Both dPSS and iPSS attempt to express the sum of the phenotypically

active ARV drugs in the patients’ new regimen. In the dPSS, the activity of each new drug in the regimen was estimated as follows: if fold-change (FC) was less than the lower CCO (i.e. susceptible), the drug contributed 1 point; if FC was higher than the lower CCO (i.e. resistant), the drug contributed 0 points to the dPSS. The iPSS was calculated in a similar fashion but also accounts for partial or intermediate susceptibility of new ARV drugs (FC between IWR1 the upper and lower CCOs): each fully active drug (FC < lower CCO) gets a score of 1, and each partially active drug (lower CCO < FC < upper CCO) gets a score of 0.5. In both dPSS and iPSS, if the FC was < 0.4 for a specific drug (i.e. the virus was considered to be hypersusceptible to the drug), that drug contributed 1.5 points to the dPSS or iPSS. The primary objective was to evaluate the predictive value of RC or PD-0332991 manufacturer either PSS for virological or immunological outcomes at weeks 12 to 48 following randomization. The sample size estimates for this substudy were based on assumptions made regarding RC changes during ARDFP. Compilation of RC data from published studies [15, 27]

and unpublished observations suggested that mean log10 RC increases by 0.3 [standard deviation (SD) = 0.38] after 2 months of ARDFP. According to these data, we estimated that the available sample size provided 90% power to detect a mean change of 0.20 in log10 RC. The intended duration of ARDFP in OPTIMA was 12 weeks, so that an increase in RC after the ARDFP greater than 0.3 might be anticipated. Pearson correlation analysis was used to analyse baseline RC in response to salvage ARV therapy and/or Aprepitant treatment interruption. Multivariate regression analysis was performed in order to evaluate changes in (a) CD4 cell count using baseline viral load, RC and PSS as independent variables and (b) viral load using baseline

CD4 cell count, RC and PSS as independent variables. P-values of < 0.05 were chosen a priori to be indicative of statistical significance. The statistical software used was sas version 9.1 (SAS Institute, Cary, NC). A total of 283 patients had samples available for RC and PSS measurements at baseline and were included in the analysis. Baseline demographic characteristics, previous and on-study ARV use, and baseline CD4 cell counts and HIV RNA of these patients are presented in Table 1. As reported elsewhere [25], no significant differences were found in the primary outcome measure by treatment arm. For the purpose of this substudy, we combined the subgroups receiving standard and mega-ARV regimens within the no-ARDFP group (n = 146) and the ARDFP group (n = 137). Mean week 0 RC was low: 50.8% (SD = 44.6) in the no-ARDFP patients and 52.4% (SD = 40.2) in the ARDFP patients. There was no significant difference in week 0 CD4 cell count, viral load or RC between groups (P = 0.774, P = 0.594 and P = 0.