Combination Therapy of HIV Infection

Ormrat Kampeerawipakorn

Combination Therapy for HIV Infection and Resistance

Introduction

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.

Factors contributing to treatment failure

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.

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 non-thymidine 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.

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).

Why does the combination therapy work better than individual therapy?

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).

Combinations of Antiretroviral Drugs

• Combinations of Nucleoside Reverse Transcriptase Inhibitors

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).