Anti‐Inflammatory Effects of a Polyphenol‐Rich Spearmint Extract in a Rat Paw Edema Model

This study assessed the anti‐inflammatory effects of a polyphenol‐rich spearmint extract containing rosmarinic acid using a rat paw edema model. Four groups of male Wistar rats received intraperitoneal administration of either a proprietary spearmint extract containing 15% rosmarinic acid at doses of 10, 30 or 100 mg/kg (n=18), rosmarinic acid standard at doses of 15 or 50 mg/kg (n=10), Indomethacin, an anti‐inflammatory agent, at 10 mg/kg (n=6), or saline alone as a vehicle control (n=10). Fifteen minutes later, subplantar injections of carrageenan (100 μl, 1%) were administered to induce paw edema in all animals with the exception of four animals in the saline control group who received a saline subplantar injection (saline‐saline group). Paw volumes were assessed by the volume displacement method immediately after carrageenan injection (baseline) and 3 and 6 hours later. Paw edema was expressed as mean percent change of paw volume at 3 and 6 hours compared to baseline. As expected for the rats in the saline‐carrageenan group, paw volume increased by 36% and 46% at 3 and 6 hours post‐injection, respectively, and was significantly higher than the saline‐saline group at the same time points (p=0.014 and p=0.014 respectively). On the other hand, rats treated with spearmint extract demonstrated limited inflammation with paw volume increases of 9%, 11% and ‐8% at 6 hours post‐injection for the 10, 30 and 100 mg/kg spearmint groups respectively, which was significantly lower than the saline‐carrageenan group (p<0.05 for all at 6 hours). In addition, the increase in paw volume in rats treated with 100 mg/kg spearmint extract at 3 hours was also significantly lower than that in the saline‐carrageenan group at 3 hours (p=0.008). Similarly, rats treated with the anti‐inflammatory agent had limited inflammation compared to the saline‐carrageenan group at 3 and 6 hours post‐injection (p=0.031 and p=0.005, respectively), with increases in paw volumes of 6% and 3% from baseline. Animals treated with 15 and 50 mg/kg rosmarinic acid showed increased paw volumes of 20% and 7% at 3 hours, and 10% and 5% at 6 hours compared to baseline, respectively. These volumes were also significantly lower than the saline‐carrageenan group at 3 hours (p=0.043, 50 mg/kg) and 6 hours (p<0.05, 15 and 50 mg/kg). When comparing the attenuation in inflammation following spearmint extract treatment to the anti‐inflammatory agent, the spearmint extract group performed equally well at attenuating local inflammation. Furthermore, when comparing the 15 mg/kg rosmarinic acid group with the spearmint extract group that contained an equivalent amount of rosmarinic acid (100 mg/kg group), the animals receiving the spearmint extract injections exhibited significantly less inflammation than the rosmarinic acid standard group at both the 3 and 6 hour time points (p=0.013 and p=0.045, respectively) indicating that additional actives in the spearmint extract have an effect on the inflammation response. In conclusion, these data show that this polyphenol‐rich spearmint extract has similar anti‐inflammatory properties to indomethacin and superior effects to rosmarinic acid alone, indicating a potential synergy between the constituents present in the spearmint extract. Support or Funding Information Supported by Kemin Industries, Inc.