Passive transdermal drug delivery has been used for more than 20 years, but faces certain limitations with the type of active molecules that may delivered through this route. Very few drugs can passively diffuse across the skin barrier at therapeutically useful rates. Cell ablation using a radio-frequency (RF) alternating electrical current is a well known, proven medical technology that can be applied to drug delivery to overcome this obstacle. RF ablation is a safe, low-cost technique and a familiar concept in medical practice. When properly modified, RF cell-ablation technology may also create microchannels in the skin's surface to enable transdermal delivery of drugs.   

RF ablation is a well-known medical technology to eliminate living cells. It is widely used to cut through tissues in minimally invasive operations or to destroy small tumors in the kidney and liver (1–5). RF ablation is performed by placing a conducting wire on a body area and passing an alternating electrical current at a frequency above 100 KHz (radio frequency) through the area. The ions in the cells adjacent to the electrodes vibrate as they try to follow the change in electrical current direction. These vibrations cause heat, which results in water evaporation and cell ablation. RF microchannels are created by placing a closely spaced array of tiny electrodes with very precise dimensions against the skin (see Figure 1). The alternating electrical current is transferred through each of the microelectrodes, ablates the cells underneath each electrode, and forms microscopic passages in the stratum corneum and outer dermis (6, 7). These RF microchannels penetrate only the outer layers of the skin, where there are no blood vessels or nerve endings. This action minimizes skin trauma and unpleasant sensations. The process is performed in seconds. Immediately after formation, the microchannels fill with interstitial fluid, which is responsible for the hydrophilic nature of the microchannels. As a result, microchannels serve as aquatic channels into the inner layers of the skin. They are embedded in the surrounding of the hydrophobic stratum corneum. For drug delivery, the microchannels may last up to 24 h. At 36 h, the delivery through treated skin returns to the values of intact skin. 

A major difficulty in penetrating the skin for drug delivery is that the texture and softness of the skin change from site to site and from person to person. To address these variations, a  unique microelectrode-array design was used. This microelectrode array design adapts to these differences in skin type within and between treatment sites. This design is essential to forming RF microchannels with consistent, well-controlled depths, enabling the drug to reach the capillary bed without unnecessary trauma to the inner layers of the skin (8).  

The "ViaDerm" (TransPharma Medical Ltd., Lod, Israel) system is an example of a microelectronic system based on RF cell ablation for transdermal drug delivery. The system consists of the device, which is used to pretreat the skin and form the RF microchannels in the outer layers of the skin, and a patch containing the drug, which is placed on top of the pretreated skin (see Figure 2). 

The device consists of a handheld electronic control unit and a microelectrode array. The control unit (see Figure 2a) is battery-operated, rechargeable, and reusable for at least 1000 applications. This particular device is available in three sizes (treatment area of 1, 2.5, or 5 cm

 ), depending on the desired dose of drug to be delivered (see Figure 3). 

The microelectrode array (see Figure 2b) contains hundreds of microelectrodes. The microelectrode array is disposable, low-cost and intended for one use only. The array is based on a proprietary design and made of biocompatible materials that are well-established in medical devices. Within a few seconds, the control unit and the array create an array of RF microchannels (see Figure 4), thereby preparing the treatment site for the patch containing the drug. After application of the patch (see Figure 2c) on the pretreated area, the drug passively diffuses from the patch through the RF microchannels into the inner layers of the skin and into systemic circulation (9). 

Transdermal delivery of large proteins is a novel and exciting method (10). No commercial technology currently available incorporates proteins into transdermal patches. The unique printed-patch technology for transdermal delivery of proteins complements RF cell ablation. The manufacturing method involves dispensing very small droplets of a concentrated protein solution on a transdermal liner in a predetermined pattern (see Figure 5). The liquid is dried, leaving a dry and thin layer of formulated protein on top of the liner. The highly water-soluble proteins are dissolved by the interstitial fluid that is secreted from the skin through the RF microchannels, thus forming a highly concentrated protein solution 

. The diffusion of the dissolved molecules occurs through the RF microchannels into the viable tissues of the skin across a steep concentration gradient. This process brings about a high delivery rate and a peak-blood profile of the drug resembling that of a subcutaneous injection (9). 

A dispensing–manufacturing technology widely used in the diagnostic industry was adapted to successfully manufacture the printed patches. This manufacturing method enables complete and flexible control of the drug load on the patch, control of patch size and shape, and high manufacturing yield with minimal protein losses. In addition, this method fully retains the stability and biological activity of the protein drug. Printed patches were used in studies in which human growth hormone, insulin, and teriparatide (hPTH

) were delivered in animals (guinea pigs and pigs) and humans (11). 

The skin's low permeability limits the types of drugs that can be delivered transdermally. Many drugs with a hydrophilic character permeate the intact skin too slowly to be of therapeutic benefit (10). Under RF cell ablation, pretreating the skin allows aquatic channels to form across the stratum corneum, which provides significant enhancement in the permeability of water-soluble compounds. Drugs that exhibit insufficient solubility in water can still benefit from the technology. By increasing solubility using various formulation approaches, such as  drug-cyclodextrin complexes or dissolving the drug in a water-alcohol mixture, the drugs are also able to permeate the skin. Table I shows the 

 skin permeability of various drugs in a dynamic diffusion-cell model using full-thickness porcine skin. The results show enhanced transdermal delivery with the hydrophilic compounds—granisetron hydrogen chloride (HCl) and lidocaine HCl. Lidocaine HCl is more water-soluble than diclofenac sodium and had higher delivery rates. The effect of the compound concentration on its delivery rate was shown with testosterone (2% versus 6% in aqueous solution) and lidocaine HCl (2% versus 5% in aqueous solution). The delivery rate increased linearly with the concentration of the loaded compound. 

Effect of molecular size on delivery rate

The effect of molecular size on the delivery rate of macromolecules through the treated skin was shown in a study using fluorescein isothiocyanate-labeled dextran molecules of various sizes: 10, 40 or 70 kDa. This 

 experiment tested the delivery of these macromolecules through a full-thickness porcine skin that had been pretreated with the device. An increase in molecular size brought about a decrease in delivery rate (see Figure 6). However, it is important to note that even the largest 70-kDa molecule was successfully delivered transdermally through the RF microchannels. 

Effect of patch technology on pharmacokinetic profiles

Based on the patch technology used, two types of drug profiles are feasible using the microelectronic system (ViaDerm).  When a patch based on a dry formulation is used (

, a protein-printed patch), a peak-drug profile is observed in the blood, which resembles a profile of a subcutaneous injection. Figure 7 shows the blood profile for delivering human growth hormone in pigs. The peak 

 (maximum concentration) is affected by the patch dose (see Figure 7a) or by microchannel density (see Figure 7b). Figure 7a shows that increasing the drug load on the patch resulted in a higher delivered amount. Figure 7b shows the effect of microchannel density on the efficiency of drug delivery. Increasing the microchannel density from 150 to 300 microchannels (MCs)/cm

  resulted in a much higher delivered amount without increasing the drug load on the patch. This technique also significantly increased the bioavailability of the drug. 

It is possible to achieve a sustained drug flux for 24 h by incorporating the active material into a moist matrix such as a hydrogel that serves as an infinite reservoir. The matrix holds the drug on the skin in a soluble form, which makes it available for delivery through the aqueous microchannels. The matrix releases the drug slowly at a rate suitable for delivery, enabling the drug concentration in the blood to be maintained as long as 24 h. This effect was shown in a human study of granisetron delivery. The study measured the transdermal delivery of granisetron (a charged, water-soluble molecule) during a period of 24 h. The drug was administered through  a small patch (5.6 cm

 ) using a microelectronic system based on RF cell ablation (the ViaDerm system). This delivery method was compared with other routes, including passive transdermal delivery, oral delivery (one tablet every 12 h), and intravenous (IV) delivery. The study was conducted on six healthy adult volunteers in each test group. The results (see Figure 8) revealed differences in the plasma-drug levels and profiles between the treatments. The IV and oral deliveries displayed a peak or peak-and-valley profile corresponding to the administration regimen. In contrast, the transdermal delivery through microchannels resulted in a concentration increase up to 9 h and a constant level up to 24 h, indicating that the channels enabled drug delivery for at least 24 h. The control group (marked as "ViaDerm untreated" in Figure 8) showed very low plasma levels across the entire time period, emphasizing that the skin pretreatment used to form microchannels enabled the transdermal delivery of this water-soluble molecule. 

The variability in drug levels in the blood of subjects treated with the system using RF cell ablation was similar to that measured for oral delivery. This finding confirms that RF microchannel formation is uniform and reproducible.   

Factors influencing bioavailability and delivered dose

A large delivered dose and high bioavailability are important for any drug-delivery method, particularly for costly macromolecule active materials. If the bioavailability of the protein using a specific delivery method is low (< 10–20%), there is a significant loss of protein. Alternative delivery routes for peptides and proteins result in low bioavailability compared with subcutaneous injection (12). A microelectronic system based on RF cell ablation using  printed-protein patches resulted in very high bioavailability of up to 40% relative to subcutaneous injection. Table II shows the  bioavailability of three drug molecules. The data show that a low (

, 6%) or high (up to 40%) relative bioavailability is attainable, depending on the ratio between the amounts of active material and microchannels. 

Drug delivery through microchannels using RF cell ablation may be affected by the molecular size of the molecule delivered, water solubility, concentration, microchannel density, duration of delivery, dosage forms, drug profile, type of patches, and drug accumulation.  

. No data exist regarding the limitation of the size of drug molecules that can penetrate the microchannels. The transdermal delivery of small-molecule drugs can be increased significantly by pretreatment. In addition, macromolecules such as peptides and proteins can also be delivered systemically through the skin using this technology. 

 The microchannels are filled with interstitial fluid. Water-soluble molecules, therefore, can be easily delivered through the microchannels of the inner layers of the skin and by systemic circulation. Water-insoluble drugs can be delivered transdermally using RF cell ablation by increasing the water solubility through a suitable formulation.  

 As in any passive delivery, the rate of delivery depends on the concentration gradient. Increasing the drug concentration on the skin in the vicinity of the microchannels will result in a higher delivery rate. 

 By increasing the microchannel density (MCs/cm

 ), a higher amount of drug can be delivered. This higher amount results in a more efficient delivery process or enables the delivery of a higher one-time dose. 

 The duration in which enhanced drug delivery can be observed is up to 24 h, after which the delivery rate will not be significantly higher compared with delivery using intact skin. The maximum patch application time should be 24 h.  

 A patch is the most convenient dosage form to use with the microelectrode system. A patch has a drug area size matching the size of the electrode array. Simpler dosage forms such as gels or creams, however, can also be used. Also, the delivery of the drug in semisolid-dosage forms on treated skin can be increased significantly compared with delivery on intact skin.  

. The result of the transdermal delivery using  RF cell ablation can be a peak-plasma profile or a constant blood level, depending on the type of patch technology used. 

 A reservoir patch, usually a water-based hydrogel, can be used to incorporate small or large molecules and apply them on the skin. A hydrogel patch maintains the skin in a hydrated state, and therefore enables drug delivery at a constant level for up to 24 h. For proteins, the use of a printed patch is advisable for stability purposes. The resulting blood profile is in a peak shape that resembles that of an injection. The use of printed patches also results in efficient and cost-effective delivery. 

 One of the most disturbing issues regarding passive delivery is the accumulation of drug in the stratum corneum because of the affinity between hydrophobic drugs and this lipidic tissue. A drug in the reservoir continues to be released into circulation long after the treatment stops, thereby decreasing the effectiveness of the treatment. The microelectronic system based on RF cell ablation used in this study delivered water-soluble drugs that cannot be accumulated in the lipidic stratum corneum. No issue of reservoir formation exists.  

Other transdermal drug delivery technologies 

Drug delivery using a microelectronic system based on RF  cell ablation offers a convenient, painless, and less invasive alternative to injection, a common method for administering large proteins and peptides. This system also allows for less costly manufacturing that may not require sterility or expensive processes such as lyophilization. In instances where multiple daily administrations are needed, the one-a-day application based on RF cell ablation avoids the need for multiple injections or continuous infusion.  

Although injection is the primary delivery method for large-molecule drugs, small molecules may also be delivered orally or through passive transdermal methods. Oral drug delivery methods suffer from low gastrointestinal (GI) absorption and high first-pass effect, which may lead to low oral bioavailability and potential adverse GI effects. A transdermal alternative overcomes these limitations and offers the benefit of immediate cessation of drug administration in case of an adverse effect or overdose. Transdermal delivery also enables verification if a drug were taken or not, thereby improving compliance and monitoring in dependent populations such as dementia patients. 

Although passive transdermal delivery methods provide these benefits, they do not allow for the delivery of water-soluble drugs, a capability inherent in RF cell ablation. As earlier discussed, passive transdermal delivery often leads to the formation of a drug reservoir in the stratum corneum because of  the affinity of the lipophilic active drug to the stratum corneum. The buildup of this reservoir results in a long lag time before therapeutic concentrations in circulation are reached, a problematic titration process, as well as a long elimination process in treatment termination. RF cell ablation overcomes this problem because of the hydrophilic nature of the drugs it is able to deliver. 

 Iontophoresis is another transdermal delivery option. This approach uses an electrical repulsion of a relatively low voltage to drive molecules through the intact skin. Iontophoresis is successful only with drug molecules of up to 1000 Da. Because delivery through the intact skin requires formulations of certain pH levels, the target drug needs to be ionized for successful delivery. Iontophoresis can only be applied for short periods of time because of possible skin irritation caused by the electrical current. Only drugs with short-delivery duration, therefore, can be used with this technology. Delivery by RF cell ablation is not limited by these factors. There is no molecular size limitation, no molecular electrical charge requirement, and no specific formulation pH constraint.  

 Microneedles represent another transdermal delivery option. Microneedles create a matrix of holes in the skin using an array of ultra-sharp microscopic needles; however, they too suffer from significant limitations. The amount of drug that can be coated on microneedles is limited, so only very potent drugs are suitable for this technology. In addition, a complex formulation is needed to enable the drug to be efficiently coated onto the microneedles. Delivery through microneedles results in a sharp peak profile, compared with the more extended release the delivery system based on RF-cell ablation.   

Articles

Drug Delivery

A microelectronic system based on radio-frequency (RF) cell ablation addresses limitations of other transdermal drug-delivery methods. This system expands the transdermal spectrum to include the delivery of water-soluble molecules, peptides, proteins, and other macromolecules.

Advances in Radio-Frequency Transdermal Drug Delivery

Various carriers have been developed for local or systemic delivery of small-molecule drugs and biologics. The general approaches include liposomes (1–2), copolymer micelles (3), hydrogels (4), microemulsion-based media (5), dendrimers (6), cyclodextrins (7), pectin (8), tocols (9), and synthetic polymer microspheres (10). The materials used to make these carriers are called biomaterials (11). The US Food and Drug Administration, and therefore the entire industry, is shifting increasingly away from poorly-defined biomaterials that may include animal products to highly-defined, animal-component-free biomaterials. Control over structure and origin has long been a selling point of synthetic polymer-based materials for use 

. Pertinent concerns in the development of a biomaterial for drug delivery include biocompatibility, efficiency of drug loading, and timescale of drug release. A broad range of methods has been developed to characterize the physical, chemical, and biological properties of biomaterials (12). Some FDA-approved biomaterials lead to side effects in some applications. The breakdown products of poly(lactide-co-glycolide) microspheres, for instance, are highly acidic, lowering the local pH and increasing the severity of inflammation in tissues (13). For such reasons the level of interest in developing novel biomaterials has never been greater. 

 Polypeptide multilayer nanofilms (PMNs) have recently been developed as multifunctional biomaterials (14). Surprisingly, perhaps, the earliest work on PMNs was for optical coatings (15) and electrochemical electrode coatings (16). It was later proposed to make PMNs out of derivatives of poly(L-lysine) in which "functional" peptides, for example hormone molecules, were grafted onto monomer side chains via the free amino group (17). PMNs are polyelectrolyte multilayer films made largely if not entirely of polypeptides (14). 

Polypeptides are interesting for biomaterials development because they are "natural." Peptides constitute half of the dry mass of an organism and they are biodegradable. Moreover, peptides can exhibit extremely high biological specificity and their breakdown products are generally not only benign, but indeed useful as food (18). Furthermore, polypeptides can be designed; an astronomical number of chemically different peptide structures can be realized, and a large number of different peptides can be prepared in huge quantities by various methods (19–20). Peptides designed for PMN fabrication can be protein inspired (21). 

Polyelectrolyte multilayer films were first studied systematically by Decher and colleagues (22–23). Polyelectrolytes suitable for multilayer film fabrications can be classified as "strong" or "weak," depending on how they are affected by pH. "Strong" polyelectrolytes show limited dependence on pH, "weak" polyelectrolytes the opposite (24). Polypeptides are weak polyelectrolytes. The isoelectric point of a peptide, the pH at which its net charge equals 0, is a strong function of amino acid composition (25). The author's research group has performed studies to determine how the PMN architecture can be altered with changing pH. 

PMNs are fabricated by electrostatic layer-by-layer self-assembly (LBL). The method is simple and repetitive (see Figure 1). Electrostatic interactions enable and limit polymer adsorption. Electrostatic attraction holds peptides together between layers, and electrostatic repulsion drives peptides apart within a layer. As a result, the researcher can control layer thickness to within about 1–10 nm. The LBL method is versatile. Many different chemical species are suitable. The method is environmentally benign: The most useful solvent for most current applications of LBL is water. 

Several charges are needed on a polymer for it to overcome the dissipative tendency of thermal energy and bind to a surface. This resembles the length requirement of an oligonucleotide to prime polymerization of the complementary DNA strand. Relatively little heat is exchanged on binding of a polyelectrolyte to an oppositely charged surface, so 

 is temperature (in degrees Kelvin), and 

 is entropy. In other words, adsorption is largely driven by an increase in entropy. This increase is as a result of the release of small counterions to solution as side-chain charges in the newly adsorbed polymers form ion pairs with side chains of opposite polarity on the surface of the nanofilm. A useful compendium of essays on experimental and theoretical aspects of multilayer films is available (26), as is a brief summary of the physics of nanofilm fabrication (34). 

It should be noted that the polyelectrolyte film fabrication method outlined above places no strict geometrical requirement for the substrate. Indeed, these films have been built on substrates of varying surface roughness and shape. Spherical particles in the nanometer to micrometer range have attracted considerable attention (27–28). There are three basic approaches with spherical particles: coating microparticles when the particles are the material of interest (29), trapping a material of interest that has been adsorbed onto template particles (30), and creating hollow spherical shells by dissolving the "dummy" template particles after nanofilm fabrication for subsequent "loading" and "release" of molecules of interest (31–33). All these approaches present interesting possibilities for systemic or local drug delivery. 

The work of the author's research group has been comprehensive, encompassing the physics, chemistry, and biology of multilayer nanofilms made of polypeptides (14, 21, 34). With regard to physics, we have investigated how the electric charge, hydrophobic surface, and side-chain hydrogen bonds of peptide structures contribute to PMN assembly, structure, and stability (35). Amino acid side chains have rather different chemical properties. With regard to biology, we have investigated how PMN designs influence cell behavior 

 (36–37). We have been especially interested in the extent to which we can control cellular phenotype 

 by encoding biological information in peptides for PMN fabrication.  

In fundamental studies of the role of electronic charge density, hydrophobic interactions, and side-chain hydrogen bonds in PMN fabrication, structure, and stability (35), we have found that changes in peptide structure can have a surprisingly large impact on PMN fabrication, overall nanofilm thickness, and nanofilm density for a given number of polymer-adsorption steps during fabrication. Control over film thickness and density are potentially useful for determining the ability of the film to act as a diffusive barrier, which can provide differential controls over the rate of release of encapsulated materials (see Figure 2). We have validated PMN fabrication on a wide range of substrates with widely ranging electronic properties. 

We have also explored coding regions of the human genome for sequences that are suitable for PMN fabrication (38). These sequences will generally have a large number of charged side chains per residue and a large net charge at neutral pH. Both conditions are needed for control over peptide solubility and binding affinity in PMN. Such sequence information is also potentially useful for controlling the immunogenicity of individual peptides and PMNs. PMNs intended for contact with blood, for instance, could be designed using sequence information from known blood proteins, tested experimentally, and modified as needed to achieve the desired properties. 

Our chemical inquiries have focused on structural alterations of PMNs with changes in pH. Naturally, pH will influence the architecture and behavior of a PMN. The choice of peptides that go into the PMN therefore must take into account their behavior in environments of various acidities, so that the desired architecture can be achieved. At neutral pH, acidic side chains (for example, the carboxylic acid group of glutamic acid) and basic side chains (for example, the amino group of lysine) are ionized with high probability. The tendency to be ionized at a given pH is even higher inside a PMN. At neutral pH, and in the absence of proteases, a PMN is in essence indefinitely stable (39). In contrast, at acidic pH, glutamic acid side chains become protonated. This raises the electric potential of the entire nanofilm, increases the electrostatic repulsion between positively-charged peptides, and creates a net flow of small negative counterions into the film. The overall effect is gradually to delaminate the PMN. The rate and overall extent of delamination can be controlled by changing the structure of peptides used for PMN fabrication. Control over PMN stability

 is potentially useful for delivery of encapsulated drugs. 

PMN stability can be controlled in at least two more ways: by incorporating recognition sequences in peptides and by peptide crosslinking (14). Various proteases are known to recognize specific amino acid sequences. Such information could be used to determine the overall susceptibility of a peptide or PMN design to proteolysis and therefore the stability of the peptide or PMN in a specific biochemical environment. Peptides for PMN fabrication could be made of nonnatural amino acids to limit proteolysis. 

The amino acid cysteine has a highly reactive thiol group in its side chain. Under oxidizing conditions, two cysteine side chains will form a disulfide crosslink, whereas under reducing conditions a disulfide bond will break (25). Disulfide bonds are therefore a way of controlling PMN stability (14, 40–41). Thiol reactivity is potentially useful for drug delivery from a PMN in at least two ways. In one approach, bioactive molecules are trapped within a disulfide crosslinked film under oxidizing conditions, for instance the extracellular environment (see Figure 3). The crosslinks are severed when the PMN enters a reducing environment, as can be found, for example, inside cellular endosomes or lysosomes. The more acidic pH can cause the PMN to disintegrate, which then releases the encapsulated bioactive molecules. In a second approach (42), bioactive molecules are conjugated to the peptides of the PMN by disulfide bonds. The bioactive molecules are then released from the PMN under reducing conditions. 

PMNs can be "functionalized" in various ways (14). We have been interested in how knowledge of various cell-receptor signals in extracellular matrix proteins could be used to alter the cytophilicity of PMNs (36–37). We have encoded various receptor signals into peptides, incorporated these peptides into PMNs, and then seeded cells on these PMNs 

. We have investigated how the resulting films influence cell adhesion, morphology, proliferation, and differentiation. We have found that some cell lines are considerably more sensitive than others to PMN architecture and thickness such as primary cells 

 transformed cells. Such knowledge may prove useful when drug makers design applications of the PMN technology. 

Cell sensitivity to PMN structure has suggested that PMNs could be useful for drug delivery 

 in localized delivery from an implantable medical device (14, 37). The development of 

 methods will be crucial for the future of stem cell therapeutics and regenerative medicine. Localized delivery of drugs from implant devices may increase the usefulness of implants such as glucose sensors. 

In conclusion, substantial progress has been made in the development of the PMN platform technology in the past few years. From multiple angles the prospects for commercializing the technology look good. The technology is promising for biomaterials development, and it has good potential to influence development of a variety of approaches to drug delivery. The technology could also be useful for development of multifunctional biomaterials. For instance, a variety of bioactive peptides could be used simultaneously to functionalize a PMN for different purposes, or various drugs could be encapsulated simultaneously for co-delivery. It is possible that other basic themes for technology development have yet to be identified. It is certain in any case that many variations on the known themes wait to be explored. 

, is vice-president of research and chief scientific officer at Artificial Cell Technologies, Inc., 5 Science Park at Yale, New Haven, CT 06511, tel. 1 203.772.3430, 

 1. A. Samad et al., "Liposomal Drug Delivery Systems: An Update Review," 

Lipospheres in Drug Targets and Delivery: Approaches, Methods, and Applications

 3. K. Kataoka et al., "Block Copolymer Micelles for Drug Delivery: Design, Characterization and Biological Significance,"

 4. A.S. Hoffman, "Hydrogels for Biomedical Applications," 

 5. M.J. Lawrence and G.D. Rees, "Microemulsion-based Media as Novel Drug Delivery Systems," 

 6. H.L. Crampton and E.E. Simanek, "Dendrimers and Drug Delivery Vehicles: Non-Covalent Interactions of Bioactive Compounds with Dendrimers

 7. R. Challa et al., "Cyclodextrins in Drug Delivery: An Updated Review," 

 8. L. Liu et al., "Pectin in Controlled Drug Delivery—A Review," 

 9. P.P. Constantinides et al., "Advances in the Use of Tocols as Drug Delivery Vehicles," 

Polymeric Drug Delivery I: Particular Drug Carriers

 (American Chemical Society, Washington, DC, 2006). 

 11. C. Alexander, "Theme Issue: Biomedical Materials,"

Biomaterials: The Intersection of Biology and Materials Science

 (Englewood Cliffs, New Jersey: Prentice-Hall, 2008). 

 13. J.M. Anderson and M.S. Shive, "Biodegradation and Biocompatibility of PLA and PLGA Microspheres," 

 14. D.T. Haynie et al., "Polypeptide Multilayer Films," 

 15. T.M. Cooper et al., "Preparation of Polypeptide-Dye Multilayers by an Electrostatic Assembly Process," 

 16. Y. Cheng and R.M. Corn "Ultrathin Polypeptide Multilayer Films for the Fabrication of Model Liquid/Liquid Electrochemical Interfaces," 

 17. J. Chluba et al., "Biomaterials with Bioactive Coatings" in US Patent Application 2004/0241292, Dec. 2, 2004. 

, 4th ed. (Garland Science, New York, 2002). 

Fmoc Solid Phase Peptide Synthesis: A Practical Approach

 (Oxford University Press, New York, 2000). 

 20. J. Howl, Ed., Peptide Synthesis and Applications" (Humana Press, Totowa, New Jersey, 2005). 

 21. D.T. Haynie et al., (2006) "Protein-inspired Multilayer Nanofilms: Science, Technology and Medicine," 

 22. R.K. Iler, "Multilayers of Colloidal Particles." 

 23. G. Decher, "Polyelectrolyte Multilayers, An Overview. In Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials (Wiley-VCH, Weinheim, Germany, 2003) 1–46. 

 24. J.W. Nicholson, The Chemistry of Polymers, 3rd Ed. (Cambridge: Royal Society of Chemistry) (2006). 

 25. J.M. Berg et al., Biochemistry, 6th Ed. (Freeman, New York, 2006). 

Investigators are exploiting the tremendous structural diversity of polypeptides and their biophysical properties to develop novel drug carriers. Peptide-based materials hold out much promise for tailor-made targeting, penetration, and release of contents in a host of biological microenvironments.

Polypeptide Multilayer Nanofilms in Drug Delivery

Fast-dissolving drug-delivery systems were first developed in the late 1970s as an alternative to tablets, capsules, and syrups for pediatric and geriatric patients who experience difficulties swallowing traditional oral solid-dosage forms (1). In response to this need, a variety of orally disintegrating tablet (ODT) formats were commercialized. Most ODT products were formulated to dissolve in less than one minute when exposed to saliva to form a solution that could then be more easily swallowed.  

Dissolvable oral thin films (OTFs) evolved over the past few years from the confection and oral care markets in the form of breath strips and became a novel and widely accepted form by consumers for delivering vitamins and personal care products. Companies with experience in the formulation of polymer coatings containing active pharmaceutical ingredients (APIs) for transdermal drug delivery capitalized on the opportunity to transition this technology to OTF formats. Today, OTFs are a proven and accepted technology for the systemic delivery of APIs for over-the-counter (OTC) medications and are in the early- to mid-development stages for prescription drugs. 

This article provides insight into the functionality and benefits of dissolvable films and reviews the current product landscape and market potential. The authors also explain how thin films are formulated and manufactured, and discuss potential future applications. 

Pharmaceutical companies and consumers alike have embraced OTFs as a practical and accepted alternative to traditional OTC medicine forms such as liquids, tablets, and capsules. OTFs offer fast, accurate dosing in a safe, efficacious format that is convenient and portable, without the need for water or measuring devices (2). OTFs are typically the size of a postage stamp and disintegrate on a patient's tongue in a matter of seconds for the rapid release of one or more APIs.  

The formulation of dissolvable films is customarily facilitated through aqueous polymer matrices that span a wide molecular weight (MW) range, thereby providing flexibility to achieve certain physical properties. With the selection of appropriate polymer excipients, these properties can be tailored to meet specific API-loading needs and dissolution rates. 

To date, the commercial launch of OTFs is primarily in OTC products addressing therapeutic categories such as cough/cold, sore throat, and antacid/gas relief as well as a number of nutritional supplement applications (2) (see Table I). In compliance with the appropriate monographs, current products deliver a specified API dose that is immediately released and ingested. Some drug forms are more easily used in OTFs than others (soluble versus nonsoluble), but we can expect API concentrations to increase as new OTF formulations are developed. 

The capabilities of the base technology to create dissolvable films continue to evolve. For example, a common misconception of the OTF format is that it is limited with regard to the loading capacity of APIs. Some researchers reference a limit of 30 mg of API content as the maximum concentration. A more accurate statement would be that OTFs have the capability to load APIs up to 50% of the unit dose mass, as demonstrated by Novartis Consumer Health's Gas-X thin film, which contains 62.5 mg of simethicone per dose (3). 

Strategic therapeutic categories for oral thin films  

In addition, the benefits of the technology continue to be studied. Some drug substances may be absorbed more rapidly through the oral mucosal and esophageal tissues via OTF formats during ingestion. Because the drug enters directly into the bloodstream and avoids hepatic first-pass metabolism, bioavailability may be improved, and the drug can be administered in smaller doses. Smaller doses translate to fewer side effects and potentially improved patient compliance. Oral mucosal delivery via OTFs could become a preferential delivery method for therapies in which rapid absorption is desired, including those used to manage pain, allergies, sleep difficulties, and central nervous system disorders. 

Functional film properties controlled during manufacturing 

The combination of the current platform capabilities, in conjunction with known and emerging benefits, suggests significant market potential for dissolvable films. According to the 2006 Technology Catalyst, Inc. (TCI) report, it is anticipated that the OTF market could reach $500 million this year and approach $2 billion in 2012. These estimates are contingent upon the launch of OTC formats in new categories such as smoking cessation products and the expected launches of generic and branded prescription drugs (3). We anticipate dramatic growth for OTFs over the next five years given several factors:  

The benefits of the thin-film format (fast, accurate, safe, and well-tolerated) are widely known and accepted by consumers (4)

The development of thin-film prescription drugs are already in the early- to mid-development stages

OTFs could potentially replace some orally dissolving tablets (ODTs) as a competitively priced alternative (3)

Consumer studies indicate patients prefer a fast-dissolving product over a standard tablet (5) 

OTFs may provide drug manufacturers with significant opportunities in life-cycle management, market expansion, and product differentiation

Acceptance of OTF technology in the OTC market has led to significant improvements in development cycle time with products potentially developed and launched within 12–16 months.

The TCI report forecasts growth only for the extension of OTF technology in oral immediate-release applications. The total market size expands if film technology is extended into emerging controlled-release and dermal or transdermal applications.  

Looking forward, there is speculation as to how generic and branded drug manufacturers will strategically introduce the OTF format, particularly given the erosion of branded ODT products to their generic counterparts. The potential exists for some manufacturers to bypass the launch of ODT products and move directly to the development of OTFs, particularly in generic products and profitable branded-product platforms (3). This approach has some regulatory merit, as the US Food and Drug Administration stated that an OTF abbreviated new drug appliation (ANDA) may be submitted as an alternative dosage form for a chewable tablet. This statement, was made in conjunction with an approved suitability petition for an ANDA for famotidine orally dissolvable film in a 10-mg dose (6). Future suitability petitions for ANDAs using OTFs to substitute for ODTs can be expected (3).  

. OTF formats can be considered for any therapeutic category in which an oral solid, liquid, or ODT format is currently offered. Ideal applications are those that use drug compounds that are potent and possess a narrow therapeutic range (7). In addition, OTFs are proven to be a more beneficial platform for compromised populations such as children or the elderly where a quick, well-tolerated platform aids administration (8). 

From a strategic standpoint, a group of therapeutic categories stand out as leading applications for the OTF format to expand beyond commercial OTC products on the market today (see sidebar, "Strategic therapeutic categories for oral thin films"). Development of a branded prescription OTF product for smoking cessation would set a precedent for the format, much like the launch of transdermal drug delivery patches for smoking cessation did in the 1990s. 

Addressing solid-dose manufacturing trends

Several emerging trends in solid-dose manufacturing may be more easily overcome by OTF formats than other solid forms. A push toward developing low-dose, high-potency formulations (6) is leading to improvements in drug formulation. These include the conversion of drug substances to new forms such as from salt- to free-base, modified crystal morphology, and nanoparticles, which may present challenges for traditional solid-dose manufacturing. For example, nanoparticle drug substances may segregate more easily than micronized counterparts during the powder-blending and powder-flow processes that lead into the direct-compression operations in solid-dose manufacturing. Because APIs typically are added into bulk fluids in various ways before casting OTFs (suspensions, emulsions, or solutions), new high-potency drug forms do not present any unique barriers to OTF formulation and manufacturing. 

A second trend is the increasing need for dosage formats using poorly soluble drugs (6). One solution to improving poorly soluble API delivery is through particle-size reduction such as nanoparticles. Other mechanisms include preparing the drug substance as a microemulsion or using co-solvents to solubilize the API. OTF formats can accommodate any of these techniques during manufacturing. 

 An inherent benefit of the OTF format to drug manufacturers is the flexibility of film formulation for a wide range of eligible APIs. A number of physical properties can be altered, including film-dissolution rates, material composition, and API absorption rates (5).  

 OTFs are produced by combining water-soluble components and additives to attain a desired dissolution rate when exposed to a fluid. By modifying the combination of film-forming polymers and film thickness, formulators can design films to release their APIs in seconds, or for controlled-release applications, to deliver the dose over a period of hours (see Figure 1). 

Formulators may select materials from several water-soluble polymers to deliver the desired film strength and disintegration properties. Examples of water-soluble polymers include natural gums, polyethelene oxide, derivatives of cellulose, polyvinyl pyrrolidone, polyvinyl alcohol, and acrylic-based polymers (7). The optimal properties of film strength and disintegration are obtained with the selection of a particular combination of hydrophilic polymers.  

Material sets are tailored to meet specific guidances and regulations. Scientists have various formulary options to address regulatory and compliance needs including, but not limited to:  

Bovine spongiform encephalopathy/transmissible spongiform encephalopathies (BSE/TSE)

Nongenetically modified organisms (non-GMO)

Analytical methods exist to characterize specific products and the potential impact of material selection on a product's performance.  

Thickness and mass play a role in the determination of dissolution rates. Constructions composed predominantly of low-MW, water-soluble polymers at different thicknesses will result in a range of rapid disintegration rates and mechanical strengths. High-MW polymers tend to impart good mechanical properties; however, disintegration time is increased (9).  

Future applications may require films that demonstrate specific dissolution rates to release drugs in different physiological conditions (e.g., neutral–alkaline pH). Such appliations can be created by adjusting film thickness or adding excipients that control the release. Dispersed-phase filler particles can be included to add bulk to the film, increase the solids portion to facilitate coating, or alter dissolution rates. When distributed throughout the film layer, air or other gases can also change the dissolution time (10).  

. The flexibility of the OTF delivery format provides an unlimited set of raw materials to select from when developing and delivering APIs. These materials include traditional aqueous-soluble and solvent-soluble (nonaqueous soluble) ingredients (2). API-containing liquid formulations may be in the form of a solution, emulsion, suspension, or dispersion.    

Solutions: all ingredients, including drug substances, are fully dissolved and soluble in the bulk liquid

Emulsions: these are typically used for aqueous formulations to which an oil-soluble ingredient such as a flavoring has been added

Suspensions or dispersions: nonsoluble APIs or other excipients may be added to the bulk-liquid formulation under careful controls before casting to assure the dispersed phase remains suspended for uniformity.

. Similar to other oral immediate- release drug delivery forms in which the dosage form contacts the tongue, taste-masking plays a critical role in the success of a product's acceptance and is an equally critical component of the film's formulation. Many of the same taste-masking techniques used to formulate syrups and soft-chew oral dosage medications can be used in OTFs. Traditional flavor and sweetener combinations, encapsulation or particle coating of the API, and complexation with ion-exchange resins are some of the techniques that may be successfully applied for masking bitter-tasting or objectionable materials in OTFs. When possible, taste-masking techniques resulting in discrete particles larger than 250 Âµm should be avoided, as these could potentially present uniformity challenges in the finished dosage form (7).  

Because OTFs may incorporate APIs with a narrow therapeutic index, tight manufacturing tolerances must be held to deliver a uniform pharmaceutical product. The manufacturing techniques used for OTFs are built upon the proven liquid-casting techniques used for decades in the commercial manufacturing of advanced polymers and coatings found in transdermal drug-delivery systems (4), 

 diagnostic devices, and other medical devices (see Figure 2). 

 Liquid casting is the preferred manufacturing method over 100% solids extrusion because it avoids exposing APIs to elevated temperatures that may cause drugs to degrade. Liquid casting also allows for film uniformity and low variability with regard to mass. A typical relative standard deviation (RSD) for uniformity testing of an oral thin-film batch prepared by liquid casting is on the order of 1–2% RSD (7).  

The casting station transfers a bulk solution from the mixing vessel into a thin film on the surface of a release liner (2) in widths of typically 30–120 cm. The preferred finished film thickness is typically 12–100 Âµm, although various thicknesses are possible to meet API loading and dissolution needs. Multiple casting techniques may be selected on the basis of the fluid rheology, desired applied mass, and required dosage uniformity. These coating techniques include knife-over-roll, reverse roll, slot-die, gravure cylinder, and Meyer rod coating.  

For pediatric and geriatric patients, fast-dissolving drug-delivery systems provide an easier way to take medications and vitamins. Oral thin films have evolved to provide systemic delivery of active pharmaceutical ingredients for over-the-counter and soon, prescription drugs. The authors review the practical benefits of dissolvable films, their manufacture, and their market potential.

Dissolvable Films: Dissolvable Films for Flexible Product Format in Drug Delivery

Commonly accepted convention within the pharmaceutical industry is that fewer blockbuster drugs will be developed in the foreseeable future as the human genome project takes effect and personalized medicine becomes more of a reality. Consequently, drug-discovery strategies have changed. More firms focus on developing new chemical entities that are targeted, have greater potency, and produce fewer side effects compared with conventional therapies. Many anticancer, antiviral, or central-nervous system drugs that are currently in clinical trials have these characteristics and will eventually require commercial production. 

Because the potency of these drugs is greater, the commercial quantities required are relatively small. Industry data suggest that around 20% of drugs in development pipelines may be classified as highly potent, up from about 5% in the 1990s (1). At the same time, high-volume manufacturing is shifting to lower-cost operations in India and the Far East, and Western manufacturers are refocusing their efforts by establishing high-potency manufacturing assets that provide multiple barriers to entry. Typically, these barriers are capital investment, production capabilities, technology, and regulatory compliance. Several Western producers of active pharmaceutical ingredients (APIs) have adopted this strategy by building or enhancing capabilities in manufacturing highly potent compounds (1). This shift is also occurring for secondary manufacturing of high-potency drugs (2).  

Liquid and semisolid encapsulation using two-piece hard-shell capsules is an ideal oral drug-delivery approach for poorly water-soluble compounds, compounds that exhibit poor bioavailability, and for highly potent drugs. From a manufacturing perspective, the production of two-piece hard-shell capsules provides an environment in which containment is easier and operates at a comparatively lower safety risk compared with powder filling or tablet manufacture that typically generate dust particulates. The authors review the challenges involved in the secondary manufacture of high-potency drugs. From their own experience, they further provide insight in designing and constructing a new high-potency secondary manufacturing facility for liquid and semisolid APIs and their manufacture into two-piece hard-shell capsules.  

High-potency APIs are classed according to their inherent toxicity characteristics and pharmacological potency, which are translated into an occupational exposure limit (OEL). Typically, APIs with OELs at or below 10 micrograms per cubic meter (μg/m

 ) of air as an 8-h time-weighted average are considered potent from an occupational health perspective. There is no universally accepted system of categorization, although most seem to use a four-category system (see Table I). The four categories range from Category I, low-potency compounds requiring conventional good manufacturing practices (GMPs), to Category IV, for which a high degree of containment is required (3). 

The quantification of OELs and category classification is typically defined by the end of Phase-II or during Phase-III clinical studies. This process, however, does not address the classification of preclinical and early-development compounds. For early-stage compounds, it is important to have all relevant toxicological or compound-comparative data available to ensure an appropriate analysis of the risk and level of containment required to maximize safety.  

Approximately 28% of the drugs currently on the market are considered potent by these criteria, according to SafeBridge Consultants. The same firm estimates that approximately 40–45% of the OELs that have been set by the pharmaceutical industry are at 10 μg/m

  or less. Supporting data relating to drug-development pipelines confirm a substantial and growing market (4). 

 Data published by management consultants Becker Associates suggest there are 300–400 highly potent compounds currently in production for either clinical evaluation or commercial manufacture (5). 

Many high-potency molecules such as cytotoxics, hormones, steroids, potential mutagenics, or ingredients that may produce toxic intermediates are frequently assigned OELs of less than 1 μg/m

 . Working to these levels requires more planning and risk analysis to ensure that every stage of manufacture and waste disposal is safe for workers and the local environment (4).  

The availability of integrated formulation and secondary manufacturing services for oral drug products is scarce within the pharmaceutical industry. There are several factors for this situation as follows:   

Capital costs for facilities and equipment

Product and waste-management containment and remediation. 

Bowtle has extensively reviewed the strategies for formulation development (6). These options have mushroomed during the past few years so that poorly water-soluble compounds are now the norm rather than the exception. The key objective is to scope out the most suitable liquid or semisolid formulation from which a final oral dose product can be produced. Liquid excipients may be used in this process and can involve the use of both off-the-shelf or proprietary excipients (see Figure 1). 

Risk-reduction in high-potency secondary manufacture

The exposure risks associated with handling solid, highly potent drug substances can be substantially reduced if a liquid or semisolid formulation can be developed. This situation is based on the reduced operator exposure through the removal of a potential dust hazard during the manufacturing process that further facilitates ease of API transfer during the formulation stage and cleaning of the production equipment. From the authors' experience, a liquid or semisolid formulation can also reduce processing time through easier handling and transportation and can drive down overall process control. 

There are several advantages to incorporating crystalline highly potent APIs into liquid, semisolid, or suspension formulations. These advantages are as follows:   

Increasing operator safety through reducing potential exposure 

Removing the potential risk of fire or explosion 

Substantially increasing the ease of handling, transfer and cleaning

Additional advantages of liquid-filled two-piece capsules are summarized in Figure 2. 

Containment, and most importantly, the safety of production and associated staff, is the key issue facing manufacturers of high-potency products. This focus means a higher level of investment is required that involves:  

Increased operator expertise through specialist training 

Validation of facility and equipment to meet target OELs. 

The construction of appropriate barrier isolation equipment such as glove boxes, isolator units, and manufacturing equipment represents a high capital investment, particularly if off-the-shelf equipment needs to be modified to meet the lower OEL target limits. 

Liquid-fill, two-piece capsules may be used in the delivery of high-potency drugs.  

Griffiths has produced a comprehensive review of safety as a design feature in secondary manufacturing that includes a "protection-cascade" approach (7). This approach involves multilevel containment by which the primary concern is the facility's ability to ensure product quality, worker safety, and protection of the external environment. It supports the concept that the project and operations teams must provide a clearly defined system of overlapping layers of protection and control throughout a facility and its operations (see Figure 3). 

The ability to efficiently clean through effective validation procedures is a critical design aspect in the construction of a general-purpose high-potency facility. The overall elements as part of the people, process, prefacility construction, and equipment planning process are as follows:   

Ensure that internal trainers are appropriately trained, and that training is maintained by using external expertise

Undertake regular training and maintain appropriate records for supervisory and operational staff

Increase the profile and importance of safety from the board down to operators

Review all and any relevant safety data and request toxicological and clinical data from the originator and evaluate the data against occupational health categories

Undertake an evaluation of the physical and chemical properties, particularly particle size, explosive or static- electricity potential

Undertake a thorough evaluation of the containment that will be required to carry out the specified processing and ensure that the processes are in place for further checking and validation

Liquid and semisolid encapsulation using two-piece hard capsules is an ideal drug delivery approach for highly potent compounds and poorly water-soluble drugs. The authors detail the factors to reduce risk when designing and operating a facility for secondary manufacturing of highly potent drugs.

Challenges in the Secondary Manufacture of Encapsulated High-Potency Drugs

Prefilled syringes have become important options in the packaging, distribution, and delivery of pharmaceutical solution products, particularly biopharmaceuticals. This container–closure system provides several practical advantages, including convenience in simplified drug administration; greater accuracy in filling (i.e., less likelihood of overfilling, which is common in other packaging–delivery options); and reduced instances of misidentification, improper dosing, and contamination (1). These advantages, along with a critical mass of commercially available syringe systems, have fueled a rapid expansion of the market for prefilled syringes, with worldwide sales estimated to exceed 1 billion units and annual sales expected to grow at a rate in excess of 10% (2, 3). 

The basic components of a prefilled syringe are illustrated in Figure 1. The syringe barrel is typically either made from Type 1 borosilicate glass or various plastic materials (e.g., polypropylene, cyclic olefin polymer, cyclic olefin copolymers). Other components (e.g., tip cap and plunger) are typically made from elastomeric materials, which may be surface coated. Syringe barrels and plungers are typically coated with an agent (e.g., silicone oil) to facilitate plunger movement. Plunger movement is accomplished by an attached piston rod (typically plastic). 

In the US Food and Drug Administration's Guidance for Industry, Container Closure Systems for Packaging Human Drugs and Biologics 

, the agency defines a container–closure system as "the sum of packaging components that together contain and protect the dosage form. This includes primary packaging components" (4). A similar definition is provided by the European Medicines Agency in its Guideline on Plastic Packaging Materials 

www.emea.europa.eu/pdfs/human/qwp/435903en.pdf

 (5). It is clear from these definitions that a prefilled syringe meets the definition of a container–closure system and thus is subject to the associated guidelines and regulations. It is noteworthy that because of its use and nature, a prefilled syringe, as an injectable product, falls into FDA's highest risk category for the likelihood of a packaging component–dosage form interaction.  

Foremost among these regulations and guidelines are the regulatory expectations that revolve around suitability for the intended use. As noted in FDA's container–closure guidance:  

Every packaging system should be shown to be suitable for its intended use: it should adequately protect the dosage form; it should be compatible with the dosage form; and it should be composed of materials that are considered safe for use with the dosage form and the route of administration. If the packaging system has a performance feature in addition to containing the product, the assembled container closure system should be shown to function properly. 

Each of these aspects of suitability for intended use will be considered in greater detail as follows. 

As a packaging system, a prefilled syringe must meet all the suitability-for-intended-use expectations. Only certain aspects of a given expectation may be applicable to prefilled syringes. For example, the expectation of protection of the dosage form is interpreted to mean that the prefilled syringe should protect the dosage form from factors that may degrade the quality of the dosage form throughout its shelf life. Some commonly cited causes of such degradation for prefilled syringes include loss of solvent, adsorption of water vapor, and microbial contamination. Other potential causes of degradation, including light exposure and contact with reactive gases, are less germane to mainstream applications of prefilled syringes because such systems are not currently marketed as light or gas barriers. 

Several interactions between the drug product and the prefilled syringe can come into play under the general category of compatibility. In essence, compatibility is achieved if the interaction of the drug product and prefilled syringe is sufficiently limited that the quality of the drug product (or the syringe) is not changed to an unacceptable degree because of the interaction. Examples of potential interactions that could lead to unacceptable changes in product quality include:  

 Loss of drug product potency owing to absorption and/or adsorption of the active drug substance or formulation excipient by the syringe

Accelerated or modified degradation of the drug substance or formulation excipient induced by a chemical leached from the syringe

Change in drug product properties (e.g, pH, biological activity)

Discoloration of either the dosage form or packaging system

The nature of their application and their mode of use mean that prefilled syringes meet the regulatory definitions of immediate packaging or container–closure systems.

Pharmaceutical Technology-04-01-2008

Advances in Radio-Frequency Transdermal Drug Delivery

Polypeptide Multilayer Nanofilms in Drug Delivery

Dissolvable Films: Dissolvable Films for Flexible Product Format in Drug Delivery

Challenges in the Secondary Manufacture of Encapsulated High-Potency Drugs

Suitability-for-Use Considerations for Prefilled Syringes