Subchronic Toxicity Study of Nitric Oxide Nano Bubbles Injection in Sprague-Dawley Rats

A 90-day subchronic toxicity study in Sprague-Dawley rats demonstrated that intravenous administration of nitric oxide nanobubbles at doses up to 0.06 mL is generally safe, showing no mortality and only mild to moderate histological changes in the liver, kidney, and spleen, thereby supporting further formulation optimization for potential human clinical trials.

The Gist

The Tiny Bubble Test: Is Nitric Oxide Safe for the Long Haul?

Imagine you have a magical, invisible gas called Nitric Oxide (NO). In the human body, this gas is like a super-efficient messenger. It tells your blood vessels to relax, helps your heart pump better, and fights inflammation. But here's the catch: this gas is like a firework spark—it disappears almost instantly (in just a few seconds).

Scientists wanted to use this gas to treat diseases, but they needed a way to keep it alive long enough to do its job. Enter Nanobubbles. Think of these as microscopic, indestructible "bubbles" or "delivery trucks" that trap the Nitric Oxide inside, protecting it and letting it travel through the body safely.

This paper is the story of a 90-day safety test to see if these Nitric Oxide nanobubbles (NONB) are safe to inject into the body over a long period.

---

The Experiment: A 3-Month Road Trip for Rats

To test this, the researchers took a fleet of Sprague-Dawley rats (the standard "test drivers" for medical science) and put them on a 90-day journey.

• The Setup: They divided the rats into groups. Some got a "placebo" (just water), while others got injections of the Nitric Oxide nanobubbles at three different strengths: a small dose, a medium dose, and a large dose.
• The Mission: They wanted to see if the bubbles would cause any harm to the rats' organs (liver, kidneys, heart, lungs, spleen) or mess up their blood chemistry over three months.
• The Safety Net: They even kept a "satellite group" of rats for an extra month after the injections stopped, just to see if any delayed side effects would pop up later.

The Results: A Clean Bill of Health (With a Few Bumps)

Here is what happened during the 90-day trip, broken down by what they checked:

1. The "Vital Signs" (Behavior and Weight)

• The Verdict: The rats were happy campers. No one died, no one had seizures, and no one got the runs. They ate well, gained weight normally, and their fur stayed shiny.
• Analogy: It's like driving a car for three months; the engine didn't blow up, and the passengers didn't get sick.

2. The "Fuel Gauge" (Blood Sugar and Cholesterol)

• The Verdict: As the dose of bubbles got bigger, the rats' blood sugar and cholesterol went up slightly.
• The Catch: Even at the highest dose, the numbers stayed within the normal range for a rat. They didn't spike into dangerous territory.
• Analogy: Imagine filling a car's gas tank. The more you pour in, the fuller it gets. The researchers were worried it might overflow, but the tank just got a little fuller without spilling over.

3. The "Engine Filters" (Liver and Kidneys)

• The Liver: The liver is the body's filter. The test showed a tiny bit of "fatty degeneration" (like a filter getting a little clogged with grease) in the rats that got the highest dose. However, the liver enzymes (the warning lights on the dashboard) stayed within safe limits.
• The Kidneys: The kidneys are the body's water filters. The test showed a slight rise in creatinine (a waste product), but again, it stayed within the "safe zone."
• The Twist: In the "satellite group" (the rats observed for an extra month), the liver looked much better. It seems the liver has a great ability to clean itself up once the bubbles stop coming.

4. The "Electrical System" (Sodium and Potassium)

• The Verdict: The bubbles caused a slight shift in the body's salt balance. Sodium went down a little, and Potassium went up a little.
• Analogy: Think of the body's salts as the electrical wiring. The bubbles tweaked the voltage slightly, but the lights didn't flicker out, and the system didn't short-circuit. By the end, the levels were still close to normal.

5. The "Special Effects" (Heart, Lungs, and Spleen)

• Heart & Lungs: These organs were perfectly normal. No damage at all.
• The Spleen: This was the only organ that showed some "scars." The spleen (which recycles old blood cells) showed some minor bleeding and iron buildup.
• Why? The researchers think the Nitric Oxide made the tiny blood vessels in the spleen relax so much that a little fluid leaked out, causing minor bruising. It wasn't a catastrophic failure, just a minor leak.

The Big Picture: What Does This Mean?

The researchers concluded that Nitric Oxide Nanobubbles are safe for subchronic use (up to 90 days) in rats, even at the highest dose tested.

• No Fatalities: No rats died.
• No Organ Failure: The vital organs kept working, even if they showed minor signs of stress.
• Recovery: The body showed a strong ability to recover once the treatment stopped.

The Final Takeaway

Think of this study as a stress test for a new type of fuel. The scientists poured a lot of this new "Nitric Oxide fuel" into the rats' systems for three months. While the engine (the body) showed a few minor warning lights (slight changes in blood sugar and minor tissue stress), the car didn't break down.

The paper suggests that this technology is ready for the next step: human clinical trials. However, the scientists recommend tweaking the formula slightly to make it even smoother before we start testing it on people.

In short: The Nitric Oxide nanobubbles passed the 90-day safety test with flying colors, proving they are a promising candidate for future medical treatments.

Technical Summary

1. Problem Statement

Nitric Oxide (NO) is a critical signaling molecule with therapeutic potential for vasodilation, anti-platelet aggregation, and inflammation reduction. However, its clinical application is hindered by an extremely short biological half-life (1–10 seconds) and rapid diffusion. While nanobubble technology has successfully stabilized hydrogen gas for medical use, the development of NO nanobubbles (NONB) for intravenous delivery remains underexplored. Previous studies indicated that NO donors could cause dose-dependent increases in liver enzymes (AST, ALT) and urea, raising concerns about hepatotoxicity and nephrotoxicity. There is a lack of comprehensive subchronic toxicity data regarding the safety profile of intravenous NONB formulations over extended periods.

2. Methodology

• Study Design: A 90-day subchronic toxicity study following modified OECD Guideline 408.
• Subjects: Male and female Sprague-Dawley rats (8–12 weeks old, 150–250 g).
• Groups: Six groups of 14 animals each (7 males, 7 females):
  • Group 1: Healthy control (no injection).
  • Groups 2, 3, 4: Received intravenous (tail vein) NONB injections at doses of 0.01 mL, 0.04 mL, and 0.06 mL, respectively.
  • Group 5: Healthy satellite control (observed for an additional 4 weeks post-treatment).
  • Group 6: Satellite group receiving the highest dose (0.06 mL), observed for an additional 4 weeks post-treatment to assess reversibility.
• Test Material: NONB solution (26 million bubbles/mL) prepared in reverse osmosis water with a buffer (1% ascorbic acid, 1% sodium bicarbonate) to maintain pH stability.
• Data Collection:
  • Clinical Observations: Daily monitoring for mortality, morbidity, behavioral changes, and physical signs (fur condition, tremors, etc.) over 90 days.
  • Body & Organ Weight: Weekly body weight; terminal organ weights (liver, kidney, heart, lung, spleen) to calculate relative organ weights.
  • Hematology: Complete blood count (RBC, WBC, platelets, etc.).
  • Serum Biochemistry: Glucose, lipids (cholesterol, triglycerides), liver function (AST, ALT), kidney function (urea, creatinine), and electrolytes (Na, K).
  • Histopathology: H&E staining of liver, kidney, heart, lung, and spleen. Organs were scored on a scale of 1 (normal) to 5 (severe damage) based on specific criteria (e.g., fatty degeneration, necrosis, fibrosis).
• Statistical Analysis: ANOVA/Kruskal-Wallis tests with post-hoc analysis (Games-Howell, LSD, Mann-Whitney) for significance ($p < 0.05$).

3. Key Results

• Mortality and Morbidity: No deaths occurred in any group during the 90-day period. No clinical signs of toxicity (seizures, tremors, diarrhea, fatigue) were observed. Body weight gain was consistent across all groups.
• Organ Weights: No significant differences were found in the relative weights of the liver, kidney, heart, lung, or spleen between treatment and control groups.
• Hematology: All hematological parameters (RBC, WBC, platelets, hemoglobin, etc.) remained within normal limits with no statistically significant differences between groups.
• Serum Biochemistry:
  • Glucose, Lipids, Liver Enzymes, Creatinine: Levels increased linearly with the dose of NONB. However, all values remained within the established normal physiological ranges for rats.
    • Glucose: Max ~76 mg/dL (Normal: 62–201 mg/dL).
    • AST/ALT: Max ~38 U/L and ~19 U/L respectively (Normal ranges: 37–205 U/L and 6–114 U/L).
    • Creatinine: Max ~1.02 mg/dL (Normal: 0.2–1.2 mg/dL).
  • Electrolytes:
    • Sodium (Na): Showed a dose-dependent decrease. While statistically significant, levels remained near or slightly below the lower limit of normal.
    • Potassium (K): Showed a dose-dependent increase but remained within the lower range of normal.
• Histopathology:
  • Heart and Lungs: Normal histology (Score 1) across all groups.
  • Liver: Dose-dependent mild to moderate fatty degeneration and focal necrosis (Scores 2–3) in treatment groups compared to controls.
  • Kidney: Mild tubular dilation and degeneration observed in male rats at the highest dose (Score 2); female rats showed no significant histological changes.
  • Spleen: Dose-dependent focal to multifocal hemorrhage and hemosiderin-laden macrophage infiltration (Scores 2–3).
• Reversibility (Satellite Groups): The satellite groups (observed 4 weeks post-treatment) showed similar biochemical and histological profiles to the high-dose group, suggesting the effects of the 0.06 mL dose persist for at least 4 weeks, though no acute deterioration was noted.

4. Key Contributions

• Safety Validation: This study provides the first comprehensive subchronic (90-day) toxicity data for intravenous NONB administration, establishing a safety window up to 0.06 mL/day.
• Dose-Response Characterization: It elucidates the dose-dependent physiological responses to NONB, specifically the linear increase in glucose, lipids, and liver enzymes, and the inverse relationship with sodium levels.
• Mechanistic Insight: The study links the observed splenic hemorrhage and hemosiderosis to potential hemodynamic instability caused by NO-induced vasodilation (arterial relaxation > venous relaxation), offering a hypothesis for the mechanism of splenic toxicity.
• Formulation Stability: Confirms that the NONB formulation (stabilized with ascorbic acid and bicarbonate) maintains stability and efficacy over a 90-day period without causing acute lethality.

5. Significance and Conclusion

The study concludes that intravenous administration of Nitric Oxide Nanobubbles at doses up to 0.06 mL is safe for subchronic exposure in Sprague-Dawley rats. While minor, statistically significant deviations were observed in biochemical markers (glucose, lipids, electrolytes) and mild histological changes (fatty liver, splenic hemorrhage), all parameters remained within or near physiological limits, and no organ failure or mortality occurred.

Implications:

• The findings support the progression of NONB technology toward clinical trials in humans, provided that formulation adjustments are made to mitigate the mild hepatic and splenic stress observed at the highest dose.
• The persistence of effects in the satellite group highlights the need for further research into the long-term metabolic clearance of NO from nanobubbles and the potential for irreversible tissue changes.
• The study bridges the gap between theoretical NO therapy and practical nanomedicine safety, validating the potential of NONB as a viable therapeutic delivery system.