Glycyrrhizic acid enhances disposition kinetics and therapeutic efficacy of oxytetracycline against experimental Chlamydia psittaci infection in pigeons.

Chlamydia psittaci (C. psittaci) is a major intracellular pathogen of pigeons that often requires prolonged antimicrobial therapy; however, high-dose oxytetracycline (OTC) may increase toxicity. Glycyrrhizic acid (GA), a bioactive licorice constituent, has anti-inflammatory, antioxidant, and antimicrobial-enhancing properties. This study evaluated whether GA improves the pharmacokinetics, therapeutic efficacy, and safety of OTC against experimental C. psittaci infection in pigeons. C. psittaci was isolated and confirmed via embryonated egg inoculation and real-time PCR. Eighty-four pigeons were allocated to seven groups: negative control (G1), infected control (G2), oral OTC (G3), intravenous OTC (G4), oral GA (G5), oral OTC plus GA (G6), and intravenous OTC plus GA (G7). Pharmacokinetics were assessed using a validated green HPLC-UV method, while antimicrobial efficacy, inflammatory and oxidative stress markers, and hepatic and renal biomarkers were evaluated. GA showed borderline synergism with OTC (fractional inhibitory concentration index, 0.499) against the tested isolate. The HPLC method showed excellent analytical performance, with an AGREE greenness score of 0.82. Compared with OTC alone, GA significantly (P < 0.05) increased Cmax and AUC₀-₂₄, improved absolute oral bioavailability from 29% to 38%, increased relative bioavailability 1.5-fold, and reduced apparent oral and systemic clearance. Combination therapy markedly reduced chlamydial burden and inclusion body formation, attenuated IL-6, TNF-α, and malondialdehyde levels, and restored glutathione and hepatic and renal biomarkers compared with monotherapies. These findings support GA as a promising adjunct to OTC therapy for avian chlamydiosis, enhancing OTC exposure and therapeutic efficacy while reducing infection-associated tissue injury, potentially mitigating adverse effects associated with prolonged OTC therapy.