Fluoroquinolones act primarily via inhibition of bacterial DNA gyrase, as well as topoisomerase IV. Unfortunately, levofloxacin, moxifloxacin & co. have a direct effect on the metabolism of fibroblasts. Tenocytes3,4, fibroblasts specific to tendons, produce collagen and elastin, with collagen responsible ce collagen and elastin, with collagen responsible
for stability and elastin for elasticity of tendons.5,1

Animal studies demonstrated that fibroblasts of the Achilles tendon, paratenon, and shoulder capsule increased matrix degradation, decreased synthesis of new matrix, and reduced cell proliferation after exposure to ciprofloxacin at physiological concentrations.5 

Matrix metalloproteinases (MMPs) are enzymes with the function of remodeling tendons.5 This remodeling is crucial for the adaptation of the tendon to activity and controls the assembly and disassembly from tissue. There is differential expression of MMPs in healthy compared to degenerative tendons in humans. Therefore, one theory is that MMPs influence the development of tendinopathies.5

MMP-3 expression and activity is increased in tendons that must withstand high mechanical loads, such as the Achilles or supraspinatus tendon, because increased remodeling is likely required due to repetitive microtrauma.5

Studies have shown that after exposure to ciprofloxacin, the expression of MMP-3 as well as MMP-1 increases in human fibroblasts from tendon cells.5 If MMPs are upregulated, this leads to thinner and reduced collagen fibrils.

This may account for the reduced amount of type I collagen, elastin, fibronectin, and beta-1 integrin in tendons after exposure to ciprofloxacin.5

Because of these changes, tendon rupture may occur acutely after treatment with fluoroquinolones, especially in tendons with reduced regenerative capacity and tendons that were already structurally compromised.5

Interestingly, attempts to inhibit matrix metalloproteinases with (natural) substances as a therapy for fluoroquinolone-induced tendon pain have shown little to no success.

The role of oxidative stress in the development of tendon disorders

Oxidative stress is significantly increased after exposure to a fluoroquinolone antibiotic. There is a causal relationship between oxidative stress and the induction of MMP-2, therefore oxidative stress is now thought to be partly responsible for tendon toxicity. This hypothesis is also supported by studies showing that the tendon toxicity of fluoroquinolone antibiotics can be reduced by the administration of antioxidants such as vitamin E or coenzyme Q10.5 

Thus, oxidative damage to mitochondria in tendon cells after exposure to fluoroquinolones has already been reported.5 In my view, this is a clear indication that secondary mitochondriopathy is also of significance in tendon diseases caused by fluoroquinolones.

Thus, general therapy with antioxidants seems to be useful in fluoroquinolone-induced tendon pain, at least as prophylaxis against further damage.

Chelation of divalent cations and its consequences

In addition to their effect on fibroblasts, fluoroquinolones are known to chelate divalent and trivalent cations such as calcium, magnesium, iron, or zinc.5 Magnesium and iron, along with vitamin C, are important co-factors in the hydroxylation of proline to hydroxyproline6 and are thus critical for collagen synthesis.

In addition, magnesium and other cations are known to have a direct effect on the regulation of integrins. Integrins are transmembrane proteins that are involved in the structural stability of cells, such as attachment of cells to the extracellular matrix or to other cells. Integrins are now thought to be essential for maintaining the integrity of musculoskeletal tissues.5

Interestingly, similar biochemical changes have been shown in animals on an artificial diet deficient in magnesium for 6 weeks compared with animals after treatment with a fluoroquinolone antibiotic.5 This strengthens the hypothesis that the toxicity of ciprofloxacin and co. is related, among other things, to the chelation of magnesium.5

Therefore, the daily high-dose administration of magnesium is recommended in the therapy of fluoroquinolone-induced tendon pain. A substitution of vitamin C also seems to be useful, especially in view of the fact that most of the patients show a low vitamin C level in laboratory tests.

The effect of fluoroquinolone antibiotics on the hydroxylation of proline to hydroxyproline in collagen synthesis

As described, fluoroquinolone antibiotics inhibit prolyl-4-hydroxylase, the enzyme that hydroxylates the amino acid proline to hydroxyproline.7 The hydroxylation of proline takes place in the rough endoplasmic reticulum and requires, among other things, ascorbic acid (vitamin C).8 As mentioned above, other cofactors are cations such as magnesium and iron as well as alpha-ketoglutarate.6 Alpha -ketoglutarate is a key product of the Krebs cycle.9

There was previously the idea of substituting the missing hydroxyproline with, for example, collagen powder. From our experience, it cannot be clearly shown so far that regular substitution has any effect on tendon pain.

Much more often it helps the affected person to substitute a high dose of magnesium. Magnesium has an important influence on the nervous system, as it has a role in the activation/inactivation of NMDA glutamate receptors in the CNS.

Above I have documented with studies that pain and damage to the tendon often do not correlate and it is believed that a local interaction between tendon tissue and peripheral as well as centra nervous system is the potential cause of the severe pain of the affected person. The observation that magnesium or drugs for nerve pain more often lead to an alleviation of tendon pain than, for example, the substitution of collagen or vitamin C supports this theory.

Source:

• Pathogenesis of tendinopathies: inflammation or degeneration? Michele Abate et al.
• Biology of tendon injury: healing, modeling and remodeling P. Sharma and N. Maffuli; 2006
• Mitochondrial Dysfunction and potential mitochondrial protectant treatments in Tendinopathy, Xueying Zhan et al. 2021 DOI: 10.1111/nyas.14599
• https://www.amboss.com/de/wissen/Tendinopathie
• Musculoskeletal Complications of Fluoroquinolones: Guidelines and Precautions for usage in the Athletic Population; Mederic M Hall, Jonathan T Finnoff, Jay Smith 
• Mitochondrien, Symptome, Diagnose und Therapie; Bodo Kuklinski 
• Nonantibiotic Effects of Fluoroquinolones in Mammalian Cells; Sujan Badal, Yeng F. Her, L. James Maher 
• Histologie das Lehrbuch, Ulrich Welsch, Wolfgang Kummer, Thomas Deller Elsevier 5. Auflage 2018
• Alpha-Ketoglutarate: Physiological Functions and Applications; Nan Wu et al.