Chronic Hepatitis C Virus Related Thrombocytopenia to Evaluate the Effects of E5501

This study had three phases: Prerandomization (Screening), Randomization (Core Study), and Open-Label Extension (OLE). The Randomization Phase included Treatment Periods A1 and A2 and a Follow-up Period (for those participants not continuing into the OLE phase). The OLE Phase included Treatment Periods B1, B2, and B3 (depending on when a participant entered the OLE), and a Follow-up Period. Participants may have been followed for sustained viral response, if appropriate. In the Core Study (randomization phase) participants were randomized (in a 1:1:1:1 ratio) to receive one of four treatments (placebo or avatrombopag \[10mg, 20mg, and 30mg\] for up to 21 days. Participants who successfully completed Treatment Period A1, (platelet count \>=100x10\^9/L) initiated antiviral treatment with pegylated interferon (PEG-IFN) alpha-2a and progressed to Treatment Period A2. Participants with a platelet count \>=150x10\^9/L initiated antiviral treatment and progressed into Treatment Period B2, study drug was interrupted then eventually restarted at 10 mg daily once their platelet counts returned to acceptable levels. Those who were not considered successful after 21 days in Treatment Phase A1 were withdrawn from the Core Study (Part A) and were eligible to enter the OLE at Treatment Period B1. Participants who chose to not enter the OLE entered into the Follow-up Phase. At the end of Treatment Period A2, eligible participants could enter the OLE at Treatment Period B3. In the OLE Phase, participants entering into Open-label Treatment Period B1 began once-daily treatment with avatrombopag at a dose of 20 mg without titration for up to 21 days. Once the participant's platelet counts were sufficient, they entered Treatment Period B2. Participant's eligible to enter into Treatment Period B2 received avatrombopag and antiviral treatment for 13 weeks. Participants who successfully completed Treatment Period A2 or B2 could continue on antiviral treatment for up to a maximum of 48 weeks (including the 13 weeks in Treatment Periods A2 or B2) and open-label avatrombopag, at the investigator's discretion. In Treatment Period B3, the dose of avatrombopag was allowed to be titrated up or down in accordance with the participant's platelet count response, within the range of a minimum of 5 mg and a maximum of 50 mg. In the Follow-up Period, participants were seen at either a single 30-day follow-up visit or followed for the full 30 days after the last dose of avatrombopag.