Here’s a brief overview of trandermals that may or may not prove useful.
Cutaneous Delivery Systems
The Basics
Why this method of delivery?
The oral administration of drugs, although convenient, does not always permit optimal or even sufficient bioavailability. Many chemicals are destroyed in the GI tract or are metabolized almost completely by the liver and never get to the rest of the body. Taking orals also requires multiple doses throughout the day to maintain a consistent amount of the drug in the system: thus when sleeping the levels of the drug will drop dramatically. Cutaneous delivery results in higher bioavailability and has the benefit of time release/sustained release. .
What does a cutaneous delivery system consist of?
There are two main components in cutaneous delivery systems, penetration enhancer(s), and carrier/solvents. Penetration enhancers allow the drug to pass more readily through the skin while the carrier/solvent is the delivery vehicle through which the drug is carried through the skin. Other components may be added to a to counteract side effects from one of the main components, all of which will be discussed in further detail herein.
How does it work?
One of the greatest difficulties in cutaneous drug delivery is permeation of the stratum corneum, the outer most layer of skin. The stratum corneum acts as a barrier to the environment thus also impeding absorption of our chemicals. Other factors also affect absorption/penetration; the size of the molecule of the drug, it’s lipophilic/hydrophilic balance and it’s polarity. The larger the molecular weight(>500) of the drug being delivered the harder it is for it to penetrate the skin. Drugs with too high or low a hydrophilic or lipophilic balance also have difficulty permeating the stratum corrneum .A substance is lipophilic if it is able to dissolve much more easily in in lipid/oil than in water. Prohormones are relatively small molecules and are lipophilic in nature making transdermal delivery viable.
Most drugs must be “encouraged” to go through the skin as very few molecules can penetrate the skin. The more water in the upper layers of the skin the more readily things will pass through. Drinking water(being adequately hydrated), using skin moisturizers or being in a high humidty environment may improve rate of absorption/penetration albiet only slightly. Another approach involves physical means such as shaving or exfoliating to reduce the thickness of the skin. Other physical means could involve electrical or sound waves to influence the integrity of the skin but no viable practical methods currently exist. The last and most popular if not effective approach is the use of chemicals to increase permability, often referred to as penetration enhancers(PE). Fairly long-chain alcohols(laurrl alcohol), fatty acids(oleic acids) and other fairly large molecules(esters such as isopropyl myristate) can increase skin permeability. These PEs tend to include aliphatic groups that make formulations oily, malodorous and lead to skin irritation.
Bioavailabilty/Effectiveness
The bioavailability of a drug delivered via cutaneous transportation will vary according to the formulation. Some products claim over 70% of the drug will reach the blood system while most claim about 30-40%. Ancedotal evidence suggests 30% bioavailability is the best case scenario.
Application
Cutaneous delivery products are best applied after a warm/hot shower. The water increases hydration of the skin and the heat dialates pores and increases blood flow. The area of application should be scrubbed/exfoliated to removed dead skin cells. Ideal application areas include the inner part of the arms, inner thighs, back of knees, tops of feet where skin is the thinner and surface area adequate. Any hair on the application site should be shaved so that none of the delivery system sticks to the hairs. Shaving also exfoliates the skin improving penetration.
The traditional delivery approach consisted of the penetration enhancers, solvents/carriers and drug all residing in one compound/bottle. One would calculate the appropriate dose and rub it into the skin and wait until dry. Newer methods of application have been developed and should increase bioavailability. The ideal method of application is a three step process. The area should be treated with the penetration enhancers first. After 10-15 mins or when dry apply the carrier/solvent/drug on. Rub this compound into the skin. Wait until dry then apply the penetration enhancers again to permeate any residual carrier/solvent/drug. If the penetration enhancers and solvent/carrier is premixed then buy two bottles and add the drug to only one of the bottles. Use the product that doesn’t contain the drug as the penetration enhancer in the aforementioned method.
Regardless of the method used to apply the delivery system certain guidelines should be followed. Any areas coming in contact with the delivery system must be clean so that no transient or unwanted chemicals are introduced into your system. Avoid sweating or getting wet for at least 30-45 mins after application to keep chemicals from washing away. Do not let others(especially children and women) come in contact with the application areas as it is possible to transmit these chemicals to them.
Cutaneous delivery products should be applied every 12 hours and the application site should be rotated to maximize effectiveness.
Now that we’ve covered the basics of cutaneous delivery systems we will delve into the specifics of these systems.
Topical(site specific/localized) vs Systematic Cutaneous Delivery Systems
A transdermal product would seem like the ideal delivery system for topical adminsitration but several limitations need to be resolved.
The stratum corneum (SC)is made up of ordered lipid bilayers separated from one another by thin layers of water. Thus diffusion through this area will be hindered by H bonding. This makes hydrophilic substances difficult to penetrate. Once past the SC we encounter the epidermis and the dermis. These layers are hydrophilic making diffusion of lipophilic coumpounds quite difficult. In this region is the dermal vasculature whereby a compound will become absorbed and distributed into the bloodstream and become systematic.
What this means is that the carrier, substance, and even penetration enhancer must exhibit dual properties. They need to be fat-soluble and then become water-soluble and vice versa as they penetrate the layers of the skin and into the desired tissues. This is considered the rate limiting step in a transdermal formula, ie impossible to overcome.
Even if someone miraculously overcomes this, isolated localized delivery is still not possible. Sub cutaneous or deep tissue injections with steroids have shown that the drug will not remain localized. It diffues through the tissue and into the bloodstream whereby it becomes localized.