Plasma concentrations of bupivacaine after brachial plexus administration of liposome-associated and plain solutions to rabbits.

Bupivacaine has been associated to multilamellar liposomes with the aim of altering circulating plasma concentrations after injection into the rabbit brachial plexus. Plasma concentrations of bupivacaine have been compared after administration of free drug (BP) or bupivacaine associated to multilamellar liposomes (BP-MLV) made of phosphatidylcholine and cholesterol (molar ratio 4:3). Under light general anaesthesia, one group of six rabbits received an axillary injection of 2.5 mg BP (1 ml, 0.25%), and a second received the same dose of BP-MLV. In both groups 3H bupivacaine was used as a marker. The brachial plexus was located using a nerve stimulator. Injection of the anaesthetic solutions invariably prevented the motor response of the paw. The arterial plasma concentrations of bupivacaine were determined after 5 to 240 min and after 24 hr by beta counting. In the MLV population, additional measurements were performed after 48 and 72 hr. The two plasma curves showed a plateau (0.2 microgram.ml-1) which was reached after five minutes in the BP group and after 90 min using BP-MLV. In the BP-MLV group, the plasma concentrations of bupivacaine were lower during the first ten minutes (P < 0.05), and higher after 24 hr (P < 0.05). Radioactivity decreased between 4 and 24 hr in the BP group and between one and two days in the BP-MLV population. It is concluded that elevated plasma drug concentrations were maintained for longer with BP-MLV than with BP. This could prolong the action of the local anaesthetic through a slow release.