In previous studies, some tetracycline (TC) antibiotics showed potential as analgesic. We investigated here the analgesic activity of new non-antibiotic TC derivatives using the formalin-induced nociceptive pain model in adult C57BL/6 mice. Specifically, we tested the effects of i.p. injections of DDMC (5, 10, 20 mg kg⁻¹) and DDOX (10, 20, 40 mg kg⁻¹), which are non-antibiotic derivatives of demeclocycline and doxycycline, respectively. Repeated treatments with DDMC remarkably reduced nociceptive pain in both phases of the test, at 10 mg kg⁻¹ its efficacy was comparable to that of 10 mg kg⁻¹ of morphine. DDOX was also effective in this paradigm but intrinsically less potent than DDMC, exerting analgesic effects between 20 and 40 mg kg⁻¹. Interestingly, a single injection of DDMC (10 mg kg⁻¹) was sufficient to produce a robust anti-nociceptive effect similar to that of morphine. A single injection of DDOX (40 mg kg⁻¹) also produced anti-nociceptive effects but only in the second phase of the test. Noticeably, male mice exhibited a better analgesic response to DDMC (10 mg kg⁻¹) than females. A single injection of DDMC (10 mg kg⁻¹) and morphine but not of DDOX (40 mg kg⁻¹), powerfully inhibited formalin-induced spinal cord c-Fos expression whereas both TC derivatives restrained the activation of Iba-1-immunoreactive cells, indicating a potential indirect effect on inflamed microglial cells. In summary, the non-antibiotic TCs, DDMC and DDOX, demonstrated notable analgesic efficacy against formalin-induced pain, suggesting their potential as alternatives for analgesic treatment.

Keywords: Antibiotics, doxycycline, morphine, pain