1. OPC Poisoning- Role of oximes, Controversies and newer compounds explored
Dr Binila Chacko
MBBS, MD Gen Med, DNB, FCICM, DM Crit Care
Professor
Medical ICU
Christian Medical College, Vellore

2. Major problem developing countries
Introduction
Major problem developing countries
Epidemic proportions in India
60% of deliberate self harm (unpublished data-Vellore)
he National Crime Records Bureau of India reported in its 2012 annual report, that 135,445 people committed suicide in India, of which 13,755 were farmers (11.2%)
One farmer’s suicide every 30 minutes

3. Mortality and morbidity
Unpublished Vellore data
92.13% required intubation and ventilation
Mean duration of ventilation 7.75 (6.07)days
Mortality 8.4%
West India data
34.21% ventilated
10.5% mortality
Data from other places
Sri Lanka 15.8%-24.8%
Pakistan 17.39%
Mega dose intoxications
Use of class I compounds
Delay in presentation to hospital
Concentrated formulations
Significant problem/generally over 30% ventilated invasively/mortality ranges from 10-20%

4. CIRCULATING ORGANOPHOSPHATE ABSORTION OF TOXIN THROUGH SKIN AND GUT
N-acetyl cysteine
Glycopyrrolate
Magnesium
DECREASE ABSORPTION
Atropine
OXIDATIVE STRESS
Bronchorrhea
Decremental
muscle response
MODULATE EFFECTS
Adenosine
Clonidine
Magnesium
Diazepam
Atropine
Atropine
Respiratory depression, Seizures
Arrhythmias, long QTc
CIRCULATING ORGANOPHOSPHATE
OP- AChE Complexes
ABSORTION OF TOXIN THROUGH SKIN AND GUT
Oximes
NEUTRALIZE TOXIN
Albumin (FFP)
BuChE (FFP)
ENHANCE ELIMINATION
Hydrolases
BIO-REMEDIATION
Alkalinisation
Hemoperfusion

5. Managing the OP poisoned patient
Reducing toxin load
Neutralizing toxin
Counteracting effects
Enhancing elimination

6. Neutralize toxin - oximes
Are oximes beneficial in human OP poisoning?
Subject of much debate & literature
Reasons for failure

7. What are they? Nucleophilic agents
Reactivate bound acetyl cholinesterase
Common – pralidoxime and obidoxime

8. Oximes - animals Early animal experiments
Oximes administered either before the poison or just after the poison
Regeneration of cholinesterase
Animals survived
Peter JV, Cherian AM. Organic insecticides. Anaesth Intensive Care 2000; 28: 11-21

9. Oximes - humans Initial reports retrospective
Prospective non-randomized trials
Randomized controlled trials
Systematic reviews and meta-analysis

10. Oximes – controlled trials
Retrospective studies – 4
Prospective (non-randomized) studies - 2
Randomized placebo controlled – 4
Randomized trials – different doses - 3

11. So how do we look at all this evidence?

12. Meta-analytic evidence
Mortality
Ventilation
Meta-analytic evidence
Peter JV, Moran JL, Graham P. Advances in the management of organophosphate poisoning. Expert Opin. Pharmacother. 2007; 8: 1-13
Oxime vs. Control arm trials only
Need for ICU

13. Oxime therapy – Pawar trial
High dose oximes – 50 gm in 48 hours
The first, performed in Maharashtra, India, recruited 200 patients to a comparison of pralidoxime.  All were given a 2 g loading dose of pralidoxime over 30 min. Patients were then randomly assigned to control and study groups. Controls were given a bolus dose of 1 g pralidoxime over 1 h every 4 h for 48 h. The study group had a constant infusion of 1 g over an hour every hour for 48 h. Thereafter, all patients were given 1 g every 4 h until they could be weaned from ventilators. Analysis was by intention to treat. 
This study showed reduced mortality, from 8% to 1% [adjusted relative risk 0.11 (95% CI 0.01, 0.84)], as well as a reduced incidence of pneumonia and intubation postrandomization, and a reduced need for atropine and ventilation 39.
Pawar KS, Bhoite RR, Pillay PP et all. Continuous pralidoxime infusion versus repeated bolus injections to treat organophosphorus poisoning: a randomised controlled trial. Lancet 2006; 368:

14. Oxime therapy – Pawar trial
Pawar KS, Bhoite RR, Pillay PP et all. Continuous pralidoxime infusion versus repeated bolus injections to treat organophosphorus poisoning: a randomised controlled trial. Lancet 2006; 368:

15. Oxime therapy Largest trial - 235 patients, PAM=121, Saline=114
2 gm loading dose over 20 min
Then 0.5 g/hr for maximum 7 days
Till atropine not required for h or death
Eddleston M, Eyer P, Worek F et al. Pralidoxime in acute organophosphorus insecticide poisoning – A randomised controlled trial. PLoS Med 6(6): e doi: /journal.pmed

16. Oxime therapy
To reduce confounding due to the ingestion of different OP insecticides, a further analysis was performed of patients with laboratory‐proven chlorpyrifos or dimethoate poisoning alone; again, this found no evidence of benefit 40. Of note, uniquely, this study measured red cell AChE activity both before and after pralidoxime administration, finding clear reactivation in patients receiving pralidoxime and proving adequacy of dosage.
Eddleston M, Eyer P, Worek F et al. Pralidoxime in acute organophosphorus insecticide poisoning – A randomised controlled trial. PLoS Med 6(6): e doi: /journal.pmed

17. Summary of oxime trials
Meta-analysis - null effect or potential harm
The largest oxime study trend to harm
Pune study - reduction in mortality with high dose
Moderate poisoning (severe to GH)
Patients presented very early (< 2.5 hours)
Used fairly high doses (50 gm)
Both studies were performed in South Asia, and recruited patients poisoned by WHO Class II toxicity OP insecticides, but found different results. One major difference between the studies was the severity of illness, with moderate to severely ill patients being excluded from the Maharashtrian study, as shown by the lower mortality in the control arm (8% vs. 14%). By contrast, the Sri Lankan study recruited all patients who required atropine, thereby excluding only the ‘least poisoned’ patients. In addition, all Indian patients were routinely admitted to the intensive care unit of a private hospital, where they received one‐to‐one nursing care and intensive support. This differed considerably from the Sri Lankan study, which was performed in a government district hospital with few intensive care beds. The majority of these patients were treated, at least partially, on the open medical wards with a dearth of nursing and medical staff.

18. Why is there no benefit?

19. Reasons for oxime failure
Study design
Timing
Dose of oximes
Type of compounds
Toxicity of antidotes

20. Study design Some retrospective and prospective studies
Only 7 RCTs (4 placebo controlled and 3 trials of different doses)
Some methodological flaws in design
More RCTs - stratified?

21. The timing CMC study Pune study
Peter JV, Moran JL, Graham P. Advances in the management of organophosphate poisoning. Expert Opin. Pharmacother. 2007; 8: 1-13

22. The dose
Initial study by Cherian – 12 gm over 3 days – considered too little
Pawar study used 50 gm – showed benefit
Sri Lankan study – relatively high dose 12 gm/day – no benefit
2 new studies (2015 & 2016 from India – high dose (n=100 & 150) – no benefit

23. Type of compound Di-methyl OP compounds Eddleston study
Rather resistant to oxime therapy.
Eddleston study
Reactivation happened with di-ethyl but not di-methyl
No difference in mortality
Median pseudocholinesterase levels were lower in survivors who received placebo than those who died with pralidoxime
Dimethylphosphoryl-inhibited human cholinesterases: inhibition, reactivation, and aging kinetics. Arch. Toxicol 1999; 73:7-14

24. Toxicity of antidote Oximes by themselves can cause muscle weakness
Rapid infusion causes dizziness, flushing, numbness
Formation of stable phosphoryl oximes (POXs) with high anticholinesterase activity
Additionally toxicity of the solvent large amount of solvents ingested
The phosphoryl oxime-destroying activity of human plasma. Arch. Toxicol 2000; 74:27-32

25. Aging of the OP-AChE complex..
Phosphoryl oximes (POXs)
De-alkylation of the OP over time with hydrolysis results in aging of the OP-AChE site..Oximes can’t mop up the OP and
Oxon inhibition of AChE and BChE occurs by phosphorylation of the active site of the enzyme, which proceeds with a half-life of about 0.7 h for dimethyl organophosphate compounds and approximately 31 h for diethyl organophosphate compounds.5
The phosphorylated enzyme can also be reactivated by treating with strong nucleophilic agents, such as oximes.
Ageing occurs spontaneously when the OP–enzyme conjugate is de- alkylated; the leaving group being either a methanol or ethanol depending on the side groups of the OP compound.
The enzyme is now aged, i.e. permanently phosphorylated and irreversibly inhibited.
The half-life of ageing for dimethyl organophosphate compounds is around 3 h and for diethyl organophosphate around 30 h.
This means oximes may be useful to regenerate the enzyme up to about 12 h for dimethyl and about 5 days for diethyl organophosphate compounds (four half lives).11
T1/2 aging Dimethyl 3 hours and diethyl 30 hours

26. Oximes Oxime therapy in humans not shown to be beneficial
Several reasons why it may not work
Early presenters (<2 h) with diethyl poisoning, there may be role
Good supportive care important for good outcome

27. Other agents

28. Neutralize toxin - Bioscavenger
Cholinesterases to sequester OP
Purified human BChE under trials in animals
Has a long shelf life (lyophilised form) > 2 years
Single dose in mice – therapeutic concentration for at least 4 days
Potential for immunological responses limited as human in origin

29. Neutralize toxin - Bioscavenger
Still not commercial
So alternative – FFP – rich in cholinesterases
Concentrations is > 4000 IU/L in FFP

30. Neutralize toxin - Bioscavenger
Preliminary study of 33 OP poisoned patients
20 received atropine and PAM and 9 in addition 2 bags of plasma daily from day 2 till they no longer needed atropine
Intermediate syndrome did not occur in plasma arm

31. Neutralize toxin - Bioscavenger
Is cholinesterase the secret ingredient in FFP?
The amount of cholinesterase in FFP alone cannot account for the clinical improvement
Recent animal studies suggest that albumin is also a bioscavenger
Our BIOSTOP study

32. BIOSTOP study

33. BIOSTOP study

34. Pyridostigmine bromide (PB), a quaternary carbamate compound was approved to manage organophosphate toxicity. People who are under the risk of organophosphate exposure were recommended to administer this compound as a prophylactic agent. The compound reversibly inhibits AChE and butylcholinesterase (BChE) without crossing blood brain barrier. Therefore, organophosphate compounds bind with AChE in a less extent. Immediately af

35. Modulate effects Anti-cholinergic agents Magnesium Clonidine Diazepam
N-acetyl cysteine

36. Role of nebulised beta 2 agonists
Facilitate fluid elimination from the lungs.
Up-regulation of sodium transport in the alveolar epithelium
However, it might also work to increase fluid elimination from the lung. Poisoned patients' lungs become filled with fluid (bronchorrhoea) during acute cholinergic crisis, a clinical feature that is treatable with atropine. However, atropine only halts the production of fluid and does not speed its removal from the lung. A treatment that increases removal, complementing atropine's cessation of fluid production, could speed the return of effective oxygen exchange.
Although the mechanism is not fully understood, it might be similar to the alveolar fluid collection seen in the acute respiratory distress syndrome. Studies of fluid transport across the alveolar epithelium indicate that the epithelium is actively involved in oedema fluid clearance 58. Upregulation of sodium transport with beta‐adrenergic agonists such as salbutamol increases fluid egress in animal models. A nebulized dose of salbutamol – available in rural hospitals worldwide – might be able to access the alveolar epithelium and increase the rate of fluid removal in OP poisoning
The recent Balti‐2 study looked at the effect of 7 days of intravenous salbutamol in patients with established acute lung injury and showed no benefit 59. However, this is unlikely to be relevant to poisoned patients because of the different route of administration (nebulized) and time course (given only during resuscitation over the first hour or so after presentation). The disadvantage of this route is that the pulmonary fluid might reduce the ability of salbutamol to access the alveolar epithelium. To gather initial pilot safety data, a small pilot study (NCT ) has been started in Bangladesh.

37. Nicotinic receptor antagonist drugs
?role in preventing intermediate syndrome
Role of neuromuscular blocking drugs
In a porcine model of OP poisoning
Encouraging protective effects on NMJ function following early administration of rocuronium (Eddleston et al., unpublished observations).
Might prevent subsequent nicotinic receptor overstimulation and NMJ dysfunction.
A pilot RCT of this approach is currently underway in Sri Lanka (NCT ).
OP‐induced NMJ dysfunction or intermediate syndrome causes patients to require mechanical ventilation for days or weeks 14, 54, 55. Its pathophysiology is uncertain but is likely to be due to overstimulation of pre‐ and/or postsynaptic nicotinic acetylcholine receptors by excess acetylcholine, their downregulation and the failure of neurotransmission 55, 56. Supportive ventilation is the only current therapy.
Atropine is given to poisoned patients to block muscarinic overstimulation. However, neuromuscular blocking agents (nicotinic acetylcholine receptor antagonists) are not currently used to prevent nicotinic overstimulation 57. In a porcine model of OP poisoning, we found encouraging protective effects on NMJ function following early administration of rocuronium (Eddleston et al., unpublished observations). This suggests that the administration of neuromuscular blocking agents to patients already requiring ventilation for the predominantly central respiratory failure typical of acute cholinergic crisis might prevent subsequent nicotinic receptor overstimulation and NMJ dysfunction. A pilot RCT of this approach is currently underway in Sri Lanka (NCT ).

38. Lipid emulsions
Lipid sink for treatment of severe cardiotoxicity from local anesthetics and other including antipsychotic agents and antidepressants 
? Role in lipid soluble OP insecticide poisoning
However, a rodent study of unformulated parathion showed little benefit
One case report suggested a benefit after parathion poisoning 
No formal Phase II/III trial done
Over recent years, the use of lipid emulsions for the treatment of severe cardiotoxicity has spread from bupivacaine to other cardiotoxins, including antipsychotic agents and antidepressants 60. The exact mechanism is unclear but proposed mechanisms include a direct effect on cardiomyocyte metabolism and an intravascular ‘sink’. Whereas some clinicians are keen advocates 61, others are more cautious, calling for better quality clinical evidence before widespread adoption 62.
Lipid emulsion has been used for the treatment of the lipid‐soluble surfactant cardiotoxicity seen in glyphosate poisoning 63. It has also been proposed for OP insecticide poisoning 64 because some OP insecticides are highly lipid soluble and formulated in lipid‐soluble solvents. However, a rodent study of unformulated parathion showed little benefit 65. One case report suggested a benefit after parathion poisoning 66. A formal Phase II/III RCT on this intervention is required

39. Enhancing elimination

40. Enhance elimination OP hydrolases
An alternative approach to treating OP insecticide poisoning would be to enhance their hydrolysis in the blood, reducing toxicity and enhancing the effect of other antidotes.
Research on medical counter‐measures for highly potent chemical nerve agents, such as sarin and tabun, has focused on pretreating people with the mammalian enzymes butyrylcholinesterase and paraoxonase, sometimes in concert with oximes, to break down OP compounds in the blood after exposure 67. However, butyrylcholinesterase is a stoichiometric inhibitor, binding on a one‐to‐one basis with the OP. As it is poorly reactivated by coadministered oximes 68, it will only be able to bind a single OP molecule, limiting its efficacy. Paraoxonase is enzymatic, and able to break down multiple OP molecules, but it is highly specific for its substrates 69, similarly limiting its usefulness.
Bacterial OP hydrolases have been isolated 70, 71 and shown after intravenous administration to be highly effective at breaking down a variety of OP insecticides in the blood 72. Their immunogenicity will limit repeated administration in the short term, similar to streptokinase, but they might be useful for treating acutely poisoned patients. However, it is unclear whether the funds are available to develop such a treatment for this patient population and there appears to be little movement towards their clinical development.

41. So where do we go from here?

42. OP management over the years..
Freeman and Epstein – Atropine
Wilson and Ginsburg –Pralidoxime
1955
Bio scavenger therapy
Toxin neutralizers
Magnesium
Diazepam
Clonidine
Beta 2 agonists
Nicotine receptor antagonists
Modulating effects of OP
Lipid emulsion
Decreasing toxin levels
What has made the difference?
We are back to atropine-in Vellore-used atropine alone-decrease in mortatliy over the years-what has made the difference-prompt resuscitation and supportive care-improvement in intensive care support of the patient. Compounds? Have they changed over the years?
Intensive care – big difference to outcome
Pre-ventilation era – mortality >50%
1999 – 2005 data CMC – 15% mortality

43. Presently – don’t expect anyone with OP poisoning to die
Improving mortality
Characteristic
2006
(44 patients)
2007
(51 patients)
Age: Mean ± SD years (Range)
27.7 ± 8.8 (14-56)
30.9 ± 11.8 (16-65)
M:F ratio
29:15
31:20
Pseudocholinesterase levels*
402 ± 204 (86-984)
245 ± 185 (95-888)
Number needing mechanical ventilation
39 (88.6%)
41 (80.4%)
Duration of mechanical ventilation (Mean ± SD days)
10.3 ± 11.0
6.7 ± 5.7
ICU length of stay (Mean ± SD days)
8.8 ± 8.0
8.2 ± 5.7
Hospital length of stay (Mean ± SD days)
15.8 ± 12.2
11.7 ± 7.0
ICU mortality
4 (9.1%)
3 (5.9%)
Presently – don’t expect anyone with OP poisoning to die
Peter JV, John G. Management of acute organophosphorus pesticide poisoning. Lancet 2008; 371: 2170

44. Conclusion Supportive care and early atropinisation Oximes debatable
Currently no evidence of benefit
Recent interest in
Rocuronium
Beta 2 agonists
Lipid emulsions
Although multiple antidotes have been proposed for the treatment of OP insecticide‐poisoned patients, only a single antidote – the muscarinic receptor antagonist atropine – has clear evidence of efficacy and an undisputed role in management

45. Acknowledgment Dr JV Peter and Dr George John
Medical ICU team CMC Vellore