Mechanism of action of the antiviral substance PDTC

Infections by respiratory viruses are among the most frequent diseases in men. In the case of human rhinoviruses (HRV) - which are the main causative agents of common cold - there are more than 100 different non-crossreacting serotypes circulating in the human population. It is therefore not possible to achieve protection by development of a general vaccine as has been successfully carried out in the case of other viruses. On the other hand the frequent infection by HRV constitutes not only a health burden to men, but also causes a considerable economic loss due to working days lost. Furthermore, infection by HRV is known to weaken the immune system and thus sets the stage for other, more dangerous viral and/or bacterial agents. Prevention of infection by HRV may therefore indirectly protect against other infectious diseases which occur frequently after a common cold. There is thus an urgent need for the development of antivirals against HRV particularly as most drugs against common cold act against the symptoms of the disease rather than against the viral infection itself. Our group has recently discovered a drug (PDTC) which inhibits HRV replication in cell culture. In contrast to other substances active against HRV, PDTC does not seem to act at a very early stage of the infection cycle, e.g., by binding directly to the viral capsid, but seems to act at a subsequent state. There is evidence that PDTC inhibits both major and minor group HRV serotypes as well as other viruses thus extending the spectrum of potential applications. So far no HRV variants resistant to PDTC have been found. This can perhaps be taken as a first indication that the drug might act on a cellular component essential for virus replication. In this project we want to investigate the mode of action of PDTC on both HRV infected and on uninfected cells. It is expected that this investigation will allow to identify the stage at which the drug acts and will help to understand the mechanism of inhibition of HRV multiplication by PDTC.

Human rhinoviruses (HRV) are the most frequent cause of the common cold and are implicated in more than 50% of upper respiratory tract infections. Although not life threatening, infections with rhinovirus can prepare the ground for more serious diseases, such as exacerbations of asthma or otitis media. In addition, the common cold is an enormous economic burden on society in terms of visits to doctors, treatments, and absences from work. Currently there is no marketed antiviral therapy available for the prevention or treatment of rhinovirus infection related illnesses apart from symptomatic treatment. In our laboratory we could show that pyrrolidine dithiocarbamate (PDTC) is a potent antiviral agent. PDTC inhibits the multiplication of human rhinovirus, of poliovirus and of influenza virus in cell culture. In this project we investigated the precise molecular basis of the inhibitory function, and the effects of PDTC on the individual steps of the infection cycle of HRV. The compound did not interfere with early steps in the viral life cycle such as receptor binding and internalisation. But we could demonstrate that both the processing of the viral polyprotein and the replication of the viral RNA was inhibited by PDTC. The antiviral effect of PDTC depends on the availability of metal ions, as addition of metal ion chelators neutralised the antiviral property. However, addition of zinc ions re-established the antiviral effect. Our experiments indicate that a change in the level of intracellular zinc ions plays a critical role. Furthermore it was found that during HRV infection apoptosis is induced, as can be seen by cellular markers such as caspase activation and DNA fragmentation. Apoptosis does not affect viral replication per se but might facilitate release of progeny virus. These experiments enhance our understanding of the complex virus-host interactions and might pave the way for the development of novel antiviral treatments.