Another potentially useful drug to prevent cyst growth in autosomal dominant polycystic kidney disease comes from the interesting observation that kidney transplant patients with native ADPKD kidneys who were given an immunosuppressant regimen containing the drug rapamycin (sirolimus) displayed less cyst growth than those who did not receive rapamycin.
The molecular mechanism of rapamycin is already known: it inhibits a cell signaling pathway known as the mTOR pathway (mTor = "mammalian target of rapamycin"). In support of its usefulness in ADPKD, the epithelium lining cysts (arrow) shows intense staining with an antibody against phosphorylated (active) mTOR, while non-cystic tubules (depicted by the arrowhead) do not show mTOR activity, as shown in this 2006 PNAS paper by Shillingford et al . We will obviously have to see how rapamycin does in clinical trials. One of the obvious caveats is that this is not a wholly benign medication as observed by many transplant nephrologists, and bear in mind that it would likely be a chronic medication taken to prevent the growth of cysts and resultant ESRD. In the meantime, an article in this month's Kidney International by Gattone et al successfully uses rapamycin to decrease cyst growth in a mouse model of nephronophthisis (the pcy mouse), providing further rationale for this approach.