MESTINON

Myasthenia gravis is an autoimmune disease involving dysfunction at the neuromuscular junction, most commonly due to autoantibodies directed against the acetylcholine receptor (AChR), which results in muscle tone loss, muscle weakness, and fatigue.

Acetylcholinesterase inhibitors have been the symptomatic treatment of choice in myasthenia gravis since the 1930s with the early use of physostigmine and neostigmine.

By inhibiting the breakdown of acetylcholine in the neuromuscular junction, they increase signalling and relieve symptoms. 2, 10, 11 Pyridostigmine is the current drug of choice, with superior pharmacokinetics and reduced side effects compared to neostigmine. 10, 11 In addition to treating myasthenia gravis, pyridostigmine is used to reverse neuromuscular blocks, relieve symptoms in congenital myasthenic syndromes, and protect against certain nerve agents, notably during the Gulf War. 3, 4, 11, 12 Pyridostigmine was granted initial FDA approval on April 6, 1955, as an oral tablet.

Possible dose forms have been expanded to include extended-release tablets, syrups, and injections, marketed under various brand and generic names. 10, 11.

Pyridostigmine is indicated for the treatment of myasthenia gravis.

When administered

Intravenous, it is indicated for the reversal or antagonism of the neuromuscular blocking effects of nondepolarizing muscle relaxants.

Pyridostigmine has also been used as a prophylactic agent against irreversible organophosphorus acetylcholinesterase inhibitors, primarily in a military capacity.

Pyridostigmine bromide, designated as 3-hydroxy-1-methyl-pyridinium bromide dimethyl-carbamate, is an Oral active reversible cholinesterase inhibitor similar to neostigmine but with a milder adverse effect profile and a longer duration of action.

Pyridostigmine may, specifically in the case of excessive administration, result in a cholinergic crisis, with symptoms mimicking a myasthenic crisis.

Administration of atropine is recommended in the case of a true cholinergic crisis or to counteract muscarinic/nicotinic effects such as bradycardia and excessive bronchial secretions. 10, 11.

Pyridostigmine administered Oral is poorly absorbed in the GI tract, with an oral bioavailability of only 10-20%.

However, this may in part be due to some metabolism by both the blood and liver. 5, 12 Approximately 1-2 hours following a single oral dose of 60 mg, the C max was determined to be 40-60 μg/L.

Pyridostigmine follows approximately linear kinetics, with a direct correlation between the dose and plasma AUC.

Pyridostigmine taken

Oral with food results in a flatter peak to the plasma concentration vs. time curve.

The peak plasma concentrations are reached ~90 minutes later than in fasted subjects but with no change in bioavailability or AUC.

Pyridostigmine administered

Intravenous in healthy, myasthenic, and surgical patients has a range of distribution volumes, between 0.53 and 1.76 L/kg.

Pyridostigmine is hydrolyzed by cholinesterases systemically, including in the blood, and by microsomal enzymes in the liver, though this remains poorly defined.

The primary hydrolysis product is 3-hydroxy-N-methyl-pyridinium (HNM), which can be glucuronidated. 5, 12 One study suggested the existence of as many as eight metabolites in the urine of patients receiving radiolabeled pyridostigmine Intravenous, including various glucuronidated, demethylated, and oxidized (quinone) metabolites. 7, 12 Another study confirmed that HNM is the main metabolite, and suggested additional possible metabolites such as a 3,4.

• or 3,6-dihydroxy-N-methyl-pyridinium or a methoxy.

• or acetoxy-N-methyl-pyridinium.

The exact products formed, apart from HNM, require further validation.

Hover over products below to view reaction partners Pyridostigmine 3-hydroxy-N-methyl-pyridinium.

Pyridostigmine is primarily renally eliminated.

Roughly 90% of an intravenous dose is recovered in the urine within 24 hours, with unchanged pyridostigmine and its main metabolite HNM recovered in an approximately 4:1 ratio. 5, 8, 12.

Pyridostigmine administered

Intravenous in healthy, myasthenic, and surgical patients has a range of elimination half-lives, between 0.38 and 1.86 hours.

Pyridostigmine administered

Intravenous in healthy, myasthenic, and surgical patients has a range of clearance values, between 0.29 and 1.0 L/h/kg.

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information regarding pyridostigmine is not readily available.

Patients experiencing an overdose are at an increased risk of severe adverse effects such as bradycardia, excessive bronchial secretions, and cholinergic crisis.

Symptomatic and supportive measures are recommended. 10, 11.

Although failure of patients to show clinical improvement may reflect underdosage, it can also be indicative of overdosage.

As is true of all cholinergic drugs, overdosage of pyridostigmine bromide may result in cholinergic crisis, a state characterized by increasing muscle weakness which, through involvement of the muscles of respiration, may lead to death.

Myasthenic crisis due to an increase in the severity of the disease is also accompanied by extreme muscle weakness, and thus may be difficult to distinguish from cholinergic crisis on a symptomatic basis.

Such differentiation is extremely important, since increases in doses of pyridostigmine bromide or other drugs of this class in the presence of cholinergic crisis or of a refractory or "insensitive" state could have grave consequences.

Osserman and

Genkins 1 indicate that the differential diagnosis of the two types of crisis may require the use of Tensilon TM (edrophonium chloride) as well as clinical judgment.

The treatment of the two conditions obviously differs radically.

Whereas the presence of myasthenic crisis suggests the need for more intensive anticholinesterase therapy, the diagnosis of cholinergic crisis, according to Osserman and Genkins 1 calls for the prompt withdrawal of all drugs of this type.

The immediate use of atropine in cholinergic crisis is also recommended.

Atropine may also be used to abolish or obtund gastrointestinal side effects or other muscarinic reactions; but such use, by masking signs of overdosage, can lead to inadvertent induction of cholinergic crisis.

For detailed information on the management of patients with myasthenia gravis, the physician is referred to one of the excellent reviews such as those by Osserman and Genkins 2, Grob 3 or Schwab 4,5.

The safety of pyridostigmine bromide during pregnancy or lactation in humans has not been established.

Therefore, use of pyridostigmine bromide in women who may become pregnant requires weighing the drug's potential benefits against its possible hazards to mother and child.

Pyridostigmine bromide tablets are contraindicated in mechanical intestinal or urinary obstruction, and particular caution should be used in its administration to patients with bronchial asthma.

Care should be observed in the use of atropine for counteracting side effects, as discussed below.

Tablet contains 60 mg pyridostigmine bromide.

The size and frequency of the dosage must be adjusted to the needs of the individual patient.

The average dose is ten 60 mg tablets daily, spaced to provide maximum relief when maximum strength is needed.

In severe cases as many as twenty-five tablets a day may be required, while in mild cases one to six tablets a day may suffice.

For information on a diagnostic test for myasthenia gravis, and for the evaluation and stabilization of therapy, please see product literature on Tensilon (edrophonium chloride).

USP, 60 mg are white, round, flat-faced, beveled edge tablets with a quadrisect functional score on one side and debossed with ‘ANI’ over ‘470’ on the other and are supplied in bottles of 100 (NDC 62559-470-01).

Store at 20° to 25°C (68° to 77°F) .

Dispense in original container.

USP in a dry place with the silica gel enclosed.

The safety of pyridostigmine bromide during pregnancy or lactation in humans has not been established.

Therefore, use of pyridostigmine bromide in women who may become pregnant requires weighing the drug's potential benefits against its possible hazards to mother and child.

Safety and effectiveness in pediatric patients have not been established.