HCV Drug Resistance Challenges in Japan: The Role of Pre-Existing Variants and Emerging Resistant Strains in Direct Acting Antiviral Therapy.

Sustained virological response (SVR) rates have increased dramatically following the approval of direct acting antiviral (DAA) therapies. While individual DAAs have a low barrier to resistance, most patients can be successfully treated using DAA combination therapy. However, DAAs are vulnerable to drug resistance, and resistance-associated variants (RAVs) may occur naturally prior to DAA therapy or may emerge following drug exposure. While most RAVs are quickly lost in the absence of DAAs, compensatory mutations may reinforce fitness. However, the presence of RAVs does not necessarily preclude successful treatment. Although developments in hepatitis C virus (HCV) therapy in Asia have largely paralleled those in the United States, Japan's July 2014 approval of asunaprevir plus daclatasvir combination therapy as the first all-oral interferon-free therapy was not repeated in the United States. Instead, two different combination therapies were approved: sofosbuvir/ledipasvir and paritaprevir/ritonavir/ombitasvir/dasabuvir. This divergence in treatment approaches may lead to differences in resistance challenges faced by Japan and the US. However, the recent approval of sofosbuvir plus ledipasvir in Japan and the recent submissions of petitions for approval of paritaprevir/ritonavir plus ombitasvir suggest a trend towards a new consensus on emerging DAA regimens.