Anti-nociceptive effects of non-antibiotic derivatives of demeclocycline and doxycycline against formalin-induced pain stimulation

Previous tetracycline (TC) studies hinted at its analgesic potential. We explored here it in new non-antibiotic TC derivatives. Using the formalin-induced nociceptive pain model in male adult C57⁄BL6 mice, we tested the effects of DDMC (5, 10, 20 mg.kg-1) and DDOX (10, 20, 40 mg.kg-1), non-antibiotic derivatives of demeclocycline and doxycycline, respectively. A sub-chronic treatment with 5 mg.kg-1 of DDMC remarkably reduced nociceptive pain in both phases of the test, comparable to morphine (10 mg.kg-1). DDOX was also effective but intrinsically less potent than DDMC as exerting analgesic effects between 20 and 40 mg.kg-1. Intriguingly, a single injection of DDMC (10 mg.kg-1) produced robust antinociceptive effects similar to morphine. A single injection of DDOX (40 mg.kg-1) also exerted anti-nociceptive effects in both phases. Male mice exhibited a better analgesic response to DDMC compared to females. Most interestingly, DDMC (10 mg.kg-1) and morphine treatments, but not DDOX, powerfully inhibited formalin-induced spinal cord c-Fos expression. Furthermore, both TC derivatives restrained the activation profile of Iba-1-immunoreactive cells, indicating a potential indirect effect on inflamed microglial cells. Summarizing, the modified tetracyclines, DDMC and DDOX, exhibit notable analgesic efficacy against formalin-induced pain, independent of their antimicrobial action, suggesting their potential as alternatives for analgesic treatment.