As I am no pharmacist, but doing formulation experiments at the moment I noticed a difference in viscosity using different amount of water/oil when making emulsions. How does that affect the formulation? Is there an upper limit of viscosity when iv administration is considered?
Yes there certainly is an upper limit of viscosity for IV administration as too high of a viscosity could lead to an occlusion.
Generally speaking if you are using a viscous solution it is because your compound will not go into solution.
If possible you should try to get the compound into solution so you do not need to depend on a suspension for delivery. Alternatively you could consider administering your drug via IP, IM or subcutaneous injection.
If you must administer via IV then if possible administer the volume very slowly rather than as a single bolus.
I agree with Dr Ratner. A viscous solution is very difficult to administer accurately and safely i.v. as it can pose significant risks to a patient/animal (occlusion as stated). The pressure required to eject it from a syringe may also cause undue tissue/vessel damage and stress to the one being injected. Suspensions that are viscous are best off either diluted further in an appropriate solvent to reduce viscosity and injected slowly or administered i.m/i.p/s.c. If you have a good emulsion with the right oil vehicle, it should absorb slowly using the non-venous routes, but again a viscous solution may have issues; oily solutions can cause pain and tissue damage, particularly i.m and s.c. If you dilute your emulsion further to reduce viscosity, the volume will obviously increase so you may need to consider slow infusion by using pump and i.v catheter. If you do need to inject i.v. i suggest you 1/ try a different emulsion formulation that is less viscous, or 2/ as already suggested by Doc Ratner, get your compound into solution and not an emulsion. LogP of your compound can often give insight into which vehicle to choose. PEGs, PG and DMSO dilutions are often a good choice (but you probably already know this one). Remember that if your emulsion/compound of interest crashes from suspension if water is added, this will highly likely happen i.v. when it hits an aqueous environment like plasma... and that will be no good for you patient/animal or results.
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