# Raman Spectroscopy to Monitor the Delivery of a Nano-Formulation of Vismodegib in the Skin
## 1 Introduction  
The epidermis, which makes up the majority of the surface of the skin, is composed of keratinocytes produced from the basal stratum and is divided into many layers. The outermost epidermal layer, known as the _stratum convenum_ (SC), is made up of corneocytes that are encased in a lipid matrix. Vismodegib (VDG) (Erivedge(r), Genentech) is a novel antineoplastic drug approved for its use by oral administration in basal cell carcinoma, a type of skin cancer [1]. In recent years, research has been conducted on the local administration of VDG through topical applications in order to minimize systemic distribution and mitigate potential side effects. [2, 3]. Transfersomes are a special kind of unilamellar liposomes, composed of a mixture of phospholipids and an edge activator that confers deformability to their membrane, and they are among the carriers that have been proven useful to deliver VDG to the viable epidermis. Their enhanced flexibility and fluidity make them especially useful for skin delivery, as they can pass through the very narrowchannels that exist in the SC to reach the viable epidermis, where they can release their cargo [4]. In order to determine whether a compound loaded into transfersomes, or any other skin delivery system, effectively overcomes the SC barrier and reaches the viable epidermis, it is necessary to assess its distribution across these skin regions. For this purpose, _in vitro_/_ex vivo_ techniques, such as Franz cell permeation studies or penetration studies using the Saarbrucken penetration model, are employed or, alternatively, in vivo incubation in animal models, which requires sacrificing the individuals prior to determination. In living human subjects, the tests can, at most, investigate the distribution within the SC, using the in vivo tape stripping technique--which causes an increase in trans epidermal water loss, is painful, and leaves the skin damaged for a period of time--without providing insight into the distribution of the compound in the layers of the viable epidermis and beyond [5].  
On the other hand, Raman spectroscopy is a sensitive and non-destructive technique that involves the use of optical filters to analyze the scattered radiation and obtain information about molecular structure, relying on the emission of photons, which form the unique fingerprint of functional groups [6]. Thus, it provides not only qualitative information, but also quantitative conformational analysis [7]. It is based on inelastic scattering, and measures the change in energy of the scattered radiation. Raman scattering is considered a weak, second-order process due to the interaction of the laser photons and the vibrational mode of the sample under analysis. It is a low-damage, high-sensitivity method, which make it suitable for studying lipids and drugs [8], and its potential to detect molecules in the skin has been pointed out in the past decade [9]. Recent advancements in Raman spectroscopy have expanded its capabilities, allowing for more precise and detailed analyses of samples. The Raman spectrum of VDG was previously studied [10] through their characteristic functional groups, such as pyrimidine, amide, sulfone, phenyl, and chlorine groups. Therefore, Raman spectroscopy could be of great utility in detecting compounds loaded in drug delivery systems for topical-cutaneous administration, allowing for safe monitoring of their effective arrival to deep skin layers in living individuals. This could contribute to improved treatment monitoring and optimization of dosages, by correlating the actual amount of the compound in the skin with measured effects. To pave the way towards that aim, in this work, the Raman technique is used to determine VDG directly from human and pig skin samples.