Net Tubular Secretion Research Articles

Conventional Pig as Animal Model for Human Renal Drug Excretion Processes: Unravelling the Porcine Renal Function by Use of a Cocktail of Exogenous Markers.

Over recent years, pigs have been promoted as potential animal model due to their anatomical and physiological similarities with humans. However, information about the contribution of distinct renal elimination processes [glomerular filtration rate (GFR), effective renal plasma flow (ERPF), tubular secretion, and reabsorption] in pigs is currently limited. Therefore, a cocktail of renal markers, consisting of iohexol (GFR), para-aminohippuric acid (ERPF and net tubular anion secretion), pindolol (net tubular cation secretion), and fluconazole (net tubular reabsorption) was administered intravenously to 7-week-old male conventional pigs. Plasma and urinary concentrations were determined using validated analytical methods. The clearance of iohexol (GFR) was 97.87 ± 16.05 ml/min/m² (mean ± SD). The ERPF, calculated as the renal clearance of PAH, was 226.77 ± 62.45 ml/min/m², whereas the net tubular secretion of PAH was 130.28 ± 52.62 ml/min/m². The net tubular secretion of R-pindolol and S-pindolol was 13.53 ± 12.97 and 18.01 ± 39.23 ml/min/m², respectively. The net tubular reabsorption of fluconazole was 78.32 ± 13.52 ml/min/m². Overall, this cocktail of renal markers was considered to be safe for use in pigs since no adverse effects were observed. Iohexol, PAH and fluconazole were considered suitable renal marker to assess the porcine renal function. Pindolol seems less appropriate due to the high degree of nonrenal clearance in pigs. The values of GFR, ERPF, and anion secretion are within the same range for both human and pig. Regarding the tubular reabsorption of fluconazole, slightly higher values were obtained for pigs. Nevertheless, these results indicate the conventional pig could be an appropriate animal model to study renal drug elimination processes in humans.

Homogentisic acid is not only eliminated by glomerular filtration and tubular secretion but also produced in the kidney in alkaptonuria.

The clinical effects of alkaptonuria (AKU) are delayed and ageing influences disease progression. Morbidity of AKU is secondary to high circulating homogentisic acid (HGA) and ochronosis. It is not known whether HGA is produced by or processed in the kidney in AKU. Data from AKU patients from four studies were merged to form a single AKU group. A control group of non-AKU subjects was generated by merging data from two non-AKU studies. Data were used to derive renal clearance and fractional excretion (FE) ratios for creatinine, HGA, phenylalanine (PHE) and tyrosine (TYR) using standard calculations, for comparison between the AKU and the control groups. There were 225 AKU patients in the AKU group and 52 in the non-AKU control group. Circulating HGA increased with age (P < 0.001), and was significantly associated with decreased HGA clearance (CLHGA ) (P < 0.001) and FEHGA (P < 0.001). CLHGA and FEHGA were increased beyond the theoretical maximum renal plasma flow, confirming renal production and emphasising the greater contribution of net tubular secretion than glomerular filtration to renal elimination of HGA. The kidneys are crucial to elimination of HGA. Elimination of HGA is impaired with age resulting in worsening disease over time. The kidney is an important site for production of HGA. Tubular secretion of HGA contributes more to elimination of HGA in AKU than glomerular filtration.

The Effect of Uremic Solutes on the Organic Cation Transporter 2

Chronic kidney disease (CKD) is characterized by the accumulation of uremic solutes; however, little is known about how these solutes affect drug absorption and disposition. The goal of this study is to evaluate the effect of uremic solutes on the organic cation transporter, OCT2, which plays a key role in the renal secretion of many basic drugs. As a second goal, we reviewed the literature to determine whether there was evidence for the effect of CKD on the renal secretion of basic drugs. We first screened 72 uremic solutes as inhibitors of [14C]-labeled metformin uptake by OCT2. Seven were identified as inhibitors and 3 of them were determined to be clinically relevant. Of the 7 solutes, dimethylamine, malondialdehyde, trimethylamine, homocysteine, indoxyl-β-d-glucuronide, and glutathione disulfide were novel OCT2 inhibitors. For 6 drugs that are known OCT2 substrates, both secretory clearance and glomerular filtration rate declined in parallel with progression of CKD from stage 2 to 4, suggesting that selective effects of uremic solutes on net tubular secretion of organic cations do not occur. Further clinical studies are warranted with a broader range of OCT2 substrates to determine whether CKD may differentially affect tubular secretion of drugs especially in patients with advanced CKD.

Determining the mechanisms underlying augmented renal drug clearance in the critically ill: use of exogenous marker compounds.

IntroductionThe aim of this study was to explore changes in glomerular filtration (GFR) and renal tubular function in critically ill patients at risk of augmented renal clearance (ARC), using exogenous marker compounds.MethodsThis prospective, observational pharmacokinetic (PK) study was performed in a university-affiliated, tertiary-level, adult intensive care unit (ICU). Patients aged less than or equal to 60 years, manifesting a systemic inflammatory response, with an expected ICU length of stay more than 24 hours, no evidence of acute renal impairment (plasma creatinine concentration <120 μmol/L) and no history of chronic kidney disease or renal replacement therapy were eligible for inclusion. The following study markers were administered concurrently: sinistrin 2,500 mg (Inutest; Laevosan, Linz, Austria), p-aminohippuric acid (PAH) 440 mg (4% p-aminohippuric acid sodium salt; CFM Oskar Tropitzsch, Marktredwitz, Germany), rac-pindolol 5 or 15 mg (Barbloc; Alphapharm, Millers Point, NSW, Australia) and fluconazole 100 mg (Diflucan; Pfizer Australia Pty Ltd, West Ryde, NSW, Australia). Plasma concentrations were then measured at 5, 10, 15, 30, 60 and 120 minutes and 4, 6, 12 and 24 hours post-administration. Non-compartmental PK analysis was used to quantify GFR, tubular secretion and tubular reabsorption.ResultsTwenty patients were included in the study. Marker administration was well tolerated, with no adverse events reported. Sinistrin clearance as a marker of GFR was significantly elevated (mean, 180 (95% confidence interval (CI), 141 to 219) ml/min) and correlated well with creatinine clearance (r =0.70, P <0.01). Net tubular secretion of PAH, a marker of tubular anion secretion, was also elevated (mean, 428 (95% CI, 306 to 550) ml/min), as was net tubular reabsorption of fluconazole (mean, 135 (95% CI, 100 to 169) ml/min). Net tubular secretion of (S)- and (R)-pinodolol, a marker of tubular cation secretion, was impaired.ConclusionsIn critically ill patients at risk of ARC, significant alterations in glomerular filtration, renal tubular secretion and tubular reabsorption are apparent. This has implications for accurate dosing of renally eliminated drugs.

SLC22A2is associated with tubular creatinine secretion and bias of estimated GFR in renal transplantation

Genome-wide association studies reported SLC22A2 variants to be associated with serum creatinine. As SLC22A2 encodes the organic cation transporter 2 (OCT2), the association might be due to an effect on tubular creatinine handling. To test this hypothesis we studied the association of SLC22A2 polymorphisms with phenotypes of net tubular creatinine secretion: fractional creatinine excretion (FEcreat) and bias of estimated glomerular filtration rate (eGFR). We also studied the association with end-stage renal disease (ESRD) and graft failure (GF) in renal transplant recipients. SLC22A2 single nucleotide polymorphisms (SNPs), rs3127573 and rs316009, were genotyped in 1,142 ESRD patients receiving renal transplantation and 1,186 kidney donors as controls. GFR was measured with (125)I-iothalamate clearance. Creatinine clearance was also assessed. FEcreat was calculated from the simultaneous clearances of creatinine and (125)I-iothalamate. Donor rs316009 was associated with FEcreat (beta -0.053, P = 0.024) and with estimated [modification of diet in renal disease (MDRD) and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI)] but not measured GFR. In line with this, donor rs316009 was associated with bias of the MDRD and CKD-EPI but not the Cockroft-Gault equation. Both SNPs were associated with ESRD: odds ratios [95% CI] 1.39 [1.16-1.67], P = 0.00065, and 1.23 [1.02-1.48], P = 0.042, for rs3127573 and rs316009, respectively. Neither SNP was associated with GF. Thus, SLC22A2 is associated with phenotypes of net tubular creatinine secretion and ESRD.

A Novel Double-Tracer Technique to Characterize Absorption, Distribution, Metabolism and Excretion (ADME) of [14C]Tofogliflozin After Oral Administration and Concomitant Intravenous Microdose Administration of [13C]Tofogliflozin in Humans

Human mass balance studies and the assessment of absolute oral bioavailability (F) are usually assessed in separate studies. Intravenous microdose administration of an isotope tracer concomitant to an unlabeled oral dose is an emerging technique to assess F. We report a novel double-tracer approach implemented for tofogliflozin combining oral administration of a radiolabel tracer with concomitant intravenous administration of a stable isotope tracer. Tofogliflozin is a potent and selective sodium/glucose cotransporter 2 inhibitor for the treatment of type 2 diabetes mellitus currently in clinical development. The objectives of the present study were to assess the systemic exposure of major circulating metabolites, excretion balance, F and contribution of renal clearance (CLR) to total clearance (CL) of tofogliflozin in healthy subjects within one study applying a novel double-tracer technique. Six healthy male subjects received 20 mg [(12)C/(14)C]tofogliflozin (3.73 MBq) orally and a concomitant microdose of 0.1 mg [(13)C]tofogliflozin intravenously. Pharmacokinetics of tofogliflozin were determined for the oral and intravenous route; the pharmacokinetics of the metabolites M1 and M5 were determined for the oral route. Quantification of [(12)C]tofogliflozin in plasma and urine and [(13)C]tofogliflozin in plasma was performed by selective LC-MS/MS methods. For the pre-selected metabolites of tofogliflozin, M1 and M5, a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) was applied to plasma and urine samples. Total radioactivity was assessed in plasma, urine and feces. Pharmacokinetic analysis was conducted by non-compartmental methods. The pharmacokinetics of tofogliflozin in healthy subjects were characterized by an F of 97.5 ± 12.3 %, CL of 10.0 ± 1.3 l/h and volume of distribution at steady-state (V(ss)) of 50.6 ± 6.7 l. The main route of elimination of total drug-related material was by excretion into urine (77.0 ± 4.1 % of the dose). The observed CL(R) of 25.7 ± 5.0 ml/min was higher than the product of the estimated glomerular filtration rate (eGFR) and fraction unbound in plasma (f(u)) (eGFR × f(u) 15 ml/min), indicating the presence of net active tubular secretion in the renal elimination of tofogliflozin. However, CLR contributed only 15.5 % to the CL of tofogliflozin, suggesting that reductions in CLR by renal impairment won't significantly affect systemic exposure to tofogliflozin. Tofogliflozin and its metabolite M1 were the only major circulating entities accounting for 46 ± 8.6 and 50 ± 8.2 %, respectively, of total circulating drug-related material, while the metabolite M5 was a minor circulating metabolite accounting for 3.0 ± 0.3 % of total circulating drug-related material. Both the M1 and M5 metabolites were excreted into urine and the major metabolite M1 did not exhibit active tubular secretion. These results demonstrate the utility of the double-tracer approach to provide essential pharmacokinetic data and excretion data for drug-related material in one study at the same dosing occasion. The data obtained allowed the characterization of absorption, distribution, metabolism and excretion of tofogliflozin. Tofogliflozin exhibited highly favorable pharmacokinetic properties as demonstrated by its high F, low CL and a low V(ss. The presence of only one major circulating metabolite of tofogliflozin was unambiguously demonstrated. As a drug targeting the kidney, luminal exposure of the kidney is achieved by renal filtration and active tubular secretion.

Renal excretion of apricitabine in rats: ex vivo and in vivo studies

Apricitabine (ATC) is a novel nucleoside reverse transcriptase inhibitor undergoing phase 2/3 clinical development for the treatment of HIV infection. In this investigation, the renal handling of ATC was evaluated in the isolated perfused rat kidney (IPK) model with follow-up in vivo studies. IPK experiments were performed to characterize the renal excretion of ATC, to probe mechanisms of ATC excretion using known inhibitors of organic cation (cimetidine) and organic anion (probenecid) transport systems, and to screen for potential drug-drug interactions between ATC and clinically relevant medications (dapsone, metformin, pentamidine, stavudine, tenofovir and ritonavir). ATC demonstrated net tubular secretion in the IPK with a baseline excretion ratio (XR) of 2.1 ± 0.56. ATC XR decreased 3.6-fold in the presence of cimetidine and 2-fold in the presence of probenecid. Among the clinically relevant medications, metformin produced the greatest inhibitory effect on ATC excretion. In vivo studies were conducted in rats to evaluate ATC disposition upon co-administration with compounds that showed a significant effect on ATC clearance in the IPK model. Co-administration of cimetidine and trimethoprim significantly reduced ATC renal clearance, but resulted in only a moderate increase in plasma exposure. Metformin had no apparent effect on ATC clearance in rats. These findings indicate that the IPK model is more sensitive to secretory inhibition as compared to in vivo. The medications screened showed minimal effects on ATC renal excretion in the IPK, and should thus be excluded as potential in vivo interactants. Overall, this study generated important information on renal handling of ATC to support its development and commercialization.

Abstract B211: Pralatrexate pharmacokinetics with intravenous administration to oncology patients

Abstract Pralatrexate is a rationally designed antifolate with high affinity for reduced folate carrier-1 (RFC-1) and folyl poly-glutamyl synthetase (FPGS), resulting in efficient uptake and retention in cancer cells. Pralatrexate is being developed for various indications, including peripheral T-cell lymphoma (PTCL), cutaneous T-cell lymphoma, non-small cell lung cancer (NSCLC), and bladder cancer. Pralatrexate is a 1:1 racemic mixture of R- and S- diastereomers. Here we summarize the pharmacokinetics (PK) of pralatrexate from 3 Phase 1–2 studies. Pralatrexate was administered weekly or bi-weekly by IV push over 3–5 minutes (n=44) or as a 60-minute infusion (n=10) at doses ranging from 30–325 mg/m2. Serial plasma and urine samples, collected for up to 72 hours after dosing, were analyzed using a validated chiral LC-MS/MS method (lower limit of quantitation: 0.5 ng/ml). PK parameters were estimated by noncompartmental PK analysis. Clinical covariates were captured at screening to explore relationships with pralatrexate PK. The final PK results were based on 54 patients; 10 with relapsed/refractory PTCL (PDX-008), 38 with advanced NSCLC (PDX-007), and 6 with advanced solid tumors and concurrent docetaxel administration (PDX-99-083). The PK population was predominantlyWhite (81%) and balanced by gender (52% female). Age ranged from 24 to 77 (mean 60) years, body weight from 43–127 (77) kg, and estimated creatinine clearance from 53 to 130 (89) mL/min. Patients had adequate hepatic and hematologic function. Both pralatrexate diastereomers showed a similar multiphasic decline in plasma concentrations with a slow terminal phase (t½term = 12∓18h, PDX-008), likely reflecting the return from deep intracellular compartments. There was no evidence of PK nonlinearities in the dose range of 30–325 mg/m2. On a weekly or biweekly administration schedule, there was no significant drug accumulation in plasma over the first 2–6 doses. Other than expected changes in Cmax and Tmax, there were no apparent differences in PK between IV administration over 3–5 minutes and 60-minute infusion. Mean (COV) values (n=41−54) for total body clearance, renal clearance and volume of distribution at steady-state were 495 mL/min (57%), 125 mL/min (71%) and 106 L (121%) for the S-diastereomer and 218 mL/min (44%), 74 mL/min (60%) and 36 L (78%) for the R-diastereomer, respectively. These results suggest net renal tubular secretion, low hepatic extraction, and limited tissue distribution for both diastereomers. Consistent with approximately 30% contribution of renal excretion to overall drug elimination, creatinine clearance and age accounted for only 10% of the observed variability in total body clearance. In conclusion, this analysis suggests that pralatrexate PK is linear and dose-proportional across the dose range tested, with no accumulation with repeated dosing. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B211.

Effects of Trimethoprim on the Clearance of Apricitabine, a Deoxycytidine Analog Reverse Transcriptase Inhibitor, and Lamivudine in the Isolated Perfused Rat Kidney

Apricitabine (ATC) is a novel deoxycytidine analog reverse transcriptase inhibitor in development for the treatment of human immunodeficiency virus infection. Studies were performed to characterize the excretion of ATC and its metabolite, BCH-335 (-1-(2-hydroxymethyl-[1,3]oxathiolan-4-yl)-1H-pyrimidine-2,4-dione), in the isolated perfused rat kidney (IPK). A second objective was to investigate the effect of trimethoprim on ATC excretion because trimethoprim inhibits the excretion of lamivudine, structurally similar to ATC, in the IPK. ATC excretion was nonlinear at doses of 80 to 1600 microg. The excretion ratio (ratio of clearance to glomerular filtration rate, assuming negligible protein binding) was greater than 1.0, indicating net tubular secretion. In contrast, the excretion of BCH-335 was independent of the dose of BCH-335. Concomitant administration of ATC and BCH-335 did not affect the excretion of either compound. Trimethoprim significantly inhibited the excretion of both ATC and BCH-335, with IC(50) values of 0.45 and 0.54 microg/ml, respectively. In the presence of trimethoprim, the excretion ratios for both compounds were less than 1.0, indicating tubular reabsorption. Trimethoprim inhibited the excretion of ATC and lamivudine to similar extents. Following concomitant administration of ATC, lamivudine, and trimethoprim, there was no evidence of an interaction between ATC and lamivudine. These results suggest that ATC undergoes active tubular secretion in the kidney. Because the renal excretion of both ATC and lamivudine is inhibited by trimethoprim to similar extents, in clinical practice exposure to ATC, it would be expected to be increased in the presence of therapeutic concentrations of trimethoprim to a similar extent as has been shown previously for lamivudine.

Role of tubular secretion and carbonic anhydrase in vertebrate renal sulfate excretion.

The renal proximal tubule of vertebrates performs an essential role in controlling plasma SO(4)(2-) concentration ([SO(4)(2-)]). Although net tubular SO(4)(2-) reabsorption is the predominate control process in terrestrial vertebrates, a facilitated secretory flux is also present. In contrast, marine teleosts obtain excess SO(4)(2-) from drinking, and increased plasma [SO(4)(2-)] is prevented predominately through net tubular secretion. Tubular SO(4)(2-) secretion is accomplished by at least two electroneutral anion exchange processes in series. Movement of SO(4)(2-) into the cell across the basolateral membrane is pH dependent, suggesting SO(4)(2-)/OH(-) exchange. Luminal HCO(3)(-) and Cl(-) can facilitate SO(4)(2-) movement out of the cell across the brush-border membrane. The molecular identities of the anion exchangers are unknown but are probably homologues of SO(4)(2-) transporters in the mammalian SLC26 gene family. In all species tested, glucocorticoids increase renal SO(4)(2-) excretion. Whereas glucocorticoids downregulate SO(4)(2-) reabsorptive mechanisms in terrestrial vertebrates, they may also stimulate a mediated secretory flux. In the marine teleost, cortisol increases the level of SO(4)(2-)/HCO(3)(-) exchange at the brush-border membrane, tubular carbonic anhydrase (CA) activity, CAII protein, and a proportion of tubular SO(4)(2-) secretion that is CA dependent. CA activity is required for about one-half of this net SO(4)(2-) secretion but is also required for about one-half of the net reabsorption in bird proximal epithelium. A CA-SO(4)(2-)/anion exchanger metabolon arrangement is proposed that may speed both the secretory and reabsorptive processes.

Effect of organic cations on the renal tubular secretion of pseudoephedrine in the rat.

1. Pseudoephedrine is a weak organic base that undergoes renal tubular secretion. The aim of the present study was to assess whether two other commonly used weak organic bases (cimetidine and morphine) inhibit the renal tubular secretion of pseudoephedrine in the rat isolated perfused kidney. 2. A total of 12 perfusions were performed with four perfusions in each of three treatment groups. In the control group, pseudoephedrine was administered as a bolus dose of [14C]-pseudoephedrine and unlabelled pseudoephedrine to achieve an initial perfusate concentration of 0.4 microg/mL. For the treatment groups, pseudoephedrine was administered as above and cimetidine or morphine was added to the perfusion medium in increasing concentrations of 0.5-12.5 and 0.2-5.0 microg/mL, respectively. 3. The mean (+/-SD) fraction unbound of pseudoephedrine alone in perfusate was 0.866+/-0.014 and was not different (P> 0.05) in the presence of cimetidine or morphine. 4. In control experiments, the renal excretory clearance (CLR) of pseudoephedrine was three-fold greater than glomerular filtration rate (GFR), yielding a ratio consistently greater than unity, which indicates extensive net tubular secretion of pseudoephedrine. The CLR and total clearance of pseudoephedrine were similar, suggesting an absence of renal metabolism of pseudoephedrine. 5. The CLR/GFR ratio for pseudoephedrine was not affected by morphine, but was significantly reduced (P < 0.05) in the presence of cimetidine. 6. The results indicate that cimetidine inhibits the renal tubular secretion of pseudoephedrine.

Single-Dose Pharmacokinetics of Indinavir and the Effect of Food

Indinavir sulfate is a human immunodeficiency virus type 1 (HIV-1) protease inhibitor indicated for treatment of HIV infection and AIDS in adults. The purpose of this report is to summarize single-dose studies which characterized the pharmacokinetics of the drug and the effect of food in healthy volunteers. Indinavir concentrations in plasma and urine were obtained by high-pressure liquid chromatography and UV detection assay methods. The results indicate that indinavir was rapidly absorbed in the fasting state, with the time to the maximum concentration in plasma occurring at approximately 0.8 h for all doses studied. Over the 40- to 1,000-mg dose range studied, concentrations in plasma and urinary excretion of unchanged drug increased greater than dose proportionally. The nonlinear pharmacokinetics were attributed to the dose-dependent oxidative metabolism of first-pass metabolism as well as to metabolism in the systemic circulation. Renal clearance slightly exceeded the glomerular filtration rate, suggesting a net tubular secretion component. At high concentrations in plasma, tubular secretion appeared to be lowered because there was a trend for a decreased renal clearance. Administration of 400 mg of indinavir sulfate following a high-fat breakfast resulted in a blunted and decreased absorption (areas under the concentration-time curves [AUCs], 6.86 microM.h in the fasted state versus 1.54 microM.h in the fed state; n = 10). However, two types of low-fat meals were found to have no significant effect on the absorption of 800 mg of indinavir sulfate (AUCs, 23.15 microM.h in the fasted state versus 22.71 and 21.36 microM.h, respectively, in the fed state; n = 11). Immediately following dosing, the concentrations of indinavir in urine often exceeded its intrinsic solubility. To reduce the risk of nephrolithiasis, it is recommended that indinavir sulfate be administered with water.

Effect of cationic drugs on the renal secretion of ranitidine in the rat isolated perfused kidney.

1. The rat isolated perfused kidney (IPK) was used to determine whether the renal tubular secretion of ranitidine is influenced by clinically relevant concentrations of other organic cationic drugs (amantadine, pseudoephedrine, triamterene and trimethoprim) that also undergo tubular secretion. 2. Ranitidine and [3H]-ranitidine were administered to the recirculating perfusion medium as a loading dose followed by a constant infusion to maintain clinically relevant perfusate ranitidine concentrations in the range 400-700 ng/mL. The renal clearance of ranitidine (CL[R]) was calculated, as was glomerular filtration rate (GFR), from the renal clearance of [14C]-inulin. 3. A total of 20 perfusions were performed and, in each case, ranitidine was administered for 80 min. In four control IPK, no drug other than ranitidine was administered. In the remaining IPK, amantadine, pseudoephedrine, triamterene or trimethoprim (n = 4 in each case) were administered to achieve low, medium and high concentrations during the 20-40, 40-60 and 60-80 min periods, respectively. 4. The mean (+/- SD) unbound fraction of ranitidine in the perfusion medium was 0.889 +/- 0.046 and was not altered (P>0.05) by the presence of the other drugs. 5. The CL(R)/GFR ratio for ranitidine in all kidneys was substantially greater than unity and had a mean value of 10.65 or greater in control kidneys, indicating extensive net tubular secretion. 6. The CL(R)/GFR was not affected (P>0.05) by amantadine, pseudoephedrine or triamterene at any concentration or by trimethoprim at the low concentration. However, medium (2000 ng/mL) and high (5000 ng/mL) concentrations of trimethoprim caused significant reductions in CL(R)/GFR of 20 and 28%, respectively (P<0.05). 7. The results indicate that at clinically relevant concentrations the renal tubular secretion of ranitidine is inhibited by trimethoprim, but not by amantadine, pseudoephedrine or triamterene.

Fractional reabsorption of calcium, magnesium and phosphate in the kidneys of freshwater North American eels (Anguilla rostrata LeSueur) following removal of the corpuscles of Stannius.

Renal function was observed in freshwater North American eels (Anguilla rostrata LeSueur) 2 weeks after the removal of the corpuscles of Stannius. There was a positive linear correlation between glomerular filtration rates and urine flow rates in both sham-operated and stanniectomized eels but there was no difference in slope or elevation between the two groups nor did urine flow rates ever exceed glomerular filtration rates. Osmolar clearance and free-water clearance were unchanged following stanniectomy. Plasma Ca2+ and K+ concentrations increased and plasma Mg2+, phosphate, Na+ and Cl- concentrations decreased following stanniectomy. Plasma ultrafilterable Ca increased and ultrafilterable Mg decreased after stanniectomy but neither changed relative to its total plasma concentration. Stanniectomy was followed by a decreased renal tubular reabsorption of Mg2+ relative to the amount filtered (CMg/CIn); the same applies to CNa/CIn. Even though the filtered load of Ca increased in conjunction with the predictable hypercalcemia, there was no change in the fraction of filtered Ca reabsorbed. Net tubular secretion of phosphate was observed in both sham-operated and stanniectomized eels together with a slight increase in Cphos/CIn following stanniectomy. Some or all of these changes in plasma electrolytes and/or the modified renal transport of Na+, Mg2+ and possibly phosphate may be caused by the changes in cardiovascular function that were recently shown to follow stanniectomy.

Toxicity and excretion of cisplatin in the avian kidney

Cisplatin is a widely applied antineoplastic drug with significant nephrotoxic potential. The purpose of this study was to define the toxic effect and excretion of cisplatin in the chicken, a species widely applied to the study of tubular transport mechanisms but little used for toxicology studies. Toxicity was assessed by the relative effect of cisplatin on renal clearances of the standard reference substrates inulin and p-aminohippurate (PAH) and by morphologic assessment of the kidneys of cisplatin-treated chickens. The data clearly support close similarities in the pattern of tubular cell damage produced in the chicken versus that reported for rats and human patients. It was further demonstrated that administration of an organic cation reduced Pt accumulation in the kidney and mitigated the toxicity as has been reported for rats. Excretion studies were carried out during unilateral renal portal infusion of cisplatin, and the results indicated that cisplatin does not undergo net tubular secretion as occurs in the rat and human. It can be concluded that, while the pattern of cisplatin-induced nephrotoxicity closely parallels that seen in mammals, the avian kidney exhibits a different pattern of urinary Pt excretion than does the mammalian kidney after cisplatin administration.

Developmental changes in renal tubular function

Use of the standard measure of renal function, glomerular filtration rate when corrected for body size, indicates fully functional renal capacity by one year of age, which remains relatively constant until the fourth decade of life, when it begins to gradually decline with advancing age. The active tubular secretion and reabsorption of cations and anions provide an occasion for drug interactions which are difficult to predict without knowledge of the exact mechanisms involved. Data support developmental changes in net tubular secretion for some substances. For digoxin, the larger ratio of digoxin clearance to creatinine clearance that is observed in children decreases during adolescence to the lower ratio observed in adults, and this decrement is better correlated with sexual maturation than with chronologic age. Thus for drugs with significant renal excretion of active drug or metabolite, the clarification of net renal tubular mechanisms would provide important clinical information.

Alterations in pharmacokinetics and protein binding behavior of cefazolin in endotoxemic rats

The possible alterations in the pharmacokinetics and protein binding behavior of the beta-lactam antibiotic cefazolin (CEZ) were investigated in endotoxemic rats induced by Klebsiella pneumoniae O3 lipopolysaccharide (LPS). LPS (250 micrograms/kg of body weight) was infused for 20 to 30 min 2 h before an intravenous administration of CEZ (20 mg/kg). Significant decreases in systemic clearance and renal clearance of CEZ were observed in LPS-treated rats without any changes in fraction of urinary excretion in unchanged CEZ (> 0.8). The volume of distribution at steady state showed a tendency to increase. The protein binding parameters of CEZ, the binding capacity, and number of binding sites on the albumin molecule were decreased by LPS, whereas the dissociation constant did not change. Significant decreases in systemic and renal clearances for unbound CEZ were observed in LPS-treated rats. The glomerular filtration rate estimated as inulin clearance was also decreased by LPS. The ratio of renal clearance of unbound CEZ to glomerular filtration rate (clearance ratio) dropped to 70% of that in control rats, and the net tubular secretion of CEZ was also dramatically reduced. The present study suggests that LPS has an effect on the pharmacokinetics of CEZ by changes which occur in renal handling and protein binding.

The relation between type of renal disease and renal drug clearance in children.

It is generally assumed that the renal clearance of drugs in patients with renal impairment are affected to a similar extent regardless of the type of renal disease (intact nephron hypothesis). We have studied the effect of underlying renal disease on the pharmacokinetics of cefotaxime and desacetylcefotaxime in two groups of children (ages 7 to 16 y) with varying degrees of renal dysfunction. Patients in group 1 (n = 5) had intrinsic renal disease and those in group 2 (n = 5) had extrinsic renal disease, as identified by the primary renal lesion. After a single intravenous dose of cefotaxime timed blood and urine samples were collected for 24 h; cefotaxime and desacetylcefotaxime were measured by HPLC. There were no significant differences between the groups in age, body surface area, urine output, creatinine clearance, total body clearance, nonrenal clearance, renal clearance, and volume of distribution at steady state of cefotaxime, and renal clearance of desacetylcefotaxime. However, the renal clearance: creatinine clearance (CLR:CLCR) ratios for both cefotaxime [1.34 in group 1 vs. 0.51 in group 2] and desacetylcefotaxime [1.58 in group 1 vs. 0.75 in group 2] were statistically significant between the two groups. Group 1 patients had an average CLR:CLCR ratio greater than 1 for both the parent compound and the metabolite, suggesting that net tubular secretion was still intact, despite a diminished glomerular filtration rate (CLCR = 24 ml.min-1.73 m-2).(ABSTRACT TRUNCATED AT 250 WORDS)

Renal clearance of pyridostigmine in myasthenic patients and volunteers under the influence of ranitidine and pirenzepine.

1. To assess the contribution of tubular secretion to the renal excretion of pyridostigmine, and its modification by other cationic drugs, six volunteers were given single oral doses of 60-mg pyridostigmine bromide with and without co-administration of ranitidine or pirenzepine. Renal clearances were determined by h.p.l.c. analysis of pyridostigmine and enzymic measurement of endogenous creatinine in plasma and urine. 2. In patients with myasthenia receiving continuous pyridostigmine therapy, renal clearance values were obtained in the same manner with and without ranitidine (10 patients) or pirenzepine (nine patients) co-medication. 3. Pyridostigmine was not bound to plasma proteins. Its renal clearance averaged 6.65 ml/min per kg (350% of the creatinine clearance) in all subjects, 74% being due to net tubular secretion. 4. Mean values for pyridostigmine renal clearance and for clearance by secretion were decreased in the presence of pirenzepine, but plasma concentrations were not affected significantly. Ranitidine caused a small non-significant decrease of the pyridostigmine clearance in patients. 5. Pyridostigmine had a higher elimination (2 h-1) than the absorption rate constant (0.23 h-1) when administered orally as a non-retarded preparation. 6. The renal clearance of creatinine was slightly increased by pyridostigmine in volunteers and slightly decreased by pirenzepine in the total group of subjects.

Enalaprilat handling by the kidney: barrier-limited cell entry.

The angiotensin-converting enzyme inhibitor enalaprilat is formed in vivo in liver and kidney by esterolysis of the antihypertensive drug enalapril. To gain insight into the renal elimination of enalaprilat, we carried out multiple-indicator dilution experiments in the isolated perfused rat kidney. Kidneys were perfused single pass with an amino acid-supplemented Krebs-Henseleit buffer containing 20% bovine red blood cells and 4% bovine serum albumin, at a flow rate of 0.11 +/- 0.02 (SD) ml.s-1 x g-1. A bolus of 51Cr-labeled red blood cells (vascular red blood cell indicator), 125I-labeled albumin (vascular plasma indicator), L-[14C]glucose (interstitial space indicator), and [3H]-enalaprilat was injected into the renal artery, and timed samples of venous blood (up to 1 min) and urine (up to 10 min) were collected. The data were analyzed using a variable-transit-time, space-distributed model with modifications accounting for glomerular filtration and the observed 14% protein binding of enalaprilat; the glomerular filtration rate (GFR) estimated from L-glucose clearance was 9.0 +/- 2.9% of total plasma flow. The ratio of renal clearance of unbound enalaprilat to GFR was 1.56 +/- 0.29, indicating both glomerular filtration and net tubular secretion of enalaprilat. Unidirectional influx from plasma to tubular cells exceeded tubular secretion by a factor of 2.2 +/- 0.5. Thus only about one-half of the enalaprilat taken up by the tubular cells was excreted into urine, with the remainder refluxing into the capillary blood stream, indicating bidirectional permeation of enalaprilat across the basolateral tubular membrane.