Lipophilicity is a critical parameter that dominates the efficacy of metalloporphyrins in blocking the development of morphine antinociceptive tolerance through peroxynitrite-mediated pathways

Abstract

Severe pain syndromes reduce the quality of life of patients with inflammatory and neoplastic diseases, partly because reduced analgesic effectiveness with chronic opiate therapy (i.e., tolerance) leads to escalating doses and distressing side effects. Peroxynitrite-mediated nitroxidative stress in the dorsal horn of the spinal cord plays a critical role in the induction and development of antinociceptive tolerance to morphine. This provides a valid pharmacological basis for developing peroxynitrite scavengers as potent adjuncts to opiates in the management of pain. The cationic Mn(III) ortho-N-alkylpyridylporphyrins MnTE-2-PyP(5+) and MnTnHex-2-PyP(5+) are among the most potent peroxynitrite scavengers, with nearly identical scavenging rate constants (approximately 10(7) M(-1) s(-1)). Yet, MnTnHex-2-PyP(5+) is significantly more lipophilic and more bioavailable and, in turn, was 30-fold more effective in blocking the development of morphine antinociceptive tolerance than MnTE-2-PyP(5+) using the hot-plate test in a well-characterized murine model. The hydrophilic MnTE-2-PyP(5+) and the lipophilic MnTnHex-2-PyP(5+) were 10- and 300-fold, respectively, more effective in inhibiting morphine tolerance than the hydrophilic Fe(III) porphyrin FeTM-4-PyP(5+). Both Mn porphyrins decreased levels of TNF-alpha, IL-1 beta, and IL-6 to normal values. Neither of them affected acute morphine antinociceptive effects nor caused motor function impairment. Also neither was able to reverse already established morphine tolerance. We have recently shown that the anionic porphyrin Mn(III) tetrakis(4-carboxylatophenyl)porphyrin is selective in removing ONOO(-) over O(2)(-), but at approximately 2 orders of magnitude lower efficacy than MnTE-2-PyP(5+) and MnTnHex-2-PyP(5+), which in turn parallels up to 100-fold lower ability to reverse morphine tolerance. These data (1) support the role of peroxynitrite rather than superoxide as a major mechanism in blocking the development of morphine tolerance and (2) show that lipophilicity is a critical parameter in enhancing the potency of such novel peroxynitrite scavengers.

Figure 1

Structures of the complexes relevant to this study: MnTE-2-PyP⁵⁺, MnTnHex-2-PyP⁵⁺, MnTBAP and FeTM-4-PyP⁵⁺.

Figure 2

Inhibition of morphine antinociceptive tolerance with MnTE-2-PyP⁵⁺ or MnTnHex-2-PyP⁵⁺. On day five, acute injection of morphine (3 mg/kg) in animals that received saline over four days produced a significant antinociceptive response when compared to responses observed in animals that received an equivalent volume of saline (A,B, Vehicle, V vs Naïve, N). On the other hand, a significant loss to the antinociceptive effect of the acute injection of morphine was observed in animals that received repeated administration of morphine over four days (Morphine group) (A,B). Co-administration of morphine over four days with MnTE-2-PyP⁵⁺ (0.3–3 mg/kg/day, n=6; A) or MnTnHex-2-PyP⁵⁺ (0.01–0.1 mg/kg/day, n=6; B) inhibited the development of tolerance in a dose-dependent manner. Results are expressed as mean ± s.e.m for six animals. °P<0.001 for Vehicle, V vs Naïve, N; *P<0.001 for Morphine alone vs Vehicle; †P<0.001 for Morphine + drug treated vs Morphine alone.

Figure 3

On day five, acute injection of different doses of morphine (0.3–3 mg/kg) in animals that received saline over four days produced a dose-dependent significant antinociceptive response when compared to responses obtained in animals receiving an equivalent volume of its vehicle. The antinociceptive response to morphine was not altered in animals that were treated over four days with MnTE-2-PyP⁵⁺ or MnTnHex-2-PyP⁵⁺ (3 and 0.1 mg/kg/day respectively, n=6) indicating lack of acute interaction between morphine and these compounds. Results are expressed as mean ± s.e.m for six animals. *P<0.001 for the vehicle group when compared to values obtained in the absence of morphine.

Figure 4

Acute injection of morphine (3 mg/kg) on day five in animals that received saline over four days (Vehicle group) did not increase dorsal horn tissue levels of TNF-α (A), IL-1β (B) and IL-6 (C) when compared to animals that received an equivalent volume of its vehicle (naïve group). On the other hand, acute administration of morphine in animals that received repeated administration of morphine (Morphine group, Mor) led to a significant increase in TNF-α, IL-1β and IL-6 in dorsal horn tissues (A–C). This increase was attenuated by co-administration of morphine over four days with MnTE-2-PyP⁵⁺ or MnTnHex-2-PyP⁵⁺ (3 and 0.1 mg/kg/day respectively, n=6) (A–C). Results are expressed as mean ± s.e.m for 6 animals. *P<0.001 for Morphine alone vs Vehicle; †P<0.001 for Morphine + drug-treated vs Morphine alone.