If you mean the loading ratio (%) of the drug into PLGA particles (mg drug/mg PLGA), it's quite easy. You can dissolve 1 mg of your drug loaded PLGA particles in 1 ml of the primary organic solvent or DMSO and after the particles are degraded, study the optical density or fluorescence intensity of the drug (depending on the features of your drug) and find the amount of it, which would equal the "Drug content per PLGA particle" ratio.

thanks Mr kianpour, but I mean the partition coefficient. it means ratio of drug solubility in PLGA to drug solubility in one solvent (such as water). I dont want the experimental method. I wanted to know if there is an article in which I can find this coefficient!

Well, I have absolutely no idea how this parameter is measured or what it's used for, but what I can say for sure is that this parameter must be totally unique for each particular drug depended on their physicochemical properties, which makes it difficult to find it in literature! So I can only wish you luck pal.

Good luck