Introduction Chronic ulceration of the skin, caused by a variety of underlying pathologies, is a problem with an increasing prevalence in Western society. Over the past 40 years, there have been a number of impressive advances made in the treatment of these ulcers. The event that likely had the most profound impact on the treatment of slowly healing wounds was the observation by Winter[1] in 1962 that wounds heal better when they are maintained in a moist environment rather than being allowed to desiccate due to air exposure. As a result, a fundamental change occurred in the “best practices” adopted for treating chronic ulcers. Following Winter’s observations, a number of other significant advances in the treatment of slowly healing wounds have been made. Some of these build upon the provision of a moist wound healing environment and include a plethora of dressings that claim various advantages for maintaining an “ideal” wound healing environment. Other products offer a variety of additional perceived benefits ranging from antimicrobial activity to added warmth to removal of exudates by suction. Several companies currently offer various cell or tissue-based products believed to enhance and/or accelerate the healing process of wounds. From on-going wound research, knowledge regarding the molecular environment of wounds in various stages of repair has also come to light. This research has yielded information on the communication network present within the wound that directs the orderly deposition of various tissue components and, in essence, the entire healing process. One of the discoveries of this vein of research was the determination that a host of growth factors play major roles in coordinating the various events in the healing process. Research directed at investigating the differences between wound healing in acute and chronic wounds has elucidated that certain proteolytic enzymes[2,3] (e.g., collagenase, stromelysin) are present in excess amounts in chronic ulcers. The activities of these proteolytic enzymes are believed to negatively impact wound healing when they are present in excess as seen in chronic wounds.[2,4] The potential for these enzymes to cleave growth factors may also be part of the reason for the less than anticipated impact of exogenous growth factors applied directly to wounds. Some preliminary work suggests modalities that reduce the activity of proteolytic enzymes are associated with an improvement in the rate of wound closure.[5,6] These advances in wound care are setting the stage for dramatic improvements in the wound care products and techniques that are available to help individuals recover from chronic wounds. However, another important aspect of wound care is also receiving added attention—namely, preparing the wound to heal. There are a variety of interventions that must be performed if wound care is going to successfully lead to a resolved wound. Some of these interventions include ensuring the nutritional status of the individual is appropriate for wound healing to take place. Studies have demonstrated that people who are undernourished simply do not heal as well as those for whom adequate nutrition is established.[7,8] Amelioration of underlying pathologies is also a prerequisite for good wound healing. This is particularly well understood when it comes to adequate blood supply and control of blood sugar levels in diabetic patients. Experience has taught that when either of these parameters is inadequate, optimal wound healing does not occur. Debridement Modalities Another step in the provision of a wound bed that is acceptable for healing is the appropriate cleansing of the wound and the removal of necrotic tissue present in the wound. Clinicians have recognized the need for appropriate wound cleansing and debridement for centuries. Devitalized tissue present in a wound bed is undesirable for a number of reasons, including the following: it may serve as a reservoir for bacterial growth; it may contain elevated levels of inflammatory mediators that promote chronic inflammation of the wound site; and the presence of devitalized tissue may impair cellular migration necessary for wound repair.[9,10] Thus, it is becoming increasingly well recognized that clearing a wound bed of nonviable tissue is an important step that may facilitate the healing process for a variety of wound types, particularly burn wounds and various types of chronic wounds.11–14 Not only has wound debridement become a recognized first step in promoting wound healing utilizing traditional means but also as a means of preparing a wound for treatment with more costly skin equivalents and growth factor therapies. Although wound debridement is an important first step in the healing process, there is not a universally accepted methodology of debridement that can be equally applied to all wound types. Due to the number of considerations that need to be assessed to select the most appropriate method of debridement, several different methodologies exist. There are four principal methods of debridement that are in current clinical use. These methodologies include autolytic, enzymatic, mechanical, and surgical or sharp debridement. Another method of debridement that is becoming more accepted is biological debridement, and this method will be described as well. Autolytic debridement. Autolytic debridement is the process by which a wound bed naturally clears itself of debris. Phagocytic cells and protein-digesting enzymes, also referred to as proteinases or peptidases, present in the patient’s own wound fluid are responsible for accomplishing this process. Autolytic debridement requires moisture in the wound bed to permit the optimal movement of phagocytic cells and to facilitate the action of proteinases. Establishment of a moist healing environment in the wound bed is the first step the clinician can take to promote autolytic wound debridement.15,16 Typically, dressings, such as hydrogels, hydrocolloids, and transparent films, are used to provide the environment for autolytic debridement.17 Periodic flushing of the wound bed to eliminate inhibitory byproducts of phagocytic and proteinase action may be necessary in some wounds in order to maintain optimal conditions over longer periods. Autolytic debridement is the least demanding and most natural form of debridement available to the clinician. Since autolytic debridement is a slow process (taking days to weeks), its use is limited to situations where there is minimal necrotic tissue contamination of the wound bed and where the risk of infection or systemic exposure to toxins is negligible. Enzymatic debridement. Enzymatic debridement involves the application of exogenously derived proteolytic enzymes to a wound to accelerate a controlled digestion and removal of necrotic tissue. Enzymatic debridement has also been classified as a type of chemical debridement due to the involvement of a chemical reaction (hydrolysis) in the degradation of the proteinaceous devitalized tissue. Today, the term chemical debridement is more commonly reserved for the application of a chemical agent, such as hypochlorite solutions (e.g., EUSOL or Edinburgh University Solution), for the degradation of the devitalized tissue. However, the use of chemical debriding agents, like EUSOL, has become uncommon in wound debridement due to the cytotoxic effects these agents may have on the surrounding tissue.18 Enzymatic debriding agents are typically used in conjunction with moist wound healing and serve as adjuncts to the autolytic debridement process.19 Exogenous enzymes typically promote a more rapid debridement than allowing the autolytic process to proceed unaided.20 Even so, the process may still require several days to a few weeks to completely debride a wound, depending upon the agent chosen, the presentation of the devitalized tissue, and the skill with which the agent is employed. Several commercially available enzymatic debriding preparations can be obtained by prescription for this purpose. Because each preparation has its own particular protein-digesting characteristics, it is important for the clinician to become familiar with the relative merits and shortcomings of these preparations as they apply to each necrotic wound condition. For example, debriding enzymes may be inhibited by certain pharmaceutical preparations (e.g., silver or other heavy metals present in some antimicrobial preparations), and some patients may exhibit sensitivity to certain components of the debriding agent. Mechanical debridement. Mechanical debridement is a process by which devitalized tissue and wound debris are physically removed from the wound bed. The simplest form of mechanical debridement involves the use of wet-to-dry gauze, but this technique is nursing time-intensive and costly. Typically, the devitalized tissue is covered with a saline-soaked gauze that remains in place to soften the devitalized tissue. As the gauze dries, the surface of the tissue becomes entangled in the fibers of the gauze and is removed as the gauze is lifted from the surface of the wound. The process is nonselective and may lead to the removal of viable tissue in addition to the target tissue.12 In addition, the process often causes bleeding and pain upon removal, which may lead to nonselective trauma to the wound.17 Other methods of mechanical debridement include irrigation, pulsatile lavage, and whirlpool therapy.17,21 Some of these methods may be contraindicated in individuals at risk for infection, as bacterial contamination of some wounds has been documented to originate from contaminated plumbing associated with whirlpool baths.22 Sharp or surgical debridement. Surgical or sharp debridement is the manual removal of necrotic tissues utilizing instruments to cut away the devitalized tissue. Surgical debridement is considered to be the fastest and most effective form of debridement, particularly for large wounds where there is a significant amount of necrotic tissue present or when the wound is extremely contaminated.20 This method allows for the wound to be trimmed to healthy, bleeding tissue that may be appropriate for skin grafting. Prior to surgical debridement, appropriate pain control must be achieved in those patients with non-neuropathic wounds.17,20 Sharp debridement is sometimes differentiated from surgical debridement, although both may employ the same instruments and both require skilled individuals to complete. When the differentiation is made, surgical debridement is referred to as a form of debridement carried out by a surgeon in an operating room; sharp debridement may be conducted at the bedside by trained clinical staff and is usually less invasive.20 Biological debridement. Biological debridement is a debridement modality that is not in widespread use in North America, although its acceptance in Europe is increasing. Biological debridement, also known as maggot debridement therapy (MDT), involves the application of contained fly larvae to a wound requiring debridement.23 After application of the larvae to the wound, a dressing “cage” is constructed to keep the larvae contained to the wound site. The larvae are believed to secrete digestive enzymes that selectively dissolve necrotic tissue.24 In one clinical study, the larvae were left in place for 48 hours with one application of this therapy per week.23 This method requires the availability of appropriate larvae at the correct stage of development as well as the acceptance of the method by both the clinical staff and the patient. Table 1 provides an overview of the relative merits and shortcomings of the four most commonly encountered methods of debridement. It is also important to consider that current best practices dictate the best form of debridement may be a combinational approach of several methods, depending upon the needs of the specific patient and the availability of appropriate resources. Enzymatic Wound Debridement Debridement may be a crucial first step for promoting the healing of wounds in some patients. Enzymatic debridement should be considered when necrotic tissue is present to the extent that wound healing may be impaired and where debridement at a rate faster than that achieved via autolytic debridement is desired. Alternatively, surgical debridement should also be considered in such cases, but the clinician may determine that surgical or sharp debridement is inappropriate for the patient. Table 2 provides a description of the contraindications for surgical debridement. Current best practices recommend the use of one or more debridement modalities as appropriate for the particular wound. Substrate specificity. All enzymatic debriding agents contain some form of proteinase or combination of proteinases and other hydrolases. These enzymes may digest very specific types of protein (e.g., collagen, fibrin, elastin) and may be unable to attack even very closely related proteins, or they may be rather nonspecific in their action and may be capable of digesting a variety of proteins with a common structural feature. Thus, enzymatic debriding agents may be considered to be either “specific” or “nonspecific,” depending upon their ability to promote the cleavage of certain peptides or amino acid sequences. A proteinase is capable of catalyzing the degradation of specific protein structures or amino acid sequences. The target structure or sequence is referred to as the enzyme’s substrate. Under standardized conditions, an enzyme is capable of catalyzing the degradation of a specific quantity of its substrate within a certain amount of time. This rate of digestion is usually expressed as “activity units.” These values are characteristic of the particular enzyme and are usually defined by international or pharmacopeial convention. For example, an IU refers to an international unit of activity and a USP unit is defined as the United States Pharmacopoeia. In order to best select and control the rate and extent of the debriding action of an enzymatic agent, correct matching of the type or types of substrate to be digested and the number of units of activity required per gram of product is very important. Sources of enzymes used as debriding agents. In addition to differences in substrate activity, the sources of debriding enzymes are also different. Some enzymes originate from animal or plant tissues, whereas others may be derived from microbial sources. Because all enzymes are proteins, it is possible for humans to become sensitized to exogenous (non-self-derived) enzymes. This is dependent, to some extent, upon a particular patient’s ability to immunologically recognize the particular enzyme when applied therapeutically. Therefore, sources of enzymes may need to be considered relative to each patient’s history of drug or dietary sensitization. It also requires that patients utilizing exogenously derived enzyme therapeutics be monitored for signs of developing sensitivity. Fortunately, the current number of available enzymatic debriding agents is fairly limited. Therefore, if the substrate, activity, and patient history requirements are known, it is possible to compare products on the basis of their content and make an informed choice as to which debriding agent is appropriate for a particular individual. The following discussion summarizes some of the important characteristics of various enzymatic debriding agents currently available and considered for use as debriding agents. Animal-derived enzymes. One class of enzymes used or considered for use as debriding agents is derived from animal sources. Included in this class of enzymes are fibrinolysin, desoxyribonuclease, trypsin, and chymotrypsin. On a commercial scale, all of these enzymes are typically derived from bovine sources. Fibrinolysin is typically derived from bovine plasma. This enzyme, when activated, specifically attacks and breaks down the fibrin component of blood clots and fibrinous exudates. Desoxyribonuclease is derived from bovine pancreatic tissue and acts specifically on the nucleoprotein components of purulent exudates. These two enzymes have been combined together in a product known as Elase® (Parke Davis), which is available in some European countries. Studies have been conducted with this product and its components with results that suggest it may be somewhat effective as a debriding agent in uncomplicated ulcers25 but not any more effective than placebo in complex wounds.26 Other animal-derived enzymes considered for use as debriding agents include trypsin and chymotrypsin. These enzymes are also produced from bovine pancreatic tissue and are fairly nonspecific enzymes with activities directed at bonds between specific amino acid residues in the target protein. Plant-derived enzymes. Enzymes useful for debridement applications have also been recovered from various plants. Bromelain is a group of enzymes derived from the stem of pineapple plants that contains three cysteine proteinases. This mixture of enzymes is effective at breaking down a variety of devitalized tissue substrates over a pH range of 5.5 to 8.5. Papain is an enzyme derived from the fruit of the papaya tree (Carica papaya). Papain is a nonspecific cysteine proteinase (an enzyme that contains a cysteine residue at the active site) that is capable of breaking down a wide variety of necrotic tissue substrates over a wide pH range (3.0 to 12.0). Ficin is an enzyme similar to papain that has activity against a wide variety of protein substrates. It is also a cysteine proteinase (derived from the ficus plant), which demonstrates preferential cleavage at tyrosine and phenylalanine residues of proteins. Microbially produced enzymes. Besides enzymes that are normally obtained from eukaryotic sources, there are also several microbially produced enzymes that may be used in debridement. This discussion pertains only to those that are naturally produced by bacteria and does not include eukaryotic enzymes that may be produced through recombinant means in prokaryotic expression systems. Sutilains are water-soluble mixtures of serine proteinases derived from the bacterium Bacillus subtilis. These enzymes are relatively nonspecific in their action and are capable of breaking down a variety of necrotic tissue types over an optimal pH range of 6.0 to 6.8. Perhaps the most widely known microbially produced enzymes that are in current use as debriding agents are the collagenases. These enzymes are a family of metal ion-requiring enzymes (metallopeptidases) that specifically attack and break down collagen. Commercial collagenases are obtained from Clostridium histolyticum. Another enzyme that is currently being explored for possible use as a debriding agent is vibriolysin. Vibriolysin is another metallopeptidase derived from the bacterium Vibrio proteolyticus and has been found to be very active for digesting collagen.27 Inactivation of enzymatic action. Since enzymes are proteins, they can be denatured or otherwise have their proteolytic action reduced or totally eliminated by a variety of means. Heat is one major inactivating agent. Enzymes can be denatured and rendered inactive by exposure to elevated temperatures. The amount of heat required to reduce the activity of an enzyme is somewhat specific for the type of enzyme. The acidity (pH) of the local environment is also critical to an enzyme’s ability to function optimally. Enzymes have pH ranges within which they demonstrate maximal activity. Outside of these ranges they tend to quickly drop in their rate of activity and may become completely inactive, depending upon the enzyme and the environmental pH. A variety of chemical inhibitors also plays a major role in decreasing the activity of enzymes. These inhibitors include hydrogen peroxide, detergents, formaldehyde, and alcohol; all are agents that may denature or damage proteins and inactivate enzymes. In addition, many drugs commonly used in wound healing, such as silver sulfadiazine (or other presentations of silver), gentamicin, and penicillin can also inhibit or inactivate some debriding enzymes. Accuzyme®: A Papain-Urea Debriding Agent Accuzyme® (Healthpoint, Ltd., Fort Worth, Texas) is a white, hydrophilic ointment that contains papain (8.3 x 105 USP units of activity per gram, based on the USP lot H reference standard) and 100mg of urea per gram of product. Physical and chemical characteristics and mode of action. Papain, the proteolytic enzyme from the fruit of Carica papaya, is an enzyme with potent activity against denatured (nonviable) protein. At the same time, clinical and laboratory experience has demonstrated that the enzyme does not harm the viable tissue surrounding the wound.28 Papain, combined with urea, produces a very effective debriding agent as demonstrated by in-vitro studies (Figure 1).29 The principle components of wound eschar include fibrin, collagen, and elastin.30 An in-vitro efficacy study has shown that Accuzyme can effectively digest these eschar proteins. Fibrinolysis studies have demonstrated that Accuzyme shows enhanced ability to digest fibrin compared to either papain or urea alone in the Accuzyme ointment base (Figure 1). Functional proteins typically exist with a high degree of tertiary structure (folds and other three-dimensional structures) that contributes to their functionality. This three-dimensional structure is maintained by hydrogen bonds within the protein. Urea, a small nonionic molecule, is capable of interfering with these bonds causing the protein to essentially relax.31 In addition, experimental evidence suggests urea may cause disruption of some of the disulfide bonds within proteins to expose particular thiol groups (–SH), which may serve as activators of papain.32 Papain itself is unusually resistant to the effects of urea and is stable in the level of urea present in Accuzyme.33 These studies also demonstrated that Accuzyme is considerably more active in digesting denatured collagen than native collagen. These studies suggest that the papain/urea debriding ointment could effectively cleave the denatured collagen found in nonviable tissues with very limited action against native protein in surrounding viable tissues. The debriding efficacy of the enzyme may depend on its delivery vehicle. The same enzyme formulated in different ointment bases under different manufacturing specifications could result in very different proteolytic activity (efficacy). Accuzyme is formulated using a proprietary emulsion system developed to be very compatible with papain and urea. It provides all the conditions necessary for the enzyme to be active. Papain is found to release from the formulation easily with full activity. The formulation also maintains the stability of papain for its labeled shelf life. The presence of penicillin and sulfonamides does not inhibit the proteolytic action of papain, whereas Terramycin® (Pfizer Labs), Aureomycin® (Lederle Lab), and Chloromycetin® (Parke-Davis) may inhibit protein digestion. Clinical Experience with Papain-Urea Debriding Preparations The long clinical history of the use of papain or papain-urea preparations for wound debridement clearly demonstrates the effectiveness of this enzyme as a debriding agent. In 1879, Wurtz and Bouchut first coined the term papain to describe the purified proteinase derived from papaya.34 By the 1880s, various pharmaceutical preparations containing this enzyme were available for many indications, such as eczema, psoriasis, and oral syphilitic ulcers. Modder in 1888 applied papain to speed the separation of slough from chronic, slow healing ulcers.35 In 1901, Kilmer, in a monograph on the pharmaceutical use of papaya fruit, noted a similar use of papaya paste in infected wounds by the indigenous peoples of areas where papaya was grown.36 Since that time, papain has been used successfully as a debriding agent in the treatment of a number of different wound types. Chronic wounds. In 1940, Glasser reported the successful use of papain treatment in 58 cases of sloughing wounds.37 The action of urea to relax folded proteins and render them more susceptible to attack by papain was subsequently recognized, and debridement preparations containing papain-urea combinations were developed. Carter38 evaluated the use of a papain-urea debridement preparation on 37 patients with ulcers resistant to conventional therapy. The types of ulcers evaluated in this study included pressure ulcers of the scalp, decubitus ulcers of the sacrum, ischium, or trochanters, traumatic leg ulcers due to poor circulation, and diabetic leg ulcers. Some of these patients had previously been treated unsuccessfully for periods ranging from 24 days to 3 years. Treatment with this preparation resulted in lesions that were clean and beginning to granulate, generally within 5 to 12 days. The results from bacterial cultures taken prior to debridement and again when the wound bed appeared to be clean and granulating showed that papain-urea debridement was not affected by the presence of various wound microbes. The results also demonstrated an inability to culture organisms from the wound bed in 11 of the patients following debridement. No adverse reactions to the medication were observed. This study concluded that daily application of this preparation was effective and allowed the nursing staff more time to devote to other patients, thereby making it more economical than other treatments requiring multiple daily applications. Similarly, Morrison and Casali reported the successful use of papain-urea preparations in 27 of 30 evaluated pressure ulcer cases.39 Debridement was generally complete within 3 to 5 days. Only in three patients demonstrating particularly extensive necrotic involvement was the papain-urea ointment unable to clean the wounds and extensive surgical debridement was required. Burke and Golden40 critically evaluated the use of a papain-urea containing preparation on 27 wound cases, including severe pressure ulcers, infected burns, diabetic ulcers, varicose ulcers, heavy calluses, chronic cellulitis with multilocular abscesses, fistulous tracts, and infected metastatic tumor sites. The duration of treatment with the debriding agent varied from 5 days to 60 days with significant improvement, if not complete healing, observed in almost all cases. Once daily application was sufficient for most cases except large sloughing wounds where applications were twice daily. They concluded that papain, in combination with urea, was an effective debriding agent in their studies. More recently, Alvarez, et al.,41 conducted a prospective clinical study in patients with pressure ulcers in nursing homes. The authors described the utilization of this type of conservative approach to wound debridement as being desirable in nursing home settings, such as where the study was undertaken. In this 26-person study, a proportion of the wounds were treated with Accuzyme (n = 14) once daily for four weeks or until complete debridement was achieved. The remaining wounds were treated with a competitive product. Morphometric measurements were used to follow the wound size and the amount of nonviable tissue associated with the wounds. In this study, Accuzyme was found to be effective in quickly debriding the nonviable tissue. As illustrated in Figure 2, only approximately 20 percent of the eschar present at the initiation of treatment was present after one week of treatment. The rapid debridement was associated with a concomitant appearance of granulation tissue (Figure 3), as determined by clinical assessment. During the conduct of the study, the authors reported there were no incidents of pain associated with the treatment, and none of the patients withdrew from the study as a result of failure of the treatment regimen. Additionally, the authors did not report any irritation to the surrounding tissue as a result of application of the enzyme preparation to the wounds accompanied by the presumed contact of the drug product with surrounding healthy tissue. The authors concluded that the results of the study suggested that debridement with Accuzyme was, “a safe and effective alternative to surgical debridement.” Burns. In 1943, Cooper, et al.,42 reported successful debridement of burn eschar with papain-containing solutions. They noted that papain required the presence of “activators” in order to function acceptably. These observations are in keeping with other data, including that in Figure 1, which suggests that papain in the absence of urea is less effective as a debriding agent. Guzman and Guzman in 195343 also reported the successful utilization of papain as a debriding agent in burns and other wounds. They also noted the absence of any negative action of papain on healthy tissue. Product Safety As discussed, there is extensive historical data supporting the use of papain-urea preparations as safe wound debridement products. In addition, in an independent study conducted in 1995, Accuzyme was applied to 59 human subjects to evaluate the level of irritation and/or sensitization produced following multiple, repeated applications (10 applications per subject). No visible signs of erythema or edema were noted for any Accuzyme-treated site relative to its corresponding untreated control site on any subject. Similarly, challenge testing conducted following a 14-day induction phase did not produce signs of sensitization in any of the subjects. The conclusion from this study was that Accuzyme did not indicate a potential for dermal irritation and/or sensitization.44 Another study was completed that served to examine the impact of clinically applied Accuzyme to intact skin. This test was conducted in an attempt to determine if there was a negative impact of Accuzyme on healthy tissue, as evidenced by delayed reduction in the transepidermal water loss (TEWL) in skin where the barrier properties of the skin had been compromised. This test was conducted in 16 volunteers who had the skin of their forearms repeatedly tape-stripped until a TEWL measurement of twice the baseline value was obtained. The tape-stripped skin was subsequently treated with Accuzyme or left untreated (control). Over time, TEWL was assessed as an indication of any treatment-related damage to the skin. The results of this study indicated that a negative impact of Accuzyme could not be detected.44 Instead, an actual improvement in the TEWL results was observed. Conclusion In an era of rapidly advancing knowledge regarding the mechanisms involved in aiding the healing process of various types of wounds, the concept of adequate preparation of the wound for healing is re-emerging as an important consideration in wound treatment. In particular, control of the microbial bioburden and removal of nonviable tissue in the wound are both recognized as important steps in the reparative process. A number of debridement modalities exist, but one modality that is universally acceptable for all wounds has not emerged. Enzymatic debridement of nonviable tissue remains an effective debridement option for some wound types. Several different enzymes have been, are, or may become useful as debriding agents. One of these enzymes is papain, a plant-derived enzyme that has a long history of successful, clinical application. In addition, this enzyme, formulated as a commercial product called Accuzyme, has been studied in a variety of laboratory and clinical settings to establish both its safety and efficacy. The results of these studies help to reinforce the cli