Toxicology Antivenom Efficacy Or Effectiveness: the Australian

Toxicology 268 (2010) 148–154

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Toxicology

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Review Antivenom efﬁcacy or effectiveness: The Australian experience

Geoffrey K. Isbister a,b,∗ a Department of Clinical Toxicology and Pharmacology, Calvary Mater Newcastle, Newcastle, New South Wales, Australia b Tropical Toxinology Unit, Menzies School of Health Research, Charles Darwin University, Darwin 810, Australia article info abstract

Article history: Despite widespread use of antivenoms, many questions remain about their effectiveness in the clinical Received 28 May 2009 setting. The almost universal acceptance of their value is based mainly on in vitro studies, animal studies Received in revised form and human observational studies. Numerous examples exist where they demonstrate clear benefit, such 19 September 2009 as consumption coagulopathy in viper envenoming, prevention of neurotoxicity in Australasian elapid Accepted 21 September 2009 bites, systemic effects in scorpion and funnel-web spider envenoming. There are also concerns about the Available online 25 September 2009 quality and efficacy of some antivenoms. However, it is important not to confuse the efficacy of antivenom, defined as its ability to bind and neutralise venom-mediated effects under ideal conditions, and the effec- Keywords: Antivenom tiveness of antivenom, defined as its ability to reverse or prevent envenoming in human cases. There are Snake envenoming numerous potential reasons for antivenom failure in human envenoming, of which antivenom inefficacy Efficacy is only one. Other important reasons include venom-mediated effects being irreversible, antivenom being Effectiveness unable to reach the site of toxin-mediated injury, or the rapidity of onset of venom-mediated effects. A Coagulopathy number of recent studies in Australia bring into question the effectiveness of some antivenoms, including Venom snake antivenom for coagulopathy, redback spider and box jellyfish antivenoms. Despite brown snake antivenom being able to neutralise venom induced clotting in vitro, use of the antivenom in human envenoming does not appear to change the time course of coagulopathy. However, it is important that apparent antivenom ineffectiveness in specific cases is correctly interpreted and does not lead to a universal belief that antivenom is ineffective. It should rather encourage further studies to investigate the underlying pathophysiology of envenoming, the pharmacokinetics of venoms and antivenoms, and ultimately the effectiveness of antivenom based on snake type, clinical effects and timing of administration. © 2009 Elsevier Ireland Ltd. All rights reserved.

Contents

1. Venoms and antivenoms ...... 148 2. Antivenom efficacy or effectiveness ...... 149 3. Australian brown snake (Pseudonaja spp.) antivenom ...... 150 4. Failure of antivenom in recovery from VICC in Australian snakebite ...... 151 5. Other Australian antivenoms ...... 151 6. Implications of antivenom efficacy without effectiveness ...... 152 Conflicts of interest ...... 152 Funding ...... 152 References ...... 152

Despite the widespread use of antivenoms over the last century almost universal acceptance of their value is based mainly on in (Hawgood, 1999), many questions remain about their effectiveness vitro studies, animal studies and observational studies in humans. in the clinical setting (Currie, 2006). There is a common perception that they are highly effective for the majority of envenoming con- 1. Venoms and antivenoms ditions. In fact, there is limited evidence to support this and the Venomous animals have specialized glands and venom apparatus to produce venom and inject or apply it parenterally into ∗ other organisms (Meier, 1995). This contrasts to poisonous animals Calvary Mater Newcastle, Edith St, Waratah NSW 2298, Australia. Tel.: +61 2 4921 1211; fax: +61 2 4921 1870. which lack this speciﬁc apparatus and usually have to be ingested E-mail address: [email protected]. to cause toxic effects (Isbister and Kiernan, 2005). Envenoming

0300-483X/$ – see front matter © 2009 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.tox.2009.09.013 G.K. Isbister / Toxicology 268 (2010) 148–154 149 syndromes most commonly and importantly result from bites or Over the last decade there have been significant concerns about stings by venomous snakes, scorpion, spiders and marine creatures. the availability and quality of snake antivenoms (Lalloo et al., 2002), Venom consists of a mixture of toxins which have a broad range mainly in the rural tropics where the burden of disease is the great- of toxic effects in vitro and in vivo. When injected into humans est. Much of the debate has focussed on the quality and efficacy of (or animals) these toxins produce unique clinical syndromes. The antivenoms, and therefore on whether antivenom manufacturers most important toxins from a clinical perspective include neuro- are producing high quality and efficacious antivenoms. These con- toxins (Isbister and Kiernan, 2005; Kuruppu et al., 2008; Nicholson cerns about snake antivenom are under-pinned by a strong belief and Graudins, 2002), haemorrhagins (Hati et al., 1999), coagulant that if antivenoms are manufactured correctly, they will be effective toxins (Isbister, 2009), myotoxins (Mebs and Ownby, 1990) and for most snake bites. Suggestions have been made as to whether necrotoxins (Gutierrez and Rucavado, 2000; Vetter and Isbister, some antivenoms in fact neutralise (or bind) to specific impor- 2008). tant toxins in the venom and their inability to do this has resulted Antivenoms are antibody preparations that are produced from in antivenom failure in the clinical setting (Lalloo and Theakston, the plasma of animals, usually horses or sheep, by injecting the 2003). The idea that snake antivenom may not actually be clinically animals with venoms (Gutierrez et al., 2003). They are therefore effective is rarely entertained. Interestingly, the belief in antivenom polyclonal antibody mixtures with numerous antibodies of vary- effectiveness is less clear for scorpion antivenoms, where there is ing titre and affinity to the different toxins in the venom. If a single far more debate about whether antivenoms are beneficial in the venom (usually from one species) is used to immunise the animal clinical setting (Abroug et al., 1999; Amaral and Rezende, 2000; then the resulting antivenom is a ‘monovalent’ antivenom, how- Gueron and Ilia, 1999). ever it is still a polyclonal antibody mixture (O’Leary and Isbister, There are limited well designed and very few placebo ran- 2009). Polyvalent antivenoms are those raised to multiple different domised controlled trials of antivenom effectiveness (Abroug et al., venoms or a mixture of monovalent antivenoms. Antivenoms can 1999; Boyer et al., 2009; Lalloo and Theakston, 2003). Numerous be either whole IgG molecules, F(ab )2 fragments or Fab fragments preclinical and in vitro studies have demonstrated that antivenoms (Chippaux and Goyffon, 1998). can bind venoms and prevent appropriate venom-mediated effects Snake antivenoms are the most important and commonly used in vitro if pre-mixed with venom (Graudins et al., 2001; Isbister et antivenoms because of the severity of snake envenoming and num- al., 2007b; Ismail and Abd-Elsalam, 1998; Laing et al., 2004; Ramos- ber of snake bite cases worldwide (White et al., 2003). Snake Cerrillo et al., 2008; Winter et al., 2009). For some venom-mediated bite is now recognised as a neglected tropical disease by the effects there are clinical studies which support antivenom being an World Health Organisation because of this burden of disease, high- effective treatment. Studies have shown effectiveness of antivenom lighted in a recent study estimating the burden of snake bite to for venom induced consumption coagulopathy (VICC) in West be at least 440,000 cases of envenoming and 20,000 deaths from African saw-scaled viper (Echis carinatus) and Russell’s viper enven- snakebite every year (Kasturiratne et al., 2008). Scorpion envenoming (Myint et al., 1985; Warrell et al., 1977). For neurotoxicity oming is the next most important condition for which antivenom there are studies demonstrating antivenom effectiveness in taipan exists due to the high morbidity and potential mortality in chil- (Oxyuranus canni) bites in Papua New Guinea (Currie, 2006; Lalloo dren (White et al., 2003). Antivenoms are also available for spider et al., 1995a; Trevett et al., 1995), scorpion envenoming (Boyer envenoming and marine envenoming, but these are either less et al., 2009; Calderon-Aranda et al., 1995), and funnel-web spi- serious or less common worldwide (Currie, 2003; Isbister et al., der envenoming (Hartman and Sutherland, 1984). Conversely, 2003). other studies have demonstrated that for at least some effects, antivenom appears to have a much more limited role, such as delayed use in neurotoxicity (Kularatne, 2002; Phillips et al., 1988; 2. Antivenom efficacy or effectiveness Pochanugool et al., 1997; Theakston et al., 1990), VICC in Aus- tralasian elapid envenoming (Isbister et al., 2009; Tanos et al., This brief review aims to explore the reasons that antivenom has 2008), myotoxicity, renal failure (Myint et al., 1985) and local tis- not been as successful as hoped in some clinical settings, despite sue effects (Dart et al., 2001; Warrell et al., 1976, 1977). Although a preclinical studies showing that antivenoms neutralise venom number of randomised controlled trials of antivenom have been effects in vitro. The review will contrast the efficacy of antivenom, undertaken (Ariaratnam et al., 2001; Dart et al., 2001; Isbister, defined here as its ability to bind and neutralise venom-mediated 2006; Isbister et al., 2008b; Otero et al., 2006; Paul et al., 2004; effects under ideal conditions (in vitro studies and animal studies Smalligan et al., 2004), the majority compare types of antivenoms, of binding and neutralisation), and the effectiveness of antivenom, antivenom dose or route of administration, and are not placebo defined as its ability to reverse or prevent envenoming in human controlled trials. The few placebo controlled trials have mainly patients. been with scorpions and have not been universally supportive of

Table 1 Potential reasons for failure of antivenom administration in human envenoming.

Antivenom inefﬁcacy The inability of the antibodies in the antivenom to bind the toxins in the venom. This appears to be rare.

Irreversible venom-mediated effects Toxin injury that is irreversible such as pre-synaptic neurotoxicity (nerve terminal destruction) (Kuruppu et al., 2008), myotoxicity or renal injury (Isbister et al., 2007a).

Inability of antivenom to reach venom target This can occur with venom-mediated injury at the bite site or when toxin molecules are much smaller than antivenom molecules (e.g. short chain neurotoxins compared to IgG molecules) (Chippaux and Goyffon, 1998).

Rapid venom onset Rapid onset of envenoming such that antivenom is unable to prevent or reverse severe or life-threatening effects, such as in cardiac toxicity from box jellyﬁsh (Winter et al., 2009).

Mismatch of venom and antivenom pharmacokinetics This will manifest as “recurrence” and is thought to occur when there is ongoing venom absorption after antivenom has been eliminated. This is usually only a problem with the rapidly eliminated Fab type antivenoms (Boyer et al., 2001). 150 G.K. Isbister / Toxicology 268 (2010) 148–154

Fig. 1. Comparison and transition from antivenom efﬁcacy to antivenom effectiveness.

the effectiveness of antivenom (Abroug et al., 1999; Boyer et al., et al., 2006, 2007b; O’Leary et al., 2006) now shows that this is 2009). unlikely to be the case and that brown snake antivenom is highly There are numerous potential reasons for the failure of efficacious. antivenom in cases of human envenoming, of which antivenom Sprivulis et al. (1996) investigated the ability of Australian snake inefficacy, i.e. the inability of the antibodies to bind the toxins in antivenom to prevent the in vitro procoagulant clotting effects of the venom, is only one. Other important reasons include venom- their respective venoms. The study used very high venom concen- mediated effects being irreversible (e.g. presynaptic neurotoxicity), trations based on the assumption that the venom concentration in antivenom being unable to reach the site of toxin-mediated injury patients can be simply calculated from the average venom yield or the rapidity of the onset of venom-mediated effects (Table 1 of the snake. In these series of experiments they were unable and Fig. 1). It is also important to consider whether the reasons for to prevent brown snake venom induced clotting in vitro with antivenom failure differ for different antivenoms and more impor- a range of increasing concentrations of antivenom and required tantly whether they differ based on particular venom-mediated much larger amounts of the other antivenoms for their respective clinical effects. Making the assumption that antivenom efficacy venoms (Sprivulis et al., 1996). The investigators suggested that always implies antivenom effectiveness, may lead to the belief that even 25 times the recommended amount of antivenom (1 vial) some antivenoms are not efficacious, or have low potency, and was insufficient to prevent clotting for brown snake. An earlier therefore in some cases to escalating doses of antivenom being used study by Tibballs and Sutherland (1991) using brown snake venom with no benefit (Isbister et al., 2008b; Yeung et al., 2004). and antivenom showed that 10–25 times the recommended dose The efficacy of a number of Australian antivenoms has been of antivenom was required to prevent coagulation abnormalities questioned over the last two decades (Currie, 2003; Madaras et using 10 ␮g/kg of venom (similar amount to Sprivulis et al.). The al., 2005; Masci et al., 1998; Sprivulis et al., 1996; Tibballs and problem with both of these studies (Sprivulis et al., 1996; Tibballs Sutherland, 1991; Yeung et al., 2004). This has resulted in sig- and Sutherland, 1991) was that the very high venom concentrations nificant increases in antivenom dose for most snake antivenoms used were not consistent with concentrations that have now been (Yeung et al., 2004) and redback spider antivenom (Ellis et al., 2005; measured in envenomed patients, even those with severe enven- Isbister et al., 2008b), which has the potential to increase antivenom oming (Isbister et al., 2007b). The amount of venom administered reaction frequency (Isbister et al., 2008a) and is extremely costly. would result in a blood concentration approximately 100–1000 Australian antivenoms will be used as examples to explore whether times that seen in patients, so the fact that Tibballs et al. demon- poor clinical response to antivenom is in fact antivenom efficacy or strated some effect of antivenom at 10–25 times the recommended antivenom effectiveness. dose is in fact reassuring that the recommended dose of 1 vial is more than sufficient. 3. Australian brown snake (Pseudonaja spp.) antivenom The laboratory studies by Sprivulis et al. (1996) and Tibballs and Sutherland (1991) introduced the idea of poor antivenom efficacy Significant concerns have been raised over the efficacy of brown and that larger doses may be required for treatment. A number of snake (Pseudonaja spp.) antivenom in Australia because a number observational studies appeared to confirm this (Barrett and Little, of animal and laboratory studies suggested that the commercial 2003; Isbister and Currie, 2003; Yeung et al., 2004) and a study of antivenom was poorly able to neutralise the prothrombin acti- brown snake bites in Western Australia recommended an initial vator component of the venom (Madaras et al., 2005; Masci et dose of 10 vials of antivenom based on the reports of increasing al., 1998; Sprivulis et al., 1996; Tibballs and Sutherland, 1991). It doses being used in patients (Yeung et al., 2004). The problem with was then suggested that this was the reason for antivenom fail- these studies was that they reported the amount of antivenom ure in some cases of human envenoming (Henderson et al., 1993) being given, rather than objectively assessing the effectiveness of and the requirement for much larger doses of antivenom to treat antivenom. The crucial error was that recovery from VICC, the major patients in some parts of Australia (Yeung et al., 2004). Over a indication for antivenom in Australia, takes up to 12–18 h to occur period of a decade this lead to increasing doses of antivenom after antivenom administration (Isbister et al., 2006, 2009). Early being administered for brown snake envenoming (Currie, 2000) testing of clotting function before improvement could be expected, and recommendations in some parts of Australia for an initial dose based on the pathophysiology of consumption of clotting factors of 10 vials of brown snake antivenom (Yeung et al., 2004) com- and the required re-synthesis, was resulting in inappropriate repeat pared to the original one vial recommended by the manufacturer antivenom dosing. These observational studies were therefore sim- (White, 2001). A critical examination of these earlier studies and ply reporting the inappropriate use of large antivenom doses and consideration of recent laboratory and clinical studies (Isbister not assessing antivenom dose. G.K. Isbister / Toxicology 268 (2010) 148–154 151

Fig. 2. Effect of antivenom on venom induced clotting times for a range of venom concentrations. Plots of clotting time (CT) for venom alone divided by the clotting time with antivenom [CTvenom/CTantivenom] versus antivenom concentration (mU/mL) are presented for a range of venom concentrations—50 ng/mL (filled circles; equivalent to concentrations in patients), 250 ng/mL (crosses), 500 ng/mL (filled triangles), 5000 ng/mL (filled diamond) and 50,000 ng/mL (grey squares; taken from Sprivulis et al. (1996)). [Redrawing of data presented in Figure 4 of Isbister et al. (2007b)].

A more recent study by our group using similar clotting studies 4. Failure of antivenom in recovery from VICC in Australian demonstrated that the equivalent of one vial of antivenom is suf- snakebite ficient to prevent the in vitro procoagulant effect of brown snake venom (Fig. 2)(Isbister et al., 2007b). This is based on using clini- A semi-mechanistic systems model of the coagulation pathway cally relevant concentrations of venom (4–95 ng/mL), determined has recently been developed that explored the effects of taipan from a study of patients with brown snake envenoming (Isbister et venom on the coagulation pathway (Tanos et al., 2008). The model al., 2007b; O’Leary et al., 2006). The same study also showed that was able to simulate clotting factor information consistent with antivenom concentrations equivalent to the administration of one data obtained from actual taipan bites (Lalloo et al., 1995b). How- vial of antivenom were able to bind all free venom in solution at ever, the model also suggested that antivenom neutralisation needs venom concentrations equivalent to those seen with human enven- to occur soon after venom enters the circulation to have an impact oming (Isbister et al., 2007b). These studies provide significant on the recovery of the coagulopathy (Tanos et al., 2008). This has evidence of antivenom efficacy in vitro, both binding to venom and concerning implications for antivenom therapy in VICC in Aus- preventing venom-induced clotting, and demonstrate the impor- tralasian snakebite, because it suggests that it may not have any tance of undertaking in vitro studies that are clinically relevant. effect on the coagulopathy if given more than an hour after the Based on these studies (Isbister et al., 2007b) an initial dose of bite. brown snake antivenom should be 1 vial and repeat doses are not This had already been reported for the coagulopathy in taipan required, and it will take 12–18 h for the coagulopathy to recover. bite in Papua New Guinea, where early antivenom did not result Concerns have also been raised about the efficacy of tiger snake in a more rapid recovery of the coagulopathy (Lalloo et al., 1995a). antivenom. In the study by Sprivulis et al. (1996) large doses of Further empirical support for this has recently been produced for antivenom were required to prevent the in vitro procoagulant effect the coagulopathy resulting from bites by brown snakes and tiger of the relevant venoms. However, similar to brown snake venom, snakes in Australia (Isbister et al., 2009). This study was unable very high venom concentrations were used, about 1000-fold those to demonstrate any relationship between the timing of antivenom expected in human envenoming (O’Leary et al., 2008), which are and the recovery of the coagulopathy (Isbister et al., 2009), suggest- again not clinically relevant. Preliminary studies of tiger snake ing that antivenom may not play a role in speeding the recovery of envenoming show that venom concentrations in patients are also VICC in Australia. in the range 5–200 ng/mL (O’Leary et al., 2008) and that tiger snake The above research and earlier studies question the commonly antivenom is able to prevent the procoagulant effect of tiger snake held belief in Australia that antivenom is effective for VICC but not venom in vitro (O’Leary et al., 2007). for presynaptic neurotoxicity (White, 2001). The administration of The use of very large amounts of venom for in vitro assays to test antivenom within 6 h of the bite in taipan envenoming appears to antivenom efficacy is a major problem as described above. Similar reduce the number of patients requiring endotracheal intubation issues have arisen when drugs have been used at much higher con- (Lalloo et al., 1995a), whereas the administration of antivenom does centrations for in vitro studies compared to those seen in human not change the course of the coagulopathy (Lalloo et al., 1995a; studies (Zar et al., 1982). This underlies the importance of under- Tanos et al., 2008). taking observational studies of human envenoming where serial venom concentrations are measured. Such studies are essential to 5. Other Australian antivenoms not only make sure that clinically appropriate venom concentrations are used for in vitro studies but provide an understanding of Problems with antivenom effectiveness in Australia are not the time course and likely reversibility of venom-mediated effects. limited to snake antivenoms and there are concerns about the effec- Fig. 2 compares in vitro clotting studies done at clinically relevant tiveness of both box jellyfish and redback spider antivenom. concentrations, to those used by Sprivulis et al., which are 1000 Chironex fleckeri, the major Australian box jellyfish, has caused times those seen in envenomed patients. over 70 deaths and continues to be responsible for paediatric deaths 152 G.K. Isbister / Toxicology 268 (2010) 148–154 in the north of Australia. Severe envenoming occurs with large envenoming, the pharmacokinetics of venoms and antivenoms and areas of tentacle contact with skin and results in rapidly develop- ultimately the effectiveness of antivenom based on snake species, ing (10–20 min) cardiovascular compromise and cardiac arrest due clinical effects, as well as dose, route, and timing of antivenom to a lethal toxin. An ovine antivenom was developed against box administration. An important example is the obvious difference jellyfish venom and has been used over the last thirty years with between the consumptive coagulopathy in Echis spp. (carpet vipers) questionable success. There have been at least four deaths despite from Africa, and Australasian elapid snakes. There is good evidence antivenom administration. Protagonists of the antivenom have from observational studies that the coagulopathy in Echis spp. bites attributed survival in cases where antivenom was used. However, may last for many days untreated, but will resolve a mean of 12 h similar patients survived prior to the introduction of antivenom after antivenom treatment (Warrell et al., 1977). It is unclear why with early and effective cardiopulmonary resuscitation, making it there is persistent coagulopathy for Echis spp. but not Australasian unclear what role antivenom has (Currie, 2003; Currie and Jacups, elapids, but it underlies the importance of assessing antivenom in 2005). Like snake antivenoms, this has lead to the idea of poor each clinical scenario. antivenom efficacy and the suggestion that specific antibodies were In Australia, funnel-web spider antivenom provides an impor- missing to the lethal toxin (Currie, 2003; Ramasamy et al., 2004). tant contrast to those discussed so far. Although funnel-web spider Two recent studies have investigated the efficacy of box jel- envenoming is rare in Australia it is potentially fatal and continued lyfish antivenom in its ability to bind and neutralise the venom to cause deaths in the late 1970s, despite care in modern inten- (Konstantakopoulos et al., 2009; Winter et al., 2009). Western blot sive care units (Isbister et al., 2005). Although no controlled trial analysis using box jellyfish antivenom or antibodies raised to the of the funnel-web spider antivenom was undertaken the dramatic venom preparation showed that both could detect the majority of response in the first few cases of severe envenoming, particularly the protein bands in the venom ranging from 10 to 200 kDa (Winter in young children, provided strong evidence that the antivenom et al., 2009). Pre-mixing antivenom (and antibodies) with the was effective against the neurotoxicity (Hartman and Sutherland, venom prevented toxicity in a cell-based assay (Konstantakopoulos 1984). With the introduction of antivenom there has been a signif- et al., 2009) and also prevented cardiovascular collapse in an in icant decrease in reports of severe envenoming requiring intensive vivo rat model (Winter et al., 2009). However, administration of care admission, decreased lengths of stay and no reports of multi- the antivenom after venom addition in the in vivo rat model did organ failure (Isbister et al., 2005). not prevent cardiovascular collapse. Even more concerning still, administration of antivenom to the rat prior to venom (not pre- Conflicts of interest mixed with venom) also did not prevent cardiovascular collapse (Winter et al., 2009). 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