Researchers are exploring whether these ubiquitous fluorinated molecules might worsen infections or hamper vaccine effectiveness.
Image credit: Shutterstock/Dmitry Naumov.
-
Communicated by John W. Daly, National Institutes of Health, Bethesda, MD, June 28, 2006 (received for review May 24, 2006)
Abstract
The nerve agents soman, sarin, VX, and tabun are deadly organophosphorus (OP) compounds chemically related to OP insecticides. Most of their acute toxicity results from the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that inactivates the neurotransmitter acetylcholine. The limitations of available therapies against OP poisoning are well recognized, and more effective antidotes are needed. Here, we demonstrate that galantamine, a reversible and centrally acting AChE inhibitor approved for treatment of mild to moderate Alzheimer’s disease, protects guinea pigs from the acute toxicity of lethal doses of the nerve agents soman and sarin, and of paraoxon, the active metabolite of the insecticide parathion. In combination with atropine, a single dose of galantamine administered before or soon after acute exposure to lethal doses of soman, sarin, or paraoxon effectively and safely counteracted their toxicity. Doses of galantamine needed to protect guinea pigs fully against the lethality of OPs were well tolerated. In preventing the lethality of nerve agents, galantamine was far more effective than pyridostigmine, a peripherally acting AChE inhibitor, and it was less toxic than huperzine, a centrally acting AChE inhibitor. Thus, a galantamine-based therapy emerges as an effective and safe countermeasure against OP poisoning.
The organophosphorus (OP) compounds soman, sarin, VX, and tabun, referred to as nerve agents, are among the most lethal chemical weapons ever developed (1). Some of them were used with catastrophic results in wars and also in terrorist attacks in Japan in the 1990s (2). The majority of insecticides are also OPs, and intoxication with these compounds represents a major public-health concern worldwide (3, 4). The possibility of further terrorist attacks with nerve agents and the escalating use of OP insecticides underscore the urgent need to develop effective and safe antidotes against OP poisoning.
The acute toxicity of OPs results primarily from their action as irreversible inhibitors of acetylcholinesterase (AChE) (5). In the periphery, acetylcholine accumulation leads to persistent muscarinic receptor stimulation that triggers a syndrome whose symptoms include miosis, profuse secretions, bradycardia, bronchoconstriction, hypotension, and diarrhea. It also leads to overstimulation followed by desensitization of nicotinic receptors, causing severe skeletal muscle fasciculations and subsequent weakness. Central nervous system-related effects include anxiety, restlessness, confusion, ataxia, tremors, seizures, cardiorespiratory paralysis, and coma.
Current therapeutic strategies to decrease OP toxicity include atropine to reduce the muscarinic syndrome, oximes to reactivate OP-inhibited AChE, and benzodiazepines to control OP-triggered seizures (5). The limitations of these treatments are well recognized (4), and alternative therapies have been sought. Among these therapies are phosphotriesterases and butyrylcholinesterase (BuChE), enzymes that act as OP scavengers (6, 7). However, potential adverse immunological reactions and the difficulty in delivering these large molecules systemically may limit progress in this field.
Pyridostigmine bromide, a quaternary carbamate that does not cross the blood–brain barrier appreciably and that reversibly inhibits AChE and BuChE with similar potencies, has been approved for use among military personnel who are under threat of exposure to nerve agents. Pretreatment with pyridostigmine prevents OP-induced irreversible AChE inhibition in the periphery, and it increases survival of animals acutely exposed to lethal doses of nerve agents, provided that atropine and oximes are administered promptly after an OP exposure (5, 8, 9). When used acutely before an OP exposure, reversible inhibitors of AChE that are capable of crossing the blood–brain barrier, including physostigmine, tacrine, and huperzine A (hereafter referred to as huperzine), afford better protection than pyridostigmine against OP toxicity, but generally this protection occurs at doses that produce significant incapacitation and central nervous system impairment (10–13).
Galantamine, a drug approved for treatment of mild to moderate Alzheimer’s disease (14), has properties appropriate for an antidotal therapy against OP poisoning. Briefly, galantamine (i) is a reversible AChE inhibitor that crosses the blood–brain barrier (14); (ii) has anticonvulsant properties (15, 16); and (iii) prevents neurodegeneration (17–19), a hallmark of OP poisoning (20). Thus, we used guinea pigs, the best nonprimate model to predict the effectiveness of antidotal therapies for OP poisoning in humans (21), to test the hypothesis that galantamine may be an effective and safe countermeasure against OP intoxication. Because of their low levels of circulating carboxylesterases, guinea pigs, like nonhuman primates, are considerably more sensitive to OPs, and they respond better than do rats or mice to antidotal therapies consisting of pretreatment with reversible AChE inhibitors and posttreatment with atropine (21).
Our results demonstrate that OP toxicity and lethality are counteracted when galantamine is administered before or soon after the acute exposure of atropine-treated guinea pigs to the nerve agents soman and sarin, or to paraoxon, the active metabolite of the OP insecticide parathion. We also provide evidence that the antidotal therapy consisting of galantamine and atropine is more effective and less toxic than alternative treatments.
Results
Combination Therapy Consisting of Galantamine and Atropine Effectively Prevents the Acute Toxicity of Lethal Doses of Soman, Sarin, and Paraoxon: Comparison with Pyridostigmine and Huperzine.
Clear signs of cholinergic hyperexcitation, including miosis, increased chewing, hypersalivation, muscle fasciculations, difficulty in breathing, and loss of motor coordination, were evident at 5–15 min after the s.c. injection of 1.5× LD50 soman (42 μg/kg of body weight) or sarin (63 μg/kg) in prepubertal male guinea pigs. Although an i.m. injection of atropine (6–16 mg/kg) immediately after the OP challenge attenuated the muscarinic signs, all animals showed tremors and intense convulsions within 15–30 min after the challenge. Atropine-treated, OP-challenged guinea pigs were euthanized when they developed life-threatening symptoms, and at 24 h after the exposure to nerve agents, only 11% of the animals (7 of 65) remained alive.
All guinea pigs that were pretreated with 5–12 mg/kg galantamine·HBr (hereafter referred to as galantamine) and posttreated with 10 mg/kg atropine survived the s.c. injection of 1.5× LD50 soman or sarin, with no toxic signs either before or after the OP exposure. The ED50 values of galantamine for 24-h survival of animals exposed to soman or sarin were 1.82 ± 0.37 or 2.2 ± 0.50 mg/kg, respectively (Fig. 1 A and B). The optimal dosage of galantamine changed as the OP levels increased. For example, in animals posttreated with 10 mg/kg atropine, the ED50 for galantamine to prevent the lethality of 2.0× LD50 soman was 5.1 ± 0.66 mg/kg (mean ± SEM; n = 8–10 animals per group), with 100% 24-h survival being achieved with ≥8 mg/kg (Fig. 1 A). Effective doses of galantamine were well tolerated; only animals that received 16–20 mg/kg galantamine showed mild adverse symptoms, which lasted 10–15 min and included increased chewing, hypersalivation, fasciculations, and tremors.
Fig. 1.
Pretreatment with galantamine prevents the acute toxicity of lethal doses of OPs: Comparison with pyridostigmine and huperzine. In all experiments, guinea pigs received an i.m. injection of selected doses of galantamine, pyridostigmine, or huperzine followed 30 min later by a single s.c. injection of 1.5× LD50 (42 μg/kg) or 2.0× LD50 (56 μg/kg) soman, 1.5× LD50 sarin (56 μg/kg), or the indicated doses of paraoxon. At 1 min after the OP challenge, all animals received atropine (1–10 mg/kg, i.m.). (A–C) Dose–response relationships for galantamine or atropine to maintain 24-h survival of animals challenged with nerve agents. (D) Dose–response relationship for paraoxon-induced decrease in 24-h survival of atropine-treated guinea pigs that were pretreated with saline or galantamine. (E) Effects of increasing doses of pyridostigmine or huperzine in maintaining 24-h survival of soman-challenged, atropine-treated animals. Each group had 8–12 animals. Percent survival represents the percent of animals that were kept alive because they presented no life-threatening symptoms.
Muscarinic blockade by atropine contributed to the antidotal effectiveness. Regardless of whether animals were pretreated with 5 or 8 mg/kg galantamine, 50% reduction of the lethality of those nerve agents was achieved with similar doses of atropine (mean ± SEM = 5.7 ± 0.47 mg/kg and 5.2 ± 0.13 mg/kg, respectively). However, a synergistic interaction occurred between galantamine and ≥6 mg/kg atropine; increasing the dose of galantamine from 5 to 8 mg/kg decreased the dose of atropine needed to protect the animals from the toxicity of soman (Fig. 1 C). Doses of galantamine and atropine required to treat OP intoxication may be optimized by using response-surface methods (22).
The acute toxicity of paraoxon was also effectively counteracted by therapy consisting of galantamine and atropine. All guinea pigs treated with atropine (10 mg/kg, i.m.) immediately after their exposure to ≥1.8 mg/kg paraoxon developed life-threatening symptoms and were euthanized. In contrast, all atropine-treated animals survived with no signs of toxicity when they received galantamine (8 mg/kg, i.m.) 30 min before their exposure to 2 mg/kg paraoxon (Fig. 1 D). Further, galantamine/atropine-treated animals that survived the challenge with 3 mg/kg paraoxon (Fig. 1 D) displayed only brief, mild signs of intoxication that included increased chewing and slight tremors.
The effectiveness of the antidotal therapy consisting of galantamine/atropine surpassed that of a combination of pyridostigmine and atropine in preventing acute OP toxicity. Only a fraction of animals pretreated with pyridostigmine (26–65 μg/kg) and posttreated with 10 mg/kg atropine survived the challenge with 1.5× LD50 soman (Fig. 1 E). The effectiveness of this therapy increased as the dose of pyridostigmine was raised to 52 μg/kg (Fig. 1 E). Increasing the dose of pyridostigmine to 65 μg/kg, however, decreased the effectiveness of the treatment, most likely because the potential benefit of increasing the protection of AChE from the actions of OPs is counteracted and eventually outweighed by the simultaneous pyridostigmine-induced inhibition of BuChE, an enzyme that serves as an endogenous scavenger of OPs (6).
The safety of the antidotal therapy consisting of galantamine/atropine was greater than that of a combination of huperzine and atropine. Approximately 80% of the animals challenged s.c. with 1.5× LD50 soman survived if they were pretreated with 100–200 μg/kg huperzine and posttreated with 10 mg/kg atropine; the minimum dose of huperzine needed to provide 100% survival of soman-challenged, atropine-treated guinea pigs was 300 μg/kg (Fig. 1 E). However, at doses ≥300 μg/kg, huperzine triggered transient, albeit incapacitating side effects that included profuse secretions, muscle fasciculations, abnormal gait, tremors, and respiratory distress. The stereotypic behavior of animals treated with huperzine was quantitatively analyzed in an open-field arena, as described below.
Galantamine Maintains Long-Term Survival of OP-Challenged, Atropine-Treated Guinea Pigs and Has No Significant Effect on Gross Behavior of the Animals: Comparison with Huperzine.
Even though all animals survived the first 24 h after the soman challenge when they were pretreated with 6 mg/kg galantamine and posttreated with 6 mg/kg atropine, only 80% of them remained alive after the 3rd day post-OP exposure (Fig. 2 A). In contrast, during the entire observation period, survival remained at 100% in animals pretreated with 5–8 mg/kg galantamine and posttreated with 10 mg/kg atropine. Increasing the dose of atropine to 16 mg/kg reduced the acute and long-term efficacy of doses of galantamine <8 mg/kg (Fig. 2 A). Thus, 10 mg/kg atropine ensured the highest long-term effectiveness of galantamine against the toxicity of 1.5× LD50 soman.
Fig. 2.
Long-term effectiveness and acute toxicity of different antidotal therapies against OP poisoning. (A) Seven-day survival of guinea pigs treated with galantamine at 30 min before and atropine at 1 min after their challenge with 1.5× LD50 soman. Each group had 8–12 animals. (B) Seven-day follow-up of the weight of animals subjected to different treatments. Weights are expressed as percent of the weights measured 1 h before a treatment. Control groups consist of animals that received a single i.m. injection of atropine, galantamine, huperzine, or saline. The soman/atropine groups consist of animals treated with galantamine or huperzine at 30 min before and atropine at 1 min after soman (n = 5–8 animals per treatment). (C) Graphs of the average total distance traveled and stereotypy of guinea pigs in an open-field arena at the indicated times after they received an i.m. injection of saline, galantamine, or huperzine (n = 6 animals per treatment). In B and C, results are presented as the mean ± SEM. Asterisks indicate that results from huperzine- and saline-treated animals are significantly different at P < 0.05 (ANOVA followed by Dunnett’s post hoc test).
Within 1 week after a single i.m. injection of saline, galantamine (8 mg/kg), or atropine (10 mg/kg), guinea pigs gained weight at similar rates, i.e., 2.51 ± 0.11% per day, 2.30 ± 0.05% per day, and 2.37 ± 0.03% per day (Fig. 2 B). In contrast, guinea pigs that received a single i.m. injection of huperzine (300 μg/kg) gained weight at a rate of 1.72 ± 0.17% per day (Fig. 2 B), which is significantly slower than that measured for saline-injected animals (P < 0.01 compared with saline-injected animals according to ANOVA followed by Dunnett’s post hoc test). Although galantamine/atropine-treated, soman-challenged animals lost, on average, 10% of their body weight at 24 h after the OP exposure (Fig. 2 B), their rate of weight gain during the remaining recovery period (2.72 ± 0.26% per day; mean ± SEM; n = 5 animals) was not significantly different from that of saline-treated animals that were not challenged with soman. Galantamine/atropine was equally effective in maintaining the rates of weight gain of guinea pigs challenged with 1.5× LD50 sarin or 3 mg/kg paraoxon at 2.53 ± 0.20% per day or 2.66 ± 0.21% per day (mean ± SEM; n = 3–5 animals per group), respectively. The acute toxicity of huperzine was not reflected in the rates of weight gain of animals that survived the OP challenge when treated with huperzine/atropine (Fig. 2 B).
In an attempt to quantify potential untoward behavioral effects of the doses of galantamine and huperzine needed to prevent acute OP poisoning, the overall ambulatory activity of guinea pigs was examined in an open-field arena. Previous studies reported that other centrally acting AChE inhibitors, including physostigmine, decrease locomotor activity and stereotypic behavior of rodents in the open field (23). Further, inhibition of the NMDA type of glutamate receptors, a mechanism that appears to contribute to the effectiveness of huperzine in preventing OP toxicity (24), is known to increase stereotypy in rodents (25).
Each guinea pig, immediately after receiving an i.m. injection of saline, galantamine (8 mg/kg), or huperzine (300 μg/kg), was placed in an open-field arena equipped with infrared sensors. At the dose tested, galantamine had no significant effect on the overall locomotor activity of guinea pigs (Fig. 2 C). However, huperzine increased the locomotor activity of the animals, and this effect became significant at 30 min after the treatment (Fig. 2 C). At this time, a distinct pattern of locomotor stereotypy, including repetitive routes of locomotion in the open-field arena, was also significantly higher in huperzine- than in saline-treated animals (Fig. 2 C). These effects of huperzine resemble those of the NMDA receptor antagonists ketamine, phencyclidine, and dizolcipine (25).
Galantamine Can Be Safely Used Pre- or Posttreatment to Counteract Acute OP Toxicity: Therapeutic Windows of Time.
An effective antidotal therapy should afford long-lasting protection for first responders who will attend a population acutely exposed to toxic levels of OPs. Thus, experiments were designed to determine how long before an exposure to OPs an acute pretreatment with galantamine would remain effective in preventing their toxicity. All atropine-treated guinea pigs that received 8 mg/kg galantamine up to 1 h before soman survived with no signs of toxicity (Fig. 3 A). As the interval between the injections of galantamine and soman increased beyond 1 h, the survival decreased (Fig. 3 A). Increasing the dose of galantamine to 10 mg/kg prolonged the time within which the antidotal therapy remained effective (Fig. 3 A).
Fig. 3.
Efficacy of galantamine as a pre- or posttreatment for OP poisoning is dose- and time-dependent. (A) Twenty-four-hour survival of animals that received a single i.m. injection of 8 or 10 mg/kg galantamine at 1, 2, 3, 4, or 5 h before the s.c. injection of 1.5× LD50 soman that was followed 1 min later by an i.m. injection of 10 mg/kg atropine. (B and C) Twenty-four-hour survival of animals that received a single i.m. injection of specific doses of galantamine at different times after their challenge with 1.5× LD50 soman or 2–3 mg/kg paraoxon, respectively. Each group had 8–10 animals.
Considering the difficulty of predicting when a person will be exposed to toxic levels of OPs under battlefield conditions, in the case of a terrorist attack, or during handling of insecticides, experiments were also designed to determine whether posttreatment with galantamine could effectively counteract the acute toxicity of OPs. All animals treated with 8 or 10 mg/kg galantamine up to 5 min after the soman challenge survived (Fig. 3 B) with no signs of intoxication; the rate of weight gain and gross behavior of these animals were indistinguishable from those of saline-treated animals that were not exposed to soman. Galantamine was no longer effective when given 10 min after 1.5× LD50 soman. Posttreatment with galantamine/atropine also prevented the acute toxicity of supralethal doses of paraoxon (Fig. 3 C). The therapeutic window of time within which posttreatment with galantamine remained effective in sustaining 100% survival of the animals decreased as the dose of the OP increased (Fig. 3 C).
No Signs of Neurotoxicity Were Observed in the Brains of Atropine-Treated Guinea Pigs That Received Galantamine Before or After the Challenge with Soman.
Neurodegeneration in three areas of the brain, the pyriform cortex, the amygdala, and the hippocampus, is characteristic of OP intoxication. The components of the antidotal therapy regimen, by themselves, were not neurotoxic. No signs of brain damage were detected at 24 h after an i.m. injection of saline (Fig. 4 A), 8 mg/kg galantamine (Fig. 4 B), or 10 mg/kg atropine (data not shown). Galantamine is a critical component of the antidotal therapy regimen because atropine alone was unable to prevent the well described neuronal death triggered by 1.5× LD50 soman (Fig. 4 C). Large numbers of shrunken neurons that were labeled with Fluoro-Jade B (FJ-B), an anionic fluorescein derivative that binds with high affinity to degenerating cells, were consistently seen in the hippocampus, amygdala, and pyriform cortex of atropine-treated guinea pigs that survived for 24 h after the challenge with 1.5× LD50 soman (Fig. 4 C). In contrast, staining with FJ-B was rarely seen in brain sections of soman-challenged, atropine-treated animals that were given 8 mg/kg galantamine at 30 min before or 5 min after the OP (Fig. 4 D and E). Further, the edema observed in the hippocampus and the marked parenchymal spongy state of the amygdala and pyriform cortex of soman-exposed, atropine-treated animals were absent in animals that received galantamine 30 min before or 5 min after the nerve agent (Fig. 4 C–E).
Fig. 4.
Soman-induced neurodegeneration is not present in the hippocampus, pyriform cortex, and amygdala of guinea pigs pre- or posttreated with galantamine. (A and B) Representative photomicrographs of the hippocampal CA1 field, the pyriform cortex, and the amygadala of guinea pigs that were euthanized 24 h after an i.m. injection of saline (A) or 8 mg/kg galantamine (B). No FJ-B-positive neurons were seen in the brains of these animals. (C) Large numbers of FJ-B-positive neurons were seen in all three index areas of the brain of a guinea pig that survived for 24 h after the challenge with 1.5× LD50 soman. (D and E) FJ-B-positive neurons were rarely seen in brain sections of animals that received galantamine (8 mg/kg, i.m.) at 30 min before (D) or 5 min after (E) soman. In C–E, all animals received atropine (10 mg/kg, i.m.) at 1 min after the OP, and they were euthanized at 24 h after the OP challenge. Photomicrographs are representative of results obtained from each group, which had five animals.
Doses of Galantamine That Effectively Prevent OP-Induced Toxicity and Lethality Are Clinically Relevant.
To help establish the clinical relevance of the doses of galantamine needed to counteract OP poisoning, plasma and brain concentrations of the drug were determined by HPLC at various times after treatment of guinea pigs with 8 mg/kg galantamine. This dose was selected because (i) in association with atropine, it afforded full protection against OP-induced toxicity and lethality, and (ii) it was half of the minimum dose at which galantamine triggered mild side effects.
In guinea pigs, as in humans (26), plasma levels of galantamine declined with first-order kinetics. After an i.m. injection of 8 mg/kg galantamine, plasma and brain levels of the drug peaked between 5 and 30 min and decayed with half-times of 71.7 ± 14.4 min and 57.8 ± 4.31 min, respectively (Fig. 5 A and B). As shown in Fig. 3 A, full protection against acute toxicity was achieved when 8 mg/kg galantamine was administered to guinea pigs up to 1 h before soman, a time when plasma and brain levels of the drug were 0.90 ± 0.01 μg/ml and 0.80 ± 0.04 μg/g, respectively (Fig. 5 A and B). Because the molecular weight of galantamine is 287.4, these findings suggest that the minimal plasma concentration of galantamine needed to prevent OP toxicity and lethality is ≈2.8 μM. Doses of galantamine recommended for treatment of patients with Alzheimer’s disease are between 8 and 24 mg/day (14), and peak plasma concentrations of 0.2–3 μM have been detected in healthy human subjects treated orally or s.c. with a single dose of 10 mg of galantamine (26, 27). Thus, doses of galantamine needed to prevent OP toxicity generate peak plasma concentrations similar to those achieved with doses clinically used to treat Alzheimer’s disease.
Fig. 5.
Differential sensitivity of brain and blood AChE activities to inhibition by galantamine in vivo and in vitro. (A and B) Concentrations of galantamine measured in blood (A) and brain (B) samples obtained at various times after treatment of guinea pigs (n = 4–6 animals per time point) with galantamine (8 mg/kg, i.m.) is plotted on a logarithmic scale against time. (C) AChE activity measured in samples from saline-treated animals was taken as 1 and used to normalize the enzyme activity measured in samples obtained at various times after treatment of animals with galantamine (8 mg/kg, i.m.). Normalized inhibition (1 − normalized activity) is plotted against the time at which samples were obtained. Asterisks indicate that results from galantamine- and saline-treated animals are significantly different at P < 0.001 (***) or < 0.01 (**) (ANOVA followed by Dunnett’s post hoc test). In A–C, the first point corresponds to results obtained at 5 min after the treatment. (D) Increasing concentrations of galantamine were added in vitro to brain homogenates and blood samples obtained from naive animals. AChE activity in untreated samples was taken as 1, and it was used to normalize activity measured in galantamine-treated samples. The graph of normalized AChE activity vs. galantamine concentrations was fitted with the Hill equation. Results are presented as the mean ± SEM (n = 4–6 animals per galantamine concentration).
In agreement with the concept that galantamine-induced AChE inhibition is reversible, the degree of AChE inhibition in brain and blood from galantamine-treated guinea pigs decreased as the galantamine levels declined in both compartments. Inhibition of AChE became negligible at 6 h after the treatment (Fig. 5 C), when plasma and brain levels of the drug were <0.1 μg/ml and 0.1 μg/g, respectively (Fig. 5 A and B). Maximal inhibition of blood AChE activity was ≈70% (Fig. 5 C), observed at 30 min after the treatment when the plasma levels of galantamine had peaked. The effectiveness of galantamine in patients with Alzheimer’s disease has been correlated with 40–70% inhibition of AChE in blood (28).
Maximal AChE inhibition in the brains of galantamine-treated animals was significantly different from that observed in their blood (Fig. 5 C). Measured peak concentrations of galantamine were 1.6 ± 0.13 μg/ml in the plasma and 1.38 ± 0.11 μg/g in the brain. These concentrations resulted in ≈70% and 25% inhibition of AChE in the blood and brain, respectively. Measured peak levels of galantamine in the plasma correspond to 5.6 ± 0.5 μM. Considering 80% of the brain weight as water, measured peak levels of galantamine in brain tissue would correspond to 3.8 ± 0.3 μM. Based on the concentration–response relationships obtained for galantamine-induced inhibition of guinea pig blood and brain AChE in vitro (Fig. 5 D), it is estimated that 5.6 μM galantamine would inhibit blood AChE activity by 68%, and 3.8 μM galantamine would inhibit brain AChE activity by 25%. In vitro, galantamine inhibited guinea pig blood and brain AChE with EC50 values of 1.8 ± 0.38 μM and 16.9 ± 9.8 μM, respectively (mean ± SEM; Fig. 5 D). In humans, blood AChE activity is also 10-fold more sensitive to inhibition by galantamine than is brain AChE activity (29).
Discussion
The present study demonstrates the remarkable potential of galantamine to improve antidotal therapy for even the most deadly OPs. In combination with atropine, well tolerated, clinically relevant doses of galantamine, administered acutely either before or soon after an exposure to the nerve agents soman and sarin or paraoxon, fully counteract the toxicity and lethality of these compounds. Although atropine alone attenuates the muscarinic syndrome resulting from the exposure of guinea pigs to the OPs, it does not afford significant protection against their lethality.
The exact mechanisms that account for the superiority of galantamine as a countermeasure against OP poisoning are yet to be fully elucidated. However, it can be postulated that the effectiveness of galantamine is related both to the higher potency with which it inhibits AChE compared with BuChE (30), an action that should help preserve the scavenger capacity of plasma BuChE for OPs, and to the protection of brain AChE from OP-induced irreversible inhibition. The finding that galantamine was essential to counteract soman-induced neurodegeneration in the brain supports the notion that AChE-related and/or -unrelated actions of this drug in the central nervous system contribute to its effectiveness. Neuronal loss in the brains of OP-intoxicated animals correlates to some extent with the intensity and duration of OP-triggered seizures (31–33). Yet, neurodegeneration and consequent cognitive impairment induced by OPs can be significantly reduced by therapeutic interventions that, although unable to suppress OP-triggered seizures, effectively decrease glutamate excitotoxicity (32). The ability of the galantamine-based therapy to prevent OP-induced convulsions and the well reported neuroprotective effects of galantamine against different insults (17–19, 34, 35) may be important determinants of the antidotal effectiveness. Because no cognitive impairment has been detected in soman-challenged animals when neuronal loss in their brains remains below a certain threshold (32), galantamine is likely to maintain normal cognitive performance of OP-exposed subjects.
Inhibition of brain AChE by >60–70% has been shown to trigger severe incapacitating effects, including seizures (36). Maximal degrees of inhibition of AChE activities observed in guinea pigs treated with doses of galantamine that effectively counteracted OP intoxication were ≈70% in blood and 25% in brain. All other centrally acting AChE inhibitors studied to date, including huperzine, acutely prevent OP toxicity when used at doses that decrease blood AChE activity by >70% (10–13). However, brain AChE activity is inhibited to a similar extent (≈70%) by these drugs (13). Therefore, a high degree of reversible and selective AChE inhibition in the blood appears to be necessary to counteract the peripheral toxic effects of OPs acutely. A low degree of reversible inhibition of brain AChE may be sufficient to protect a significant pool of the enzyme from OP-induced irreversible inhibition, and it may be critical to limit the occurrence of untoward side effects of centrally acting reversible AChE inhibitors.
Development of effective and safe antidotes against OP toxicity will help improve the treatment of the victims of a terrorist attack with nerve agents, and it will help reduce the mortality associated with OP insecticide poisoning worldwide. The demonstration that an acute galantamine-based therapy effectively and safely counteracts OP poisoning is, therefore, of utmost relevance for farm workers and others who handle OP insecticides, for the general population under threat of OP exposure in terrorist attacks, and for soldiers, who, despite the Geneva Protocol, may be exposed to deadly nerve agents in the course of battle.
Materials and Methods
Animal Care and Treatments.
Male albino guinea pigs [Crl(HA)Br; Charles River Laboratories, Wilmington, MA] weighing 350–420 g (5–6 weeks old) were used. Galantamine, pyridostigmine, or huperzine was injected in one hindlimb, and atropine was injected in the other. The nerve agents, diluted in sterile saline, and paraoxon, diluted in DMSO, were injected s.c. between the shoulder blades of the animals. All injections (≈0.5 ml/kg) were performed by using disposable tuberculin syringes with 25- to 26-gauge needles. Handling and disposal of nerve agents were according to the rules set forth by the U.S. Army. All conditions for animal maintenance conformed to the regulations of the Association for Assessment and Accreditation of Laboratory Animal Care, complied with the standards of the Animal Welfare Act, and adhered to the principles of the 1996 Guide for the Care and Use of Laboratory Animals (37). Atropine sulfate, pyridostigmine bromide, (±)-huperzine A, and paraoxon were purchased from Sigma–Aldrich (St. Louis, MO). Soman and sarin were obtained from the U.S. Army Medical Research and Development Command (Fort Detrick, MD). Galantamine·HBr was a generous gift from Alfred Maelicke (Galantos, Mainz, Germany).
Histopathological Analyses.
Guinea pigs were anesthetized at appropriate times after their treatments and transcardially perfused with 0.9% saline (70 ml/min) until blood was cleared and subsequently perfused with 10% formalin. Their brains were then removed, placed in 10% formalin for no longer than 48 h, dehydrated, and embedded in paraffin. Sections 5 μm thick were cut and then dried in an incubator at 37°C for 12 h before they were stained with FJ-B (38). After it was mounted, the tissue was examined under an epifluorescence microscope with blue (450–490 nm) excitation light and a filter for fluorescein isothiocyanate. Photomicrographs were taken with a digital microscope camera (AxioCam; Zeiss, Jena, Germany).
Analysis of Galantamine Concentrations in the Brain and Plasma of Guinea Pigs.
At various times after treatment with galantamine (8 mg/kg, i.m.), animals were anesthetized with CO2. Blood (5–10 ml) was collected by cardiopuncture with a plastic heparinized system and kept in dry ice. Immediately after cardiopuncture, the animals were exsanguinated by carotid artery transection. Their brains were removed, superfused with 0.9% saline, and snap frozen in liquid nitrogen. Frozen blood samples and brains were kept at −80°C until further processing. Brain and plasma levels of galantamine were measured by using a modified HPLC method (39).
Radiometric Enzymatic Assay.
Pulverized brain tissue was mixed with buffer containing antiproteases (0.5 unit/ml aprotinin, 30 μg/ml leupeptin, 1 mg/ml bacitracin, 2 mM benzamidine, and 5 mM N-ethylmaleimide) and sonicated for 20 s on ice. Aliquots of the resulting suspensions and of blood samples were used for determination of protein concentration (micro BCA protein assay; Pierce, Rockford, IL). Measurements of AChE activity were performed in the presence of the BuChE inhibitor tetraisopropyl pyrophosphoramide (1 mM) with a modified two-phase radiometric assay (40) using 20 pM [3H]acetylcholine iodide [specific activity, 76 Ci/mmol (1 Ci = 37 GBq); PerkinElmer Life Sciences, Boston, MA], which produced ≈200,000 cpm when totally hydrolyzed by eel AChE (2 units).
Behavioral Assays.
Locomotor activity and stereotypy of guinea pigs were analyzed in an open-field arena equipped with infrared sensors (AccuScan Instruments, Columbus, OH), as described by June et al. (41). Counts obtained from the total number of interruptions of the infrared beams were automatically compiled every 5 min and processed for measures of total distance traveled and stereotypy.
Acknowledgments
We thank Dr. Harry L. June and Dr. Jacek Mamczarz (Department of Psychiatry, University of Maryland School of Medicine) for guidance and assistance with the behavioral experiments. We are grateful to Dr. Robert Bloch (Department of Physiology, University of Maryland School of Medicine), Dr. David Burt (Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine), Col. George Korch (U.S. Army Medical Research Institute of Infectious Diseases, Fort Detrick, MD), Col. Dr. David Moore (Strategic Research Program Development, Office of the Commander, U.S. Army Medical Research Institute of Chemical Defense), and Dr. Susan Wonnacott (Department of Biology and Biochemistry, University of Bath, Bath, U.K.) for helpful comments. We are also indebted to Ms. Mabel Zelle for technical and editorial support and to Ms. Christina M. Tompkins and Ms. Denise M. Fath for excellent assistance with the histological techniques. This work was supported by U.S. Army Medical Research and Development Command Contract DAMD-17-95-C-5063, Battelle Scientific Services Contract TCN 03132, U.S. Public Health Service Grant NS25296 from the National Institutes of Health, and National Institutes of Environmental Health Sciences/National Institutes of Health Training Grant T32 ES07263 (all to E.X.A.). The use of galantamine as an antidote against OP poisoning is protected under the International Patent Application PCT/US05/33789 filed on September 23, 2005.
Footnotes
- †To whom correspondence should be addressed. E-mail: ealbuque{at}umaryland.edu
-
Author contributions: E.X.A., E.F.R.P., J.A.R., and M.A. designed research; E.X.A., E.F.R.P., Y.A., W.P.F., M.O., W.R.R., T.A.H., and R.K.K. performed research; E.X.A., E.F.R.P., Y.A., W.P.F., W.R.R., and R.K.K. analyzed data; and E.X.A. and E.F.R.P. wrote the paper.
-
Conflict of interest statement: No conflicts declared.
- Abbreviations:
- AChE,
- acetylcholinesterase;
- BuChE,
- butyrylcholinesterase;
- FJ-B,
- Fluoro-Jade B;
- OP,
- organophosphorus
-
Freely available online through the PNAS open access option.
- © 2006 by The National Academy of Sciences of the USA
References
Effective countermeasure against poisoning by organophosphorus insecticides and nerve agents
Sign up for Article Alerts
Jump to section
You May Also be Interested in
Recent experiments and simulations are starting to answer some fundamental questions about how life came to be.
Image credit: Shutterstock/Radoslaw Lecyk.
Archaeologists have long tried to define the transition between the two time periods.
Image credit: Ceri Shipton.
Jonathon Yuly, David Beratan, and Peng Zhang investigate how electron bifurcation reactions work.
A study finds that giant pandas roll in horse manure to increase their cold tolerance.
Image credit: Fuwen Wei.