Fluoroquinolones are an effective, broad-spectrum antibiotic used to treat an array of bacterial infections. However, they are associated with an increased risk of tendinopathy and tendon rupture even after discontinuation of treatment. This condition is known as fluoroquinolone-associated tendinopathy, the underlying mechanisms of which are poorly understood. While many factors may be involved in the pathophysiology of tendinopathies in general, changes in tenocyte metabolism and viability, as well as alteration of proteoglycan metabolism are prominent findings in the scientific literature. This study investigated the effects of ciprofloxacin, a common fluoroquinolone, on cell viability, proteoglycan synthesis, and proteoglycan mRNA expression in equine superficial digital flexor tendon explants after 96 h treatment with between 1–300 µg/mL ciprofloxacin, and again after 8 days discontinuation of treatment. Ciprofloxacin caused significant reductions in cell viability by between 25–33% at all dosages except 10 µg/mL, and viability decreased further after 8 days discontinuation of treatment. Proteoglycan synthesis significantly decreased by approximately 50% in explants treated with 100 µg/mL and 300 µg/mL, however this effect reversed after 8 days in the absence of treatment. No significant mRNA expression changes were observed after the treatment period with the exception of versican which was down-regulated at the highest concentration of ciprofloxacin. After the recovery period, aggrecan, biglycan and versican genes were all significantly downregulated in explants initially treated with 1–100 µg/mL. Results from this study corroborate previously reported findings of reduced cell viability and proteoglycan synthesis in a whole tissue explant model and provide further insight into the mechanisms underlying fluoroquinolone-associated tendinopathy and rupture. This study further demonstrates that certain ciprofloxacin induced cellular changes are not rapidly reversed upon cessation of treatment which is a novel finding in the literature.

Fluoroquinolone antibiotics are associated with increased risk of tendinopathy and tendon rupture, which can occur well after cessation of treatment. We have previously reported that the fluoroquinolone ciprofloxacin (CPX) reduced proteoglycan synthesis in equine tendon explants. This study aimed to determine the effects of CPX on proteoglycan catabolism and whether any observed effects are reversible. Equine superficial digital flexor tendon explant cultures were treated for 4 days with 1, 10, 100 or 300 µg/mL CPX followed by 8 days without CPX. The loss of [35S]-labelled proteoglycans and chemical pool of aggrecan and versican was studied as well as the gene expression levels of matrix-degrading enzymes responsible for proteoglycan catabolism. CPX suppressed [35S]-labelled proteoglycan and total aggrecan loss from the explants, although not in a dose-dependent manner, which coincided with downregulation of mRNA expression of MMP-9, -13, ADAMTS-4, -5. The suppressed loss of proteoglycans was reversed upon removal of the fluoroquinolone with concurrent recovery of MMP and ADAMTS mRNA expression, and downregulated TIMP-2 and upregulated TIMP-1 expression. No changes in MMP-3 expression by CPX was observed at any stage. These findings suggest that CPX suppresses proteoglycan catabolism in tendon, and this is partially attributable to downregulation of matrix-degrading enzymes.

Introduction Ciprofloxacin, a commonly prescribed fluoroquinolone antibiotic, has been shown to have a deleterious effect on the tendon extracellular matrix. The aims of this study were to determine the effects of short term exposure of ciprofloxacin on 1) tenocyte metabolism, 2) glycosaminoglycan content, and 3) mRNA expression levels of extracellular proteoglycans, and to correlate these changes with the production of newly synthesised proteoglycans in explant cultures of equine tendon. Methods Normal superficial digital flexor tendon from 6 yr old Thoroughbred horses were incubated for 4 days in DMEM alone (control), or DMEM containing up to 300 µg/ml ciprofloxacin (CPX). The effect of CPX on the metabolism of tenocytes was determined by lactate production and Alamar blue assay and glycosaminoglycan content was determined by a dimethylene blue assay. Levels of gene expression of aggrecan, versican, decorin, biglycan and fibromodulin were determined by quantitative PCR. The production of newly synthesised proteoglycans was determined by 35S-sulfate incorporation at the end of the treatment period. Results Higher levels of CPX (≥100 µg/ml) reduced tenocyte metabolism by 30–40% as measured by lactate production and Alamar blue assay. There was no change in glycosaminoglycan content nor mRNA expression levels of aggrecan, versican, decorin, biglycan or fibromodiulin in CPX-treated tendon explants compared with control. However, CPX suppressed the synthesis of both 35S-labelled large and small proteoglycans in a dose dependent manner. Discussion The suppression of newly synthesised proteoglycans in tendon explants treated with CPX suggests that these changes are driven by changes in metabolism rather than expression of its associated genes.