Distribution concentrates in skeletal muscle 7 15

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Distribution Concentrates in skeletal muscle (7-15 times plasma levels) and cardiac muscle (15-30% times plasma levels). This is really a drug which concentrates where it is needed. Probably eventually gets into whole body water, but slowly. Elimination Mostly eliminated unchanged by kidneys. Note: Elimination is slower in patients with kidney disease, etc. ADR’s ADR’s seen in up to 25% of patients. Most important: cardiac dysrhythmias. (“Any drug in excess…….”). Also: headache, fatigue, drowsiness, nausea and vomiting, visual disturbances, etc. Dysrhythmias can be treated with phenytoin, etc. Interactions Administration with diuretics may cause potassium depletion. Several drugs (quinidine, etc.) raise blood levels of digoxin (effect on renal elimination?). Neostigmine (= “non-proprietatry” name = “official” name) Galenical Source Physostigmine (eserine) derived from the calabar bean of West Africa. Neostigmine is a quaternary amine, a semi-synthetic form. Brand Name Prostigmin Drug Class Anticholinesterase, parasympathetomimetic (because it raises ACh levels). Related Drugs Physostigmine and other anticholinesterases. Mechanism of Action Reversible (competitive) inhibition of acetylcholinesterase. Some direct agonist action on nicotinic receptors of the neuromuscular synapse. pKa 12.0 Basic. (Would be charged at all physiological pHs.) Mean Vd 0.5-1.0 L/kg (some extravascular reservoir?) Plasma Protein Binding None
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Oral Bioavaility Probably less than 5% (but it can be taken by mouth). (Note: oral and injected doses of drugs may be very different.) Half-Life 50-90 minutes. Why so short? It’s partly broken down in the blood (plasma esterases), and would also be actively transported into the renal tubules since it’s a quaternary amine. Slower in patients with renal impairment. Absorption Poor oral absorption. (See above.) Distribution Probably to the whole body water eventually, but slowly . (Crosses membranes with difficulty.) Would not reach meaningful levels in the brain because of the BBB. Elimination 30% metabolized in the blood (plasma esterases and other enzymes). 70% excreted unchanged. (What would happen in a patient who showed an exaggerated response to succinylcholine?) ADR’s Most relate to increased ACh levels (extensions of the drug’s mechanism of action). Related to the muscarinic receptors of the smooth muscle : nausea, vomiting, diarrhea, abdominal cramps, bronchial secretion, miosis, sweating. Related to the nicotinic receptors of the nerve-muscle junction : muscle cramps, fasciculation. Interactions Additive with other parasympathomimetics. Phenytoin (= “non-proprietary” name = “official” name ) Galenical Source None. A pure synthetic, derivived from phenobarbital, another pure synthetic. Drug Class Anticonvulsant, antiarrhythmic (certain anticonvulsants can also be used to calm an over-excitable heart) Related Drugs Mephenytoin, ethotoin (two other hydantoins, both now obsolete) Mechanism of Action Binds to and slightly inhibits the voltage-dependent sodium channel – the channel that initiates action potentials in neurons and heart muscles.
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