, notably to manage rabies and hepatitis B virus infections, rather than by the use of specific antiviral chemotherapy. However, there is an ever-increasing list of useful antiviral drugs (see Chapters 6 and 7) and, as knowledge of the molecular biology of viral replication increases, more potential targets are being identified. In general, the herpes viruses and HIV virus infections are the most widely treated virus infections.
All eight herpesviruses that infect man (Table 27.1) exhibit the phenomenon of latency. Following acute infection, virus is not eliminated from the body, but viral DNA remains quiescent without causing any apparent damage to the latently infected cells. Later, virus may be reactivated; the virus replicates, and disease may re-emerge. Herpesvirus infections can therefore be classified as primary, the host's first encounter with the virus; or secondary, due to reactivation of latent virus. Occasionally, reinfection with a new exogenous virus may occur. The clinical features of primary and secondary herpesvirus infections are often very different.
Aciclovir is very active against certain herpesviruses and was the first truly selective antiviral agent. Two more oral antiherpetic drugs have been developed since the introduction of aciclovir into clinical practice: valaciclovir (an oral pro-drug of aciclovir) and famciclovir (a pro-drug of penciclovir) (p. 100). These newer agents exhibit greater oral bioavailability, which allows an easier dosing schedule.
Table 27.1 Human herpesviruses
Herpesviruses differ in their sensitivity to aciclovir and penciclovir (Table 27.1). Cytomegalovirus (CMV) and human herpesvirus type 6 do not possess thymidine kinase, and cannot activate the drugs efficiently. In practice, these drugs are useful only in the treatment of herpes simplex virus (HSV), in varicella-zoster virus infection, and in certain unusual manifestations of Epstein-Barr virus infection. Clinical trials in various manifestations of herpes simplex and varicella-zoster infection have shown the two compounds to be equipotent. Which drug should be prescribed—aciclovir, valaciclovir, or famciclovir—depends on the circumstances. Aciclovir is the only agent licensed for intravenous use, and thus must be used in life-threatening diseases necessitating intravenous therapy. For oral therapy, the choice lies between valaciclovir and famciclovir. As there are no clinical trials evaluating the newer drugs in the treatment of ocular herpes simplex or zoster, aciclovir is still the drug of choice for these conditions.
Herpes simplex virus infections
Although most HSV infections are asymptomatic, there can be a wide range of clinical manifestations.
Primary orolabial infection
Symptomatic primary infection, which usually occurs in children, presents with extensive painful blistering and ulceration of the lips, tongue, and mucous membranes of the mouth, sometimes with marked cervical lymphadenopathy and fever. Left untreated, the infection is self-limiting with complete healing in 2-3 weeks. Oral valaciclovir or famciclovir therapy reduces the manifestations of systemic disease, the formation of new lesions, the period of virus excretion, and the time to healing; they are of unquestioned therapeutic benefit.
Recurrent orolabial infection
Resolution of primary disease is followed by the establishment of latency of HSV within the trigeminal nerve ganglion. This virus can be reactivated in later life and track back down the nerve to reach the mouth. The manifestations of reactivated disease in an immunocompetent host are much milder: there is no systemic upset, only a few lesions (cold sores), localized to a small area on the lips; the inflammatory response is less, with less pain and swelling, and the lesions heal in about 5-6 days. Aciclovir therapy enhances resolution of lesions by about 24 h. Given the mild nature of recurrent disease (as compared with primary infection), and the marginal benefit of aciclovir, routine systemic treatment of cold sores is not recommended.
Intact skin is a very efficient barrier to the spread of viruses. However, in patients with chronic dermatitis or eczema, this barrier is not intact, and virus can spread freely. Eczema herpeticum is a severe complication of primary or secondary orolabial HSV infection in patients with eczema. Lesions extend to large areas on the face, neck, and upper chest. Virus may also gain access to the bloodstream through cracks in the skin, putting the patient at risk of death from disseminated HSV infection. Antiviral therapy is mandatory. Patients should be given supplies of valaciclovir or famciclovir so that they can self-medicate in the early stages of recurrent disease.
Ocular herpes simplex virus infection
HSV is the commonest infectious cause of blindness in the UK. Children are particularly prone to self-inoculating the eye with virus-infected oral secretions. Spread of virus to the cornea results in keratitis and may lead to severe inflammation and chronic ulceration. Management should include systemic and topical aciclovir and prompt referral for specialist ophthalmological assessment.
The clinical manifestations and management of primary and secondary genital herpes are analogous to those of orolabial disease. Thus primary infection results in extensive bilateral, painful ulceration with spread to adjacent skin, inguinal adenopathy, and fever, lasting 2-3 weeks. Patients may experience difficulty in passing urine. Cervicitis with profuse discharge is common in women. Oral valaciclovir or famciclovir is of proven benefit. Recurrent disease is much more localized, with no systemic upset, and shorter-lasting; oral antiviral therapy is of marginal value.
Some unfortunate individuals may suffer severe recurrent attacks of genital herpes as frequently as once a month. The immunological basis for this debilitating and depressing condition is not understood. One approach to the management of these patients is to give them continuous prophylactic antiviral therapy. This results in a considerable decrease in the frequency of attacks and rarely gives rise to significant side effects.
Pregnant women with genital herpes are at risk of passing on the infection during childbirth. Neonatal HSV infection is a potentially devastating disease, as virus readily disseminates to internal organs, and such babies may die of HSV hepatitis, pneumonitis, or encephalitis. Survivors are almost invariably left with severe neurological sequelae. Although mortality from neonatal herpes has improved with the use of intravenous aciclovir, overall morbidity has scarcely been affected, presumably because by the time the diagnosis is made, and therapy initiated, the damage has already been done.
Herpes simplex encephalitis
Herpes simplex encephalitis is the commonest form of sporadic viral encephalitis (Chapter 26), with an annual incidence of about 1 case per million population. Most cases occur in individuals with evidence of prior infection with HSV. Thus the disease is usually a manifestation of secondary infection, although the route by which virus gains access to the brain is not clear. Untreated, mortality is high, and survivors suffer severe long-term damage. Diagnosis is difficult but should be suspected in all patients with features of acute encephalitis with focal signs, especially when confirmed by computed tomography scans or magnetic resonance imaging of the brain. The cerebrospinal fluid usually shows a pleocytosis, but it is very rare to succeed in isolating the virus from cerebrospinal fluid. HSV DNA may be detected in cerebrospinal fluid during the acute stage by the polymerase chain reaction, but this test is not universally available. Intravenous aciclovir, in high dosage, improves the prognosis dramatically, provided therapy is started as soon as the diagnosis is suspected, without waiting for laboratory confirmation.
Patients who are immunocompromised for whatever reason are at risk of severe primary or secondary HSV disease. All HSV infections in this group of patients should be treated promptly with aciclovir, if necessary by the intravenous route. In bone marrow transplant recipients, prophylactic aciclovir is recommended because of the potentially severe and life-threatening nature of such infections.
The primary manifestation of infection with varicella-zoster virus is chickenpox. This is usually a trivial disease of children, despite the dramatic rash. The most important complication is varicella pneumonia, which can be life-threatening. This is considerably more common in adults than in children, and seems to be more common in pregnant women who develop chickenpox. Adults with chickenpox are at risk of severe pneumonitis and should be referred promptly to hospital for intravenous aciclovir therapy if evidence of respiratory involvement occurs.
It has been suggested that all adults, and even all children, with chickenpox should be given aciclovir. The arguments in favour of this blanket approach are economic rather than medical. Aciclovir allows resolution of the disease 24-48 h earlier than would otherwise be the case, enabling adults, or carers of sick children to return to work that much sooner. The frequency of complications in adults, although greater than in children, remains uncommon and even large-scale clinical trials have failed to prove that aciclovir therapy reduces their incidence.
Pregnant women who develop chickenpox in late pregnancy may pass this infection on to their baby if it is born within a week of onset of the maternal rash since there is insufficient time to allow the mother to generate protective antibodies that can be transferred across the placenta. Neonatal chickenpox is a feared disease, and babies at risk should be given passive immunization with hyperimmune zoster immune globulin. It is also reasonable to give such babies prophylactic aciclovir by mouth, although some withhold the drug until the baby develops signs of disease.
The site of latency of varicella-zoster virus is the dorsal root ganglion. The systemic nature of varicella infection means that dorsal root ganglia up and down the spinal cord become latently infected. Reactivation of infection results in virus travelling down the dorsal nerve route in question, to reach the dermatome supplied by that nerve. This accounts for the characteristic rash of shingles, or herpes zoster, the clinical manifestation of reactivated varicella infection. The onset of rash is often preceded by pain or abnormal sensation in the distribution of the dermatome. The rash usually heals uneventfully, but pain in the area of the rash may persist long after the rash itself has resolved. This post-herpetic neuralgia is more likely to occur in elderly patients, and can be extremely debilitating.
Other complications of secondary varicella infection occasionally occur: reactivation of virus in the ophthalmic branch of the trigeminal nerve may result in keratitis and damage to the cornea; facial nerve involvement may cause a form of Bell's palsy; motor nerve damage may also be evident in zoster affecting the limbs; involvement of sacral ganglia may result in urinary and anal retention.
A prolonged or atypical attack of herpes zoster may be the presenting feature of a number of diseases in which host immune responses are impaired. These include malignancy of the reticulo-endothelial system and HIV infection. In any immunocompromised patient with zoster, dissemination of the virus in the bloodstream may occur, with the appearance of lesions beyond the initial dermatome. This spread of virus also puts the patient at risk of life-threatening internal organ infection.
The aims of antiviral therapy include alleviation of the pain and discomfort of the rash, and the prevention of complications, including post-herpetic neuralgia and dissemination. Antiviral therapy given early in the course of zoster reduces the incidence of post-herpetic neuralgia and accelerates the resolution of pain. It is therefore logical to use antiviral therapy (aciclovir or its congeners) in the following types of zoster, provided this is started within 72 h of onset of the rash (or longer for immunocompromised patients):
- any form of zoster in an immunocompromised patient, no matter how mild the attack appears to be on first presentation;
- ophthalmic zoster;
- zoster involving motor nerves, including the facial nerve;
- sacral zoster;
- zoster occurring in patients at 50 years of age or above who are therefore at increased risk of post-herpetic neuralgia.
Considerably larger doses of aciclovir are necessary to inhibit Epstein-Barr virus replication than herpes simplex or varicella-zoster virus in vitro. This presumably reflects the differing efficiency of the phosphorylation of the drug by these viruses. The results of clinical trials of aciclovir in patients with infectious mononucleosis have been disappointing. The only manifestation of Epstein-Barr virus infection in which aciclovir is useful is oral hairy leukoplakia. This unusual disease arises only in immunocompromised patients, most commonly those with HIV infection. It presents as a whitish coating on the tongue or buccal mucosa, which may resemble candidiasis. The lesions are packed with replicating virus. The lesions may respond to aciclovir therapy, but reappear when treatment is stopped, so that long-term use of the drug may be necessary.
Primary CMV infections of immunocompetent individuals are usually asymptomatic, although a small minority result in the infectious mononucleosis syndrome (glandular fever). The site of virus latency is currently unknown, and reactivation fails to induce any recognizable disease in the individual concerned. There are two groups of patients, however, in whom the virus is a significant pathogen: babies infected in utero, and immunocompromised patients.
Transfer of CMV across the placenta may arise from both primary and reactivated maternal infection. Congenital infection affects about 1 in 300 live births in the UK. Most of these babies develop normally, but about 5-10% are born with so-called cytomegalic inclusion disease, which is often fatal. A further 5-10% are normal at birth, but later develop abnormalities such as deafness, impaired neurodevelopment, or learning difficulties. There is currently no effective strategy for the prevention of congenital infection. In theory, antiviral therapy may be beneficial for normal babies who are destined to develop abnormalities, but there is no way of identifying the babies at risk, and the toxicity of the available anti-CMV drugs precludes their widespread use for this purpose.
Symptomatic CMV disease may arise from primary or secondary infection in immunocompromised patients, including transplant recipients and those infected with HIV. In these patients, active CMV infection is a multisystem disease. In transplant recipients, it often presents with fever and leucopenia; other complications include hepatitis, myositis, and pneumonitis. In HIV-infected patients, retinitis is the commonest manifestation, accounting for 85% of all CMV disease in patients with AIDS, but infection may also involve any part of the gastrointestinal tract, the brain, lungs, liver, adrenals, and peripheral nerves. Diagnosis is dependent upon clinical suspicion, virus isolation from normally sterile body samples, biopsy, and qualitative measurements of CMV viraemia.
Although several drugs are now available, the goal of a safe, easily administered and effective therapy for CMV infection has yet to be attained. Ganciclovir, foscarnet, and cidofovir must all be given by intravenous infusion, but fomivirsen is administered by intraocular injection. Side effects may be serious: bone marrow suppression (ganciclovir), nephrotoxicity (foscarnet and cidofovir), and painful penile ulceration (foscarnet) are the most notable.
Ganciclovir is the drug of first choice, with foscarnet as an alternative. Experience with cidofovir and fomivirsen is so far restricted to the management of CMV retinitis. Not all complications of CMV infection respond equally well to therapy. Drugs can save the sight of patients with retinitis, and infections of the bowel and liver also respond well, but treatment of CMV pneumonitis in bone marrow transplant recipients is often unsuccessful because immune responses to the virus may also contribute to the disease process.
Therapy does not eliminate the virus, and in patients who remain immunosuppressed, CMV disease often recurs when therapy is stopped. This is a particular problem for AIDS patients with retinitis in whom relapse is almost universal and some form of maintenance therapy is needed. Regular intravenous injections of ganciclovir or foscarnet are effective, but far from optimal, since the drugs penetrate the eye poorly. This has led to the use of intravitreous injections, and the development of intraocular devices designed to deliver drug continuously at the site of disease. A further possibility about which there is still controversy is to give oral valganciclovir, an oral pro-drug of ganciclovir, as maintenance treatment. The restoration of immune function in HIV-infected patients treated with antiretroviral regimens has resulted in a marked decrease in the incidence of CMV-associated disease in these individuals.
Avoiding transplantation of material from latently infected donors to uninfected recipients can significantly reduce serious CMV disease in solid organ transplant recipients. In contrast, bone marrow recipients most at risk of life-threatening CMV disease are those who are seropositive themselves, but receive marrow from a seronegative donor. However, matching of CMV serostatus between donor and recipient in order to prevent the risk of disease is not a practicable solution to the problem. If recipients are deemed to be at risk, administration of CMV prophylaxis is appropriate. The exact regimen is a matter of debate, but trials in different transplant settings have demonstrated the benefit of CMV immunoglobulin, oral ganciclovir, and, surprisingly, oral aciclovir or valaciclovir. Serum levels of aciclovir in these patients are well below the concentrations necessary to inhibit CMV replication, but sufficiently high levels may be achieved inside infected cells.
Human herpesviruses 6, 7, and 8
Human herpesvirus type 6 is the causative agent of roseola infantum, one of the many rashes of childhood. Occasional cases are associated with hepatitis or encephalitis. The virus is sensitive in vitro to ganciclovir, but not aciclovir; however, the value of treatment is unknown. No disease has yet been associated with either primary or secondary infection with human herpesvirus type 7.
Human herpesvirus type 8, also known as Kaposi's sarcoma-associated herpesvirus, is found in almost all forms of Kaposi's sarcoma, although strict proof that the virus is the cause of this malignant disease has not yet been adduced—the virus could be an innocent passenger within the tumour tissue. The weight of evidence does suggest an aetiological role in tumour formation, though any role for antiviral drugs remains to be defined.
Respiratory tract infections
Upper respiratory tract infections
Infections of the upper respiratory tract with rhinoviruses (of which there are over 100 serotypes), corona-, entero-, adeno-, respiratory syncytial and parainfluenza viruses are extremely common, but, fortunately, result in very little serious morbidity or mortality. Despite considerable effort, no specific antiviral therapy has been successfully developed for treatment or prevention of these infections. Intranasal interferon spray is effective in the treatment and prophylaxis of common colds that are caused by rhinoviruses, but patients experience nasal stuffiness, irritation, and bleeding, which negates any possible therapeutic benefit. This approach to the management of the common cold has now been abandoned.
Lower respiratory tract infections
Viral infection of the lower respiratory tract is potentially much more dangerous. The commonest viral infections giving rise to bronchiolitis and pneumonia are those due to respiratory syncytial and influenza viruses. Varicella pneumonia and CMV pneumonitis have been referred to above. In addition, giant cell pneumonia is a rare and fatal complication of measles, usually in leukaemic children who escaped vaccination.
Respiratory syncytial virus infection of infants
Bronchiolitis and pneumonia due to respiratory syncytial virus infection are relatively common in infants under 1 year of age. Ribavirin reduces viral shedding, hastens resolution of fever, improves respiration, and shortens stay in hospital. The main difficulty is that the drug needs to be administered by use of a small particle aerosol generator. Moreover, for maximum benefit, the infant must breathe nebulized drug for at least 12 h each day. Thus, ribavirin is not usually used in otherwise uncomplicated infection in an immunocompetent host. In babies with congenital immunodeficiency or heart or lung defects, ribavirin can be life-saving.
Influenza A (but not B) virus is susceptible to amantadine and rimantadine. Clinical trials in boarding schools and other places in which large numbers of individuals are crowded together have shown that amantadine is effective in the treatment and prophylaxis of influenza A. Those treated with the drug have a milder, shorter-lasting infection. About 70% of contacts given the drug are protected from infection. However, amantadine is unpopular in the elderly, who suffer the brunt of serious influenza, because of its stimulatory actions on the central nervous system. Patients become confused and agitated, and may be unable to sleep.
Amantadine may be of more use in the prevention of spread of infection in an institution in which influenza has already occurred. Vaccination of non-infected individuals may take 2 weeks to induce protection, whereas protection with amantadine is demonstrable after a few hours. The drug is also an alternative for those in whom vaccination is contra-indicated because of egg allergy or for other reasons.
Rimantadine, a derivative of amantadine, is said to be equipotent in its anti-influenzal properties but less prone to side effects. However, it is not currently licensed in the UK. Viral resistance emerges rapidly in patients treated with either of these drugs.
Influenza A and B viruses are sensitive to the neuraminidase inhibitors, zanamivir and oseltamivir, which are the preferred drugs to manage influenza virus infection. Controlled trials have shown a shorter time to the resolution of symptoms and lower usage of antibiotics among patients treated with these drugs. However, they must be given within 48 h of onset of symptoms for maximum benefit—a time frame not often met by patients suffering influenzal symptoms. Moreover, zanamivir has poor oral bioavailability, is not licensed for children under 12 years of age and has to be given by oral or nasal inhalation. Oseltamivir, in contrast, can be administered by mouth in capsules and a 5-day course is recommended. Currently in the UK, it is recommended that neuraminidase inhibitors be used when influenza is circulating in the community for ‘at-risk’ persons, which includes those with chronic respiratory, renal or cardiovascular disease, and diabetes mellitus together with the immunocompromised and those age 65 years or older.
Oseltamivir may also be prescribed prophylactically to prevent influenza in ‘at-risk’ children aged 1 year or older with similar criteria as above, provided prophylaxis can be started within 48 h of exposure. This is only practically possible when influenza virus is known to be circulating in the community.
Chronic viral hepatitis
Patients infected with hepatitis B or C viruses may become chronic carriers. These patients are at risk of continuing inflammation in the liver, leading to chronic hepatitis, cirrhosis, and ultimately, hepatocellular carcinoma.
All chronic carriers of hepatitis B virus have detectable surface antigen (HBsAg) in their peripheral blood. However, HBsAg-positive individuals can be subdivided further into those in whom HBeAg (a breakdown product of the viral core antigen) can be detected, and those with antibodies to this antigen, anti-HBe. HBeAg is a surrogate marker of active viral replication in hepatocytes. HBeAg-positive patients are considerably more infectious, and are at much greater risk of the long-term deleterious consequences of hepatitis B virus carriage, than are anti-HBe positive patients. For these reasons, therapy for chronic infection is targeted at those HBsAg carriers who are HBeAg positive.
Both standard interferon-α and pegylated formulations (p. 106-107) are beneficial in the treatment of chronic hepatitis B infection, but their action is immunomodulatory, rather than antiviral. Interferon enhances the expression of human leucocyte antigen (HLA) class I molecules on the surface of infected hepatocytes resulting in the efficient elimination of these cells by circulating cytotoxic T lymphocytes. This in turn reduces the amount of virus in the liver, and the production of infectious virions. HBeAg in the peripheral blood is replaced by anti-HBe.
A typical response of a hepatitis B virus carrier to interferon is shown in Fig. 27.1. At the onset of therapy, the patient is HBsAg and HBeAg positive, and has a raised alanine aminotransferase level, which indicates continuing liver cell damage. Some weeks after the initiation of interferon therapy, there is a marked rise in alanine aminotransferase. Paradoxically, this indicates a good therapeutic response, since it shows that hepatocytes now express sufficient HLA Class I molecules to allow recognition and destruction by cytotoxic lymphocytes. At the end of therapy, liver function returns to normal and the patient is now anti-HBe positive. Note that the patient remains HBsAg positive. Thus virus has not been completely eliminated, but the risk of ongoing liver damage is much reduced, and the patient is much less of an infection risk to sexual partners and family members. After a number of years, many patients do in fact eventually lose HBsAg.
Interferon is administered for up to 48 weeks. Even so, not all carriers respond to interferon. Those who have been carriers of the virus from birth (the vast majority world-wide) and patients who are immunocompromised (including those with HIV infection, in whom carriage of hepatitis B virus is common, as the routes of transmission of the two viruses are similar) respond poorly. In other patients, response rates are > 30%.
Fig. 27.1 Treatment of chronic hepatitis B infection with interferon showing the change in alanine aminotransferase (ALT) levels, loss of hepatitis B e antigen (HBeAg), and the appearance of antibody to HBe (anti-HBe). Hepatitis B surface antigen (HbsAg) is not eliminated.
New forms of therapy of chronic hepatitis B virus infection are becoming available. These include lamivudine, adefovir, tenofovir, and entecavir, which produce best results when combined with interferon. Lamivudine requires prolonged (> 1 year) oral therapy to produce a 100-fold reduction in viral DNA titre, and histological improvement of liver biopsy appearance in more than 80% of patients. However, seroconversion rates to anti-HBe are much lower, at best 20%. Cessation of therapy leads to recurrence of viral replication. More prolonged therapy may improve the seroconversion rate, but this benefit must be weighed against the increased risk of the emergence of resistant virus. Adefovir and tenofovir are active against lamivudine-resistant mutants. Entecavir is the most potent agent to date.
Co-infection with HIV and hepatitis B virus is increasingly recognized and requires specialist management since agents such as lamivudine and tenofovir are active against both viruses and both infections require multidrug regimens for their management.
Another area in which these newer agents have shown promise is in the prophylaxis and treatment of hepatitis B positive patients undergoing liver and other forms of transplantation. There is a reduction in risk of severe disease in the case of a grafted liver and a reduced rate of virus reactivation associated with other procedures.
The relative success of interferon therapy in chronic hepatitis B infection led to trials of its use in chronic infection with hepatitis C virus. About 40-50% of such patients respond to a year's course of interferon, as judged by improvement in liver function and loss of viral RNA from serum. However, about half relapse when therapy is stopped, resulting in an overall cure rate of less than 20%. The mechanism of action of interferon in hepatitis C infection appears to be different from that in hepatitis B, as response is not accompanied by a rise in alanine aminotransferase levels.
Ribavirin therapy provides modest benefit in liver function in chronic hepatitis C disease. When combined with interferon, response rates are much better than with interferon alone, so that interferon monotherapy is no longer justifiable. The pegylated interferons are more potent and easier to administer and are now standard therapy.
Combination therapy with pegylated interferon and ribavirin is recommended for adults with moderate to severe chronic hepatitis C. Patients are assessed pretreatment for prognostic markers. A poor response is likely in: the presence of genotype 1 infection (hepatitis C virus exists in at least six distinct genotypes); high viral load; age>40 years; pre-existing fibrosis on biopsy; and male sex. Such patients require prolonged treatment (48 weeks). Genotype 2 and 3 infections are more responsive to treatment; 24 weeks' therapy is generally sufficient.
The management of co-infection with HIV and hepatitis C virus is particularly challenging and requires specialist selection and supervision. The timing of treatment of hepatitis C in relation to the stage of HIV infection is crucial.
The management of HIV-infected patients has undergone enormous changes in the past few years. Advances in drug design and in understanding the replication cycle of HIV have led to the development of an increasing number of antiretroviral agents (see Table 7.1, p. 110). Moreover, application of molecular biological techniques allows accurate monitoring of the amount of HIV RNA in peripheral blood (viral load) in individual patients. Treatment regimens and management protocols are becoming ever more complex. In order to achieve some sort of standardization of clinical practice, bodies such as the British HIV Association publish consensus guidelines for antiretroviral treatment of HIV-seropositive individuals. Such is the pace of change that these guidelines need annual revision. It is nevertheless possible to discern some important underlying principles governing the appropriate use of antiretroviral therapy, although their detailed application may vary from place to place and over time.
Initially zidovudine (AZT) was the only option available. However, resistance invariably develops through the selection of mutations in the error-prone reverse transcriptase gene, and AZT monotherapy loses its clinical benefit after about 6 months.
With the advent of more antiretroviral drugs, it became possible to model the treatment of HIV infection on that used for the treatment of tuberculosis: use of multiple agents to reduce the chances that the virus will become resistant to all of the drugs simultaneously. This principle has now been validated for a variety of combination therapies and has become the standard of care.
Triple drug combinations are highly effective in reducing viral load, both in degree and in duration of the effect. With the choice of agents including nucleoside analogues, non-nucleoside reverse transcriptase inhibitors, protease inhibitors, and a fusion inhibitor (Table 7.1, p. 110), there are numerous possible triple drug combinations. The continuing emergence of new agents will complicate this even further. It is clearly not possible to test each one of these combinations in full-scale clinical trials, but several regimens have proved to be highly effective. Such combination therapies are now collectively referred to as ‘highly active antiretroviral therapy’ (HAART).
Regimens recommended in the UK include two nucleoside reverse transcriptase inhibitors plus either a non-nucleoside reverse transcriptase inhibitor or a protease inhibitor. The reason for not selecting one drug of each category, which might appear intuitively to be the best idea, is to reserve one class of drugs for the time when initial therapy begins to fail, as it inevitably does. At this time, the virus present within the patient should at least be fully sensitive to all drugs of the unused class. In the absence of such ‘class-sparing’, the options for salvage therapy in the event of treatment failure are much reduced, as cross-resistance between drugs of the same class is common.
The exact choice of drugs for an individual patient depends on a number of variables. It cannot be assumed that virus in a patient who has received no previous antiretroviral therapy will be sensitive to all drugs, as the patient may have acquired a resistant strain. Such primary drug resistance now affects more than 10% of newly diagnosed patients in the UK. Moreover, adverse events are often unpredictable and regimens may need tailoring to individual patients. Clearly, the more drugs a patient takes, the greater the risk of adverse reactions. For example, there is particular concern about the long-term safety of protease inhibitors, owing to their effect on lipid metabolism.
Monitoring of therapy
For many years, the antiviral effect of drugs used in HIV-infected individuals could be determined only by measuring the level of circulating CD4-positive T cells as a surrogate marker of infection. The advent of viral load testing provides a much more direct measure of the efficacy of an antiviral drug. Viral load is of prognostic importance: the higher the load in an individual patient, the sooner that individual will develop AIDS and die. Thus the goal of antiretroviral therapy is to reduce viral load to undetectable levels. It is now possible routinely to detect as few as 50 copies of viral RNA per ml of plasma.
Failure of treatment for whatever reason can be detected by a rise in viral titres. Thus, long-term antiretroviral therapy is monitored by serial viral load measurements, and action taken if these levels rise. If failure is due to the emergence of resistance to one or more of the agents being taken, then appropriate changes in regimen should be instituted. It is possible to analyse the genome of the HIV present in a patient who is failing therapy, and identify which resistance mutations are present. Drug-resistance genotyping is expensive but has become integral to defining initial therapy and for identifying resistance arising during treatment. Comparative trials have shown that genotypic resistance testing confers a significant benefit on the virological response when choosing therapeutic alternatives.
In parallel with the reduction in viral load, CD4 counts rise, and there is at least a partial restoration of the previously damaged host immune system. As the immunodeficiency recedes, then the risk of those complications of HIV infection that define the acquired immune deficiency syndrome also recede. As a consequence, it may be possible to discontinue prophylaxis against infections such as pneumocystosis, toxoplasmosis, and cryptococcosis in patients responding successfully to HAART.
In addition to monitoring viral load and CD4, patients stabilized on HAART are reviewed every few months for clinical assessment, lipid analysis, and evidence of organ or bone marrow toxicity arising from treatment or complications of their HIV infection.
When to treat?
Given that there are now drugs available that can substantially decrease the viral load within an individual patient, and that this can be monitored, the question arises as to when antiretroviral therapy should be started. This decision is an important one. It requires commitment from the patient to take the medication for a prolonged period, usually for life. This may incur both physical and psychological morbidity, disadvantages that have to be weighed against the potential therapeutic benefits to be gained. At present, UK guidelines recommend starting therapy in any patient with symptomatic HIV infection or AIDS regardless of their viral load or CD4 count. In those who are asymptomatic the CD4 count is the major determinant for starting therapy. All those with counts < 200 cells/mm3 are treated. Those with counts between 200 and 350 cells/mm3 are evaluated further for viral load, rate of CD4 decline, evidence of coexisting conditions and patient preference. Currently, treatment is deferred for those with CD4 counts above 350 cells/mm3. Evidence for long-term survival benefit of early treatment of primary HIV is lacking and this is no longer an indication for treatment.
A special consideration is the use of antiretroviral drugs in pregnancy. There is compelling evidence that the risk of vertical transmission of HIV from mother to baby is proportional to the maternal viral load during pregnancy. Reduction of maternal viral load by use of antiretroviral agents significantly reduces the risk of transmission. Thus, all pregnant HIV carriers should be offered antiretroviral therapy regardless of whether or not they would otherwise qualify for such treatment. The choice of regimen will depend on the patient's past history of antiretroviral usage. Delivery should preferably be by caesarian section to further reduce the risk of virus transmission to the newborn. Breast feeding is not advised. In the UK, antiretroviral drugs are started between 28-32 weeks of pregnancy and during labour. The newborn is also treated for 6 weeks post-partum.
Importance of compliance
Drugs are only effective for as long as their concentration in the tissues is high enough. Failure to take the requisite regular doses of an antiviral drug leads to a fall in tissue levels, with a consequent risk of escape of the virus from inhibition of replication, and an increased likelihood of mutation to resistance. It is important, therefore, that patients adhere to the prescribed regimens. The more doses that are missed, the faster resistant virus will emerge.
Compliance is a significant problem, especially with multidrug regimens, in which some tablets should be taken with food and others on an empty stomach; some twice a day, others three or four times a day. By combining drugs in fixed dose formulations, compliance can be increased and is now the preferred treatment approach. Patients may also be on a variety of other agents; e.g. for prophylaxis against pneumocystis pneumonia or recurrent herpes simplex disease. Moreover, in any disease, and HIV infection is no exception, adherence to treatment is poorer in patients with symptomfree disease. Failure to take the tablets must always be considered as an explanation for a sudden rise in viral titre.
The complexity of HIV disease and its attendant complications, most notably of opportunistic infections (Chapter 28), require specialist management over many years. While HAART has substantially improved the survival and quality of life of those affected by HIV/AIDS, the disease remains incurable. New therapeutic approaches are needed, with novel modes of action and fewer side effects. This will remain the situation until an effective vaccine is developed, which despite an enormous research effort, remains an elusive goal.
HIV-positive patients are at risk of a wide range of serious infections. These are discussed in Chapter 28.
Papillomavirus infections (warts) are amenable to physical (cryotherapy; surgery) and chemical (podophyllin and derivatives; salicylic acid) therapies. Interferons may have a place in the management of recalcitrant warts. About 50% of warts disappear following intralesional or intramuscular interferon, but they often recur. Topical application of imiquimod, a new imidazoquinoline that stimulates interferon and other cytokines, appears to be modestly effective.
Progressive multifocal leucoencephalopathy, a disease that occurs almost exclusively in HIV-positive and other immunosuppressed patients, is a manifestation of reactivation of a polyomavirus in the brain. Although it is of limited availability, the cytotoxic drug, cytosine arabinoside (Ara C) has been successfully used. Cidofovir (p. 101) may prove to be more potent and reliable.
This large group of viruses causes a wide range of clinical disease, from skin rashes in children to paralytic polio. The more serious infections include meningitis and, in immunodeficient patients, chronic enteroviral encephalitis, a rare but extremely debilitating disease. Antiviral therapy presently has little to offer these patients, but this may change. The investigational drug, pleconaril, prevents enteroviral entry into target cells by interfering with the binding of virus to its cellular receptor. Clinical trials are currently being conducted.
Ribavirin, taken orally, is very effective in the treatment and prophylaxis of Lassa fever, a haemorrhagic virus infection with a high mortality rate encountered in certain parts of West Africa.
The treatment of viral gastroenteritis is considered in Chapter 21.