HIV Drug Resistance Why is it important to take ARVs in the correct way?

Transcription

1 HIV Drug Resistance Why is it important to take ARVs in the correct way? At the end of this session, the student should be able to: - explain the difference between resistant virus and wild-type virus - explain the genetic basis for resistance - estimate differences in risk for developing a virus resistant to each of the ARV drugs (pharmacological and genetic barriers) - know which drugs have cross resistance - understand the dangers of mono/bitherapy - explain the principle of a weak virus versus a fit virus - decide on a likely effective second-line therapy - decide when a salvage therapy is needed This lecture is divided into the following chapters: 1. HIV resistance 2. Quasispecies 3. Resistance to ARVs 4. Adherence and resistance 5. Why is it important to take the right dose of medications? 6. Where does drug-resistant HIV come from? 7. Reverse transcriptase mutations: NRTIs 8. Reverse transcriptase mutations: NNRTIs 9. Protease inhibitor resistance 10. Genetic barrier 11. Treatment failure 12. Holding regimen 13. Choice for second-line regimens 14. References 1. HIV resistance As HIV copies itself (replicates), spontaneous mistakes occur: these mistakes are called mutations. A virus with a mutation is slightly different from the original virus, and sometimes this difference allows the virus to replicate even in presence of ARVs. 2. Quasispecies The high rate of HIV replication (several billion new viruses are produced every day!) combined with the high mutation rate during each cycle of infection ensures that patients naturally have a complex and diverse mixture of viral quasispecies, each differing by one or more mutations. Thus, if most copies of HIV are identical, about 1% of the HIV population in a body will naturally contain mutations. In the absence of treatment, the virus carrying these mutations is usually not as strong as the original wild type. Wild-type virus is the most efficiently replicating and most prevalent quasispecies in the replicating viral population. If a patient takes drugs inconsistently and the HIV carries resistance to one drug, such as NVP, under selective drug pressure (and thus not a fully suppressive ART regimen!), the NVP-resistant variants

2 will emerge because of the replication advantage under these selection conditions. 3. Resistance to ARVs The most-used ARVs in low-resource settings target two enzymes that help HIV to replicate: reverse transcriptase and protease. ARVs work by preventing these enzymes from doing their jobs, thereby preventing HIV from copying itself. When HIV treatment is started, the ARVs will dramatically lower the amount of HIV in the body, provided that the patient takes all drugs on time and that the virus is not already drug resistant. If the ART regimen does not fully suppress viral replication, there will be selection for drug-resistant quasispecies that will have a replication advantage and will be stronger than wild-type virus. 4. Adherence and resistance If a patient takes ARVs inconsistently (i.e., if a patient is not adherent to the HIV treatment), HIV will have a chance to make more copies of itself: resistant viruses can proliferate, and the antiretroviral drugs will no longer be able to stop the enzymes from making more copies of HIV. Many of these new copies may have mutations, some of which might be resistant to one or more of the ARVs: antiretroviral drugs at this point can no longer block HIV replication. 5. Why is it important to take the right dose of medications? Taking the right amount of drugs at the right time will keep drug levels in the blood high enough for the antiretrovirals to do their job, blocking replication of the HIV virus and keeping it from mutating. Missing doses of medications allows drug levels to drop. When levels get too low, they may no longer be sufficient to prevent viral replication. It is unlikely that this scenario would arise with a single missed dose, but over time, accumulated missed doses could allow the virus to replicate again. When this happens, the strains that are resistant to the drugs will be the strongest in this context of non-suppressive ART, and they will have the replication advantage. When the patient returns to taking medications properly, most HIV strains will be suppressed. However, the small number of viruses that have mutated and become resistant to the drugs during the period of nonadherence can now survive and replicate even though the patient is back to taking antiretroviral drugs. Eventually the resistance will lead to a rise in the HIV viral load. Instead of having treatable, drug-sensitive viruses, there will be resistant viruses that no longer respond to the drug treatment. The resistant virus may still be treatable with other antiretroviral drugs, but there will be fewer options to choose from. 6.Where does drug-resistant HIV come from? By keeping the replication of the virus under control, antiretroviral drugs help to keep the immune system healthy. Once HIV has mutated so that antiretroviral drugs no longer affect it, the virus will be able to replicate and infect other CD4 cells without interference. The virus eventually destroys infected CD4 cells, weakening the immune system. If the HIV viral load is detectable under treatment, then HIV can replicate under selective conditions and can develop resistance to treatment. Therefore, the best chance to avoid development of resistance is to reach and maintain an undetectable viral load! 7. Reverse transcriptase mutations: NRTIs

3 When HIV infects a cell, the reverse transcriptase copies the viral single-stranded RNA genome into a double-stranded viral DNA. The viral DNA is then integrated into the host chromosomal DNA, which then allows host cellular processes, such as transcription and translation, to reproduce the virus. The nucleoside reverse transcriptase inhibitors block the enzyme s function and prevent complete synthesis of the double-stranded viral DNA, thus preventing HIV from multiplying. Because NRTIs lack the 3 OH found on normal deoxynucleoside triphosphates (dntps), they act as chain terminators when incorporated into the growing viral DNA chain. Resistance to NRTIs can occur by two mechanisms: 1) Enhanced ability to discriminate between the NRTI and the natural substrate prior to incorporation into the viral DNA. 2) Enhanced excision of the incorporated NRTI. The M41L, D67N, K70R, L210W, T215Y/F, and K219Q/E mutations are known as TAMs. TAMs (thymidine analogue mutations) are a subset of NAMs, or nucleoside analogue-associated mutations, mutations associated with resistance to numerous NRTIs. TAMs are selected by the thymidine analogues ZDV and d4t and accumulate in step-wise fashion. Their emergence is delayed by the concomitant use of 3TC and, conversely, is rapid with use of ZDV/ddI and ddi/d4t. TAMs confer resistance effects against all available NRTIs, although their impact is greater for ZDV, d4t, ddi, ABC, and TDF than for 3TC. Overall, the greater the number of TAMs, the greater the degree of NRTI resistance and cross-resistance. The presence of 4 TAMs typically causes >100-fold decreased susceptibility to ZDV, five- to sevenfold decreased susceptibility to ABC, and two- to fivefold decreased susceptibility to d4t, ddi, and TDF. Not all TAMs have the same effect on NRTI resistance, however; pathway-1 TAMs, including M41L, L210Y, and T215Y, have greater resistance effects than pathway-2 TAMs, which include K70R, T215F, and L219W. M184V emerges rapidly in nonsuppressive regimens containing lamivudine or emtricitabine. It is associated with high-level phenotypic resistance to lamivudine and emtricitabine in vitro, as well as with an ongoing reduction in replication capacity and an HIV RNA level that is less than the pretreatment baseline level. Mutation M184V also confers low-level phenotypic resistance to ABC and ddi but in isolation does not compromise virologic responses to these drugs. However, M184V in combination with 3 TAMs or with mutations at positions 65, 74, or 115 causes significant resistance to ABC. M184V also delays the emergence of TAMs when combined with ZDV and causes increased susceptibility to ZDV, d4t, and TDF through inhibition of primer unblocking. These properties are generally described in terms of reduced viral fitness and resensitization effects. - Viruses carrying the combination of mutations are less likely to become fully sensitized to ZDV when M184V is present. - Those carrying the cluster of mutations are more likely to become fully sensitive to ZDV when M184V is present.

4 The K65R mutation can emerge under treatment with tenofovir, abacavir, or didanosine, but it is the signature mutation of tenofovir. The K65R mutation is associated with a reduced virologic response to tenofovir in vivo. HIV-1 with TAMs shows reduced susceptibility to all NRTIs, most notably AZT, whereas HIV-1 with K65R shows reduced susceptibility to all NRTIs except AZT. K65R and TAMs rarely occur together in patients. However, when present together, K65R can restore susceptibility to AZT. Look at the picture below: a patient inconsistently taking first-line ART will be at risk for developing the initial NNRTI and 3TC mutations (i.e., K103N and 184V); a patient kept under the failing regimen for a while will start to develop TAMs. Once one resistance has developed, 3TC and NNRTI will not work anymore, and the number of TAMs will determine the susceptibility of the virus to the other ARVs. In the far right column, note that when a patient takes a TDF first-line containing regime, what the susceptibility of the virus will be to other ARVs if K65R is present!

5 8. Reverse transcriptase mutations: NNRTIs NNRTI-associated resistance mutations, the most common of which is K103N, are common at the time of virologic failure and may occur as the first resistance mutations, even preceding detection of M184V in first-line regimens such as AZT/D4T-3TC-NNRTI. Most NNRTI resistance is associated with high-level crossresistance to other drugs in the class. It had been proposed that efavirenz would be active in cases of nevirapine failure because of the Y181C mutation, but poor results have been observed with clinical sequencing, possibly because of the presence of low levels of K103N not detected by commercial assays. 9. Protease inhibitor resistance Resistance mutations in the protease gene are classified as either major or minor. Major mutations in the protease gene are defined in general either as those selected first in the presence of the drug or those shown at the biochemical or virologic level to lead to an alteration in drug binding or an inhibition of viral activity or viral replication. Major mutations affect drug susceptibility phenotype. In general, these mutations tend to be the primary contact residues for drug binding. Minor mutations generally emerge later than major mutations and by themselves do not significantly affect phenotype. In some cases, their effect may be to improve replicative fitness of the virus containing major mutations (compensatory mutations). 10. Genetic barrier The genetic barrier is the threshold above which ART drug resistance develops. A number of factors determine this threshold, including the number of critical mutations required for loss of activity, the level of pre-existing resistance, and the rate of replication of these pre-existing resistant strains. Defining the genetic barrier entails more than simply counting mutations; it also involves determining the effect of single mutations or combinations of mutations on HIV s susceptibility to the drugs in the regimen. Regimens with a high genetic barrier to resistance are those that require a greater number of critical mutations to render treatment ineffective, include boosted protease inhibitor (PI)- and thymidine analogue triple-art containing regimens. Regimens with a low genetic barrier to resistance are those that require fewer critical mutations to render treatment ineffective and may be associated with rapid virologic failure and the development of resistance. One recent example of low genetic barrier regimens with high virologic failure rates have been non thymidine-containing triple nucleoside/nucleotide reverse transcriptase inhibitor (NRTI/NtRTI) combination regimens (e.g., ABC + 3TC + TDF and ddi + 3TC + TDF).

6 It should be appreciated, however, that not all regimens with a low genetic barrier have a high rate of early virologic failure; in fact, a number of the most effective antiretroviral combinations contain agents against which only one or two key mutations are required to confer resistance in HIV (e.g., NRTI + 3TC + EFV). (Andrew D. Luber, Genetic Barriers to Resistance and Impact on Clinical Response, ejias: ejournal of the International AIDS Society, Posted 07/07/2005) 11. Treatment failure When patients start with ZDV/3TC, d4t/3tc, or the triple-nrti combination of ZDV/3TC/ABC, they first develop M184V and subsequently develop thymidine-associated mutations. When ABV/3TC is chosen as the initial therapy, M184V is the first mutation to develop and is usually followed by L74V and sometimes K65R. When TDF/FTC is the first choice, M184V emerges first, followed shortly thereafter by K65R. The important take-home point from this slide is that the M184V resistance mutation emerges first, followed by a gradual accumulation of either thymidine-associated mutations or either L74V or K65R, depending on the initial NRTIs chosen. In treatment failure after initial NNRTI-based regimens (i.e., AZT/d4T-3TC-NNRTI), NNRTI resistance and/or the development of M184V emerge(s) first. Although the NNRTI component of the regimen has lost activity, the 3TC (or FTC) + NRTI combination still exerts a significant antiviral effect.

7 12. Holding regimen For a patient with few treatment options, there is clearly a benefit to continuing a nonsuppressive regimen. Joel Gallant from Johns Hopkins calls this a holding regimen. Guidelines for the use of the holding regimen are as follows: 1. Never use an NNRTI. NNRTIs do not have a favorable effect on fitness, and accumulation of additional mutations may result in cross-resistance to secondgeneration NNRTIs. 2. Always use lamivudine or emtricitabine because they are simple and welltolerated drugs. We know from several studies, as discussed before, that M184V decreases fitness and that these drugs increase the activity of zidovudine, stavudine, and tenofovir. 3. Select other NRTIs and PIs based on resistance and tolerability issues. 4. Continue therapy until the availability of at least two new active drugs in order to avoid the problem of serial monotherapy*. *This situation is in strong contrast to early virologic failure, when we have many options and in which we should switch promptly to avoid selecting for broader resistance. 13. Choice of second-line regimens WHO recommends that the entire regimen be changed if treatment failure occurs. The new second-line regimen must involve drugs that retain activity against the patient's virus strain and should ideally include a minimum of three active drugs, one drawn from at least one new class, to increase the likelihood of treatment success and minimize the risk of cross-resistance. The PI class is thus reserved for second-line treatments, preferably supported by two new NRTIs. Because the PI component represents a new class and therefore a new working mechanism, it is the key element of the new regimen. To maximize its potency for a successful virological suppression and durable treatment response, the recommendation is to use a ritonavir-boosted PI regimen such as Kaletra, boosted atazanavir, indinavir, fosamprenavir, saquinavir, or indinavir. When (d4t or ZDV) + 3TC are used as part of the first-line regimen, nucleoside cross-resistance may compromise the potency of alternative nucleoside/tide components in the second-line regimen, in particular in the presence of longstanding virologic failure. Thus, when failure has been identified clinically or immunologically, many patients can be expected to have significant NRTI resistance at the time of switching.

8 A proposed schema for second-line ART options was discussed in Geneva in May References 1. Clavel F, Hance AJ. HIV drug resistance. N Engl J Med Mar 4;350(10): Johnson VA et al. Update of the drug resistance mutations in HIV-1: Top HIV Med Aug-Sep;15(4): The body. A guide to HIV drug resistance Prioritizing second-line antiretroviral drugs for adults and adolescents: a public health approach. Report of a WHO working group meeting. 2007