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AIDS is still an incurable disease, although patients’ suffering can certainly be alleviated. Counselling and support are important and include:

• discussing the diagnosis
• how transmission of the virus takes place
• how transmission can be prevented
• how one can best preserve health for as long as possible
• how to tackle the many AIDS problems that occur

Above all, listen to the patient. Patients will benefit from a friendly doctor sympathetically listening to their story, being able to give a clear and logical explanation for their discomfort and reassuring them that they are not alone in this.

The various antiviral drugs are practically unavailable in developing countries although this situation is changing rapidly.

There are several types of reverse transcriptase inhibitor

• nucleoside analogues
• non-nucleoside analogues
• nucleotide analogues

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A nucleoside consists of a sugar or saccharide (ribose or deoxyribose) and a base (pyrimidine or purine). Nucleoside compounds have to be metabolised to triphosphate compounds before they become active, in contrast to non-nucleoside compounds, which are directly active. As nucleoside analogues have to be phosphorylated in the cell before becoming active, antagonism between molecules that use the same phosphorylation pathways should be taken into account. Hence AZT and d4T as well as ddC and 3TC are not combined with each other. Furthermore, combinations of drugs with a similar toxicity profile, such as ddC + ddI or ddC + d4T should be avoided.

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Various drugs in this class can disturb the gamma-DNA polymerase in the mitochondria, resulting in mitochondrial dysfunction. This is characterised by myopathy, cardiomyopathy, neuropathy, liver steatosis and/or lactate acidosis. [Humans have 5 different DNA polymerases: alpha (replication lagging strand nuclear DNA), beta (repair nuclear DNA), gamma (replication mitochondrial DNA), delta (replication leading strand nuclear DNA), epsilon (repair nuclear DNA). Do not confuse these with the prokaryotic DNA polymerases I, II and III].

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The intracellular half-life of the nucleoside-analogue triphosphates shows considerable variation:

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These substances bind to reverse transcriptase and inhibit the enzyme.

• Nevirapine (Viramune®). The recommended dose is one 200 mg tablet/day for 2 weeks, followed by one 200 mg tablet twice daily. This initial regimen is necessary as the compound induces its own breakdown. If initially 400 mg per day is given this would result in overdosing. Hypersensitivity reactions with skin rash occur. The blood level falls by 35% if St John’s wort is taken at the same time (interaction via CYP3A4). The drug is destined to play an important role in the prevention of neonatal transmission in developing countries. In July 2000 Boehringer Ingelheim Company decided to offer Viramune® free for a period of five years for the prevention of transmission of HIV-1 from mother to child in certain developing areas. In some countries, like Thailand, certain generic fixed drug-combinations, such as d4T, 3TC and nevirapine (GPO-Vir) are supplied in one pill. Taking a pill twice per day costs half a Euro (i.e. half a US dollar).

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• Tenofovir (Viread®). Tenofovir disoproxil fumarate (Viread®) is administered as a 300 mg tablet once a day, preferably with a meal. It has less side-effects than most other drugs. Nucleotide analogues closely resemble nucleoside analogues such as Retrovir® or Epivir®. The only difference is that nucleotide analogues are chemically preactivated and thus have to undergo less biochemical processing in the body before becoming active. Tenofovir is studied as a preventive once-per-day pill for high-risk groups such as prostitutes. If the results of these studies would show a positive preventive effect, it would be important not to loose sight of condom-use as a preventive stategy.

In contrast to AZT and similar molecules, the protease inhibitors inhibit maturation of HIV proteins. They inhibit a viral enzyme (protease) that cleaves viral protein precursors into their separate components. Protease inhibitors have the disadvantage that a large number of pills have to be taken. There are numerous side-effects. Multiple interactions with other drugs are possible. Lipodystrophia with redistribution of body fat, hypercholesterolaemia, hypertriglyceridaemie and diabetes mellitus can occur. Insulin resistance may occur due to the blockade of certain cellular saccharide transport molecules (glucose transporter 4). There is evidence that protease inhibitors such as indinavir and saquinavir inhibit the development of Kaposi's sarcoma, not only because of their antiviral effect but also by direct interference with the angiogenesis in the tumour.

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• Saquinavir (Invirase®, Fortovase®). Invirase® is poorly absorbed from the gastrointestinal tract (bioavailability about 4%). Boosting with ritonavir raises the blood level of saquinavir. Ingestion together with grapefruit juice considerably increases the absorption (see also cytochrome P450). An alternative is Fortovase® 1000 mg twice daily, each time with 100 mg ritonavir as a booster. Fortovase® can be kept for up to 3 months without refrigeration. Invirase 500 mg tablets are expected to become available soon.
• Ritonavir (Norvir®). This is very potent antiviral product, but is frequently poorly tolerated due to gastrointestinal side-effects. Nausea and perioral paraesthesias are frequent. Ritonavir needs to be given only twice per day, preferably with meals. The syrup contains 40% alcohol. The bitter taste can be masked by mixing it with chocolate milk. It is a potent cytochrome P450 (CYP3A4) inhibitor, both in the intestinal wall and liver. This is the most important enzyme for the metabolism of protease inhibitors. There is a substantial first-pass metabolism. There are many interactions with other medications, including, among others, saquinavir and indinavir, so that the dose of these latter substances can be reduced (inhibition of breakdown). It is at present mainly used for its "booster effect" and not so much for its direct antiviral effect. Kaletra® (see further ) is an example of this.
• Indinavir (Crixivan®). This substance is usually tolerated quite well, though 3 doses each of 800 mg (if without Norvir®) are required per day. It should be taken away from meals. Indinavir penetrates into the cerebrospinal fluid and should also play a part in the prevention of AIDS dementia (HIV-encephalopathy). There are many interactions with other medications. If St John’s wort is taken at the same time the blood level falls by 80%. Kidney stones, renal insufficiency and haemolytic anaemia can occur. It is advisable to drink plenty of fluid when taking Crixivan®.
• Nelfinavir (Viracept®). The 250 mg tablets are initially taken in a dose of 750 mg three times a day. Undesirable side-effects of nelfinavir mesylate include gastrointestinal disorders such as diarrhoea. Monitoring liver function is advised. Easier regimens such as five 250 mg tablets twice per day, have been subsequently worked out. Introduction of 625 mg tablets is expected.
• Amprenavir (Agenerase®). This drug is best used in combination with ritonavir. In this way, a reduced number of tablets can be taken (600 mg twice per day instead of 8 x 150 mg capsules twice per day).
• Lopinavir/ritonavir (Kaletra®). Lopinavir is a protease inhibitor. The compound has been marketed in a fixed combination with ritonavir (both drugs in one tablet: Kaletra®). Ritonavir is not actually used here as an antiviral agent as such, but serves to raise the plasma concentration of lopinavir via inhibition of breakdown of the latter. Kaletra® can be kept unrefrigerated for only one month.
• Atazanavir (Reyataz®, earlier name Zrivada). Atazanavir is a novel protease inhibitor that needs to be given only once per day (400 mg). The medicament does not cause any hyperlipidaemia, in contrast to other drugs of the same class. Hyperbilirubinaemia can occasionally occur (without liver function disturbances).
• Tipranavir. Tipranavir is very promising as it shows no cross-resistance with the first protease inhibitors. It is a potent and non-peptidic HIV-1 protease inhibitor, the first of its kind. It can be "boosted" by subtherapeutic levels of ritonavir (e.g. TPV/r 500/200 mg twice per day). The most common side effects are gastrointestinal disturbances (diarrhoea, nausea, vomiting).

Resistance to various antiviral agents is increased by underdosing, monotherapy and irregular dosings. New drugs will be necessary. There are various drugs with antiviral activity that are at present still in an experimental stage. Lodenosine, entecavir, emivirine, capravirine, calanolide A and diaminopurine-dioxalane are just a few of a wide range of drugs. An Indian company produces the combination nevirapine 200 mg + stavudine 30/40 mg + lamivudine 150 mg under the name Triomune®. Fluorocytidine shows activity against both HIV and HBV. A once-daily dose of 200 mg reduced HIV viral load by 1.9 log copies/ml.

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Hydroxyurea (Hydrea®) has an antiviral activity and is synergistic with ddI, but potentiates the toxicity of the latter (idem D4T). The mode of action is still not clear, though hydroxyurea probably inhibits the synthesis of deoxynucleotides by blocking the enzyme ribonucleotide reductase. Its routine use is not recommended, since its clinical efficacy has not yet been demonstrated.

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The following products are not often used:

• Delavirdine (Rescriptor®): not available in Europe. It is associated with maculopapulous rash and itching, fever, conjunctivitis and joint pain.
• Loviride is no longer used.
• Adefovir dipivoxil (Preveon®). This is a nucleotide-analogue and has been abandoned. This substance contains a phosphoryl group and does not need to be phosphorylated (does not require any intracellulair transformation to form an active molecule). Due to possible side-effects on the mitochondria, it was best taken together with L-carnitine. The latter compound has an important role in the transport of long-chained lipids through the membranes of mitochondria. These latter have a role in energy production (beta-oxidation) and in the production of cardiolipine, a phospholipid (diphosphatidylglycerol).

Combinations of drugs are at present being used in the West (compare with tuberculosis and leukaemia chemotherapy). Various cocktails, that often have to be individually adapted, are used. As first-line treatment the WHO advises a scheme with 2 nucleoside analogues in combination with an NNRTI ("highly active antiretroviral therapy" or "HAART"). Alternatively, the combination of 2 nucleoside analogues and a protease-inhibitor can be used. Zidovudine, stavudine, lamivudine, abacavir, nevirapine and efavirenz penetrate into the cerebrospinal fluid and can thereby have a preventive effect on the occurrence of AIDS dementia. Any combination treatment should, therefore, contain one of these molecules. Videx® does not penetrate very well into the cerebrospinal fluid.

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The recommended therapy is a triple therapy consisting of:

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Initial treatment regimens with minimal patient inconvenience:

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Combinations of reverse transcriptase inhibitors to be avoided:

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Clinicians are aware that the development of resistance is a significant problem in achieving long-term success with highly active antiretroviral therapy (HAART). However, resistance is not an all or none phenomena, but instead comes in gradations. These gradations are defined by the increase in the inhibitory concentration of a particular drug to a level higher than that of wild-type viruses. In some cases viral resistance can be overcome by increasing plasma concentrations, something that is routinely done when ritonavir is used to slow down degradation of protease inhibitors ("boosted PI). The use of the C-trough/IC50 ratio (Inhibitory quotient or "IQ") has been found to be a useful marker of antiretroviral efficacy.

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Medication which has to be taken only once daily is easier to take than medication which requires multiple dosing. Once daily regimens improve compliance.

Can be given once daily : Viread®, Emtriva®, Videx EC®, Zerit XR®, Stocrin®, Reyataz®

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The viral load is measured with various techniques such as quantitative PCR (HIV RNA or bDNA, branched DNA) and is expressed as the number of viral RNA copies per millilitre of blood. Combination antiviral treatment with at least 1 protease inhibitor reduces the viral load in 80% of treated patients to below the present detection limit. However, it is still not yet known how long this effect lasts. A viral load of >30,000 copies of RNA/ml is often regarded as an indication for antiviral drug therapy. A drug is considered effective when a reduction of at least 90% of the initial viral load can be achieved. It is of course best to achieve an undetectable viral load. Most current techniques have a detection limit of 50 RNA copies per ml. The question to what extent the viral load in the blood reflects the viral load in various tissues has not yet been satisfactorily answered. Some HIV strains are difficult to detect with the currently available commercial techniques.

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Nobody has a hard and fast answer to this and the recommendations differ somewhat. Some experts advise an early start (as soon as the viral loading exceeds 5,000-10,000 copies/ml) regardless of the CD4-cell count. This means that the great majority of patients should be treated. Is this desirable? Arguments based on virology and immunology (with the necessary uncertainty and regular new information) sometimes conflict with the clinicians' findings (also with their uncertainties, subjectivity and doubts). HIV is a chronic infection and the question arises whether patients can take these cocktails for 10 or 15 years and tolerate the side-effects for all this time. Compliance (or non-compliance) with therapy is a very important issue. Furthermore, there is the cost of all this. On the other hand experience with the drugs is limited in time and new clinical and therapeutic data are constantly appearing. Tritherapy is usually started in symptomatic patients, patients with less than 350 CD4 cells/m l and patients with a viral load of more than 30,000 copies/ml. However, these criteria serve only as guidelines. They should be individually adjusted and a decision made for each individual patient. Antiviral treatment is potentially dangerous for the foetus in the first three months of pregnancy.

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In developing countries it is best to wait before starting antiviral therapy, certainly if the patient has insufficient financial means to continue the treatment. Moreover, protease inhibitors are difficult to combine with the classical antituberculosis treatments. The instability of certain products in high temperature environments should be taken into account (e.g. Kaletra).

Many interactions between protease-inhibitors and other drugs have been described. When in doubt, an up to date table should be consulted. It is however useful to have some idea of which drugs cause no major interactions. The following are likely to be safe:

Metabolism is an important elimination pathway for lipophilic drugs. The biotransformation of drugs takes place mainly in the liver, but also in the intestinal mucosa and lungs. A number of drugs undergo partial or complete biotransformation in the liver and/or the intestine before they get into the systemic circulation. This is the first-pass metabolism. Oxidation is often an important stage in the biotransformation and takes place via mono-oxygenases or "mixed-function" oxidases. Cytochrome P450 is the key enzyme in these reactions. The enzyme uses NADPH and oxygen. The term cytochrome P450 (CYP) in fact covers a large number of isoenzymes which are subdivided into families and subfamilies based on similarities in amino acid composition. These families are designated with a number (e.g. CYP3). Subfamilies are designated with a letter (e.g. CYP3A). Individual isoenzymes are again designated with a number (e.g. CYP3A4).

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More than 30 CYP-isoenzymes have been identified in man. CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4 are especially important for biotransformation of many clinically used drugs. There is also a substantial variability in the enzyme activity of cytochrome P450. This may be responsible for the variability in pharmacokinetic parameters and for the therapeutic response. Genetic factors and interactions with drugs or other substances (inhibition or induction) are two important causes of this variability. For many drugs the expression of the enzymes responsible for the biotransformation is polygenetically controlled. This means that several genes control their metabolism. This is responsible for part of the unpredictable "spontaneous" interindividual variability in biotransformation. For some medicines the expression of enzymes responsible for the metabolism is monogenetically controlled. This means that only one gene controls the metabolism. For these medicines the interindividual differences in pharmacokinetic parameters and in therapeutic response are much more clearly identifiable. If there are two or more variants with different activity (genetic polymorphism) for such a gene, there must be at least three genotypes and two phenotypes at the population level, e. g. homozygotes, heterozygotes; rapid metabolisers and slow metabolisers. Genetic polymorphism for cytochrome P450 has hitherto been reported for oxidative reactions catalysed by CYP2D6 and CYP2C19. Between 5 and 10% of the Caucasian population are slow metabolisers of medicines metabolised by CYP2D6 (dextromethorphan for example). Approximately 2 to 5% are slow metabolisers of medicines metabolised by CYP2C19. In a slow metaboliser, administration of the usual dose can lead to excessively high plasma levels of the drug, due to a decreased elimination or first-pass metabolism. On the other hand the effectiveness of the drug can be reduced in a slow metaboliser when the effect of a medicine is due to its metabolite. This is demonstrated, for example, by codeine which, under the influence of CYP2D6 is normally metabolised to morphine which causes the analgesic effect. In slow metabolisers, in whom the gene coding for CYP2D6 is defective or absent, this transformation does not take place, resulting in a reduction of the analgesic effect.

Some medicines are potent inhibitors of a certain isoenzyme. For instance, all protease inhibitors inhibit cytochrome P450. Ritonavir is an important inhibitor of CYP2D6 and CYP3A4. Hence, a low dose of ritonavir can be used in order to reduce the dose of another protease inhibitor (basis of the combination Norvir®-Crixivan® and Kaletra®). Competition can occur when two drugs that are substrates for the same CYP isoenzyme are administered simultaneously. This can lead to inhibition of the metabolism of one or both medicines. Some medicines inhibit the activity of a certain CYP isoenzyme without themselves being a substrate for it. An example is quinidine, which is a potent inhibitor of CYP2D6, but which is itself metabolised by CYP3A4. Foodstuffs can also interfere with the metabolism of medicines. Certain constituents in grapefruit juice inhibit CYP3A4, thereby increasing the bioavailability of terbinafine, calcium antagonists of the dihydropyridine group and cyclosporine, among others.

In most developing countries antiviral agents are scarcely available, if at all. Determining resistance in this context is, therefore, not very meaningful. Nevertheless, it is useful to be familiar with the underlying principles. In the West resistance is determined: (1) in a pregnant woman before initiating or after failure of therapy when a new treatment is being considered, (2) upon initiating antiviral therapy in a child, (3) after failure of therapy when a modification of the therapy is being considered. It should be considered (4) for an as yet untreated patient before the start of therapy, especially if this is at the time of seroconversion. Viral mutants can in fact "disappear" after the acute phase (they are less easily detected) as they have a lower fitness, in the Darwinian sense of the word. If resistance is not determined in the acute phase of the disease, it is nevertheless advisable to store plasma so that such an investigation can be carried out later. In (5) post-exposure prophylaxis an attempt should be made to get a sample from the index case. In each case one should not wait until the results of the resistance determination are known before starting treatment. However, when the results become available, treatment can be adjusted as necessary.

Tests for resistance determination should be carried out with viral RNA derived from plasma, before therapy is stopped and before starting new therapy. It is only possible if there is a detectable viral load (preferably > 1000 RNA copies/ml plasma).

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• Sequencing: amplification by RT-PCR and automatic sequencing. There is significant interlaboratory variability. In half of the laboratories it has been found that at least 25% of the virus population must consist of mutants before they can be detected.
• LIPA: Line Probe Assay for RT. Detects better some mixed virus populations but can miss certain mutations.

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• Virus culture: difficult and time consuming.
• Recombinant virus: the RT and Protease genes of the virus that is to be tested are integrated in a laboratory virus from which these genes have been deleted. This virus is then cultured. With this technique 25% of the virus population must also consist of mutants before any difference in sensitivity can be detected.
• Virtual phenotype: sequencing of the mutant virus and use of a software program to predict sensitivity/resistance.

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