DHODH inhibitors: What will it take to get them into the clinic as antivirals?

The emergence of new human viruses with epidemic or pandemic potential has reaffirmed the urgency to develop effective broad-spectrum antivirals (BSAs) as part of a strategic framework for pandemic prevention and preparedness. To this end, the host nucleotide metabolic pathway has been subject to intense investigation in the search for host-targeting agents (HTAs) with potential BSA activity. In particular, human dihydroorotate dehydrogenase (hDHODH), a rate-limiting enzyme in the de novo pyrimidine biosynthetic pathway, has been identified as a preferential target of new HTAs. Viral replication in fact relies on cellular pyrimidine replenishment, making hDHODH an ideal HTA target. The depletion of the host pyrimidine pool that ensues the pharmacological inhibition of hDHODH activity elicits effective BSA activity through three distinct mechanisms: it blocks viral DNA and RNA synthesis; it activates effector mechanisms of the host innate antiviral response; and it mitigates the virus-induced inflammatory response. However, despite the spectacular results obtained in vitro, the hDHODH inhibitors examined as mono-drug therapies in animal models of human viral infections and in clinical trials have produced disappointing levels of overall antiviral efficacy. To overcome this inherent limitation, pharmacological strategies based on multi-drug combination treatments should be considered to enable efficacy of hDHODH-targeted antiviral therapies. Here, we review the state-of-the-art of antiviral applications of hDHODH inhibitors, discuss the challenges that have emerged from their testing in animal models and human clinical trials and consider how they might be addressed to advance the development of hDHODH inhibitors as BSA for the treatment of viral diseases.