Objective- The purpose of the present study was to investigate the viability of equine fibroblast-like synoviocytes (FLSs) treated with doxycycline. Design- Experimental study. Animals - FLSs from metacarpophalangeal joints of six skeletally mature horses. Methods- FLSs were established from synovial fluids of healthy joints. The cells were treated with varoius concentrations (1, 5, 10, 50, 100, 150, 300, 400 μg/ml) or without doxycycline for 48-hour. Viability of FLSs was determined using MTT assay and the trypan blue dye exclusion method. Results- No significant differences were observed between viability of FLSs cultures treated with doxycycline until 150 μg/ml and control group (P>0.05). Doxycycline at 300 and 400 μg/ml significantly decreased FLSs viability (P<0.05). FLSs viability was 74.28% and 59.07% in 300 and 400 μg/ml, respectively, when measured by the MTT assay. Also FLSs viability at 300 and 400 μg/ml of doxycycline were 68.10% and 43.26%, respectively, with trypan blue exclusion method. Conclusion and Clinical Relevance- These findings demonstrated that doxycycline was not toxic for equine FLSs at concentration ≤150 μg/ml in vitro. Further studies are needed to investigate the safety, efficacy and detrimental effects of doxycycline in equine joint.

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