Targeted Therapies
In GeneReviews, a targeted therapy is one that addresses the specific underlying mechanism of disease causation (regardless of whether the therapy is significantly efficacious for one or more manifestation of the genetic condition); would otherwise not be considered without knowledge of the underlying genetic cause of the condition; or could lead to a cure. —ED
Targeted therapy for PH1 includes pyridoxine (for individuals with specific AGXT missense variants), RNA interference (RNAi) therapeutics to reduce hepatic overproduction of oxalate, and liver and kidney organ transplantation (either combined or performed sequentially).
Note: Use of RNAi therapeutics to reduce hepatic oxalate production in pre- and post-transplantation management of kidney recipients is complicated; information to date is limited to case reports [Stone et al 2021, Sellier-Leclerc et al 2023, Bacchetta et al 2024]. As clinical experience increases, best practices for management of organ transplantation will evolve rapidly [Devresse et al 2020, Bacchetta et al 2024], making published practice guidelines [Groothoff et al 2023, Michael et al 2024] and centers/clinicians experienced in PH1 management the best sources of information.
Because disease manifestations, including CKD or even kidney failure, are often present at the time of diagnosis, several approaches to targeted therapy are necessary (see Table 7).
Table 7.
Primary Hyperoxaluria Type 1: Targeted Therapies
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Treatment Class | Treatment | Therapy Candidates | Mechanism | Comment |
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Small molecule therapy
| Pyridoxine (vitamin B6 analog) | Persons w/missense AGXT variants, esp homozygotes for p.Phe152Ile, p.Gly170Arg, & p.Ile244Thr (See Genotype-Phenotype Correlations & Table 10.) | Enhances residual activity of AGT (a pyridoxal 5'-phosphate-dependent enzyme) 1 | ~30%-50% of individuals w/PH1 are pyridoxine responsive. Since there are multiple mechanisms of pyridoxine response & only a few pathogenic variants have been tested, a trial of pyridoxine should be considered in all persons w/1 or 2 missense AGXT pathogenic variants, incl those w/advanced CKD or kidney failure. 2
|
Gene therapy (RNAi)
| Lumasiran (Oxlumo®) | Effective in all persons w/PH1, independent of specific AGXT pathogenic variants | siRNA therapeutic agent that ↓s glyoxylate substrate available for metabolic conversion to oxalate by targeted reduction of hepatic glycolate oxidase | FDA & EMA approved for persons of all ages Experience using lumasiran in persons w/advanced CKD, on dialysis, or following kidney transplantation alone is limited.
|
Nedosiran (Rivfloza®) | Effectiveness demonstrated in PH1 independent of specific AGXT pathogenic variants | siRNA therapeutic agent that ↓s conversion of glyoxylate to oxalate by targeted reduction of hepatic LDHA | FDA approved in persons w/PH1 age >9 yrs w/eGFR >30 mL/min/1.73 m2 |
Organ transplantation
| Liver transplant | Persons w/GFR <25-30 mL/min/1.73 m2 | Liver transplantation restores normal AGT activity. | When kidney replacement therapy is needed, the decision needs to be made between kidney transplant alone or liver & kidney transplant simultaneously. 3 |
AGT = alanine-glyoxylate aminotransferase; CKD = chronic kidney disease; eGFR = estimated glomerular filtration rate; EMA = European Medicines Agency; FDA = US Food and Drug Administration; GFR = glomerular filtration rate; LDHA = lactate dehydrogenase A; RNAi = RNA interference; siRNA = small interfering RNA
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Liver transplant is considered "targeted therapy" because it restores normal hepatic AGT function. Kidney transplant replaces kidney function but does not address the underlying cause of the disorder, and thus is considered supportive care.
Pyridoxine (vitamin B6). Pyridoxine, available for oral administration as pyridoxine hydrochloride, can reduce oxalate production in some individuals with PH1 by enhancing residual activity of alanine-glyoxylate aminotransferase (AGT), a pyridoxal 5'-phosphate-dependent enzyme [Cochat & Rumsby 2013, Garrelfs et al 2021b, Fargue & Acquaviva Bourdain 2022].
Pyridoxine response is assessed by comparing the 24-hour urine oxalate excretion rate before treatment and after at least three months of pyridoxine treatment at a minimum dose of 5 mg/kg/day. Reduction of ≥30% or normalization of urinary oxalate excretion while receiving pyridoxine indicates responsiveness [Cochat et al 2012]. Note: Due to the difficulty of accurate 24-hour urine collections in infants and small children, comparison of random urine oxalate:creatinine measurements before and after treatment may be used.
Most individuals who are pyridoxine responsive show maximum benefit at a dose of 5-8 mg/kg/day [Monico et al 2005, Hoyer-Kuhn et al 2014]. Several authors have observed little additional benefit with pyridoxine doses >10 mg/kg/day and thus recommend close monitoring for adverse effects with use of higher doses [Groothoff et al 2023].
A starting pyridoxine dose of 5 mg/kg/day is recommended. Stepwise increases in pyridoxine dose to a maximum of 10-20 mg/kg/day with assessments of response by measurement of urine oxalate excretion at each step determines the minimum effective dose.
Pyridoxine responsiveness can be difficult to determine in individuals with advanced CKD or end-stage kidney disease (ESKD), whose urinary oxalate excretion rates may be influenced by the low glomerular filtration rate (GFR). When the GFR is <30 mL/min/1.73 m2, assessing changes in plasma oxalate concentration may be more effective.
Certain AGXT pathogenic variants are known to be pyridoxine responsive (see Genotype-Phenotype Correlations).
Because 30%-50% of individuals with PH1 are pyridoxine responsive (and only a few AGXT variants have been tested), assessment of pyridoxine responsiveness should be considered in all individuals who have one or two missense AGXT variants, including those with advanced CKD or kidney failure. Following treatment with pyridoxine, p.Gly170Arg homozygotes may have improved kidney function with recovery from dialysis dependence [Lorenz et al 2021] and good outcomes following kidney transplantation without liver transplantation [Lorenz et al 2014].
Individuals responsive to pyridoxine should continue this therapy indefinitely or until successful orthotopic liver transplantation (when a liver with normal AGT enzyme activity replaces the liver with deficient AGT enzyme activity). Additionally, the recent availability of lumasiran and nedosiran requires comparing pyridoxine and siRNA therapeutic agents regarding efficacy, durability of effect, long-term tolerability, safety, and use under special circumstances such as pregnancy (see Pregnancy Management).
Pyridoxine has an excellent safety profile in individuals with PH1, even after decades of use. In contrast, peripheral neuropathy (including paresthesias) has been reported in individuals who do not have PH1 who were receiving very large doses of pyridoxine (typically, adults receiving 1-2 g/day) [Toussaint 1998]. Of note, paresthesias reported in one individual with PH1 on a dose of pyridoxine of 2.1 mg/kg/day resolved following its discontinuation.
RNA interference (RNAi) therapy. Early experience with lumasiran, a small interfering RNA (siRNA) therapeutic agent that targets glycolate oxidase approved in late 2020 by the US Food and Drug Administration (FDA) and European Medicines Agency (EMA), demonstrates that it significantly reduces hepatic oxalate production and urinary oxalate excretion in individuals with PH1 [Scott & Keam 2021, Garrelfs et al 2023].
Nedosiran, a second siRNA therapeutic agent approved for PH1 in late 2023, targets a separate hepatic enzyme, lactate dehydrogenase A (LDHA) [Baum et al 2023]. However, preliminary data regarding efficacy of lumasiran and nedosiran suggest individual variability in response; thus, complete normalization of oxalate production does not occur in all treated individuals [Baum et al 2023, Gang et al 2023, Garrelfs et al 2023].
Additional data are needed to confirm long-term safety and efficacy of lumasiran and nedosiran in reversing the kidney-related effects of hepatic overproduction of oxalate.
Organ transplantation. Prior to targeted therapy, organ transplantation was the only option to prevent systemic oxalosis once GFR was lower than 25-30 mL/min/1.73 m2.
Until 2020 whole liver transplantation was – despite limited organ availability, high cost, and significant morbidity and mortality – the only intervention capable of restoring normal hepatic AGT enzyme activity and thus normalizing oxalate production. Liver and kidney transplantation were often performed either as a single combined procedure or sequential procedures, with similar outcomes achieved using either approach [Metry et al 2021].
With recent availability of RNAi therapeutic agents capable of reducing hepatic oxalate production, some have suggested that liver transplantation may no longer be needed [Devresse et al 2020]. However, RNAi agents are costly and not universally available. Early data regarding outcomes of treatment with lumasiran and nedosiran suggest individual variability in response; thus, complete normalization of oxalate production does not occur in all [Baum et al 2023, Gang et al 2023, Garrelfs et al 2023]. Pending long-term data on sustained efficacy and safety of these agents, recommendations regarding liver transplantation in PH1 may change over time.
Points to consider regarding transplantation include the following:
When systemic oxalosis is present prior to transplantation, mobilization of systemic oxalate places the kidney allograft at risk until tissue oxalate stores are completely cleared [
Bacchetta et al 2024]; thus, transplant recipients must be monitored closely to assure maintenance of high urine volumes and use of crystal inhibitor medication. Dialysis may be required if plasma oxalate concentrations are high due to delay or compromise of kidney allograft function [
Michael et al 2024].
Liver transplantation for PH1 should always be performed with complete removal of the native liver.
Pyridoxine supplementation in individuals who have been pyridoxine responsive can be discontinued at the time of liver transplantation, since normal AGT enzyme activity will have been restored.
Although most publications report transplantation of organs from deceased donors, living donor kidney or liver allografts are viable alternatives in some situations. It is important to note that the appropriateness of using a parent or sib who is heterozygous for an AGXT pathogenic variant as a donor remains unclear. Although heterozygotes can have reduced AGT enzyme activity in the liver, they typically have normal urine oxalate excretion and remain free of stones or oxalate-related CKD.
Supportive Care
Early diagnosis of PH1 with initiation of supportive care (also referred to as conservative treatment) aims to prevent crystal injury to kidneys, reduce stone formation and stone-related kidney damage, preserve kidney function, and prevent systemic oxalosis (see Table 9).
Table 9.
Primary Hyperoxaluria Type 1: Supportive Care
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Objective | Treatment | Consideration/Other |
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Prevent crystal injury to kidneys by ↓ crystal formation |
| Infants or small children may need gastrostomy tube placement. |
Reduce stone formation
|
| Use imaging studies at regular intervals to guide mgmt. |
Reduce stone-related kidney damage
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|
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Preserve kidney function
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| Use IV fluid if needed to assure high urine volume (e.g., during vomiting or diarrhea). |
Prevent systemic oxalosis
| Monitor plasma oxalate concentration during transient or permanent periods of low GFR. Initiate dialysis promptly to ↓ oxalate concentration. 1, 2
| Dialysis, most often used as a bridge to kidney recovery or transplantation, often requires ≥4 dialysis sessions per week to maintain plasma oxalate concentrations that minimize risk of oxalosis. 2, 3 |
GFR = glomerular filtration rate; NSAID = nonsteroidal anti-inflammatory drug; PH = primary hyperoxaluria
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Reduce calcium oxalate crystallization in the urine. Reduction of calcium oxalate crystals helps to minimize crystal injury to kidney tissue and reduces stone formation [Groothoff et al 2023, Michael et al 2024]. Recommendations include the following:
High oral fluid intake. Drinking large volumes of fluid (2-3 L/m2/24 hours) at regular intervals over the entire day/night minimizes calcium oxalate crystal formation through urinary dilution.
Small children may require gastrostomy tube placement or nasogastric tube for feeding and fluid supplementation.
Extreme care should be taken during any illness that could lead to hypovolemia or decreased oral fluid intake; individuals should be advised to seek early medical attention to initiate intravenous fluids if needed to maintain urine volume.
Inhibition of calcium oxalate crystallization. If the GFR is preserved, increase urine citrate excretion using oral potassium citrate at a dose of 0.1-0.15 mg/kg or 0.3-0.5 mmol/kg/day in three to four divided doses. If the GFR is reduced or blood potassium concentration is elevated, sodium citrate can be used.
In individuals with good kidney function (eGFR >30 mL/min/1.73 m2), oral pyrophosphate-containing solutions (at 20-30 mg/kg/day of phosphate in divided doses) can be used to inhibit crystal formation.
In PH1 excess oxalate results from endogenous metabolism, not dietary intake of oxalate, and dietary restriction of oxalate intake is of little benefit. Thus, avoiding specific foods or beverages with very high oxalate content, without strict limits, is appropriate.
Reduce stone formation and stone-related kidney damage. Monitoring new stone formation and stone size at regular intervals helps clinicians adjust fluids and medications and allows early identification of stones at risk of causing urinary obstruction.
Since recurring symptomatic stone events requiring urologic interventions are common, consultation with a urologist experienced in the care of individuals with PH1 helps individualize care and improve outcomes, particularly when specialized equipment and techniques may be required for infants and small children [Pais & Assimos 2005, Carrasco et al 2015].
Calcium oxalate stones can be resistant to shock wave lithotripsy (SWL); nephrocalcinosis can also complicate stone management. Because stone removal can result in acute kidney failure, interventions least likely to cause kidney injury are preferred [Carrasco et al 2015, Kohli & Kurtz 2022].
Surgical modalities used for stone management include the following:
Ureteroscopy, which effectively removes stones with minimal complications, is least likely to be associated with kidney injury.
Shockwave lithotripsy (SWL). Although a reasonable option, SWL has a lower success rate than endoscopic lithotripsy or endoscopic stone removal, thus increasing the need for repeated procedures [
Kohli & Kurtz 2022]. Calcium oxalate monohydrate stones, typical of PH1, are among the hardest stones, making them more likely to be resistant to SWL [
Kohli & Kurtz 2022]. Particularly stones in the lower pole of the kidney or parenchyma do not respond as well to SWL as stones in other locations [
Al-Abadi & Hulton 2013]. Multiple treatments are often needed; incomplete stone clearance may lead to their rapid enlargement [
Kohli & Kurtz 2022].
Percutaneous nephrolithotomy may be needed for larger, bulky stone burden.
Prevent systemic oxalosis by reducing the plasma oxalate concentration. This can be accomplished by dialysis to remove oxalate and/or reduction in oxalate production. Hemodialysis, which is more effective at oxalate removal than peritoneal dialysis, is most often used as a bridge to transplantation in individuals with PH1; however, it may also be an adjunct therapy when kidney allograft function following kidney transplantation is delayed or poor. In countries without access to organ transplantation, hemodialysis may be used for long-term management.
Hemodialysis is the primary dialysis modality used in persons with PH1 with kidney failure to reduce plasma oxalate concentration to prevent or treat oxalosis; however, absent targeted therapy to reduce hepatic oxalate production, the high rate of oxalate production (often 2-7 mmol/1.73 m2/day) exceeds the ability of hemodialysis to prevent or treat oxalosis. In some individuals both hemodialysis and peritoneal dialysis are required.
Current guidelines suggest reducing and maintaining plasma oxalate concentration below 30-45 µmol/L (the calcium oxalate supersaturation threshold at which tissue deposition occurs) as long as possible between dialysis sessions. Although most individuals with PH1 require four or more dialysis sessions per week to maintain acceptable plasma oxalate concentrations, significant individual variation in oxalate production requires individualization of dialysis prescriptions [Tang et al 2014]. Detailed kidney dialysis recommendations can be found in published practice guidelines [Groothoff et al 2023, Michael et al 2024]. It has been noted that such intensive dialysis regimens are difficult for patients, their families, and nephrology providers [Lawrence & Wattenberg 2020].
Following increasing clinical experience with RNAi therapeutics effective in reducing hepatic oxalate production in individuals with PH1, it is expected that guidelines for PH1 [Groothoff et al 2023, Michael et al 2024] will continue to evolve.