There are several clinical trials regarding the slowing of progression of polycystic kidney disease currently being conducted; amongst the drugs being studied are V2 receptor antagonists. The rationale for this is that cyclic AMP (cAMP) is known to be required for cyst growth; as ADH signaling involves an elevation of intracellular cAMP levels in the collecting duct, V2 receptor antagonists (e.g., the "vaptans") would be predicted to lower intracellular cAMP concentration and thereby lead to decreased cyst growth.
An even more economic treatment logically presents itself: drink more water! It is not known whether or not this would have a long-term benefit, but the recommendation to drink 3L of water a day (a recommendation which is fairly routine for stone-formers) would be predicted to decrease ADH levels by approximately 50%.
PKD patients also should be advised to avoid health products which contain forskolin--a potent stimulator of cAMP, which would therefore be predicted to accelerate cyst growth. Forskolin is an ingredient in various bodybuilding supplements and other non-FDA-regulated health care products.