"Strong reasons make strong actions"--The antiviral efficacy of NS3/4A protease inhibitors

Preclinical characteristics of the hepatitis C virus NS3/4A protease inhibitor ITMN-191 (R7227)

Future treatments for chronic hepatitis C virus (HCV) infection are likely to include agents that target viral components directly. Here, the preclinical characteristics of ITMN-191, a peptidomimetic inhibitor of the NS3/4A protease of HCV, are described. ITMN-191 inhibited a reference genotype 1 NS3/4A protein in a time-dependent fashion, a hallmark of an inhibitor with a two-step binding mechanism and a low dissociation rate. Under preequilibrium conditions, 290 pM ITMN-191 half-maximally inhibited the reference NS3/4A protease, but a 35,000-fold-higher concentration did not appreciably inhibit a panel of 79 proteases, ion channels, transporters, and cell surface receptors. Subnanomolar biochemical potency was maintained against NS3/4A derived from HCV genotypes 4, 5, and 6, while single-digit nanomolar potency was observed against NS3/4A from genotypes 2b and 3a. Dilution of a preformed enzyme inhibitor complex indicated ITMN-191 remained bound to and inhibited NS3/4A for more than 5 h after its initial association. In cell-based potency assays, half-maximal reduction of genotype 1b HCV replicon RNA was afforded by 1.8 nM; 45 nM eliminated the HCV replicon from cells. Peginterferon alfa-2a displayed a significant degree of antiviral synergy with ITMN-191 and reduced the concentration of ITMN-191 required for HCV replicon elimination. A 30-mg/kg of body weight oral dose administered to rats or monkeys yielded liver concentrations 12 h after dosing that exceeded the ITMN-191 concentration required to eliminate replicon RNA from cells. These preclinical characteristics compare favorably to those of other inhibitors of NS3/4A in clinical development and therefore support the clinical investigation of ITMN-191 for the treatment of chronic hepatitis C.

Nonresponse to treatment for hepatitis C: current management strategies

Chronic hepatitis C affects >170 million people worldwide, causing cirrhosis and liver cancer in a sizeable proportion of patients. Substantial progress has been made in the treatment of chronic hepatitis C. More than 50% of patients can achieve sustained virological response after 24-48 weeks of interferon and ribavirin combination therapy, making chronic hepatitis C a potentially curable disease. However, a large proportion of patients with chronic hepatitis C do not clear the virus after current standard therapy. Hepatitis C virus develops two pathways to counteract the antiviral effect of interferon. Some chronic hepatitis C patients may have a virus that is more resistant to interferon therapy, while other patients appear to have defective immune responses or poor tolerance or compliance to interferon-based antiviral therapy. The possible strategies to improve antiviral efficiency in these nonresponders are to increase the dosage, prolong the duration of treatment and improve the compliance of patients. A total of 6-15% of prior nonresponders to standard interferon plus ribavirin therapy will respond to re-treatment with peginterferon plus ribavirin, while 32-50% of patients who have relapsed will respond to re-treatment. New small molecules are under development to treat chronic hepatitis C and may be important particularly in the treatment of prior nonresponders to current standard therapy.

Old and emerging therapies in chronic hepatitis C: an update

The main goal of therapy in hepatitis C virus (HCV) infection is to achieve a sustained virological response currently defined as undetectable HCV-RNA in peripheral blood determined with the most sensitive polymerase chain reaction technique 24 weeks after the end of treatment. This goal is practically equivalent with eradication of HCV infection and cure of the underlying HCV-induced liver disease. The current standard in hepatitis C treatment consists in combination regimens of pegylated interferon-alpha (Peg-INF-alpha) with Ribavirin (RBV). Such treatment schemes are quite successful in patients with HCV genotypes 2 and 3 infections achieving HCV eradication rates of 75-90%. However, they are much less effective in patients with genotypes 1 and 4 infections with eradication rates ranging between 45% and 52%. Moreover, they have several, and sometimes severe, adverse effects and contraindications, further limiting their efficacy and applicability in an appreciable number of patients with chronic HCV-induced liver disease. Therefore, the need for improvement of existing therapies and for development of new effective, safe and tolerable drugs is a matter of great clinical relevance and importance. In this article, recent improvements in the current standard of therapy with IFN-alpha and RBV in various subsets of patients with chronic hepatitis C and in the clinical development of new emerging drugs, particularly small molecules, will be reviewed and commented. The article is divided in two main parts: (i) improvements in the standard combination therapies and schemes of approved Peg-INF-alpha with RBV and expectations from new interferons, interferon inducers and alternatives to RBV; (ii) new drugs for HCV in clinical development focusing mostly on specific inhibitors of HCV and less so on other drugs including immune therapies.

The therapeutic potential of NS3 protease inhibitors in HCV infection

Hepatitis C virus (HCV) infection is a serious cause of chronic liver disease worldwide and afflicts > 170 million people. The HCV-encoded NS3 protease is essential for viral replication and has long been recognised as a prime target for antiviral drugs. However, the peculiar active site structure of this enzyme, a shallow dent on the surface of the protein, has rendered the development of small-molecule inhibitors a highly challenging task. Nevertheless, perseverance and creativity has led to significant progress in this field over the last few years resulting in three compounds that are reported to enter the clinic. The impressive reduction of HCV RNA plasma levels observed with two of these inhibitors (ciluprevir and VX-950) in clinical trials has undoubtedly illustrated the potential of this viral enzyme-targeted drug discovery approach.

Challenges and successes in developing new therapies for hepatitis C

Hepatitis C virus (HCV) will continue to be a serious global health threat for many years to come because of the chronic nature of the infection, its high prevalence and the significant morbidity of the resulting disease. Recently, a small number of molecules have produced encouraging results in proof-of-concept clinical trials. At the same time, preclinical evidence is accumulating that development of resistance will eventually limit the efficacy of new drugs. Thus, combinations of multiple agents will be required to treat chronic HCV infection.