Drug repurposing high-throughput screen identifies candidate antiviral compounds against Puumala Orthohantavirus

This study conducted a drug repurposing high-throughput screen in human cell lines using live Puumala orthohantavirus, identifying and validating 70 candidate antiviral compounds that target diverse host pathways, including heat shock proteins, the mTOR pathway, and nucleotide synthesis.

The Gist

Imagine a city under siege. The invaders are Hantaviruses (specifically the Puumala virus), tiny, invisible spies that sneak into human cells, hijack the machinery, and turn the cell into a virus factory. This causes severe diseases like kidney failure or lung syndrome. The scary part? We have no approved "weapons" (drugs) to stop them. Doctors can only treat the symptoms, like putting out fires after the building has already burned down.

This paper is about a team of scientists who decided to try a different strategy: Drug Repurposing.

The Big Idea: The "Used Car Lot" Approach

Instead of trying to build a brand-new, custom-made weapon from scratch (which takes years and billions of dollars), the scientists went to the "used car lot" of medicine. They looked at a massive library of 5,256 drugs that are already approved, safe, and sitting on pharmacy shelves for other diseases like cancer, heart disease, or infections.

Their question was simple: "Could any of these existing drugs accidentally stop the virus, even if they weren't designed for it?"

The Experiment: A High-Tech "Look-See"

To test this, the scientists set up a massive, automated experiment in a lab.

1. The Setup: They used two types of human cells as "battlefields."
   • A549 cells: Like lung cells (where the virus often starts).
   • HUVECs: Like blood vessel lining cells (the virus's main target in the body).
2. The Process: They put these cells in tiny trays (384 wells) and added one of the 5,000+ drugs to each well. Then, they introduced the virus.
3. The Detective Work: 24 hours later, they used high-powered microscopes and special glowing dyes to count how many cells had been infected.
   • If a drug worked, the cells would look healthy, and the virus would be gone (or very low).
   • If a drug failed, the cells would be full of glowing red virus signals.

The Results: Finding the Hidden Gems

Out of the thousands of drugs tested, they found 70 "winners" that successfully stopped the virus in at least one of the cell types.

Here is what they discovered, broken down into simple categories:

• The "Starvation" Strategy (Nucleotide Synthesis Inhibitors):
  Viruses need building blocks (nucleotides) to copy their genetic code. Some of the winning drugs were like cutting off the supply line. They stopped the cell from making these building blocks, so the virus literally ran out of parts to build itself. This included drugs like Mycophenolic acid, which are already used for organ transplants.

• The "Traffic Controller" (mTOR Inhibitors):
  Cells have a master switch called mTOR that tells the cell when to grow and when to make proteins. The virus tries to hijack this switch to make more of itself. The scientists found drugs that locked the switch, confusing the virus and stopping its production line. Interestingly, these worked better in lung cells than in blood vessel cells, showing that the virus plays differently depending on the neighborhood it's in.

• The "Stress Manager" (Heat Shock Protein Inhibitors):
  When a cell gets stressed (like when a virus attacks), it uses "chaperone proteins" (HSPs) to help fix broken parts. The virus uses these chaperones to fold its own proteins correctly. The winning drugs disabled the chaperones, leaving the virus's proteins messy and useless.

• The Surprise Hit: Antibiotics!
  This was the most unexpected discovery. They found that certain antibiotics (usually used to kill bacteria) also stopped the virus. It's like finding out that a tool meant for fixing a car engine also happens to stop a burglar. We don't know exactly how yet, but it's a huge clue that we need to investigate.

• The "Double Agents" (Proviral Compounds):
  They also found 21 drugs that did the opposite: they made the infection worse. Most of these were drugs that tweak how DNA is read (epigenetics). This is a warning sign: if we use these drugs for other things, we might accidentally make a hantavirus infection more dangerous.

Why This Matters

This study is like finding a map of the virus's weaknesses.

1. Speed: Because these drugs are already approved for humans, if one of them works well in the next stage of testing, it could be used to treat patients much faster than creating a new drug from scratch.
2. New Targets: It shows us that the virus relies on specific parts of our own body (like the stress managers or the building block factories). We can now focus on those specific pathways to design better treatments.
3. The Antibiotic Surprise: The fact that antibiotics worked suggests there are hidden connections between how our body fights bacteria and how it fights viruses.

The Bottom Line

The scientists didn't just find a needle in a haystack; they found a whole box of needles. They proved that by looking at old, familiar drugs with fresh eyes, we can find powerful new ways to fight a deadly virus that currently has no cure. It's a hopeful step toward turning the tide against hantaviruses.

Technical Summary

1. Problem Statement

• Clinical Gap: Hantaviruses (Orthohantaviruses) cause severe zoonotic diseases, specifically Hemorrhagic Fever with Renal Syndrome (HFRS) and Hantavirus Pulmonary Syndrome (HPS). Currently, there are no FDA- or EMA-approved vaccines or specific antiviral therapies; treatment is limited to supportive care.
• Research Limitations: Previous drug discovery efforts have relied on pseudotyped viruses or focused on specific infection stages. These approaches often fail to account for complex host dependencies required for productive, live-virus infection and replication.
• Objective: To systematically identify host-directed modulators of Puumala virus (PUUV) infection using a live-virus, high-throughput drug repurposing screen to discover novel antiviral candidates.

2. Methodology

The study employed a phenotypic screening strategy using the Drug Repurposing Hub library (5,256 FDA-approved, clinical, and pre-clinical compounds).

• Cell Models:
  • A549 cells: Human lung epithelial cells (primary screening and validation).
  • HUVECs: Primary human umbilical vein endothelial cells (validation, as hantaviruses primarily target endothelial cells).
• Screening Platform:
  • Assay: High-throughput immunofluorescence microscopy.
  • Protocol: A549 cells were seeded in 384-well plates with pre-spotted compounds (10 µM). PUUV was added directly to the growth medium. After 24 hours, cells were fixed, immunostained with convalescent patient sera (anti-viral proteins), and counter-stained with Hoechst 3342 (nuclei) and CellTracker (cell body).
  • Optimization: The protocol was simplified to remove washing steps and virus removal, enabling automation while maintaining a high-quality screening window (Z' = 0.7 in optimization; 0.41 in full screen).
  • Analysis: Images were analyzed using CellProfiler. Infection rates were calculated as the ratio of virus-positive cells to total cells. Viability was assessed via nuclei count.
• Hit Selection Criteria:
  • Antivirals: Compounds reducing infection rate by ≥2-fold with ≤50% reduction in cell viability.
  • Provirals: Compounds increasing infection rate by ≥2-fold with ≤50% reduction in viability.
• Validation: Primary hits were re-tested in A549 cells and HUVECs at three concentrations (0.83, 2.5, and 10 µM).
• Functional Clustering: Validated hits were grouped by annotated targets and mechanisms of action (e.g., mTOR inhibitors, HSP inhibitors, antibiotics).

3. Key Results

• Screening Output:
  • From 5,256 compounds, 150 antiviral candidates and 25 proviral candidates were identified in the primary screen.
  • Validation: 70 of the 150 antiviral candidates were successfully validated across the two cell systems:
    • 36 active in both A549 and HUVECs.
    • 24 specific to A549.
    • 10 specific to HUVECs.
  • Proviral Validation: 21 of the 23 primary proviral hits were validated in A549 cells. (Proviral effects could not be reliably assessed in HUVECs due to a naturally high baseline infection rate of ~80%).
• Functional Clustering & Mechanisms:
  • Known Pathways: Validated hits clustered into known antiviral mechanisms, including:
    • Nucleotide Synthesis: Inhibitors of Inosine Monophosphate Dehydrogenase (IMPDH) (e.g., Mycophenolic acid derivatives).
    • Signaling: mTOR pathway inhibitors (showed higher efficacy in A549 than HUVECs).
    • Proteostasis: Heat Shock Protein (HSP90) inhibitors.
  • Novel Findings:
    • Antibiotics: Several beta-lactam antibiotics (non-ribosome targeting) were identified as potent, low-toxicity antivirals. This is a novel finding, as beta-lactams are not classically associated with antiviral activity.
    • Proviral Agents: The proviral hits were enriched for epigenetic modifiers, specifically Histone Deacetylase (HDAC) inhibitors, suggesting a potential link between chromatin state and hantavirus replication/persistence.
• Reproducibility: There was a moderate but significant correlation ($r=0.45$) between primary screen and validation data in A549 cells, and substantial concordance between A549 and HUVEC datasets.

4. Significance and Contributions

• Systematic Host-Targeting Map: The study provides the first systematic map of host pathways influencing PUUV infection using a live-virus screen across two distinct human cell types.
• Novel Therapeutic Avenues:
  • The identification of beta-lactam antibiotics as antivirals offers a completely new class of compounds for repurposing against hantaviruses, warranting immediate mechanistic investigation.
  • The confirmation of IMPDH inhibitors and mTOR inhibitors reinforces the importance of nucleotide availability and translational control in hantavirus replication.
• Insight into Persistence: The discovery that HDAC inhibitors promote infection suggests that epigenetic regulation may play a role in hantavirus latency or persistence, a critical area for understanding long-term infection in humans.
• Resource for Future Research: The dataset serves as a reference framework for prioritizing host pathways for mechanistic studies and accelerates the development of therapeutic strategies for HFRS and HPS.

Conclusion

This research successfully bridges the gap between hypothesis-driven studies and broad phenotypic screening. By utilizing a live-virus high-throughput approach, the authors identified 70 validated host-directed antiviral compounds, including novel candidates like beta-lactam antibiotics, and highlighted specific cellular dependencies (nucleotide synthesis, mTOR, HSP90) that are critical for PUUV replication. These findings provide a robust foundation for developing the first effective treatments for hantavirus infections.