Primary hyperoxaluria type 1 (PH1) is a rare metabolic disease caused by a deficiency of the liver-specific enzyme alanine/glyoxylate aminotransferase, which results in an increased endogenous oxalate production (EOP),1Cochat P. Rumsby G. Primary hyperoxaluria.N Engl J Med. 2013; 369: 649-658Crossref PubMed Scopus (345) Google Scholar leading to kidney stones, nephrocalcinosis, and eventually kidney failure. In recent years, promising new therapies based on RNA interference have been developed.2Garrelfs S.F. Frishberg Y. Hulton S.A. et al.Lumasiran, an RNAi therapeutic for primary hyperoxaluria type 1.N Engl J Med. 2021; 384: 1216-1226Crossref PubMed Scopus (147) Google Scholar,3Hoppe B. Koch A. Cochat P. et al.Safety, pharmacodynamics, and exposure-response modeling results from a first-in-human phase 1 study of nedosiran (PHYOX1) in primary hyperoxaluria.Kidney Int. 2022; 101: 626-634Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar One of these drugs, OXLUMO (lumasiran), inhibits glycolate oxidase (GO), which converts glycolate to glyoxylate, the direct precursor of oxalate. The phase 3 ILLUMINATE-A trial reported a mean reduction of 65.4% in 24-hour urinary oxalate excretion and near normalization in 84% of patients after 6 months of lumasiran treatment.2Garrelfs S.F. Frishberg Y. Hulton S.A. et al.Lumasiran, an RNAi therapeutic for primary hyperoxaluria type 1.N Engl J Med. 2021; 384: 1216-1226Crossref PubMed Scopus (147) Google Scholar Complete normalization of urinary oxalate excretion was seen in 52% of patients. This suggests individual differences in treatment response, which is pivotal to understand to improve therapeutic strategies. Hypothetically, persistent hyperoxaluria under lumasiran treatment could be due to (i) less effective GO inhibition, (ii) high EOP at baseline, or (iii) the continued contribution from precursors other than glycolate to EOP. To investigate the role of each of these mechanisms in individual patients taking lumasiran, we included 4 patients with PH1 who underwent stable isotope assessments, as described previously.4van Harskamp D. Garrelfs S.F. Oosterveld M.J.S. et al.Development and validation of a new gas chromatography-tandem mass spectrometry method for the measurement of enrichment of glyoxylate metabolism analytes in hyperoxaluria patients using a stable isotope procedure.Anal Chem. 2020; 92: 1826-1832Crossref PubMed Scopus (6) Google Scholar Hereby, we assessed the in vivo consequences of inhibiting GO in patients with PH1. Four adult patients with PH1 (patients 1–4) underwent 2 stable isotope assessments: one before lumasiran treatment initiation and one while on active treatment for a duration of 17 to 24 months (Table 1).5Garrelfs S. van Harskamp D. Peters-Sengers H. et al.Endogenous oxalate production in primary hyperoxaluria type 1 patients.J Am Soc Nephrol. 2021; 32: 3175-3186Crossref PubMed Scopus (5) Google ScholarTable 1Patient characteristics and changes in oxalate and glycolate metabolism, at time of first and second stable isotope infusion protocolVariablePatient 1a(M + 1) enrichments did not completely reach steady state in patients 1 and 2.Patient 2a(M + 1) enrichments did not completely reach steady state in patients 1 and 2.Patient 3Patient 4AGXT genotypec.33dupC/c.454T>AbPyridoxine-sensitive mutation (c.454T>A corresponds to p.Phe152Ile, and c.508G>A corresponds to p.Gly170Arg).c.33dupC/c.508 G>AbPyridoxine-sensitive mutation (c.454T>A corresponds to p.Phe152Ile, and c.508G>A corresponds to p.Gly170Arg).c.454T>A/c.508G>A +1007T>AcAlanine/glyoxylate aminotransferase double mutant, results in a serious pathogenic effect.Ex5_11del/Ex5_11delPyridoxine, doseYes, 600 mg/dNoYes, 100 mg/dNoAge, yrdAge at time of lumasiran treatment initiation.20.319.835.331.4Time taking lumasiran, mo17172324PrePostChange, %PrePostChange, %PrePostChange, %PrePostChange, %Characteristics Weight, kg73.365.2–78.385.1–63.866.0–107.3115.5– Serum creatinine, μmol/L103109–103110–115114–105101– eGFR, ml/min per 1.73 m26862–>9082–5352–8183–Oxalate Ra oxalate, mmol/d1.560.84–463.161.87–411.811.05–422.851.04–63 Urine oxalate, mmol/deMean, multiple 24-hour collections.1.530.92–402.161.56–281.300.76–423.130.92–71 Plasma oxalate, μmol/L14.57.1–5111.46.5–4311.06.8–3812.03.7–69Glycolate Ra glycolate, mmol/d6.3218.74+1968.7047.35+4449.8515.98+627.512.3+65 Urine glycolate, mmol/deMean, multiple 24-hour collections.1.115.68+4132.1514.75+5871.953.96+1035.19.4+86 Plasma glycolate,μmol/L90.2279+209135385+25448255+43182159.3+94Contribution of glycolate to EOP ASR, mmol/d0.970.18–821.120.50–551.000.09–911.390.15–90 ASR/Ra oxalate, %fContribution of glycolate to oxalate production, expressed in percentage, calculated from both using the ASR of (M + 1) oxalate and the oxalate Ra.62.421.0–6635.426.9–2455.18.6–8448.613.9–71ASR, absolute synthetic rate; EOP, endogenous oxalate production; eGFR, estimated glomerular filtration rate; Pre, measurement at time of first stable isotope infusion protocol (before treatment); Post, measurement at time of second isotope infusion protocol (after treatment); Ra, rate of appearance.For reference values in healthy volunteers, please refer to our previous publication.5Garrelfs S. van Harskamp D. Peters-Sengers H. et al.Endogenous oxalate production in primary hyperoxaluria type 1 patients.J Am Soc Nephrol. 2021; 32: 3175-3186Crossref PubMed Scopus (5) Google ScholarBold and italicized data indicate xxx.a (M + 1) enrichments did not completely reach steady state in patients 1 and 2.b Pyridoxine-sensitive mutation (c.454T>A corresponds to p.Phe152Ile, and c.508G>A corresponds to p.Gly170Arg).c Alanine/glyoxylate aminotransferase double mutant, results in a serious pathogenic effect.d Age at time of lumasiran treatment initiation.e Mean, multiple 24-hour collections.f Contribution of glycolate to oxalate production, expressed in percentage, calculated from both using the ASR of (M + 1) oxalate and the oxalate Ra. Open table in a new tab ASR, absolute synthetic rate; EOP, endogenous oxalate production; eGFR, estimated glomerular filtration rate; Pre, measurement at time of first stable isotope infusion protocol (before treatment); Post, measurement at time of second isotope infusion protocol (after treatment); Ra, rate of appearance. For reference values in healthy volunteers, please refer to our previous publication.5Garrelfs S. van Harskamp D. Peters-Sengers H. et al.Endogenous oxalate production in primary hyperoxaluria type 1 patients.J Am Soc Nephrol. 2021; 32: 3175-3186Crossref PubMed Scopus (5) Google Scholar Bold and italicized data indicate xxx. Isotopic steady state was reached on both measurement days (Supplementary Figure S1). Oxalate rate of appearance (Ra) (mean [SD]) decreased from 2.34 (0.78) mmol/d at baseline to 1.20 (0.46) mmol/d (P < 0.05) while receiving lumasiran treatment. This amounted to a mean reduction of 47.9% (range, 40.7%–63.4%), in line with the mean reduction in 24-hour urinary oxalate excretion (–44.9%) and plasma oxalate concentration (–50.3%). During the baseline measurement, isotopic steady state was nearly, although not completely, reached in patients 1 and 2 (Supplementary Figure S2). During the second measurement, all patients reached isotopic steady state. Following treatment with lumasiran, mean (SD) plasma glycolate concentrations increased from 88.8 (35.8) to 269.6 (92.7) μmol/L. In line with this, glycolate Ra and urinary glycolate excretion increased to a certain degree in all patients (Table 1). Following infusion with [1-13C]glycolate, the rate of incorporation of glycolate in oxalate (mean [SD]) was 8.08 (1.52) %/h at baseline versus 2.77 (1.37) %/h in the treated group (fractional synthetic rate; Supplementary Figure S3). The mean (SD) absolute synthetic rate (ASR), reflecting [1-13C]oxalate production from glycolate as a precursor, decreased from 1.12 (0.19) to 0.23 (0.19) mmol/d while receiving lumasiran treatment. The mean reduction in ASR, reflecting GO inhibition, was 79.3%. There were significant differences between patients; the reduction in ASR ranged from 55% in patient 2 to 91% in patient 3. The mean quantitative contribution (percentage) of glycolate to oxalate production (ASR/oxalate Ra) was reduced by 61.6% (range, 24%–84%). Supplementary Figure S4 provides [D5]glycine enrichments. Mean (SD) Ra of glycine was 260.3 (83.0) μmol/kg per hour while receiving lumasiran treatment. Under lumasiran treatment, glycine (M + 1) enrichments remained below the detection limit (<0.1 MPE) in all patients, irrespective of pyridoxine responsiveness, which is indicative of limited or no contribution of the [1-13C]glycolate tracer as a precursor for glycine (Supplementary Figure S5). Stable isotope assessments, performed in 4 patients with PH1, served as a proof of mechanism of lumasiran. The contribution of glycolate to oxalate production was reduced by nearly 80%, indicating effective GO inhibition. However, we found important individual differences in the extent of GO inhibition, which partly explained the remaining extent of hyperoxaluria. The reduction in oxalate Ra (47.9%), reflecting the reduction in EOP, corresponded well to the reduction in 24-hour urinary oxalate excretion (44.9%) and plasma oxalate concentration (50.4%). Absolute reductions in oxalate Ra were larger in pyridoxine-unresponsive than in pyridoxine-responsive patients (–1.5 vs. –0.7 mmol/d), most likely attributable to the higher baseline oxalate Ra. Previously, we found that glycolate serves as a precursor of oxalate for at least 50% of oxalate Ra in patients with PH1.5Garrelfs S. van Harskamp D. Peters-Sengers H. et al.Endogenous oxalate production in primary hyperoxaluria type 1 patients.J Am Soc Nephrol. 2021; 32: 3175-3186Crossref PubMed Scopus (5) Google Scholar Because only GO is known to convert glycolate to glyoxylate,1Cochat P. Rumsby G. Primary hyperoxaluria.N Engl J Med. 2013; 369: 649-658Crossref PubMed Scopus (345) Google Scholar,6Mackinnon S.R. Bezerra G.A. Krojer T. et al.Novel starting points for human glycolate oxidase inhibitors, revealed by crystallography-based fragment screening.Front Chem. 2022; 10844598Crossref PubMed Scopus (0) Google Scholar the change in contribution of glycolate to oxalate production (i.e., the reduction in ASR) reflects GO inhibition and, thus, the efficacy of lumasiran. As glycolate is not directly converted to oxalate itself, but oxidized via glyoxylate, the (M + 1) enrichment of oxalate measured following infusion of [1-13C]glycolate may be an underestimation due to dilution of the (M + 1) labeled glyoxylate pool. Consequently, the calculated FSR and ASR may in reality be even higher. Although lumasiran treatment lowered the ASR by 79.3%, indicating effective GO inhibition, hyperoxaluria persisted in all patients to a certain extent. Stable isotope assessments provided insight in the role of 3 potential explanatory factors for the persistent hyperoxaluria observed, namely (i) less effective GO inhibition (i.e., less ASR reduction), (ii) high EOP at baseline (high oxalate Ra at baseline), and (iii) the contribution of precursors other than glycolate to EOP (low ASR/Ra at baseline). These factors combined explain why hyperoxaluria after treatment was most prominent in patient 2, who had the lowest reduction in ASR (55%), the highest EOP at baseline (3.2 mmol/d), and the lowest contribution of glycolate to oxalate production at baseline (35.4%). In patients 1, 3, and 4, urinary oxalate excretion had decreased to 0.8 to 0.9 mmol/d, thus almost reaching near-normal values (defined as <1.5 times the upper reference limit of normal; i.e., 1.5 × 0.46 mmol/d). The remaining extent of hyperoxaluria in patient 1 was mainly due to slightly less effective GO inhibition; in patient 3, it was due to continued contribution of precursors other than glycolate to EOP; and in patient 4, it was due to high urinary oxalate at baseline. Patients 1 and 4 both had urinary oxalate excretion rates of 0.9 mmol/d while receiving lumasiran treatment. Expressing urinary oxalate excretion using the percentage reduction from baseline instead of absolute values would thus give additional information in this case. Glycine enrichments remained below the detection limit for all patients, including the 2 pyridoxine-responsive patients who had previously shown to have residual alanine/glyoxylate aminotransferase activity. This can be explained by the depletion of glyoxylate secondary to GO inhibition. These findings also suggest that transaminases other than alanine/glyoxylate aminotransferase play a negligible role in converting glycolate to glycine. Only 4 patients were included in this study because of the low prevalence of PH1 and the fact that lumasiran was, until recently, only available for clinical trial participants. Second, following [1-13C]glycolate infusion, isotopic steady state was not achieved in all participants. Nevertheless, (M + 1) glycolate enrichments were used in the FSR/ASR calculations as they almost reached equilibrium. This may have slightly influenced the results by underestimating the enrichment of the precursor at baseline. Our data confirm the potential of inhibiting GO activity to reduce oxalate production in patients with PH1. They also reveal individual differences in GO inhibition (range, 55%–91%). Stable isotope assessments could be used to establish clinical efficacy of lumasiran in patients in whom this is questioned. In the near future, this method may serve as an instrument to decide between treatment continuation or crossing over to other RNA-interference agents based on individual responses. ELM, SFG, MJSO, and JWG report an unconditional grant from both Dicerna Pharmaceuticals and Alnylam Pharmaceuticals, during the conduct of the study. All the other authors declared no competing interests. Figure 1 The authors thank all the patients with primary hyperoxaluria type 1 for their participation. This work was funded by the Amsterdam UMC; personal PhD scholarship for SFG; Alnylam Pharmaceuticals (unconditional grant); and Metakids (grant 2019-04-UMD). Download .pdf (.57 MB) Help with pdf files Supplementary File (PDF) Supplementary Methods. Supplementary References. Supplementary Table S1. Tracer infusion rates (μmol/kg per hour). Supplementary Figure S1. Plasma enrichments of (M + 2) oxalate during the 10-hour primed continuous infusion of [U-13C2]sodium oxalate in 4 patients with primary hyperoxaluria type 1 (PH1) untreated (A) and treated (B) with lumasiran. Supplementary Figure S2. Plasma enrichments of (M + 1) glycolate during the 6-hour primed continuous infusion of [1-13C]glycolate in patients with primary hyperoxaluria type 1 (PH1) untreated (A) and treated (B) with lumasiran. Supplementary Figure S3. Plasma enrichments of (M + 1) oxalate during the 6-hour primed continuous infusion of [1-13C]glycolate in patients with primary hyperoxaluria type 1 (PH1) untreated (A) and treated (B) with lumasiran. Supplementary Figure S4. Plasma enrichments of [D5]glycine during the 6-hour primed continuous infusion of [D5]glycine in patients with primary hyperoxaluria type 1 (PH1) treated with lumasiran. Supplementary Figure S5. Plasma enrichments of (M + 1)glycine during the 6-hour primed continuous infusion of [1-13C]glycolate in patients with primary hyperoxaluria type 1 (PH1) untreated (A) and treated (B) with lumasiran.
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