Autosomal dominant polycystic kidney disease (ADPKD) is a widespread genetic disease that leads to renal failure in the majority of patients. The very first pharmacological treatment, tolvaptan, received Food and Drug Administration approval in 2018 after previous approval in Europe and other countries. However, tolvaptan is moderately effective and may negatively impact a patient's quality of life due to potentially significant side effects. Additional and improved therapies are still urgently needed, and several clinical trials are underway, which are discussed in the companion paper Müller and Benzing (Management of autosomal-dominant polycystic kidney disease-state-of-the-art) Clin Kidney J 2018; 11: i2-i13. Here, we discuss new therapeutic avenues that are currently being investigated at the preclinical stage. We focus on mammalian target of rapamycin and dual kinase inhibitors, compounds that target inflammation and histone deacetylases, RNA-targeted therapeutic strategies, glucosylceramide synthase inhibitors, compounds that affect the metabolism of renal cysts and dietary restriction. We discuss tissue targeting to renal cysts of small molecules via the folate receptor, and of monoclonal antibodies via the polymeric immunoglobulin receptor. A general problem with potential pharmacological approaches is that the many molecular targets that have been implicated in ADPKD are all widely expressed and carry out important functions in many organs and tissues. Because ADPKD is a slowly progressing, chronic disease, it is likely that any therapy will have to continue over years and decades. Therefore, systemically distributed drugs are likely to lead to potentially prohibitive extra-renal side effects during extended treatment. Tissue targeting to renal cysts of such drugs is one potential way around this problem. The use of dietary, instead of pharmacological, interventions is another.

Mild reduction in food intake was recently shown to slow polycystic kidney disease (PKD) progression in mouse models, but whether the effect was due to solely reduced calories or some other aspect of the diet has been unclear. We now show that the benefit is due to the induction of ketosis. Time-restricted feeding, without caloric reduction, strongly inhibits mTOR signaling, proliferation, and fibrosis in the affected kidneys in a PKD rat model. A ketogenic diet had a similar effect and led to regression of renal cystic burden. Acute fasting in rat, mouse, and feline models of PKD results in rapid reduction of cyst volume, while oral administration of the ketone β-hydroxybutyrate (BHB) in rats strongly inhibits PKD progression. These results suggest that cystic cells in PKD are metabolically inflexible, which could be exploited by dietary interventions or supplementation with BHB, representing a new therapeutic avenue to treat PKD.