Treatment of Chronic Ulcers in Diabetic Patients with a Topical Metalloproteinase Inhibitor, Doxycycline

Gloria A. Chin, MD, MS; Tera G. Thigpin; Karen J. Perrin, BSN, ARNP; Lyle L. Moldawer, PhD; Gregory S. Schultz, PhD

Disclosures

Wounds. 2003;15(10) 

In This Article

Discussion

The concept of treating chronic lower-extremity ulcers in diabetic patients with topical inhibitors of MMPs and TACE is based on multiple reports of elevated levels of inflammatory cytokines and proteases in wound fluids and biopsies, including diabetic foot ulcers,[7] and on the ability of the proteases to degrade growth factors and matrix proteins that are essential for wound healing.[8] Therefore, it is logical to hypothesize that reducing elevated levels of TNF and MMPs in chronic wounds should promote healing.[10] Clinical evidence supporting this hypothesis was provided by Trengove and colleagues[6] who reported that levels of cytokines and proteases decreased as chronic venous ulcers began to heal, and by Ladwig and colleagues[29] who reported that optimal healing of chronic pressure ulcers correlated with low values of the ratio of MMP-9/TIMP-1. Fortunately, both TACE and MMPs are members of the metalloproteinase superfamily of proteinases due to the presence of a zinc ion in the active center of the enzymes. Thus, an optimal approach would be to use a single drug that can inhibit both TACE and MMP activities. As seen in Figures 1 and 2, doxycycline is able to significantly inhibit both TACE and MMP activities in a dose dependent manner in vitro. However, the data also reveal that doxycycline is not a very potent inhibitor of TACE or MMPs, since the IC50 values are in the range of mg/mL concentrations. Fortunately, doxycycline can be formulated in physiologically compatible vehicles at concentrations in the range of 10mg/mL (1% w/v), which is 100 to 1,000 times higher than the IC50 values determined by the in-vitro reactions.

Serine proteases, especially neutrophil elastase, have also been reported to be elevated in many chronic wounds, suggesting that optimal healing of chronic wounds may require the reduction of both serine proteases and metalloproteinases.[6] Although doxycycline does not directly inhibit neutrophil elastase, it may indirectly inhibit elastase activity by preventing the destruction alpha-1 protease inhibitor (A1-PI), the major natural inhibitor of elastase, by MMPs.[30] Thus, topical doxycycline may favorably alter the activities of three proteases in chronic wounds: TACE, MMPs, and neutrophil elastase.

Another approach to reducing protease activities in chronic wounds is the use of dressings that combine absorbent materials, such as oxidized regenerated cellulose (ORC) or alginate, which contain high concentrations of substrates for wound proteases, such as collagen fibers. As wound fluid is absorbed into the dressing, it is proposed that the proteases bind and degrade the collagen substrate molecules in the dressing, rather than diffusing back into the wound bed and breaking down the collagen molecules, growth factors, and receptors that are essential for healing.[31] Thus, these dressings act as a sink for proteases in wound fluids. Results from a randomized controlled trial of chronic diabetic plantar ulcers treated with Promogran® collagen/ORC dressing (Johnson & Johnson Wound Management, Somerville, New Jersey) suggested a trend for increased healing.[32]

A major objective of the pilot clinical study was to evaluate the safety of topical one-percent doxycycline hydrogel treatment of chronic lower-extremity ulcers in diabetic patients. Since MMPs play important roles in migration of epidermal cells and in neovascularization, it is possible that topical doxycycline might retard epithelial healing or impair formation of granulation tissue in chronic wounds.[33,34,35] Topical treatment of suction blisters in human volunteers with the potent MMP inhibitor, ilomastat, retarded epithelial healing about 30 percent,[36] and minocycline, a tetracycline family member, reduced angiogenesis in a rabbit corneal angiogenesis model.[37] However, all four chronic wounds treated with doxycycline formed granulation tissue and epithelialized within 30 weeks of treatment. Another concern of long-term topical treatment with one-percent doxycycline was the possibility of developing doxycycline-resistant bacteria. However, development of antibiotic resistance is favored by long-term use of an antibiotic at low levels that approach the minimal inhibitor concentration for 50-percent reduction of growth (MIC50). The MIC50 for doxycycline with susceptible bacteria is approximately 0.1mg/mL, and for intermediate sensitivity bacteria is 8mg/mL (doxycycline hyclate package insert, Mutual Pharmaceuticals, Philadelphia, Pennsylvania). The concentration of one-percent doxycycline is 10,000mg/mL, which is substantially higher than these MIC50 values, and no infections developed in the doxycycline-treated wounds.

A second major objective of the pilot clinical study was to evaluate the efficacy of topical one-percent doxycycline in a CMC hydrogel on healing of chronic lower-extremity ulcers in diabetic patients. The Chi-squared analysis indicated that topical one-percent doxycycline treatment healed the chronic diabetic ulcers better than the vehicle treatment. Comparison of healing percentages in the doxycycline arm (75%) and control arm (30%) after 20 weeks of treatment were in general agreement with healing rates reported for placebo (30%) and PDGF (50%) arms in the initial becaplermin (Regranex®, Johnson & Johnson Wound Management) study of 61 patients.[38]

The apparent beneficial effect of topical one-percent doxycycline treatment on healing of the chronic ulcers was probably not due to the antibiotic action of doxycycline, since bacterial levels at the time of enrollment of all the chronic wounds were below the level that is associated with impaired healing. The authors speculate that the beneficial effect on healing was due to reducing levels of TNF, MMPs, and elastase activities in the chronic wound environment, which improved the actions of endogenous growth factors. Obviously, a much larger randomized clinical trial must be conducted to conclusively establish the safety and efficacy of topical one-percent doxycycline on healing of chronic lower extremity ulcers in diabetic patients, but this pilot study suggests that topical one-percent doxycycline is sufficiently safe and efficacious to justify further investigations.

Comments

3090D553-9492-4563-8681-AD288FA52ACE

processing....