Effect of probenecid on blood levels and renal elimination of furosemide and endogenous compounds in rats: Discovery of putative organic anion transporter biomarkers

Transporter-mediated drug-drug interactions (DDIs) are assessed using probe drugs and in vitro and in vivo models during drug development. The utility of endogenous metabolites as transporter biomarkers is emerging for prediction of DDIs during early phases of clinical trials. Endogenous metabolites such as pyridoxic acid and kynurenic acid have shown potential to predict DDIs mediated by organic anion transporters (OAT1 and OAT3). However, these metabolites have not been assessed in rats as potential transporter biomarkers. We carried out a rat pharmacokinetic DDI study using probenecid and furosemide as OAT inhibitor and substrate, respectively. Probenecid administration led to a 3.8-fold increase in the blood concentrations and a 3-fold decrease in renal clearance of furosemide. High inter-individual and intra-day variability in pyridoxic acid and kynurenic acid, and no or moderate effect of probenecid administration on these metabolites suggest their limited utility for prediction of Oat-mediated DDI in rats. Therefore, rat blood and urine samples were further analysed using untargeted metabolomics. Twenty-one m/z features (out of >8000 detected features) were identified as putative biomarkers of rat Oat1 and Oat3 using a robust biomarker qualification approach. These m/z features belong to metabolic pathways such as fatty acid analogues, peptides, prostaglandin analogues, bile acid derivatives, flavonoids, phytoconstituents, and steroids, and can be used as a panel to decrease variability caused by processes other than Oats. When validated, these putative biomarkers will be useful in predicting DDIs caused by Oats in rats.

Local perfusion of capillaries reveals disrupted beta-amyloid homeostasis at the blood-brain barrier in Tg2576 murine Alzheimer's model

BACKGROUND

Parenchymal accumulation of beta-amyloid (Aβ) characterizes Alzheimer's disease (AD). Aβ homeostasis is maintained by two ATP-binding cassette (ABC) transporters (ABCC1 and ABCB1) mediating efflux, and the receptor for advanced glycation end products (RAGE) mediating influx across the blood-brain barrier (BBB). Altered transporter levels and disruption of tight junctions (TJ) were linked to AD. However, Aβ transport and the activity of ABCC1, ABCB1 and RAGE as well as the functionality of TJ in AD are unclear.

METHODS

ISMICAP, a BBB model involving microperfusion of capillaries, was used to assess BBB properties in acute cortical brain slices from Tg2576 mice compared to wild-type (WT) controls using two-photon microscopy. TJ integrity was tested by vascularly perfusing biocytin-tetramethylrhodamine (TMR) and quantifying its extravascular diffusion as well as the diffusion of FM1-43 from luminal to abluminal membranes of endothelial cells (ECs). To assess ABCC1 and ABCB1 activity, calcein-AM was perfused, which is converted to fluorescent calcein in ECs and gets actively extruded by both transporters. To probe which transporter is involved, probenecid or Elacridar were applied, individually or combined, to block ABCC1 and ABCB1, respectively. To assess RAGE activity, the binding of 5-FAM-tagged Aβ by ECs was quantified with or without applying FPS-ZM1, a RAGE antagonist.

RESULTS

In Tg2576 mouse brain, extravascular TMR was 1.8-fold that in WT mice, indicating increased paracellular leakage. FM1-43 staining of abluminal membranes in Tg2576 capillaries was 1.7-fold that in WT mice, indicating reduced TJ integrity in AD. While calcein was undetectable in WT mice, its accumulation was significant in Tg2576 mice, suggesting lower calcein extrusion in AD. Incubation with probenecid or Elacridar in WT mice resulted in a marked calcein accumulation, yet probenecid alone had no effect in Tg2576 mice, implying the absence of probenecid-sensitive ABC transporters. In WT mice, Aβ accumulated along the luminal membranes, which was undetectable after applying FPS-ZM1. In contrast, marginal Aβ fluorescence was observed in Tg2576 vessels, and FPS-ZM1 was without effect, suggesting reduced RAGE binding activity.

CONCLUSIONS

Disrupted TJ integrity, reduced ABCC1 functionality and decreased RAGE binding were identified as BBB alterations in Tg2576 mice, with the latter finding challenging the current concepts. Our results suggest to manage AD by including modulation of TJ proteins and Aβ-RAGE binding.

Probenecid affects muscle Ca2+ homeostasis and contraction independently from pannexin channel block

Tight control of skeletal muscle contractile activation is secured by the excitation-contraction (EC) coupling protein complex, a molecular machinery allowing the plasma membrane voltage to control the activity of the ryanodine receptor Ca2+ release channel in the sarcoplasmic reticulum (SR) membrane. This machinery has been shown to be intimately linked to the plasma membrane protein pannexin-1 (Panx1). We investigated whether the prescription drug probenecid, a widely used Panx1 blocker, affects Ca2+ signaling, EC coupling, and muscle force. The effect of probenecid was tested on membrane current, resting Ca2+, and SR Ca2+ release in isolated mouse muscle fibers, using a combination of whole-cell voltage-clamp and Ca2+ imaging, and on electrically triggered contraction of isolated muscles. Probenecid (1 mM) induces SR Ca2+ leak at rest and reduces peak voltage-activated SR Ca2+ release and contractile force by 40%. Carbenoxolone, another Panx1 blocker, also reduces Ca2+ release, but neither a Panx1 channel inhibitory peptide nor a purinergic antagonist affected Ca2+ release, suggesting that probenecid and carbenoxolone do not act through inhibition of Panx1-mediated ATP release and consequently altered purinergic signaling. Probenecid may act by altering Panx1 interaction with the EC coupling machinery, yet the implication of another molecular target cannot be excluded. Since probenecid has been used both in the clinic and as a masking agent for doping in sports, these results should encourage evaluation of possible effects on muscle function in treated individuals. In addition, they also raise the question of whether probenecid-induced altered Ca2+ homeostasis may be shared by other tissues.

Probenecid slows disease progression in a murine model of autosomal dominant polycystic kidney disease

Development of autosomal dominant polycystic kidney disease (ADPKD) involves renal epithelial cell abnormalities. Cystic fluid contains a high level of ATP that, among other effects, leads to a reduced reabsorption of electrolytes in cyst-lining cells, and thus results in cystic fluid accumulation. Earlier, we demonstrated that Pkd1RC/RC mice, a hypomorphic model of ADPKD, exhibit increased expression of pannexin-1, a membrane channel capable of ATP release. In the current study, we found that human ADPKD cystic epithelia have higher pannexin-1 abundance than normal collecting ducts. We hypothesized that inhibition of pannexin-1 function with probenecid can be used to attenuate ADPKD development. Renal function in male and female Pkd1RC/RC and control mice was monitored between 9 and 20 months of age. To test the therapeutic effects of probenecid (a uricosuric agent and a pannexin-1 blocker), osmotic minipumps were implanted in male and female Pkd1RC/RC mice, and probenecid or vehicle was administered for 42 days until 1 year of age. Probenecid treatment improved glomerular filtration rates and slowed renal cyst formation in male mice (as shown in histopathology). The mechanistic effects of probenecid on sodium reabsorption and fluid transport were tested on polarized mpkCCDcl4 cells subjected to short-circuit current measurements, and in 3D cysts grown in Matrigel. In the mpkCCDcl4 epithelial cell line, probenecid elicited higher ENaC currents and attenuated in vitro cyst formation, indicating lower sodium and less fluid retention in the cysts. Our studies open new avenues of research into targeting pannexin-1 in ADPKD pathology.

Interaction of Organic Anion Transporter 3-Mediated Uptake of Steviol Acyl Glucuronide, a Major Metabolite of Rebaudioside A, with Selected Drugs

Organic anion transporter 3 (OAT3) plays a critical role in the renal excretion of many xenobiotics. Because steviol acyl glucuronide (SVAG), an OAT3 substrate, is the major circulating metabolite after oral ingestion of steviol glycosides and is excreted into the urine, inhibition of OAT3 activity may alter pharmacokinetic profiles of SVAG. The present study showed that drugs such as probenecid and glimepiride displayed potent inhibition toward the OAT3-mediated SVAG transport, with IC₅₀ values of 4.9 and 0.8 μM, respectively. No species differences were observed. Probenecid and glimepiride could significantly elevate plasma concentrations of SVAG after oral administration of rebaudioside A, with significant increases in plasma maximum (C_max) and area under the plasma time-concentration curve values. The inhibitory effect on the OAT3-mediated SVAG transport exemplified a unique case between drugs and the metabolite of a food additive. Our data suggest that caution should be exercised when giving steviol glycoside products to human subjects with compromised renal function.

Effect of Known Inhibitors of Ion Transport on Pendrin (SLC26A4) Activity in a Human Kidney Cell Line

BACKGROUND/AIMS

Pendrin is a Cl-/I-/HCO3- exchanger playing a fundamental role in controlling blood pressure and airway function, therefore representing an attractive target for the treatment of hypertensive states and respiratory distresses. A review of the literature regarding the ability of some compounds (namely several known inhibitors of ion transport) to block pendrin activity revealed discordant findings. These incongruous findings may be due, in part, to the concentration of compound and/or the nature of the model system used in the study.

METHODS

Pendrin activity was evaluated by measuring pendrin-dependent iodide influx following overexpression of the transporter in a human kidney cell line, in the presence of selected test compounds or the respective vehicles.

RESULTS

Pendrin activity was significantly hampered by 0.1 mM 5-nitro-2-[(3-phenylpropyl)amino]benzoic acid (NPPB), niflumic acid and tenidap, but was resistant to 0.1 mM 4, 4'-diisothiocyano-2, 2'-stilbene-disulfonic acid (DIDS), furosemide and probenecid.

CONCLUSIONS

The results of the present study indicate that clinically effective non-steroidal anti-inflammatory drugs (niflumic acid and tenidap) directly inhibit pendrin activity.

Isolation and characterization of polyspecific mouse organic solute carrier protein 1 (mOscp1)

We succeeded in isolating the cDNA-encoding mouse organic solute carrier protein 1 (mOscp1) from a mouse testis cDNA library. mOscp1 consisted of 1137 base pairs that encoded a 379-amino acid protein, and the amino acid sequence was 85% identical to that of human OSCP1 (hOSCP1). Northern blot analysis revealed that the gene coding for mOscp1 is highly expressed in the testis, but not in other tissues. When expressed in Xenopus laevis oocytes, mOscp1 mediated the high-affinity transport of p-aminohippurate (PAH) (K(m) = 18.8 +/- 4.1 microM) with Na(+) independence. mOscp1 transported various kinds of structurally dissimilar drugs and chemicals such as probenecid, dehydroepiandrosterone sulfate, and glutarate with some differences in substrate specificity compared with hOSCP1. Cyclophosphamide inhibited the mOscp1-mediated PAH uptake. Immunohistochemical analysis revealed that the mOscp1 protein is localized in the plasma membrane side of Sertoli cells in the testis. Our results indicate that isolated mOscp1 is a polyspecific organic solute carrier protein and may be a key molecule for the testicular handling of organic solutes.

Probenecid: Its Chromatographic Determination, Plasma Protein Binding, and in vivo Pharmacokinetics in Dogs

Pharmacokinetics (PK) of probenecid including plasma probenecid concentrations, in vitro plasma protein binding properties, and in vivo PK parameters were determined in dogs. Probenecid concentrations were best determined by HPLC, which showed good linearity and good recovery with simple plasma preparation. The quantification limit of probenecid was approximately 50 ng/ml at S/N ratio = 3, by simple procedure with HCl and methanol treatment. Probenecid showed two types of binding characteristics, i.e., high-affinity with low-capacity and low-affinity with high-capacity binding. This result indicated 80-88% of probenecid was bound to plasma protein(s) at observed concentrations (< 80 microg/ml) in vivo at an intravenous dose of 20 mg/kg. Plasma probenecid concentration-time profile following i.v. administration in dogs showed biphasic decline and well fitted a two-compartment open model. The total body clearance was 0.34 +/- 0.04 ml/min/kg, volume of distribution at steady-state was 0.46 +/- 0.07 l/kg, elimination half-life was 18 +/- 6 hr, and mean residence time (MRT) was 23 +/- 6 hr. Since probenecid has been known as a potent inhibitor of renal tubular excretion of acidic drugs and highly binds to plasma proteins, our observation in relation to plasma protein binding and PK parameters will serve as the basic information concerning drug-drug interactions in dogs and in other mammalian species.

The protease inhibitor ritonavir inhibits the functional activity of the multidrug resistance related-protein 1 (MRP-1)

BACKGROUND

Efflux pumps situated on the plasma membrane, such as P-glycoprotein (Pgp) and the multidrug resistance related-protein 1 (MRP-1), have been shown to extrude HIV protease inhibitors from the cell. MRP-1 is present on many barrier sites throughout the body, such as the blood-brain and blood-testis interfaces and could reduce the concentration of protease inhibitors in these sanctuary sites for HIV-1 replication. Factors that modulate efflux pump function in vivo are poorly defined.

OBJECTIVE

To analyze the inhibitory potential of the anti-retroviral drugs indinavir, amprenavir, ritonavir, lamivudine or zidovudine to modulate MRP-1 function.

METHODS

Effect of anti-HIV drugs on the efflux pump activity of MRP-1 was evaluated in the presence of increasing concentrations of human plasma, using UMCC-1/VP cells which stably over-express MRP-1. MRP-1 activity was abrogated by probenecid. The potential of blocking MRP-1 function for an extended (3 day) time period, was also examined in MRP-1 over-expressing cells cultured with either probenecid or the anti-retroviral drugs and a cytotoxic compound (etoposide) that is transported by MRP-1.

RESULTS

Ritonavir inhibited the functional activity of MRP-1 similarly to probenecid, as demonstrated by re-sensitization of MRP-1 over-expressing cells to cytotoxic effects of etoposide. Inhibition by ritonavir was inversely related to the concentration of human plasma added to the cells (r2 = 0.89). Other anti-HIV drugs didn't affect the MRP-1 mediated efflux of etoposide.

CONCLUSIONS

These data may be exploitable to further improve sanctuary site concentrations of anti-HIV or anti-cancer drugs by using ritonavir as a lead compound to develop more potent MRP-1 inhibitors.

The influence of inhibition of probenecid sensitive transporters on the central nervous system (CNS) uptake and the antinociceptive activity of morphine-6-glucuronide in rats

In light of a recent demonstration that probenecid enhanced the central nervous system (CNS) uptake of morphine-3-glucuronide (M3G), we investigated the consequences of probenecid co-administration for the spinal cord concentrations and antinociceptive effects of morphine-6-glucuronide (M6G) in rats. Spinal cord tissue concentrations of M6G were assessed by in-vivo microdialysis. Ten rats were administered M6G by intravenous infusion for 8 h, five of them additionally received an infusion of probenecid. Antinociceptive effects of M6G were assessed by means of formalin tests during the 8th h of the M6G infusion in another five rats that received M6G together with probenecid. Five additional rats per groups received probenecid only, M6G only and placebo. The inhibition of probenecid-sensitive transporters caused a raise in both plasma (statistically significant) and spinal cord tissue (not statistically significant) concentrations of M6G by a factor of 1.3, without affecting the tissue to plasma concentration ratio of M6G (0.0812 +/- 0.034 for M6G alone, 0.0824 +/- 0.021 for M6G plus probenecid). Co-administration of probenecid with M6G resulted in a significant reduction of the number of flinches by a factor of 2.5 as compared to M6G alone. The study showed that probenecid sensitive transporters play a role for the CNS concentrations of M6G via an increase of its plasma concentrations, but the effect of probenecid co-administration is small as compared to the reported effects on the CNS concentrations of M3G in rats.

Inhibition of biliary excretion of methotrexate by probenecid in rats: quantitative prediction of interaction from in vitro data

This study was designed to establish a strategy to predict drug interactions involving biliary excretion. The interaction between methotrexate and probenecid was examined as an interaction model since this interaction has already been clinically reported. Coadministration of probenecid reduced the biliary clearance of methotrexate in a dose-dependent manner in rats. This inhibition by probenecid was confirmed in vivo both in the uptake and excretion processes of methotrexate across sinusoidal and canalicular membranes, respectively. That is, both hepatic uptake clearance, assessed in integration plot analysis, and steady-state biliary clearance defined with respect to hepatic unbound methotrexate, were reduced in the presence of probenecid. Probenecid inhibited the active transport of methotrexate both in isolated hepatocytes and canalicular membrane vesicles, confirming the interaction at those two membranes. The degree of inhibition of the uptake and excretion processes found in vivo was comparable with the predicted values using the inhibition constant assessed in isolated hepatocytes and canalicular membranes, respectively. This suggests that the interaction at each membrane transport process can be quantitatively estimated from in vitro data. We have also proposed the method to predict the degree of inhibition of the net excretion from circulating plasma into the bile, the predicted values being also comparable with the inhibition actually found in vivo. The present analysis demonstrates a strategic rationale for predicting drug interactions involving biliary excretion using in vitro systems to avoid any false negative predictions.

Identification of the 1-cyano-3,4-epithiobutane-derived urinary mercapturic acid N-acetyl-S-(4-cyano-2-thio-1-butyl)-cysteine in male Fischer 344 rats

1-Cyano-3,4-epithiobutane (CEB), a naturally occurring nitrile derived from cruciferous plants, causes nephrotoxicity in male Fischer 344 rats. Nephrotoxicity induced by CEB is dependent on glutathione (GSH) conjugation and bioactivation. Conjugation with GSH and subsequent metabolism leads to the formation of specific urinary metabolites. The objectives of the present study were to identify CEB-derived urinary metabolites and quantify urinary non-protein thiols and thioethers in male Fischer 344 rats. Animals received 125 mg kg(-1) of CEB alone or following pretreatment with one of three selective inhibitors of GSH metabolism: acivicin, probenecid or aminooxyacetic acid. Total non-protein urinary thiol and urinary thioether concentrations were elevated in all treated groups at 12 and 24 h; however, elevations in non-protein thiols were not significantly greater in rats administered CEB alone as compared to negative controls. A single predominant urinary metabolite was identified as the CEB-derived mercapturic acid N-acetyl-S-(4-cyano-thio-1-butyl)-cysteine. Evidence for other CEB-derived metabolites was also demonstrated. These findings represent the identification of a unique compound and provide further evidence for the importance of GSH conjugation as a significant pathway in CEB metabolism.

Clinical pharmacokinetics of the antiviral nucleotide analogues cidofovir and adefovir

Cidofovir and adefovir are members of a new class of antiviral compounds. They are acyclic phosphonate analogues of deoxynucleoside monophosphates. Both compounds undergo intracellular activation to form diphosphates that are potent inhibitors of viral DNA polymerases. Cidofovir has broad spectrum antiviral activity against herpesviruses, papillomaviruses and poxviruses, whereas adefovir has potent activity against retroviruses and certain DNA viruses, including herpesviruses and hepadnaviruses. Intravenous cidofovir is approved for treatment of cytomegalovirus retinitis in patients with AIDS. Cidofovir and adefovir are dianionic at physiological pH and have low oral bioavailability in animals and humans. After intravenous administration to HIV-infected patients, the pharmacokinetics of both drugs are independent of dose and are consistent with preclinical data. Systemic exposure is proportional to the intravenous dose and both drugs are cleared by the kidney and excreted extensively as unchanged drug in the urine. Intracellular activation of a small fraction (< 10%) of the dose by cellular kinases leads to prolonged antiviral effects that are not easily predicted from conventional pharmacokinetic studies. The observed rate of elimination of cidofovir and adefovir from serum may not reflect the true duration of action of these drugs, since the antiviral effect is dependent on concentrations of the active phosphorylated metabolites that are present within cells. For both drugs, > 90% of an intravenous dose is recovered unchanged in the urine over 24 hours. Metabolism does not contribute significantly to the total clearance of either drug. Concomitant oral probenecid decreases both the renal clearance of cidofovir and the incidence of nephrotoxicity, presumably by blocking its active tubular secretion. This is the basis of the clinical use of concomitant probenecid as a nephroprotectant during cidofovir therapy. Subcutaneous administration produces exposure equivalent to that following intravenous administration. Drug interaction studies with cidofovir are ongoing, but there is no evidence of an interaction between zidovudine and either cidofovir or adefovir. Clearance of cidofovir in patients with renal impairment showed a linear relationship to creatinine clearance. The low oral bioavailability of adefovir has led to the development of an oral prodrug, adefovir dipivoxil, currently in development for the treatment of HIV and hepatitis B infections.

Expression of a renal type I sodium/phosphate transporter (NaPi-1) induces a conductance in Xenopus oocytes permeable for organic and inorganic anions

Two distinct molecular types (I and II) of renal proximal tubular brush border Na+/Pi cotransporters have been identified by expression cloning on the basis of their capacity to induce Na+-dependent Pi influx in tracer experiments. Whereas the type II transporters (e.g., NaPi-2 and NaPi-3) resemble well known characteristics of brush border Na+/Pi cotransport, little is known about the properties of the type I transporter (NaPi-1). In contrast to type II, type I transporters produced electrogenic transport only at high extracellular Pi concentrations (> or =3 mM). On the other hand, expression of NaPi-1 induced a Cl- conductance in Xenopus laevis oocytes, which was inhibited by Cl- channel blockers [5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) > niflumic acid >> 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid]. Further, the Cl- conductance was inhibited by the organic anions phenol red, benzylpenicillin (penicillin G), and probenecid. These organic anions induced outwardly directed currents in the absence of Cl-. In tracer studies, we observed uptake of benzylpenicillin with a Km of 0.22 mM; benzylpenicillin uptake was inhibited by NPPB and niflumic acid. These findings suggest that the type I Na+/Pi cotransporter functions also as a novel type of anion channel permeable not only for Cl- but also for organic anions. Such an apical anion channel could serve an important role in the transport of Cl- and the excretion of anionic xenobiotics.

Saturable transport of valproic acid in rat choroid plexus in vitro

In order to obtain in vitro evidence for a specific transport system of valproic acid (VPA) at the blood-cerebrospinal fluid (CSF) interface, the uptake of VPA by isolated rat choroid plexus was investigated. The uptake clearance of [3H]VPA decreased with the increase of the unlabeled VPA concentration in the incubation medium. Kinetic analysis yielded an apparent Km of 10.0 mM, Vmax of 0.0871 mumol s-1 g-1 and Kns, a permeability coefficient of the nonsaturable component, of 6.85 microL s-1 g-1, indicating that both saturable and nonsaturable systems may contribute to VPA uptake by choroid plexus. Organic anions, penicillin G, p-aminohippurate, salicylate, and probenecid significantly inhibited VPA uptake by choroid plexus. We suggest that VPA translocation through choroidal membrane is partly operated by the organic anion transport system. A significant decrease of VPA uptake induced by 2,4-dinitrophenol, stilbenedisulfonate, and hypothermia (10 degrees C) indicates the involvement of an energy-dependent, carrier-mediated transport system. These results demonstrate that VPA is actively transported through the rat choroidal epithelium via a saturable system probably shared by organic anions.

Fluid phase endocytosis by isolated rabbit lacrimal gland acinar cells

It is well known that lacrimal gland acinar cells retrieve secretory vesicle membrane constituents from their apical plasma membranes after stimulated exocytosis of secretory proteins. There have also been indications of a recycling traffic involving the basal-lateral plasma membranes. In an effort to document this traffic, determine how it is regulated, and discern whether it involves more than one intracellular compartment, we studied internalization of the fluid phase marker, Lucifer Yellow, and its relationship to protein release in acinar cells isolated from rabbit lacrimal glands. Loading of intracellular vesicles was apparent with fluoresence microscopy. Stimulation with carbachol increased both the rate of internalization and the intracellular volume equilibrating with extracellular fluid, suggesting the loading of two compartments. A carbachol concentration of 10 microM increased uptake by 80% during 20-min incubations at 37 degrees C. Increasing the carbachol concentration to 1 mM reduced the response by 50%, and it appeared to do so by decreasing the intracellular volume accessible to extracellular fluid, rather than the rate of endocytosis. Carbachol affected protein release differently, increasing it by 50% at 10 microM and 80% at 1 mM. Acceleration of endocytosis by 10 microM carbachol was transient, becoming negligible after 60 min. Vasoactive intestinal peptide (VIP) and isoproterenol increased internalization 35% and 25% respectively; neither reduced uptake at the highest concentrations tested; and only isoproterenol significantly affected protein secretion. Combinations of VIP and carbachol exerted synergistic effects on both fluid phase internalization and protein release. Steady-state uptake at 18 degrees C in the presence of 10 microM carbachol was equal to uptake at 37 degrees C in the absence of carbachol, suggesting a temperature block in the pathway to at least one endocytic compartment. Decreasing the temperature to 18 degrees C eliminated the inhibitory action of excessive carbachol, suggesting that the compartment whose loading was impaired by excessive carbachol was positioned distal to the temperature block. Carbachol accelerated release of marker from preloaded cells, indicating that it stimulated recycling between the plasma membranes and endocytic compartments. This effect was maximal at a concentration of 10 microM and unchanged with increasing concentrations. In accord with the hypothesis that traffic into and out of a certain compartment was particularly dependent on stimulation, a fraction of the marker taken up by optimally stimulated cells at 37 degrees C was retained unless carbachol or VIP was present in the efflux medium.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence of human serum albumin beta-lactamase activity

The beta lactamase activity of serum albumin was measured using a chromogenic cephalosporin: 3-(2,4 dinitrostyryl) -(6 R, 7R) - 7 -(2 thienacetamido) ceph 3 - em 4 carboxylic acid) (nitrocefin). It was found that albumin of different species and purity, obtained from a variety of sources, showed a significant beta lactamase activity. The reaction presented a saturation kinetics, with a KM of 1.42 10(-5) M. Oleic acid and ibuprofen inhibited the reaction, suggesting that the cephalosporin and these ligands are bound to the same site in albumin. The tertiary structure of albumin is necessary for the catalytic activity to be present probably unprotoned lysine residues in the catalytic site of albumin are related with this activity.

Interindividual variation in the capacity-limited renal glucuronidation of probenecid by humans

A dose of 1,000 mg probenecid was administered orally to 14 human volunteers in order to quantify the maximal rate of formation and excretion of probenecid acyl glucuronide in the urine. Probenecid showed dose-dependent pharmacokinetics. Plasma protein binding of probenecid was high, being somewhat higher in males (90.7 +/- 1.4%) than in females (87.9 +/- 1.4%; p = 0.0019). It was shown that probenecid is metabolized by cytochrome P-450 into at least two phase I metabolites. Each of the metabolites accounted for less than 12% of the dose administered; the main metabolite probenecid acyl glucuronide, representing 42.9 +/- 13.2% of the dose, was only present in urine and not in plasma. The renal excretion rate-time profile of probenecid acyl glucuronide showed a plateau value in the presence of an acidic urine pH. This plateau value was maintained for about 10 h at the dose of 1,000 mg. The height of the plateau value depended on the individual and varied between 250 and 800 micrograms/min (15-50 mg/h). It was inferred that probenecid acyl glucuronide is formed in the kidney during blood-to-lumen passage through the tubular cells. We conclude that the plateau value in the renal excretion rate of probenecid glucuronide reflects its Vmax of formation.

Contribution of the human kidney to the metabolic clearance of drugs

OBJECTIVE

To demonstrate that the human kidney is capable not only of filtering and secreting drugs and their metabolites, but also of carrying out conjugation reactions such as acyl glucuronidation, N-glucuronidation, and glycination.

DATA SOURCES

Plasma concentrations and renal excretion rates of drugs are measured and renal clearance is calculated in a series of selected pharmacokinetic studies in healthy human volunteers (some studies were conducted in the authors' laboratory and others were reported in the literature). BACKGROUND THEORY: It is generally agreed that the liver plays the dominant role in drug metabolism, and that the function of the kidneys is limited to excretion of parent drug and metabolites. This can be easily understood when a metabolite is present in both plasma and urine. When the metabolite is present in urine but is not measurable in plasma, then the possibility exists that the metabolite is formed by the kidneys.

RESULTS

"Simple" excretion by the kidneys is demonstrated for sulfatroxazole/sulfamethoxazole. Ether glucuronides of codeine are formed in the liver, and the resulting glucuronide is excreted by the kidneys. Possible formation of N1- and N2-glucuronides by the kidneys is demonstrated for sulfadimethoxine, sulfametomidine, and sulfaphenazole. Acyl glucuronidation of probenecid and nalidixic acid is carried out by the kidneys. The acyl glucuronidation of probenecid shows a capacity-limited formation/excretion rate of 46 mg/h, which is subject dependent. During this process, the acyl glucuronidation of co-administered nalidixic acid is reduced from 53 to 16 percent compared with that of nalidixic acid alone. Probenecid and its acyl glucuronidation do not inhibit the ether glucuronidation of codeine in the liver, but only interfere with the active tubular secretion process. The acyl glucuronidation of the nonsteroidal antiinflammatory drug naproxen and its metabolite, O-desmethylnaproxen, may be carried out by the liver and kidneys. Glycination of benzoic acid and salicylic acid is carried out in both the liver and kidneys.

CONCLUSIONS

It is difficult to recognize renal drug metabolism in the intact human body (in vivo); the glucuronides or conjugates must be measured via direct HPLC analysis. In cases where the metabolite is present in high concentrations in urine but not in blood, there may be an indication that the kidneys are responsible for the formation of the metabolite. Impaired kidney function not only affects renal excretion but may also affect renal metabolism.

Kidney-selective prodrugs of 6-mercaptopurine: biochemical basis of the kidney selectivity of S-(6-purinyl)-L-cysteine and metabolism of new analogs in rats

Recently, we have reported that S-(6-purinyl)-L-cysteine (PC) is a kidney-selective prodrug of 6-mercaptopurine. In the present study, the in vivo metabolism of PC and the biochemical basis of its renal selectivity were further investigated. In addition, several PC analogs were synthesized and evaluated as prodrugs of 6-mercaptopurine by determining the concentrations of 6-mercaptopurine and its metabolites, 6-methylmercaptopurine and 6-thiouric acid, in urine after rats were given the analogs. At 30 min after PC treatments, kidney metabolite concentrations were dependent on the PC dose at 40 to 130 mumol/kg and were not increased when a 400 mumol PC/kg dose was given. At the 400 mumol PC/kg dose, metabolite concentrations in the kidneys were higher at 30 min than at 1 or 3 hr, and were nearly 2.5- and 100-fold higher than those in liver and plasma, respectively. Rates of PC in vitro metabolism by liver and kidney cytosolic cysteine conjugate beta-lyases (beta-lyases) were similar, but metabolism by renal mitochondrial beta-lyase occurred at a 3-fold higher rate than the rate obtained with hepatic mitochondrial beta-lyase. When rats were given aminooxyacetic acid (500 mumol/kg) or probenecid (270 mumol/kg) before PC (130 mumol/kg), total kidney metabolite concentrations were reduced by 55 and 36%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid high-performance liquid chromatographic assay for the simultaneous determination of probenecid and its glucuronide in urine. Irreversible binding of probenecid to serum albumin

A reversed-phase high-performance liquid chromatographic (HPLC) assay for the simultaneous determination of probenecid and its glucuronide in urine has been developed. The genuine glucuronide conjugate was isolated from urine by the use of solid-phase extraction on Amberlite XAD-2 and finally purified by the use of preparative HPLC on a Sepharon Hema 1000 RP-18 column. The purity of the product obtained was 88.9%. The isolated glucuronide was used as a standard sample. Of a p.o. dose of 500 mg to two volunteers, 26 and 29% were excreted as the ester glucuronide, while 1.0 and 2.7% were excreted unmetabolized. The stability of the ester glucuronide was investigated in aqueous buffers, buffered urine and human serum albumin solutions. The glucuronide was unstable in neutral and mildly alkaline solutions, and special precautions have to be taken during sampling and sample treatment in order to preserve the genuine glucuronide. The presence of human serum albumin in the solution stabilized the glucuronide against isomerization/rearrangements but catalysed the hydrolysis of the glucuronide. When incubating human serum albumin with the ester glucuronide, probenecid was shown to be covalently bound to the protein probably via a transacylation reaction.

Penetration of vascular contrast media into brain tissue and cerebrospinal fluid. An experimental study in rats

Recent studies suggest that intravenously administered contrast media (CM) penetrate into cerebrospinal fluid (CSF), and the concentration of CM in CSF increases significantly following acetazolamide or probenecid pretreatment. However, corresponding levels of CM in brain tissue and extracellular fluid (ECF) have not been reported. Adult anesthetized rats were injected intravenously with sodium/meglumine diatrizoate and iohexol. The control group received no pretreatment, and the pretreated group received acetazolamide and probenecid before the CM. The amount of each contrast agent was measured in brain tissue and in CSF by high performance liquid chromatography. Pretreated animals attained significantly higher CSF concentrations of diatrizoate and iohexol than control animals. However, tissue ECF concentrations in pretreated animals were not significantly different than in control animals for either agent. The results are consistent with the idea that a flushing action of CSF helps to remove CM from the brain ECF.

Hepatic and renal clearances of probenecid in the rat

The elimination kinetics of probenecid in the rat was studied by using an in vitro liver perfusion system to estimate its basic elimination and in vivo to estimate its renal excretion by catheterization of the urinary bladder. In the liver perfusion study different amounts of probenecid were added to the perfusion medium yielding different initial concentrations (40, 200 and 400 micrograms/ml), and 4 different intravenous bolus doses (50, 75, 100 and 200 mg/kg) were administered in vivo to rats in order to evaluate the renal excretion. The results obtained were described by one-compartment models, with Michaelis-Menten elimination by the liver Vm = 67.4 +/- 14.0 (SD) micrograms/min and a slightly decreasing Km with increasing initial concentrations [from 76.5 +/- 9.0 to 53.2 +/- (SD) 18.4 micrograms/ml]. The excretion by the kidney was characterized by a saturable pathway in parallel with first-order elimination, the maximum rate of the active transports Tm = 0.04 +/- (SD) 0.09 micrograms/min, Km,r = 100.3 +/- (SD) 12.3 micrograms/ml and a linear renal clearance of 0.0008 +/- (SD) 0.0002 ml/min.

Clavulanate-potentiated amoxycillin: in vitro antibacterial activity and oral bioavailability in calves

The minimal inhibitory concentrations (MIC) of amoxycillin and clavulanate-potentiated amoxycillin (amoxycillin:clavulanic acid, 4:1 by weight) were compared for 171 Salmonella, 170 Escherichia coli, and 32 Pasteurella isolates recovered from infected neonatal calves. In the presence of clavulanic acid, the MIC of amoxycillin was reduced to levels less than or equal to 12.5 micrograms/ml for all the Salmonella group B, all the Pasteurella, and for 12 out of the 44 E. coli isolates which were resistant to amoxycillin (MIC greater than or equal to 100.0 micrograms/ml). For isolates sensitive to amoxycillin (MIC less than or equal to 1.56 microgram/ml) there was no change in MIC values in the presence of clavulanic acid. A small proportion of Salmonella and E. coli isolates were resistant to clavulanate-potentiated amoxycillin. In a cross-over trial involving 10 preruminant (2 weeks old) calves, amoxycillin trihydrate and clavulanate-potentiated amoxycillin were administered orally at 10 mg/kg. An analysis of serum amoxycillin level data showed that the pharmacokinetic parameters t1/2ab, Cmax, t1/2 beta, AUC, Cp degree, and f' (estimated drug absorption ratio) were the same after treatment with amoxydrate and clavulanate-potentiated amoxycillin. Administration of clavulanate-potentiated amoxycillin and probenecid resulted in elevation and prolongation of serum amoxycillin levels. Computations showed that in preruminant calves serum amoxycillin concentrations sufficient to inhibit sensitive pathogens can be maintained by oral clavulanate-potentiated amoxycillin treatment at 10 mg/kg TID. At two times that dose rate serum drug concentrations capable of inhibiting 50% of all types of pathogens examined can be maintained.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinations of 5-hydroxyindoleacetic acid and homovanillic acid in human CSF with monitoring of probenecid levels in CSF and plasma

The accumulation of 5-HIAA and HVA in cerebrospinal fluid (CSF) was studied in eight healthy volunteers after oral administration of probenecid. Simulation indicated that a dose of 4.5 g probenecid should be used to achieve probenecid plasma concentrations between 200 and 400 micrograms/ml. Almost complete inhibition of the active transport of the acidic metabolites was assumed to be obtained at these concentrations. Probenecid 4.5 g was administered in two doses (2.5 g and 2 g), separated by 4 h. Plasma samples were drawn at varying intervals over a period of 46 h and lumbar puncture (LP) was performed at either 14 h or 20 h after the first administration of probenecid. The concentration of probenecid, 5-HIAA and HVA in CSF was estimated and the probenecid-induced accumulation of 5-HIAA and HVA was compared with their baseline values. There were no statistically significant differences (P greater than 0.05) in the accumulation of the monoamine metabolites between the two LP (14 h and 20 h), neither were there any differences in CSF concentrations of probenecid at the time of LP. There were only small differences in probenecid plasma concentrations, although statistically significant. Due to maximum blockade of the active transport system no correlation was observed between the CSF concentration of probenecid and the induced accumulation of 5-HIAA and HVA, respectively. The range of probenecid-induced accumulation for 5-HIAA and HVA in these volunteers was 156-429% and 183-600%, respectively. The suggested monitoring of probenecid plasma levels is proposed as a suitable model to investigate central neuronal activity of dopamine and serotonin in the central nervous system.

Non-linear elimination and protein binding of probenecid

Six healthy volunteers were given probenecid 0.5, 1 and 2 g p.o. and 0.5 g i.v. The protein binding of probenecid at different concentrations in human plasma was estimated by equilibrium dialysis. The free fraction was found to increase nonlinearly with increasing total probenecid concentration, up to a maximum free fraction of 26%. The plasma concentration-time data after the oral doses were described by a one-compartment open model with first-order absorption and Michaelis-Menten elimination. The mean absorption rate constant 0.0072 min-1 was dose-independent, and the maximal rate of elimination (mean 1429 micrograms/min) did not differ between doses whether calculated from the total or free concentrations. The Michaelis-Menten constant constant decreased significantly from 67.1 to 55.5 micrograms/ml as the dose increased from 1 g to 2 g, while the unbound Michaelis-Menten constant remained unchanged. The elimination of probenecid after the 0.5 g dose was in the linear region of the Michaelis-Menten elimination when calculated from the total and the free concentrations. The volume of distribution increased only slightly from 9.5 to 11.41 as the dose increased from 0.5 to 2 g, but the unbound volume of distribution decreased significantly from 164 to 99 1. Absorption was complete and was independent of the dose administered.

Probenecid disposition by parallel Michaelis-Menten and dose-dependent pseudo-first-order processes

Re-evaluation of published data on the urinary excretion of probenecid [4-(dipropylamino)sulfonylbenzoic acid, 1] and its metabolites as a function of orally administered dose has revealed that the elimination process is comprised of five parallel pathways. Excretion of the major metabolite, the acyl glucuronide 2 (35-45% of dose), follows Michaelis-Menten kinetics. The three oxidized metabolites, the mono-N-propyl, carboxylic acid, and secondary alcohol derivatives 3, 4, and 5, respectively, (totaling 30% of dose), each adhere to pseudo-first-order kinetics in which the elimination rate constant, as a result of product inhibition, is a function of the administered dose. Four to 13% of the dose is eliminated unchanged in apparent first-order fashion. A mathematical relationship between elimination half-life and dose, under conditions of product inhibition, is derived. Computer simulation of the elimination process confirms the experimental observation that the fraction of dose excreted in the form of each metabolite remains relatively constant over the dose range of 0.5 to 2 g. This study is believed to represent the first application of parallel Michaelis-Menten and dose-dependent pseudo-first-order processes to drug disposition. It demonstrates that the observation of constant proportions of excreted metabolites over a relatively wide range of doses does not provide evidence that the elimination process remains first-order.

Kinetic evidence for a common transport route of benzylpenicillin and probenecid by freshly prepared hepatocytes in rats. Influence of sodium ion, organic anions, amino acids and peptides on benzylpenicillin uptake

Hepatic transport system of benzylpenicillin was characterized by using freshly prepared rat hepatocytes. Uptake of benzylpenicillin and cefpiramide did not require the presence of sodium ion in the incubation medium. No influence of several kinds of amino acids (leucine, histidine, phenylalanine, valine, alanine, glutamic acid and glycine) and peptides (prolyl-leucine, glycyl-glycine, glycyl-sarcosine, glycyl-leucine and gamma-glutamyl-cysteinylglycine) was observed for benzylpenicillin uptake into hepatocytes. Taurocholic acid and cholic acid significantly inhibited benzylpenicillin uptake. The kinetic study revealed that taurocholic acid inhibited benzylpenicillin uptake in a noncompetitive fashion. A similar effect of benzylpenicillin on taurocholic acid uptake was observed, suggesting that the affinity site of the hepatic transport carrier of benzylpenicillin is distinct from that of taurocholic acid. It is noteworthy that p-aminohippuric acid and p-acetylaminohippuric acid did not inhibit benzylpenicillin uptake. In contrast to the mutual inhibition behavior of benzylpenicillin and taurocholic acid, benzylpenicillin is fully and competitively inhibited by the simultaneous addition of probenecid. The inhibition constant Ki value of probenecid was calculated to be 0.322 mM. The uptake of probenecid was also significantly inhibited by benzylpenicillin. It is postulated that the affinity site of benzylpenicillin transport carrier is the same as that of probenecid and that the whole process of the benzylpenicillin transport system is common, at least in part, to the probenecid transport process in the liver.

Pharmacokinetics of probenecid and the effect of oral probenecid administration on the pharmacokinetics of cefazolin in mares

The pharmacokinetics and bioavailability of probenecid given IV and orally at the dosage level of 10 mg/kg of body weight to mares were investigated. Probenecid given IV was characterized by a rapid disposition phase with a mean half-life of 14.0 minutes and a subsequent slower elimination phase with a mean half-life of 87.8 minutes in 5 of 6 mares. In the remaining mare, a rapid disposition phase was not observed, and the half-life of the elimination phase was slower (172 minutes). The mean residence time of probenecid averaged 116 minutes for all 6 mares and 89.2 minutes for the 5 mares with biphasic disposition. The total plasma clearance of probenecid averaged 1.18 +/- 0.49 ml/min/kg, whereas renal clearance accounted for 42.6 +/- 9.3% of the total clearance. The steady-state volume of distribution of probenecid averaged 116 +/- 28.2 ml/kg. Plasma protein binding of probenecid was extensive, with 99.9% of the drug bound at plasma probenecid concentrations of 10 micrograms/ml. The maximum plasma probenecid concentration after 10 mg/kg orally averaged nearly 30 micrograms/ml. The half-life of probenecid after oral administration was approximately 120 minutes. Oral bioavailability was good with greater than 90% of the dose absorbed. The effect of probenecid on tubular secretion of organic anions was evaluated by determining the pharmacokinetics of IV cefazolin (11 mg/kg) administered alone and 15 minutes after probenecid (10 mg/kg orally). Treatment with probenecid did not affect pharmacokinetic values of cefazolin. This failure of probenecid to alter the pharmacokinetics of cefazolin may be caused by insufficient plasma probenecid concentrations after the oral dose.

Pharmacokinetics of probenecid in sheep

Six Merino ewes were given 1 g (27 g/kg) probenecid by the intravenous (i.v.), intramuscular (i.m.) and subcutaneous (s.c.) routes. After i.v. injection, the biological half-life was 1.55 h and apparent volume of distribution at the steady state (Vdss) 0.18 l/kg. Body clearance (ClB) and renal clearance (ClR) were 0.12 l/h/kg and 0.03 l/h/kg, respectively. Approximately 28% of unchanged probenecid was excreted in urine. Plasma probenecid concentrations after i.v., i.m. and s.c. injections were 133, 37, and 31 micrograms/ml, respectively, at 15 min; 76, 36, and 34 micrograms/ml at 1 h; and 43, 23 and 34 micrograms/ml at 2 h. The average bioavailability of probenecid given by i.m. and s.c. injection was 46% and 34%, respectively. However, after 2 h, probenecid plasma concentrations remained higher when it was given subcutaneously than when it was given intramuscularly. Urine output was correlated positively (P less than 0.05) with kel and ClB. Urine pH increased significantly (P less than 0.01) for the first 2 h, and then steadily declined over the subsequent 6 h. The results suggested that probenecid in sheep was rapidly eliminated because it was rapidly excreted in the normal but alkaline urine. Subcutaneous administration of probenecid in animals may be a useful alternative to oral or i.v. administration.

Protein binding as a component of drug interaction in cellular pharmacokinetic studies. Effects of probenecid on transport and accumulation of methotrexate in Ehrlich ascites tumor cells in vitro

The organic acid probenecid has been shown to interfere with the active extrusion of methotrexate (MTX) from L1210 tumor cells in vitro leading to enhanced MTX accumulation and increased formation of MTX polyglutamate derivatives. In the presence of serum albumin (4 g/100 ml), to which probenecid is bound, the inhibition by probenecid of [3H]MTX efflux from the Ehrlich ascites tumor cell was reduced markedly. While half-maximal inhibition of MTX efflux occurred with 0.12 mM probenecid in the absence of albumin, 1.45 mM probenecid was required in its presence. The presence of albumin also modified the probenecid-induced elevation of steady-state MTX levels in the tumor cell. Maximal elevation of cellular MTX levels occurred with 0.5 mM probenecid in the absence of albumin, and 3 mM probenecid in its presence. Serum albumin further reversed the effects of probenecid on MTX influx. While probenecid inhibited influx of 1 microM [3H]MTX in the absence of albumin (half-maximal inhibition at approximately 1 mM probenecid), probenecid stimulated MTX influx in its presence (half-maximal effect at 0.5 to 1 mM). Equilibrium dialysis studies demonstrated that probenecid displaced MTX from albumin, increasing the effective free concentration of MTX in the incubation medium, and hence the rate of MTX influx. Therefore, probenecid may enhance the accumulation of MTX in the tumor cells by increasing the level of free (as opposed to albumin bound) MTX in the extracellular medium as well as by direct inhibition of MTX efflux. These observations may provide an additional explanation for probenecid enhancement of the therapeutic efficacy of MTX in tumor bearing mice and highlight the importance of assessing drug-protein interactions in an in vitro experimental model.

Pharmacokinetics of cefotaxime and probenecid in sheep with normal and reduced renal function

The pharmacokinetics of cefotaxime and probenecid, given by intravenous injection, were determined in six Merino ewes which had been subjected to a 75% reduction in renal mass. These results were compared with results previously determined in sheep with normal renal function. In the sheep with reduced renal mass, the following significant changes in parameter values for cefotaxime were observed. The elimination rate constant (kel) decreased by 47%, the apparent volume of the central compartment (Vc) decreased by 59%, the steady state volume (Vss) decreased by 50%, and the total body clearance (ClB) decreased by 78%. The rate constant for distribution of drug into tissues (k12) increased 6.9 times, the rate constant for distribution out of tissues (k21) increased 3.7 times, and the area under the plasma concentration-time curve (AUC) increased by a factor of 4.9. The parameter values, determined in sheep with reduced renal mass, for probenecid plasma half-life, Vss and the rate constants k12, k21, and kel were not significantly different from the values obtained previously in sheep with normal renal mass. However, the rate constant for renal excretion of probenecid (ke), renal clearance (ClR), ClB and Vc decreased by 79, 90, 54 and 36%, respectively. The results indicate that reduced renal mass increased the plasma half-life for cefotaxime as well as increasing its diffusion into tissue. In the case of probenecid the overall distribution and elimination kinetics were not altered by reduced renal mass; however, the rate of urinary excretion of the drug was reduced.

Dose-dependent kinetics of probenecid in rhesus monkeys--infusion studies

14 infusion studies to steady state with probenecid were carried out in 2 monkeys. Plasma clearance decreased with increasing rate of infusion. The urinary excretion data showed a decrease in the fraction of the dose excreted as probenecid conjugate, an increase in the total N-depropyl metabolites, and no change in the 2-hydroxy metabolites with increasing probenecid dose. Pooled Michaelis-Menten constants for probenecid metabolism were estimated from a plot of the linearized Michaelis-Menten equation using infusion rates of probenecid versus the plasma clearances of the drug.

Coupling between renal tubular secretion and effect of bumetanide

The relationship between renal tubular secretion of bumetanide and its saluretic effect was studied in six healthy subjects before and after probenecid (1 gm IV). Bumetanide was determined in serum and urine by HPLC. Continuous intravenous infusion of bumetanide (200 micrograms/hr) gave an average diuresis at steady state of 15 +/- 3 ml/min. Corresponding plasma concentration, urinary excretion rate, and renal clearance of bumetanide averaged 14.3 +/- 2.3 ng/ml, 64 +/- 31 micrograms/30 min, and 145 +/- 59 ml/min. After probenecid there was a marked change in bumetanide kinetics. Average plasma concentration rose to 41.7 +/- 8.1 ng/ml, whereas renal clearance and urinary excretion rate fell to 15.1% and 29.5% of control. There was also a concomitant decrease in diuresis and saluresis to 47% and about 40% of control. Probenecid also reduced the renal clearance of para-aminohippurate and inulin to 67% and 75% of control. Since the fractional water and sodium chloride excretion was also reduced about 33% and 42%, it is concluded that a large part of the diuretic effect of bumetanide depends on its active tubular secretion. As with furosemide and piretanide, bumetanide diuresis is elicited from the luminal side of the human nephron.

Pharmacokinetics of imipenem in healthy volunteers

The pharmacokinetics of imipenem were evaluated in four studies involving 49 healthy men, several of whom participated in more than one study. Within a dose-range of 150 to 1000 mg, imipenem was found to give high plasma concentrations, proportional to the size of the dose. The half life in the beta-phase was about 1 h in 48 subjects with normal renal function and about 80 min in one subject with a glomerular filtration rate (GFR) of about 50 ml/min/1 X 73 m2. The volume of distribution in the central compartment was about 101. Co-administration of imipenem with probenecid resulted in a slight but significant increase of the plasma half life and a corresponding increase of the area under the plasma concentration curve (AUC). The renal excretion of imipenem was characterized by low urinary recovery (UR) of imipenem. That was in agreement with findings by others that in animals, imipenem undergoes renal metabolism by a dipeptidase, dehydropeptidase I, located to the brush border of the proximal tubular cells. There was a very high degree of between-subject variability of the UR with values varying from about 5% to more than 40% of the dose. Comparing the results obtained after several administrations of imipenem to the same subjects, a small within-subject variability was found. Co-administration of imipenem with inhibitors of the dehydropeptidase MK0789 or MK0791 (cilastatin), resulted in a uniform increase of the imipenem UR to about 70% of the dose irrespective of the degree of metabolism when imipenem was given alone. The effects of the inhibitors on the plasma kinetics of imipenem were an increase of the AUC by about 20% and a proportional decrease of the plasma clearance (VClp) while the plasma half life remained unaffected. Testing various ratios of imipenem and the inhibitors and using incremental data, it could be demonstrated that an increase of the imipenem/cilastatin ratio resulted in a prolonged inhibition of the renal metabolism. Optimal inhibition seemed to be achieved at a ratio of 1:1 between imipenem and cilastatin. A practical consequence of the inhibition of renal metabolism by cilastatin was that high urine concentrations were maintained for longer periods when the combination was given than when imipenem was administered alone. In all subjects, imipenem and the inhibitors were well tolerated and the only adverse reaction observed was nausea during infusion, observed in one subject.

A renal mechanism for the clofibric acid-probenecid interaction

A rabbit model has been used to investigate two hypotheses for the decrease in clofibric acid plasma clearance on probenecid coadministration in man. Rabbits administered probenecid (10 mg/kg followed by 10 mg/kg/hr) showed a mean reduction from control values of 68% for clofibric acid plasma clearance, 80% for clofibric acid glucuronide renal clearance and an elevation in the area under the plasma clofibric acid glucuronide concentration-time curve of 5.2-fold. These findings support the hypotheses that the decrease in clofibric acid plasma clearance involves the inhibition of the renal secretion of clofibric acid glucuronide by probenecid and the subsequent hydrolysis of the glucuronide as the major mechanism, rather than direct inhibition of clofibric acid glucuronide synthesis by probenecid, but do not exclude a mechanism in which both processes operate.

Effect of probenecid on the pharmacokinetics of ceftizoxime

The effect of probenecid on the pharmacokinetics of ceftizoxime was studied. Twelve healthy male volunteers first received 1 g of ceftizoxime either by iv rapid injection or by im injection. One week later, each subject was again given a similar dose of ceftizoxime, one hour after the ingestion of 1 g of probenecid. Serum and urinary concentrations were determined by a HPLC method. Ceftizoxime half-life was increased by probenecid from 1.7 to 2.3 h in the iv group, and 1.9 to 2.8 h in the im group (P less than 0.05, paired t-test). Probenecid increased the area under the serum concentration-time curve by 49% in both groups. There was no significant change in the volume of distribution following probenecid. This study confirms that ceftizoxime is eliminated by glomerular filtration and tubular secretion and that the latter is inhibited by probenecid.

Pharmacokinetics of sulbactam and ampicillin following oral administration of sultamicillin with probenecid

Eight volunteers each received lg of probenecid followed immediately by 1.5 g sultamicillin, both given by the oral route. Plasma concentrations of ampicillin and sulbactam were measured using a differential bioassay method. An ampicillin mean peak plasma concentration of 23.1 mg/l was found typically 1.5 h after dosing; the mean peak plasma concentration of sulbactam (10.0 mg/l) occurred at the same time. The mean plasma half-life of ampicillin was 1.45 h and that of sulbactam 1.3 h. The mean urinary recovery of ampicillin was 65% and that of sulbactam 62%. Five of the volunteers reported minor changes in bowel habits.

Determinants of bumetanide response in the dog: effect of probenecid

The pharmacokinetics and pharmacodynamics of intravenous bumetanide (0.250 mg/kg), alone (treatment I) and after probenecid pretreatment (treatment II), were studied in four mongrel dogs. Lactated Ringer's solution was administered by vein throughout both treatments at a flow rate of 2 ml/min to avoid fluid and electrolyte depletion. Bumetanide and probenecid concentrations were analyzed by HPLC, sodium by flame photometry, and creatinine by colorimetry. Although the probenecid markedly reduced the plasma and renal clearances of bumetanide, as well as the fraction excreted unchanged in the urine, there was no significant difference between treatments I and II in the 4-hr natriuretic and diuretic responses. However, analysis of the dose-response curves between treatments I and II showed that sodium excretion was better correlated with bumetanide urinary excretion rate than with plasma concentration. The reasons for a poor correlation between treatments during the early time periods are discussed.

5-hydroxytryptamine and depression: a model for the interaction of normal variance with pathology

1 Theories linking 5-hydroxytryptamine (5-HT) with depression are briefly reviewed. The various experimental strategies adopted to investigate this relationship, examination of autopsy data, CSF metabolite data, 5-HT re-uptake patterns in human blood platelets and imipramine binding studies in human platelets, are discussed. 2 Recent studies of 5-hydroxyindole acetic acid (5-HIAA) levels in cerebrospinal fluid have revealed a linkage between low 5-HIAA levels and suicide, aggression and impulsivity. Decreases in the number of imipramine binding sites have also been found in brains of suicide victims. 3 The available data lead to the conclusion that decreased 5-hydroxytryptaminergic function may be associated with an increased risk of depression, suicide, and some types of aggression.

Pharmacokinetics of probenecid following oral doses to human volunteers

The pharmacokinetics of probenecid were examined following single 0.5-, 1.0-, and 2.0-g oral doses to healthy male volunteers. Doses were administered following overnight fast, according to a randomized design. Plasma levels of probenecid were determined by high-pressure liquid chromatography (HPLC), using sulfamethazine as the internal standard. Mean peak probenecid levels of 35.3, 69.6, and 148.6 micrograms/ml were obtained at 3-4 hr following the 0.5-, 1.0-, and 2.0-g doses, respectively. Probenecid levels from the 0.5- and 1.0-g doses declined in apparent monoexponential fashion, with mean elimination half-lives of 4.2 and 4.9 hr. Interpretation of the 2.0-g data by a kinetic model incorporating first-order elimination resulted in a plasma drug half-life of 8.5 hr. When first-order elimination was replaced by a Michaelis-Menten-type function, the mean value of the resulting Vm/Km ratios was 0.20, equivalent to a plasma drug half-life [0.693/(Vm/Km)] of 3.8 hr. Plasma probenecid curves from all three dosages were successfully fitted to the saturable elimination model using nonlinear regression and numerical integration routines. The results suggest that probenecid elimination may be saturable at therapeutic dose levels.

Is there a probenecid sensitive transport system for monoamine catabolites at the level of the brain capillary plexus?

Probenecid inhibits the transport of the small monocarboxylic acids lactate and propionate from blood to brain but does not affect the transport of 5HIAA or HVA. Neither lactate, 5HIAA, HVA, nor probenecid itself inhibits probenecid uptake into brain from blood and neither lactate nor 5HIAA itself inhibits 5HIAA uptake. These results indicate first that probenecid inhibits the lactate carrier but is itself not transported by that carrier and second that 5HIAA and probenecid are independently transported from blood to brain by a low affinity system, probably by diffusion. Preloading animals with both tryptophan and probenecid increased the apparent transport of lactate, probenecid and 5HIAA but not the transport of glucose. This indicates that the transport of 5HIAA, lactate and probenecid from brain to blood involves a common, saturable carrier. These two sets of data indicate that either the brain capillary transport system is asymmetric or that probenecid-inhibited transport of monoamine catabolites from brain occurs at sites other than the capillary transport system of the blood-brain barrier.

Failure of probenecid to alter the pharmacokinetics of ceforanide

This investigation evaluated the effect of probenecid on ceforanide concentrations in eight healthy volunteers. Each volunteer was given 1 or 2 g of ceforanide either alone or with 1 g of probenecid. Concentrations of ceforanide in plasma, urine, and saliva were then measured. Probenecid did not alter the plasma concentrations of ceforanide, nor did it affect the urinary excretion of this agent. Ceforanide was not secreted into saliva in any detectable amount either when administered alone or with probenecid. It is not clear why probenecid has a negligible effect on ceforanide concentrations in plasma. It may be that tubular secretion plays less of a role in the excretion of ceforanide than expected, or that the physical properties of ceforanide prevent probenecid from affecting its excretion.

[Pharmacokinetic interference between furosemide and cephalosporins]

The effect of furosemid was studied on rats in comparison to that of etamid with respect to circulation in the body of some cephalosporins, such as cephalotin, cephaloridin, cephazolin, cephacetrile, cephapirin, cefradin and cefalexin. Furosemid and etamid were administered to the rats intraperitoneally in a dose of 100 mg/kg 30 minutes before exposure to the cephalosporins. Cefalexin was administered intragastrically in a dose of 100 mg/kg and the other cephalosporins were administered intramuscularly in a dose of 20 mg/kg. It was found that furosemid increased the blood levels of all cephalosporins in the rats and especially those of cephaloridine and cephacetrile, the levels being increased 5-6 times. The effect of etamid was analogous but less pronounced. Furosemid significantly decreased the renal excretion of some cephalosporins, i.e. cephaloridine, cephalotin, cephacetrile and cephazolin and had an insignificant effect on renal excretion of the other antibiotics. Etamid did not inhibit renal excretion of the cephalosporins.

Clinical pharmacokinetics of probenecid

A review of the clinical applications and of the disposition of probenecid in man, including drug interactions, is presented. Probenecid is the classical competitive inhibitor of organic acid transport in the kidney and other organs. There are 2 primary clinical uses for probenecid: as a uricosuric agent in the treatment of chronic gout and as an adjunct to enhance blood levels of antibiotics (such as penicillins and cephalosporins). Adsorption of probenecid is essentially complete following oral administration. The drug is extensively metabolised by glucuronide conjugation and by oxidation of the alkyl side chains; oxidation of the aromatic ring does not occur. The half-life of probenecid in plasma (4 to 12 hours) is dose-dependent. Renal excretion is the major route of elimination of the metabolites; excretion of the parent drug is minimal and is dependent on urinary pH. Probenecid and its oxidised metabolites are extensively bound to plasma proteins, mainly to albumin. Tissue concentrations (based on animal studies) are generally lower than plasma concentrations. Most of the drug-drug interactions involving probenecid are due to an effect on the kidney-block of transport of acidic drugs. Similarly probenecid affects the tubular secretion of a number of acidic endogenous substances by the kidney. Probenecid is also involved in the block of transport of acidic metabolites of catecholamines, for example homovanillic and hydroxyindoleacetic acids, in the brain. There are a number of analytical procedures for the assay of probenecid. These are based on spectrophotometry, spectrofluorometry, gas and liquid chromatography and radioimmunoassay.