acquired immunodeficiency syndrome

Pharmacology & Therapeutics 89 (2001) 17 ± 27

Antiretroviral therapeutic possibilities for human immunodeficiency virus/ acquired immunodeficiency syndrome G.A. Balint* Laboratory of Clinical Pharmacology, Department of Psychiatry, New Clinics, University of Szeged, Medical Faculty, G.P.O. Box 427, H-6701 Szeged, Hungary

Abstract In human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) illness, the reverse transcriptase and protease (PRT) enzymes of HIV are currently the targets of antiretroviral (ARV) therapy. Nucleoside analogues were the first group of ARV drugs that exerted antiviral activity in patients. More recently, PRT inhibitors have provided new approaches in the treatment of HIV infection and AIDS. Impressive clinical results have been obtained with combination therapies of three ARV drugs, including one PRT inhibitor. It is worth mentioning also that apart from these two main drug groups, there are many new compounds under development, including a vaccine(s) against HIV. D 2001 Elsevier Science Inc. All rights reserved. Keywords: HIV; AIDS; Therapy; Drug resistance Abbreviations: AIDS, acquired immunodeficiency syndrome; ARV, antiretroviral; AZT, zidovudine, azidothymidine; ddA, dideoxyadenosine; ddC, zalcitabine, dideoxycytidine; ddI, didanosine, dideoxyinosine; HIV, human immunodeficiency virus; IND, indinavir, MK-639; PRT, protease; RIT, ritonavir, ABT-538; RTC, reverse transcriptase; SQV, saquinavir, Ro-31-8959; 3-TC, lamivudine, thiacytidine, BCH-189

Contents 1. Introduction . . . . . . . . . . . . 2. General aspects . . . . . . . . . . 3. Reverse transcriptase inhibitors . . 3.1. Nucleoside analogues . . . 3.1.1. Zidovudine . . . . 3.1.2. Zalcitabine . . . . 3.1.3. Didanosine . . . . 3.1.4. Lamivudine . . . . 3.2. Non-nucleoside analogues . 3.2.1. Nevirapine . . . . 4. Protease inhibitors. . . . . . . . . 4.1. Saquinavir . . . . . . . . . 4.2. Ritonavir . . . . . . . . . . 4.3. Indinavir . . . . . . . . . . 5. Drug resistance . . . . . . . . . . 6. Combined therapy. . . . . . . . . 7. Future aims . . . . . . . . . . . . 8. Summary . . . . . . . . . . . . . References . . . . . . . . . . . .

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* Tel.: +36-62-545-366; fax: +36-62-545-973. E-mail address: [email protected] (G.A. Balint). 0163-7258/01/$ ± see front matter D 2001 Elsevier Science Inc. All rights reserved. PII: S 0 1 6 3 - 7 2 5 8 ( 0 0 ) 0 0 1 0 1 - 7

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G.A. Balint / Pharmacology & Therapeutics 89 (2001) 17±27

``. . . Prevention, prevention, prevention . . .'' Dr. Gro Harlem Brundtland, Director-General of W.H.O., UNAIDS Programme Coordinating Board, May 25, 2000

1. Introduction The acquired immunodeficiency syndrome (AIDS) was first recognised in 1981, and the human immunodeficiency virus (HIV), the virus that causes AIDS, was identified in 1983. Since the start of the global HIV/AIDS epidemic, more than 30 million people are thought to have been infected with HIV,  30 million adults and 2.6 million children. Of these, an estimated 5 million adults and 1.5 million children have already died. Today,  23 million persons are estimated to be living with HIV infection or AIDS. Of these, 21 million are adults and 1 million are children (WHO (UNAIDS), 1996). The virus is now being transmitted virtually in all countries. Taking into consideration the above-mentioned facts, the development of antiviral drugs against HIV has had high priority, but so far, has resulted in only two classes of drugs that have moved into general clinical practice. In this review, the author aims to give an account of these drugs from the pharmacological, as well as from the clinical, point of view. Moreover, the author wishes to emphasise that he is fully aware that the problems mentioned are much more complex and have many more and other aspects Ð mainly, but not entirely, in the field of new drug development Ð than those dealt with in this review.

The main steps of both types of HIV multiplication cycles are presented in Fig. 1. Currently, the reverse transcriptase (RTC) and protease (PRT) enzymes of HIV are the targets of antiretroviral (ARV) therapy. Nucleoside analogues were the first class of drugs that demonstrated antiviral activity in treated patients. More recently, PRT inhibitors have provided new possibilities in the treatment of HIV infection. At present, the ARV drugs listed in Table 1 are in clinical use. The HIV-RTC is a polymerase and a very error-prone and relatively not very specific enzyme, which creates problems in clinical practice through the rapid development of resistance. However, the nature of this enzyme also allows the

2. General aspects Two distinct retroviruses, HIV-1 and HIV-2, cause HIV infection in humans. Both are enveloped, positive-strand RNA viruses, belonging to the lentivirus subfamily. These RNA viruses (with the help of different enzymes) integrate their genetic material into host cells, causing a long course of (chronic) infection and disease. HIV-1 is the predominant type of HIV throughout the world. HIV-2 was first identified in West Africa (in the mid1980s), and is still found primarily there. HIV-1 and HIV-2 are structurally similar, are transmitted by the same routes, and are diagnosed with comparable laboratory (serologic or virologic) assays. Cross-reactivity between the two types on serologic assays is common, and, therefore, virus-specific assays are required to distinguish the two infections. HIV-2 has proven to be a less aggressive virus than HIV1, both in terms of transmission and pathogenesis. Both sexual and perinatal (vertical) transmission rates of HIV-2 are lower than those of HIV-1.

Fig. 1. The most important steps of HIV infection and HIV replication. Step 1: Binding to the surface receptors of CD4 + cells. Step 2: The HIV-RNA passes into the cell. Step 3: The HIV genome (RNA), with the help of RTC (and in the presence of nucleotide phosphates), incorporates into the DNA of the host cell. The RTC enzyme inhibitors (RTC-inhib.) act on the third step of this process. Step 4: Due to the reverse transcription, the HIV-RNA became a part of the DNA of the host cell. Step 5: The infected cell operates like a virus factory. In this step, the HIV-PRT process, the immature virus particles form infectious virions. This fifth step is the site of action of the PRT-inhibitor (PRT-inhib.) drugs.

G.A. Balint / Pharmacology & Therapeutics 89 (2001) 17±27 Table 1 ARV drugs in use RTC inhibitors: Nucleoside analogues: AZT, ddC, ddI, 3-TC Non-nucleoside analogues: nevirapine PRT inhibitors: SQV, RIT, IND

possibility of inhibition, using analogues of its normal substrates, the deoxynucleoside triphosphates. The HIV-RTC copies the viral RNA first to RNA-DNA hybrid, then it removes the RNA component by RNAse hybrid, which is part of the RTC dimer. Finally, it forms viral double-stranded DNA. Subsequent integration of viral DNA into cellular DNA is mediated by HIV endonuclease, a possible target for inhibitors that has not been utilised yet. The integrated viral DNA is then transcribed by cellular polymerases and translated by the cellular machinery to viral proteins. On the other hand, HIV-PRT is also an essential component of the replicative cycle of HIV, performing the post-transitional processing of the gag and gag± pol gene products into the functional core of proteins and viral enzymes. Inhibition of this enzyme leads to the production of immature, noninfectious viral progeny, and, hence, the prevention of a further round of infection. Structurally, the enzyme is a homodimer, consisting of 2 identical 99 amino acid chains.

3. Reverse transcriptase inhibitors

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dated in the liver and is excreted in the urine (Yarchoan et al., 1986). In the initial clinical trial (Fischl et al., 1987), patients received 1500 mg/day of AZT, which was modified to 1200 mg/day at the time of registration. Subsequent studies have shown that 400 ±600 mg/day have the same effect as higher doses (Sandstrom & Oberg, 1993a, 1993b). In clinical practice, the dosage regimen of 200 mg every 8 hr has come into widespread use. According to the pertinent literature, the essence of HIV infection is the highly significant decrease of the so-called CD4 + lymphocytes, sometimes down to 0. (The normal cell count is  1500 ± 2000/mm3.) The CD4 + lymphocytes are lymphocytes of thymus origin, with a helper phenotype. They have an important role(s) in different cellular immune responses. During AZT treatment, the CD4 + cell count increases. Therefore, the patient's immune response improves. Moreover, the HIV-characteristic p24 antigen level simultaneously decreases. According to the Expert Committee of the National Institute of Allergy and Infectious Diseases (Bethesda, MD, USA), the introduction of AZT therapy is strongly advised: 1. when the HIV-positive patient's CD4 + cell count decreases below 500, regardless of whether the patient has or has not any symptom(s); 2. currently, there is no better (laboratory or other) indicator of the commencement of AZT therapy than the observation of CD4 + cell count (Editorial, 1990).

3.1. Nucleoside analogues The chemical structures of the nucleoside analogues are shown in the Fig. 2. 3.1.1. Zidovudine Zidovudine (azidothymidine, AZT) was first synthesised by Horwitz et al. in 1964, and was found to have ARV activity in 1974 by Ostertag et al. Its HIV-inhibitory effect was proven by Mitsuya et al. in 1985. AZT is a prodrug that is phosphorylated intracellularly by kinase enzymes to an active triphosphate metabolite (Mitsuya et al., 1985). This AZT-triphosphate has a specific, selective affinity for HIV-RTC, causing a chain termination and inhibition of this enzyme. The dose that causes 95% viral inhibition (ID95) is below 1 mmol/L (Sandstrom & Oberg, 1993a, 1993b). The oral bioavailability of AZT is  66%, but this may differ greatly among individuals. It can be found in the cerebrospinal fluid, too, and its protein binding is  35% (Klecker et al., 1987). AZT enters the cells by passive diffusion. Its plasma halflife (t1/2) is of the order of 1 hr. AZT is largely glucuroni-

Fig. 2. The chemical structure of the nucleoside analogue RTC inhibitor drugs.

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G.A. Balint / Pharmacology & Therapeutics 89 (2001) 17±27

Apart from its positive effect on the CD4 + cell count and on the frequency of opportunistic infections, AZT is capable of influencing the different neurologic abnormalities in HIV/AIDS infection in a positive way (e.g., encephalopathies, meningitis, myelopathies, muscle disease, etc.) (Balint, 1997). It is worth mentioning that AZT is almost the only drug that is effective in the HIV-infection-attached thrombocytopenia. The recent Concorde trial (Concorde Coordinating Committee, 1994) raised the possibility that AZT therapy (alone, as a monodrug therapy) perhaps will not lengthen the survival time in clinically full-blown AIDS, but taking into consideration the above-mentioned facts (neurologic pathologies, as well as thrombocytopenia), it seems better to maintain the above-mentioned basic principles of the National Institute of Allergy and Infectious Diseases. On the other hand, one must not forget that AZT can be toxic, mainly in higher doses, i.e., > 1000 mg/day. Nausea and vomiting, together with headache, are frequently encountered at the beginning of therapy. Other unwanted side effects are, or can be: (1) Haematological disorders: anaemia, myelosuppression, decreased white blood cell count; (2) Gastrointestinal disorders: nausea, vomiting, disturbed liver function tests; (3) Others: myopathies, malaise, hyperpigmentation of the skin, Stevens ±Johnson syndrome. It is necessary to also mention that apart from myelosuppressive drugs, very few hypothetical interactions with other drugs seem of clinical importance. AZT can safely be combined with cytostatic treatment for Kaposi's sarcoma or antimycobacterial drugs for opportunistic infections (Sandstrom & Oberg, 1993a, 1993b). As monodrug therapy in HIV disease, AZT is clearly not optimal. It can only inhibit the infection of new cells, while the production of new viruses and immunosuppressive materials is not affected. Moreover Larder et al. (1989) documented that resistance may develop after a longer (more than 1 year) treatment. It is very important to note that the drug, i.e., AZT, is not suitable for prophylactic purposes of HIV infection. 3.1.2. Zalcitabine The anti-HIV activity of zalcitabine (dideoxycytidine, ddC) was discovered at the same time as AZT, and was found to be more potent (Mitsuya & Broder, 1986). In 1992, ddC was approved for use in combination with AZT in the United States. The basic mechanism of action of ddC is considered to be the same as for AZT, i.e., phosphorylation to the triphosphate (step by step) and then inhibition of RTC and proviral-DNA chain termination.

It is desirable to achieve a concentration of  1 mmol/L of free ddC in clinical practice, but this concentration may not be sufficient for complete inhibition of HIV in resting cells (Richman et al., 1987). ddC is absorbed rapidly, but there are large individual variations, with an oral bioavailability of 86% (Yarchoan et al., 1988). Its accumulation is negligible. The relatively high bioavailability indicates minimal first-pass metabolism. The t1/2 of ddC in the plasma is  1.2 hr on average. Plasma protein binding is < 4%. ddC is cleared primarily in an unmetabolised form by renal elimination. Seventy percent is excreted in the urine and  10% in the faeces within 24 hr. It is worth mentioning that the pharmacokinetics of ddC are not affected by co-administration of AZT. Most probably, this is true for AZT also. In early studies, dosages above 0.01 mg/kg every 8 hr caused an unacceptable high frequency of adverse reactions (Yarchoan et al., 1988). There was a decrease in p24 antigen level, and a trend toward increasing CD4 + cell count. In further dose-finding studies (ACTG-114, ACTG-119, ACTG-047, and ACTG-106 [AIDS Clinical Trials Group of NIAID, USA]), it was established that ddC is particularly suitable for combined therapy with AZT because it seems evident that toxicity prohibits the use of ddC as a monodrug therapy (adequate plasma concentrations cannot be maintained). The maximally tolerated monodrug dosage is 0.01 mg/kg or 0.75 mg every 8 hr. In a combined therapy, this dose is added to 50 ±200 mg AZT every 8 hr. Alternating regimens allow higher doses. It is claimed that resistant HIV strains have not been identified to date. It seems very probable that if there is resistance toward ddC, it may develop considerably more slowly than toward AZT. Two major categories of adverse reactions have been reported (Yarchoan et al., 1988): (1) early mucocutaneous syndrome and (2) (a treatment-limiting) peripheral neuropathy with axonal degeneration. The toxicity symptoms consist of fever, malaise, rash, and sometimes oesophageal pathology (ulcers). These signs usually are not treatment-limiting ones, and they frequently resolve during the first month of therapy, despite continued treatment. Pancreatitis is a concern with ddC treatment [as it is with didanosine (dideoxyinosine, ddI) also] (Schwartz & Brandt, 1989). It is worth mentioning that the AZT-ddC combined therapy is one of the most widely used therapies in clinical practice. 3.1.3. Didanosine Together with AZT, it was shown at the same time that dideoxyadenosine (ddA) is also active against HIV, although less potent than AZT (Balint, 1994). Warner (1977) stated that ddA administered p.o. is nephrotoxic, and, therefore, ddI was developed as a prodrug of ddA. Like AZT and ddC, ddI has to be converted intracellularly to a

G.A. Balint / Pharmacology & Therapeutics 89 (2001) 17±27

triphosphate, and in addition, the inosine moiety has to be converted to adenosine (Faulds & Brogden, 1992). The known mechanism of action of ddI is similar to that of AZT and ddC, i.e., after phosphorylation to the triphosphate, it replaces the normal substrate of RTC and causes inhibition and chain termination. ddI is an acid-labile compound, and, therefore, its absorption is reduced by food. Consequently, it has to be administered with an antacid on an empty stomach. The bioavailability under these conditions is  40%, with large individual variation (Knupp et al., 1991). The plasma t1/2 of ddI is  1 hr, but the active metabolite, the ddA triphosphate, has an intracellular t1/2 of  12 hr (Ahluwalia et al., 1988). The concentration of ddI in the cerebrospinal fluid is  20% of the corresponding plasma concentration. Protein binding of ddI is < 5%. ddI is metabolised to uric acid, different purine metabolites, and hypoxanthine. About 40 ±50% is found unaltered in the urine (Knupp et al., 1991; Hartmann et al., 1990). The usual p.o. daily dose of ddI is 200 ± 250 mg, administered with some kind of antacid. The dose is the same in combination with AZT. Phase I clinical pharmacological investigations and the monitoring of patients have shown that ddI seems to cause early discomfort and (later) more severe complications than either AZT or ddC. The early symptoms include: (1) Gastrointestinal complaints, mainly diarrhoea. Later there may be pancreatitis (in  5% of the patients); (2) Neuropathy; and (3) Rarely, hepatitis, excitation or depression, heart failure (Faulds & Brogden, 1992). 3.1.4. Lamivudine Lamivudine (thiacytidine, 3-TC, BCH-189) is structurally related to ddC. It has been reported to inhibit HIV in cell cultures (Soudeyns et al., 1991). Both enantiomers of the material have anti-HIV activity, but the ( ÿ )-enantiomer (i.e., 3-TC) was found to be less cytotoxic (Coates et al., 1992b). The racemic mixture is known as BCH-189. 3-TC is inhibitory to HIV-1 and HIV-2 at concentrations of 0.002 ±0.67 mmol/L (Coates et al., 1992a). According to Soudeyns et al. (1991), 3-TC is less potent than AZT and ddC, but it has also been shown to have less cellular toxicity. Moreover, 3-TC is active against AZTresistant HIV. The mechanism of action of 3-TC seems to be similar to that of AZT (Hart et al., 1992). 3-TC has  80% oral bioavailability in humans. Its usual dosage is 200± 400 mg/day p.o. In such doses, the compound is well-tolerated. 3.2. Non-nucleoside analogues 3.2.1. Nevirapine The chemical structure of nevirapine is given in Fig. 3.

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Fig. 3. Chemical structure of nevirapine, a non-nucleoside analogue RTC inhibitor drug.

Nevirapine was first described by Merluzzi et al. (1990) as a selective inhibitor of HIV-1-RTC. Nevirapine inhibits HIV-1 in cell cultures at a concentration of 0.04 mmol/L, and is toxic to cells at  300 mmol/L. Its mechanism of action was found to be noncompetitive inhibition of deoxynucleoside triphosphates, and an uncompetitive inhibition of the primer template of HIV-RTC. Thus, its mechanism of action is similar to that of other non-nucleoside RTC inhibitors, i.e., thione and tetrahydro-imidazo-benzodiazepinone derivatives, which are not in clinical practice yet (Debyser et al., 1991). Nevirapine is synergistic with AZT, and clinical evaluations, either alone or in combination with AZT, have shown a dose-related reduction of the p24 antigen. The usual dosage of nevirapine is in the range of 100 ± 400 mg daily. In some patients, dose-limiting rashes occurred above the 400-mg dose (Cheeseman, 1992). Although the non-nucleoside RTC inhibitors such as nevirapine are safe, well-tolerated, and quite effective in early trials and early clinical use, resistant strains of HIV emerge rapidly, and this fact limits their use as monotherapy (Maletic-Neuzil, 1994). 4. Protease inhibitors Attempts to develop HIV-PRT inhibitors have been ongoing for several years. Recent efforts have been aided by the availability of the three-dimensional structure of the enzyme (based on X-ray crystallographic studies). These data were the basis of rational drug design, i.e., the design of inhibitors using computer modelling. The initial drugs were nonhydrolysable peptide analogues, but the use of these compounds was limited because of their poor oral absorption, rapid (biliary) clearance, extensive serum ±protein binding, low bioavailability, and last, but not least, because of their poor stability. Moreover, these initial compounds, with a peptic nature, have a complex structure and, therefore, are costly to produce.

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G.A. Balint / Pharmacology & Therapeutics 89 (2001) 17±27

The so-called second generation of HIV-PRT inhibitors now have less (minimal) or even no peptide characteristics, with an improved bioavailability and a more affordable production cost. PRT inhibitors that have entered clinical use all exert in vitro activity, inhibiting HIV-1 in doses ranging from 1 nmol/L to 100 nmol/L. These agents have minimal cytotoxicity at these doses. Virions released by HIV-infected cells treated with a PRT-inhibitor drug resemble the immature, noninfectious particles produced by viruses defective in the PRT gene. Moreover, PRT inhibitors are able to reduce production of infectious particles from chronically infected cells. Considering that latently infected cells probably constitute a reservoir for HIV within the organism, this property represents an advantage over RTC inhibitors, which inhibit virus replication only in newly infected cells (Meek et al., 1990; McDonald & Kuritzkes, 1997). Combinations of various PRT inhibitors with both types of RTC inhibitors have been studied extensively in vitro (Kageyama et al., 1992; McDonald & Kuritzkes, 1997). Interestingly, antagonistic interactions have not been reported; the combinations have been minimally additive, but usually synergistic. 4.1. Saquinavir Saquinavir (Ro-31-8959, SQV) is a peptide derivative that inhibits the HIV-PRT, preventing post-translational processing of viral polyproteins. It was the first agent of this type of drug to become available for the treatment of HIV infection. Its chemical structure is given in Fig. 4. Of the available drugs, SQV has the least favourable bioavailability because of poor absorption and a significant first-pass hepatic metabolism. Its bioavailability is only 4%. Considering that SQV is metabolised by a hepatic mixedfunction microsomal oxygenase system (mainly by cytochrome P450), co-administration of inducers of cytochrome P450 (e.g., rifampicine, carbamazepine, etc.) will further

Fig. 4. Chemical structure of saquinavir.

decrease the plasma concentration of SQV. In contrast, administration of SQV with food significantly enhances its absorption. Therefore, it is recommended to be taken within 2 hr before eating (Muirhead, 1992). The currently approved maximal dose of SQV is 600 mg 3 times daily p.o. In this dose, SQV is generally well-tolerated, either as monotherapy or in combination with RTC inhibitors (Noble & Faulds, 1996). Laboratory and clinical studies have documented that SQV, in combination with RTC inhibitors, is effective in the treatment of advanced HIV infection, in spite of the emergence of SQV-resistant HIV strains. 4.2. Ritonavir Ritonavir (ABT-538, RIT) is an HIV-PRT inhibitor with a resistance profile similar to that of indinavir (MK-639, IND), but different from that of SQV. RIT has a good oral bioavailability and has a significant inhibitory effect on cytochrome P450 and thus, can increase concentrations of drugs metabolised through this system, including the other PRT inhibitors (Lea & Faulds, 1996). In patients with HIV-1 infection, RIT markedly reduced viral load within 2 weeks of onset of treatment and it also increased the CD4 + cell counts (Lea & Faulds, 1996). The usual dose of RIT is 1500 ± 2000 mg daily, and it is cleared primarily via hepatobiliary elimination (Denissen et al., 1997). The most common adverse reactions that occur with the use of RIT are gastrointestinal disturbances Ð nausea, vomiting, and diarrhoea. More severe unwanted side effects have also been reported; namely, hyperlipidaemia, renal failure, and gout secondary to RIT treatment (Hoetelmans et al., 1997). Triple therapy with RIT plus AZT, in combination with 3-TC or ddC, reduced HIV viraemia to below detectable levels in most patients with acute, and some patients with advanced, HIV infection (Lea & Faulds, 1996). 4.3. Indinavir IND is also well-absorbed after oral administration, but its absorption is significantly decreased when it is coadministered with high amounts of protein or fatty foods. Therefore, IND should be taken on an empty stomach. IND is also metabolised by the cytochrome P450 pathway, but it is a considerably weaker inhibitor of cytochrome P450 than RIT (Moyle & Gazzard, 1996; McDonald & Kuritzkes, 1997; Deeks et al., 1997). The usual daily dose of IND is in the same range as that of SQV and RIT. IND appears to be well-tolerated. The most common toxic side effects noted include evidence of hepatic abnormalities and nephrolithiasis, which is due to the crystallisation of the drug in the urine. Other, more severe adverse side effects include hepatitis and nephropathy (Brau et al., 1997; Tashima et al., 1997).

G.A. Balint / Pharmacology & Therapeutics 89 (2001) 17±27

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5. Drug resistance

6. Combined therapy

It is a well-known fact that following prolonged treatment with RTC inhibitors, especially with nucleoside analogues, resistant viral strains appear that show mutations in the gene-codifying RTC. These mutations cause changes in amino acids, which gradually increase resistance. Viruses typically contained amino acid substitutions V75F, F77L, F116Y, and Q151M in the RTC gene (Schmit et al., 1998). Associated with the high turnover rate of HIV, frequent RTC errors lead to a wide range of mutations. Certain mutations can modify the response to antiviral agents. Persistent replication, like HIV, whatever the cause, favours the emergence of drug-resistant mutants (Bossi et al., 1998). Apart from many point mutations in the HIV-RTC, which confer resistance to ARV drugs, there are also inserts and/or deletions in the gene of RTC. Winters et al. (1998) found such RTC inhibitor-treated patients to have HIV-1 strains possessing a 6-bp insert between codons 69 and 70 of the RTC gene. Known drug-resistant mutations were also observed in these strains, with T215Y appearing in all such strains. Site-directed mutagenesis studies confirmed the role of the inserts alone conferring reduced susceptibility (i.e., resistance) to most RTC inhibitors. In addition, other factors, e.g., cellular factors, may contribute to the waning efficiency of chemotherapy. For example, AZT may induce decreased activity of cellular thymidine kinase. This leads to the hypothesis that due to long-term treatment with nucleoside analogues, altered drug metabolism (in the host and/or in the target cells, respectively) may contribute to inefficient activation (i.e., phosphorylation) of nucleoside analogues in HIV-positive patients. Thus, intracellular subtherapeutic levels of active compounds may occur. In such an (intracellular) environment, selection of resistant viral strains may be promoted (Groschel et al., 1997). On the other hand, resistance to HIV-PRT inhibitors, e.g., IND, involves the accumulation of multiple amino acid substitutions in the viral PRT. A minimum of 11 amino acid positions have been identified as potential contributors to (phenotypic) resistance. Three or more amino acid substitutions in the PRT enzyme are required minimally before resistance becomes measurable (Condra, 1998). This result may have serious implications for therapy. It predicts that viral variants resistant to PRT inhibitors are unlikely to preexist in PRT-naive patients, and further, that high-level resistance can only develop if the HIV is allowed to replicate in the presence of the drug. The use of IND (and other PRT inhibitors as well) in combination with other ARV agents, e.g., AZT, has been demonstrated to dramatically reduce the incidence of resistant mutations, suggesting that with maximal suppression of viral replication, longterm control of HIV infection may be achieved. Thus, the goal of therapy must be to never (?) allow the virus to replicate (Condra, 1998).

There is a growing consensus that the appropriate goal of antiviral therapy for HIV/AIDS is the maximal suppression/ inhibition of virus replication. It is also well known that HIV replication is tightly coupled with CD4 + cell destruction, and that the constant virus turnover goes together with an extraordinary ability of HIV to generate drug-resistant strains. When drugs that only partially inhibit HIV replication are administered, there is a greater possibility of the emergence of drug-resistant variants. Theoretically, this drug resistance can be prevented, or at least delayed, when combination-drug regimens are introduced. With the recent developments of ARV treatment, i.e., the approbation of PRT inhibitors by the United States Food and Drug Administration, in patients with HIV/AIDS illness, impressive results have been obtained (Saag et al., 1996). These clinical trials of combination therapies usually include three drugs, two RTC inhibitors and one PRTinhibitor compound. According to the World Health Organization, the newer quantitative techniques to measure viral RNA expression in serum have been found essential for deciding when to start treatment (Saag et al., 1996). In 1996, an international expert panel established that recent data on HIV pathogenesis, methods to determine plasma HIV-RNA, clinical trial data, and availability of new drugs (e.g., PRT inhibitors) point to the need for new approaches to treatment. Therapy is recommended based on the CD4 + cell count, plasma HIV-RNA level, or clinical status of the patient. Preferred initial drug regimens include nucleoside ± analogue combinations. At present, PRT inhibitors probably are best reserved for patients at higher progression risk (Carpenter et al., 1996; Gao et al., 2000). On the other hand, combined therapies are very expensive, more than $1000 (US) per month. Moreover, they require a difficult and rigorous daily regimen in order to avoid the emergence of drug resistance. Resistance may occur even with triple therapy, and the virus may hide in the Table 2 New drugs under development for antiretroviral therapy 1. RTC inhibitors Non-nucleoside analogues: Chromanone derivatives (12-oxo-calanolideA), 2-aryl-substituted-benzimidazoles, 1-(2, 6-difluoro-benzyl)-2-(2, 6difluoro-phenyl)-4-methyl-benzimidazole Mono-methyl-substituted deoxycytidine kinase analogues: 3-4-diO-( ÿ )-camphanoyl-(+)-cis-khellactone Betulin-derivatives 2. PRT inhibitors Cyclic-urea HIV-PRT inhibitors (DMP-850 and DMP-851), others 3. New approaches Hydrazide-containing inhibitors of HIV integrase: N-(2-hydroxybenzoyl)-N-(2-hydroxy-3-phenoxy-propyl)-hydrazine Arylamide inhibitors of HIV integrase: 6,7-dihydroxy-2-naphtoic acid HIV-accessory protein inhibitors Inhibition of HIV entry into the CD4 + cells

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Table 3 Commonly used drug dosages for ARV treatment Drug

Adult dosage

Major toxicity

AZT ddC ddI 3-TC Nevirapine

300 mg b.i.d. or 200 mg t.i.d. 0.75 mg t.i.d. 200 mg b.i.d. 150 mg b.i.d. 200 mg q.i.d. for 14 days, then 200 mg b.i.d. 600 mg t.i.d. (hard gel caps) with meal 600 mg every 12 hr 800 mg every 8 hr, 1 hr before or 2 hr after meals

Bone marrow suppression Peripheral neuropathy Pancreatitis, neuropathy Minimal Rash, hepatitis

SQV RIT IND

CNS. In addition, when treatment is interrupted, strong viral rebound effects, leading to rapid clinical deterioration, have been observed (Saag et al., 1996). Considering the complex nature of combined HIV/AIDS therapy, the patients compliance is low,  50%, according to the opinion of the World Health Organization. Nevertheless, cautious optimism is still warranted, as long-term effects on clinical outcomes of ARV drugs and treatments are not yet known (WHO (Office of HIV/AIDS and Sexually Transmitted Diseases), 1997). 7. Future aims No doubt, the final and ideal solution of the HIV/AIDS problem would be a highly potent, simple, and effective vaccine, at an affordable cost. Research goes along this line, but the final success is yet to come. The discussion of this problem is not in the frame of this review, but the interested reader may find data in the recent literature (Parren et al., 1997; Lamb-Wharton et al., 1997; Heyward et al., 1998; Clements-Mann, 1998; Miramontes, 1998). On the other hand, it is evident that treatment for HIV/ AIDS disease with newer agents has changed considerably. We know that in most cases, monotherapy is not the best strategy to combat rapid turnover of virus and development resistance, and, therefore, various combined-drug regimens, as well as new drugs, are being explored (Lipsky, 1996; Fox, 1997). The present efforts include the development of not only new RTC inhibitors, preferably with novel, non-nucleoside analogue structures (e.g., chromanone derivatives [Xu

Abdominal pain, headache Nausea, vomiting, hepatitis Nephrolithiasis, rash, blurred vision, dizziness, thrombocytopenia

et al., 1998], 2-aryl-substituted benzimidazoles [Roth et al., 1997], mono-methyl substituted deoxycytidine kinase analogues [Xie et al., 1998], as well as betulin derivatives [Sun et al., 1998]), but new PRT inhibitors are also of interest to researchers (e.g., cyclic urea HIV-PRT inhibitors [Rodgers et al., 1998], and others [Kiso, 1998]). Completely new approaches were also introduced as therapeutic goals. Inhibition of other enzymes in the process of HIV replication are targeted, e.g., hydrazide-containing inhibitors of integrase (Zhao et al., 1997), or HIV accessory proteins are considered as therapeutic targets (Miller & Sarver, 1997). Moreover, there are efforts to inhibit the HIV entry into the CD4 + cells (Chan & Kim, 1998) (Table 2). While many industrialised countries are now witnessing a drop in AIDS deaths as a result of the new ARV combination therapy, only a very small proportion of people in the developing world have access to these treatments. Another problem is the cost of drugs. Very often the local resources for drugs are insufficient, particularly in SubSaharan Africa, where the situation is the worst (WHO (UNAIDS), 1999). Therefore, some multinational pharmaceutical companies are already committed to making ARV drugs more affordable at a substantially reduced cost. Taking into consideration the above facts, the question inevitably arises: Is monotherapy still acceptable?, and if yes, when? The answer is not easy. Theoretically, the answer should be ``no,'' but the reality dictates ``yes.'' Although there is very significant research work all over the world for newer drugs for ARV therapy, it seems that for the foreseeable future, AZT will remain the cornerstone of HIV/AIDS

Table 4 Commonly used drug regimens for ARV treatment Therapy

Drug

Monodrug therapy Multidrug therapy

AZT or nevirapine (vertical transmission!) Two RTC inhibitors, usually AZT plus one other or two nucleoside analogue RTC inhibitors plus nevirapine Two RTC inhibitors (one is always AZT) plus one PRT inhibitor Two RTC inhibitors plus two PRT inhibitors (usually the newest drugs, without resistance)

Combined therapy Highly active ARV therapy

Although monodrug therapy is obsolete, among special circumstances (developing countries), it is still used and can be justified.

G.A. Balint / Pharmacology & Therapeutics 89 (2001) 17±27

therapy. AZT increases the life span of HIV-positive patients, decreases the incidence of opportunistic infections, and enhances the quality of life. The current recommendation is to use the drug at a dose of 500 mg daily in patients with a CD4 + cell count of < 500. Another aspect of a possible monotherapy is a recent joint Ugandan± American study. It has found that a single oral dose of nevirapine administered to an HIV-infected woman in labor and another to her baby within 3 days of birth reduces the (vertical) transmission by one-half, compared with a similar, but longer, course of AZT. If implemented widely in developing countries, this intervention by nevirapine as monotherapy potentially could prevent some 300,000 ±400,000 newborns per year from beginning life infected with HIV. Moreover, this nevirapine regimen seems to be considerably cheaper than the longer AZT course used for the same reason (Guay et al., 1999; Marseille et al., 1999). On the other hand, there is no doubt that the so-called highly active ARV therapy has reduced the incidence of AIDS in HIV-infected persons. However, this (quite complicated) regimen, which includes the latest (newest) drugs in a form of a combined therapy, seems to be enormously expensive. Therefore, in developing countries, this form of therapy is difficult (or even impossible) to apply because of the widespread lack of patient compliance and because local resources for drugs are (tragically) insufficient (Strickland, 1999). It is now also evident that the epidemic data show shocking increases in the incidence of HIV/AIDS and serious underreporting of the disease in the last 5 years. However, there is also another side of truth; namely, HIV/ AIDS remains the only serious infectious disease for which antimicrobial treatment is deliberately delayed (Phillips et al., 1996; Vittinghoff et al., 1999). 8. Summary In summary, the author wishes to show the commonly used drug regimens for ARV treatment, with the note that the HIV/AIDS pandemic is as powerful as ever, and the situation calls for an unprecedented global effort (Piot, 1997) (Tables 3 and 4). References Ahluwalia, G., Cooney, D. A., Mitsuya, H., Fridland, A., & Flora, K. P. (1988). Cellular pharmacology of the anti-HIV agent 2,3-dideoxyadenosine. Proc Am Acad Cancer Res 29, 349P. Balint, G. A. (1994). A szerzett immunhianyos tunetegyuttes gyogyszeres kezelesenek jovobeni lehetosegei. Orv Htlp 135, 1235 ± 1242. Balint, G. A. (1997). Central and peripheral neurologic abnormalities in HIV-infection. Trop Med 39, 39 ± 44. Bossi, P., Calvey, V., & Bricaire, F. (1998). Resistance to antiretroviral agents: mechanism and study methods. Presse Med (suppl. 5), 18 ± 19.

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