Micropore Particle Technology Promotes Wound Healing, Whereas Polyhexamethylene Biguanide Causes Tissue Degeneration: A Case Report

Case Report. A 72-year-old woman with a nontraumatic spinal cord injury developed eschar on her lower right back. An underlying abscess was identified, which upon surgical debridement left a large wound extending down to the hip bone. In addition, the hip suffered from chronic osteomyelitis and was exposed at the bottom of the wound. The wound was initially treated for 5 weeks with Manuka honey but deteriorated further. Next, micropore particle technology (MPPT) was used. It cleared the wound of necrotic tissue based on autolytic debridement and removed the soft tissue infection; over a 3-month period, the wound reduced 50% in volume. Treatment approach was changed to polyhexamethylene biguanide (PHMB) and was applied as a gel once every second day to the wound. After 6 days, it was observed to cause tissue degeneration, disruption of the structure of the exposed bone, and the appearance of froth coming through the hip bone. A pain syndrome developed and the use of PHMB was terminated on day 10. After a wash-out period, the use of MPPT was reinitiated. Over the following 8 months, MPPT continued to control the infectious debris coming from the hip bone and promote healing without affecting the bone or causing side effects. Conclusions. It is generally assumed that the cytotoxic properties of antiseptics seen in cell culture experiments do not occur on wounds. The present case shows these cytotoxic properties are expressed on wounds, and they do disrupt tissues and tissue regeneration. 

Polyhexamethylene biguanide (PHMB), also known as polyhexanide or polihexanide, is an antiseptic frequently used in wound care, usually in concentrations of 0.1% to 0.5%.¹ While the mode of action has not been fully elucidated, it is suggested that it involves membrane disruption and DNA binding.² The spectrum of biocidal activity includes bacteria and fungi, as well as mammalian cells.²

A recurring question in wound care is why antiseptics, despite their cytotoxicity, do not appear to interfere with tissue regeneration in wounds. Using an in vivo porcine wound model, the investigators^3,4 found PHMB in a noninfected healing wound did inhibit the formation of granulation tissue. However, these studies were conducted in an animal model, and it is possible that different effects could be seen in humans. Recently, a case presented itself in which the effects of PHMB on tissue regeneration in a patient’s healing wound could be evaluated.

Both prior to and following application of PHMB, the wound was treated with micropore particle technology (MPPT; Amicapsil/Acapsil; Willingsford Ltd., Southampton, UK). Micropore particle technology is anti-infective and supports wound healing without exerting antimicrobial effects.⁵ It consists of fine porous particles that, by capillary evaporation, remove wound exudate⁵; this action also removes toxins and enzymes secreted by bacteria and fungi and disrupts the structure of biofilm. In turn, the immune system regains control of the wound environment and healing process. To date, MPPT has not shown any indication of cytotoxic effects.⁵ In a comparative clinical study,⁶ MPPT reduced the time to reach a healing wound free of infection by 60% compared with both a topical antibiotic and an antiseptic. 

A 72-year-old patient had received radiotherapy 35 years prior to treat cervical cancer. As revealed by an MRI, this resulted in the development of wide-spread osteomyelitis in the hip and right femur, a large abscess on top of the hip bone, a sinus extending from the abscess down the right femur, and a second sinus extending from the other end of the abscess up to and into the spinal cavity. Over the past 20 years, the patient had gradually become paraplegic but retained some level of sensation and movement. The abscess had remained undetected until March 2017, when eschar suddenly appeared on the right-hand side of the lower back. This initially was treated with Manuka honey as a pressure ulcer, but in late May 2017, surgical debridement was performed. 

Debridement removed the eschar and most of the “cheesy” slough that filled the abscess and left a 6.5-cm deep wound with a 10 cm x 2.5 cm opening directly on top of the right hip bone. The wound was packed with Manuka honey dressings once daily to both resolve the remaining necrotic tissue and promote healing. However, after a brief appearance of a few granulation buds at 1 edge during the third week, the wound regressed. By late June (Figure 1A), the wound had increased in size to 12 cm x 3 cm and formed a leathery layer of “soft eschar” on top of the entire wound, including a 1-cm thick, hard, leathery cap on top of the osteomyelitic hip bone. The measurable depth of the wound was 6 cm. The necrosis-covered, infected bone constituted 25% of the wound surface, and infected material drained from the hip bone and tunnel along the osteomyelitic femur into the wound. Severe undermining, measuring 2 cm to 4 cm in depth, along half of the perimeter of the wound was present (left and cranial edges). In addition, the skin appeared dark red to purple in color, thickened, and significantly hardened, resembling the consistency of cardboard. Cellulitic edges were evident. The wound was highly infected and malodorous, and the patient showed signs of toxemia. 

At this stage, the tunnel exiting from the bottom of the abscess down along the right femur measured 20 cm in depth and 3 cm in width. The tunnel ran along the main vessels and nerves and was tightly filled with slough. The femur also suffered from chronic osteomyelitis that drained into the tunnel. The tunnel was not emptied surgically, because it considered too risky. 

At this stage, 5 weeks after surgical debridement, the use of Manuka honey was stopped due to worsening of the condition and once daily application of MPPT was initiated. The MPPT powder was applied once daily in a 1-mm to 3-mm layer to all accessible wound surfaces, including exposed osteomyelitic bone, undermining between the skin and soft tissue, and as far into the tunnel as possible (ie, until the slough blockage). The wound opening was covered with a permeable contact layer dressing (Mepitel; Mölnlycke Health Care, Gothenburg, Sweden) and 1 layer of 2-ply gauze. The wound progressively cleared of the necrotic tissue on top of the hip bone over the following 4 weeks, and both granulation tissue and new epithelium around the wound edges appeared and gradually grew in size and thickness (Figure 1). The malodorous infection of the entire wound resolved within the first week with MPPT.

With autolytic debridement and concomitant steady granulation, the wound built up considerable amounts of new tissue, thus markedly decreasing wound volume. The broad brim of dark red- to purple-colored skin surrounding the wound edges, which showed signs of cellulitis, contracted in width and gained a healthy pale pink hue. The hardened skin started to recover and became softer and bendable, at which time the skin began to fuse with the soft tissue, thereby slowly clearing the undermining. In addition, new epithelium, showing an off-white color, generated on the wound edges, thus decreasing the wound opening. The area on top of the infected hip bone (previously covered by strong, dry slough) cleared, and the fascia covering the hip bone developed a healthy, structured appearance. Granulation tissue had slowly begun to settle and expand on top of this fascia across the exposed osteomyelitic bone area. 

By week 9 (ie, start of September), the 20-cm deep tunnel was free of slough, and the application regimen was changed to once daily every other day. By week 12 (ie, mid-October), the wound had reduced by about 50% in volume, and there was no sign of soft tissue infection. The cranial part of the wound was still steadily removing necrotic structures, including nonviable vessels, but the area was contained, noninfected, and reducing. The patient overall was feeling well and had returned to daily activities. The wound was ready for surgical removal of the osteomyelitis, followed by tertiary closure of the wound.

By week 12, the treatment goal for MPPT had been reached, ie, to remove the soft tissue infection and reach a controlled, healing wound to enable surgical closure. The use of MPPT was stopped, and, 2 days later, the treatment protocol was changed to a once daily application every other day of PHMB (Prontosan Wound Gel X; B. Braun Medical Inc, Bethlehem, PA), containing 0.1% PHMB hydrochloride (also known as polyaminopropyl biguanide) and an undecylenamidopropyl betaine surfactant. First, a cellulose-based gelling fiber ribbon (KerraCel Gelling Fiber; Crawford Healthcare, Knutsford, UK [now owned by 3M+KCI, San Antonio, TX]) was folded into the sinus, extending down the leg. Next, 3 cm x 2 cm of PHMB was applied onto a gelling sheet (10 cm x 10 cm) and then placed on the necrotic tissue of the cranial area. Following this, 2 x 2 cm of PHMB was applied onto a gelling ribbon (5 cm x 5 cm) and then placed on the exposed hip bone. The gelling ribbon was folded into the remainder of the void and secured with a foam dressing (Tegaderm Foam Adhesive; 3M, St. Paul, MN). This procedure was performed once every other day. 

The changes to the wound resulting from the use of PHMB are shown in Figure 2. On day 6 of PHMB application, froth appeared in 1 area of the exposed hip bone, indicative of gas entering the wound through holes in the facia and underlying infected bone, which most likely formed after starting PHMB. The froth grew in amount over the subsequent 4 days. After the start of application, the granulation tissue gradually began to lose its proliferation layer (ie, the outermost layer of white germ-cells) and gained an increasingly red, slightly irritated appearance. 

On day 6, regression of the granulation tissue was observed. Over the following days this led to the disappearance of the white germ-cell proliferation layer and, in areas covering the bone, the disappearance of pink granulation tissue. Areas that had already undergone strong tissue regeneration and maturation showed loss of structure, vitality, and bud formation, leading to flattening of the areas. This overall marked regression was indicative of damage to the cells. The wound edges changed from bulging, whitish-pink into a shriveling appearance with red to purple coloring. The fascia on top of the exposed hip bone lost its structured appearance and regressed in certain places (Figure 2). The changes to the fascia resulted in increased direct exposure of the underlying infected bone, which did result in increased release of infective material into the wound as well as increased difficulty for granulation tissue to migrate across. Exudate and slough increased in volume and changed in color from pale yellow to dark brown to black. 

On day 10 after the start of PHMB application, an acute pain syndrome developed in the wound, where after the use of PHMB was immediately stopped. During the 7-day wash-out period, the effects caused by PHMB remained. After the 7-day wash-out period, the patient returned to once daily MPPT application. A reduced level of wound exudate, renewed generation of granulation tissue, and gradual closing of the wound had only fully resumed after 3 weeks of daily MPPT applications. Daily use of MPPT continued for 8 months. During this 8-month period, the wound continued to improve and reduce in size by around 90% relative to start of MPPT; however, it was unable to close due to the osteomyelitis. At 12 months (ie, July 2018) after the first MPPT application, the patient was scheduled for surgery to remove parts of the infected bone and to close the wound by tertiary intention. The operation was successful.

The present case shows that over an application period of nearly 12 months, MPPT continued to treat the soft tissue infection, manage the continuous inflow of infection from the osteomyelitic hip and femur, and promote tissue regeneration; no side-effects were seen even with direct application onto chronic osteomyelitic bone. In contrast, within 6 days of PHMB application, gas production emerged through a previously unseen hole in the exposed bone and, gradually over the following days, the wound regressed, showing damage to granulation tissue, fascia, and epithelializing wound edges. These changes continued following the discontinuation of PHMB and were only gradually reversed after returning to MPPT. The wound continued to improve with daily use of MPPT; after 8 months, the patient underwent surgery for wound closure by tertiary intention.

Using 8 different cell-lines, Chindera et al² found cells began to accumulate PHMB internally at 0.0001% and all cells scored positive at 0.0003% (3 µg/mL). Creppy et al⁷ reported that after 3 hours PHMB is cytotoxic to intestinal, neuronal, and hepatic cells, with an IC50 of 0.002%. Yabes et al⁸ found cytotoxic effects on fibroblasts, keratinocytes, and osteoblasts increased with duration of exposure, which could correlate with the reported internal accumulation². Polyhexamethylene biguanide is highly stable in water¹⁰, and a 2018 toxicology study¹¹ reported that, at 0.05% (500 mg/L) and after 4 weeks of exposure, PHMB caused liver changes (mild centrilobular hypertrophy) in rats when receiving PHMB in their drinking water. Polyhexamethylene biguanide readily binds to many materials, and the bioavailability is low if given in food.¹² Studies of cell types related to wound healing have shown PHMB is biocidal to fibroblasts, keratinocytes, osteoblasts,⁸ and chondrocytes⁹; PHMB has been found to be toxic to all cell types on which it has been evaluated. Also, in a porcine wound model, investigators^2,3 reported that PHMB prevented the formation of granulation tissue when given in the instillation fluid in combination with NPWT. 

The present case provides further evidence that PHMB has cytotoxic effects on wound healing; overall, there is a high level of data consistency between this report and previously published data cited herein. In this case, PHMB caused the existing granulation tissue layer to rapidly deteriorate and the germ-layer, responsible for generating new tissue, to disappear. Therefore, PHMB caused degeneration of the tissue responsible for closing the wound. Polyhexamethylene biguanide also affected the structure of the exposed bone and caused holes to appear, thereby destabilizing the structure. The emergence of froth was observed, indicating an increase in gas-producing bacteria within the bone. Given that PHMB is toxic to all cells above 0.002%,⁵ it is highly likely that PHMB also would have damaged immune cells in the area, consequently interfering with the body’s ability to fight infection.

In pharmaceutical research and development, a rule of thumb for normal drugs is that the therapeutic index (ie, ratio between the recommended clinical concentration divided by the concentration at which toxicity is first seen) should be at least 10. If this principle was applied to wound healing, it would mean the maximal concentration at which PHMB could be used would be 0.0002% (ie, 500 times lower than it is currently being used). However, PHMB is normally used in wound care in concentrations of 0.1% or higher,¹ but it has consistently been shown to be cytotoxic to all tested cell lines with an IC50 around 0.002%.^2,13 This means PHMB is used at a concentration 50 times higher than the level at which it kills 50% of the cells. 

The use of PHMB 0.1% concentration raises several questions. The exposed hip bone in the present case suffered from chronic osteomyelitis and was thought to already be fragile. The periosteum covering healthy bone contains fibroblasts in its outer layer, and these are known to be highly sensitive to PHMB.⁸ Therefore, exposure could have potentially affected and weakened the periosteum, thereby increasing the risk of osteomyelitis in healthy bone. The case also demonstrated tissue degeneration as a result of PHMB, which means PHMB could interfere with wound closure and, in some cases, may even expand a wound or cause it to penetrate deeper layers. Further, most toxicological experiments have studied^11,12 PHMB in the diet, but PHMB readily binds to many substrates (ie, solid food), which means toxicology studies in relation to wound healing must involve direct tissue application.^8,9 Also, considering PHMB accumulates in cells^2,7,13 and is highly stable in water,¹⁰ the effective half-life in tissue will be long and is a factor that should be taken into consideration.

The findings raise the question as to why the cytotoxic effects of PHMB on wounds have not previously been reported. In the present case, the wound was closely monitored on a daily basis, which made it possible to observe the changes. In addition, the tissue was actively granulating on top of exposed bone, making it possible to directly observe the degeneration of the granulation tissue and of the bone structure caused by PHMB. Normally, antiseptics are used on infected wounds with the aim to remove infection, where the baseline level of tissue regeneration would be low, making it difficult to observe a reduction. Also, in many cases, the main parameter measured is bioburden (ie, level of colonization), and while this parameter is likely to be affected by PHMB, it is not an indicator of healing but rather only of biocidal activity. These observations suggest the cytotoxic potential of wound care products should be evaluated on actively granulating and epithelializing wounds (against an untreated control) to allow observation of negative effects on the level of granulation and epithelization; in wound healing studies, the outcome measure should be healing.

In the present case, PHMB was used on a healing wound, and it may be argued that PHMB and antiseptics in general are primarily indicated for infected wounds with the aim to remove infection. However, the clinical literature^14-19 shows antiseptics have limited treatment effects on wound infections and in supporting healing. The US Food and Drug Administration²⁰ even concluded dressings with antimicrobials are without efficacy against wound infections and supporting healing. A common argument for using antiseptics in wound care is their ability to remove antibiotic-resistant infections and their inability to contribute to antimicrobial resistance (AMR). However, newer data^21-23 show several bacterial strains have developed resistance to PHMB and other antiseptics, and several antiseptics cause bacteria to develop cross-tolerance to other antiseptics as well as to antibiotics.²³ This cross-resistance is based on general tolerance mechanisms many bacteria can express, which means bacteria, over a period of a few days,²³ can become resistant to entire classes of disinfectants and antibiotics, essentially making the tools for treating bacterial infections ineffective.²³ Finally, all antiseptics show cytotoxicity in vitro,^7,8,13,24 and Carroll et al^3,4 reported in a porcine wound healing model that PHMB, as well as silver and octenidine (octenidine dihydrochloride), reduced the formation of granulation tissue in vivo. From this information and the present case, there is evidence to indicate the toxicity that occurred with this patient’s healing wound could be generalized across the class of antiseptics.

Finally, both antibiotics and antiseptics give rise to antimicrobial resistance^21-23 and in an area hosting a microbiome, antimicrobials can create an advantage for the resistant strains by eliminating the antimicrobial-sensitive strains while minimally affecting the resistant strain. This can give the resistant strains room to expand. As antimicrobial resistance and virulence are often linked, the use of antimicrobials in these areas can actively support a resistant, often more harmful infection, instead of fighting it, and may consequently place the patient at risk.

In vitro and in vivo findings have reported cytotoxicity of PHMB, and the present case suggests these effects can be seen in the clinical setting as well. Polyhexamethylene biguanide, in this case, was found to cause tissue degeneration, including degeneration of granulation tissue and bone structure. In contrast, over a 12-month period, MPPT continued to promote wound healing, and it was not associated with any negative effects on soft tissue, epithelium, facia, vessels, or the exposed bone structure. 

Authors: Jeanette Sams-Dodd, BSc, BScVet; and Frank Sams-Dodd, PhD, Dr.med.

Affiliation: Willingsford Ltd, Southampton, Hampshire, United Kingdom

Correspondence: Frank Sams-Dodd, PhD, Willingsford Ltd, NFEC, Rushington Business Park, Chapel Lane, Totton, Southampton, Hampshire SO40 9LA United Kingdom; fsd@willingsford.com

Disclosure: Mrs. Sams-Dodd and Dr. Sams-Dodd are paid employees of Willingsford Ltd (Southampton, Hampshire, United Kingdom).