ARTICLES 192 VOLUME 22 NUMBER 2 FEBRUARY 2004 NATURE BIOTECHNOLOGY Skin, the largest organ of the human body, offers a painless and com- pliant interface for systemic drug administration 1 . As compared with injections and oral delivery routes, transdermal drug delivery increases patient compliance, avoids metabolism by the liver, and pro- vides sustained and controlled delivery over long time periods. And yet, after nearly four decades of extensive study, the success of this technology remains limited, with only a few transdermal products available in the market, all of which are based on low molecular weight lipophilic drugs 2 . Development of transdermal products for macromolecules is hindered primarily by low skin permeability. Evolved to impede the flux of toxins into the body, skin naturally offers a very low permeability to the movement of foreign molecules across it 3 . A unique hierarchical structure of lipid-rich matrix with embedded corneocytes in the upper strata (15 μm) of skin, stratum corneum (SC), is responsible for this barrier 3 . Overcoming this bar- rier safely and reversibly is a fundamental problem in the field of transdermal delivery. Several technological advances have been made in the past couple of decades to overcome this barrier: iontophoresis 4 , sonophoresis 5 and chemical penetration enhancers 6,7 , to name a few. CPEs provide several advantages such as design flexibility with formulation chem- istry, patch application over a large area (>10 cm 2 ) and the absence of external physical delivery mechanisms. Several different classes of CPEs, including surfactants 8 , fatty acids and fatty esters 9 , have been studied for permeation enhancement. However, only a few induce a therapeutic enhancement of drug transport. This problem is aggra- vated for high molecular weight drugs, which are especially difficult to deliver. In addition, potent enhancers are also potent irritants to the skin at concentrations necessary to induce sufficient penetration enhancement 10 . Thus, improved enhancement with single enhancers inevitably leads to a compromise on safety issues. Attempts have been made to synthesize novel CPEs, such as Azone 11,12 ; however, achiev- ing sufficient potency without irritancy has proved challenging. Potent CPEs usually enhance skin permeability by altering the stra- tum corneum structure 13,14 . Because the stratum corneum consists of nonviable, keratinized cells, disruption of its structure does not induce irritation. However, CPEs are usually not selective towards stratum corneum lipids, and eventually they affect the viable epider- mal cells, thereby inducing irritation by interstitial release of cytokines and by triggering other inflammatory responses 15 . Our tests of 32 individual enhancers, chosen from a list of >250 older and newer CPEs, did not find any that achieved therapeutic lev- els of macromolecular skin permeability without irritation. However, these CPEs might still be used as building blocks to construct combi- nations of CPEs that are more potent. Here we describe the search for a new family of potent and safe permeation enhancers that we desig- nate synergistic combinations of penetration enhancers (SCOPE). SCOPE formulations are rare mixtures of known CPEs that exhibit high potency upon contact with the stratum corneum but a relative lack of irritation in the epidermis because of the differential retention of components in the stratum corneum. SCOPE formulations are dis- tinct from empirically formulated enhancer combinations reported in the literature 16,17 , which are usually designed from a limited num- ber of experiments and based on additive rather than synergistic effects. We describe examples of SCOPE formulations for transder- mal drug delivery and a method for discovering them. NLS:S20 and SLA:PP, two leading SCOPE formulations discovered via this methodology, increased the flux of macromolecules such as LHRH and heparin in vitro by 50- to 100- fold. The SLA:PP formulation also delivered leuprolide acetate in hairless rats without skin irritation. Department of Chemical Engineering, University of California, Santa Barbara, California 93106, USA. Correspondence should be addressed to S.M. (samir@engineering.ucsb.edu). Published online 4 January 2004; doi:10.1038/nbt928 Discovery of transdermal penetration enhancers by high-throughput screening Pankaj Karande, Amit Jain & Samir Mitragotri Although transdermal drug delivery is more attractive than injection, it has not been applied to macromolecules because of low skin permeability. Here we describe particular mixtures of penetration enhancers that increase skin permeability to macromolecules (∼1–10 kDa) by up to ∼100-fold without inducing skin irritation. The discovery of these mixtures was enabled by an experimental tool, in vitro skin impedance guided high-throughput (INSIGHT) screening, which is >100-fold more efficient than current tools. In vitro experiments demonstrated that the mixtures delivered macromolecular drugs, including heparin, leutinizing hormone releasing hormone (LHRH) and oligonulceotides, across the skin. In vivo experiments on hairless rats with leuprolide acetate confirmed the potency and safety of one such mixture, sodium laureth sulfate (SLA) and phenyl piperazine (PP). These studies show the feasibility of using penetration enhancers for systemic delivery of macromolecules from a transdermal patch. © 2004 Nature Publishing Group http://www.nature.com/naturebiotechnology