Assessment of Repurposed Compounds for Antiviral Activity Against Measles Virus

This study demonstrates that among four repurposed compounds tested, quercetin (enhanced by ascorbic acid), isoquercetin, and zafirlukast exhibit potent dose-dependent therapeutic antiviral activity against measles virus in vitro, supporting their further evaluation as candidate treatments.

The Gist

The Measles "Locksmith" Hunt: A Story of Repurposed Keys

Imagine the Measles virus as a highly skilled, invisible burglar. It's incredibly contagious (one burglar can infect 12 to 18 neighbors!) and once it breaks into your house (your body), it can cause serious damage like pneumonia or brain inflammation.

Currently, we have a great security system called the MMR vaccine. If everyone uses it, the burglar can't get in. But sometimes, the burglar slips through the cracks (breakthrough cases), or the security system doesn't work for everyone (like people with weak immune systems). Worse yet, if the burglar does get inside, we have no special "anti-burglar spray" or "emergency lock" to kick them out. We can only offer a cup of tea and a blanket (supportive care) while the body fights it out.

The Big Question: Can we find a "key" that already exists in our medicine cabinet to lock the burglar out or kick them out?

The Experiment: Testing Four "Keys"

The scientists in this study decided to test four existing drugs and supplements to see if they could stop Measles. They treated the virus like a puzzle and the drugs like potential solutions:

1. Azelastine: An allergy medicine (like a key for a different lock).
2. Zafirlukast: An asthma medicine (another key for a different lock).
3. Quercetin: A natural plant compound found in onions and apples (a nature-made key).
4. Isoquercetin: A slightly modified version of Quercetin (a nature-made key with a tiny upgrade).

They tested these keys in two ways:

• The "Pre-Game" Test: Putting the key on the door before the burglar arrives (Preventative).
• The "Mid-Game" Test: Putting the key on the door after the burglar is already inside (Therapeutic).

The Results: Who Opened the Door?

Here is what happened when they tried these keys on the Measles burglar in the lab:

• Azelastine (The Allergy Med): It didn't work at all. It was like trying to open a digital lock with a physical key. It didn't stop the virus, and it actually made the cells (the "house") a bit sick, which confused the results.
• Zafirlukast (The Asthma Med): This one worked! It didn't stop the burglar from entering, but once the burglar was inside, this drug helped the house fight back. It reduced the virus significantly.
• Quercetin (The Onion Power): This was the star of the show. It was the most effective at kicking the virus out of the cells. However, there was a catch: in the lab, Quercetin is a bit unstable, like a flower that wilts quickly in the sun.
• The "Vitamin C" Trick: The scientists realized that Quercetin gets "rusty" (oxidizes) in the lab liquid, which makes it less effective and slightly toxic to the cells. So, they added Vitamin C (Ascorbic Acid), which acts like a rust-proof shield.
  • Result: With the Vitamin C shield, Quercetin became even stronger! It became the most powerful weapon against Measles in the test.
  • Isoquercetin also worked well with the Vitamin C shield, though it wasn't quite as strong as Quercetin.

The Takeaway: What Does This Mean for Us?

Think of this study as a speed test for finding a new medicine.

1. We found a potential hero: Quercetin (especially with Vitamin C) and Zafirlukast showed they can fight Measles in the lab. They are like finding a spare key that fits the Measles lock.
2. Timing matters: These drugs worked best when given after the infection started (therapeutic), rather than just trying to prevent it.
3. The next steps: Just because a key works in a test tube (the lab) doesn't mean it will work perfectly in a human body. The lab is a simple, controlled room, but a human body is a complex city with traffic, weather, and different neighborhoods.

In simple terms: The scientists found that some old drugs and natural supplements might be able to treat Measles if we give them to people after they get sick. Quercetin, boosted by Vitamin C, looks like the most promising candidate. Now, the real work begins: testing these in animals and eventually in humans to see if they are safe and effective enough to become a real medicine.

The Bottom Line: We don't have a cure for Measles yet, but this study gave us a very strong hint about where to look next. It's like finding a flashlight in the dark; it doesn't solve the problem of the dark, but it shows us exactly where to walk to find the exit.

Technical Summary

1. Problem Statement

• Unmet Medical Need: There is currently no approved antiviral therapy for Measles Virus (MeV). Management is limited to supportive care, despite the virus's high contagion (R0 of 12–18) and potential for severe complications (pneumonia, encephalitis, death).
• Epidemiological Context: Despite high vaccine coverage, MeV outbreaks are recurring globally and in the US (with cases rising significantly from 2024 to 2026), driven by unvaccinated populations and waning immunity.
• Vulnerable Populations: Immunocompromised individuals often respond poorly to the MMR vaccine, and breakthrough infections occur.
• Objective: To rapidly identify candidate drugs for clinical evaluation by repurposing existing compounds with known broad antiviral activities. The study specifically evaluated azelastine hydrochloride, zafirlukast, quercetin, and isoquercetin.

2. Methodology

The study utilized an in vitro screening platform using Vero/hSLAM cells (stably expressing the human MeV receptor, CD150).

• Virus Strain: MeV isolate (MVs/Ohio.USA/17.14/3 [D9]).
• Experimental Settings: Compounds were tested in four distinct modes to evaluate preventative and therapeutic potential:
  1. Prophylactic: Cells pre-treated with compounds before infection (tests interference with early entry).
  2. Virucidal: Compounds pre-mixed with virus before infection (tests direct viral particle inactivation).
  3. Steady-state (Therapeutic): Infection followed by daily replenishment of compounds (tests post-entry inhibition).
  4. Persistent (Therapeutic): Infection followed by a single dose of compounds (tests sustained post-entry inhibition).
• Assay Conditions:
  • Viral loads and cell viability were measured 48 hours post-infection.
  • Dose-response curves (50 µM to 3.13 µM) were generated to calculate EC50 values.
  • Flavonoid Optimization: Quercetin and isoquercetin were re-tested in the presence of 1 mM ascorbic acid to prevent oxidation-induced cytotoxicity (hydrogen peroxide generation) common in cell culture.
• Quantification:
  • Viral Load: Measured via qRT-PCR targeting the MeV hemagglutinin (MeV-H) gene.
  • Cell Viability: Measured via luminescence (CellTiter-Glo®).
  • Data Analysis: EC50 values were derived using a four-parameter log-logistic (4PL) model.

3. Key Contributions

• First Evaluation of These Compounds against MeV: This study provides the first data on the specific antiviral activity of azelastine, zafirlukast, quercetin, and isoquercetin against MeV.
• Optimization of Flavonoid Assays: The study identified that ascorbic acid is critical for accurately assessing the therapeutic potential of quercetin and isoquercetin by mitigating false-positive cytotoxicity caused by flavonoid oxidation.
• Differentiation of Mechanisms: The study distinguished between compounds that act via direct virucidal activity versus those requiring post-entry therapeutic intervention.

4. Results

• Azelastine Hydrochloride:
  • Outcome: No clear antiviral activity observed.
  • Note: While viral loads decreased at high concentrations, cell viability declined proportionately, suggesting the observed effect was likely due to general cytotoxicity rather than specific antiviral activity.
• Zafirlukast:
  • Outcome: Showed clear therapeutic activity in both steady-state and persistent settings.
  • Efficacy: No activity in prophylactic or virucidal settings.
  • EC50: 21.2 µM (steady-state) and 23.0 µM (persistent).
• Quercetin:
  • Outcome: Demonstrated the most potent therapeutic activity among all tested compounds.
  • Enhancement: Activity was significantly enhanced in the presence of ascorbic acid.
  • EC50 (with Ascorbic Acid): 4.22 µM (steady-state) and 4.77 µM (persistent).
  • Mechanism: Activity was strictly therapeutic; no prophylactic or virucidal effects were observed.
• Isoquercetin:
  • Outcome: Showed therapeutic activity, though slightly less potent than quercetin.
  • Enhancement: Activity improved with ascorbic acid.
  • EC50 (with Ascorbic Acid): 10.7 µM (steady-state) and 10.9 µM (persistent).
  • Note: Some reduced cell viability was still observed, but the therapeutic window was established.

5. Significance and Conclusion

• Candidate Identification: The study identifies quercetin as the lead candidate for MeV therapy, followed by isoquercetin and zafirlukast.
• Clinical Relevance: These compounds are already clinically approved (azelastine, zafirlukast) or in clinical development with GRAS (Generally Recognized As Safe) status (quercetin, isoquercetin), allowing for rapid advancement to clinical trials if in vivo results are positive.
• Mechanistic Insight: The lack of prophylactic/virucidal activity suggests these compounds likely act on post-entry stages of the viral life cycle (e.g., replication or assembly) rather than blocking entry.
• Future Directions: The authors recommend further in vivo evaluation to confirm efficacy, noting that while in vitro platforms are excellent for rapid screening, they do not fully capture the complexity of MeV infection in a whole organism. The combination of isoquercetin and zafirlukast is also highlighted as a potential synergistic strategy.