Effect of ion pairing on in vitro transcorneal permeability of a Δ9-tetrahydrocannabinol prodrug: Potential in glaucoma therapy

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  • Effect of Ion Pairing on In Vitro Transcorneal Permeabilityof a 9-Tetrahydrocannabinol Prodrug: Potentialin Glaucoma Therapy


    1Department of Pharmaceutics, School of Pharmacy, The University of Mississippi, University, Mississippi 38677

    2Research Institute of Pharmaceutical Sciences, The University of Mississippi, University, Mississippi 38677

    3National Center for Natural Product Research, The University of Mississippi, University, Mississippi 38677

    4ElSohly Laboratories, Inc., Oxford, Mississippi 38655

    Received 21 February 2011; revised 10 August 2011; accepted 27 September 2011

    Published online 11 October 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jps.22791

    ABSTRACT: The aim of the present study was to evaluate and improve the in vitrotranscorneal permeability characteristics of 9-tetrahydrocannabinol (THC) through prodrugderivatization and formulation approaches. In vitro corneal permeability of THC and itshemisuccinate (THC-HS) and hemiglutarate (THC-HG) ester prodrugs and WIN 55-212-2(WIN), a synthetic cannabinoid, was determined using isolated rabbit cornea. The formula-tions studied included hydroxypropyl beta cyclodextrin (HP$CD) or randomly methylated betacyclodextrin (RM$CD), as well as prodrugion-pair complexes with L-arginine or tromethamine.Corneal permeability of WIN was found to be two-fold higher than THC in the presenceof HP$CD. THC-HS and THC-HG exhibited pH-dependent permeability. In the presence ofHP$CD, at pH 5 (donor solution pH), both prodrugs exhibited six-fold higher permeability com-pared with THC. However, permeability of the prodrugs was about three-fold lower than that ofTHC at pH 7.4. RM$CD, at pH 7.4, led to a significant improvement in permeability. Formationof ion-pair complexes markedly improved the solubility and permeability of THC-HG (seven-fold and threefold greater permeability compared with THC and WIN, respectively) at pH 7.4.The in vitro results demonstrate that the use of an ion-pair complex of THC-HG could be aneffective strategy for topical delivery of THC. 2011 Wiley Periodicals, Inc. and the AmericanPharmacists Association J Pharm Sci 101:616626, 2012Keywords: absorption; drug transport; formulation; passive diffusion/transport; permeability


    In 1971, Hepler and Frank1 published a report thatlinked marijuana smoking to a significant drop in

    Abbreviations used: BCH, 2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid; CB1, cannabinoid receptor type 1; CB2, cannabi-noid receptor type 2; DPBS, Dulbeccos phosphate buffered saline;HP$CD, hydroxypropyl beta cyclodextrin; IOP, intraocular pres-sure; IPBS, isotonic phosphate buffered saline; RM$CD, randomlymethylated beta cyclodextrin; THC, 9-tetrahydrocannabinol;THC-HS, 9-tetrahydrocannabinol hemisuccinate; THC-HG, 9-tetrahydrocannabinol hemiglutarate; THC-HGARG, ion pair of9-tetrahydrocannabinol hemiglutarate and L-arginine; THC-HGTRIS, ion pair of 9-tetrahydrocannabinol hemiglutarate andtromethamine; WIN, WIN 55-212-2.

    Correspondence to: Soumyajit Majumdar (Telephone: +662-915-3793; Fax: +662-915-1177; E-mail: majumso@olemiss.edu)Journal of Pharmaceutical Sciences, Vol. 101, 616626 (2012) 2011 Wiley Periodicals, Inc. and the American Pharmacists Association

    intraocular pressure (IOP). Because of its implica-tions in the treatment of glaucoma, this report stim-ulated intense research toward identification of theconstituents responsible for this pharmacological ac-tion. It was established that 9-tetrahydrocannabinol(THC, Fig. 1a), a primary active constituent of mar-ijuana, is one of the components responsible for theIOP lowering effects.2 During the course of further in-vestigations, a reduction in IOP was observed whenTHC was administered either orally or intravenouslybut not when applied topically.3,4 This lack of topicalactivity, although some reports did demonstrate thattopical delivery of THC significantly lowered IOP,5 ledresearchers to conclude that the IOP lowering mech-anism of THC was probably because of its centrallyacting hypotensive effect and not due to activation oflocal ocular receptors.



    Figure 1. Chemical structures of (a) 9-tetrahydrocannabinol (THC), (B) 9-tetrahydrocannbinol hemisuccinate (THC-HS), (c) 9-tetrahydrocannbinol hemiglutarate(THC-HG), (d) WIN 55-212-2 (WIN).

    However, recent studies suggest that THC canlower IOP and act as a neuroprotective agent bybinding to the cannabinoid receptors expressed inthe ocular tissues. In the 1990s, two cannabinoid re-ceptors type 1 and 2, CB1 and CB2, were identifiedand cloned.6 THC acts as an agonist for both CB1and CB2 receptors. Affinity values for the CB1 andCB2 receptors are 5.05 and 3.13 nM, respectively,7

    whereas the half-maximal effective concentration val-ues for the CB1 and CB2 receptors are 6 and 0.4 nM,respectively.8 Although the distribution of cannabi-noid receptors in the body, since their identifica-tion, has been largely delineated, only recently havecannabinoid receptors been identified in the oculartissues. CB1 receptors are expressed in the trabec-ular meshwork, iris, ciliary body, and the retina,912

    whereas CB2 receptors have been found in the retinaand trabecular meshwork.13 These locally expressedcannabinoid receptors are now believed to be involvedin the IOP lowering and neuroprotective activity of anumber of endocannabinoid and synthetic cannabi-noid derivatives.

    There are two major pathways for the drainageof aqueous humor from the anterior ocular seg-ment: drainage through the Schlemms canal or theuveoscleral route. Activation of the CB1 receptorsin the ciliary muscle, by CB1 receptor agonists, in-duces contraction of the ciliary muscle.14 Contractionof the ciliary muscle leads to widening of the inter-cellular spaces in the trabecular meshwork and en-hances outflow of aqueous humor.15 Recently, bimato-prost, a prostaglandin analog that enhances uveoscle-ral outflow, has been shown to contract the humanciliary muscle through CB1-mediated mechanism.16

    Furthermore, activation of CB1 receptors leads tofragmentation and reduction of actin stress fibersin the trabecular meshwork, further enhancing out-flow of aqueous humor.9 Thus, THC can reduce IOPthrough the local CB1/CB2 receptors. This can alsobe inferred from a previous clinical study. Merrittet al.5 demonstrated that 0.1% THC in mineral oilwhen given topically led to a 5.4 mm drop in IOP butwas accompanied with a 12 mm drop in systolic bloodpressure. A 10 mm drop in systolic blood pressure,

    following systemic THC administration, should be as-sociated with less than a 1 mm drop in IOP.17 Also,0.05% THC in mineral oil, topically administered, ledto a 4.8 mm drop in IOP with no systemic hypotensiveeffect. Thus, these data suggest that topical THC isprobably acting locally through the ocular cannabi-noid receptors to reduce the IOP, and not through thesystemic pathway.

    In glaucoma, a reduction in IOP is often not enoughto prevent or arrest the development or progressionof glaucoma-related optic neuropathy.18,19 Vision losscontinues even after significant IOP reduction hasbeen achieved. It has been suggested that neuroglialcell cytotoxicity in the optic nerve and retina leadsto visual field loss in glaucoma.20 Neuroglial cell tox-icity impairs macroglial glutamate metabolism andcauses microglia to release inflammatory cytokinesfollowing ischemia because of compression or vascu-lar occlusion. The released glutamate acts on recep-tors, including the N-methyl-D-aspartate subtype, onthe retinal ganglion cells to induce calcium influx andthe release of toxic reactive oxygen species, leading toapoptosis.20 Recent studies have demonstrated thatCB2 or nonspecific CB1/CB2 agonists were able toprotect retinal cells from oxidative stress but specificCB1 agonists had no effect.21 Intravitreally adminis-tered THC has been demonstrated to act as a neuro-protective in a rat model of glaucoma.20,22 Hampsonet al.23 also reported that the neuroprotective activ-ity of THC could also be independent of CB1 receptoractivation. Although the mechanism of neuroprotec-tion of THC is not yet clearly understood, it could bedue to the activation of CB2 receptors, its antioxidanteffect, or some other mechanism.2325 The current ev-idence clearly suggests that THC possesses both IOPlowering and neuroprotectant activity, which are in-dependent of each other.

    Although significant efforts have been directed to-ward understanding the pharmacology of THC, de-sign of effective topical delivery strategies for THChas not seen much activity. In 1977, Green et al.26

    published a paper comparing corneal penetration ofTHC from different oils and found that light mineraloil was the best of the four vehicles tested. The study



    reported a 20% drop in IOP of normotensive rabbitswhen a 50:L dose (0.1%, w/w THC) was adminis-tered topically. However, plasma drug concentrationswere not reported. All further pharmacological stud-ies, with THC administered topically, were carried outusing light mineral oil as the vehicle. However, THCbeing a highly lipophilic molecule, with an aqueoussolubility of only 12 :g/mL and a log P of 6.3,27 itseffective partitioning from the oily vehicle into thetear film would be a suspect. Kearse and Green28 com-pared in vitro corneal permeability of THC from dif-ferent vehicles and observed that the permeability ofTHC from light mineral oil was only 1.86 108 cm/s.Thus, the lack of topical activity observed in the ear-lier in vivo reports could be due to the i