A randomized, controlled study to investigate the efficacy and safety of a topical gentamicin-collagen sponge in combination with systemic antibiotic therapy in diabetic patients with a moderate or severe foot ulcer infection

DFUIs are responsible for frequent healthcare visits, a large number of diabetes-related hospital bed-days and a potential predictor of amputation [1]. A recent study [2] of 5.6 billion ambulatory care visits between 2007 and 2013 found that 785 million involved diabetes and 6.7 million were for diabetic foot ulcers (0.3%) or diabetic foot infections (0.5%). Multivariable analyses indicated DFUIs, compared to other diagnoses, were associated with a 6.7 times higher odds of direct emergency referral or hospitalization and 1.5 times more visits in the previous 12 months [2].

Successful treatment of DFUIs requires multidisciplinary management, including attention to wound care, revascularization, off-loading and metabolic abnormalities, as well as antimicrobial therapy and often surgery [3]. These measures are evidence-based or acknowledged by most experts [3]. However, because the infection may still not respond to treatment in a substantial minority of cases, investigators have explored various auxiliary techniques. These may be aimed at enhancing infection cure and/or ulcer healing, and include such methods as patient education and hyperbaric oxygen therapy [3, 4]. Most of these auxiliary measures are not evidence-based sensu strictu, but their use is widespread throughout the world [3, 4].

One of these auxiliary approaches is to combine treatment with systemic (intravenous or oral) antibiotic therapy with adjunctive or local antiseptic or antibiotic agents [5]. A recent Cochrane systematic review concluded that randomized controlled trial data on the effectiveness and safety of topical antimicrobial treatments for diabetic foot ulcers is limited by the availability of relatively few, mostly small, and often poorly designed trials [6]. The authors stated “given the high, and increasing, frequency of diabetic foot wounds, we encourage investigators to undertake properly designed randomized controlled trials in this area.”

One product that has been studied for treatment of DFUIs is the combination of a collagen sponge with gentamicin. We previously had performed and published an open-label, multicenter study of 56 patients combining daily application of the gentamicin-collagen-sponge for up to 28 days with systemic antibiotic therapy versus to systemic antibiotic therapy alone in the treatment of moderate DFUI [7]. Although the study failed to meet its primary endpoint (clinical cure at day 7), the gentamicin-sponge showed superior efficacy in terms of eradication of baseline pathogens and, most importantly, a significant higher cure rate at the final visit [7]. Thus, we thought it would be useful to reexamine the potential benefits of the gentamicin-collagen sponge in a similar, but larger single center study, assessing safety and clinical cure rates after more than 1 month instead of 1 week. Our hope was that this pilot study would allow assessing the feasibility of a much larger future (second) multicenter trial.

The Geneva University Hospital system is the only public hospital in Geneva and the tertiary center for parts of neighboring France. The institution has been using a clinical pathway [1] for DFUI since 2013, which includes input and consultation from diabetologists, podiatrists, angiologists, infectious diseases physicians, orthopedic and foot surgeons [4], radiologists [8], vascular surgeons, and specialized physiotherapists and surgical nurses (AT) for ortheses and patients’ education.

Definitions and study criteria

The diagnosis criteria for diabetes mellitus for this study stem from the American Diabetes Association and basically included a glycolysed hemoglobulin A1 level of ≥6.5%, a fasting glycemia level of 7.0 mmol/L, and/or the presence of an anti-diabetic drug as current treatment for diabetes mellitus [9]. We included patients presenting to or patients in our hospital who were found to have a moderate or severe DFUI, based on the criteria developed for IDSA foot infection guidelines [10]. Briefly, a diabetic foot infection is defined as having ≥2 manifestations of inflammation (local swelling or induration, erythema, tenderness or pain, warmth, or purulent discharge (thick, opaque to white or sanguineous secretion). Moderate infection was defined as local DFUI with surrounding erythema > 2 cm, or involving structures deeper than skin and subcutaneous tissues (e.g., abscess, osteomyelitis, septic arthritis, fasciitis) but no evidence of systemic inflammatory response syndrome. Severe infection was defined as local DFUI accompanied by ≥2 systemic signs of inflammatory response syndrome. Cure was defined as absence of microbiological, clinical, and imaging evidence of the original infection. Improvement was defined as a significantly better wound score and a decrease in the number of manifestations of inflammation, however without complete resolution. Regarding the evolution of the wound, we used a custom-made DFUI assessment to describe the evolution of the wound, summing up various elements of a wound in a single wound score (Additional file 1). The elements used for that score were all quantified, and consisted of the wound size, wound depth, the degree of undermining of wound edges, the presence of discharge and pus, erythema, induration, tenderness, pain and local warmth (Additional file 1).

A patient was eligible for study participation if he/she met the following criteria: is aged ≥18; has been diagnosed with diabetes mellitus; has an open foot wound of ≥1 cm² located on or below the malleolus that has findings of infection; has undergone any appropriate surgical intervention needed to remove necrotic and to drain an abscess (Fig. 1); and, if female, is nonpregnant and nonlactating. Of note, surgical debridement was not a must. Patients could participate in the study without prior surgical debridement if there was no clinical need to remove necrosis or to drain an abscess. The exclusion criteria were: a known history of hypersensitivity to gentamicin or bovine collagen [11]; a DFUI associated with prosthetic material or any implanted device; peripheral arterial insufficiency clinically requiring urgent limb revascularization after inclusion; and, having received > 48 h of potentially effective antibiotic therapy and showing improvement in wound infection. Of note, a chronic arterial insufficiency with a clinically preserved foot in absence of vascular necroses was not an exclusion criterion. If a patient had received an antibiotic within 72 h, but deep-tissue culture results indicated that the infecting pathogen was not susceptible to that antibiotic, the patient could be enrolled. Further exclusion criteria were: mild DFUI (as defined by the IDSA criteria [10]); severe immune-suppression; extensive necrosis requiring amputation (Fig. 2); residual osteomyelitis (after any debridement) requiring more than 28 days of antibiotic treatment [1]; a history of myasthenia gravis or other neurological conditions precluding gentamicin use; a history of epilepsy; and, active or recent alcohol or substance abuse.

Gentamicin-collagen sponges

Innocoll Pharmaceuticals Ltd. provided us with the bioresorbable gentamicin-collagen sponges. These are 5 × 5 cm in size and contain type 1 bovine collagen and 50 mg of gentamicin sulfate (equivalent to 32.5 mg of gentamicin base), and are commercially available in Switzerland under the trade name GARAMYCIN® Sponge (Fig. 3; unpacked sponge). We made an educational videotape to demonstrate the correct application of the sponge (Additional file 2: Video S1).

Our randomized, controlled trial was designed to determine if, for patients with a DFUI, the addition of a topical gentamicin-collagen sponge to systemic antibiotic therapy plus local wound care and surgery as needed, was beneficial. Our results showed no significant differences, between the group that received the sponge and the group that did not, in the rate of clinical cure, clinical improvement of DFUI or pathogen eradication. Thus, the gentamicin-collagen sponge arm did not prove to be superior to the control arm (no gentamicin-collagen sponge). In our previous study, the gentamicin-collagen sponge group had a higher proportion with total cure and a higher rate of eradication of baseline pathogens [7].

Interestingly, in our patients there was an apparent tendency to a more rapid or more pronounced ulcer healing in the gentamicin-collagen sponge group during the second half of the study (weeks three to five), compared to the control arm. Even if this difference was not statistically significant, a similar finding was noted in two previous studies of the gentamicin-collagen sponge in patients with diabetic foot wounds [7, 17]. Both randomized trials observed a shortened wound healing time in favor of the gentamicin-collagen sponge. In the study by Varga et al. with 50 patients who had undergone a minor foot amputation, the median time difference was 2 weeks [17]. As reported in previous studies [7, 17], the gentamicin-collagen sponge was well tolerated with no adverse effects of concern. Although our results did not show a significant benefit for the gentamicin-collagen sponge as adjunctive therapy for treating DFUIs, we have demonstrated the feasibility of a potential larger multicenter trial.

We terminated this pilot study earlier than scheduled (after enrolling 88 patients instead of the planned 144), because it was more difficult than we expected to find eligible cases based on our enrollment criteria. Among all DFUI patients we screened, despite the fact that only 3% (11/375) declined to participate, only 23% were eligible for enrollment due to exclusion criteria. Using our power calculation that was based on a success of 90% in the sponge arm and a power of 80%, our final sample size would have detected a 17% difference in a sponge superiority study, and 9% difference in a control non-inferiority study (with the difference set at 10%).

There have been many published reports of topical antimicrobial treatments for infected ulcers, including of the diabetic foot. Beyond anecdotal reports [18], available studies have not found any significant benefit of local antiseptic or antibiotic agents in healing the ulcers per se or in preventing infection [19]. These topical agents might have a role in curing superficial infections [7], but they do not appear to add benefit to a concomitant systemic antibiotic therapy in terms of overall cure [20, 21]. A recent Cochrane systematic review and meta-analysis of topical antimicrobial therapy for diabetic foot ulcers concluded that randomized controlled trial data on the effectiveness and safety of topical antimicrobial treatments for diabetic foot ulcers was limited to a relatively few, mostly small, and often poorly designed trials [6]. Indeed, the authors noted that in view of “the high, and increasing, frequency of diabetic foot wounds, we encourage investigators to undertake properly designed randomized controlled trials … to evaluate the effects of topical antimicrobial treatments for … the treatment of infection in these wounds and ultimately the effects on wound healing.”

The antimicrobial effect of the gentamicin-collagen sponge relies on gentamicin, which is delivered by the topical, rather than the more usual systemic, route. Gentamicin and other aminoglycosides can be released from bovine collagen like with the sponge used in our study [11], they can be delivered by the polymer chitosan [22, 23], or by other means such as calcium sulfate pellets [24]. Theoretically, advantages in wound healing times could be associated with the collagen composition of the dressing instead of the gentamicin compound, for which we found no evidence in the literature. Since both compounds are usually mixed together in one sponge, formally the different author groups cannot sharply distinguish between the individual benefits of the compounds. However and logically, the anti-microbiologically active compound is the gentamicin and not the bovine collagen.

Topical anti-infective agents have many potential advantages [5] and can employ innovative agents and approaches to therapy. For example, a novel investigational agent, 1% pexiganan acetate cream, has been studied in mild DFUIs. Pexiganan is a peptide disrupting the integrity of bacterial cell membranes. Two prospective, blinded, randomized, controlled, multicentre trials compared topical pexiganan with oral ofloxacin in a total of 835 (for the two studies combined) DFUIs. The clinical response rate at end of 7–42 days of therapy was 85–90% for both groups; the microbiological response and wound healing rates were equivalent [25]. More recently, however, preliminary results from two large, multicenter, randomized controlled trials comparing topical pexiganan gel to a placebo (the gel vehicle) for mild DFUIs found no significant difference in clinical or microbiological outcomes [26]. Other studies of topical therapy for DFUI have employed a variety of antibiotics (e.g., mupirocin, bacitracin, neomycin, chloramphenicol, polymyxin B, and gentamicin) as well as antiseptics [3].

In this second pilot study performed by our group, the gentamicin-sponge did not significantly influence overall clinical cure or the eradication of wound pathogens. The gentamicin-sponge was very well tolerated without attributable adverse events. If necessary, our pilot study confirms the feasibility of a much larger second multicenter clinical trial.