A Singapore study led by the Novartis Institute for Tropical Diseases (NITD) has shown that a novel small-molecule inhibitor of the dengue virus (DENV) nonstructural protein 4B (NS4B) has favorable pharmacokinetics and efficacy in preclinical animal models, the authors reported in the February 3, 2021, edition of Science Translational Medicine.

With its demonstrated pan-serotypic activity, efficacy in both acute and delayed in vivo models, and favorable pharmacokinetics and safety, the new NS4B inhibitor, designated NITD-688, represents a promising preclinical candidate for treating dengue fever (DF).

A mosquito-borne flavivirus with four distinct serotypes, DENV infects about 390 million people per year, with symptoms ranging from flu-like DF to severe, life-threatening dengue shock syndrome (DSS) or dengue hemorrhagic fever (DHF).

"Besides high fever, DF is associated with debilitating pain," said study leader Feng Gu, head of the NITD in Emeryville, California.

"Most patients eventually recover after often a long period of fatigue, although approximately 2-5% of infections can lead to severe DSS or DHF, which are characterized by bleeding, organ impairment and/or plasma leakage," Gu told BioWorld Science.

Of concern, dengue incidence has increased 30-fold over the past 50 years, with severe outbreaks having a significant health and economic burden.

"The incidence of dengue has increased dramatically over recent decades, presumably exacerbated by climate change and human migration," noted Gu.

"Over half the world is now estimated to be at risk for dengue infection," he said, hence the need for effective control.

Effective control

Effective dengue control requires combining preventative strategies, including mosquito vector control and vaccines, with improved clinical care including antivirals.

Although vector control strategies show promise, they have not shown long-term efficacy in containing epidemics or preventing global spread.

Moreover, the only quadrivalent dengue vaccine, Dengvaxia (Sanofi Pasteur), is recommended only for use in previously infected individuals, and currently there is no available specific DENV antiviral treatment.

DENV antiviral

Besides having pan-serotypic safety and efficacy, an effective DENV antiviral should be orally administered, fast-acting and stable in tropical conditions.

Further, because severe disease is associated with higher viremia, decreasing viremia should reduce DF symptoms and duration, and progression to DSS or DHF.

Historically, small-molecule DENV inhibitor development has mainly focused on target-based and phenotype-based approaches.

The target-based approach has included extensive targeting of viral proteins together with high-throughput screens (HTSs) and structure-based rational design.

Cell-based phenotypic HTSs have also successfully identified candidate inhibitors against both host and viral targets, with NS4B being the most common.

However, anti-DENV drug development has been hampered by factors including limited pan-serotypic efficacy, safety, and lack of drug-like physicochemical or pharmacokinetic properties, despite extensive research.

"Unfortunately, despite great efforts from many scientists around the world, we still don't have a specific antiviral treatment available for dengue," noted Gu.

This prompted the new study, in which Gu and his team at NITD performed a phenotypic HTS using the Novartis compound library to identify candidate DENV inhibitors, which were optimized to improve anti-DENV potency and solubility.

"A large group of scientists at NITD has been working for many years to develop a dengue antiviral, screening 1.5 million chemical compounds for activity against DENV, in order to select growth inhibitory compounds," said Gu.

"After rounds of chemical optimization, the final molecule, NITD-688, was shown to be a potent inhibitor of all four serotypes of dengue when tested in cell culture," he said.

This is a significant finding, "as outbreaks of one or more DENV serotypes occur worldwide, so it is important that any anti-dengue drug candidate has pan-serotypic inhibitory activity."

As well as showing strong pan-phenotypic potency, NITD-688 also demonstrated excellent oral efficacy in DENV-infected AG129 mice.

A 1.44-log reduction in viremia was seen when mice were treated with oral NITD-688 30 mg/kg twice daily for 3 days from the time of infection, compared with a 1.16-log reduction when mice were treated 48 hours after infection.

Selection of resistance mutations and binding studies with recombinant proteins indicated that NS4B was the target of NITD-688, which studies in rats and dogs showed to have a long half-life and good oral bioavailability.

"We believe that the target of NITD-688 is the viral NS4B protein, which is an important component of the viral replication complex, so the interaction of NITD-688 and NS4B inhibits viral replication, killing the virus," said Gu.

Moreover, extensive in vitro safety profiling, combined with rat and dog toxicology studies, demonstrated that NITD-688 was well tolerated after 7-day repeat dosing.

"NITD-688 showed good pharmacokinetic properties, suggesting that it is suitable for oral dosing and was well tolerated in animal models," said Gu.

"Together, these data show that NITD-688 is a potent, pan-serotype dengue virus NS4B inhibitor, which is a promising preclinical candidate for the treatment of dengue," said Gu.

"We will now study the molecule in human clinical trials, as we believe NITD-688 has the potential to decrease the DENV viral load in patients, which could alleviate the painful and debilitating DF symptoms and possibly reduce progression to severe life-threatening disease."