Introduction
    Hepatitis C was referred to as non-A non-B hepatitis when it was initially discovered. Since it is transmitted through the blood, it accounted for virtually all the cases of post-transfusion hepatitis cases. As culture in test tubes was not possible and chimpanzees were the only animals available as infection experimental models, the identification of the causative virus gene was extremely difficult. However in 1989 the American company Chiron succeeded in identifying the virus gene and its structure became known. The hepatitis C virus (HCV) consists of a positive single-stranded RNA of a total length of approximately 9.6 kb as a genome, and given the resemblance of its structure, it is classified in the flavivirus family. An antibody determination system using the viral protein as the antigen has been established. Approximately 1~2% of the world population is estimated to be infected. HCV infection becomes chronic in a significant number of cases, and since it progresses to cirrhosis and liver cancer, great expectations are being placed on the development of an effective approach to treatment. We will briefly describe in the following the recent advances achieved in the treatment of HCV.
HCV and approaches to the treatment
    HCV possesses a non-translated region of about 340 base-pairs (5'UTR) and a non-translated region of about 230 (3'UTR) at the two ends of the genome. After translation of the virus proteins into one polyprotein, it is cleaved by the cells and the viral protease, which generates virus proteins and ensures replication (Figure 1). It is made up of structural proteins composing the viral particles (core, E1, E2) and of the non-structural proteins required for replication (NS2, NS3, NS4A, NS4B, NS5A, NS5B).
    The approach to the treatment used so far most frequently to fight off HCV infection is interferon (IFN)-alpha and beta. However the efficacy of the treatment with IFN varies with the HCV genotype, the viral load inside the body of the patients and the degree of mutation of the ultravariable domain of the virus envelope protein (E1, E2). In single administration, the effectiveness is reportedly 30% or so on average. Besides since IFN is rapidly eliminated from the body after its administration, it is necessary to repeat the injections several times, which oftentimes restricts the social life of the patients and is a very painful experience. Attempts are being made to enhance the activity and to improve the sustained response to IFN. One product belonging to the former category is consensus IFN. It has been synthesized artificially so as to compare the current eleven gene arrangements and to have the highest number of common arrangements. The results of clinical trials conducted so far suggest that the efficacy in terms of viral RNA negativeness is higher than in groups administered with IFN alpha 2. The latter category includes pegylated IFN in which inert type polyethylene glycol (PEG) with a molecular weight of about 40,000 has been attached to IFN-alpha. A half-life in the blood of at least 90 hours has been achieved by slowing renal excretion, and a sustained response at least ten times as high as conventional IFN has been obtained. Its anti-IFN antibody induction capacity is also low, and it is effective in about 50% of HCV infected individuals. Furthermore agents such as ribavirin, thymosin alpha, corticosteroids, etc., show efficacy in combination with IFN, although they are not effective when used in monotherapy. Amid these agents, ribavirin, a guanosine analog, has been found to be effective in approximately 50% of infected individuals when combined with IFN. Its efficacy is reported frequently in Western countries where the IFN dosage is particularly low.
    In addition to IFN, attempts have also been made to develop therapeutic agents by inhibiting the functions of HCV protein and of nucleic acid and directly blocking virus replication. NS3 protein of HCV is a chymotrypsin-like serine protease. It is maturated by truncating each viral protein using NS4A as a coupling factor. A serine protease inhibitor, which blocks it, has been developed and a clinical trial has been kicked off. In addition, an RNA-dependent RNA polymerase inhibitor is also under development using NS5B synthesizing viral RNA as the target. Furthermore antisense nucleic acid and ribozymes using as the target the internal ribosomal entry site (IRES) functioning at the start of the translation present in 5'UTR of viral RNA have been developed, and clinical trials are ongoing. Although it is not a therapeutic agent, a vaccine using the envelope protein is also under development. This is not easy, since HCV has many genotypes and a hypervariable domain is also present. A clinical trial using E1 protein as a target is in progress in Belgium. Lactoferrin, which is present in large quantities in milk, and glucosidase inhibitors are also being investigated, as they may display an anti-viral activity. RNA interference (RNAi) using the HCV genome as the target looks also promising as an efficacious approach to treatment, although the technique for introducing it into the body of patients has not yet been established.
Conclusion
    Recent reports have clearly shown that in fact over 80% of patients with liver cancer are HCV-antibody positive. However it remains in many respects uncertain why HCV, a long-lasting infection through replication of the cytoplasm, is linked to a high incidence of liver cancer. Since the HCV infection system has not been ascertained, we have established a HCV persistent manifestation cell line for analysis, and we have found that the virus persistent manifestation proper is also related to the increase in the tumorigenicity of the cells. We are willing to contribute to the progress of the diagnosis of the condition of hepatitis C and to the treatment of liver cancer through the elucidation of these mechanisms.

[Co-researchers] Michinori Obara (Institute of Medical Science, Tokyo)
Chieko Kai (Institute of Medical Science, University of Tokyo)