Mechanism of Lidocaine Release From Carbomer–Lidocaine hydrogels

Rheology, acid‐base behavior, and kinetics of lidocaine release of carbomer–lidocaine (C-L) hydrogels are reported. A series of (C-L)x (x = mol% of L = 25, 50, 75, 100) that covers a pH range between 5.33 and 7.96 was used. Concentrations of ion pair ([R‐COO−LH+]) and free species (L) and (LH+) were determined by the selective extraction of (L) with cyclohexane (CH) together with pH measurements, i.e., CH in a ratio CH/hydrogel 2:1 extracted 48% of the whole concentration of lidocaine [LT] of a (C-L)100, {[LT] = ([R‐COO−LH+]) + (L) + (LH+)}. The remaining species in the aqueous phase were distributed as: (L) 3.82%, (LH+) 14.5%, and [R‐COO− LH+] 81.7%. Rheology and pH as a function of (C-L) concentration are also reported. Delivery rates of free base L were measured in a Franz‐type bicompartmental device using water and NaCl 0.9% solution as receptor media. (C-L) hydrogels behave as a reservoir that releases the drug at a slow rate. pH effects on rate suggest that, under the main conditions assayed, dissociation of [R‐COO−LH+] is the slow step that controls releasing rates. Accordingly, release rate was increased upon addition of a second counterion (i.e., Na+), or through the diffusion of neutral salts such as NaCl, into the matrix of the gel. © 2002 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:267–272, 2002