An Antifungal for Antidiuresis?
Authors: Joseph G. Verbalis
Journal: Journal of the American Society of Nephrology (2019)

The article by Vukicevic et al.1 in this issue of the Journal of the American Society of Nephrology reports effects of fluconazole to increase osmotic water transport in renal collecting ducts by arginine vasopressin (AVP)–independent effects on trafficking of aquaporin-2 (AQP2) water channels. This paper is notable for several reasons, including (1) that drugs with AVP-independent effects on water transport are potential treatments for nephrogenic diabetes insipidus (NDI), a syndrome for which only suboptimal treatment exists; (2) that the fluconazole effects described provide additional support to mechanisms thought to underlie AQP2 ubiquination and trafficking in renal collecting duct cells; and (3) that, perhaps most importantly, the report illustrates the potential for repurposing existing US Food and Drug Administration (FDA)–approved drugs for new indications. Each of these points is deserving of further consideration. NDI occurs when the collecting duct principal cells cannot respond to AVP with increased water resorption as a result of translocation of AQP2 channels into the apical membrane of the cells. Treatments for congenital and acquired NDI include diuretics (thiazide and amiloride) that act to increase solute and water reabsorption in more proximal nephron segments, low-solute diets to decrease obligate free water excretion, and PG cyclo-oxygenase inhibitors that enhance AVP actions. However, these treatments have limited efficacy in many patients, requiring combination therapy to achieve clinically meaningful decreases in urine output. Consequently, therapies to increase water transport independent of AVP could represent a marked improvement in therapy for patients with NDI. Potential mechanism-based therapies for NDI caused by AVPR2 mutations include pharmacologic chaperones to partially correct mutant AVPR2 cellular misprocessing2 and drugs to bypass the effects of AVPR2 in collecting duct principal cells, including PG receptor inhibitors, β3-adrenoreceptor agonists, and secretin receptor agonists.3 However, none of these potential therapies have yet come to fruition for practical clinical use. Therefore, any new candidate therapy for this challenging disorder, such as fluconazole, should be viewed with guarded enthusiasm. AVP-stimulated signal transduction is responsible for the majority of AVP-induced antidiuretic effects in the kidney, but potential contributory roles of non-AVP modulators of AQP2 trafficking, such as PG E2, nitric oxide, and adenosine, remain less certain. Although the molecular pathways involved with AVP-mediated activation of AVPR2 are well understood,4 the molecular mechanisms involved with AQP2 trafficking into the apical membrane of collecting duct cells as well as the retrieval and internalization of the AQP2 channels are still being elucidated. The comprehensive in vitro studies of Vukicevic et al.1 demonstrated an increased abundance of AQP2 in IMCD cells, but the absence of changes in AQP2 mRNA indicated that this was not due to enhanced transcription of the AQP2 gene. Rather, a combination of cellular effects seems to be responsible for the increased osmotic permeability produced by fluconazole, including inhibition of phosphorylation of AQP2 at S261 leading to decreased ubiquination, and inhibition of RhoA activity with decreased polymerization of F-actin, thereby potentially removing a physical barrier to the transport of AQP2 vesicles to the apical plasma membrane. The similarity of these findings with those produced by forskolin strengthens the importance of these effects to increase trafficking of AQP2 to the plasma membrane as important modulators of the antidiuretic effects of AVP.5 Knowledge of the molecular basis of diseases has provided increased opportunities to translate these research findings into new drugs. However, drug discovery requires an increasingly large amount of effort, money, and time due to multiple requirements of the drug approval process to ensure efficacy and safety of approved drugs. It has been estimated that it takes approximately 12 years and $2.6 billion to discover a candidate compound and take it through all of the preclinical and clinical testing required for FDA approval.6 Furthermore, most candidate drugs fail to win FDA approval because of unanticipated adverse effects and toxicities. Consequently, strategies to reduce the time required for drug approval, decrease the costs involved, and improve eventual success rates would be advantageous. Drug repurposing is a strategy that can accomplish these goals.7 Agents already approved by the FDA have been tested in humans, and therefore, detailed information is available regarding their pharmacology, formulation, and toxicities. If new indications are found for such already approved drugs, new candidate therapies would be ready for clinical trials more quickly, thereby speeding their review and potential approval by the FDA for treatment of different diseases. Many examples of successful drug repurposing have already occurred. Early successful cases of drug repurposing resulted from serendipitous discoveries. Sildenafil was tested as a treatment for erectile dysfunction only after enhanced erections were discovered as a side effect in phase 1 trials of the drug for cardiovascular disease. The antihypertensive drug minoxidil was reformulated into a topical cream after patients experienced hair regrowth with drug use. More recent discoveries include the potential use of the oncology drug Nilotinib for treatment of neurodegenerative disorders, such as Parkinson disease.8 Researchers are now taking more focused approaches to identify potential new uses for existing and failed drugs using high-throughput techniques, such as large-scale cell-based screens and bioinformatics strategies to discover candidate drugs. The study by Vukicevic et al.1 demonstrates this approach. The authors screened a library of 17,700 small molecules in a cell-based assay to identify modulators of AQP2 localization; this high-throughput screen identified the antifungal drug, fluconazole, as a potential candidate.9 The first step in assessing potential candidate drugs is to perform the requisite preclinical in vitro and in vivo studies to determine if the drug produces the desired biologic effects: in this case, increased osmotic water transport in renal collecting duct cells. The studies reported in this issue nicely confirm an antidiuretic action of fluconazole, but only clinical trials can verify efficacy in humans. A common uncertainty is whether the doses used in in vitro and in vivo preclinical studies replicate the range of blood levels produced by therapeutic doses of the repurposed drug. In this regard, it is of some concern that there is an absence of reports of hyponatremia using fluconazole, which might be expected for an agent producing significant antidiuretic effects. However, confirmatory clinical trials can be conducted quickly and in a more focused manner, because this is an FDA-approved drug with known toxicities. This leads to the possibility that a more effective treatment for NDI may be forthcoming in a relatively short period of time. The National Center for Advancing Translational Science (NCATS) has taken a leading role in drug repurposing though a variety of programs. The Therapeutics for Rare and Neglected Diseases Repurposing Screening Group collaborates with investigators at the National Institutes of Health, universities and other nonprofit research institutions, and private sector companies to identify drug targets or disease phenotypes for assay development and to develop and optimize assays for high-throughput screening (https://ncats.nih.gov/trnd). Similar services are available through many of the 58 US academic health centers with Clinical and Translational Science Awards (CTSAs) from the NCATS. An additional important service provided by CTSA hubs is the availability of Clinical Research Units to advance newly identified repurposed compounds to clinical trials for the treatment of rare and neglected diseases, because repurposing generic drugs for new uses is usually not a priority of pharmaceutical companies due to limited possibilities for financial return on investment. Even if clinical trials in humans show significant antidiuretic effects of therapeutic doses of fluconazole, several limitations will need to be recognized. (1) NDI caused by AQP2 mutations will likely not respond, because AQP2 water permeability is required for physiologic collecting duct function and therefore, cannot be bypassed. (2) The magnitude of antidiuretic effect produced by agents that affect AQP2 trafficking without activation of AVP2R will be limited, because in the absence of AVP stimulation of AQP2 gene transcription, cellular AQP2 abundance is reduced, thereby reducing maximal antidiuresis.10 (3) The known toxicities of agents, such as ketoconazole, may become more problematic with prolonged use, which will be necessary for chronic treatment of NDI. Nonetheless, any treatment that has the potential to ameliorate the polyuria and hyperosmolality of NDI would represent an important advance in the therapy of this challenging disorder. Disclosures None.