Cerium nitrate in the management of burns

Burns 31 (2005) 539–547 www.elsevier.com/locate/burns

Review

Cerium nitrate in the management of burns J.P. Garner a,*, P.S.J. Heppell b a

Chesterfield Royal Hospital, Calow, Chesterfield, Derbyshire S44 5BL, UK b The Radcliffe Infirmary, Woodstock Road, Oxford OX2 6HE, UK

Abstract Background: The introduction of early excision of the burn eschar has contributed to a reduction in burn-related mortality but is not appropriate in all circumstances. Cerium nitrate has been used since 1976, usually in combination with silver sulphadiazine, to improve outcome where early excision is not performed. However, has still not gained universal acceptance. The evidence for its use is reviewed. Methods: A MEDLINE search was performed for the years 1966–2003 using keywords ‘cerium’, ‘sulphadiazine’, ‘Flammacerium’, ‘lanthanides’ and ‘topical therapy for burns’. The reference lists of key articles were then sifted for other relevant articles. Results: Cerium has been shown to reduce mortality and morbidity in the treatment of severe burns. This benefit is derived from its action on the burn eschar. It binds and denatures the lipid protein complex liberated from burnt skin that is responsible for the profound immunosuppression associated with major cutaneous burns. It has only limited antimicrobial properties. Conclusions: Cerium nitrate is an excellent topical treatment for most cutaneous burns not undergoing immediate excision and closure. # 2005 Elsevier Ltd and ISBI. All rights reserved. Keywords: Cerium nitrate; Silver sulphadiazine; Mortality

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical chemistry of cerium . . . . . . . . . . . . . . . 3.1. General chemistry . . . . . . . . . . . . . . . . . . 3.2. Interaction with calcium-dependent systems. 3.3. Cellular interactions . . . . . . . . . . . . . . . . . Antimicrobial actions of cerium. . . . . . . . . . . . . . 4.1. In vitro activity . . . . . . . . . . . . . . . . . . . . 4.2. Synergy with silver sulphadiazine. . . . . . . . The immunological effects of thermal injury . . . . . 5.1. Burn toxin . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Alterations in cell-mediated immunity . . . . 5.3. Cytokine dysregulation . . . . . . . . . . . . . . . 5.4. Animal and human studies . . . . . . . . . . . . Effects in the eschar. . . . . . . . . . . . . . . . . . . . . . 6.1. Physical hardening . . . . . . . . . . . . . . . . . .

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* Corresponding author. Present address: 6 Kensington Court, Lodge Moor, Sheffield S10 4NL, UK. Tel.: +44 114 230 1237. E-mail address: [email protected] (J.P. Garner). 0305-4179/$30.00 # 2005 Elsevier Ltd and ISBI. All rights reserved. doi:10.1016/j.burns.2005.01.014

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6.2. Biological dressing . . . . . . . Clinical outcome studies . . . . . . . . Adverse effects of Cerium therapy . Current standing . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .

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1. Introduction

2. Methodology

Major burns still carry considerable mortality and morbidity despite decades of research. Mortality may be divided into early and late deaths. Early mortality is generally attributed to the circulatory shock [1] that accompanies a large burn or the acute respiratory effects of inhalational injury [2]. Late deaths have classically been ascribed to sepsis [3], but it is increasingly recognised that these deaths may result from immune failure in the absence of documented sepsis [4]. Many of the later immune problems from burns are ameliorated by early excision of the burn, a technique now practiced widely in burns units worldwide, removing the agent responsible for immunosuppression. Whilst early burn excision is an ideal and practical solution in many units, it is recognised that situations exist where early excision is not feasible. Early excision of large burns usually requires coverage with something other than autologous skin grafts [5], as these will be in limited supply. The solution in the developed world is the use of combinations of stored allograft skin [6], xenografts [7] and tissue engineered biosynthetic products that achieve temporary wound coverage [8]. In many parts of the world, such expensive products are not available and early excision is limited by the paucity of autologous donor sites. In other cases, the burn patient will also have concomitant injuries to other organ systems or co-existent morbidities that make it undesirable for early operation under general anaesthesia [9]. There are also specific situations, such as burns in the military combat environment, in which early burn excision is not feasible. Burn wound excision and closure in a field hospital is limited by environmental conditions, the absence of biosynthetic dressings, a dearth of blood products, time and experienced personnel and the real possibility of mass burn casualties overwhelming limited resources. Over the course of 30 years, some burns units in Europe and the United States have pursued the use of the rare earth metal cerium as an adjunct to topical treatment of cutaneous burns. In those centres, the results have been excellent but widespread adoption of cerium has been slow. This article examines the chemical characteristics of cerium that may be relevant to burn management, the historical development of its use and the evidence for its efficacy.

A MEDLINE search was performed for the years 1966– 2003. Keywords used were ‘Cerium’, ‘sulphadiazine’, ‘Flammacerium’, ‘Lanthanides’ and ‘topical therapy for burns’. Only English language papers were used and all reports of cerium nitrate use were examined including case reports and review articles. The reference lists of key articles were then sifted for other relevant articles, particularly pertaining to the chemical and microbiological characteristics of cerium.

3. Physical chemistry of cerium 3.1. General chemistry Cerium (58Ce) is the second lightest Lanthanide or rare earth element. Discovered in 1803, Cerium is a trivalent atom although unique among the Lanthanides, it also has a quaternary valency. Despite its epithet, Cerium is not particularly rare, being naturally more abundant than lead. It is found in a variety of ‘rare earth’ minerals such as bastnaesite and monazite, from which it is extracted commercially for use in the glass making industry, flint manufacture and radiation shielding. It forms a variety of salts, with the nitrate (as a colourless, aqueous hexahydrate [Ce(NO3)3
6H2O] solution) of principal medical interest. As a group, the Lanthanides have no known physiological role and are unable to penetrate intact mammalian cell membranes, but can enter the cytoplasm of effete cells [10]. Given the lack of a natural biological role, the extent to which Lanthanides interact with biological systems is surprising. The interaction with calcium-mediated systems is particularly powerful. Due to greater charge-to-volume ratios, and despite their different valencies, all the Lanthanides readily displace calcium from biological systems and this effect is heavily implicated in cerium’s therapeutic role. 3.2. Interaction with calcium-dependent systems All lanthanides behave similarly, with differences in effect being quantitative rather than qualitative. Individual chemical studies of Cerium are rare. Lanthanide interactions with Calcium are neither universally inhibitory nor stimulatory; both effects are observed

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across different systems and even within the same system at different Lanthanide concentrations. In the activation of trypsinogen to trypsin, Calcium stimulates the reaction by stabilisation of the substrate against autolysis [11] and Lanthanide substitution increases this action [12]. In contrast, Lanthanide substitution inhibits other Calciumdependent processes including activation of factor X, phospholipase A2 and cellular Ca2+-ATPase channels [13]. It has been suggested that if Calcium plays a structural role within the biological system, then rare earth substitution allows residual activity. Conversely, replacement of ‘functional-calcium’ amounts to competitive antagonism with little or no residual function [14]. The avid displacement of Calcium from within numerous biological systems raises the possibility that interaction with dermal and epidermal Calcium-dependent events may represent one mechanism by which Cerium exerts a beneficial effect in burn patients. Calcium is important in keratinocyte maturation and wound healing after injury and calmodulin, a metal-binding protein implicated in Calcium homeostasis and wound repair has been shown to bind cerium [15]. Modulation of the calmodulin/Calcium action in wound regeneration by Cerium may potentially contribute to improved outcomes after burn injury. 3.3. Cellular interactions Lanthanides do not penetrate living mammalian cell membranes, but by interaction with Calcium-dependent transmembrane signalling channels, they can interfere with intracellular events. This is the likely mechanism for Lanthanide inhibition of stimulus-coupled cellular secretion, such as angiotensin-induced secretion of aldosterone from the adrenal cortex and the release of trypsin from the pancreas in response to a secretagogue [10]. Cerium (and lanthanum) have a particularly potent inhibitory effect on the degranulation of mast cells and the release of histamine from both mast cells and basophils, again by interference with a cell membrane ATPase pump [16]. This effect is also seen in epidermal Langerhans cells [17]. It has been proposed that cerium could be used as a topical treatment for atopic eczema, in which the antigen-presenting function of Langerhans cells and histamine release of mast cells are crucial factors. The observations that both collagen and G-actin polymerise in an almost physiological manner under the influence of Lanthanides raises a further potential role for Cerium within the realm of wound healing.

4. Antimicrobial actions of cerium 4.1. In vitro activity The bacteriostatic properties of the rare earth elements were first recognised at the end of the 19th century, and

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these early reports of the effects of the Lanthanides on a variety of organisms have been reviewed elsewhere [18]. The first systematic analysis of these effects was published in 1947 [19] when Cerium and Lanthanum (and the nonrare earth element thallium) were tested against a panel of 39 bacterial species across 16 genera, including Staphylococcus aureus and Pseudomonas aeruginosa. Cerium nitrate was an effective bacteriostatic agent against the whole spectrum of bacteria, but was pH dependent, with the greatest effect at slightly acidic pH values. The most susceptible genera were the pseudomonads, with inhibition of growth at concentrations of between 0.001 and 0.004 M of Cerium nitrate; Escherichia and Salmonella species required concentrations of approximately 0.005 M for bacteriostasis, whilst S. aureus required almost twice that concentration. Further tests using Cerium chloride against 35 common fungi failed to inhibit fungal growth at a concentration higher than that previously shown to inhibit all strains of bacteria. No attempts were made to identify or postulate on the mechanism whereby bacteriostasis was effected. Twenty years later, Sobek and Talburt [20] studied the effects of Cerium nitrate on the morphology and cellular function of a single organism, Escherichia coli. There was ready uptake of Cerium into the cell cytoplasm, in stark contrast to mammalian cells, with marked inhibition of cellular respiration, oxygen uptake and glucose metabolism. Morphologically, the cell wall remained intact but developed ‘knob-like protrusions’ when viewed under the electron microscope, which the authors had previously noted on fungi similarly affected by Cerium and associated with eventual membrane disruption [21]. 4.2. Synergy with silver sulphadiazine Synergy describes the situation where the benefits of a combination treatment are greater than the simple additive effects of the individual components. Silver sulphadiazine (SSD) was introduced in 1968 [22], by substitution of a single hydrogen ion of sulphadiazine by silver. It was an attempt to replicate the known antimicrobial properties of silver nitrate in burns patients without the problems previously encountered of electrolyte disturbances, toxicity and staining of both burn and bedding. Its widespread activity against a variety of microbes including staphylococci, pseudomonas and fungi soon made it the topical agent of choice. The increased efficacy of SSD over sulphadiazine alone was attributed to the incorporation of the dissociated silver ion into the bacterial nucleotide sequences with inhibition of replication [23]. The first suggestions of a synergistic interaction between cerium nitrate (CN) and SSD came in the original report of its use [24]. The eight burn patients treated with CN–SSD had an incidence of negative wound cultures twice that of patients treated with Cerium alone. Overall, there was a 50% reduction in mortality compared to

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predicted death rates from probit tables [25] after the use of CN–SSD. It was presumed that the improvement in both outcome and bacterial counts was due to an additional antimicrobial effect of Cerium; thereafter other studies attempted to precisely define this action. In vitro assessment of Cerium’s activity against a broth containing P. aeruginosa [26] found little anti-pseudomonal activity when Cerium was applied as a solution or cream, and only minimal synergism when used with SSD. Further tests using a model of pseudomonas cultures instilled into a wound on the dorsum of laboratory animals showed no antibacterial effect of Cerium. It reduced the efficacy of SSD [26]. Further in vitro testing [27] against a panel of 37 common burn unit pathogens assessed antimicrobial activity by measuring the diameter of the induced zone of inhibition. Agar plates of bacterial cultures were treated with SSD and CN–SSD. In only 3 out of the 37 cases did the addition of Cerium increase the zone of bacterial inhibition; in 4 cases there was no difference and in 30/37 cases the degree of inhibition was reduced by the concomitant use of Cerium. A European study yielded contrasting results. The addition of Cerium reduced the minimum inhibitory concentration (MIC) of SSD required for bacteriostasis by a mean of two and a half times in over 75% of samples [28]. In the remaining samples, the MIC of SSD remained unchanged despite the addition of Cerium, implying no decrease in the antimicrobial efficacy of silver sulphadiazine. The greatest synergistic effect, representing an eight-fold reduction in MIC, occurred with two staphylococci (aureus and epidermidis) against which silver sulphadiazine had been relatively inactive both experimentally [29] and clinically [24]. There were no in vitro antibacterial effects of Cerium alone [28]. The in vitro testing of Cerium is complicated by the absence of a reliable technique. Cerium’s avid binding of protein and phosphates induces precipitation in many liquid test media, reducing the concentration of active Cerium, potentially influencing the results. Conversely, silver sulphadiazine is poorly soluble and can give erroneously poor results in agar disc diffusion tests [30]. The method of in vitro testing may therefore influence the results. A clinical study examining the relative effects of the two agents on 31 children with burns [31] found no benefit of using Cerium-enhanced SSD over plain silver sulphadiazine. Gram-negative cultures were prominent from Cerium-treated children in contrast to earlier studies [24,32]. Recent literature confirms Cerium has limited antibacterial activity against common burn unit pathogens [33], and that there is little additional antibacterial effect from Cerium in combination with silver sulphadiazine [33,34]. A single contrary report [35] claimed Cerium nitrate to have one of the best bacteriostatic profiles of eight topical agents tested against both staphylococci and pseudomonas.

5. The immunological effects of thermal injury 5.1. Burn toxin It is now clear that much of the morbidity and mortality from major burns is due to alterations in immune function. Immunosuppression is partly responsible for the burn victim’s susceptibility to infection in addition to the effects of disruption of the barrier function of an intact epidermis. This immune failure is also likely to be responsible for the late burn deaths from multi-organ failure syndrome (MOFS), several weeks after injury when systemic or burn wound infection is absent. The clinical observation that early excision of burn wounds decreases mortality [36] reinforces the idea that the burn wound or eschar is responsible for the generation of immunosuppression. Experimental evidence concurs with this view: sterile homogenates of burnt mouse skin were injected into the abdomen of healthy mice producing an 80% mortality rate at 10 days, whereas injection of unburnt skin had no effect [37]. The burn toxin responsible for this immunosuppression was isolated in 1975 and has subsequently been identified as a high molecular weight (3,000,000 Da) lipid protein complex (LPC) formed by heat-induced polymerisation of six skin polypeptides. The individual precursor elements are not toxic, and it appears to be the polymerisation of all six subunits together that renders the toxic effects [38]. 5.2. Alterations in cell-mediated immunity The mechanism of LPC-mediated immunosuppression is still not entirely clear. Alterations in the ratios of the various T-cell sub-types after burn were first discovered in 1984. Monoclonal antibody labelling demonstrated a significant reduction in the T-cell helper/suppressor ratio after burns in mice. This was probably due to an increase in the T-cell suppressor subpopulation [39]. These changes could be influenced by a variety of anti-inflammatory agents such as cimetidine, ibuprofen, indomethacin and cyclophosphamide given intraperitoneally and by the topical application of cerium nitrate; they all produced a significant elevation of the helper/suppressor ratio [40]. The same author also reported improved clinical outcomes following therapyinduced maintenance of T-cell subset ratios by demonstrating significantly decreased mortality using the same agents as before in a model of cutaneous burns complicated by a standardised septic insult [41]. The degree of ear swelling in response to an in vivo challenge of 0.2% 2,4-dinitrofluorobenzene (DFNB) in previously sensitised laboratory mice, has been shown to be a sensitive marker of cell-mediated immunity (CMI), being dependent on an intact T-cell mechanism. Using this as a marker, Peterson et al. [42] compared the effects of treatment with cerium on the immunosuppressive effects of burn injury on CMI with controls. At 14 days postinjury—the point in time previously shown to correlate with

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the greatest degree of immunosuppression [43]—the control treatment, eucerin ointment, was able to restore CMI to only 50% of pre-burn levels whilst silver sulphadiazine faired little better (55%). However, treatment with Cerium nitrate restored CMI to 90% of control levels and combined CN– SSD achieved a 99% restoration of cell-mediated immunity. This effect was most marked when the Cerium was applied early in the post-burn period (i.e. within 2 h) but some immunomodulatory effect was maintained if treatment occurred within 24 h. In addition, although a single treatment of Cerium improved CMI (to 71% of normal— an improvement significantly better than eucerin ointment alone), the best results were obtained when Cerium was applied on a daily basis for a week. 5.3. Cytokine dysregulation Thermal injury may, like other traumas, be expected to induce a cytokine response, with elevations of Interleukin (IL)-6, granulocyte colony stimulating factor (G-CSF) and occasionally IL-8 being observed [44]. The cytokine most closely studied and apparently most heavily implicated in the response to thermal injury is IL-2. Interleukin-2 acts as a growth factor for all T-lymphocyte subpopulations, activates natural killer cells and B-cell antibody production and induces other cytokines, notably IL-1, TNF-a and TNF-b and interferon-g. It is a central regulator of the immune response and acts as a marker of T-cell-mediated immunity; it is consistently elevated in serum samples taken from burned patients. However, T cells drawn from burnt patients exhibit virtually no response to specific endogenous IL-2 stimulation in vitro in the first days after injury. This suggests that the immune system may initially be maximally stimulated after which function declines markedly [45]. In patients who ultimately survive, the IL-2 production is later seen to rise to near normal levels, whereas non-survivors fail to restore IL-2 levels. The LPC is a potent inhibitor of IL-2dependent cell growth, thus it appears that at least part of the immunosuppressive action of LPC is by interference in T-cell activation, a process seemingly essential for recovery from major burn injury [46]. Introduction of a single bath in Cerium nitrate solution into clinical management obviated many of the detrimental effects of the burn on activated lymphocyte function, with a maintenance of IL-2 receptor activity at higher levels than traditionally treated controls. It also maintained in vitro cellular IL-2 production within the normal range in response to stimulation [45]. Similar benefits in the modulation of the levels of the pro-inflammatory cytokines, IL-6 and TNF-a have also been demonstrated in rat models [47,48]. 5.4. Animal and human studies The lipid protein complex appears to be responsible for changes in the cellular immune system, with rises in levels of pro-inflammatory cytokines, leukocyte and endothelial

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cell activation [49] and inhibition of activation of IL-2dependent systems. In vitro, Cerium nitrate appears to be able to modulate these immune changes and two studies have translated this immunomodulatory effect into survival advantage. Kistler et al. [50] excised healthy skin from laboratory rodents, burnt it and replaced it on the donor animals. Prior to regrafting, the burnt skin patches were treated with either saline or Cerium nitrate solution. The improvement in survival after Cerium treatment compared to controls was highly significant ( p < 0.0001) and equated to the survival rates found in burnt animals who underwent immediate burn wound excision and autografting. In a human trial aiming to demonstrate that survival advantage was due to improved immune function rather than any direct antimicrobial effect, 64 patients with a total burn surface area (TBSA) greater than 30% were bathed in a 0.04 M solution of Cerium nitrate for 30 min on arrival at a single burns unit [51]. Within this cohort there were only 3 deaths, none of which could be attributable to sepsis, MOFS or immunosuppression, whereas over 15 patients had a predicted mortality of over 80%. Whilst these outcomes are exceptional, the use of predictive mortality comparators such as this warrants a degree of caution in interpretation of the results. An extensive discussion of the immunological effects of cutaneous thermal injury including cytokine dysfunction, heat shock protein modulation and antigenicity in burns is beyond the scope of this review but the interested reader is referred to Allgo¨ wer et al. [52] for an excellent review of this area.

6. Effects in the eschar 6.1. Physical hardening How does Cerium exert this potent immunomodulatory effect? The striking effect of Cerium on the burn eschar has been an almost universal finding since the earliest reports of is use. Topical Cerium renders the eschar firm and impermeable, often described as leather-like in appearance with a greenish yellow discolouration. The eschar is firmly adherent to the wound beneath, in contrast to the soft macerated eschar generated by silver sulphadiazine treatment. This physical hardening of the eschar brings benefits in nursing terms, as soak-through and dressing changes are minimised and are less labour intensive [51]; excision is in most cases easier [3,6]. Despite the longstanding recognition of this effect on the eschar, there is only a single report that has examined histologically the changes within the eschar and investigated the mechanisms involved. Using patients as their own controls, Boeckx et al. [53] compared the effects on the eschar of SSD with and without the addition of cerium. Twenty-two patients over an 8-month period with deep dermal burns (average TBSA 21.7% [range 10–77%]) were studied by selection of two site-matched areas, typically either the upper or lower limbs, which were

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treated with SSD on one side and CN–SSD on the other. Thereafter, treatment was identical and punch biopsies were taken daily from each site for histological examination. On routine haemotoxylin and eosin staining, the Cerium-treated specimens showed a thin eosinophilic surface layer and a marked basophilic banding at the junction of the papillary and reticular dermis. The banding increased in width and intensity from the first post-burn day onwards. These changes were not present in the SSD group, which did however have a marked inflammatory infiltrate and evidence of early wound healing, something notably absent from the Cerium specimens. Subsequent biopsies, taken up to 10 weeks after burn injury showed no signs of wound healing beneath an intact Cerium crust. Further examination with specialist staining techniques identified deposits, evident from the first post-burn day, of insoluble pyrophosphate and carbonate salts, as well as Calcium deposits within the upper dermal layers. All these deposits were within the upper 1 mm of the biopsy and present only in the Cerium-treated group; Calcium was never found in the absence of Cerium in any specimen. Boeckx et al. postulated that Cerium may bind tissue pyrophosphate, removing the inhibition provided to local calcium deposition in a situation analogous to the pyrophosphate–Calcium interactions within cancellous or cortical bone. In addition, Cerium did not produce these effects in superficial burns where the basal membrane remained intact, suggesting that interaction with dermal collagen could play an important role in Calcium deposition. Exposed dermal collagen may act as a nidus for Cerium pyrophosphate and subsequent Calcium crystallisation; the proclivity of Lanthanide–Calcium interactions in general would support this hypothesis. 6.2. Biological dressing This histological explanation of how the Cerium crust is formed does not entirely explain the beneficial effects on mortality. It may be that the impermeable crust prevents bacterial colonisation of the burn wound, reducing sepsis and improving outcome. Alternatively, rather than acting primarily as a barrier to the ingress of bacteria, the eschar may function as a barrier to the egress of burn toxin into the systemic circulation. By its effects on sucrose gradient gel diffusion tests, Cerium can be shown experimentally to bind the LPC and its precursors and denature the toxic components [54]. Whichever mechanism is responsible, and it is likely to be a combination of both, the effects on the eschar are remarkable: it becomes firmly adherent to the wound for many weeks with a minimal incidence of subeschar infection [55]; eschars have been left in situ for 6 [5], 12 [53] and 14 weeks [55] without detriment. There is little tendency to spontaneous eschar-separation, and when finally excised, the wound beneath is generally clean, healthy and ready to accept a skin graft. Graft take rates of 90% are reported [9]. There appears to be inhibition of granulation and contraction of full thickness wounds beneath the Cerium

eschar [9], which acts as a biological dressing. There is a single report of slightly delayed re-epithelialisation of superficial wounds for which Cerium is probably not indicated [3].

7. Clinical outcome studies The first report of Cerium use came from William Monafo [24] in Missouri in 1976; he suggested that the antimicrobial properties of the rare earth metals may have a positive benefit when used on severely burnt patients. Cerium was chosen because of its abundance and ease of purification. Sixty patients, ranging in age from 6 months to 92 years, were treated with various formulations of Cerium, including eight treated with Cerium nitrate–silver sulphadiazine cream in the latter part of the study. The standard treatment protocol included early excision of burns, usually within 5 days of injury, and coverage with autograft. Cerium treatment was well tolerated, and left wounds that readily accepted grafts. The most striking feature of Cerium treatment was a 50% reduction in mortality rate against that predicted from probit tables for the severity of injury. Cerium also altered the burn wound bacterial profile, with a marked decrease in Gram-negative colonisation in general and pseudomonal infection in particular. S. aureus became the predominant organism. Overall, there was a decrease in the density of bacterial counts from wound cultures, a shift away from Gram-negative bacteria and a reduction in invasive burn wound infection. Three further reports [32,55,56] from the same institution over the following 7 years reiterated similar findings in successively larger groups of patients. In these later series, CN–SSD was the treatment of choice and burn excision was usually delayed until at least the third week. Improvements in bacteriostasis and mortality were similar to those reported initially, although all four reports used either historical or predicted mortality rates as comparison. A prospective randomised controlled trial of Cerium nitrate therapy [57] at a different institution attempted to address the criticisms of using historical controls. In 60 patients with TBSA greater than 10%, there was no difference in overall or sepsis-related death rates between CN–SSD- and SSD-treated patients. However, no comparison was made to predicted death rates. There was no alteration in the spectrum of bacterial colonisation between the two groups, although the SSD-treated group did have a significantly higher number of positive wound biopsies. This result was discounted by the authors as a reflection of the longer duration of hospitalisation of some of the SSDtreated group. This explanation ignored the alternative interpretation that Cerium treatment reduced colonisation and shortened hospital stay. Since that prospective trial, sporadic case series have generally supported the use of Cerium by detailing improvements in overall outcome. One burns unit achieved

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a 39% reduction in mortality when compared to historical data [5] and another reported a 59% reduction in mortality against predicted death rates. This was accompanied by a fall in the level of wound colonisation and septic complications [9]. One randomised trial comparing CN– SSD against SSD did report higher mortality and levels of bacterial colonisation with Cerium treatment, but despite randomisation there was a marked surfeit of older and more severely burned patients in the Cerium-treated group [6]. The most recent randomised trial [58] to compare SSD and CN–SSD involved 60 patients in two groups in a small but well constructed prospective study. Re-epithelialisation of partial thickness wounds occurred a mean of 8 days earlier when treated with Cerium. An identical reduction in the healing time of partial thickness wounds had been reported nearly 40 years previously when the wounds were kept moist as compared to those that were allowed to dry out [59], suggesting another possible mechanism for improved outcomes after Cerium treatment. Excised full thickness wounds were ‘‘ready to receive grafts’’ 11 days earlier than SSD-treated patients, although it is not clear in the original study as to what this means and whether it was a function of the patients general health, satisfactory vascularity, or bacteriological status of the excised wound bed or some other feature. Average hospital stay was reduced by 7 days in the Cerium group, but the was no statistical difference in mortality between the two groups. Anecdotally, however, the one Cerium-treated patient who died had healthy wounds at post-mortem, whilst three of the four SSD-treated patients died with ‘highly deteriorated’ wounds. Despite the enthusiasm of individual clinicians, and favourable results from case reports over three decades, no prospective randomised trial has yet confirmed improvements in mortality from burn injury after treatment with cerium. This may be because no such benefit truly exists, or, that the methodological difficulties of constructing and recruiting to a trial sufficiently well powered to reliably detect improvements in mortality in groups as heterogeneous as burns victims has confounded these efforts.

8. Adverse effects of Cerium therapy By virtue of the fact that the rare earth elements do not penetrate the living mammalian cell membrane, adverse effects are rare. Early in vitro testing established toxicity profiles for all the Lanthanides via a variety of routes of administration with the LD50 dosages being greatest for oral dosing and lowest for the intravenous route [60,61]. Scheidegger et al. [51] calculated the maximum concentration likely in humans after bathing in Cerium and found the human levels to be many thousands of times lower than the lowest LD50 from rats. Estimation of systemic Cerium absorption is not performed routinely as it is time consuming, costly and requires neutron activation analysis to yield results. When it has been performed, Cerium has

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either not been detected [57] or has been present in extremely low concentrations (<10 mg/100 ml) [32] reflecting the virtual absence of systemic absorption. A single report [62] has been published in Japanese with an English abstract that examined visceral levels of Cerium and silver after 3 months of treatment with topical CN–SSD. Significant amounts of silver were identified in both the kidneys and liver of the patient, who subsequently died, but only a trace of Cerium was evident in hepatic tissue, with none at all found in the kidneys. The clinical significance of these findings is unknown. Early clinical reports [24,57] documented occasional cases of transient methoglobinaemia following Cerium nitrate therapy. It is likely that it is due to the bacterial reduction of nitrate in the wounds, in a similar manner to that documented with silver nitrate use [63]. Interestingly, no recent reports have described any cases of methoglobinaemia. The only commonly reported adverse effect in clinical practice is a stinging sensation after application that was reported in up to 87% of patients in one series but was easily controlled with oral analgesics [56].

9. Current standing Initially used as a soak solution for gauze dressings, cerium nitrate has for the majority of the 28 years since its introduction, been used in combination with silver sulphadiazine cream. Two companies market commercial formulations of micronised 1% silver sulphadiazine in a hydrophilic cream base with 2.2% Cerium nitrate, giving final concentrations of 0.029 and 0.05 M, respectively [Flammacerium (Solvay Duphar, The Netherlands) and Dermacerium (Silvestre Laboratories, Rio de Janeiro, Brazil)]. Flammacerium is registered and marketed in several western European countries, notably Belgium, the Netherlands, France and Spain and on a ‘named patient only’ basis in United Kingdom. Despite having its origins in the United States, Cerium nitrate–silver sulphadiazine (Flammacerium) remains in regulatory terms, an orphan drug, a designation it received in 1999, although FDA granted special permission for extended use after the terrorist attacks of September 11th, when it was used widely across New York [64]. Dermacerium is licensed in South America, where it is used on chronic wounds, such as venous stasis ulcers, as well as burns [65]. Although only available on a named patient basis in UK, a recent postal survey revealed that is currently used in approximately 70% of all the adult burns units in Great Britain and those units believed that Flammacerium should be fully licensed in UK [66]. In Europe, there is little clinical or pre-clinical research being undertaken into Cerium treatment, as the manufacturer has no plans to expand their European marketing programme beyond its current scope and the overall market is small. The situation is different in the USA, where the manufacturer of Flammacerium is undertaking a large prospective randomised trial of

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CN–SSD versus SSD in severe burns (TBSA + age > 90) where predicted mortality is greater than 80%. When fully recruited this study will be sufficiently powered to prove a reduction in mortality of 20–30% following treatment with cerium and is aimed at providing the evidence to support a full FDA submission for Flammacerium.

10. Summary Cerium nitrate has been used with good results in individual burn units across Europe, United Kingdom and the United States for nearly 30 years, although the rationale for its use has changed over time. Initially used in the belief that the addition of Cerium nitrate enhanced the antimicrobial activity of silver sulphadiazine, there is now considerable evidence to the contrary although the differences in experimental methods and problems inherent in in vitro testing of Cerium cloud the issue. Its hardening effect on the eschar is universally accepted and may help prevent bacterial ingress and maintain a moist wound environment—two further possible ways in which Cerium, acting as a biological dressing, aids wound healing. Nursing care of the burn wound is also made easier. Recent evidence suggesting a major effect of Cerium in binding and denaturing the immunosuppressive lipid protein complex generated by burned skin seems now to be its predominant mode of action in improving survival—this of course is obviated by early total burn wound excision and closure. Despite extensive clinical usage of Cerium, unequivocal evidence of improvements in human burn mortality from randomised controlled trials remains absent. Those reports that demonstrate survival advantage have used either predicted or historical mortality rates as comparators, generally at a time when non-operative management of severe burns had a higher mortality than today. There are five prospective studies of the effects of Cerium, only three of which are randomised, but none have shown definite survival advantage from Cerium and in fact one of the nonrandomised trials detailed a worsening of outcome. These studies are however also open to methodological criticisms by virtue of lack of or unequal randomisation or small sample size. The current US trial is aimed at confronting these issues and when fully recruited will be sufficiently powered to demonstrate a distinct survival advantage with Cerium nitrate compared to silver sulphadiazine alone—if such an advantage truly exists.

11. Conclusion In the industrialized world where the resources and expertise are available to undertake early total wound excision and closure, the indications for the use of cerium

are limited. Those patients in whom such excision is either inadvisable because of concomitant disease or not indicated because of a mixed depth pattern of injury could benefit from its use. In other less developed or affluent parts of the world where early excision and coverage of the burn wound is not feasible because of clinical, economic or logistic constraints, then treatment with Cerium nitrate–silver sulphadiazine represents a safe and efficacious alternative. The benefits of a firm adherent biological dressing that can be safely left in situ for many weeks in a patient with severe concurrent injuries or in a situation of mass burn casualties are considerable. Conflict of interest statement There are no conflicts of interest, financial personal or institutional with regard to this work.

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