Laboratory Testing Blog

Practically, in vivo skin absorption measurement studies are rare due to ethical, economical, and analytical concerns. Therefore, a shift in focus has been made to developing and validating alternative in vitro test methods. The diversity of existing in vitro and in vivo techniques shows the difficulties of comparing results between different methods, species, ages, as well as healthy and diseased skin.

Transdermal patches are designed to slowly deliver the drug substances through the intact skin, resulting in a prolonged and adequately constant systemic absorption rate.  The rate limiting step for systemic absorption of the drug substance is usually the absorption through the skin.  Absorption may also be limited by incorporating or dissolving the drug substance in a semisolid reservoir with a membrane to control the release and the diffusion of the drug substance(s) from the transdermal patch.  A transdermal patch can also be formulated combining both drug delivery principles as the means of controlling drug delivery to the surface of the skin.

Recombinant monoclonal antibodies (mAbs) are large, hetero­geneous proteins that have emerged as therapeutics due to their predictable properties, controlled functions and long circulating life.  mAbs represent a class of advanced, but expensive, medi­cines.  Recently, there has been increasing interest by both innovator and generic drug companies in developing less costly biosimilar versions of mAbs.

Quality control testing of human skin models is a crucial element to consider when performing in vitro percutaneous absorption studies.  This fact is reflected in various toxicological regulations such as the European Union's Scientific Committee on Consumer Safety (SCCS) test guidelines for for the in vitro assessment of dermal absorption of cosmetic ingredients.

One of the most important considerations in the drug manufacturing process is safety, not only of the drug itself, but also impurities and degradation products.  Impurities present in the active pharmaceutical ingredient (API) have to be identified to ensure no mutagenic or toxic substances will be administered to patients.  Drug product degradation profiles need to be established to guide stable formulation development and provide suitable drug shelf-life assessment.  Drug regulatory agencies also have requirements for characterization of the impurity profile of a pharmaceutical.

In the European Union and some other countries, it is general practice while performing in vitro skin penetration studies, to exclude the amount of Test article that was found in the first (upper) two tape strips at study completion both in vitro and in vivo.

Driven by regulatory requirements, skin penetration and absorption studies are becoming essential for safety and efficacy evaluation of test substances or formulations.  In vivo, substances topically applied onto the skin surface can diffuse into and penetrate through the skin using three distinct pathways, namely the intercellular and the transcellular route across the stratum corneum as well as the route along the skin appendages (hair follicles and sweat glands).  Both the percutaneous penetration and the absorption into different skin compartments of topically applied test substances can be evaluated by using a Franz diffusion cell apparatus.

The United States Food and Drug Administration (U.S. FDA) recently released a guidance document for stability testing of Abbreviated New Drug Application (ANDA) products in an effort to spur discussion on what the new guidelines should contain.  The guidance describes the FDA’s current thinking on this topic and are only recommendations at this time.  In this guidance document, FDA suggests that ANDA's submitted and the Drug Master Files (DMF's) that support ANDA's should follow the stability recommendations provided in International Conference on Harmonization (ICH) stability guidances.

In vitro method validation procedures for skin absorption and drug release studies can be somewhat challenging if not approached in the appropriate manner.  Below are 5 steps to ensure that method validation work for skin absorption and drug release studies will proceed in an orderly manner:

Impure substances present in pharmaceutical products have to be identified to make sure no mutagenic or toxic substances will be administered to patients.  Impurity profiling of pharmaceutical products must be established to guide stable formulation development and provide suitable drug shelf life assessment.  Impurity profiling of pharmaceuticals i.e., the identification, isolation, characterization and purification of these impurities is required.  The traditional approach in impurity identification involves isolation and purification by semi-preparative HPLC, followed by characterization using NMR spectroscopy, accurate MS or LC-MS/MS methods.