People often cite the fact that emulsifiers like kolliphor el will cause hypersensitivity reactions and use to justify liposomal delivery as an alternative. However, stable liposomes that evade RES use PEG, which is also documented to cause hypersensitivity. Since liposomes are significantly more expensive to produce than O/W emulsions for example, I don't see what the big deal is? I tested the size of a 1100 MW drug in ethanol:kolliphor 1:1 diluted in water today and saw it makes uniform (PdI< .1) droplets about 13nm. Compared to a liposomal formulation of the same drug, which is ~90nm and same PdI. Isn't smaller better? Isn't the microemulsion still "technically" a nanosize delivery system? Or would the field define this as "free" drug? Because it's not really free, is it? It's trapped in a droplet of oil.
1.By chemical composition, liposomes are similar to natural cell membranes
2.There are 70% water in the human body. Such an environment is an ideal medium for the existence of liposomes, rather than microemulsions.
3.Critical parameter of the packing of surfactants for microemulsions (micelles) N
I agree with Yuri. However N3: the packing parameter of microemulsions can vary from less than 1/3 to more than 1 (direct microemulsions, bicontinuous microemulsions and inverse microemulsions respectively)
if you 'saw' droplets (if there are droplets) then it is a nanoemulsion, not a micro'emulsion'. The latter is a thermodynamically stable solution, not an emulsion containing defined oil droplets. It needs high surfactant concentration relative to solubilized oil, even more than a nanoemulsion. The smaller the droplets the more surfactant is needed for stabilization because overall interface to be stabilized is growing exponentially. Liposomes do not necessarily contain PEG. Classic liposomes are based on phospholipids.
Thank you all for your very informative comments. I'm not sure what I am seeing with the kolliphor:ethanol formulation, however I did find that without drug in the mixture, it still appeared to form uniform droplets or nanoemulsion or whatever (I don't know what the correct terminology would be). I am mainly curious if 80-90nm PEGylated liposomes containing my drug should be better than the former, and if so, then why? For example, are the known pharmacokinetics properties for liposomes of this size significantly better than the castor oil based 13nm things? EPR effect? Clearance rate? Etc.
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