In part I of the study, a 48-week boceprevir regimen achieved a 75% sustained virologic response (SVR) rate (n=77/103) in patients who received four weeks of Pegintron and Rebetol (P/R) followed by the addition of boceprevir (800mg TID) for 44 weeks (boceprevir P/R lead-in regimen). This represents a near doubling of the 38% SVR rate (n=39/104) for patients in the control group (p<0.0001), the company said. In a 28-week boceprevir P/R lead-in regimen 56% of patients (n=58/103) achieved SVR (p=0.005). Importantly, the likelihood of attaining SVR was greater for patients who received the boceprevir P/R lead-in regimens compared to the no lead-in arms. Of patients in the boceprevir P/R lead-in arms who achieved a rapid virologic response (RVR), 94% in the 48-week regimen and 82% in the 28-week regimen achieved SVR. RVR is defined as undetectable virus (HCV RNA) in plasma at four weeks after the addition of boceprevir. In the lead-in arms, 64% of patients achieved RVR. Fewer patients in the lead-in arms discontinued treatment due to viral breakthrough, the company said. Part II of the hepatitis C virus (HCV) Sprint-1 study explored a low-dose ribavirin strategy in which boceprevir was given in combination with Pegintron and low-dose Rebetol for 48 weeks. SVR for the low-dose Rebetol arm was 36% (n=21/59) compared to 50% for a 48-week control arm with Pegintron and standard-dose Rebetol plus boceprevir (n=8/16). In contrast to the results seen in part I, the low-dose Rebetol regimen was associated with increased viral breakthrough during treatment, higher relapse rates after the end of treatment and lower SVR, strongly indicating that standard-dose ribavirin is required to optimize response, the company noted. Another key finding of the HCV Sprint-1 study is that treatment-emergent anemia appeared to be associated with higher SVR, with anemic patients (hemoglobin decreasing to less than 10g/dl) having higher SVR rates than those without anemia (hemoglobin did not decrease to less than 10g/dl). Paul Kwo, lead investigator of the study, said: "These results are very exciting and provide important insights to help further define response guided therapy using a P/R lead-in boceprevir regimen with peginterferon and ribavirin backbone treatment. Building on these results, the boceprevir Phase III clinical program individualizes treatment based on response, utilizing RVR criteria at week four of boceprevir treatment to determine overall duration of therapy. Based on the RVR rate seen in this Phase II study, we are hopeful that the majority of patients can be treated with 28 weeks of therapy."