Cutaneous Biodistribution: A High-Resolution Methodology to Assess Bioequivalence in Topical Skin Delivery

Therapeutic-driven framework for bioequivalence assessment of complex topical generic drug products

Despite the continuous research on understanding how topical drugs and the skin interact, the development of a topical generic product remains a challenge. Due to their local action effect rather than systemic, establishing suitable frameworks for documenting bioequivalence between reference and test formulations is anything but straightforward. In previous years, clinical endpoint trials were considered the gold standard method to demonstrate bioequivalence between topical products. Nevertheless, significant financial and time resources were required to be allocated owing to the inherent complexity of these studies. To address this problem, regulatory authorities have begun to accept alternative approaches that could lead to a biowaiver, avoiding the need for clinical endpoint trials. These alternatives encompass various in vitro and/or in vivo techniques that have been analysed and the benefits and drawbacks of each method have been considered. Furthermore, other factors like the integration of a quality by design framework to ensure a comprehensive understanding of the product and process quality attributes have also been taken into account. This review delves into international regulatory recommendations for semisolid topical products, with a focus on those established by the European Medicines Agency, as well as the Food and Drug Administration. Both approaches were carefully examined, discussing aspects such as acceptance criteria, sample size, and microstructure evaluation. Additionally, novel and innovative therapeutic-driven approaches based on in vitro disease models for the rapid and effective development of topical generic products are presented.

Finite sample corrections for average equivalence testing

Average (bio)equivalence tests are used to assess if a parameter, like the mean difference in treatment response between two conditions for example, lies within a given equivalence interval, hence allowing to conclude that the conditions have "equivalent" means. The two one-sided tests (TOST) procedure, consisting in testing whether the target parameter is respectively significantly greater and lower than some pre-defined lower and upper equivalence limits, is typically used in this context, usually by checking whether the confidence interval for the target parameter lies within these limits. This intuitive and visual procedure is however known to be conservative, especially in the case of highly variable drugs, where it shows a rapid power loss, often reaching zero, hence making it impossible to conclude for equivalence when it is actually true. Here, we propose a finite sample correction of the TOST procedure, theα $$ \alpha $$ -TOST, which consists in a correction of the significance level of the TOST allowing to guarantee a test size (or type-I error rate) ofα $$ \alpha $$ . This new procedure essentially corresponds to a finite sample and variability correction of the TOST procedure. We show that this procedure is uniformly more powerful than the TOST, easy to compute, and that its operating characteristics outperform the ones of its competitors. A case study about econazole nitrate deposition in porcine skin is used to illustrate the benefits of the proposed method and its advantages compared to other available procedures.

Cutaneous Delivery and Biodistribution of Cannabidiol in Human Skin after Topical Application of Colloidal Formulations

The objective of this study was to investigate the cutaneous delivery of cannabidiol (CBD) from aqueous formulations developed for the targeted local treatment of dermatological conditions. CBD was formulated using a proprietary colloidal drug delivery system (VESIsorb®) into an aqueous colloidal solution at 2% (ACS 2%) and two colloidal gels (CG 1% and CG 2%, which contained 1% and 2% CBD, respectively). Two basic formulations containing CBD (5% in propylene glycol (PG 5%) and a 6.6% oil solution (OS 6.6%)) and two marketed CBD products (RP1 and RP2, containing 1% CBD) were used as comparators. Cutaneous delivery and cutaneous biodistribution experiments were performed using human abdominal skin (500-700 µm) under infinite- and finite-dose conditions with 0.5% Tween 80 in the PBS receiver phase. The quantification of CBD in the skin samples was performed using a validated UHPLC-MS/MS method and an internal standard (CBD-d3). The cutaneous deposition of CBD under finite-dose conditions demonstrated the superiority of CG 1%, CG 2%, and ACS 2% over the marketed products; CG 1% had the highest delivery efficiency (5.25%). Cutaneous biodistribution studies showed the superiority of the colloidal systems in delivering CBD to the viable epidermis, and the upper and lower papillary dermis, which are the target sites for the treatment of several dermatological conditions.

Enhanced topical paromomycin delivery for cutaneous leishmaniasis treatment: Passive and iontophoretic approaches

Conventional treatments for cutaneous leishmaniasis, a neglected vector-borne infectious disease, can frequently lead to serious adverse effects. Paromomycin (PAR), an aminoglycoside antibiotic, has been suggested for the topical treatment of disease-related lesions, but even when formulated in high drug-loading dosage forms, presents controversial efficacy. The presence of five ionizable amino groups hinder its passive cutaneous penetration but make PAR an excellent candidate for iontophoretic delivery. The objective of this study was to verify the feasibility of using iontophoresis for cutaneous PAR delivery and to propose a topical passive drug delivery system that could be applied between iontophoretic treatments. For this, in vitro iontophoretic experiments evaluated different application durations (10, 30, and 360 min), current densities (0.1, 0.25, and 0.5 mA/cm2), PAR concentrations (0.5 and 1.0 %), and skin models (intact and impaired porcine skin). In addition, 1 % PAR hydrogel had its penetration profile compared to 15 % PAR ointment in passive transport. Results showed iontophoresis could deliver suitable PAR amounts to dermal layers, even in short times and with impaired skin. Biodistribution assays showed both iontophoretic transport and the proposed hydrogel delivered higher PAR amounts to deeper skin layers than conventional ointment, even though applying 15 times less drug. To our knowledge, this is the first report of PAR drug delivery enhancement by iontophoresis. In summary, the association of iontophoresis with a topical application of PAR gel seems appropriate for improving cutaneous leishmaniasis treatment.

Development of an UHPLC-MS/MS method to determine cutaneous biodistribution of cannabidiol after topical application of cannabidiol gel assisted by iontophoresis

Cannabidiol has potential for use in skin disease therapy, so it is important to know the cutaneous biodistribution of cannabidiol after topical application of cannabidiol formulations. However, currently existing quantification methods for the investigation of cannabidiol skin distribution are not optimal. This study aimed to establish a method for the determination of cannabidiol in skin samples by UHPLC-MS/MS. A BEH C18 (50.0 × 2.1 mm, 2.5 μm) column was used; the mobile phase consisted of acetonitrile-0.1% formic acid (70:30, v/v), the flow rate was 0.2 μl·min-1 and the column temperature was 30°C. Positive-ion mode with multiple reaction monitoring detection was used to quantify cannabidiol (m/z 315.1 → 193.1) while diphenhydramine (m/z 256.3 → 167.08) served as the internal standard. Good linearity was shown in the range of 1-200 ng·ml-1 for cannabidiol with correlation coefficients of >0.999. The LLOQ was 1 ng·ml-1 . The intra-day and inter-day RSDs of cannabidiol were all <2%. A cryo-sectioning technique combined with the UHPLC-MS/MS method was used to successfully determine cannabidiol levels in a series of very thin skin layers.

Rational Design of Topical Semi-Solid Dosage Forms-How Far Are We?

Specific aspects of semi-solid dosage forms for topical application include the nature of the barrier to be overcome, aspects of susceptibility to physical and chemical instability, and a greater influence of sensory perception. Advances in understanding the driving forces of skin penetration as well as the design principles and inner structure of formulations, provide a good basis for the more rational design of such dosage forms, which still often follow more traditional design approaches. This review analyses the opportunities and constraints of rational formulation design approaches in the industrial development of new topical drugs. As the selection of drug candidates with favorable physicochemical properties increases the speed and probability of success, models for drug selection based on theoretical and experimental approaches are discussed. This paper reviews how progress in the scientific understanding of mechanisms and vehicle-influence of skin penetration can be used for rational formulation design. The characterization of semi-solid formulations is discussed with a special focus on modern rheological approaches and analytical methods for investigating and optimizing the chemical stability of active ingredients in consideration of applicable guidelines. In conclusion, the combination of a good understanding of scientific principles combined with early consideration of regulatory requirements for product quality are enablers for the successful development of innovative and robust semi-solid formulations for topical application.

Lipid-based nanoformulation optimization for achieving cutaneous targeting: niosomes as the potential candidates to fulfill this aim

The present study screened the utility of topically-applied nanoformulations to target the drugs/actives into the skin reservoir with the reduction of possible systemic absorption. The lipid-based nanoformulations selected in this study included solid lipid nanoparticles (SLNs), nanostructured lipid carriers (NLCs), nanoemulsions (NEs), liposomes, and niosomes. We loaded flavanone and retinoic acid (RA) as the penetrants. The prepared nanoformulations were assessed for their average diameter, polydispersity index (PDI), and zeta potential. An in vitro permeation test (IVPT) was utilized to determine the skin delivery into/across pig skin, atopic dermatitis (AD)-like mouse skin, and photoaged mouse skin. We found an increased skin absorption of lipid nanoparticles following the increase of solid lipid percentage in the formulations (SLNs > NLCs > NEs). The use of liposomes even reduced the dermal/transdermal selectivity (S value) to lessen the cutaneous targeting. The niosomes resulted in significantly greater RA deposition and reduced permeation in the Franz cell receptor compared to the other nanoformulations. The S value of the RA delivery via stripped skin was increased by 26-fold in the niosomes compared to the free RA. The dye-labeled niosomes displayed a strong fluorescence in the epidermis and upper dermis through the visualization of fluorescence and confocal microscopies. The cyanoacrylate skin biopsy manifested greater hair follicle uptake of the niosomes compared to the free penetrants by 1.5 to three-fold. The 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay indicated an increase in antioxidant ability from 55% to 75% after flavanone entrapment in the niosomes. In the activated keratinocytes, the niosomal flavanone could suppress the overexpressed CCL5 to the baseline control because of the facile cell internalization. After the formulation optimization, the niosomes with higher phospholipid amount had a superior effect in delivering penetrants into the skin reservoir, with limited permeation to the receptors.

Influence of Molecular Structure and Physicochemical Properties of Immunosuppressive Drugs on Micelle Formulation Characteristics and Cutaneous Delivery

The aim of this study was to investigate whether subtle differences in molecular properties affected polymeric micelle characteristics and their ability to deliver poorly water-soluble drugs into the skin. D-α-tocopherol-polyethylene glycol 1000 was used to prepare micelles containing ascomycin-derived immunosuppressants-sirolimus (SIR), pimecrolimus (PIM) and tacrolimus (TAC)-which have similar structures and physicochemical properties and have dermatological applications. Micelle formulations were prepared by thin-film hydration and extensively characterized. Cutaneous delivery and biodistribution were determined and compared. Sub-10 nm micelles were obtained for the three immunosuppressants with incorporation efficiencies >85%. However, differences were observed for drug loading, stability (at the highest concentration), and their in vitro release kinetics. These were attributed to differences in drug aqueous solubility and lipophilicity. Differences between the cutaneous biodistribution profiles and drug deposition in the different skin compartments pointed to the impact of differences in thermodynamic activity. Therefore, despite their structural similarities, SIR, TAC and PIM did not demonstrate the same behaviour either in the micelles or when applied to the skin. These outcomes indicate that polymeric micelles should be optimized even for closely related drug molecules and support the hypothesis that drugs are released from micelles prior to skin penetration.

The effectiveness of synthetic methoxylated isoflavones in delivering to the skin and alleviating psoriasiform lesions via topical absorption

This study was conducted to appraise the possible potential of synthetic isoflavones (SIFs) on psoriasis treatment. A practical and easy-to-operate approach was employed in synthesizing a series of SIFs, considering that acquiring flavonoids from natural resources is usually expensive, time-consuming, and non-eco-friendly. Seven SIFs derived from daidzein were produced with differences in the location of the hydroxyl groups and degree of methoxylation. The in vitro and in vivo skin absorption of topically applied SIFs was estimated. Further, keratinocytes (HaCaT) were employed as the model to investigate the anti-inflammatory activity of the isoflavones. The lipophilicity was increased from SIF-1 to -7. Noteworthily, there was a parabolic relationship between lipophilicity and skin absorption, with SIF-5 (4',7-dihydroxyisoflavone, daidzein) and SIF-6 (7-hydroxy-3',4'-dimethoxyisoflavone, cladrin) demonstrating the highest retention in pig skin. The methoxylated isoflavone SIF-5 showed the greatest permeation into barrier-deficient skin among the compounds tested, with a 6- and 8-fold increase after lipid and protein removal. The cell-based study exhibited the capability of SIFs to restrain the overexpressed IL-6, IL-8, and CXCL1 in stimulated HaCaT. The therapeutic index (TI) predicted the potential candidates of SIF-5 and SIF-6 for topical application to treat psoriatic inflammation. The imiquimod (IMQ)-driven psoriasiform murine model manifested the inhibition of hyperplasia and immune cell infiltration by topically administered SIF-5 and SIF-6. The epidermal thickness of IMQ-treated skin was decreased from 172 to 40 μm by both isoflavones. This effect was comparable with that of betamethasone, the positive control. The topical treatment of SIF-6 significantly reduced cytokine/chemokine upregulation by IMQ. The methoxylated isoflavone with dramatic anti-inflammatory activity is promising for the development of an antipsoriatic agent.

Effect of mRNA Delivery Modality and Formulation on Cutaneous mRNA Distribution and Downstream eGFP Expression

In vitro transcribed messenger ribonucleic acid (mRNA) constitutes an emerging therapeutic class with several clinical applications. This study presents a systematic comparison of different technologies—intradermal injection, microneedle injection, jet injection, and fractional laser ablation—for the topical cutaneous delivery of mRNA. Delivery of Cy5 labeled mRNA and non-labeled enhanced green fluorescent protein (eGFP) expressing mRNA was investigated in a viable ex vivo porcine skin model and monitored for 48 h. Forty 10 µm-thick horizontal sections were prepared from each skin sample and Cy5 labeled mRNA or eGFP expression visualized as a function of depth by confocal laser scanning microscopy and immunohistochemistry. A pixel-based method was used to create a semi-quantitative biodistribution profile. Different spatial distributions of Cy5 labeled mRNA and eGFP expression were observed, depending on the delivery modality; localization of eGFP expression pointed to the cells responsible. Delivery efficiencies and knowledge of delivery sites can facilitate development of efficient, targeted mRNA-based therapeutics.

DESI-MS imaging to visualize spatial distribution of xenobiotics and endogenous lipids in the skin

The cutaneous biodistribution method (CBM) yields a high-resolution quantitative profile of drug deposition as a function of skin depth. However, it provides limited details about drug spatial distribution or penetration pathways. Mass spectrometry imaging (MSI) can complement the detailed quantitative data generated by CBM studies. The objectives of this work were to use desorption electrospray ionization (DESI)-MSI to (i) investigate the spatial cutaneous distributions of a topically applied drug and excipient and relate them to skin structures and (ii) image endogenous skin components and combine these results to gain insight into drug penetration routes. Porcine skin was used to compare two bioequivalent creams of econazole nitrate (ECZ) and a micelle formulation based on D-α-tocopheryl succinate polyethylene glycol 1000 (TPGS). DESI-MSI successfully imaged the cutaneous spatial distribution of ECZ and TPGS in 40 µm-thick horizontal sections and vertical cross-sections of the skin. Interestingly, clinically bioequivalent formulations did not appear to exhibit the same molecular distribution of ECZ in XY-horizontal sections. DESI-MSI also enabled visualization of TPGS (m/z 772.4706), mainly in the upper epidermis (≤80 µm). In conclusion, through co-localization of drugs and excipients with endogenous elements of the skin, DESI-MSI could further our understanding of the cutaneous penetration pathways of xenobiotics.

Polymeric micelle formulations for the cutaneous delivery of sirolimus: A new approach for the treatment of facial angiofibromas in tuberous sclerosis complex

Facial angiofibromas are benign tumors characteristic of tuberous sclerosis complex. The disease involves the mTOR pathway and the cutaneous manifestation responds to topical treatment with sirolimus (SIR). However, there are no approved topical SIR products and extemporaneous formulations have been sub-optimal. The aims of this study were (i) to develop aqueous formulations of SIR loaded in polymeric micelles prepared using D-α-tocopherol polyethylene glycol 1000 succinate (TPGS) and (ii) to use the cutaneous biodistribution method, in conjunction with a new statistical approach, to investigate the feasibility of SIR delivery to the viable epidermis. Optimized micelle solutions and hydrogels (0.2%) were developed and stable at 4 °C for at least 6 and 3 months, respectively. Cutaneous delivery experiments (infinite and finite dose) using porcine skin demonstrated that both formulations increased SIR cutaneous bioavailability as compared to the control (ointment 0.2%). Moreover, studies with the micellar hydrogel 0.2% demonstrated SIR deposition in the viable epidermis with no transdermal permeation. These encouraging results confirmed that polymeric micelles enabled development of aqueous SIR formulations capable of targeted epidermal delivery. Furthermore, the cutaneous biodistribution provided a detailed insight into drug bioavailability in the different skin compartments that could complement/explain clinical observations of formulation efficacy.

Effect of continuous and multi-phasic current profiles on the iontophoretic transport of pramipexole, rasagiline and huperzine A: Depicting temporal variation and biodistribution in the skin

Transdermal iontophoresis is an interesting option for the non-invasive controlled delivery of therapeutic agents to treat neurodegenerative diseases. The current profile controls drug delivery kinetics and enables complex drug input profiles to be obtained. The aim of this study was to investigate the temporal variation in transport of pramipexole (PRA), rasagiline (RAS) and huperzine A (HUP) using continuous and multi-phasic current profiles by measuring cumulative permeation, transdermal flux and drug retention in the skin upon modulation of the applied current profile during a single experiment in vitro. Initial experiments with continuous current were conducted to establish a correlation between total delivery of PRA, RAS and HUP (i.e. sum of the cumulative permeation and skin deposition) and the amount of charge transferred. Subsequent experiments with multi-phasic current profiles, confirmed that the relationship between amounts of charge transferred and total delivery was able to predict the total delivery of each drug. Experimental values were within ± 15% of the predicted values. Current density and duration of current application were also shown to have a significant impact on the skin biodistribution of PRA. These results also provide insight into the rate of formation of iontophoretic drug reservoirs in the skin.

Plasma pharmacokinetics and distribution of ruxolitinib into skin following oral and topical administration in minipigs

This preclinical study compared plasma concentrations and distribution of ruxolitinib in the skin of Göttingen minipigs following twice a day oral (40 mg/kg) versus topical administration (1.5% w/w cream applied to 10% of body surface area). Following oral administration, the plasma area-under-the-curve was approximately 31-fold and maximum drug concentration was 38-fold higher than those observed following topical application. Following ruxolitinib cream application, the average plasma concentration at steady-state was 2.7 ± 1.8 nM, a concentration that is not pharmacologically relevant. The average total dermis concentration of ruxolitinib at steady-state after topical administration was 507-fold higher versus that following oral dosing, while the ratio for the total epidermal concentration following topical vs oral dosing was 1989-fold. The concentration of unbound ruxolitinib in the dermis after topical application was predicted to result in sustained and near-complete inhibition of Janus kinase/signal transducer and activator of transcription proteins (JAK/STAT) signaling in this tissue. In contrast, only partial inhibition of downstream signaling was predicted to occur after oral dosing. In conclusion, ruxolitinib cream affords an attractive disposition profile in minipigs, wherein dermis concentrations of ruxolitinib are fully effective whereas corresponding plasma concentrations are negligible. Consequently, this distribution profile should maximize the efficacy of ruxolitinib cream in the skin while minimizing the potential for deleterious systemic effects.