I think that you need to prepare a 50 mg/ml stock solution of Verteporfin in DMSO to start. If the product in your hands has not the necessary solubility you should find a different source.

Try to dilute your Verteporfin stock solution with Tween 80 and Water to have the solution for injection. (i.e. stock solution:Tween:Water = 20:10:70 to have 10 mg/ml)

Best wishes. 

You can try the first stock concentrated solution in DMSO then try to dilute it in different mixtures: tween80:water, tween 80:propylene glycol,  inclusive adding water can work 

You can use 1-5% water solution PEG (for example 6 000) for dilute concentrated solution in DMSO. 

Thank you eveybody for your kind and helpful answers! I'l try.

Juliette