Rapid reversal of non-depolarising neuromuscular block by the novel reversal agent Org 25969, which forms tight complexes with rocuronium bromide A.H. Bom, K. McMillan, J. Cottney. Department of Pharmacology, Organon Laboratories, Newhouse, Lanarkshire, ML1 5SH, Scotland, UK.

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Since the introduction of curare in anaesthetic practice in 1942, reversal of non-depolarising neuromuscular block can be obtained by the administration of an acetylcholinesterase inhibitor, often in combination with a muscarinic receptor antagonist. In 1997, we developed the idea that, by modifying cyclodextrins, tight complexes between neuromuscular agents and these sugars could be created, resulting in reversal of neuromuscular block. Using commercially available cyclodextrin derivatives, a structure-activity relationship was established and the concept was published (Bom et al., 2002)Based on this information, a chemical synthesis project was started resulting in the discovery of Org 25969 (Adam et al., 2002). Aim of this study was to compare the reversal effect of a single administration of Org 25969 with that of cumulative administration of Org 25969 in vitro.

Male mice (ICR, body weight: 20-30 g) were humanely killed. Each hemi-diaphragm with its phrenic nerve was placed on a tissue holder in a tissue bath filled with a modified Krebs-Henseleit buffer at 37ºC, bubbled with 95% oxygen and 5% carbon dioxide. The phrenic nerve was stimulated continuously using a Grass S88E stimulator (rectangular pulses of 0.2 ms every 20 sec at a supra-maximal voltage of 2.5 V) and isometric force was recorded using Grass FT03 transducers and a Grass 79D recorder. After a stimulation period of at least 30 min, 3.60 µM of rocuronium bromide was administered to induce neuromuscular block.

After 30 minutes, the preparations received either a single administration of 4.68 µM Org 25969 (Batch V) or cumulative administrations of Org 25969 at intervals of 10 minutes. Rocuronium was dissolved in a mannitol-phosphate buffer and Org 25969 was dissolved in distilled water.

After obtaining 99 ± 1 % neuromuscular block, single administration of Org 25969 resulted in 110 ± 2 % recovery of twitch height after 140 ± 22 sec (mean values ± SEM; n = 4).Cumulative administration of Org 25969 caused a maximum reversal of 88 ± 3 % at 2.43 µM Org 25969. 50% reversal was obtained at 1.69 µM Org 25969 (n = 4).

Administration of Org 25969 rapidly decreases the amount of free rocuronium molecules in the solution. This creates a gradient between the rocuronium concentration in the tissue and in the solution, resulting in a rapid removal of rocuronium from the neuromuscular junction. The rapid creation of a very large gradient after single administration of Org 25969 might explain the higher efficacy of reversal in comparison to the reversal observed after cumulative administration of Org 25969.

It should also be realised that not all rocuronium molecules will have to be complexed with Org 25969 to obtain complete reversal. The occupation of the nicotinic receptors in the neuromuscular junction only has to be decreased to ~75% to obtain complete reversal (Paton et al., 1967).

Adam et al. (2002) J Med Chem 45: 1806-1816.
Bom et al. (2002) Angew. Chem Int Ed 41: 265-270.
Paton et al. (1967) J.Physiol 191: 59-90.