Presence Of Probenecid Research Articles

Probenecid Affects the Pharmacokinetics of Ofloxacin in Healthy Volunteers

The aim of this randomised, double-blind, crossover study was to evaluate the effect of single-dose probenecid on the pharmacokinetics of ofloxacin in eight healthy male volunteers. After an overnight fast, and according to a randomised sequence, each volunteer received either single oral ofloxacin 200mg (Hoechst Marion Roussel Ltd., Mumbai, India) or both ofloxacin (1 x 200mg) and probenecid (1 x 500mg) [Geno Pharmaceutical Ltd., Mumbai, India]. Blood samples were collected at regular intervals until 24 hours. Serum concentration versus time profiles for ofloxacin were generated and pharmacokinetic parameters were calculated by noncompartmental model analysis. Elimination half-life, mean residence time and area under the curve were significantly increased (4.86 vs 5.26h; 7.23 vs 7.95h; 10.28 vs 11.9 mg/L . h) [p < 0.01], whereas the total clearance was decreased (19.66 vs 16.95 L/h) [p < 0.01] in the presence of probenecid. Other pharmacokinetic parameters were not significantly affected by coadministration of probenecid. Concomitant administration of probenecid with ofloxacin may result in a decreased elimination half-life and consequently increased bioavailability of ofloxacin. Probenecid may be co-prescribed with ofloxacin; patients taking this combination should be closely monitored and dosage reduction should be considered if warranted in high-risk patients.

Renal transepithelial secretion of ochratoxin A in the non-filtering toad kidney

Renal excretion is an important way of elimination for the nephrotoxin ochratoxin A (OTA). Because binding to proteins hinders filtration, excretion is mainly due to proximal tubular secretion. The goal of this study was to investigate the kinetics of secretion as well as the extent of urine and tissue accumulation in situ using the non-filtering amphibian kidney model. Transepithelial secretion is a saturable process ( K m = 0.63 · 10 −6mol/l) and leads to a concentration-dependent accumulation of OTA in the tubular lumen and in renal tissue. Maximum accumulation achieved is ∼ 14-fold as compared to the perfusate concentration. There was no accumulation in the tubular lumen as compared to renal tissue (lumen-to-tissue concentration ratio ∼ 1). Tissue and tubular lumen accumulation were reduced to ∼ 40% of control in the presence of 10 −3 mol/l p-aminohippurate (PAH). Addition of 10 −3 mol/l α-ketoglutarate (KG) to PAH-containing perfusate did not lead to a further reduction of secretion. By contrast, addition of 10 −2 mol/l l-phenylalanine ( l-Phe) reduced secretion further to ∼25% of control. In the presence of 10 −3 mol/l probenecid tissue accumulation was reduced to 7% and tubular lumen accumulation to 1% of control. Lumen-to-tissue concentration ratio decreased to 0.15 in the presence of probenecid, indicating an inhibitory action at the luminal membrane. Addition of albumin to the perfusate, reduced secretion to only 50% of control, whereas the concentration of free OTA was reduced below 1% as compared to control. The results of this study show that transepithelial secretion is an effective way for accumulation of OTA in the tubular lumen and thus its urinary excretion. Transport via the basolateral organic anion and a basolateral amino acid carrier are the active steps in transepithelial secretion. Luminal exit of OTA is a passive process. Furthermore, tissue accumulation by the active transport across the basolateral membrane supports the toxic action of OTA on proximal tubular cells. Due to the qualitative similarity of organic anion transport our findings should also apply for the mammalian kidney.

Evidence for the Involvement of an Anion Exchanger in Cortisol Release from Bovine Adrenocortical Cells

In this study the influence of probenecid as inhibitor of the renal excretory system for organic anions was tested on the release of cortisol from primary cultures of bovine adrenocortical cells. ACTH-stimulated cortisol release (30 min to 24 h) was inhibited by probenecid (Ki = 0.093 mM). Analysis of steroid synthesis with ³H-pregnenolone as precursor indicated that part of the inhibition resulted from an attenuation of steroid synthesis by probenecid. The increased intracellular cortisol levels in the presence of probenecid however indicate an inhibitory effect on an efflux mechanism. Cortisol efflux was trans-stimulated by <i>p</i>-aminohippurate (PAH) and tetrafluorosuccinate, substrates of the renal PAH/anion exchanger. Further evidence for the PAH exchanger provided the expression of a probenecid-sensitive PAH transporter after injection of mRNA from adrenal cells into <i>Xenopus laevis </i>oocytes.

Role of intracellular pH during cytoprotection of proximal tubule cells by glycine or acidosis.

Lowering extracellular pH to less than 7.0 strongly protects isolated proximal tubules against ATP depletion and Ca(2+)-induced injury, but there is little information about alterations of intracellular pH (pHi) in renal tubules during either injury or its modification by decreasing medium pHi or other potent protective factors such as glycine. pHi was assessed with 2',7'-bis-(2-carboxyethyl)-5-carboxyfluorescein during proximal tubule injury produced by simple ATP depletion with the electron transport inhibitor antimycin or by large increases of cytosolic free Ca2+ induced by treatment with the calcium ionophore ionomycin, alone and in combination with antimycin. Freshly isolated rabbit proximal tubules studied under superfusion conditions in the presence of probenecid were suitable for monitoring pHi during relatively prolonged and severe injury states. Probenecid, used to promote the retention of intracellular fluorophores, only minimally modified the injury response by transiently delaying lactate dehydrogenase release during antimycin treatment. The tubules did not exhibit spontaneous decreases of pHi during simple ATP depletion, but pHi fully equilibrated with cytoprotective decreases of medium pH. Irrespective of the presence of antimycin, ionomycin induced intracellular alkalinization in Ca(2+)-replete medium, which may have further enhanced the severity of injury. When medium Ca2+ was buffered to 100 nM, ionomycin induced intracellular acidification, which likely resulted from a combination of Ca2+/H+ exchange activity of the ionophore and H+ uptake during Ca(2+)-ATPase-mediated extrusion of Ca2+ released by ionomycin from intracellular pools. Alterations of pHi did not contribute to glycine cytoprotection because glycine did not affect the behavior of pHi during treatment with antimycin, ionomycin, or both agents in combination.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of carbachol on calcium entry and release in CHO cells expressing the m3 muscarinic receptor

Calcium signalling was examined in CHO-k1 cells that stably express the m3 subtype of the muscarinic receptor. The calcium indicator Fura-2 was retained in these cells only in the presence of probenecid (1 mM), suggesting that Fura-2 efflux was mediated by an organic anion transporter. The addition of carbachol (CCh) to Fura-2 loaded cells in suspension caused a rapid transient increase in intracellular calcium [Ca] i followed by a smaller sustained plateau phase. The transient rise in [Ca] i was dose-dependent with a threshold response of 89 ± 18 nM above baseline with 10 nM CCh and a maximum stimulation of 734 ± 46 nM with 10 μM CCh. This phase was accompanied by a similar dose-dependent stimulation of total inositol phosphate production and was assumed to be generated by release from intracellular stores of the endoplasmic reticulum (ER). The sustained increase in [Ca] i was generated by entry from the extracellular bath since it was blocked by pretreatment with La 3+ (1 μM) and was absent when bath calcium was chelated with EGTA. This phase was not dependent on CCh dose, and a stimulation of [Ca] i of ∼90 nM above baseline was observed with CCh concentrations between 50 nM and 10 μM. With this dose range, the rate of Mn 2+ quenching of Fura-2 at the Ca-insensitive excitation wavelength of 360 nm was likewise maximally stimulated. At lower CCh concentrations (10–50 nM), it was clear that the activation of Ca entry could not be dissociated from a threshold release of Ca from intracellular stores. The phorbol ester PMA, which uncouples the muscarinic receptor from phospholipase C, reduced the transient rise in [Ca] i by ∼50% with little or no effect on Ca entry at higher CCh levels (≥1 μM). At lower CCh concentrations (≤ 100 nM) however, pretreatment with PMA completely blocked all Ca mobilization and supports the contention that Ca entry is coupled to Ca release from stores or to store depletion. The emptying of inositol trisphosphate-sensitive stores with thapsigargin (10 nM) stimulated Ca entry and also the rate of Mn 2+ quenching. Store depletion by incubation in Ca-free media likewise stimulated Mn 2+ uptake without a rise in [Ca] i. Our data are therefore consistent with a ‘capacitative’ coupling model, whereby the activation of the plasma membrane receptor leads to an InsP 3-induced change in the degree of filling of the ER Ca pool. This ‘activated’ form of the ER then transmits a signal which increases the plasma membrane Ca permeability.

Mechanism of methylmercury efflux from cultured astrocytes

To study the mechanism of methylmercury (MeHg) efflux from the central nervous system cells, cultured astroglia obtained from neonatal rats were incubated with 10 μM MeHg-cysteine (CySH) for 30 min. After being washed four times, cell were incubated in Hg-free medium, and the release of MeHg from the cells was monitored. The amount of MeHg released in the medium approached a plateau level ( ca.31% of the loaded amount) at 4hr. Treatment of the cells with a CySH precursor, 2-oxothiazolidine-4-carboxylic acid (OTC), resulted in a significant increase of cellular levels of CySH and gluthione (GSH). OTC also increased 1.5-fold the MeHg efflux from the loaded cells. Another GSH enhancer, GSH isopropyl ester, also stimulated MeHg export from the cells. Ion-exchange column chromatography using DEAE-Sephadex revealed that the MeHg metabolite thus released was exclusively MeHg-GSH conjugate, both with and without OTC. Since the MeHg efflux was suppressed significantly by the presence of probenecid, the efflux occured via the probenecid-sensitive organic acid transport system. Even though the cellular GSH levels were depleted drastically by treatment with l- buthionine-(S,R)- sulfoximine (BSO), a considerable level (90% of the control) of Hg efflux was detected. Since neither GSH- nor CySH-MeHg was detected in the culture medium of the BSO-treated cells, GSH depletion may trugger some other secretion system(s) in the cells. These results suggest that conjugation with GSH is the major pathway for MeHg efflux in rat astroglia, and that elevation in the cellular GSH level would possibly be a logical therapy for MeHg poisoning, promoting the accelerated elimination of MeHg from the critical tissues.

Active Transport of Benzylpenicillin Across the Blood‐Milk Barrier

Passage of benzylpenicillin across the mammary gland epithelium was studied both after systemic administration of benzylpenicillin in four goats and after intramammary infusion in three goats and two cows. The results after benzylpenicillin administration alone were compared with the results after combined administration of benzylpenicillin and probenecid. The studies on passage of benzylpenicillin through the blood-milk barrier showed, when milk to plasma ultrafiltrate ratios of benzylpenicillin were plotted versus time for each half of the udders, that active transport takes place from blood to milk during steady-state plasma concentrations. Active transport was demonstrated by inhibition with probenecid: (1) The entry of benzylpenicillin into milk was slowed down under the influence of probenecid. The AUC (area under the curve)-values during the first 30 min. were reduced by 66 +/- 9% compared with the AUC-values found without coadministration of probenecid. (2) In the presence of probenecid and during equilibrium between blood and milk concentrations, benzylpenicillin reached concentration ratios in the ultrafiltrates of milk and plasma which corresponded to those expected for diffusion alone. Without probenecid these ultrafiltrate concentration ratios were more than two times higher indicating an active transport of benzylpenicillin from blood to milk. After intramammary infusion of benzylpenicillin, active transport was demonstrated from milk into blood. The absorption rate for benzylpenicillin from the mammary gland was reduced by probenecid, as measured by the ratio of benzylpenicillin to urea absorption half-life, which was increased by 40-50% in the presence of probenecid.

Pharmacokinetic Drug Interactions with ACE Inhibitors

Angiotensin converting enzyme (ACE) inhibitors which have active moieties excreted mainly in urine require adjustment of either the dose or the interval between doses in patients with moderate to severe renal dysfunction or severe congestive heart failure. Those agents such as temocapril (CS 622) and fosinopril, excreted both in urine and bile, may not require such adjustment. Renal clearance of ACE inhibitors may be reduced and some accumulation may occur in elderly patients with mild renal dysfunction or congestive heart failure. The bioavailability of ACE inhibitors is reduced by concomitant food or antacids which may slow gastric emptying and raise gastric pH. Pharmacokinetic interactions with ACE inhibitors are unlikely in patients receiving thiazide or loop diuretics. When ACE inhibitors are given hyperkalaemia may occur in patients with renal insufficiency, those taking potassium supplements or potassium-sparing diuretics, and in diabetic patients with mild renal impairment. While no pharmacokinetic interaction precludes use of this combination, the pharmacokinetics of some ACE inhibitors are subject to greater variability when patients also receive beta-blockers. Calcium antagonists and ACE inhibitors have additive anti-hypertensive effects and pharmacokinetic interactions between these agents are unlikely. One report exists of a significant effect of coadministered hydralazine on the pharmacokinetics and urinary excretion of lisinopril. Data on interactions between ACE inhibitors and digitalis are contradictory. There is no evidence that the concomitant use of ACE inhibitors and digoxin is associated with an increased risk of digitalis toxicity. ACE inhibitors are mainly excreted by glomerular filtration and renal tubular secretion. Possible interactions between ACE inhibitors and probenecid have been noted, with renal and total body clearance of ACE inhibitors being potentially reduced in the presence of probenecid. Probenecid pretreatment may enhance the pharmacodynamic response of ACE inhibitors. Few but contradictory data exist on the effects of H2-blockers on ACE inhibitor pharmacokinetics and little information is available on interactions between ACE inhibitors and hypoglycaemic drugs. Some case reports link ACE inhibitors with the induction of lithium toxicity. Coadministration of lithium should be undertaken with caution, and frequent monitoring of lithium concentrations is recommended with all ACE inhibitors. Nonsteroidal anti-inflammatory drugs (NSAIDs) may attenuate the haemodynamic actions of ACE inhibitors. NSAIDs reduce renal excretion of ACE inhibitors, with a corresponding increase in circulating drug concentrations. There is little information available on the pharmacokinetic interaction with ACE inhibitors and cyclosporin, but caution should be employed when they are used together.(ABSTRACT TRUNCATED AT 400 WORDS)

Accumulation of Salicylic Acid and Indomethacin in Isolated Proximal Tubular Cells of the Rat Kidney

The handling and accumulation of salicylic acid (SA) and indomethacin was examined in freshly isolated proximal tubular (PT) cells of the rat kidney in order to determine whether these cells provide a useful tool for studying accumulation of nonsteroidal anti-inflammatory drugs. A PT cell suspension was prepared by collagenase digestion, followed by filtration and isopycnic centrifugation. SA uptake was concentration-dependent and could be inhibited by probenecid. SA accumulated in the PT cells, and therefore, uptake is probably an active process. In the presence of probenecid, no SA accumulation was observed. Indomethacin uptake increased with time up to 2 min. Thereafter, a sharp decrease occurred, probably caused by inhibition of the oxidative phosphorylation. In the presence of probenecid, uptake was significantly reduced and no longer time-dependent. Indomethacin accumulated in the PT cells by a factor of more than 25. In the presence of probenecid, accumulation was decreased but was still considerable (approximately 10), probably as a result of binding to cellular protein. We conclude that as a result of carrier-mediated transport which is probenecid-sensitive, SA and indomethacin accumulate in the PT cells. The observed accumulation values are in accordance with previously observed values in the isolated perfused rat kidney. Therefore, freshly isolated PT cells appear to be a simple and useful model for studying the accumulation process of drugs like SA and indomethacin.

Preparation and biological characteristics of 99mTc-diol a renal agent

The present work concerns the production of 1,2-dihydroxypropyl-1-phosphonic acid (diol) by acid hydrolysis of (- cis) 1,2-epoxypropylphosphonic acid (phosphomycin), and its formulation as a kit easily labeled with [ 99mTc]pertechnetate. Biodistribution studies and whole-body autoradiographies in mice show that 99mTc-diol has a specific affinity for the kidneys: it is rapidly cleared from the blood and excreted in urine (12.09 ± 6.40% ID are excreted in urine at 5 min and 70.81 ± 2.41% ID at 30 min post-injection). Part of the injected activity remains in the kidney cortex sufficiently long to permit kidney imaging (5.66 ± 0.91% ID is still in kidneys 1 h post-injection). In comparison with other agents which also localize in the kidney cortex, such as 99mTc-DMSA and 99mTc-glucoheptonate ( 99mTc-GHA), the main differences are the following: the peak of renal activity is reached early in the 5 min post-injection period for 99mTc-diol, only at about 10 min post-injection for 99mTc-GHA and after 3 h post-injection for 99mTc-DMSA. The uptake of 99mTc-diol by other organs, especially by bones, is much smaller than in the case of 99mTc-DMSA (1.25 ∓ 0.11% ID of 99mTc-diol compared to 11.31 ± 1.17% ID of 99mTc-DMSA, 1 h post-injection). Unlike 99mTc-DMSA, the biodistribution of 99mTc-diol is not significantly influenced by acid-base imbalance, in addition, its renal uptake decreases in the presence of probenecid whereas its urinary excretion increases. Moreover, the retention effect of hyperuricemia on 99mTc-diol blood clearance and kidney excretion could be the result of a competition mechanism between this radiopharmaceutical and uric acid for a common transport system. The above biological characteristics suggest that 99mTc-diol is a kidney radiopharmaceutical with an ability for functional and imaging studies.

Effect of probenecid on disposition kinetics of ampicillin in horses.

The effect of an oral dose of probenecid on the disposition kinetics of ampicillin was determined in four horses. An intravenous bolus dose (10 mg/kg) of ampicillin sodium was administered to the horses on two occasions. On the first occasion the antibiotic was administered on its own, and on the second occasion it was administered one hour after an oral dose of 75 mg/kg probenecid. The plasma concentration of probenecid reached a mean (+/- se) maximum concentration (Cmax) of 188-6 +/- 19.3 micrograms/ml after 120.0 +/- 21.2 minutes and concentrations greater than 15 micrograms/ml were present 25 hours after it was administered. The disposition kinetics of ampicillin were altered by the presence of probenecid and as a result the antibiotic had a slower body clearance (ClB; 109.4 +/- 6.71 ml/kg hours compared with 208.9 +/- 26.2 ml/kg hours) a longer elimination half-life (t1/2 beta 1.198 hours compared with 0.701 hours) and consequently a larger area under the plasma concentration versus time curve (AUC 92.3 +/- 5.09 mg/ml hours compared with 35.95 +/- 3.45 mg/ml hours) when compared with animals to which ampicillin was administered alone. The ampicillin concentrations observed suggest that the dosing interval for horses may be increased from between six and eight hours to 12 hours when probenecid is administered in conjunction with the ampicillin.

Influence of probenecid and paracetamol (acetaminophen) on zidovudine glucuronidation in human liver in vitro.

The effects of probenecid and paracetamol on zidovudine glucuronidation were investigated, in vitro, using human liver microsomal preparations. The presence of probenecid in the incubation medium significantly reduced the maximum reaction velocity for zidovudine glucuronide formation by more than 60 per cent, and the Km was reduced by 47 per cent, suggesting an uncompetitive inhibition of zidovudine glucuronidation. In contrast, paracetamol had no significant effect on zidovudine glucuronidation. The maximum reaction velocity for zidovudine glucuronide formation and the Km were unchanged when paracetamol (5 mM) was present in the incubation medium. The effects of probenecid and paracetamol on zidovudine metabolism in vitro correlates closely with those observed in vivo. The in vitro system of human liver microsomes may have a useful role in predicting the possible interaction of other drugs with zidovudine metabolism.

Pharmacokinetic Evaluation of Drug Interactions with Zidovudine. I: Probenecid and Zidovudine in Monkeys

Pharmacokinetic evaluation of a drug interaction between zidovudine (AZT) and probenecid was conducted in monkeys. Six animals received 20 mg/kg of AZT as single intragastric (ig) and iv doses in the absence and presence of 50 mg/kg of probenecid administered ig. Plasma concentrations of AZT and its 5’-glucuronide metabolite (AZTG) were quantitated for 12 h by HPLC. Amounts of AZT and AZTG in urine were also measured, as were probenecid plasma concentrations. Non-compartmental methods were used to obtain pharmacokinetic parameters for AZT and AZTG. In the presence of probenecid, the total clearance of AZT decreased by 50%, renal clearance decreased, and elimination half-life increased. The volume of distribution at steady-state and systemic bioavailability of AZT were not significantly altered by probenecid. The areas under the plasma concentration–time curves and terminal half-lives of AZTG were increased, and renal clearances of AZTG were decreased. The alterations in AZT and AZTG pharmaco-kinetic parameters are consistent with inhibition of metabolism and renal tubular secretion by probenecid. Since AZT was administered by both oral and iv routes, clearance, volume of distribution, and bioavailability parameters were independently determined. Based on data reported for humans on the zidovudine–probenecid interaction, monkeys appear to be appropriate animal models for the evaluation of zidovudine drug interactions.

Modulation of 3'-azido-3'-deoxythymidine catabolism by probenecid and acetaminophen in freshly isolated rat hepatocytes

Metabolic studies of 3'-azido-3'-deoxythymidine (AZT) in humans have demonstrated that this compound is primarily eliminated as a 5′- O-glucuronide, 3'-azido-3'-deoxy-5'-β- d-glucopyranuronosylthymidine (GAZT), accounting for approximately 80% of the administered dose. Recently, we characterized the complete catabolic pathway of AZT in freshly isolated rat hepatocytes in suspension, demonstrating extensive formation of three catabolites, including GAZT, 3'-amino-3'-deoxythymidine (AMT), and 3'-amino-3'-deoxy-5'-β- d-glucopyranuronosylthymidine (GAMT). The present study evaluated the effects of probenecid (PROB) and acetaminophen (ACET), two agents which are also metabolized by UDP-glucuronyltransferase, on the metabolism and transmembrane distribution of AZT in rat hepatocytes. Pre-exposure of cells to 350 μM PROB 30min prior to the addition of 10 μM [ 3H]AZT decreased intracellular GAZT levels by approximately 10-fold. Interestingly, AMT formation was enhanced approximately 1.5-fold in the presence of PROB, probably resulting from increased AZT availability. In contract, pre-exposure to 50 μM ACET 30 min prior to addition of 10 μM [ 3H]AZT did not substantially alter AZT glucuronidation. Additionally, decreased AZT catabolism by PROB did not contribute to the formation of 5'-phosphorylated derivatives of AZT. Agents which undergo glucuronidation may thus not necessarily affect AZT conversion to GAZT, and their potential interactions should be investigated using in vitro systems prior to co-administration with AZT.

Effect of Probenecid on the Pharmacokinetics and Pharmacodynamics of Procainamide

Renal tubular transport of organic anions and cations is assumed to be mutually exclusive. However, results of a number of in vitro and in vivo studies suggest an interaction between the organic anion, probenecid, and various organic cations in the proximal renal tubule. To evaluate the clinical importance of such an interaction, the authors investigated the pharmacokinetics and pharmacodynamics of procainamide, an organic cation with a low therapeutic index that is excreted in part by active secretion in the proximal tubule, in the presence and absence of probenecid. In a randomized crossover study, six healthy subjects received a single 750-mg IV dose of procainamide, with and without prior probenecid administration (2 g orally). Blood and urine samples were obtained and pharmacokinetic parameters of procainamide were determined in each treatment period. QT intervals were measured from ECG recordings that were obtained at blood collection times for pharmacodynamic evaluation. Coadministration of probenecid did not result in any significant change in the overall disposition of procainamide. In particular, renal clearance was not significantly different (488 +/- 95 mL/min without probenecid vs. 478 +/- 69 mL/min in the presence of probenecid). Our data suggest an interaction between probenecid and procainamide in the proximal renal tubule does not exit. Reasons for this lack of interaction are discussed.

Pharmacokinetics of 3'-azido-3'-deoxythymidine and its catabolites and interactions with probenecid in rhesus monkeys

The pharmacokinetics and metabolism of 3'-azido-3'-deoxythymidine (AZT) were investigated in rhesus monkeys after subcutaneous administration of 33.3 mg of AZT per kg of body weight alone or in the presence of 100 mg of probenecid per kg. In addition to unchanged drug, two catabolites, 5'-O-glucuronide (GAZT) and 3'-amino-3'-deoxythymidine (AMT), were detected in plasma within 30 min. GAZT exhibited a kinetic profile similar to that of AZT, with an elimination half-life of approximately 1 h, while AMT was more variable, with an apparent half-life of 1.6 +/- 1.5 h. Approximately 90% of the total administered dose was recovered in urine within 24 h as AZT, GAZT, AMT, and the 5'-O-glucuronide of AMT. AZT and AMT demonstrated similar cerebrospinal fluid (CSF) penetration 1 h after AZT treatment, while GAZT poorly crossed the blood-brain barrier. Concomitant administration of probenecid greatly altered the pharmacokinetics of AZT, GAZT, and AMT, resulting in prolongation of their apparent elimination half-lives, increased concentrations in plasma, and marked reduction in renal clearances. In addition, the CSF/plasma concentration ratios for AZT and its catabolites were greatly increased, suggesting that probenecid inhibits efflux of AZT and its catabolites from CSF to plasma. The substantial levels of AMT in plasma suggest that this catabolite affects the pharmacodynamic properties of AZT in relation to its activity against human immunodeficiency virus replication and cytotoxicity to host cells. Enhanced AMT levels in plasma in the presence of probenecid may decrease the therapeutic efficacy of the AZT-probenecid combination.

Moment analysis of drug disposition in kidney. V:In Vivo transepithelial transport ofp-aminohippurate in rat kidney

A new method that can assess the kinetics of in vivo transepithelial transport in rat kidney has been established. The method is based upon a multiple-indicator dilution experiment and the moment analysis theory. After simultaneous bolus injections of p-aminohippurate (PAH) and inulin into the right renal artery, blood samples were taken from the carotid artery and urine was separately collected from right and left ureters. The characteristic response for the first passage of drugs through the right kidney was evaluated by taking blood circulation into consideration. To determine the mean artery-to-vein transit time and the extraction ratio in the kidney, an intravenous injection was also performed as a reference experiment for deconvolution. The urinary excretion curve corresponding to the first passage was obtained as the difference between both kidneys. The mean artery-to-lumen transit time (mean transepithelial transit time, Tcell) was computed by subtracting the mean urinary transit time of inulin from that of secreted PAH. Since transport across the luminal membrane into the lumen from tubular epithelial cells can influence the cellular residence time of drugs, Tcell and the single-pass mean residence time in epithelial cells (Tcell,sp) can be thought of describing luminal membrane transport. The value of Tcell obtained for 0.1 mM PAH was 22 sec and it was prolonged to 61 sec in the presence of probenecid, suggesting an inhibitory effect on transport across the luminal membrane. On the other hand, antiluminal membrane transport into cells from blood is characterized by the volume of distribution in the kidney (VdPAH). VdPAH was remarkably decreased by treatment with probenecid, indicating an inhibitory effect on antiluminal membrane transport. The effects of probenecid on both sides of epithelial cell membrane transport were first demonstrated in vivo. The present method is useful for the analysis of in vivo transepithelial transport including antiluminal and luminal membrane transport for drugs excreted via tubular secretion.

Ceftriaxone binding to human serum albumin: Indirect displacement by probenecid and diazepam

In vitro protein binding studies were conducted to examine the interaction between ceftriaxone (CEF), probenecid (PROB) and diazepam (DIAZ). The presence of PROB and DIAZ at concentrations equal to molar albumin concentration caused a decrease in CEF affinity from 3.7 × 10 4M −1 (control) to 1.1 × 10 4 (PROB) and 2.6 × 10 4 (DIAZ) M −1, but not in binding capacity in pooled human plasma. PROB and DIAZ at five times the molar albumin concentration also caused a decrease in CEF affinity from 4.5 × 10 4M −1 (control) to 0.45 × 10 4 (PROB) and 3.0 × 10 4 (DIAZ) M −1 in isolated human serum albumin. DIAZ and PROB displaced one another, confirming their common binding site (Site II, the benzodiazepine site) on serum albumin. By contrast, CEF was unable to displace either PROB or DIAZ from defatted albumin. In the presence of elevated free fatty acid concentrations ( four times the albumin concentration), CEF decreased the binding of both drugs. CEF free fraction (fp) in isolated human serum albumin (CEF fp = 7.7%) was increased by drugs which bind to Site I: sulfisoxazole (CEF fp = 68.1%), warfarin (CEF fp = 56.0%) and furosemide (CEF fp = 55.0%). At ten times the molar concentration of albumin, CEF displaced both warfarin (warfarin fp from 0.99 to 2.20%) and phenytoin (phenytoin fp from 17.7 to 23.4%) from defatted albumin. CEF appeared to bind to Site I (the warfarin site) on human serum albumin, and was displaced by PROB and DIAZ via a mechanism which did not involve direct competition at a common binding site.

Technetium-99m amino acid chelates: Correlation of their physico-chemical and physiological parameters. Part I.

Ten α-aminocarboxylic acids were chelated with 99mTc and purified by sephadex gel chromatography, Their hepatobiliary and renal excretion patterns were determined in rats. It was observed that lipophilicity of these chelates is the only determinant in governing their hepatobiliary excretion, whereas their renal excretion is dependent on some other factors in addition to lipophilicity. Depression of renal excretion in presence of probenecid indicated an interaction of renal tubular transport enzymes with these chelates, which was explained from their hexacoordinated dioxotechnetium (V) chelate structure (II).

Size selected mRNA induces expression of P-aminohippurate transport in Xenopus oocytes.

Xenopus oocytes were injected with size-fractionated mRNA isolated from the renal cortex of rabbit kidney and after 4 days incubation, PAH uptake in oocytes injected with mRNA (0.7-1.3 kb) was 8 to 45 fold that of the water injected controls. The oocyte to medium ratio of accumulated PAH was 1.95. The Km and Vmax for transport were 333 microM and 66.6 nmoles.oocyte-1.min-1, respectively. This Km is similar to that reported for PAH transport in intact kidneys and slices. The uptake of PAH was unaffected by the absence of Na+ or the presence of probenecid. Expression of the transport represents the first step in an effort to clone and identify the gene for PAH transport.