H2-gamendazole: a new therapeutic lead for the treatment of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) affects ∼600,000 people in the United States and as many as 1 in 400–1,000 individuals worldwide. ADPKD is associated with the formation of large fluid-filled renal cysts that typically cause pain from early adulthood, kidney failure by the sixth decade, and numerous extrarenal symptoms including hypertension and hematuria (1). Although the progression of ADPKD is slow, it has proven extremely difficult to identify effective treatments, with drugs assessed either lacking efficacy or not well tolerated by patients due to challenging side effect profiles (2). One factor complicating the identification of a useful treatment is the fact that the germline mutations in polycystin 1 (PKD1) and polycystin 2 (PKD2) that cause ADPKD abnormally activate multiple signaling pathways (involving the epidermal growth factor receptor, ERBB2, AKT, and other proteins) that affect numerous cellular processes, including controls of proliferation, polarity control, migration, and survival (3). Hence, a challenge has been to identify drugs that are sufficiently broad acting to counteract the signaling defects associated with ADPKD while nevertheless maintaining the high safety profile required for drugs with chronic use.

In a study published in a recent issue of the American Journal of Physiology-Renal Physiology, Sundar et al. (4) identified H2-gamendazole (H2-GMZ) as a potentially valuable new agent for the treatment of ADPKD (Fig. 1). The authors selected this compound for evaluation because of an intriguing profile of potential biological activity. H2-GMZ is a derivative of lonidamine, an indazole-carboxylic acid previously shown to block cAMP-dependent cystic fibrosis transmembrane conductance regulator (CFTR)-dependent anion secretion, which is important for filling cysts in ADPKD (5). Gamendazole, closely related to H2-GMZ, retains CFTR inhibitor activity in addition to having new activities in blocking activity of the molecular chaperone heat shock protein (HSP)90 and also eukaryotic elongation factor eukaryotic translation elongation factor 1α1 (eEF1A1), a protein with separable activities as a translation elongation factor and an actin-bundling protein (6). Inhibition of HSP90 has been previously shown to have valuable activity in controlling cyst growth in ADPKD, based on the reduced stability or activity of numerous signaling proteins that depend on HSP90 chaperone activity (7); eEF1A1 influences both cell growth and cell migration. In theory, a compound that simultaneously inhibits CFTR, HSP90, and eEF1A1 might be ideally designed for the treatment of ADPKD.

Figure 1.

H2-gamendazole (H2-GMZ) inhibition of autosomal dominant polycystic kidney disease (ADPKD) kidney cell signaling. Mutation of polycystin 1 (PKD1) and polycystin 2 (PKD2) (encoding a heterotetramer of polycystins localizing to the primary cilium in renal cells) affects signaling in numerous ways that promote cyst formation. Altered cellular Ca²⁺ homeostasis leads to elevated levels of cAMP, which triggers PKA to activate cystic fibrosis transmembrane conductance regulator (CFTR). CFTR pumps Cl⁻ through the apical cell surface, with enhanced CFTR activity in ADPKD causing increased fluid secretion into cysts. Separately, activation of ERBB2-dependent proliferative signaling dependent on RAS, RAF, phosphatidylinositol 3-kinase (PI3K), and other effectors enhances ADPKD renal cell proliferation. These and other signaling proteins that promote cell growth depend on binding the heat shock protein (HSP)90 chaperone for stability and/or activity. H2-GMZ has dual activity in inhibiting HSP90 and CFTR, potently restricting growth of ADPKD cells, and also alters organization of the actin cytoskeleton, suggesting potential inhibition of eukaryotic elongation factor eukaryotic translation elongation factor 1α1 (eEF1A1). CDK, cyclin-dependent kinase; EGF, epidermal growth factor; EGFR, epidermal growth factor receptor; ENaC, epithelial Na⁺ channel; IP3R, inositol 1,4,5-trisphosphate receptor; mTOR, mammalian target of rapamycin; RyR, ryanodine receptor.

In their study, Sundar et al. (4) first demonstrated that the new compound, H2-GMZ, was more effective than lonidamine in inhibiting forskolin-induced, CFTR-dependent Cl⁻ secretion in monolayers of human ADPKD cyst-lining epithelial cells, based on measurements of short-circuit current in the presence of benzamil (an inhibitor of the epithelial Na⁺ channel, which mediates cation resorption). They then showed that H2-GMZ reduced the proliferation and mitotic index of ADPKD cells induced by treatment with either cAMP or epidermal growth factor, an activating ligand for the epidermal growth factor receptor. Exploring the mechanism underlying growth inhibition, the authors found that H2-GMZ treatment reduced the expression of multiple HSP90 client proteins that are important for proliferation, including ERBB2, AKT, and CDK4, in ADPKD cells, based on an increased rate of proteasome-dependent degradation. Interestingly, levels of CFTR were also reduced, reflecting the fact that this protein also is a HSP90 target, and somewhat complicating the understanding of whether channel inhibition or degradation was the primary means by which H2-GMZ reduced CFTR-dependent anion transport.

Sundar et al. (4) performed a number of experiments that revealed promising biological activities of H2-GMZ pertinent to therapy of ADPKD. Most important, H2-GMZ limited cAMP-dependent cyst growth and enlargement in embryonic metanephric kidneys from Pkd1^m1Bei mice (bearing a disabling allele of Pkd1) following culture in vitro. H2-GMZ reduced the expression of proliferating cell nuclear antigen, a marker of proliferation, in cultured kidneys and was active at a much lower dose than lonidamine. Furthermore, the authors investigated H2-GMZ activity in Pkd1^flox/flox:Pkhd1-Cre mice, which spontaneously develop cysts soon after birth and succumb at around 4 wk of age. Daily administration of H2-GMZ from postnatal days 8−18 had striking effects, very significantly reducing the cystic index, kidney size, and blood urea nitrogen index and prolonging average survival from 28.5 to 67.8 days.

Overall, the data presented by Sundar et al. (4) support the idea that H2-GMZ is worth further investigation as a candidate lead for ADPKD. Additional results from the study suggest that the drug has a promising safety profile, although the next steps would include evaluation of long-term tolerability and application of a broader series of toxicity assays. It is notable (and somewhat surprising) that despite reducing expression of multiple HSP90 clients and evidence of impairment of HSP90 activity, H2-GMZ did not induce HSP90 or HSP70 expression; this is a potentially valuable property, as induction of the heat shock response would likely induce undesirable side effects. On the other hand, the authors also found that H2-GMZ reduced motility and caused cytoskeletal reorganization not only in ADPKD cells but also in the relatively normal murine M-1 collecting duct cell line, used as a control. Overall, further investigations of the comparative activity of H2-GMZ in ADPKD versus normal cells are necessary. As a particularly significant issue, the authors noted that gamendazole, closely related to H2-GMZ, was originally developed as a male contraceptive, and treatment with gamendazole led to a loss of fertility in male rats, in some cases, irreversibly, through action on Sertoli cells (8). Given that symptoms of ADPKD first develop in young adults, this would pose a hurdle to widespread early use of H2-GMZ, if it has a similar activity.

Overall, the early results are intriguing. Evaluation of the combination of low doses of H2-GMZ with other therapies that have shown some promise for ADPKD would be another logical next step. Certainly, given the broad activity profile of H2-GMZ already defined in targeting CFTR, HSP90, and eEF1A1, it will be essential to more extensively profile the binding and inhibitory specificity of this compound, which may result in the identification of still further targets. Given the great need for an effective therapy of ADPKD, it will be of considerable interest to observe the development of H2-GMZ and related compounds as agents for slowing disease progression and ameliorating symptoms.

GRANTS

The authors were supported by National Cancer Institute Core Grant P30CA006927 (to Fox Chase Cancer Center), Department of Defense No. W81XWH2110487, and the William Wikoff Smith Charitable Trust (to E.A.G.).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

S.P. prepared figures; S.P. and E.A.G. drafted manuscript; E.A.G. edited and revised manuscript.