Treatment of HCV in the setting of HIV infection remains a clinical challenge. A recent meta-analysis suggested that only 33% of subjects treated achieve SVR[9
]. However, high disease prevalence, and increased rates of fibrotic progression in coinfected patients supports attempts of active intervention among selected patients. The relative toxicities associated with PEG-IFN ± ribavirin regimens mandate that investigators continue to explore better predictors of treatment response that might permit earlier termination of treatment or that might provide guidance to the patient and clinician during the pre-treatment consent process. In this regard, we explored the role of quasispecies complexity as a marker of treatment response.
Quasispecies complexity in HCV/HIV coinfected subjects prior to treatment intervention has been a controversial topic, with data showing greater complexity compared to those with HCV monoinfection, as well as no difference from those with monoinfection or even less complexity [11
]. Since quasispecies complexity may also be affected by duration of infection, viral load, histology, and whether the subject is a progressor vs. non-progressor in terms of liver disease, it is likely that the variability in reporting reflects differences in comparison groups randomly selected for cross-sectional evaluation [11
]. Variation over time was clearly demonstrated in the longitudinal cohort described by Qin et al. [14
]. Over a 10 year period, progressors narrowed their quasispecies display, while non-progressors did not. Thus, a cross-section study that included more patients with advanced liver disease would show lower complexity across the group than one heavily weighted by patients with milder liver disease.[14
] However, another source of variability may be related to the methodology used to determine quasispecies complexity. In this study we utilized a well-validated non-isotopic assay. Other investigators have utilized isotopic heteroduplex assays and single-strand conformational polymorphism (SSCP) methodologies. There are limited data that permit direct comparison between assays, though our group compared HCA to SSCP and found that correlation between assays was present, but that SSCP reproducibility was low (data not shown). In this study, we noted a trend toward higher baseline complexity among subjects with HCV/HIV coinfection, but this failed to reach statistical significance, and may be reflective of the small sample size of our control population.
In contrast, we found that baseline complexity was highly associated with treatment outcomes, particularly with regard to EVR. Reasons for failure to achieve SVR appear to be significantly more complex than EVR because while virtually all patients that achieve SVR have EVR, the converse is not true. Several studies demonstrate that SVR rates among those who achieve EVR are more dependent upon drug adherence and total ribavirin dose than the initial ability to respond favorably to the therapeutic intervention [19
]. The current regression analysis suggests that among both coinfected and monoinfected subjects, baseline HCV complexity is an independent factor in achieving EVR. Other investigators have noted an association between treatment response and baseline quasispecies complexity among HCV monoinfected subjects [20
].There was no difference in quasispecies complexity as analyzed at baseline prior to treatment assignment in the entire cohort.(t-test; p= 0.76) Therefore, while treatment affected ability to achieve both EVR and SVR, this effect was independent of the quasispecies influence on the outcome overall. There were insufficient samples in the substudy to further separate the effect of treatment type.
Recently, rapid viral response (RVR) has emerged as a much more reliable predictor of SVR among monoinfected and coinfected subjects than EVR. However, there has been little effort to characterize the relationship between quasispecies changes following initiation of HCV therapy and the occurrence of RVR. Furthermore, the relationship between viral kinetic parameters and quasispecies selection has not been previously described. Herein, we demonstrated that a decrease in quasispecies complexity is highly associated with development of RVR. The narrowing of quasispecies is akin to prior observations in the setting of spontaneous HCV clearance following acute infection[13
]. In our population, this finding was independent of viral load and supportive of the concept that the population of virus prior to treatment is made up of a mix of codominant species that have variable sensitivity to interferon. While a nearly infinite number of single nucleotide changes may be generated daily during the replication process, only a limited number of species have the ability to emerge as dominant or codominant. This pool represents the primary target for initial response following administration of interferon-based therapy.
The relationship between viral kinetic parameters and quasispecies clearance is also interesting. While the efficacy parameter ( ) is predictive of RVR, it appears to be independent of the quasispecies complexity. Since this parameter is calculated during the first 48 hours of viral decline, it reflects an overall change in viral load, which as noted above seems to be independent of baseline complexity which also factors into the development of RVR. Decreased HCV complexity is highly correlated with the slope of the first phase decline. These data further support the concept that both viral load and degree of complexity independently affect viral clearance, as reflected by both RVR and SVR. Identification of specific HCV signature motifs that are more or less prone to early clearance may improve our understanding of mechanisms of interferon resistance, though other groups have found it difficult to reliably identify such motifs[23
This study is limited by the small number of subjects for whom RVR could be documented. Unfortunately, early sampling was only performed on the subset of subjects within the ACTG 5091 substudy. The larger cohort did not save evaluable samples between baseline and week 12. However, newer studies being designed within the ACTG and elsewhere will provide samples at critical timepoints that will permit extension of our observations. Larger studies will also permit development of predictive algorithms which take into account baseline quasispecies complexity, adding to the array of tools already used in the treatment decision process (e.g. genotype, viral load, fibrosis). Though complexity assays are only available as research tools at this time, it is possible that they could achieve wider clinical application if their value is validated. Finally, there is growing evidence that early viral selection plays a critical role in treatment outcomes when small molecule antiviral agents are utilized in conjunction with PEG-IFN. There are limited data suggesting that such selection will need to be incorporated into viral kinetic models to ensure predictive accuracy as the field of HCV treatment continues to evolve.
In conclusion, HCV quasispecies complexity prior to treatment appears to be an important and independent predictor of both RVR and SVR and early quasispecies selection is critical in development of RVR. These findings have important implications for predictive modeling and for advancement of our understanding of HCV viral clearance in HCV/HIV coinfected patients.