AIDS was first recognized in 1981 and HIV was identified in 1983. HIV is known as a type of retrovirus. It is a single stranded RNA virus, which can infect a number of different cells, including CD4 bearing macrophages and T-helper lymphocytes within the host. There are several steps in the viral replication, which may be advantage in the developing antiretroviral therapies such as fusion, reverse transcription, integration, transcription, translation, assembly and maturation. Form the various potent targets in the viral replication, an effective arsenal of drugs are developed for inhibit the infection of
HIV and help many people with HIV disease live longer and healthier lives. In early anti-
HIV treatment, the only drugs available for treating HIV infection, were nucleoside reverse transcriptase (RT) inhibitors. These drugs interfere with the action of specific HIV enzyme, reverse transcriptase, involved in the replication cycle of HIV. Unfortunately,
HIV rapidly develops resistance to these and other anti-HIV drugs. Researchers have faced to the problem of drug resistance, which is particularly harmful because of HIV’s high rate of replication and mutation (1).
These rationales have been used in the design of combination regimens for HIV infection. The various stages in the viral life cycle have been identified as potential targets for antiretroviral therapy, therefore combination therapy for HIV usually entails simultaneous therapy with drugs that have different sites or mechanisms of action (2).
Initially, the treatment of AIDS, by using regimens of multiple anti-HIV drugs, was the combinations of AZT and other nucleoside analogues such as ddC, ddI, D4T and 3TC. It was found that the combination of nucleoside analogues was more effective than treatment with AZT alone. However, HIV develops resistance to these drugs. In 1995, new drugs called protease inhibitors were approved and revolutionize the treatment of
AIDS (3). The target of these drugs is HIV-protease enzyme. The protease enzyme is required for the cleavage of viral polyprotein precursors that generates functional proteins in HIV-infected cells. These drugs slow down or block the action of HIV-protease, resulting in arrest of the maturation of infectious virus (4). The protease inhibitors are administered with the reverse transcriptase inhibitors for reducing severity of AIDS. This combination therapy is known as a highly active antiretroviral therapy (HAART). It was found that within two months of beginning the HAART, the viral load drops to undetectable level (3).
From these rationales, it was interesting to know about the factors contributing to treatment failure. I will focus on researchers decide to use the regimens of multiple anti-
HIV drugs, including studies of HIV-drug combination in the population.
Numerous factors contribute to treatment failure of antiviral therapy, including limited potency of anti-HIV drugs, poor adherence, pharmacological factors and continued immunologic deterioration in the face of ongoing virus replication. In the past, limited antiviral potency was the principal cause of treatment failure. Complete suppression of HIV-1 replication was rarely achieved when nucleoside reverse transcriptase (RT) inhibitors were used alone or in combination. In the absence of drug resistance, failure to completely inhibit HIV-1 with such regimens had adverse

consequences. Progressive depletion of CD4+ lymphocytes was slowed by partially suppressive regimens. In some patients, persistent HIV-1 replication leads to the emergence of more virulent T-cell tropic variants of HIV-1, which are associated with an accelerated loss of CD4+ lymphocytes and confer a significantly increased risk of disease progression and death (5).
The success of anti-HIV therapy depends on an individual’s adherence to a prescribed treatment. Poor adherence can delay the onset of symptoms or the progression of disease. Factors that contribute to poor adherence are miss timing doses, reducing the frequency of doses or the number of medications taken and drug toxicity or side effects
(6). The relative short half-life of most anti-HIV drugs requires frequent administration (at least twice daily) of these drugs. Therefore, the anti-HIV drugs must be taken reliably to ensure that they reach and are maintained at high enough concentrations in infected cell to inhibit HIV replication. Missing even a single dose can result in a drop in plasma drug concentration required to inhibit virus replication and allows the virus to continue replicating and provides the emergence of drug resistance (Figure 1). Side effects, due to drug toxicity, can cause patients living with HIV to suffer. They decide to reduce or miss doses by themselves because they feel reassured that missing doses is relatively harmless
(6). The side effects for each anti-HIV drug are summarized in Table 1 (7).
Table 1. The summary of side effects for each anti-HIV drug.
Drugs Potential side effects
Nucleoside Reverse Transcriptase Inhibitors
Zidovudine (ZDV)
Neutropenia, anemia, nausea, myopathy, malaise, headache
Didanosine (ddI)
Zalcitabine (ddC)
Pancreatitis, peripheral neuropathy, diarrhea, abdominal pain, rash
Peripheral neuropathy, pancreatitis, aphthous ulcers, anemia, elevated liver enzymes
Stavudine (d4T)
Peripheral neuropathy, elevated liver enzymes, nausea, diarrhea, myalgia
Mild rash, headache, diarrhea, hair loss Lamivudine (3TC)
Non-Nucleoside Reverse Transcriptase Inhibitors
Nevirapine
Rash, fever, thrombocytopenia, elevated liver enzymes
Delavirdine
Protease Inhibitors
Rash, fever, elevated liver enzymes
Saquinavir
Ritonavir
Indinavir
Nelfinavir
Headache, nausea, diarrhea
Nausea, vomiting, diarrhea, taste disturbance, paresthesias, elevated triglycerides
Nephrolithiasis, hyperbilirubinemia, fatigue, headache
Mild diarrhea

Pharmacological factors also may contribute to treatment failure. Limited penetration of drug into sanctuary sites such as the central nervous system may permit rekindling of HIV-1 infection from viral reservoirs even if therapy is effective at completely suppressing replication of the virus in peripheral lymphoid tissues. For example, AZT and d4T can get inside the central nervous system but ddI, ddC and 3TC do not have this ability. Difference in the intracellular metabolism of nucleoside analogues between resting and activated cells can also account for incomplete suppression of apparently sensitive viruses. All nucleoside reverse transcriptase inhibitors (NRTIs) require activation to their triphosphate forms, which are the actual inhibitors of RT.
Phosphorylation of thymidine analogues such as zidovudine (AZT; ZVD) and stavudine
(D4T) is much more efficient in activated cells as compared to resting cells, whereas nonthymidine analogues such as lamivudine (3TC) and didanosine (ddI) are efficiently phosphorylated even in resting cells. Drug-drug interactions are another potential cause of drug failure because they interfere with absorption or enhance elimination of antiviral agents (5). For example, all protease inhibitors are metabolized by the hepatic cytochrome
P450 system, drugs that induce cytochrome P450 3A4 activity (eg, rifampin and rifabutin) will reduce drug level of protease inhibitors. In contrast, the increase plasma concentration of protease inhibitors increases when they are coadministered with ketoconazole (3). The interaction of anti-HIV drugs and other drugs was summarized in Table 2 (7).
Table 2. Drug interactions between antiretroviral agents and other drugs.
Drugs Drug interactions
Nucleoside Reverse Transcriptase Inhibitors
Zidovudine (ZDV)
Didanosine (ddI)
Increased risk of neutropenia with ganciclovir and TMP-SMX
Buffer affects dapsone, ketoconazole, protease inhibitors, quinolones (should be taken 2 hours after ddI); ganciclovir increases ddI levels
Zalcitabine (ddC)
Stavudine (d4T)
Overlapping toxicity with ddI, d4T, and other drugs causing peripheral neuropathy
Overlapping toxicity with drugs that cause peripheral neuropathy
Lamivudine (3TC) TMP-SMX increases 3TC levels
Non-Nucleoside Reverse Transcriptase Inhibitors
Nevirapine
Delavirdine
Decreased protease inhibitor levels
Increased protease inhibitor levels
Protease Inhibitors
Saquinavir
Ritonavir
Rifabutin, rifampin decrease saquinavir levels; ketoconazole, itraconazole, ritonavir increase saquinavir levels; terfenadine, astemizole increase risk of arrhythmias
Opiate analgesics, oral contraceptives, saquinavir, theophylline, rifampin, rifabutin, terfenadine, and astemizole may interact with

Indinavir ritonavir
Ketoconazole increases indinavir levels; rifabutin, rifampin decrease indinavir levels; astemizole, terfenadine, cisapride, triazolam increase risk of arrhythmias
Nelfinavir
Rifampin, rifabutin decrease nelfinavir levels; terfenadine, astemizole, cisapride increase risk of arrhythmias
Drug resistance can be more appropriately termed "altered drug susceptibility." It is a phenomenon that can occur in vivo and in vitro, in response to the exposure of HIV to a drug or to a combination of drugs. The high rate of viral replication found throughout the course of HIV infection and the high frequency of virus mutations occurring during each replication cycle due to the lack of proofreading mechanisms, are the basis for the emergence of drug-resistant variants under the selective pressure of antiretroviral drugs.
With daily production of perhaps 108 to 1010 virions and a mutation rate of 3 x 10
-5 nucleotides per replication cycle, it is likely that any single mutation already exists before any drug is introduced (5,8,9,10). In addition, if two HIV viruses infect the same cell, both viruses can contribute one strand of RNA to virions produced by that cell (Figure 2).
During the next round of infection, if the reverse transcriptase that carries mutation can jump from one RNA genome to the other, it can produce a mosaic of two genomes. This process, called recombination, can allow HIV to spice two partially resistant viruses to produce a highly resistant virus. These can adapt rapidly to drug selection pressure through the emergence of drug-resistant variants. This process has substantially limited the long-term benefit of anti-HIV drugs (9).
Resistance to antiretroviral drugs is determined by mutations in the genes that encode the protease and reverse transcriptase enzymes. Some mutations selected by antiretroviral drugs directly affect viral enzymes and cause resistance via decreased drug binding, whereas others have indirect effects. It is useful to categorize resistance mutations as primary or secondary (Figure 3). Primary mutations are generally selected early in the process of resistance mutation accumulation, are relatively inhibitor specific, and may have a discernible effect on virus drug susceptibility. Secondary mutations accumulate in viral genomes already containing 1 or more primary mutations. Many secondary mutations alone have little or no discernible effect on resistance magnitude but may be selected because they improve viral fitness (ability to replicate) rather than decrease drug binding to target enzymes (5,11).
The distinction between primary and secondary mutations depicted in Figure 3A may help explain protease inhibitor cross-resistance. There seems to be little overlap in primary mutations selected by different protease inhibitors (eg, saquinavir-selected L90M and G48V; nelfinavir-selected D30N; and amprenavir-selected I50V). By themselves, these primary mutations may not cause cross-resistance to other protease inhibitors.
However, there is an overlapping spectrum of secondary mutations in the protease gene selected by all protease inhibitors. Many of the secondary changes are compensatory, improving fitness of virus containing primary mutations without actually increasing inhibitor resistance. The mutations may improve enzymatic function by altering protease catalytic activity or by affecting protease substrates (eg, making sites in gag or other viral precursor polypeptides more easily cleavable).

Cross-resistance among NRTIs can be mediated by inhibitor-specific mutations and less specific secondary mutations, especially among drugs that bind to similar or adjacent viral target residues (evident for didanosine and zalcitabine, which select for similar mutations [Figure 3B]). Similarly, the primary mutation commonly selected by lamivudine confers high-level phenotypic resistance to this drug as well as low-level phenotypic resistance to didanosine, zalcitabine, and abacavir in vitro. The clinical significance of cross-resistance among these drugs has not been determined.
The NNRTIs (nevirapine, delavirdine, and efavirenz) select for mutations in 2 different reverse transcriptase regions (codons 98 to 108 and 179 to 190). None of the mutations overlaps with mutations conferring resistance to NRTIs (Figure 3B). However, some of the mutations cause broad cross-resistance among all members of the NNRTI drug class (eg, K103N) (11).
Scientists and physicians have several reasons for deciding to use multiple antiretroviral drugs in AIDS’s patients. These are the most important ones:

It can decrease or stop HIV progression . HIV can make new copies of itself inside infected cell at very fast rate. Every day, billions of new copies of HIV are made. It also makes millions of infected T-cell die every day. Although, one drug can slow down the fast rate of infection, two drugs can slow down more (4). Multiple drugs often have additive or synergistic effects against the inciting infection (2). In 1995,
NIAID started the treatment with combinations of AZT and other nucleoside analogue
RT inhibitors to suppress HIV progression. It was found that combination therapy was more effective than with AZT alone. From a report of CDC, AIDS deaths in the
United States declined significantly in the past 2 years after the combination regimen started in 1995 as shown in Figure 4 (1).

Anti-HIV drugs from different drugs can attack the virus in different ways.
HIV is classified in a group of viruses called “retroviruses” (4). The various stages in the
HIV’s life cycle have been identified as potential targets for antiretroviral drugs as shown in Figure 5 (2,4). The viral reverse transcriptase is a potent target for drugs, which inhibit this enzyme ability. This enzyme is necessary for HIV to catalyze the early transcription of RNA into DNA prior to nuclear integration. These drugs suppress the viral replication. Another important target of anti-HIV drugs is HIV protease enzyme, which is required for cleaving protein precursors and generating functional proteins. In the use of these agents, HIV protease can be successfully blocked, resulting in arrest of the maturation of infection (2). Hitting two targets increases the probability of HIV suppression and protects new cells from infection.
From the powerful abilities of these drugs, reverse transcriptase inhibitors and protease inhibitors are often used to work together in the combination regimens as known in a highly active antiretroviral therapy (HAART).

Different anti-HIV drugs can attack the virus in different types of cells and in different part of the body. As a result of the strong immune defense from cytotoxic
B and T lymphocytes, the number of viral particles in the blood stream declines.
Little virus can now be found in the bloodstream or in peripheral blood lymphocytes.

Nevertheless, the virus persists elsewhere, particularly in follicular dendritic cells in lymph nodes and here viral replication continues. Virus can become trapped in the follicular dendritic cell network of lymphoid tissues and also in brain tissues. If latent
HIV in these tissues reactive, they can spread into blood stream. The progression of
HIV infection occurs again and might be more violent (12). It was found that some nucleoside analogues (AZT and d4T) and non-nucleosides analogues (nevirapine) can get inside spinal cord and brain better than others (13). Besides, the activity of anti-
HIV drugs after get into cell is important to decline the progression of HIV.
Laboratories studies showed that AZT and d4T worked best in infected cell that are actively producing new copies of HIV, while the nucleoside ddI, ddC and 3TC worked best in resting cell (3). Therefore, one of these drugs is used in the combination therapy for protecting hiding of HIV in these specific tissues.

Combination of anti-HIV drug may overcome or delay resistance.
HIV is a double-stranded RNA virus with an error-prone reverse transcriptase enzyme. Each time HIV infects a new cell, the reverse transcriptase makes an average of one mutant base per viral genome, while converting the viral RNA to DNA before integration into the host cell DNA. Thus, all possible single and double mutants may already exist in every patient who has prolonged HIV infection (9). When one drug is given by itself, sooner or later HIV makes the necessary changes to resist that drug. But if two drugs are given together, it takes longer for HIV to make the changes necessary for resistance. In the study of Harrigan et al ., combination therapy with abacavir plus zidovusine was more effective in reducing virus load than abacavir monotherapy and was associated with a lower frequency of mutation at codons K65R, L74V, or M184V as shown in Figure 6. Mutations, which decrease in HIV susceptibility to abacavir, were present in most subjects in whom virus suppression was incomplete. Further more, those with double mutations, in particular the combination of L74V and
M184V, tended to show the greatest reduction in efficacy as measure in virus load
(14).

The nucleoside reverse transcriptase inhibitors were the first anti-HIV drugs available. The combinations of two nucleoside analogues are the best-studied double therapies for HIV infection such as zidovudine (AZT) plus zalcitabine (ddC) or didanosine (ddI) (4). Zalcitabine, didanosine and lamivudien (3TC) are used in this regimen because they lack cross-resistance with zidovudine and had preferential phosphorylation in resting cells, as opposed to the preferential activity of zidovudine and stavudien (d4T) in active cells (2). Large studies in the United States, Europe and
Australia showed that AZT plus ddI or AZT plus ddC worked better than AZT alone
(15,16,17). Smaller studies showed that AZT plus 3TC, ddI plus d4T and d4T plus
3TC are effective in lowering amounts of virus in blood and helping raise CD4 cell count (18,19).
In the United States, NIAID conducted the randomized, double blind, comparative trial, ACTG 175. 2467 patients with a diagnosis of AIDS and CD4 count between 200 and 500 cell/mm
3
were studied. In all pateints, 1067 were antiretroviral therapy naïve and the remainders were AZT experienced. Subjects were randomized to receive AZT

monotherapy (600 mg daily), ddI monotherapy (400 mg daily) and AZT plus ddI or
AZT plus ddC (2.25 mg daily) at standard doses. The primary end points were a 50% decline in CD4 count, an AIDS-defining event, or death. The mean follow up was 143 weeks. It was found that the major endpoints in either of combination therapy were similar in AZT and ddI monotherapy. Overall, the combination of AZT plus ddI and
AZT plus ddC were significantly more effective than AZT monotherapy as shown in
Figure 7 (15).
In NUCA 3001, 366 zidovidine-naïve patients (CD4 counts of 200-500 cells/mm
3
) were randomized to receive either 300 mg lamividine bid, 600 mg zidovidine /day or combinations of either high dose (300 mg bid) or low dose (150 mg bid) lamividine with standard dose of zidovidine (200 mg tid). With in 4 weeks, both combinations produced a 1.5 log reductionin viral RNA versus a 1.2 log reduction with lamividine and a 0.5 log reduction with zidovidine monotherapy. The CD4 counts of patients receiving combination drugs also increased greater than monotherapy (18).
In Thailand, there are several studies about the combinations of NRTs in Thai HIV infected population (20,21). Thai Red Cross Research Center and Siriraj Hospital conducted a randomized, open-label, comparative trial of AZT/3TC versus
AZT/3TC/ddI in 106 antiretroviral naïve HIV-1 patients. In this study, the treatment was divided in 2 arms; first, AZT 300 mg plus 3TC 150 mg bid; second, AZT 300 mg plus 3TC 150 mg plus ddI 200 mg bid and the endpoint was a suppression of plasma
HIV-RNA from baseline including changes in CD4 cell count. Reduction in viral load was significantly greater in the AZT plus 3TC plus ddI arm compared to the AZT plus
3TC (21).
It can conclude that the different activity of each nucleoside analogue is used to design the combination therapy for HIV infection. Drugs effecting in active infected cells are often administered with drugs effecting in resting cells such as the combination of AZT and ddI or d4T and 3TC. Several large studies in the Unite
Stated, Europe and Thailand showed high antiviral efficacy of the combination therapy of nucleoside analogues in treatment-naïve AIDS patients and treatmentexperience AIDS patients. It was found that plasma CD4 cell level increases and plasma HIV-RNA level decrease in both AIDS patients groups after treatment with the combination of nucleoside analogues.

Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs) (Nevirapine,
Delavirdine and Efavirenz) can selectively inhibit HIV-1 at nanomolar concentration.
The NNRTIs have rapid effect on HIV viral load and CD4 count whereas resistance and loss of activity quickly ensue. The NNRTIs are synergistic with zidovidine and other nucleoside analogues and are active against zidovudine resistant HIV-1 isolated.
However, the co-administration of zidovudine does not prevent or delay the emergence of NNRIs resistant virus, although it may reduce the degree of resistance in some cases (2).
The INCAS study compared treatment with AZT plus ddI with or without nevirapine or AZT plus nevirapine in 151 antiretroviral-naïve patients with CD4 counts 200-600 cell/mm
3
. At 24 weeks of treatment, mean viral load changes were
1.65 log in the triple therapy versus 1.3 log in the AZT plus ddI and 0.4 log in the
AZT plus nevirapine. Plasma HIV-RNA levels had been reduced to less than 500

copies/ml in 60% of patients in the triple therapy group and in 30% of patients in the
AZT plus ddI group (3).
Stuart and collegues studied the safety and antiviral efficacy of a novel combination consisting of efavirenz, nelfinavir and one or more nucleoside reversetranscriptase inhibitors in children. 57 chlidren with CD4 cell counts 699 cell/mm 3 and
HIV-RNA 10,000 copies/ml plasma were treated with combinations of NNRTIs and nucleoside analogues. In an intention-to-treat analysis, 76 % of children had plasma
HIV-RNA level of less than 400 copies/ml after 48 weeks of the therapy and 63% had levels of less than 50 copies/ml (22).
Staszewski and collegues compared efavirenz and indinavir, both in combination with two nucleoside reverse transcriptase inhibitors in HIV-1 infected adults. In an open-label study, 450 antiretroviral-naïve patients were randomly assigned to one of three regimens: efavirenz (600mg daily) plus zidovudine (300 mg twice daily) and lamivudine (150 mg twice daily); the protease inhibitors indinavir (800mg every 8 hours) plus zidovudine and lamivudine or efavirenz plus indinavir (100mg every 8 hours). In the intention-to-treat analysis, the regimen of efavirenz, zidovudine and lamivudine was proved to be more effective in adults than the standard highly active antiretroviral therapy regimen (indinavir, zidovudine and lamivudine). The each time point in reducing the viral load to less than 50 HIV-RNA copies/ml was shown in
Figure 8 (23).
According to the same target as nucleoside analogues, non-nucleoside reverse transcriptase inhibitors are administered with nucleoside analogues. The synergistic effect can inhibit the replication of HIV. Several studies showed the combination of two nucleoside analogues and one non-nucleoside reverse transcriptase inhibitor could decrease plasma HIV-RNA level.

Because of the rapid resistance of HIV to NRTIs and NNRTIs, the HIV protease enzyme is a new potential target for antiretroviral therapy. Inhibition of protease activity in vitro causes generation of immature non-infectious virus and interruption of viral spread. The protease inhibitor was first approved in 1995 (3). Saquinavir was the first protease inhibitor to be licensed by FDA for use in combination with nucleoside analogue therapy in the treatment of HIV infection. Other agents, such as ritonavir and indinavir, which have also been designed specifically to mimic the binding site of the protease enzyme are of even greater potency than saquinavir (2).
Several studies in the United Stated and Europe showed the beneficial effect of protease inhibitors when were used with other anti-HIV drugs (24,25,26). In the
United States, NIAID conducted a 24-week trial comparing the combinations of zidovudine and zalcitabine or zidovudine and saquinavir to the triple combination of zidovudine, zalcitabine and saquinavir, also known as ACTG 229. The study included
302 patients who had been on zidovudine therapy for almost 2 years. The triple combination was associated with greater increase in CD4 cell counts and larger reductions in viral load than the former regimen. At 24 week, the CD4 cell counts of the patients receiving the triple combination returned to baseline level, compared with
37% of the saquinavir and zidovudine patients and 55% of those receiving zidovudine and zalcitabine (25).

In a Swiss cohort study, Opravil and his colleagues assessed the effect of anti- retroviral therapy on cell-free and cell-associated load in blood and lymphoid tissues
(27). The effect of double drugs, zidovudine (2

300 mg/day) plus laminvudine (2

150 mg/day), was compared with the effect of triple drugs, zidovudine plus laminvudine plus ritonavir (2

600 mg/day), in treatment-naïve, asymptomatic patients with CD4 count > 400 cells/μl. The end points were HIV-RNA concentration in plasma, peripheral blood mononuclear cells (PBMC) and sequential tonsil or lymph node. At
24 weeks, the proportion with plasma HIV-RNA <50 copies/ml was 16% and 74% among 42 randomized patients (Figure 9). Similary in plasma, cell-associated HIV-
RNA decreased sharply by approximately 1.3 log10 copies/ml in PBMC and 1.7 log10 copies/ml in the lymphoid tissue in patients on triple therapy. These decreases were less pronounced in patients on double therapy (0.5 log 10 copies/ml).
Indinavir has been studied at various doses and dosing schedules and has been shown to exert a clear dose response in term of antiviral activity and immunologic effect. In Hirsch et al ., a randomized, double-blind, multicenter study of indinavir, zidovudine and lamivudine was conducted in 320 adults with HIV-1, 50 CD4 cells/mm 3 and extensive prior zidovudine therapy. Patients received indinavir, 800 mg every 8 hour; zidovudine, 200 mg every 8 hour, and lamivudine, 150 mg twice daily or all 3 drugs for 24 weeks. At week 24, patients in the indinavir, zidovudine and lamivudine group had the greatest mean decrease in plasma HIV RNA level and mean increase in plasma CD4 cell count as shown in figure 10 and 11 (28).
It can summarize that the combination of protease inhibitors and reverse transcriptase inhibotrs has higher efficacy on the inhibition of HIV replication because they can inhibit the activity of HIV reverse transcriptase and protease. Several studies showed that the combination of protease inhibitors and reverse transcriptase inhibitors can suppress the HIV replication.
The potent antiretroviral regimens now available have proven capable of effecting a dramatic suppression of HIV viral replication and a delay in the emergence of drug resistance.
________________________________________________________________________

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Hirsch M, Steibegel R, Stazewski S, et.al. A randomized, controlled trial of indinavir, zidovudine and lamivudine in adults with advanced human immunodeficiency virus type 1 infection and prior antiretroviral therapy. J Infect Dis 1999; 180: 659-665


Most experts recommend starting antiretroviral therapy if you have significant symptoms related to HIV disease, if your CD4 cell ("T-cell") count is less than 500, or if your viral load titer (level of virus in blood) is more than 5,000 to 10,000.
There is ongoing debate about the best course of action if viral load is between
5,000 and 10,000 and patients have no symptoms and a normal CD4 count. Doctor may recommend either starting therapy or monitoring the CD4 count and viral load titer every three months off of therapy.

In general, a combination of two NRTIs and a PI is recommended. Examples of such a regimen include AZT/ddI/nelfinavir and d4T/3TC/indinavir. Occasionally other types of combinations, such as two NRTIs and an NNRTI or dual protease inhibitor therapy, may be suggested. An example of the former regimen is AZT/3TC/efavirenz, and an example of the latter is saquinavir/ritonavir. Decisions about which agents to use should be based not only on their effectiveness but also on their potential side effects and patient’s ability to take them reliably.

Before starting combination antiretroviral therapy, doctor will perform baseline laboratory studies, including a complete blood count, chemistries, CD4 count, and viral load titer. These laboratory studies will be repeated about four weeks later. An effective regimen should lower the viral load significantly, gradually raise the CD4 count, and not cause important side effects or laboratory abnormalities. If an antiretroviral regimen is successful, it will be continued, with laboratory studies repeated every three to four months thereafter. The roles of genotypic testing (profile of virus mutations) and phenotypic testing (profile of virus resistance pattern to drugs) in clinical care have yet to be determined.

The combination of antiretroviral drugs should be changed if patients are experiencing serious side effects, if viral load titer is rising and CD4 count is dropping, or if patients develop significant new complications of HIV disease.

There are no firm recommendations for modifying antiretroviral treatments that are not working. However, there are some generally accepted rules for doing so based upon an understanding of HIV pathophysiology. These include: 1) changing at least two agents at the same time; and 2) choosing drugs that are unlikely to share cross-

resistance with current ones. Cross-resistance refers to when a viral strain resistant to one drug is also resistant to another.
Figure 1.
The importance of regular dosing. As drug levels in the body drop, due to missed or delayed dosing, replication and mutation takes place. Mutations lead to drug resistance, which increases the amount of drug needed to prevent replication (IC100). The time spent when the body’s drug concentration is lower than the IC100 therefore increases, allowing replication to continue for longer. Eventually, the IC100 will rise above the maximum drug concentration in the body and the drug will have no affect on viral replication..
(6)
Figure 2.
A highly drug resistant viral strain can emerge through recombination.

Figure 3 . The most common human immunodeficiency virus 1 mutations selected by protease inhibitors (A), and nucleoside and nonnucleoside reverse transcriptase inhibitors (B). For each amino acid residue listed, the letter above the listing indicates the amino acid associated with the wild-type virus. The italicized letter below the residue indicates the substitution that confers drug resistance. The drug-selected mutations are categorized as "primary" (black bars) or "secondary"
(white bars). (The black-and-white bar indicates a mutation selected in vitro, but rarely seen in specimens from patients in whom therapy fails.) Primary mutations generally decrease inhibitor binding and are the first mutations selected. For indinavir, the mutations listed as primary may not be the first mutations selected, but they are selected in most patients' isolates in combination with other mutations. For zalcitabine, all mutations are listed as secondary because of inadequate clinical data to determine a common initial mutation. For nevirapine and delavirdine, each mutation can occur as either an initial or subsequent mutation and affect inhibitor binding. The asterisk indicates that the mutation has been reported in vitro, but relevance for clinical drug failure is uncertain. Amino acid abbreviations are as follows: A, alanine; C, cysteine; D, aspartate;
E, glutamate; F, phenylalanine; G, glycine; H, histidine; I, isoleucine; K, lysine; L, leucine; M, methionine; N, asparagine; P, proline; Q, glutamine; R, arginine; S, serine; T, threonine; V, valine;
W, tryptophan; Y, tyrosine. Multinucleoside resistance viruses have phenotypic resistance to most nucleoside reverse transcriptase inhibitors.

50
AIDS Deaths ( x 1000)
40
30
20
10
0
1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
Year
Figure 4 .
The AIDS death includes deaths for all ages, races and both gender since 1987.
Figure 5.
Nucleoside and non-nucleoside drugs interfere with the action of an HIV enzyme called reverse transcriptase, just after HIV enters a cell. Reverse transcriptase is necessary for HIV to

change its genetic material into a form that gets inside the cell nucleus, where it becomes part of the cell's genetic material and makes long chains of proteins. The HIV enzyme protease cleaves these long chains into short chains. Short protein chains are needed to form active new copies of
HIV. Protease inhibitors stop protease from cutting up the long chain of proteins. As a results, the new copies of HIV are empty an can’t go on to infect new cells.
Figure 6. Virus load responses in human immunodeficiency virus type 1infected subjects shown as percentage of maximal observable drop to limit of detection (400 RNA copies/mL) at 12 weeks ( y axis) in relation to treatment monotherapy arm (12 weeks of followed by 8 weeks abacavir monotherapy or 4 weeks of abacavir of abacavir/zidovudine combination therapy) compliance and development of abacavir mutations. *Abacavir dose reductions of any duration >24 h; filled bars, presence of abacavir mutations; ND, not determined; WT, wild type.
50% decline CD4 counts (%)
50
38
40
26
27
30
23
22
17
20 14
10
10
0
AZT ddI AZT+ddI AZT+ddC
Treatment
Treatment-naive volunteers Treatment-experienced volunteers

Figure 7.
100
80
60
40
20
0
0 2 4 8 12 16
Week
20 24 36 48
Figure 8 . Percentage of patients with plasma HIV-RNA levels of less than 400 copies per
+3TC
Enfavirenz+I efavirenz plus 3TC plus indinavir group.
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 4 12 24 36 48 60 72 84 96
Weeks
AZT+ 3TC
AZT + 3TC + RTV

Figure 9.
Proportion of patients with serum HIV-1 RNA in the AZT/3TC/RTV group compared with the AZT/3TC group.