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Topical delivery of actives
Wiki states this:
Water-in-oil creams are more difficult to handle but many drugs which are incorporated into creams are hydrophobic and will be released more readily from a water-in-oil cream than an oil-in-water cream.Is this always true?hydrofobic active - use a W/Ohydrophilic - use an O/W? -
6 Replies
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No.
Remember, water is volatile, so as you rub a cream into skin, the water evaporates off, no matter what your emulsion type is.The difference is whether or not you are left with an occlusive, coherent film of oil. -
Good point but I am not sure I understand it right. Are you referring to that certain O/W emulsions turn W/O once on the skin?
Or do you mean that the emulsion type is unimportant and/or other factors weigh more heavily when formulating for optimal delivery of an active? -
@ David If you are concerned about topical delivery, have you considered looking into penetration enhancers?
Common penetration enhancers are propylene glycol and ethanol, but there is a wide variety of molecules that use various functions to help deliver actives. Please be cautious when using penetration enhancers though (and especially with associated claims) as you do not want to turn your product into a drug.
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@ gfeldman that is also of interest, but as a first step I have to choose between W/O or O/W.
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@David, I was trying to say that the emulsion type is less important when formulating for optimum delivery. Because of the emulsions structure, a w/o emulsion is much more likely to deliver a continuous (and therefore occlusive) layer on the skin. This gives a w/o emulsion an advantage when trying to deliver an oil-soluble ingredient.
Delivering a water-soluble ingredient effectively is harder with a w/o emulsion, and it is also difficult to get a w/o emulsion to feel elegant on skin. -
@David I would suggest selecting the continuous phase as the phase that the active is soluble in.
@Bobzchemist I do believe that the phase the active is added to could make a difference, I am still uncertain about delivery, but for efficacy…
Rationale (mostly speculation based on my work with proteins, small molecule drugs, and microsphere controlled release); If you add the active to oil or water phase before mixing, then during droplet formation the active will likely partition between both phases multiple times (this also introduces mechanical stress). When the active is at the interface between the oil and water phase it will be under significant stress which could very well degrade the molecule (chemical stress).
If you add the active after the droplets have formed then it will easily solublize and disperse into the continuous phase with less mechanical stress and chemical stress. Alternatively, if it were not soluble in the continuous phase then I could imagine a mess of actives stuck at the interface or even forming small nanodroplets of themselves surrounded by water, either way.. not ideal.
For a great leap forward in active delivery/stability I would suggest a W/O/W or O/W/O delivery system, but that is beyond most formulators needs.
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