PPP: Perspectives in Percutaneous Penetration

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PPP: Perspectives in Percutaneous Penetration

Published: May 7, 2008
AltTox invited Dr. William Dressler, a consultant and toxicologist retired from the Procter & Gamble Company, to provide a summary of the 11th International Conference on Perspectives in Percutaneous Penetration (PPP), which he attended. The conference was held March 26-29, 2008 in France. Though certain of the subject matter is somewhat technical, we think the many toxicologists dealing with dermal penetration studies who could not attend the meeting will find it useful. As for others, Bill’s report illustrates the wide usage of in vitro percutaneous penetration methods to satisfy regulatory and informational needs using a variety of protocols.

Approximately 170 delegates from more than 20 countries attended the 11th PPP meeting. This conference is held biennially, more recently with a broader theme than an earlier focus on the “prediction” of percutaneous penetration. The meeting participants typically include a core group of experts, several of whom share their knowledge and insights in a pre-conference course on the “Fundamentals of Percutaneous Penetration” attended by relative newcomers to this field.

The purpose of this conference, as stated by the organizers (Keith Brain and Ken Walters, Cardiff, UK) is to provide a forum for the presentation and discussion of current research, development, and technology concerning the penetration of exogenous compounds through the skin. As always, there was considerable focus on in vitro skin permeation approaches, while the in vivo testing that was discussed usually related to human studies associated with drug delivery or research methodologies.

This year’s program featured a plenary session on the skin penetration of nanoparticles in vitro and on the physical enhancement of penetration in human subjects using iontophoresis. A special symposium on the “Bioavailability and Bioequivalence of Topical Dosage Forms” included talks on in vitro approaches. Another session focused on dermal risk assessment and an International Committee on Harmonization workshop was held on skin notation (hazard labeling). Poster sessions complemented the oral presentations.

Of particular relevance to in vitro alternative approaches was a Topical Discussion entitled “In Vitro Permeation Studies: We have the guidelines now what about the protocol?” Speakers in this session (chaired by Howard Maibach and Gordon Flynn) noted the development of various guidelines and workshop recommendations over the last 30 years for skin permeation studies ranging from FDA/AAPS, ECVAM, COLIPA, SCCNFP, OECD, EPA and UN/ILO/WHO.

The diversity of such guidelines was attributed to the broad utility of this in vitro methodology for a variety of applications in different fields. The chemical, agrochemical, pharmaceutical, and cosmetic industries all utilize in vitro skin permeation methodology. Industry objectives vary from the development, comparison, and optimization of formulations to promote (or retard) skin penetration, the development of databases for modeling, and/or the generation of data for risk assessments used for regulatory submissions. The advantages for this in vitro approach extend from reductions in animal usage, to the ability to use the most relevant (human) tissue with formulations applied under simulated use conditions, to the flexibility in using radiolabeled compounds. In addition, the ability to study materials early in the development stage before  toxicological characterization has been completed can impact the scope and optimize the design of subsequent toxicity testing, possibly reducing animal usage even further.

Given the diversity of applications, the discussion consensus was that there is unlikely to be a “one size fits all” protocol and that protocol designs should likely remain in the domain of the skin researchers rather than regulatory agencies alone. A poster presented by Raney, Lehman, and Franz illustrated areas where there was either consensus, general agreement, or an apparent lack thereof with respect to in vitro study design parameters across 10 published in vitro skin permeation guidelines.

There was consensus (70-100% agreement) on: options to use either static or flow-through diffusion cells; the use of split-thickness skin (human preferred, frozen acceptable) from the trunk (abdomen, breast, back); use of a minimum of three donors with 6 replicates per donor; maintaining a 32o C skin temperature; a need for a pre-study skin integrity test (tritiated water flux, transepidermal water loss, transcutaneous resistance); and on an 8-24 hour study duration. A consensus also extended to receptor fluid options of buffered saline, tissue culture media, and surfactant addition [up to 6% polyoxyethylene(20)oleyl ether] to ensure no more than a 10% maximum saturation of the receptor fluid as well as to a need for continuous stirring of the receptor medium. Guidelines were typically open to the use of finite or infinite dosing and to employing radioisotope or HPLC/MS analytical approaches.

Less widely, but generally accepted (50-60% agreement) parameters related to: the use of pig skin as an alternative to human skin; application of semisolids in the range of 2-5 mg/cm2 and liquids in the range of 5-10ul/cm2; GC/MS analytical methods; maxima for ethanol (50%) and bovine serum albumen (4%) receptor fluid additions; and 90-110% as an acceptable mass balance range.

Parameters specified for less than half of the guidelines related to: limitations on the storage time for frozen skin (3-12 months); acceptability of study durations of 48-72 hours; criterion for maximum water flux for integrity checks (2.5×10-3 cm/hr); and a minimum of 10 sampling time points.

Open protocol and data reporting/interpretation issues, some of which were voiced in the Topical Discussion included: tissue harvesting time parameters and donor demographics; methods of skin preparation and storage conditions/duration; acceptable study durations and materials; and methods for tape stripping. A key interpretive point for risk assessment applications that remains open is the lack of clear criteria for inclusion or exclusion of skin amounts in the estimation of systemic exposure. Suggestions on how this may be approached have been discussed in the open skin permeation literature, however.

Though not a guidance topic, the importance of an option to use viable skin in vitro as an alternative to frozen skin in selected circumstances was emphasized in another PPP presentation (G. Nohynek, L’Oreal). For example, a common oxidative hair dye ingredient, p-aminophenol, was quantitatively N-acetylated in viable skin resulting in systemic exposure to only the acetylated form, N-acetyl p-aminophenol (commonly known as acetaminophen) and not to the parent compound.

In sum, in vitro approaches enjoy wide applications in the skin permeation area. This has been driven primarily by the ability to utilize relevant human skin and early research showing excellent concordance with in vivo data obtained under similar exposure conditions with a range of compounds with varying permeability. The various guidelines and recommendations published over the last 30 years still lack accord on certain parameters, considered by some as important or even critical to achieving better inter-laboratory agreement for in vitro results. Systematic study on the impact of these parameters on the consistency of study results across laboratories may be warranted. In any event, it is important that in vitro skin permeation guidelines maintain the requisite flexibility to best address individual study objectives.

The PPP meeting and the traditional pre-conference course serve as useful forums to learn and discuss the latest developments and applications of in vitro skin permeation methods and relevant research modalities. Further information about the meeting and the forthcoming proceedings can be found at www.pppconference.org.