Future Treatments for Hyperoxaluria

Abstract

Purpose of Review

The review of potential therapies in the treatment of hyperoxaluria is timely given the current excitement with clinical trials and the mounting evidence of the importance of oxalate in both kidney stone and chronic kidney disease.

Recent Findings

Given the significant contribution of both endogenous and dietary oxalate to urinary oxalate excretions, it is not suprising therapeutic targets are being studied in both pathways. This article covers the existing data on endogenous and dietary oxalate and the current targets in these pathways.

Summary

In the near future, there will likely be therapies targeting both endogenous and dietary oxalate, especially in subsets of kidney stone formers.

Introduction

The prevalence of kidney stones is increasing in the United States with the National Health and Nutrition Examination Survey (NHANES) data demonstrating an increase from 5.2% (1988 to 1994) to 8.8% (2007 to 2010).[1, 2] Approximately 70 – 80% of the stones formed contain oxalate - an end product of metabolism and an ubiquitous element of human diets.[3, 4] Urinary oxalate is derived almost equally from diet and endogenous oxalate synthesis, the latter thought to mostly occur in the liver.[5] The importance of urinary oxalate has been demonstrated in multiple studies. Small increases in urinary oxalate can increase calcium oxalate crystal formation and thus stone disease.[6, 7] The relative risk of forming a stone increased approximately 2.5 – 3.5 fold as urinary oxalate excretion doubled from 20 to 40 mg/day, the latter value frequently used as the lower thresholed for defining hyperoxaluria in adults. This was based on an analysis of three large epidemiologic cohorts. The results also showed that small changes in the amount of oxalate excreted each day, as little as 4 mg/day, could increase the risk of developing an incident kidney stone by 60 – 100%.[8] Urinary oxalate excretion has also been recently demonstrated to be positively correlated to progression of chronic kidney disease including ESRD.[9] Despite the importance of oxalate to kidney stone disease and chronic kidney disease, there are few treatment options targeting the absorption, excretion, and/or synthesis of oxalate. However, multiple clinical trials are currently underway that target both the dietary and endogenous contributions to urinary oxalate excretion. In this article, we will discuss the future of therapeutics that reduce urinary oxalate excretion.

Materials and Methods

We conducted a Medline and a PubMed search from 1980 to 2019 to identify publications related to oxalate and kidney stone disease. Keywords used for the search were: “oxalate”, “hyperoxaluria”, “endogenous”, and “dietary” and “kidney stone”.

Endogenous Synthesis

Despite decades of research, the biochemical pathways involved in oxalate production are still poorly understood. Two major sources of endogenous oxalate production in humans have been recognized. The first is an enzymatic pathway in hepatocytes that links the oxidation of glycolate to glyoxylate in peroxisomes with either the conversion of this glyoxylate to glycine by alanine:glyoxylate amino transferase (AGT) in peroxisomes, the reformation of glycolate catalyzed by GRHPR (GR) in the cytosol, or the oxidation of glyoxylate to oxalate catalyzed by LDH in the cytosol.[10] Glyoxylate is thought to be the major precursor to oxalate synthesis and is directly produced from the metabolism of hydroxyproline[11], glyoxal[12], glycolate, and glycine[13]. Little is known about alterations in these pathways outside of studying inborn errors of metabolism associated with Types 1–3 Primary Hyperoxaluria (PH). Type 1 disease, associated with the highest level of urinary oxalate excretion, is caused by mutations in AGT, Type 2 by mutations in GR, and Type 3 by mutations in HOGA (encoding 4-hydroxy-2-oxoglutarate aldolase), an enzyme involved in mitochondrial hydroxyproline metabolism.[14] Type 1 and 2 diseases create deficiencies in the removal of glyoxylate, and we have suggested that Type 3 may be similar as we have shown that increases in the concentration of HOG (4-hydroxyoxoglutarate) inhibit GR activity.[15] Hydroxyproline is a major precursor of oxalate synthesis as demonstrated by infusion experiments with [15N,13C5]-Hydroxyproline in normal adults and those afflicted with PH 1–3;contributions to the urinary oxalate pool being 15% in normal;18 % in PH1; 47% in PH2; 33% in PH3.[16] From this analysis it is clear that there are sources of oxalate that have not been identified, particularly in normal individuals and those with PH1.

Another source of oxalate production in humans is the breakdown of ascorbic acid (AA)/Vitamin C (Vit C) to oxalate.[17] One report published over 50 years ago where an oral dose of ¹³C₆-Vit C was administered as a single dose in an adult indicated that the contribution of ascorbate breakdown to oxalate was 40% [18] Despite this report indicating Vit C as the major contributor to endogenous oxalate synthesis, little is understood about the factors that influence the turnover of ascorbic acid to oxalate. One hypothesis is that ascorbic acid is broken down to oxalate as it acts as an antioxidant. One treatment may involve approaches that increase levels of antioxidants other than AA as this may reduce AA turnover to oxalate. For example, we have identified that orally dosed Pro-Cysteine (OTZ), an agent that in rodent studies increases tissue levels of the antioxidant glutathione [19], decreased oxalate excretion by 20 – 30% in healthy volunteers [20].

Treatment options targeting the glyoxylate to oxalate pathway are limited; a small subset of PH1 patients are treated and respond to vitamin B6/pyridoxine – a cofactor for AGT. Response to this agent can be predicted by the causative mutation.[21] The endogenous oxalate pathways indicate that three enzymes, hydroxyproline dehydrogenase, glycolate oxidase (GO) and lactate dehydrogenase (LDH), are suitable targets for oxalate reduction therapy, as individuals with deficiencies in these enzymes are healthy. [22–24] There are multiple routes for delivering effective treatments including oral, subcutaneous, and intravenous. There are ongoing clinical trials where RNA interference targeting GO and LDH have yielded significant reductions in urinary oxalate excretion in those afflicted with PH1 and less so with PH2.[25, 26] Lipid nanoparticles (LNPs) and N-acetyl galactosamine (GalNAc) have allowed targeted delivery of siRNA to the epicenter of the excessive endogenous oxalate synthesis in these individuals, the liver.

Liebow et al demonstrated subcutaneous administration of RNAi to GO resulted in potent, dose-dependent, and durable silencing of the mRNA encoding GO in wild-type mice, rats, and nonhuman primates with near normalization of urinary oxalate excretion in murine models of PH1.[27] Lai and colleagues targeted LDH using a similar technology and demonstrated a similarly profound effect on urinary oxalate excretion.[28] Wood and colleagues have demonstrated possible metabolic effects of targeting LDH with RNAi, including alterations in TCA cycle metabolites, suggesting the consequences of these effects should be monitored.[29] These studies have led to current clinical trials in primary hyperoxaluria type 1 for GO targeting (NCT03681184, NCT03905694) and type 1 and 2 for LDH targeting (NCT03847909). Final Phase 1/2 study results (n=20 PH1 patients), lumasiran (RNAi targeting GO) demonstrated a mean maximal reduction in urinary oxalate of 75% with acceptable safety and tolerability.[25] In ongoing studies, DCR-PHXC (RNAi targeting LDH) has demonstrated similar mean maximal reductions in urinary oxalate excretion in PH 1 patients (71%, n=6) as well as a 42% reduction in PH2 (n=3).[26] If RNAi therapy continues to prove successful in these rare diseases, these agents may have a role in the management of select other patients with hyperoxaluria, even perhaps, idiopathic calcium oxalate stone formers.

CRISPR/Cas9-Mediated gene knockout of both lactate dehydrogenase A (LDHA) and hydroxyacid oxidase 1 (HAO1) has been reported in a PH1 mouse model and may allow for a more durable treatment option relative to the current RNAi approach.[30] Others have reported the use of oral inhibitors to LDH as a possible treatment option.[31] These treatments are currently relegated to clinical trials involving patients afflicted with PH. Again, this could extend out to idiopathic calcium oxalate stone formers. Figure 1 summarizes the possible targets to endogenous oxalate synthesis.

Figure 1:

Potential targets and pathways to address endogenous oxalate synthesis. Within the hepatocyte, potential targets include those enzymes involved in endogenous oxalate synthesis. Targeting the non-enzymatic breakdown of ascorbic acid is a potential therapy. Understanding the precursors and transporters of oxalate could yield future targets.

Dietary Oxalate

For many years it was thought that dietary oxalate contributed 10 – 20% of urinary oxalate excretion. Carefully controlled diets demonstrate this contribution is much more substantial and ranges from 10 −80%.[5, 32] The variability is related to a number of factors including dietary calcium, intestinal absorption, gut bacteria, and solubility (bio-availability) of oxalate. Several studies demonstrate that increasing dietary calcium decreases urinary oxalate excretion in individuals on controlled diets.[33–35] We have shown that reducing dietary calcium from 1,000 to 400 mg/day on a 250 mg/day oxalate diet increases mean oxalate excretion by 20.3% in Oxalobacter formigenes colonized individuals and 50.3% in noncolonized individuals.[36] In combining all of our diet controlled studies, urinary oxalate excretion increases 1.7 mg for every 100 mg of dietary oxalate ingested when calcium intake is held constant at 1000 mg a day in adults.[37]

Oxalate is absorbed by both para- and transcellular mechanisms, and their relative contributions may vary in different intestinal segments. SLC26 transporters are thought to play an important role in transcellular transport, with SLC26A3 possibly regulating intestinal absorption.[38] The amount of soluble oxalate is important when discussing intestinal absorption. Humans ingest on average 15–25 mmol of calcium per day compared with 1–3 mmol of oxalate, suggesting that, in the intestine, the bulk of oxalate is insoluble. Further, there are uncertainties about the impact of calcium absorption, gastrointestinal motility, colonization with components of the intestinal microbiome including Oxalobacter formigenes, and other potential modifiers.

One future treatment option has focused on altering the microbiome with oxalate degrading bacteria, such as Oxalobacter formigenes. In patients with enteric hyperoxaluria this may be a way to reduce oxalate absorption and thus urinary oxalate excretion. Enteric hyperoxaluria is seen in those with ileal disease, chronic inflammatory bowel disease, malabsorptive bariatric surgery, exocrine pancreatic insufficieny and other causes of fat malabsorption/steatorrhea.[39] OxThera is using a lyophilized version of Oxalobacter formigenes in clinical trials to evaluate its effects on primary hyperoxaluria.[40] Another clinical trial is ongoing at the University of Alabama at Birmingham with live Oxalobacter formigenes and converting non-colonized subjects to colonized subjects (NCT03752684). Future therapies may also arise when there is better knowledge of what drives gastrointestinal oxalate absorption and secretion.

Oxalobacter formigenes uses a complex biochemical pathway to catabolize oxalate, relying on oxalyl-CoA decarboxylase coupled to formyl-CoA transferase.[39] However, there are other enzymes that can degrade oxalate, one being oxalate decarboxylase. Multiple companies are studying the delivery of this enzyme to the gut to degrade oxalate. OxThera has a product called Oxazyme. Nephure is a proprietary oxalate decarboxylase enzyme developed and marketed as an over the counter treatment (NCT03661216). In July of 2018, parent company Captozyme announced planning the initiation of a prospective, double blinded, randomized, placebo controlled, cross-over study of this enzyme. Another oxalate decarboxylase therapy, Reloxaliase (formerly, ALLN-177), is being evaluated in subjects with enteric hyperoxaluria (NCT03456830).

Other potential future treatments for hyperoxaluria include regulation of the absorption and secretion of oxalate in the gut, possibly via manipulation of the SLC transporters.[41] Despite decades of research, there is still limited data on the mechanisms responsible for absorption and secretion of oxalate and its role in kidney stone disease. Research in mice suggests that SLC26A3 (DRA: DownRegulated in Adenoma) is the major transporter involved in transcellular oxalate absorption. SlC26A6 (PAT1: Putative Anion Transporter 1) is likely involved in transcellular oxalate secretion as an apical transporter.[41, 42] Manipulating these transporters may lead to reduction of oxalate absorption. Approaches to limit paracellular oxalate transport may also be effective. Figure 2 illustrates these potential therapies.

Figure 2:

Potential targets to address dietary oxalate absorption and secretion. Transporters involved in oxalate transport could be upregulated or down-regulated. The solubility of oxalate and the available oxalate within the lumen of the gut can be targeted.

Little is understood about renal oxalate handling, primarily because of the difficulty in measuring plasma oxalate. A report by Bergsland and colleagues involving 8 normal healthy controls and 19 hypercalciuric subjects suggested renal tubular secretion of oxalate as a key mediator of hyperoxaluria in calcium stone formers, potentially as a means of maintaining plasma oxalate in a tight range.[43] However, we have found no significant differences in renal oxalate clearance ratio’s following dietary oxalate loads between 6 recurrent calcium oxalate stone formers, 2 of which were hypercalciuric, and 12 normal healthy controls.[44] It is clear further research is still needed to better understand renal oxalate handling and its role in hyperoxaluria and calcium oxalate kidney stone disease.

Conclusion

In summary, the recent development of targetted approaches to decrease endogenous oxalate synthesis and intestinal absorption of oxalate, many of which are now being tested in clinical trials, may lead to new treatments for hyperoxaluria in the near future. However, further research on oxalate transport, renal oxalate handling, and the role of oxidative stress, antioxidants and ascorbic acid metabolism is still needed.

Key Points.

• Inhibitors of enzymes in the endogenous oxalate pathway are promising therapies in primary hyperoxalturia

• More research is needed to understand the precursors of oxalate and the contribution of ascorbic acid

• Oxalate degrading enzymes and bacteria are promising therapies in enteric hyperoxaluria

• The role of targeting these pathways in idiopathic stoneformers is not known