Transdermal delivery of antiparkinsonian agent, benztropine. I. Effect of vehicles on skin permeation

Abstract

The influence of pH and various lipophilic and hydrophilic vehicles on the epidermal permeation of benztropine (BZ) free base and its mesylate salt were studied in vitro using the hairless mouse (HLM) and human cadaver (HC) skin membranes. The pH-partition behavior of BZ base (p K a=10) was examined using n-octanol and Britton–Robinson buffers over the pH range of 5–12. Unexpectedly, the ionized species of BZ yielded a high partition coefficient (log K octanol/water=2.14), which was reflected by its relatively high skin permeability ( P=1.6×10 −2cm h −1). BZ base delivered from a lipophilic vehicle with a solubility parameter range of 7.1–10.3 (cal cm 3) 1/2 exhibited a significantly enhanced rate of permeation as compared to that attained from a hydrophilic vehicle of solubility parameter range between 12.5–23.4 (cal cm 3) 1/2. Among the neat solvents examined, a lipophilic carrier, isopropyl myristate (IPM) provided the most enhancing effect on the permeation of BZ base. In addition, the neat IPM carrier offered the maximum BZ base flux of 150 μg per cm 2 h −1 across HC skin, which was approximately 16 times greater than the target delivery rate of BZ from a 10-cm 2 device. In comparison, BZ base exhibited a 2–60 times greater flux than BZ mesylate when delivered from the neat solvents. However, interestingly enough, the binary cosolvents consisting of IPM and short-chain alkanols such as ethanol (EtOH), isopropanol (iPrOH), and tertiary butanol (tBtOH), in particular a 2:8 combination, produced a marked synergistic enhancement of BZ flux from the mesylate salt, whereas a retarding effect was noticed for the permeation of BZ base. The enhancement potency for the BZ mesylate permeation increased linearly with the carbon number of the branched alcohols present in the binary mixtures. A tBtOH-IPM (2:8) combination produced the highest BZ flux from the mesylate salt, i.e., 2016 mg per cm 2 h −1, which was 100-fold greater than from water and 44–540-fold greater than the individual neat solvents, respectively. The observed permeation enhancement of BZ mesylate by the alkanol-IPM mixtures was probably as a result of a combination of decreasing barrier ability of the stratum corneum by the binary vehicles and moderately partitioning BZ mesylate through the viable epidermis/dermis.