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Article

Sodium Thiosulfate Pharmacokinetics in Hemodialysis Patients and Healthy Volunteers

Stefan Farese,* Emilie Stauffer,* Robert Kalicki,* Tatjana Hildebrandt,† Brigitte M. Frey,* Felix J. Frey,* Dominik E. Uehlinger,* and Andreas Pasch*

Summary Background and objectives Vascular calcification is a major cause of morbidity and mortality in dialysis pa- *Department of tients. Human and animal studies indicate that (STS) may prevent the progression of Nephrology and vascular calcifications. The pharmacokinetics of STS in hemodialysis patients has not been investigated yet. Hypertension, Inselspital, University of Design, setting, participants, & measurements STS was given intravenously to 10 hemodialysis patients on- Bern, Bern, Switzerland, and and off-hemodialysis. Additionally, STS was applied to 9 healthy volunteers once intravenously and once †Institute for Plant orally. Thiosulfate concentrations were measured by using a specific and sensitive HPLC method. Genetics, Leibnitz University, Hannover, Results In volunteers and patients, mean endogenous thiosulfate baseline concentrations were 5.5 Ϯ 1.82 versus Germany 7.1 Ϯ 2.7 ␮mol/L. Renal clearance was high in volunteers (1.86 Ϯ 0.45 ml/min per kg) and reflected GFR. Non- Correspondence: renal clearance was slightly, but not significantly, higher in volunteers (2.25 Ϯ 0.32 ml/min per kg) than in an- Ϯ Ϯ Dr. Stefan Farese, uric patients (2.04 0.72 ml/min per kg). Hemodialysis clearance of STS was 2.62 1.01 ml/min per kg. On Department of the basis of the nonrenal clearance and the thiosulfate steady-state serum concentrations, a mean endogenous Nephrology and thiosulfate generation rate of 14.6 nmol/min per kg was calculated in patients. After oral application, only 4% of Hypertension, University STS was recovered in urine of volunteers, reflecting a low bioavailability of 7.6% (0.8% to 26%). of Bern, Inselspital, 3010 Bern, Switzerland. Phone: ϩ41 31 632 96 Conclusions Given the low and variable bioavailability of oral STS, only intravenous STS should be pre- 29; Fax: ϩ41 31 632 44 scribed today. The biologic relevance of the high hemodialysis clearance for the optimal time point of STS 36; E-mail: stefan.farese@ dosing awaits clarification of the mechanisms of action of STS. insel.ch Clin J Am Soc Nephrol 6: 1447–1455, 2011. doi: 10.2215/CJN.10241110

Introduction Before new xenobiotics can be applied, their pharma- Vascular calcification is associated with increased mor- cokinetics have to be established in a restricted number bidity and mortality in dialysis patients (1,2) and new of patients from the target population. Surprisingly, this therapeutic strategies are needed to tackle this chal- was not done in the case for STS. One can only speculate ⅐ why no formal pharmacokinetic studies have been pre- lenge. Sodium thiosulfate (STS, Na2S2O3 5H2O, MW 248 g) is an old drug used for decades as an viously performed. First, decades ago when STS was against poisoning (3), because STS supplements first used (3,18) the legal prerequisites for new drugs were the cyanide-detoxifying enzyme (4,5), which is not as stringent as those today. Second, when STS is given part of the endogenous thiosulfate (TS) synthesis pathway as a cyanide antidote, the application is of short duration (6). Recently, STS has been prescribed for the treatment of and occurs in patients with normal renal function. soft-tissue calcifications in patients with calciphylaxis (7– With the new indications and the prolonged use of STS, the pharmacokinetics have to be investigated for 12). Furthermore, systematic studies in rats (13) and hu- rational dosing, potential pharmacokinetic-pharmaco- mans (14) have investigated the effect of STS on aortic and dynamic modeling, and safety reasons. Therefore, we coronary artery calcifications. These studies support the investigated the kinetics of STS in dialysis patients on- notion of vascular calcification, slowing or even preventing and off-hemodialysis and in healthy volunteers (HV) properties of STS. The underlying mechanism for the effi- and present the numbers derived from classic pharma- cacy of STS is not clear. Apparently, the beneficial effects cokinetic calculations and from a newly developed STS of STS are mediated by a combination of calcium- population pharmacokinetic model. solubilizing, antioxidative, acidosis-inducing, and possibly other not-yet-identified mechanisms (13,15–17). Given the Materials and Methods favorable tolerability even in long-term use along with its Patients, HV, and Study Protocol low cost, STS has the potential to become a more widely All patients and HV (Table 1, Table 2) were studied applied therapeutic agent for patients predisposed to vas- at the Department of Nephrology/Hypertension of cular calcifications. the University-Hospital Bern, Switzerland. The study www.cjasn.org Vol 6 June, 2011 Copyright © 2011 by the American Society of Nephrology 1447 48Ciia ora fteAeia oit fNephrology of Society American the of Journal Clinical 1448

Table 1. Dialysis patients characteristics and individual dialysis treatment parameters

Urine Volume Blood Flow Treatment Dialyzer Surface Patient Age (yr) Gender Body BMI Renal Comorbiditiyb (ID) (f/m) Weight (kg) (kg/m2 ) Diseasea (ml/24 h) (ml/min) Time (minutes) Areac (m2 ) 1 76 f 92 32.6 3 0 4 350 210 2.2 2 53 m 86.4 26.4 2 0 5 300 240 2.2 3 57 m 68.4 24.2 4 0 6 350 240 1.8 4 42 m 80.6 26 4 600 2 425 210 1.8 5 63 f 53.7 22.1 4 1800 6 300 240 1.8 6 56 f 48 20 4 0 3 350 210 1.8 7 78 m 84.6 26.1 1 0 6 250 240 1.8 8 56 m 84.4 28 2 0 5 380 240 2.2 9 51 m 84 31.8 1 0 5 350 210 1.8 10 77 m 74 26 1 0 6 300 240 1.8 56 3/7 76 Ϯ 15 26 Ϯ 3.8 0 5 335 Ϯ 49 240 1.9 Ϯ 0.2 (42 to 78) (0 to 1800) (2 to 6) (210 to 240)

Mean (ϮSD) or median (range) values of all parameters are given at the end of the table. ID, identification number; BMI, body mass index. aRenal disease: 1, diabetic nephropathy; 2, glomerulonephritis; 3, nephroangiosclerosis; 4, other renal diseases. bCharlson Comorbidity Index (45). cAll high-flux polysulfone dialyzers.

Table 2. Characteristics and pharmacokinetic parameters in healthy volunteers (HV) after8gofintravenous STS (32,250 ␮mol)

Body 24-Hour Urinary TS Total Body Clearance Renal Clearance Nonrenal Clearance Gender ␮ ⅐ HV (ID) Age (yr) (f/m) Weight AUC03 ␮ ( mol min/L) Excretion after iv STS (ml/min per kg (ml/min per kg (ml/min per kg (kg) (␮mol/24 h) body wt) body wt) body wt) 11 64 m 74 10,5573 12,620 4.13 2.08 2.05 12 39 m 66 85,901 13,760 5.60 2.41 3.2 13 57 f 65 133,773 15,670 3.71 1.82 1.9 14 48 f 80 81,953 14,840 4.92 2.28 2.64 15 63 f 78 98,145 14,910 4.21 1.99 2.22 16 60 f 74 109,293 14,260 3.99 1.82 2.17 17 45 f 56 144,313 15,310 3.99 1.99 2 18 54 f 69 136,553 13,850 3.42 1.48 1.94 19 59 f 53 202,486 9600 3.00 0.91 2.06 57 (39 to 64) 7/2 68 Ϯ 9.4 121,999 Ϯ 37473 13,869 Ϯ 1847 4.11 Ϯ 0.77 1.86 Ϯ 0.45 2.25 Ϯ 0.42

Mean (ϮSD) or median (range) values of all parameters are given at the end of the table. AUC, area under the curve; TS, thiosulfate; STS, sodium thiosulfate. Clin J Am Soc Nephrol 6: 1447–1455, June, 2011 Sodium Thiosulfate Pharmacokinetics, Farese et al. 1449

was approved by the ethics committee and registered acid (Fluka, Buchs, Switzerland) and proteins were re- (NCT01008631). Ten patients and nine HV were enrolled moved by recentrifugation. Urine and dialysate samples after giving written consent. Inclusion criteria for patients were processed identically but without the delipidation were as follows: Age 18 to 80 years and dialysis vintage Ͼ3 step. months. Hemodialysis was performed 3 times per week HPLC was performed using a Waters-2695 module, cou- through an arteriovenous fistula with high-flux mem- pled to an RF10-AXL (Shimadzu, Kyoto, Japan) fluores- branes using the Fresenius-5008 device. Dialysate flow rate cence detector (excitation 380 nm, emission 480 nm) and a was set at 600 ml/min. Merck-LiChro-CART-125-4, LiChrospher-60, RP-select-B (5 The protocol for patients involved two single intrave- ␮m) reverse-phase column. PIPES (10 mM, pH 6.6) and nous doses of 8 g (32,250 ␮mol) of STS (Ko¨hler-Chemie, (gradient) were used as eluant at a flow rate of 1 Bensheim, Germany), diluted in 50 ml of 0.9% NaCl and ml/min. Area under the curves (AUCs) of the TS peaks infused over 8 minutes. A detailed protocol overview is were integrated by the software of the Waters-2695 device. given in Figure 1. Using this method, we obtained linearity from 2 to 100 Similarly, HV between 18 and 80 years without regular ␮M and a maximal intraday and interday variability of medications and a normal renal function according to the 8.6% and 9.3%, respectively. The lower detection limit was Modification of Diet in Renal Disease (MDRD) formula approximately 2 ␮M. (GFR Ͼ70 ml/min per 1.72 m2) were included. During the study, individual creatinine clearances were calculated from the 24-hour urine collection. Intravenous and oral Pharmacokinetic and Statistical Analyses pharmacokinetics were investigated with a washout pe- Individual estimates of pharmacokinetic parameters riod of 7 days between the two applications. Intravenous were calculated by a noncompartmental approach (21,22). STS (8 g) was infused as described above for dialysis The total body clearance was calculated by dividing the patients. Blood samples were collected at baseline and 15, intravenous dose by the corresponding AUC, the renal 30, 60, and 180 minutes after the end of the STS infusion. clearance by dividing the amount of TS recovered in urine Urine was collected for 24 hours. by the AUC, and the nonrenal clearance by subtracting the For oral application, 5 g (20,000 ␮mol) of the intravenous renal from the total body clearance (23). For population solution was diluted in 100 ml of water and rapidly in- pharmacokinetic modeling, the serum concentration time gested. Sampling was performed as described for the in- data and TS amounts recovered in urine and dialysate of travenous application. All samples were processed imme- dialysis patients and from HV after intravenous STS ap- diately and frozen at Ϫ80°C. plication were pooled and analyzed by a nonlinear mixed- effects modeling approach using NONMEM-6.2 (Globo- Determination of Thiosulfate Concentrations in Blood, Max LLC, Hannover, Germany), double precision, with the Urine, and Dialysate g95-FORTRAN compiler on a Windows-NT-6.0 platform. Thiosulfate (TS) was determined by a specific HPLC The NONMEM runs were executed using PsN-2.3.0 (24). method as described previously (19,20). To prevent the Serum TS concentrations were fitted to a one-compartment premature clotting of the HPLC column by lipids, we in- model using user self-supplied differential equations (AD- troduced a delipidation step. Thereby, the life span of the VAN6). Interindividual variability of estimated parameters column was enhanced for several hundred samples. was best described by an exponential model whereas the Briefly, serum samples (150 ␮l) were delipidized with residual error was implemented with an intercept-slope dichloromethane by centrifugation. Twenty-five microli- residual variability model. Data were analyzed using the ters of the supernatant was derivatized with 5 ␮lof46mM first-order conditional estimation with INTERACTION. monobromobimane, 25 ␮l of acetonitrile, and 25 ␮lof160 Statistical comparison of nested models was based on a mM HEPES/16 mM EDTA pH 8 buffer (Invitrogen, Carls- ␹2-test of the difference in the objective function (OFV). A bad, CA) for 30 minutes in the dark. Derivatization of thiol decrease in OFV of 3.84 units (P Ͻ 0.05) per 1 additional groups was stopped by 50 ␮l of 65 mM methanosulfonic degree of freedom (df) was considered significant. Good-

Figure 1. | Study flow sheet for dialysis patients. Eight grams of STS was given after the first dialysis session of the week and before the next mid-week dialysis session. Thirty minutes after the start of the mid-week dialysis, a single dialysate specimen was obtained. Dialysate was collected continuously during the whole treatment session. Numbers on the x-axis indicate time points of blood collection in minutes. 1450 Clinical Journal of the American Society of Nephrology

ness of fit and visual predictive checks were performed Results using PsN-2.3.0 and XPose-4 (24,25). Posterior empirical STS Pharmacokinetics in Hemodialysis Patients Bayesian estimates were further used for statistical infer- Off-Hemodialysis. STS was given intravenously imme- ence if calculated shrinkage was lower than 20%. diately after the first dialysis session of the week. Endog- The R.2.10.1 (R-Development Core-Team [2010]) was enous baseline TS serum concentrations before and after used to compute AUCs based on the linear trapezoidal this dialysis session were comparable (6.6 Ϯ 2.4 and 6.2 Ϯ rule. GraphPad5 (GraphPad, La Jolla, CA) was used for 2.8 ␮mol/L). Serum TS concentrations after the single dose statistics and figures. given off-dialysis are shown in Figure 2A and individual

Figure 2. | Time course of measured TS serum concentrations. TS concentrations (mean Ϯ SD) are shown after an 8-g iv dose given to (A) dialysis patients off-hemodialysis, (B) dialysis patients on-hemodialysis, and (C) healthy volunteers. In (D) the values after a 5-g oral dose administered to healthy volunteers are given. The horizontal bar in (B) represents the duration of the dialysis treatment. The horizontal, dotted lines in (A through D) indicate the mean endogenous TS concentrations. Clin J Am Soc Nephrol 6: 1447–1455, June, 2011 Sodium Thiosulfate Pharmacokinetics, Farese et al. 1451

estimates of pharmacokinetic parameters obtained by non- filtered from the blood was 1.00 Ϯ 0.41 (median 0.93, range compartmental analyses in Table 3. Mean total body clear- 0.28 to 1.7). ance was 2.04 Ϯ 0.72 ml/min per kg. This value largely The serum concentrations at the end of the dialysis treat- reflects the nonrenal clearance because minimal residual ment and 30 minutes later were comparable (24.5 Ϯ 8.7 renal function (GFR Ͻ6 ml/min) was present in only two versus 20.5 Ϯ 8.4 ␮mol/L, NS), suggesting absence of a patients. pronounced TS rebound and possibly ongoing nonrenal Assuming a constant endogenous production (G)ofTS, clearance. G can be estimated from the steady-state concentration and the total body clearance. G was estimated to be 1.05 ␮mol/ STS Pharmacokinetics in HV min (14.6 Ϯ 6.1 nmol/min per kg), a value used for further STS Kinetics after Intravenous Application. STS (8 g) calculations. was given intravenously to nine HV. Mean endogenous With use of the population pharmacokinetics approach baseline TS serum concentrations were 5.5 Ϯ 1.8 ␮mol/L described above, simulated time-serum concentration pro- and not significantly different from those of dialysis pa- files of a single 25-g STS dose (which represents the usually tients. The AUC03ϰ was significantly smaller in HV than applied dose in the literature [7–10]) in dialysis patients are in patients off-dialysis (P Ͻ 0.001; Table 2, Table 3, Figure shown in Figure 3A. 2C). The renal clearance (1.86 Ϯ 0.45 ml/min per kg) On-Hemodialysis. Mean endogenous baseline TS serum accounted for about 50% of the total body clearance (4.11 Ϯ concentrations before the mid-week session were 7.6 Ϯ 3.0 0.77 ml/min per kg; Table 2). Urinary recovery of the ␮mol/L and therefore comparable with the values ob- intravenous TS dose of 32,250 ␮mol was 43% (13,869 Ϯ tained before and after the first dialysis session of the 1847 ␮mol). week, indicating that the STS applied after the first dialysis The simulated expected time-concentration profile after had been cleared. When all endogenous TS concentrations an assumed single dose of 25 g in HV is displayed in were summed up, the mean value was 7.1 Ϯ 2.7 ␮mol/L. Figure 3B. TS serum concentrations during dialysis are shown in Fig- STS Kinetics after Oral Application. Each HV ingested ure 2B. Thirty minutes after starting dialysis, simultaneous a single STS dose of 5 g. The time-serum concentration arterial (TSa) and venous (TSv) samples from a pre- and profile is shown in Figure 2D. The peak TS concentrations postdialyzer tubing port as well as dialysate (TSd) samples were variable and only slightly higher than the usually were obtained from the dialysis circuit. TSa, TSv, and TSd measured steady-state endogenous serum concentrations. were 748 Ϯ 191, 309 Ϯ 90, and 236 Ϯ 77 ␮mol/L, respec- Within 180 minutes after STS intake, peak TS serum con- tively. For the calculation of the amount of TS lost from centrations (Cmax) were observed in only 5 of 9 HV. Thus, serum and recovered in the dialysate the blood flow (Qb), the calculation of the bioavailability by dividing the oral ultrafiltration (UF), dialysate flow (Qd), and partitioning AUCs by the intravenous AUCs was prone to yield inac- into the red blood cells have to be considered. Because no curate results. Therefore, bioavailability was estimated by rebound of STS at the end of dialysis was observed, we two alternative methods, both taking into account the dif- used a one-compartment model without unequal partition- ferent doses of STS given orally and intravenously. First, ing and derived the amount of STS recovered from the the 24-hour urinary TS excretion after oral dosing was following equation: TSd*Qd ϭ TSa*Qb Ϫ TSv*(Qb Ϫ UF*t). divided by the excretion after intravenous dosing. This The ratio between STS recovered in the dialysate and STS method yielded a low bioavailability ranging from 0.8%

Table 3. Pharmacokinetic parameters in dialysis patients after 8 g (32,350 ␮mol) of intravenous STS off-hemodialysis and on-hemodialysis

Off-Hemodialysis On-Hemodialysis

Total Body Total Body Estimated Hemodialysis a Patient AUC03␮ Clearance AUC03␮ Clearance Clearance (ID) (␮mol⅐min/L) (ml/min per kg (␮mol⅐min/L) (ml/min per kg (ml/min per kg body wt) body wt) body wt) 1 261,753 1.34 87,421 4.01 2.67 2 336,691 1.11 114,564 3.25 2.14 3 197,000 2.39 110,925 4.25 1.86 4 156,360 2.56 49,629 8.05 5.5 5 253,618 2.28 129,545 4.45 2.17 6 212,027 3.17 120,489 5.56 2.4 7 154,521 2.47 108,358 3.51 1.04 8 163,867 2.33 109,394 3.5 1.17 9 338,150 0.92 95,368 4.0 3.1 10 236,159 1.85 92,550 4.71 2.86 231,015 Ϯ 67,969 2.04 Ϯ 0.72 101,824 Ϯ 22,414 4.53 Ϯ 1.40 2.49 Ϯ 1.3

Mean (ϮSD) values of all parameters are given at the end of the table. aEstimated hemodialysis clearance ϭ total body clearance on-hemodialysis Ϫ total body clearance off-hemodialysis. 1452 Clinical Journal of the American Society of Nephrology

to 26% (median 7.6%; Table 4). Second, with use of the population pharmacokinetic approach (see below), a sim- ilar bioavailability ranging from 2.3% to 11.2% (median 6.6%; Table 5) was found. The mean urinary recovery of oral STS was 4%. A summary of all pharmacokinetic esti- mates obtained from noncompartmental analysis (21,22) is given in Table 4.

Population Pharmacokinetic Model All intravenous data from dialysis patients during and between dialyzes as well as those from HV were combined into one population pharmacokinetics model, reflecting the mean of all participants. Because no rebound was observed 30 minutes after dialysis sessions, a one-compart- ment distribution of STS was assumed. G was set at 1.05 ␮mol/min. Mean population estimates of the final model are given in Table 5. The validity of the model is depicted in Figure 4. In a final step, all model parameters were fixed at their estimates, and as indicated above, a median bio- availability of 6.6% was estimated by adding the oral data from HV.

Discussion The present investigation revealed a low and highly variable oral bioavailability of STS in HV (0.8 to 26%). We therefore decided not to expose dialysis patients to oral STS until a suitable galenic formulation of STS with an enhanced bioavailability is available. The mechanism for the low bioavailability is open to speculation. It might be at least partially explained by the STS degradation in the ϩ 3 acidic environment of the stomach (Na2S2O3 2HCl ϩ ϩ ϩ 2NaCl H2O S SO2). Alternatively, the low and variable bioavailability may be due to STS degradation by intestinal bacteria (26) and/or different expression levels of a putative TS transporter in the gut mucosa, which might exist in analogy to a -inhibitable TS transporter in the dog renal tubule (27). Interestingly, despite the very low oral bioavailability of STS, the successful prevention of renal stones in humans (28), and rats (29), and of the Figure 3. | Simulated time course of TS serum concentrations after progression of calciphylaxis (30) and nephrocalcinosis (31) an intravenous dose of 25-g STS. TS serum concentrations (A) in has been reported by comparable oral STS doses as those dialysis patients off-hemodialysis and (B) in healthy volunteers. The used in our investigation. expected median concentrations as well as the confidence intervals More than 50 years ago, a good correlation between (CI) and the minimal/maximal concentration versus time curves are renal TS and inulin or creatinine clearance was shown depicted. (32,33). This was explained by the assumption that TS mainly undergoes glomerular filtration without quantita-

Table 4. Summary of individual pharmacokinetic estimates in dialysis patients and HV

Dialysis Patients (Mean Ϯ SD) HV (Mean Ϯ SD) Thiosulfate generation (nmol/min per kg) 14.6 Ϯ 6.1 10.8 Ϯ 3.6 Renal clearance (ml/min per kg body wt) 1.86 Ϯ 0.45 Hemodialysis clearance (ml/min per kg 2.49 Ϯ 1.3 body wt) Nonrenal clearance (ml/min per kg body 2.04 Ϯ 0.72 2.25 Ϯ 0.42 wt) Total body clearance (ml/min per kg Off-dialysis: 2.04 Ϯ 0.72 4.11 Ϯ 0.77 body wt) On-dialysis: 4.53 Ϯ 1.4 Bioavailability (%) 7.6 (0.8 to 26)

Values represent the mean (Ϯ SD) or the median with range. HV, Healthy volunteers. Clin J Am Soc Nephrol 6: 1447–1455, June, 2011 Sodium Thiosulfate Pharmacokinetics, Farese et al. 1453

Table 5. Summary of population pharmacokinetic model estimates in dialysis patients and HV

Between-Subject Dialysis Patients HV Variability (%) Thiosulfate generation (␮mol/min) 1.05 (fixed) Distribution volume (L/kg) 0.226 (0.220 to 0.249)a 17.8 Renal clearance (ml/min per kg body wt) 1.36 (1.15 to 1.58)a 15.2 Hemodialysis clearance (ml/min per kg body wt) 2.62 (2.22 to 3.23)a Nonrenal clearance (ml/min per kg body wt) 2.04 (1.86 to 2.13)a Bioavailability (%) 6.6 (2.3 to 11.2)b

Values represent the mean (Ϯ SD) or the median with range. a5% to 95% confidence interval from parametric bootstrap. bFrom a parametric bootstrap on the full pharmacokinetic model. HV, Healthy volunteers.

lyzed (35–37). Here, we showed for the first time that the nonrenal clearance of TS is similar in hemodialysis patients and in HV (Table 4). The vast proportion of TS cleared by nonrenal mechanisms appears to be metabolized to sulfate (4,29,38) possibly predominantly in the liver but also in other tissues (39). TS is considered to be the principal, rapidly disappearing precursor of sulfate in mammalians (40). Thus, our study suggests that the TS is unaltered in patients with chronic renal failure. Surpris- ingly, however, when we measured endogenous serum concentrations of TS during dialysis treatment without administration of exogenous STS, the TS concentrations remained stable and did not decline as expected during dialysis treatment, although TS is well dialyzable (Table 3). Patients did not consume TS-containing food during dial- ysis treatment. Therefore, the stable TS concentrations dur- ing dialysis are difficult to interpret. One can only specu- late whether the TS synthesis is upregulated during dialysis or the removal of TS by the dialysis is negligible at Figure 4. | Identity plot of observed versus predicted TS serum low concentrations. concentrations. A high goodness of fit suggests the validity of the On the basis of our population kinetic modeling, we can model. predict the concentration versus time curves in patients and HV and the model accurately reflects the measured values tive tubular secretion or reabsorption. In our HV the mean (Figure 4). The concentrations predicted from a given dose creatinine clearance was in the same range (101 Ϯ 28 are potentially useful for targeting a final or intermediate ml/min) as the renal clearance of TS. Despite the excellent therapeutic endpoint (41). The therapeutic endpoint of STS correlation between renal clearance of TS and the values of with respect to calciphylaxis or vascular calcification is the the GFR assessed by using creatinine, endogenous TS can- disappearance of calcium deposited in soft tissues and the not be used as an estimate of the GFR for two reasons. vasculature. TS serum concentrations between 5 and 10 First, the endogenous TS generation is variable (Table 4) mmol/L have been shown to transiently lower ionized and depends among other factors on the diet; and second, serum calcium in vitro and in vivo (13). Such high concen- renal and nonrenal mechanisms contribute about equally trations are achieved only for Ͻ30 minutes when the usual to the elimination of STS in normal HV (Table 2). The dose of 25 g of STS is given to patients off-dialysis (Figure nonrenal clearance varies even between HV (Table 2, Table 3A). Thus, it remains unknown whether the presumed 4). Therefore, the GFR determined by Newman and Gil- mechanism of calcium solubilization by chelation is thera- man (32,33) required the exogenous administration of STS peutically relevant (13). As a second, alternative mecha- and the simultaneous measurement of TS in serum and nism the induction of a high anion-gap acidosis (8,13) by urine. STS, which directly inhibits the precipitation of calcium The nonrenal elimination of TS accounts for about 50% and phosphate, has been proposed (16). However, this in HV, an observation consistent with previous investiga- mechanism has been questioned by the observation that tions in humans and dogs (4,34). When the nonrenal clear- vascular calcifications can be prevented by STS even in the ance of other xenobiotics was compared between healthy presence of alkaline serum values in rats (13). As a third subjects and dialysis patients, the values observed in dial- mechanism for the efficacy of STS, an anti-oxidative effect ysis patients were either higher, lower, or identical de- improving endothelial dysfunction and promoting vasodi- pending on the agent and the metabolic pathways ana- latation has been discussed (42). The biologically relevant 1454 Clinical Journal of the American Society of Nephrology

concentrations of TS for this effect are unknown at the in children and young adults. Clin J Am Soc Nephrol 1: present time. A fourth mechanism may be related to STS- 1161–1166, 2006 induced changes in serum inhibitors of vascular calcifica- 12. Mataic D, Bastani B: Intraperitoneal sodium thiosulfate for the treatment of calciphylaxis. Ren Fail 28: 361–363, 2006 tion (43,44), as described for the matrix-Gla-protein in pre- 13. Pasch A, Schaffner T, Huynh-Do U, Frey BM, Frey FJ, Farese vious studies with STS-treated rats (13). However, the S: Sodium thiosulfate prevents vascular calcifications in ure- concentrations required for this effect are unknown, too. mic rats. Kidney Int 74: 1444–1453, 2008 Thus, for future rational dosing of STS, concentration- 14. Adirekkiat S, Sumethkul V, Ingsathit A, Domrongkitchaiporn S, Phakdeekitcharoen B, Kantachuvesiri S, Kitiyakara C, Kly- effect studies are mandatory. prayong P, Disthabanchong S: Sodium thiosulfate delays the In conclusion, the present investigation established a progression of coronary artery calcification in haemodialysis kinetic model for predicting TS concentrations after intra- patients. Nephrol Dial Transplant 25: 1923–1929, 2010 venous dosing of STS. Oral application cannot be recom- 15. Schlieper G, Brandenburg V, Ketteler M, Floege J: Sodium mended at the present time, given the low and variable thiosulfate in the treatment of calcific uremic arteriolopathy. Nat Rev Nephrol 5: 539–543, 2009 bioavailability. The mechanisms of this low bioavailability 16. Mendoza FJ, Lopez I, Montes de Oca A, Perez J, Rodriguez have to be clarified for the development of strategies to M, Aguilera-Tejero E: Metabolic acidosis inhibits soft tissue improve the bioavailability of oral STS. 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