I'm planning to transfect double strand oligo into prostate cancer cell line, can the oligo last in the cells for 4 days? Can you define the half life of oligo with partial modification?

You may need to do multiple treatments with the antisense oligo if you need to reduce expression for four days.  You may also want to consider using a chimeric oligo to extend the half life.  2'O Me RNA bases have been used successfully to extend the halflife of ASOs in cell lines.  The two articles listed below should be helpful in designing your oligos.   

Zellweger T, et al.  J Pharmacol Exp Ther 2001 Sep;298(3):934-40
Gleave M, et al.  Cancer Metastasis Rev 2002;21(1):79-92 

Application Support Topics