Mivacurium chloride (formerly recognized as BW1090U81, BW B1090U or BW1090U) is a short-duration non-depolarizing neuromuscular-blocking drug or skeletal muscle relaxant in the category of non-depolarizing neuromuscular-blocking drugs, used adjunctively in anesthesia to facilitate endotracheal intubation and to provide skeletal muscle relaxation during surgery or mechanical ventilation. Structure Mivacurium is a symmetrical molecule existing as a mixture of three of twenty possible isomers: the isomerism stems from chirality at the C-1 carbon position of both the tetrahydroisoquinolinium rings, as well as both the positively charged nitrogen (onium) heads, and the E/Z diastereomerism at the C=C double bond of the oct-4-ene diester bridge. Thus, owing to the symmetry and chirality, the three isomers of mivacurium are (E)-1R,1'R,2R,2'R, (identified as BW1217U84), (E)-1R,1'R,2R,2'S, (BW1333U83) and (E)-1R,1'R,1'S,2'S, (BW1309U83). These are also known as cis-cis, cis-trans and trans-trans mivacurium. The proportions are; (E)-cis-cis 6% of the mixture, (E)-cis-trans 36% of the mixture and (E)-trans-trans 56% of the mixture. Unlike the potency of the cis-cis isomer of atracurium (also known as 51W89 and eventually produced as the drug cisatracurium), the cis-cis isomer of mivacurium has by far the lowest potency as a muscle relaxant when compared with its other two stereoisomers. It has approximately 10% of the activity of each of the other two structures. Mivacurium belongs to a class of compounds that is commonly and erroneously referred to as "benzylisoquinolines;" mivacurium is in fact a bisbenzyltetrahydroisoquinolinium agent, often abbreviated to bbTHIQ. The orientation of the two O atoms in the bridge is to the THIQ side of the carbonyl C=O group, whereas in atracurium the O atom is on the bridge side. Atracurium's groups are "reversed ester" linkages. This makes ester hydrolysis degradation by plasma cholinesterase more favourable. Pharmacology Having ten methoxy -OCH3 groups, mivacurium is a more potent neuromuscular blocking drug than atracurium (which has eight), but is less potent than doxacurium (which has twelve). Like other non-depolarizing neuromuscular blocking agents, the pharmacological action of mivacurium is antagonism to nicotinic acetylcholine receptors. However, unlike other non-depolarizing neuromuscular blockers, it is metabolized by plasma cholinesterase (similar to the depolarizing neuromuscular blocking agent succinylcholine). Availability Mivacurium is available worldwide. It became unavailable in the United States in 2006 due to manufacturing issues, but was reintroduced in 2016. History Mivacurium represents the second generation of tetrahydroisoquinolinium neuromuscular blocking drugs in a long lineage of nicotinic acetylcholine receptor antagonists synthesized by Mary M. Jackson and James C. Wisowaty, PhD