Lidocaine tripotassium phosphate complex laden microemulsion for prolonged local anaesthesia: In vitro and in vivo studies

Lidocaine is widely used as a local anaesthetic in the clinical practice to manage pre- and post-operative pain, skin burns, etc. However, the short duration of action ( < 2 h) of marketed dosage forms limit their ability to meet clinical needs. Herein, we prepared a lidocaine-tPP(tri potassium phosphate)-complex loaded micro emulsion to achieve greater penetration, followed by destabilization of microemulsion in the skin layer to precipitate oil -complex to produce a depot effect in the skin for prolonging the effects of anaesthesia. The lidocaine-tPP-complex-microemulsion was compared with lidocaine base loaded microemulsion, marketed ointment USP and lidocaine HCl. The pseudo ternary phase diagrams at three S-mix ratios (1:2, 1:3 and 1:4; Pluronic F127: PEG 400) were constructed using Capmul MCM C8 EP as oil phase. The S-mix at 1:4 ratio showed large microemulsion area in comparison to 1:2 and 1:6 ratio. The lidocaine base (LD-1:4-ME10O45SM and LD1:4-ME20O45SM) and lidocaine-tPP-complex (LDC-1:4-ME10O45SM and LDC-1:4-ME20O45SM) loaded microemulsion batches (1:4 ratio) were thermodynamically stable. The ex vivo diffusion study showed sustained release up to 12 h with microemulsion batches, in comparison to lidocaine HCl (4 h) and ointment base (7 h). The selected LDC-1:4-ME20O45SM batch was non-irritating on the rabbit skin. In drug retention studies, LD-1:4-ME20O45SM and LDC-1:4-ME20O45SM batches showed 2.68- and 3.93-fold greater lidocaine retention in comparison to ointment USP. The radiant heat tail-flick test showed prolong local anaesthesia using LDC-1:4ME20O45SM in comparison to ointment USP. The findings suggest that lidocaine-tPP-complex loaded micro emulsion could be a potential strategy for providing prolong local anaesthesia.