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  International HIV and Hepatitis Drug Resistance Workshop
June 8-12, 2010,
Dubrovnik Croatia
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Intl Resistance Workshop Report
by Mark Wainberg, PhD, McGill University, Montreal

 
 
  June 8-12, Dubrovnik, Croatia
 
The International HIV and Hepatitis Virus Workshop on Drug Resistance and Curative Strategies dealt with a wide array of important subjects. Almost all of the conference possessed important clinical relevance. The subject areas that were covered were multiple and included each of resistance issues in regard to influenza virus, HIV, and both Hepatitis B and Hepatitis C viruses.
 
International HIV and Hepatitis Drug Resistance Workshop
 
June 8-12, 2010, Dubrovnik Croatia
 
1. Influenza virus
 
The conference began with a plenary presentation by Dr. M. de Jong of the University of Amsterdam in the Netherlands who discussed resistance issues concerning influenza virus, especially in regard to H1N1 and H5N1 strains. Dr. de Jong stressed that resistance to anti-influenza drugs such as Tamiflu and Relenza are more common among children than among adults. He further indicated that it is vital to treat children infected by influenza as early as possible in order to avoid probems of resistance and possible morbidity and mortality.
 
Since the most common drugs used to treat influenza virus are neuraminidasc (N) inhibitors, it is logical that resistance against these drugs will result from mutations within the N gene. Different strains of influenza virus may have different mutations in the N gene that are associated with neuraminadasc resistance. For example, mutations at positions H274Y and N294S are associated with resistance to Tamiflu in N1 strains while mutations at E119V and R292K are predominant in N2 viruses. These mutations may act by diminishing levels of Neuraminidase at the cell surface and may also impact on viral replication fitness. However, these effects may be countered by secondary mutations located elsewhere within the N gene such as at positions V234M and R222Q. It has also been demonstrated that some degree of pre-existing drug resistance may exist among circulating strains of influenza, especially against older drugs such as amantadine. When asked about the effectiveness of the recent vaccination campaign against H1N1 influenza, Dr. de Jong responded that the epidemic had likely already peaked by the time that vaccine roll-out took place and that the vaccination campaign had probably not played a major role in ending the epidemic.
 
2. Hepatitis B Virus (HBV)
 
A plenary lecture on this topic was presented by Professor Stephen Locarnini of Melbourne, Australia. He showed that resistance against all anti-HBV drugs has been demonstrated in the clinic and that the occurrence of resistance can be anticipated. The primary reason for resistance against nucleoside and nucleotide inhibitors of HBV lies in mutagenesis (process of mutation) of the viral reverse transcriptase enzyme. Resistance rates in regard to both Lamivudine and Adefovir are already very high. Some evidence of transmission of resistance-associated HBV has already been reported in the literature. It is also upsetting that cross-resistance and multi-drug resistance has been reported to occur. There has been great hope that the development of Tenofovir (TFV) would be effective at controlling replication of HBV that has been non-responsive to earlier therapy with Adefovir. However, this is not the case. A very common mutational profile associated with such cross-resistance is N236T/A181T. Nor is Entecavir effective at controlling the replication of HBV that is resistant against Lamivudine, a finding that is largely attributable to the M204V mutation. It has now been demonstrated that M204I/V substitutions can alter the morphology of viral particles and that these mutations can result in altered binding between antiviral antibodies and the HBV surface antigene.
 
Concern was expressed that the accumulation of mutations within the polymerase gene of HBV may ultimately affect and compromise the efficacy of the vaccine that is currently in use to protect against HBV infection. The basis for this is that the catalytic domain of the polymerase enzyme overlaps within the viral genome with the neutralization domain of the viral surface antigen. Studies on chimpanzees have now shown that vaccination with drug resistant viruses may yield antibody responses that may not easily neutralize wild-type viruses. The A181T mutation associated with drug resistance may result in a C-terminal stop codon in the envelope gene that may be responsible for some of the effects of drug resistance that have been observed. Recently, the World Health Organization has convened meetings to address this subject and is concerned about the future viability of the anti HBV vaccines now in common use. Clearly, this topic is one of important public health interest.
 
The above notwithstanding, it is also evident that the judicious use of combinations of anti HBV drugs can both forestall virus replication and provide important clinical benefits. Dr. Locarnini is a strong advocate for the use of combination therapy that includes such drugs as TFV and Entecavir, since clinical results on the use of these combinations have been excellent. The ultimate goal of therapy is to achieve deconversion in regard to the HBV surface antigen, which would provide evidence for clearance of the virus from the body. It is possible that a form of interferon termed IFN lambda may provide additional clinical benefit as this molecule appears to possess anti-HBV activity.
 
These results were reinforced in Abstract 15 by V. Svicher et al who analyzed different phenotypes of HBV for potential to develop drug resistance. This group reported that all genotypes were similar in regard to likelihood of development of drug resistance and that the mutations to emerge fastest are those with the lowest genetic barrier for resistance. Transitions at position 204 were found to be more common than transversions and the presence of natural polymorphisms in both genotypes A and G may render future drug resistance more common in these genotypes than in others. One important caveat is that these investigators did not look at the effect of viral load on the potential to develop drug resistance, but such studies are ongoing.
 
3. Hepatitis C Virus (HCV)
 
Prof. Jean-Michel Pawlotsky of Paris delivered a plenary lecture in which he discussed resistance against anti-HCV drugs. He began with the expectation that HCV disease can be cured based on the fact that this is a non-integrating virus. Indeed, excellent results have been obtained in the clinic with combination therapy using pegylated interferon alpha together with ribavirin, two compounds that are still only poorly understood in terms of mechanism of action. In general, the use of interferon seems to promote a first phase decline in viral load that is enhanced by the presence of ribivirin leading to an additional second phase decline. Better overall success rates have been obtained for genotypes 2 and 3 in which 80% cure rates have been achieved in comparison with genotype 1 for which only 45% cure rates have resulted from years of combination therapy as outlined above.
 
The field of anti-HCV drug development is at an all-time high, with multiple drugs having been developed against both the NS3/4A protease and NS5A polymerase proteins of the virus. At least 10 drugs that antagonize NS3/4A are now in various stages of clinical trials, with many having shown drops in viral loads approaching 4 logs over periods of 7-10 days. This notwithstanding, rebounds in viral loads have also been reported in these studies, resulting in an accumulation of drug resistance mutation that are associated with diminished antiviral activity. Almost all of the mutations identified thus far in regard to protease inhibitors are at or close to the enzymatic catalytic site. On occasion, combinations of various mutations have given rise to higher levels of replicative fitness than would be possible for viruses containing only one drug resistance mutation. This is a similar situation to that which has been observed for HIV, particularly insofar as resistance against integrase inhibitors are concerned. The concept of compensatory mutations that play roles in both drug resistance and viral replicative fitness is therefore important for both HCV as well as HIV. Mutational positions in regard to the NS3/4A protease are at codons 36, 54, 156, 168 and 155. In particular, the combination of mutations at positions 36 and 155, associated with resistance against telaprevir, are associated with higher level replicative fitness as well as increased levels of drug resistance.
 
In regard to nucleoside compounds, R7128 is a leading candidate. Unfortunately, an important mutation at position S282T has already been identified that is associated with decreased viral replicative fitness in the replicon model. Non- nucleoside reverse transcriptase inhibitor-like compounds that antagonize the viral polymerase have also been identified. As of this date, such drugs are now in various stages of clinical development; some can yield viral load reductions of as much as 3 logs within only 3 days. Again, resistance seem inevitable with mutations at position M423 potentially becoming clinically relevant.
 
A different category of anti-HCV drug is symbolized by cyclophilin inhibitors. Cyclophilins are involved as part of the viral replication complex and peptides derived from cyclophilin might block virus replication. Certain of these compounds have been studied in clinical trials and have yielded reductions in viral load after 14 days of therapy. However, mutations may arise in both the NS2 and NS5 proteins that might yield resistance against these compounds. Professor Pawlotsky made clear that initial clinical trials will follow a model of using drugs such as telaprevir in combination with interferon/ribavirin vs the use of interferon/ribavirin plus placebo. In one substudy, the combination of all three compounds was evaluated for either 8 or 12 weeks, following which patients were switched to receive interferon plus ribavirin alone. Twelve weeks of initiation of telaprevir resulted in this study in a 75% cure rate vs only a 60% cure rate if telaprevir was used in lead-in for only 8 weeks as assessed after a total treatment period of 48 weeks. It should be noted that individuals who had failed therapy with interferon/ribivirin might be rescued using telaprevir. However, a concern will be that potential development of resistance against telaprevir over long periods cannot be ignored.
 
Based on the results obtained to date with combination therapy involving drugs such as telaprevir plus ribivirin/interferon, many observers now predict that the extensive cure of HCV disease using rationally designed and acting antiviral drugs could potentially be accomplished within a decade. Indeed, some individuals predict that not all of the anti-HCV drugs that have been developed until now and that show promise in small scale clinical studies will be tested in the context of large phase 3 randomized clinical trials because of the likelihood that clinical success will be achieved with earlier developed products. A further consideration is that the occurrence of drug toxicity in regard to HCV disease is likely to be as significant as it is in the context of HIV disease where patients must be treated for life. If HCV disease can indeed be cured, than patients may be more willing to put up with a variety of relatively minor toxicities for relatively short time periods without issues such as adherence to anti-HCV drug regimens and development of drug resistance becoming problematic. Of course, it will be the outcome of future clinical trials that will inform us whether such predictions may be correct.
 
In Abstract 16, R. Barnard of Merck presented data in regard to a novel NS3/4A protease inhibitor termed MK7009. In a phase 2a randomized placebo controlled double blind study in treatment-na´ve patients, this compound was administered for 28 days together with IFN/ rivavirin for 20 days, following which patients continued therapy with IFN/ RBB alone for an additional 44 weeks. 4 of 9 patients in the MK7009 arm experienced virological breakthrough by day 28 and all had virus that was shown to have reduced susceptibility for this drug. The most common mutation associated with resistance was R155K that persisted for as long as patients continued to receive MK7009. However, wild-type virus began to re-emerge following cessation of MK7009 therapy, suggesting that this substitution impacted negatively on viral replicative fitness. Although mutations at position D168 was also identified in some individuals, these substitutions also reverted following cessation of MK7009. It appears as though R155K containing viruses possess higher levels of viral replicative fitness compared with those that contain mutations at position D168.
 
Analysis of Resistance in Virologic Failures during IFN/RBV Therapy Post 28-Day Treatment with MK7009: Results through Week 24 of the Phase 2A Dosing Finding Study in Genotype 1 Patients - (06/12/10)
 
In Abstract 18, DL Wyles presented data on a novel HCV inhibitor termed ODE-S-MPMPA. This compound possesses important antiviral activity in the replicon model. However, mutations associated with resistance could be selected at positions Q49L, K50N, and Q58L, all of which are within or close to a highly conserved nucleotide triphosphate tunnel of the polymerase enzyme. Cross resistance was also identified in regard to a previously identified polymerase active site mutation at position S282T. Combination of mutations at positions S282T and K50N were also observed under some circumstances, suggesting that K50N may be able to function as a compensatory mutation in regard to increased replicative fitness and, as well, higher potential levels of drug resistance.
 
In Abstract 19, O. Lenz and colleagues from Tibotec presented on a novel HCV NS3/4A PI termed TMC435. This compound has been shown to attain significant reductions in viral load and to be effective in the replicon model. Following the model of therapy outlined above, patients were treated with a combination of TMC435 together with IFN/Ribivirin for 28 days following which maintenance was carried with IFN/RBV alone. Rebounds in viral load were observed and, in some cases, resistance associated mutations were observed in the protease gene at positions 80, 155 and 168. Further analysis will be necessary in order to determine the clinical significance of these substitutions following viral rebound.
 
Deep sequencing analysis of baseline and on-treatment samples from HCV genotype-1 patients treated for 5 days with TMC435 monotherapy and subsequently re-treated with TMC435 in combination with PegIFNα-2a/ribavirin - poster presentation - (06/14/10)
 
Abstract 20 by AM Lam and colleagues from Pharmasset, Inc. dealt with combinations of compounds that antagonize both the HCVNS3/4A protease as well as NS5B polymerase, the latter through use of nucleoside or nucleotide inhibitors. These scientists were able to show synergy in the replicon model between several compounds such as HCV796 together with VX950, a PI and NNRTI respectively. Similarly, synergy results have been obtained in some cases with combinations of NNRTIs and nucleoside inhibitors, both of which antagonize the viral NS5B polymerase. These studies suggest a likely essential role for combination therapy involving compounds that are directed against varying targets within the HCV replication cycle and even against common targets that involve different binding sites of antiviral drugs, as has now been shown to occur in the replicon model with respect to NS5B polymerase.
 
An analysis of potential resistance in HCV treated patients receiving combination therapy with telapravir/IFN/RBV was reported by JC Sullivan and colleagues at Vertex Pharmaceuticals. Abstract 21 Telapravir is a NS3/4A protease inhibitor that has now been studied in studies termed PROVE 1 and PROVE 2. Thorough biological failures due to breakthrough were few, failure can be associated with a combination of mutations at positions V36M and R15K, particularly in subtype 1a, that augment rates of viral replicative fitness. In Abstract 23, S. Chevaliez and colleagues from Paris supported the key role that the resistance mutation at position V36C might play in regard to diminished HCV sensitivity to telapravir.
 
Finally, in Abstract 22, R. Ralston and colleagues from Merck presented data on combination therapy involving Boceprevir, also an inhibitor of the HCV NS3/4A protease. Most patients treated with this compound have achieved biological success, but the occurrence of resistance conferring mutations could also be demonstrated. Of particular importance were mutations at positions V36M, R155K, T54S, and T54A. Following cessation of Bosapravir therapy, various resistance-associated mutations disappeared from dominant viral species over varying periods of time and at differential rates. This study also confirms that V36M was lost the fastest and at higher proportions of all resistance-associated mutations. These findings again confirm that V36M is probably the resistance-associated NS3/4A mutation that impacts to the greatest extent on viral replicative fitness of all NS3/4A mutations evaluated to date.
 
Long-term Follow-up of Patients Treated with Boceprevir in Combination with PEG-Intron/Ribavirin (P/R): Durability of Responses and Rates of Reversion of Resistance Mutations - (06/12/10)
 
4. HIV drug resistance and the K65R mutation
 
Abstract 43 presented by AMJ Wensing et al from the Netherlands, working in collaboration with colleagues in South Africa, presented data on the rapid accumulation of drug resistance in South African patients treated with the combination of Stavudine/3TC/Nevirapine. Although initial viral suppression to below 50 copies viral RNA per ml was achieved in 77% of individuals within 3 months, RNA rebounds to > 1000 copies per ml were subsequently observed in 148 individuals after two years of treatment. The lack of alternative drugs meant that a high proportion of such individuals, i.e. 40%, continued with their same first line regimen, despite the likelihood of drug resistance. The first series of drug resistance mutations to be identified were those conferring resistance against NNRTIs, as would have been predicted, while resistance against 3TC i.e. the M184V mutation, was also detected at high levels. Of greatest concern was the presence of the K65R mutation that was observed in approximately 8% of individuals failing therapy over the two year time period of the study. The expectation is that K65R rates will continue to rise, unless a change in therapy is introduced. These findings substantiate earlier studies on higher rates of K65R development in individuals with subtype C viruses who receive stavudine as a cornerstone of therapy, substantiating earlier clinical trial results from Malawi, South Africa, and Botswana.
 
Abstract 5 by D. Coutsinos et al presented biochemical data to explain the basis whereby subtype C viruses are more prone to development of K65R than are subtype B viruses. In brief, the explanation for this distinction lies within the viral RNA template in the form of polymorphisms at positions 64 and 65 in reverse transcriptase that are located within subtype C but not subtype B viruses. These investigators further showed that reverse transcriptase enzymes derived from each of these viruses are minor and that the source of the RT itself was not responsible for the differential clinical therapy results that were obtained. There is now widespread consensus that the continuing use of Stavudine in antiviral therapy is inappropriate and, indeed, the World Health Organization has recently made a recommendation to this effect. Despite this, Stavudine continues to be used in many countries in which alternative forms of treatment are not available. The long-term danger is that the K65R mutation will limit future treatment options, since it confers broad cross-resistance to a wide range of nucleoside analogues and that the K65R mutation will presumably be shown to be sexually transmitted.
 
In Abstract 39, JA Johnson and colleagues from the Center for Disease Control in the USA have examined 13 individuals who appear to contain at least one transmitted drug resistance mutation, using plasma samples obtained during the 1990s. They carried out this work using realtime-PCR assays for each of the M41L, K75R, and K65R mutations. The results indicate the presence of each of these mutations at low frequencies i.e. less than 1%, suggesting, that they may have been sexually transmitted at time periods during which antiretroviral therapy is not as effective as it is today. In each instance, the presence of K65R was only detectable prior to but not after seroconversion in these samples obtained from newly infected individuals. The use of more recent samples from newly- infected persons failed to reveal the presence of these mutations at frequencies as high as those detected during the period 1997-2000, presumably as a result of more effective suppression of viral load in recent years, concomitant with a lower likelihood of transmission of both wild-type as well as drug resistant variants of HIV.
 
5. Use of ultra-sensitive methodologies and interpretation of results.
 
In a review lecture, Dr. R. W. Shafer of Stanford University presented information on the use of ultrasensitive techniques such as pyrosequencing to detect drug resistance mutations, when the latter are present as members of minority populations. This may occur under circumstances in which such mutations are present at frequencies below 1% and can only be detected by ultrasensitive methods. In general, the presence of low frequency mutations such as those that can only be detected by such methodologies has not yet been shown to have clinical relevance. Furthermore, it appears as though certain of the techniques employed for detection may introduce errors, including those associated with the polymerase chain reaction. In particular, such errors may occur in regard to detection of the K65R mutation. It is important, however, to distinguish this situation from that reported by multiple groups in regard to subtype C viruses in which K65R is in fact detected by bulk-sequencing as a dominant viral population among individuals failing antiretroviral drugs. This notwithstanding, the paper by Dr. Shafer indicates the need to interpret results of minority species resistant populations with caution. It is hoped that additional studies of clinical significance will follow from more extensive analyses that will also be carried out following improvements in sample preparation.
 
In Abstract 25, W. Shao and colleagues from Frederick, Maryland also documented the importance of pyrosequencing to detect low level minority species in clinical samples. This theme was also sounded in Abstract 26 by MJ Kozal and colleagues at Yale University who used ultrasensitive analysis to detect the K65R mutation at frequencies below 1% in a variety of antiretroviral-na´ve subjects. Although certain of these cases will require clinical confirmation in regard to relevance, these investigators have also documented that K65R is more frequently observed in drug na´ve patients infected by subtype C rather than subtype B variants. The investigators have concluded that this likely represents the sexual transmission of K65R mutation-containing variants. Further, their analysis revealed that 4 of 5 treatment failures with subtype C viruses who received combination therapy with TDF/FTC plus a boosted protease inhibitor all had pre-existing K65R. In contrast 8 of 13 such patients infected with subtype B viruses did not fail a similar regimen, again underlining the likelihood that K65R will be far more problematic in patients with subtype C viruses than in those infected with subtype B.
 
Abstract 27 was on the same theme as CL Wallis and colleagues, working in South Africa, have also now reported a high prevalence of K65R by ultra-deep sequencing, particularly in subtype C variants. The presence of this mutation at low frequency among subtype C-infected individuals who have not yet started antiretroviral therapy suggests that there is a high likelihood of transmitted resistance having occurred in at least some of these cases. However, PCR amplification and error may also have resulted in the presence of K65R in certain cases, as suggested in the presentation by RW Shafer in Abstract 24, cited above.
 
6. Dynamics of HIV drug resistance
 
In Abstract 3, M. Miller et al presented data showing the temporal order of emergence of resistance against different drugs following use of combination therapy with 3TC plus tenofovir in combination with either efavirenz, or raltagravir in treatment na´ve patients (Merck trial 004). The study only examined individuals who failed with increased levels of viral load, indicating probability of drug resistance. First, the study demonstrated that only 10 of 441 patients in the raltagravir arm of the study (3TC/TDF/RAL) i.e. 2% failed vs 3% of failures in the arm that included 3TC/TDF/EFV. The patterns of appearance of individual mutations were noted including those in which resistance to EFV developed prior to the presence of mutations against either 3TC or FTC. In other cases, resistance against both 3TC and RAL were reported at the same time. In some other instances, resistance against RAL never developed throughout the 96 weeks, despite the fact that patients remained on the same regimen with a non-suppressed viral load. Questions were raised as to why some of these patients were maintained on their initial regimens despite having evidence of virus replication. It is also important to note that viral load as a predictor of development of drug resistance has yet to be analyzed in the context of trying to better understand these results.
 
Temporal Order of Resistance Emergence for Lamivudine/Emtricitabine, Efavirenz, and Raltegravir in Treatment-Na´ve Patients - (06/10/10)
 
Resistance Emergence With Raltegravir Versus Efavirenz in Treatment Naive - written by Mark Mascolini - (06/10/10)
 
7. Integrase inhibitors
 
In Abstract 4, P. Yoshinaga and colleagues from the Shionogi Company of Japan, presented novel data on their integrase inhibitor that is being co-developed together with ViiV/GSK for treatment of HIV disease. This work has previously documented that signature mutations associated with resistance against some integrase inhibitors, i.e. the Q148 and N155 mutations, do not impact on the potency of their drug termed 572 but are associated with resistance against both Raltagravir (RAL) and elvitagravir (ELV). In the current study, the investigators have demonstrated that compound 572 seems to have both stronger and faster binding to drug resistant forms of integrase than does raltegravir. Furthermore, compound 572 may dissociate from the integrase enzyme at a much slower rate than does RAL. Although compound 572 continues to look very exciting on the basis of both clinical and tissue culture studies as well as biochemical profile, it is obvious that further long term follow-up clinical studies will be necessary in order to fully understand the potential use of this drug in anti-HIV disease and, as well, in order for it to gain approval from regulatory agencies.
 
These results were reinforced in presentation 8 by Kendra Hightower and colleagues who worked with a pre-formed complex of radio-labeled compound 572 and HIV integrase. Their data showed a slow dissociation of compound 572 from wild type integrase with a half-life that was remarkably approximately 100 hours in comparison to about 10 hours for RAL. The presence of mutations at positions N155H, Q140H, or Q148R diminished the half-life of the compound 572 integrase complex, but the respective half-lives were still almost 20 times longer than those associated with RAL. It remains uncertain whether or not it may be possible to select for resistance against compound 572 in tissue culture and further studies will be necessary to clarify this point.
 
· ResisWksp: Strong Inhibition of Wild-Type and Integrase Inhibitor (INI)-Resistant HIV Integrase (IN) Strand Transfer Reaction by the Novel INI S/GSK1349572 - (06/15/10)
 
· ResisWksp: Strong Inhibition of Wild-type and Integrase Inhibitor (INI)-Resistant HIV Integrase (IN) Strand Transfer Reaction by the Novel INI S/GSK1349572, poster report: "S/GSK1349572 had stronger and faster binding to mutant IN than RAL" - (06/15/10)
 
· ResisWksp: S/GSK1349572 Demonstrates Significantly Slower Dissociation Rates than Raltegravir when Comparing Wild Type and Raltegravir Resistant Integrase Protein - (06/15/10)
 
· ResisWksp: HIV Integrase Resistance Profiles and S/GSK1349572 Baseline Phenotypic Susceptibility for Subjects Ex periencing Virologic Failure on Raltegravir (RAL) in the VIKING Study (ING112961) - (06/15/10)
 
· ResisWksp: S/GSK1349572 Demonstrates Significantly Slower Dissociation Rates than Raltegravir When Comparing Wild Type and Raltegravir Resistant Integrase Protein - (06/15/10)
 
· ResisWksp: Structural Models of HIV-1 Integrase and DNA in Complex with S/GSK1349572, Raltegravir and Elvitegravir: Structure-based Rationale for INI Resistance Profiles - (06/15/10)
 
· GSK Integrase Following Raltergavir Use: When Is It Risky to Use S/GSK1349572 After Raltegravir? - written by Mark Mascolini - (06/10/10)
 
Differing Integrase Dissociation Rates With S/GSK1349572 and Raltegravir - written by Mark Mascolini - ( 06/10/10)
 
8. Resistance against CCR5 inhibitors.
 
One important question is whether or not genotypic analysis may be used to predict resistance against Maraviroc and other CCR5 antagonist. In Abstract 9, LC Swenson and colleagues presented an analysis of pyrosequencing of the viral V3 loop among individuals with R5 tropic viruses who participated in the MOTIVATE studies in which Maraviroc was used as an antiviral drug. Some of these patients went on to experience treatment failure. No major differences were reported between the use of pyrosequencing analysis versus Trofile phenotypic testing for potential use of Maraviroc in this analysis. Sequencing of the V3 loop by genotyping seems to be as predictive of the likelihood of ultimate treatment failure as does phenotypic analysis in almost all cases. Although there were some discordant results, it should also be noted that genotyping was also able to determine future failure as well as phenotyping in equal numbers of cases.
 
Similar findings were reported in Abstract 12 by V. Svicher and colleagues who showed on the basis of genotyping analysis that positions 5, 11, and 25 within V3 may be especially important in regard to HIV tropism and eligibility to be treated with CCR5 antagonists. Also largely in agreement were results in Abstract 11 presented by W. Huang and colleagues from Monogram Biosciences who showed that multiple and sequential amino acid changes in the V3 loop are usually required for resistance against Maraviroc to occur. The results of ultra deep sequencing also showed that such changes were usually subtle as well as dependent on the viral backbone. In addition, results presented in Abstract 13 by D. Chapman et al signified that the use of genotyping may be appropriate to select individuals who are likely to respond to a CCR5 antagonist. Moreover, there are now other studies, including those from the group of R. Harrigan in Vancouver, British Columbia, that suggest that ordinary population-based sequencing of the V3 loop may be adequate to determine eligibility for Maraviroc usage in almost all cases.
 
In Abstract 10, JM Strizki and colleagues also carried out a comparative analysis of phenotypic tropism testing vs genotype-based analysis of HIV V3 loops to determine potential eligibility to receive Vicriviroc. The results again demonstrate a seeming equivalence between both of these methods in regard to success at predicting a positive response to therapy with a CCR5 inhibitor.
 
Finally, in Abstract 14, R. Kaiser and colleagues, representing a consortium of scientists, showed a high rate of concordance in regard to genotypic analysis for prediction of tropism among different European laboratories. Moreover, no major differences were reported between B vs non-B subtypes in regard to the results obtained. These findings confer an important degree of confidence with regard to the future use of genotyping to determine eligibility of patients to receive CCR5 antagonists as a cornerstone of future HIV therapeutic regimens.
 
Genotypic Analysis of V3 in Patients Experiencing Virologic Failure on Maraviroc-containing Regimens and Correlation with Pre-treatment Deep Sequencing Results - (06/11/10)
 
Clinical, Virological, and Immunological Characteristics of Patients With Discordant Phenotypic and Genotypic (Ultra-Deep Sequencing) Tropism Test Results: Analysis of Tropism Calls in MOTIVATE and 1029 - (06/11/10)
 
Use of the Enhanced Sensitivity Tropism Assay (ESTA) to Predict On-Treatment Detection of CXCR4-using Virus and Impact on Virologic Outcomes in a Vicriviroc (VCV) Phase 2 Treatment Experienced Study (VICTOR-E1) ;- (06/11/10)
 
Pairwise Comparison of g enotypic tropism prediction between one reference center and 10 European laboratories - (06/10/10)
 
"Deep" Sequencing Finds X4 Virus in Screening Samples of Maraviroc Failures - written by Mark Mascolini - (06/10/10)
 
Debate Follows Data on Genotypic Predictions of HIV Tropism - written by Mark Mascolini - (06/10/10)
 
9. Dynamics of Drug Resistance
 
Finally, M. Pingen and colleagues from the Netherlands presented on the transmission of single drug resistance mutations in comparison with transmission of multiple mutations. In particular, analysis of the M184V mutation, that impacts negatively on viral replicative fitness, has shown that detection of this mutation in newly infected individuals occurs most commonly when this mutation is co-transmitted with other drug resistance mutations. This result makes good sense, since, in all likelihood, the M184V mutation when transmitted alone is likely to quickly back-mutate to wild type and, as a consequence, viruses containing M184V will rapidly be overgrown by more wild type, replication-competent variants. In contrast, transmission of linked mutations that include M184V will result in a situation in which persistence of mutations will be likely to occur over a much longer period of time, since it will take at least months or years for all linked mutations on a common viral backbone to back mutate to wild type. Accordingly, the likelihood that reversion to more wild type virus will quickly occur will be far more remote, with the consequence that mutations that confer impaired viral fitness such as M184V will continue to be detected, even by bulk sequencing, over far longer periods of time that would otherwise be the case. Similar data have also been reported by other groups.
 
Are Single Transmitted Mutants the Tip of a Resistance Iceberg? - written by Mark Mascolini - (06/11/10)
 
In Abstract 41, D. Dunn and colleagues showed that transmission of drug resistant viruses containing the M184V mutation may be associated with diminished initial viral load, independent of other considerations. Although it has long been known that the M184V substitution can impact on viral replicative fitness, this is the first report to show that transmission of M184V/I may be related to both diminished rates of disease progression as well as lower viral loads. Further analysis of these data showed that individuals not presenting with any drug resistance mutations or with mutations other than M184V/I had viral loads in the range of 4.6 logs. In contrast, the presence at baseline of M184V/I was associated with a viral load of approximately 3.3 logs (P <0.001).
 
Infection With 3TC/FTC Mutant Virus Favors Lower Pretreatment Viral Load - written by Mark Mascolini - (06/12/10)
 
In Abstract 42, LL Ross and colleagues from GSK have reported on large datasets of newly infected individuals and showed declines in rates of transmission of all drug resistance mutations in recent years. Significantly, these declines include the likelihood of transmission of mutations associated with the NNRTI family of drugs e.g. K103N, Y818C. This result is intriguing and suggests that the transmission of mutations that do not impact on replicative fitness may also be declining. The basis for this is likely the fact that much more effective suppression of viral load will mean lesser likelihood of development of drug resistance, a higher proportion of individuals are now properly managed with modern anti-HIV regimens than was previously the case. In brief, lower rates of development of novel mutations associated with drug resistance will result in fewer transmissions of all resistance mutations.
 
Mutation Rate in Antiretroviral-Naive Falling in One US Patient Population (GSK studies in USA) - written by Mark Mascolini - (06/11/10)
 
Abstract 56 by S. Fransen et al is the first work to demonstrate that resistance mutations associated with integrase inhibitors can be sexually transmitted. Virus sequenced in a newly infected single patient were shown to possess mutations at positions G140S and Q148H in advance of commencing therapy with Raltagravir. It should be noted that this individual also possessed multiple against other drugs including members of the NRTI, NNRTI and PI families of drugs. Confirmation of this report will be necessary
 
10. HIV prevention
 
In Abstract 31, JG Garcia-Lerma and colleagues have studied the use of Truvada (TDF/FTC) to protect rhesus macaques against challenge with pathogenic variants of SHIV that contained the 184V mutation. These investigators have shown that a simple regimen of oral Truvada three days prior to virus exposure, followed by a second dose two hours after exposure can protect these animals against repeated rectal exposures with a wild type SHIV. In this study, they have now demonstrated protection as well against viruses containing the M184V substitution under circumstances in which prophylaxis with Truvada (PREP) was administered prior to exposure to an intra-rectal challenge of SHIV. The results show efficacy against both wild type viruses as well as those containing the M184V substitution and suggest that these prophylactic approaches are likely to have relevance in regard to prophylaxis of humans against HIV transmission.
 
Truvada Protects Macaques From Rectal Exposure to FTC-Resistant Virus - written by Mark Mascolini - (06/11/10)