Transdermal delivery of highly lipophilic drugs: in vitro fluxes of antiestrogens, permeation enhancers, and solvents from liquid formulations

Strategies to Improve the Transdermal Delivery of Poorly Water-Soluble Non-Steroidal Anti-Inflammatory Drugs

The transdermal delivery of non-steroidal anti-inflammatory drugs (NSAIDs) has the potential to overcome some of the major disadvantages relating to oral NSAID usage, such as gastrointestinal adverse events and compliance. However, the poor solubility of many of the newer NSAIDs creates challenges in incorporating the drugs into formulations suitable for application to skin and may limit transdermal permeation, particularly if the goal is therapeutic systemic drug concentrations. This review is an overview of the various strategies used to increase the solubility of poorly soluble NSAIDs and enhance their permeation through skin, such as the modification of the vehicle, the modification of or bypassing the barrier function of the skin, and using advanced nano-sized formulations. Furthermore, the simple yet highly versatile microemulsion system has been found to be a cost-effective and highly successful technology to deliver poorly water-soluble NSAIDs.

Cannabinoids for the treatment of autoimmune and inflammatory skin diseases: A systematic review

In recent years, the medical use of cannabinoids has attracted growing attention worldwide. In particular, anti-inflammatory properties of cannabinoids led to their emergence as potential therapeutic options for autoimmune and inflammatory disorders. Recent studies have also shown that cannabinoid receptors are widely expressed and have endogenous ligands in the skin, suggesting that the skin has its own endocannabinoid system. The aim of this review is to discuss the potential therapeutic effects of cannabinoids in autoimmune and inflammatory skin diseases. Following an overview of cannabinoids and the endocannabinoid system, we describe the cellular and molecular mechanisms of cannabinoids in skin health and disease. We then review the clinical studies of cannabinoids in autoimmune and inflammatory skin diseases including systemic sclerosis (SSc), dermatomyositis (DM), psoriasis (Pso) and atopic dermatitis (AD). A primary literature search was conducted in July 2023, using PubMed and Web of Science. A total of 15 articles were included after excluding reviews, non-human studies and in vitro studies from 389 non-duplicated articles. Available evidence suggests that cannabinoids may be beneficial for SSc, DM, Pso and AD. However, further studies, ideally randomized controlled trials, are needed to further evaluate the use of cannabinoids in autoimmune and inflammatory skin diseases.

Glycols: The ubiquitous solvent for dermal formulations

Glycols stand out as one of the most commonly employed safe and effective excipients for pharmaceutical and cosmeceutical products. Their widespread adoption can be attributed to their exceptional solvency characteristics and their ability to interact effectively with skin lipids and keratin for permeation enhancement. Notably, propylene glycol enjoys significant popularity in this regard. Ongoing research endeavours have been dedicated to scrutinising the impact of glycols on dermal drug delivery and shedding light on the intricate mechanisms by which glycols enhance skin permeation. This review aims to mitigate the discordance within the existing literature, assemble a holistic understanding of the impact of glycols on the percutaneous absorption of active compounds and furnish the reader with a profound comprehension of the foundational facets pertaining to their skin permeation enhancement mechanisms, while simultaneously delving deeper into the intricacies of these processes.

Transdermal Drug Delivery of Tazarotene: Determining Tazarotene's Potential in Local Transdermal Therapy

Retinoid-based drugs, while effective, are associated with systemic toxicity. Topical alternatives offer a safer option, and tazarotene, a third-generation synthetic retinoid, holds promise. This study investigates tazarotene's transdermal delivery potential, focusing on its application for joint-related conditions. The aim of this study was to investigate the suitability of tazarotene as a candidate for transdermal delivery into joints. In vitro permeation studies, using porcine skin, assessed tazarotene's transdermal drug delivery from solution and gel formulations. A tape-stripping analysis determined stratum corneum retention and a pilot study using porcine joints assessed tazarotene's ability to reach articular cartilage. Ultra Performance Liquid Chromatography coupled with a mass detector method was used to quantify tazarotene and tazarotenic acid permeation. The results validate that tazarotene can permeate porcine skin and accumulate in articular cartilage in detectable amounts. The detection of tazarotene and tazarotenic acid in both the in vitro permeation studies and the pilot study on porcine joints validate the drug's potential therapeutic use for hand osteoarthritis. This study lays the groundwork for future research, contributing insights into tazarotene's potential for transdermal drug delivery and guiding further exploration in topical retinoid applications.

The Potential of Pharmaceutical Hydrogels in the Formulation of Topical Administration Hormone Drugs

Hormones have attracted considerable interest in recent years due to their potential use in treatment of many diseases. Their ability to have a multidirectional effect leads to searching for new and increasingly effective drugs and therapies. Limitations in formulating drug forms containing hormones are mainly due to their low enzymatic stability, short half-life and limited bioavailability. One of the solutions may be to develop a hydrogel as a potential hormone carrier, for epidermal and transdermal application. This review discusses the main research directions in developing this drug formulation. The factors determining the action of hormones as drugs are presented. An analysis of hydrogel substrates and permeation enhancers that have the potential to enhance the efficacy of hormones applied to the skin is reviewed.

Transdermal Route: A Viable Option for Systemic Delivery of Antidepressants

The high rise in the population suffering from depression depicts the need for improved and highly effective treatment options for this condition. Efforts to develop existing drugs into user-friendly dosage forms with a number of advantages in major depressive states, including but not limited to: sustained drug release, reduced drug dosing frequency, improved tolerance and adherence, suitability for use in diverse populations and different treatment scenarios, as well as less central nervous system side effects are required. One such non-invasive drug delivery route that could provide the aforementioned benefits in the treatment of depression is the transdermal route. A number of conventional and emerging transdermal delivery strategies have been investigated for some potent antidepressants and results depict the potential of this route as a viable means for systemic delivery of therapeutically relevant doses of the tested agents, with Emsam®, the commercially available patch of selegiline, being an evidence for the same. The investigated approaches include the formulation of transdermal patches, use of vesicular drug carriers, pro-drug approach, microemulsification, chemical as well as physical enhancement technologies. This review provides a comprehensive account of the rationale, developments made till date, scope and future prospects of delivering antidepressants via the transdermal1 route of administration.

Enhanced Transdermal Delivery of Bisoprolol Hemifumarate via Combined Effect of Iontophoresis and Chemical Enhancers: Ex Vivo Permeation/In Vivo Pharmacokinetic Studies

Bisoprolol hemifumarate (BH) is an antihypertensive drug that is used as first-line treatment for chronic hypertension and angina pectoris. Our study was performed to enhance the transdermal delivery of BH, a hydrophilic drug active with high molecular weight, through differently prepared hydrogels. The synergistic effect of permeation enhancers and iontophoresis was investigated via both ex vivo and in vivo permeation studies. Ex vivo iontophoretic permeation studies were performed by using male albino Wistar rat skin. Cellosolve^® hydrogel (F7) showed a 1.5-fold increase in Q₁₈₀, Jss, and FER compared to F5 (lacking permeation enhancer). BH pharmacokinetic data were studied in human volunteers, following transdermal delivery of F7, using Phoresor^® Unit II iontophoresis device, compared to conventional oral tablets. F7 showed 1.9- and 2-fold higher values of C_max and AUC_0–40, respectively compared to Concor^® tablets, as well as a smaller T_max (2.00 ± 2.00 h). The relative bioavailability of F7 was found to be 201.44%, relative to Concor^® tablets, demonstrating the significantly enhanced transdermal permeation of BH from the selected hydrogel by iontophoresis, in human volunteers. Finally, results showed the successful utility of permeation enhancers combined with iontophoresis in significantly enhanced transdermal permeation of BH, despite its large molecular weight and hydrophilic nature. Therefore, this strategy could be employed as a successful alternative route of administration to conventional oral tablets.

Delivering therapeutic cannabinoids via skin: Current state and future perspectives

Adequate evidence exists in the literature indicating a relatively positive shift with regards to the legal acceptance of cannabis and cannabis-derived products for medicinal purposes in some countries. Concomitantly, scientists are showing renewed interest in cannabis-related research work. Over the years, clinical and preclinical studies have demonstrated the therapeutic significance of cannabinoids for diverse indications. Additionally, efforts are being made to develop cannabis-related products into acceptable prescription products. FDA authorization for the commercial use of four cannabinoid-derived products, available as oral dosage forms is a significant progress already. However, there are certain drawbacks associated with the conventional delivery forms of cannabinoids. These include low oral bioavailability due to hepatic degradation, gastric instability, poor water solubility, and the side effects experienced upon the use of high doses of psychotropic cannabinoids associated with heightened plasma concentrations of the drug. These are however, limitable with the aid of transcutaneous drug delivery. Emerging topical and transdermal strategies could be exploited for the successful development of highly effective delivery systems for cannabinoids. This review discusses the feasibility of delivering therapeutic cannabinoids via skin and provides a comprehensive account of the supporting research studies that have been reported in the literature till date.

Characterization of microneedles and microchannels for enhanced transdermal drug delivery

Microneedle (MN)-based technologies are currently one of the most innovative approaches that are being extensively investigated for transdermal delivery of low molecular weight drugs, biotherapeutic agents and vaccines. Extensive research reports, describing the fabrication and applications of different types of MNs, can be readily found in the literature. Effective characterization tools to evaluate the quality and performance of the MNs as well as for determination of the dimensional and kinetic properties of the microchannels created in the skin, are an essential and critical part of MN-based research. This review paper provides a comprehensive account of all such tools and techniques.

Non-invasive drug delivery technology: development and current status of transdermal drug delivery devices, techniques and biomedical applications

Pay-load deliveries across the skin barrier to the systemic circulation have been one of the most challenging delivery options. Necessitated requirements of the skin and facilitated skin layer cross-over delivery attempts have resulted in development of different non-invasive, non-oral methods, devices and systems which have been standardized, concurrently used and are in continuous upgrade and improvements. Iontophoresis, electroporation, sonophoresis, magnetophoresis, dermal patches, nanocarriers, needled and needle-less shots, and injectors are among some of the methods of transdermal delivery. The current review covers the current state of the art, merits and shortcomings of the systems, devices and transdermal delivery patches, including drugs' and other payloads' passage facilitation techniques, permeation and absorption feasibility studies, as well as physicochemical properties affecting the delivery through different transdermal modes along with examples of drugs, vaccines, genes and other payloads.

Topical Delivery of Coenzyme Q10-Loaded Microemulsion for Skin Regeneration

The aim of this study was to develop a coenzyme Q10 (CoQ10) microemulsion system with improved solubility, penetration, and wound healing efficacy. Based on the pseudo-ternary diagram, microemulsions containing isopropyl myristate (IPM), Cremophor EL^®, and Transcutol^® HP were selected and confirmed to be nanosized (<20 nm) and thermodynamically stable based on the dilution and thermodynamic stability tests. The CoQ10-loaded microemulsion with a surfactant/co-surfactant (S/CoS) ratio of 2:1 (w/w %) demonstrated a higher permeation efficacy compared to microemulsions with S/CoS ratio of 3:1 or 4:1 (w/w %). Additionally, the CoQ10-loaded microemulsion with an S/CoS ratio of 2:1 demonstrated a relatively rapid wound healing effect in keratinocytes and fibroblasts. Overall, these data suggest that a microemulsion based on IPM, Cremophor EL^®, and Transcutol^® HP could be an effective vehicle for the topical administration of CoQ10 and could be utilized for the application of other therapeutic agents that have difficulty in penetrating the skin.

Development of a novel topical formulation of glyceryl trinitrate for the treatment of erectile dysfunction

Erectile dysfunction (ED), defined as the inability to initiate or maintain an erection sufficient for satisfactory sexual intercourse, is common, particularly in men aged ≥50 years. Existing treatments have significant limitations, and there remains a need for a fast-acting (to facilitate spontaneity during intercourse) and well tolerated local therapy. Topical glyceryl trinitrate (GTN) may meet this need because GTN undergoes rapid metabolism in penile smooth muscle and endothelial cells to produce nitric oxide, which plays a key role in the development of erection. This paper describes the rationale for the development of MED2005, a topical GTN formulation using DermaSys® technology, which is undergoing clinical trials for the treatment of ED. Pharmacokinetic studies have shown that MED2005 provides rapid delivery of GTN following application to the glans penis, and a Phase 2(a) trial in men with ED showed that MED2005 produced significant improvements in erectile function, compared with placebo. MED2005 was well tolerated in this trial, with only 21 cases of headache in 1003 intercourse attempts. It is anticipated that MED2005 will provide an effective therapy for ED, with a fast onset of action, good local tolerability, and fewer contraindications than phosphodiesterase 5 inhibitors, the current cornerstone of ED therapy.

Skin Delivery and Irritation Potential of Phenmetrazine as a Candidate Transdermal Formulation for Repurposed Indications

Phenmetrazine, a selective dopamine and norepinephrine releaser, previously available as an oral anorectic, is prone to be abused. This study aimed to assess the feasibility of delivering phenmetrazine via the transdermal route for a new indication, while also minimizing its abuse potential. The passive permeation of phenmetrazine through dermatomed human cadaver skin was evaluated using static Franz diffusion cells at 10 mg/mL for the fumarate salt, and at 20, 40, and 80 mg/mL for the free base in propylene glycol for 24 h. Further, oleic acid (5% w/w), oleyl alcohol (5% and 10% w/w), and lauric acid (10% w/w) were investigated as chemical permeation enhancers to enhance the delivery. Skin irritation potential was assessed using EpiDerm™ in vitro reconstructed human epidermal model. The free base showed superior 24-h delivery (8.13 ± 4.07%, 10.6 ± 2.5%, and 10.4 ± 1.4% for groups with 20, 40, and 80 mg/mL of the free base, respectively) to phenmetrazine fumarate salt (undetectable). The successful screening of effective chemical enhancers, oleyl alcohol (5% and 10% w/w), oleic acid (5% w/w), and lauric acid (10% w/w) resulted in significant enhancement of delivery. The calculated therapeutic relevant flux for the potential indication, attention deficit hyperactivity disorder, 20 μg/cm²/h was met, where a 24-mg daily dose from a 50-cm² patch was projected to be delivered to a 60-kg individual. Irritation study results suggest that formulations with therapeutically relevant delivery are likely to be non-irritant. In conclusion, it is feasible to deliver therapeutically relevant amounts of phenmetrazine via the transdermal route.

Recent progress in tissue optical clearing for spectroscopic application

This paper aims to review recent progress in optical clearing of the skin and over naturally turbid biological tissues and blood using this technique in vivo and in vitro with multiphoton microscopy, confocal Raman microscopy, confocal microscopy, NIR spectroscopy, optical coherence tomography, and laser speckle contrast imaging. Basic principles of the technique, its safety, advantages and limitations are discussed. The application of optical clearing agent on a tissue allows for controlling the optical properties of tissue. Optical clearing-induced reduction of tissue scattering significantly facilitates the observation of deep-located tissue regions, at the same time improving the resolution and image contrast for a variety of optical imaging methods suitable for clinical applications, such as diagnostics and laser treatment of skin diseases, mucosal tumor imaging, laser disruption of pathological abnormalities, etc.

In Vitro Drug Transfer Due to Drug Retention in Human Epidermis Pretreated with Application of Marketed Estradiol Transdermal Systems

Study objective was to assess skin-to-skin drug transfer potential that may occur due to drug retention in human epidermis (DRE) pretreated with application of estradiol transdermal drug delivery systems (TDDS) and other estradiol transdermal dosage forms (gels and sprays). TDDS (products-A, B, and C) with varying formulation design and composition, and other estradiol transdermal products (gel and spray) were applied to heat separated human epidermis (HSE) and subjected to in vitro drug permeation study. Amounts of DRE were quantified after 24 h. The DRE with product-B was significantly (P < 0.001) higher than that with product-C, product-A, gel, and spray. However, products-A and C, gel, and spray showed almost the same (P > 0.05) amounts of DRE. A separate in vitro permeation study was carried out to determine amounts of drug transferred from drug-retaining epidermis to untreated HSE. The amounts of drug transferred, due to DRE after 8 h, with product-C were significantly (P < 0.001) higher than those with products-A and B, gel, and spray. The in vitro study results indicate a high potential of skin-to-skin drug transfer due to the DRE after labeled period of using estradiol TDDS, though the clinical relevance of these findings is yet to be determined.

Formulation and evaluation of transdermal drug delivery of topiramate

Background:

Transdermal drug delivery system (TDDS) was designed to sustain the release and improve the bioavailability of drug and patient compliance. Among the various types of transdermal patches, matrix dispersion type systems disperse the drug in the solvent along with the polymers and solvent is allowed to evaporate forming a homogeneous drug-polymer matrix. The objective of the present study was to design and formulate TDDS of topiramate (TPM) and to evaluate their extended release in vitro and ex vivo.

Materials and Methods:

In the present study, an attempt has been made to develop a matrix-type transdermal therapeutic system comprising TPM with different ratios of hydrophilic and hydrophobic polymeric combinations using solvent casting technique.

Results:

The physicochemical compatibility of the drug and the polymers was studied by Fourier transform infrared spectroscopy. The results obtained showed no physical-chemical incompatibility between the drug and the polymers. The patches were further subjected to various physical evaluations along with the ex vivo permeation studies using pig ear skin.

Conclusions:

On the basis of results obtained from the physical evaluation and ex vivo studies the patches containing the polymers, that is, Eudragit L 100 and polyvinylpyrrolidone, with oleic acid as the penetration enhancer were considered as the best formulations for the transdermal delivery of TPM.

Topical formulations of miltefosine for cutaneous leishmaniasis in a BALB/c mouse model

Cutaneous leishmaniasis (CL) is caused by several species of the protozoan parasite Leishmania and affects approximately 10 million people worldwide. Currently available drugs are not ideal due to high cost, toxicity, parenteral administration and suboptimal efficacy. Miltefosine is the only oral treatment (Impavido®) available to treat CL, given over a period of 28 days with common side effects such as vomiting and diarrhoea.

OBJECTIVE

To explore the local application of miltefosine as a topical formulation to enhance activity and reduce the drug's adverse effects.

METHODS

The antileishmanial activity of miltefosine was confirmed in vitro against several Leishmania species. The permeation of miltefosine, in different solvents and solvent combinations, through BALB/c mouse skin was evaluated in vitro using Franz diffusion cells. The topical formulations which enabled the highest drug permeation or skin disposition were tested in vivo in BALB/c mice infected with L. major.

KEY FINDINGS

The overall permeation of miltefosine through skin was low regardless of the solvents used. This was reflected in limited antileishmanial activity of the drug formulations when applied topically in vivo. All topical formulations caused skin irritation.

CONCLUSIONS

We conclude that miltefosine is not an appropriate candidate for the topical treatment of CL.

Topical delivery of hexamidine

Hexamidine diisethionate (HEX D) has been used for its biocidal actions in topical preparations since the 1950s. Recent data also suggest that it plays a beneficial role in skin homeostasis. To date, the extent to which this compound penetrates the epidermis has not been reported nor how its topical delivery may be modulated. In the present work we set out to characterise the interaction of HEX D with the skin and to develop a range of simple formulations for topical targeting of the active. A further objective was to compare the skin penetration of HEX D with its corresponding dihydrochloride salt (HEX H) as the latter has more favourable physicochemical properties for skin uptake. Candidate vehicles were evaluated by in vitro Franz cell permeation studies using porcine skin. Initially, neat solvents were investigated and subsequently binary systems were examined. The solvents and chemical penetration enhancers investigated included glycerol, dimethyl isosorbide (DMI), isopropyl alcohol (IPA), 1,2-pentanol (1,2-PENT), polyethylene glycol (PEG) 200, propylene glycol (PG), propylene glycol monolaurate (PGML) and Transcutol(®)P (TC). Of a total of 30 binary solvent systems evaluated only 10 delivered higher amounts of active into the skin compared with the neat solvents. In terms of topical efficacy, formulations containing PGML far surpassed all other solvents or binary combinations. More than 70% of HEX H was extracted from the skin following application in PG:PGML (50:50). Interestingly, the same vehicle effectively promoted skin penetration of HEX D but demonstrated significantly lower uptake into and through the skin (30%). The findings confirm the unpredictable nature of excipients on delivery of actives with reference to skin even where there are minor differences in molecular structures. We also believe that they underline the ongoing necessity for fundamental studies on the interaction of topical excipients with the skin.

Investigating transdermal delivery of vitamin D3

Transdermal delivery of therapeutic amounts of vitamin D3 is proposed to overcome its variable oral bioavailability, especially for people who suffer from fat malabsorption. The main challenge for this delivery route is to overcome the barrier properties of skin, especially for very lipophilic compounds such as vitamin D3. In this study, the effect of different penetration enhancers, such as oleic acid, dodecylamine, ethanol, oleic acid in propylene glycol, isopropyl myristate, octyldodecanol, and oleyl alcohol in propylene glycol were evaluated in vitro for their effectiveness in delivering vitamin D3 through polyamide filter, polydimethylsiloxane membrane, and porcine skin. A diffusion cell was used to study the transdermal permeability of vitamin D3. Ointment formulations of vitamin D3 were prepared containing the most widely used penetration enhancers, oleic acid, and dodecylamine. The ointment containing oleic acid as chemical penetration enhancer did not improve delivery compared to control. On the other hand, the formulation containing dodecylamine as a penetration enhancer did improve the transdermal delivery of vitamin D3. However, statistical significance and an amount high enough for nutritional supplementation purposes were reached only when the skin was pretreated with 50% ethanol. In these conditions, the ointment delivered an amount of 760-ng vitamin D3 per cm(2) of skin. The research shows promise that transdermal delivery could be an effective administration route for vitamin D3 when ethanol and dodecylamine are used as penetration enhancers.

Novel dendritic derivatives of unsaturated fatty acids as promising transdermal permeation enhancers for tenofovir

Novel dendritic ester derivatives of unsaturated fatty acids as potential transdermal permeation enhancers.

Evidence that bisphenol A (BPA) can be accurately measured without contamination in human serum and urine, and that BPA causes numerous hazards from multiple routes of exposure

There is extensive evidence that bisphenol A (BPA) is related to a wide range of adverse health effects based on both human and experimental animal studies. However, a number of regulatory agencies have ignored all hazard findings. Reports of high levels of unconjugated (bioactive) serum BPA in dozens of human biomonitoring studies have also been rejected based on the prediction that the findings are due to assay contamination and that virtually all ingested BPA is rapidly converted to inactive metabolites. NIH and industry-sponsored round robin studies have demonstrated that serum BPA can be accurately assayed without contamination, while the FDA lab has acknowledged uncontrolled assay contamination. In reviewing the published BPA biomonitoring data, we find that assay contamination is, in fact, well controlled in most labs, and cannot be used as the basis for discounting evidence that significant and virtually continuous exposure to BPA must be occurring from multiple sources.

Holding thermal receipt paper and eating food after using hand sanitizer results in high serum bioactive and urine total levels of bisphenol A (BPA)

Bisphenol A (BPA) is an endocrine disrupting environmental contaminant used in a wide variety of products, and BPA metabolites are found in almost everyone's urine, suggesting widespread exposure from multiple sources. Regulatory agencies estimate that virtually all BPA exposure is from food and beverage packaging. However, free BPA is applied to the outer layer of thermal receipt paper present in very high (∼20 mg BPA/g paper) quantities as a print developer. Not taken into account when considering thermal paper as a source of BPA exposure is that some commonly used hand sanitizers, as well as other skin care products, contain mixtures of dermal penetration enhancing chemicals that can increase by up to 100 fold the dermal absorption of lipophilic compounds such as BPA. We found that when men and women held thermal receipt paper immediately after using a hand sanitizer with penetration enhancing chemicals, significant free BPA was transferred to their hands and then to French fries that were eaten, and the combination of dermal and oral BPA absorption led to a rapid and dramatic average maximum increase (Cmax) in unconjugated (bioactive) BPA of ∼7 ng/mL in serum and ∼20 µg total BPA/g creatinine in urine within 90 min. The default method used by regulatory agencies to test for hazards posed by chemicals is intra-gastric gavage. For BPA this approach results in less than 1% of the administered dose being bioavailable in blood. It also ignores dermal absorption as well as sublingual absorption in the mouth that both bypass first-pass liver metabolism. The elevated levels of BPA that we observed due to holding thermal paper after using a product containing dermal penetration enhancing chemicals have been related to an increased risk for a wide range of developmental abnormalities as well as diseases in adults.

Formulation and biopharmaceutical evaluation of a transdermal patch containing letrozole

The purpose of this study was to formulate a drug-in-adhesive (DIA) transdermal patch containing letrozole, a third generation aromatase inhibitor for the treatment of breast cancer, using pressure-sensitive-adhesives (PSAs) and to evaluate the percutaneous penetration and pharmacokinetics of letrozole after transdermal administration, compared with that for the oral route. The formulation factors for such a patch, including the PSAs, enhancers and amount of drug loaded were investigated. Among the tested preparations, the formulation with DURO-TAK 87-4098, Azone and propylene glycol showed the highest letrozole permeation. The pharmacokinetic characteristics of an optimized DIA patch containing letrozole were determined using rats, while orally administered letrozole in solution was used as a control. The pharmacokinetic parameter, such as the mean residence time (MRT) was significantly (p<0.05) different following transdermal administration compared with oral administration. The in vivo results observed with the patches in rats were in good agreement with the plasma concentrations predicted from the in vitro penetration data. As a patient-friendly, convenient, high local drug concentration and sustained dosing therapeutic system, the transdermal patches incorporating letrozole provide a useful strategy for the prevention and treatment of breast cancer.

Enhancement of transdermal drug delivery via synergistic action of chemicals

Transdermal drug delivery is an attractive alternative to conventional techniques for administration of systemic therapeutics. One challenge in designing transdermal drug delivery systems is to overcome the natural transport barrier of the skin. Chemicals offer tremendous potential in overcoming the skin barrier to enhance transport of drug molecules. Individual chemicals are however limited in their efficacy in disrupting the skin barrier at low concentrations and usually cause skin irritation at high concentrations. Multicomponent mixtures of chemicals, however, have been shown to provide high skin permeabilization potency as compared to individual chemicals without necessarily causing irritation. Here we review systems employing synergistic mixtures of chemicals that offer superior skin permeation enhancement. These synergistic systems include solvent mixtures, microemulsions, eutectic mixtures, complex self-assembled vesicles and inclusion complexes. Methods for design and discovery of such synergistic systems are also discussed.

Chemical penetration enhancers: a patent review

BACKGROUND

Ever since transdermal drug delivery came into existence, it has offered great promises, although most of them are yet to be fulfilled owing to some intrinsic restrictions of the transdermal route. On the positive side, transdermal drug delivery systems present advantages including non-invasiveness, prolonged therapeutic effect, reduced side effects, improved bioavailability, better patient compliance and easy termination of drug therapy. The greatest hindrance in the percutaneous delivery is the obstruction property of the stratum corneum, the outermost layer of the skin, in addition to usual problems such as skin binding, skin metabolism, cutaneous toxicity and prolonged lag times.

OBJECTIVE

This paper reviews investigations on the feasibility and application of penetration enhancers as described in recent patents, which help in the selection of a suitable sorption promoter(s) for enhanced delivery of medicaments through the skin.

METHOD

The patents granted under various categories of penetration enhancers have been discussed including fatty acids, terpenes, fatty alcohol, pyrrolidone, sulfoxides, laurocapram, surface active agents, amides, amines, lecithin, polyols, quaternary ammonium compounds, silicones, alkanoates and so on.

CONCLUSION

Scores of promising chemicals have been harnessed for their skin permeation promoting capacity as mentioned earlier. In future, many more chemicals and putative enhancers are likely be documented and patented.

Analgesic synergy between topical opioids and topical non-steroidal anti-inflammatory drugs in the mouse model of thermal pain

The main aim of the study was to examine analgesic effects of the topical opioids and non-steroidal anti-inflammatory drugs (NSAIDs) in a radiant heat tail-flick nociception model. Also, we have tested whether the addition of lauric acid to propylene glycol improves skin permeation for the opioids and NSAIDs. We found that the addition of lauric acid to propylene glycol dramatically improves the penetration of the drugs, measured by the drug's ED(50). We observed a significant dose response shift to the left for all tested drugs. So, morphine's ED(50) was decreased by 19-fold. The duration of the analgesic activity of morphine dissolved in a combination of propylene glycol and lauric acid was much longer compared with the same dose of the drug dissolved in propylene glycol only. Methadone and hydrocodone also produced analgesic activity in this experimental paradigm. We then assessed potential interactions between opioids, ibuprofen and diclofenac using a fixed, low dose of each. The inclusion of either S-ibuprofen or diclofenac to a fixed, low dose of morphine raised the analgesic response from around 20% to 50% and 80%, respectively. Topical methadone and diclofenac alone produced analgesia in 30% of mice. The combination produced analgesia in 100% of mice (100% versus 60%, P<0.001) and the analgesic effect was observed for 90 min. Alone, topical methadone and S-ibuprofen produced analgesia in 25% and 30% of mice, respectively. The combination elicited analgesia in 100% of mice (100% versus 55%, P<0.001) and this analgesic effect lasted for 120 min. Our current findings support the supra-additive interaction of topical mu opioids, S-ibuprofen and diclofenac in a model of moderate to severe pain, radiant heat tail-flick assay.

Overcoming the Stratum Corneum: The Modulation of Skin Penetration

It is preferred that topically administered drugs act either dermally or transdermally. For that reason they have to penetrate into the deeper skin layers or permeate the skin. The outermost layer of the human skin, the stratum corneum, is responsible for its barrier function. Most topically administered drugs do not have the ability to penetrate the stratum corneum. In these cases modulations of the skin penetration profiles of these drugs and skin barrier manipulations are necessary. A skin penetration enhancement can be achieved either chemically, physically or by use of appropriate formulations. Numerous chemical compounds have been evaluated for penetration-enhancing activity, and different modes of action have been identified for skin penetration enhancement. In addition to chemical methods, skin penetration of drugs can be improved by physical options such as iontophoresis and phonophoresis, as well as by combinations of both chemical and physical methods or by combinations of several physical methods. There are cases where skin penetration of the drug used in the formulation is not the aim of the topical administration. Penetration reducers can be used to prevent chemicals entering the systemic circulation. This article concentrates on the progress made mainly over the last decade by use of chemical penetration enhancers. The different action modes of these substances are explained, including the basic principles of the physical skin penetration enhancement techniques and examples for their application.

Transdermal absorption of propofol in rats

Propofol (PF), a highly lipophilic anesthetic, has several desirable properties, such as the rapid onset and cessation of its effects upon intravenous infusion. In this study, the transdermal absorption of PF was investigated with the aim of the development of an alternative route of administration. PF solutions containing isopropyl myristate (IPM), ethanol or propylene glycol (PG) at various concentrations were prepared and applied to the abdominal skin of rats. Petrolatum and fatty alcohol propylene glycol (FAPG) ointments containing PF were also prepared and applied to the dorsal skin. Eyelid opening was measured and the ratio of the measured value to the initial value was calculated to evaluate the level of the pharmacological effect of the preparation. The PG solution containing 80% PF achieved higher plasma PF concentrations than the 100% PF solution. The PF-FAPG ointment produced a higher plasma PF concentration than the PF-petrolatum ointment. Furthermore, a drowsy state was confirmed after transdermal administration of 42% PF-FAPG ointment. These results indicate that the combination of PF and PG was appropriate for the transdermal absorption of PF, and PF was absorbed through the rat skin to an extent sufficient to cause a continuous sedative effect.

The transdermal revolution

Historically, developments in transdermal drug delivery have been incremental, focusing on overcoming problems associated with the barrier properties of the skin, reducing skin irritation rates and improving the aesthetics associated with passive patch systems. More-recent advances have concentrated on the development of non-passive systems to aid delivery of larger drug molecules, such as proteins and nucleotides, as the trend for discovering and designing biopharmaceuticals continues. Fundamentally, improvements in transdermal delivery will remain incremental until there is wider acceptance of this route of administration within the pharmaceutical industry. Only then will the transdermal revolution live up to its true potential.

Drug delivery systems for oestrogenic hormones and antagonists: the need for selective targeting in estradiol-dependent cancers

The pleiotropic activity of oestrogens and their mechanism of action via their binding to the two oestrogen receptors alpha (ER alpha) and beta (ER beta) subtypes in the different tissues where oestrogens exert their action have been briefly described. The fate of these compounds trapped into different galenic forms is discussed with regard to their therapeutic applications. Firstly, the advantages and disadvantages of the different forms (pills, i.v. forms and transdermal patches) used in contraception are compared. Secondly, the therapeutic use of formulated oestrogens for the post-menopausal hormone replacement therapy (HRT) is analysed through the various results obtained in different trials. The link between HRT and the risks of breast cancer and cardiovascular disease is underlined. Finally, comparing the activity of selective oestrogen receptor modulators such as tamoxifen and pure anti-oestrogens such as RU58668 and ICI182780, we analysed the reasons leading to the need for a tumor targeting of the latters, but not of the former for the treatment of oestrogen-dependent breast cancer. Different injectable and biodegradable formulations, that lead to a remarkable anti-tumor efficiency in xenografts, have been recently developed and we believe that they may represent promising new administration ways of added therapeutic values for anti-oestrogens. Such devices could be extended to the delivery of other anti-cancer drugs with more aggressive activities than anti-oestrogens.

Penetration enhancers

One long-standing approach for improving transdermal drug delivery uses penetration enhancers (also called sorption promoters or accelerants) which penetrate into skin to reversibly decrease the barrier resistance. Numerous compounds have been evaluated for penetration enhancing activity, including sulphoxides (such as dimethylsulphoxide, DMSO), Azones (e.g. laurocapram), pyrrolidones (for example 2-pyrrolidone, 2P), alcohols and alkanols (ethanol, or decanol), glycols (for example propylene glycol, PG, a common excipient in topically applied dosage forms), surfactants (also common in dosage forms) and terpenes. Many potential sites and modes of action have been identified for skin penetration enhancers; the intercellular lipid matrix in which the accelerants may disrupt the packing motif, the intracellular keratin domains or through increasing drug partitioning into the tissue by acting as a solvent for the permeant within the membrane. Further potential mechanisms of action, for example with the enhancers acting on desmosomal connections between corneocytes or altering metabolic activity within the skin, or exerting an influence on the thermodynamic activity/solubility of the drug in its vehicle are also feasible, and are also considered in this review.

Discovery of transdermal penetration enhancers by high-throughput screening

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 (approximately 1-10 kDa) by up to approximately 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 oligonucleotides, 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.

Development of matrix patches for transdermal delivery of a highly lipophilic antiestrogen

The aim of this study was to develop matrix-type transdermal systems (TDSs) containing the highly lipophilic (log P = 5.82) antiestrogen (AE) and the permeation enhancers propylene glycol and lauric acid. For that purpose, permeation of AE from various adhesive matrices through excised skin of hairless mice was evaluated. It was found that pretreatment of the skin with permeation enhancers raised the transdermal flux of subsequently applied antiestrogen. Highest steady-state transdermal fluxes (1.1 microg cm(-2) h(-1)) were obtained from Gelva, polyacrylate adhesive, followed by 0.55 microg cm(-2) h(-1) from Oppanol polyisobutylene, 0.31 microg cm(-2) h(-1) from BIO-PSA silicone, and 0.12 microg cm(-2) h(-1) from Sekisui polyacrylate matrices. In order to develop TDS with high content of fluid permeation enhancer propylene glycol, two different strategies were investigated. One strategy was the addition of hydroxypropyl cellulose (HPC) as thickening agent to Gelva matrices. This allowed for propylene glycol loading levels of up to 30%, resulting in transdermal AE fluxes of 0.09 microg cm(-2) h(-1). On the other hand, a fleece-laminated backing foil was loaded with the described permeation enhancer formulation and laminated with polyacrylate adhesive layer, resulting in transdermal AE fluxes of 0.06 microg cm(-2) h(-1). However, application of these TDSs on skin pretreated with permeation enhancers raised the fluxes to 2.6 microg cm(-2) h(-1) from Gelva/HPC and 0.46 microg cm(-2) h(-1) from fleece/Sekisui.

In vitro evaluation of patch formulations for topical delivery of gentisic acid in rats

Gentisic acid (GA) is used in cosmetics as a skin-whitening agent for the treatment of skin pigmentary disorders by influencing the synthesis of melanin through inhibition of melanosomal tyrosinase activity. In order to achieve effective topical delivery of GA to the active site in the skin, a matrix-type transdermal delivery system was developed. The in vitro skin permeation as well as skin deposition of GA was studied in rats. Among the five pressure-sensitive adhesives tested, DuroTak 87-2510 was the most effective to achieve the highest permeation rate of GA. Dodecylamine showed the most potent enhancement among the enhancers tested, and significantly increased the permeation rate of GA up to 112.99 (+/-30.12) microg/cm(2) per h at the concentration of 1%, when 6% GA was incorporated in DuroTak 87-2510. Moreover, a linear relationship was observed between the skin permeation rate of GA and the amount of the skin deposition after 12 h of permeation (r(2)=0.95). Thus, the in vitro skin permeation data may be useful to determine the amount of GA actually deposited in the skin.

In-vitro release and transdermal fluxes of a highly lipophilic drug and of enhancers from matrix TDS

Transdermal systems (TDS) are a well-known application form for small, moderately lipophilic molecules. The aim of this study was to investigate the possibility of applying a highly lipophilic drug, the antiestrogen AE (log P=5.82) transdermally by polyacrylate-based matrix TDS. For this purpose, two effects of both drug and enhancer concentration in TDS were investigated: in-vitro release and transdermal permeation of drug and enhancers. In the TDS investigated, in-vitro release as well as in-vitro permeation of AE through excised skin of hairless mice was found to be independent of concentrations of both drug and enhancers. The steady-state fluxes observed were low (about 50-100 ng cm(-2) h(-1)). But skin pretreatment with permeation enhancers resulted in a markedly enhanced permeability (1400 ng cm(-2) h(-1)). Therefore, the permeation of this highly lipophilic drug seems to be limited by the stratum corneum barrier function. In contrast, the transdermal permeation of the enhancers was dependent on the TDS composition. Increase in enhancer content resulted in a higher permeation of enhancers, whereas skin pretreatment did not. In conclusion, it was shown that the highly lipophilic antiestrogen can be administered transdermally by pretreating the skin with the fluid permeation enhancer combination propylene glycol-lauric acid (9+1) and then applying a matrix TDS.