In order for renal cysts to grow in patients with polycystic kidney disease, it turns out this requires the pumping of chloride ion into the cyst lumen. This is carried out by the CFTR channel, the same chloride channel which is mutated in patients with cystic fibrosis.
Interestingly, there are some patients who were unlucky enough to inherit both CFTR and PKD1 mutations--giving them both cystic fibrosis as well as polycystic kidney disease. Although the data is limited, it appears that these individuals with dual mutations had reduced cyst growth (Xu et al, J. Nephrol, 2006).
More recently, in this month's issue of JASN, a group (Yang et al, JASN 2008) published data looking at small molecule inhibitors of CFTR--and were able to reduce cystic growth in a mouse model of cystic kidney disease, suggesting that this may be yet another useful therapeutic target for ADPKD.