Inhibition of Fosfomycin Resistance Protein FosB from Gram-Positive Pathogens by Phosphonoformate.

The Gram-positive pathogen Staphylococcus aureus is a leading cause of antimicrobial resistance related deaths worldwide. Like many pathogens with multidrug-resistant strains, S. aureus contains enzymes that confer resistance through antibiotic modification(s). One such enzyme present in S. aureus is FosB, a Mn2+-dependent l-cysteine or bacillithiol (BSH) transferase that inactivates the antibiotic fosfomycin. fosB gene knockout experiments show that the minimum inhibitory concentration (MIC) of fosfomycin is significantly reduced when the FosB enzyme is not present. This suggests that inhibition of FosB could be an effective method to restore fosfomycin activity. We used high-throughput in silico-based screening to identify small-molecule analogues of fosfomycin that inhibited thiol transferase activity. Phosphonoformate (PPF) was a top hit from our approach. Herein, we have characterized PPF as a competitive inhibitor of FosB from S. aureus (FosBSa) and Bacillus cereus (FosBBc). In addition, we have determined a crystal structure of FosBBc with PPF bound in the active site. Our results will be useful for future structure-based development of FosB inhibitors that can be delivered in combination with fosfomycin in order to increase the efficacy of this antibiotic.

Phosphate Overload Stimulates Inflammatory Reaction via PiT-1 and Induces Vascular Calcification in Uremia.

OBJECTIVE: Vascular calcification (VC) is an important risk factor for cardiovascular disease in maintenance hemodialysis (MHD) patients. Hyperphosphatemia and microinflammation statement are known major contributors to the development of VC; however, the mechanisms are unknown. The aims of this study were to explore the risk factors of VC in MHD patients and to explore whether high phosphate could increase the secretion of inflammatory cytokines via PiT-1 in monocytes. METHODS: A cross-sectional study was conducted on 65 MHD patients to assess the relevance of coronary artery calcification (CAC), inflammatory factors, serum phosphate, and sodium-dependent phosphate cotransporter (NPT) mRNA expression of peripheral blood mononuclear cells (PBMCs). Multivariate logistic regression analysis was used to analyze the predictors of CAC. The calcification effects of high phosphate (HP), TNF-α, and supernatants of healthy human monocytes treated with HP were further evaluated in cultured HASMCs. RESULTS: Diabetes, longer dialysis vintage, higher serum TNF-α levels, and PiT-1 mRNA expression of PBMCs) were independent risk factors of CAC in MHD patients. The mRNA levels of PiT-1 in PBMCs were positively correlated with serum phosphate, CAC scores, and Pit-2 mRNA levels of PBMCs. The expressions of TNF-α, IL-6, and PiT-1 in human monocytes were significantly increased in a dose-dependent manner after treatment with HP, which was subsequently inhibited by NPT antagonist phosphonoformic acid. Neither TNF-α alone nor supernatants of monocytes stimulated with HP promoted the expression of osteopontin and Runt-related transcription factor 2 (Runx2) or caused mineralization in human aortic smooth muscle cells, but combined with HP intervention, the calcification effects were markedly increased in human aortic smooth muscle cells and ameliorated by phosphonoformic acid treatment. CONCLUSION: Hyperphosphatemia directly increased the synthesis and secretion of TNF-α by monocytes may via PiT-1 pathway, resulting in elevated systemic inflammatory response, which may further aggravate VC induced by phosphate overload in MHD patients.

Foscarnet-related Hypercalcemia During CMV Treatment in an Infant With SCID: A Case Report and Review of Literature.

Foscarnet is a main treatment for disseminated cytomegalovirus infection in immunocompromised patients. One of its documented side effects is hypocalcemia. Hypercalcemia, in contrast, was described anecdotally before, almost exclusively in adults with human immunodeficiency virus infection or posttransplantation. We describe a case of severe hypercalcemia during foscarnet treatment in an infant with IL-7 Rα deficient severe combined immunodeficiency, resolved after treatment cessation. We speculate that this unusual side effect is caused by foscarnet binding to the inorganic matrix of bone.

Glutathione-S-transferase FosA6 of Klebsiella pneumoniae origin conferring fosfomycin resistance in ESBL-producing Escherichia coli.

OBJECTIVES: The objectives of this study were to elucidate the genetic context of a novel plasmid-mediated fosA variant, fosA6, conferring fosfomycin resistance and to characterize the kinetic properties of FosA6. METHODS: The genome of fosfomycin-resistant Escherichia coli strain YD786 was sequenced. Homologues of FosA6 were identified through BLAST searches. FosA6 and FosA(ST258) were purified and characterized using a steady-state kinetic approach. Inhibition of FosA activity was examined with sodium phosphonoformate. RESULTS: Plasmid-encoded glutathione-S-transferase (GST) FosA6 conferring high-level fosfomycin resistance was identified in a CTX-M-2-producing E. coli clinical strain at a US hospital. fosA6 was carried on a self-conjugative, 69 kb IncFII plasmid. The ΔlysR-fosA6-ΔyjiR_1 fragment, located between IS10R and ΔIS26, was nearly identical to those on the chromosomes of some Klebsiella pneumoniae strains (MGH78578, PMK1 and KPPR1). FosA6 shared >99% identity with chromosomally encoded FosA(PMK1) in K. pneumoniae of various STs and 98% identity with FosA(ST258), which is commonly found in K. pneumoniae clonal complex (CC) 258 including ST258. FosA6 and FosA(ST258) demonstrated robust GST activities that were comparable to each other. Sodium phosphonoformate, a GST inhibitor, reduced the fosfomycin MICs by 6- to 24-fold for K. pneumoniae and E. coli strains carrying fosA genes on the chromosomes and plasmids, respectively. CONCLUSIONS: fosA6, probably captured from the chromosome of K. pneumoniae, conferred high-level fosfomycin resistance in E. coli. FosA6 functioned as a GST and inactivated fosfomycin efficiently. K. pneumoniae may serve as a reservoir of fosfomycin resistance for E. coli.

Effects of Acyclovir, Foscarnet, and Ribonucleotides on Herpes Simplex Virus-1 DNA Polymerase: Mechanistic Insights and a Novel Mechanism for Preventing Stable Incorporation of Ribonucleotides into DNA.

We examined the impact of two clinically approved anti-herpes drugs, acyclovir and Forscarnet (phosphonoformate), on the exonuclease activity of the herpes simplex virus-1 DNA polymerase, UL30. Acyclovir triphosphate and Foscarnet, along with the closely related phosphonoacetic acid, did not affect exonuclease activity on single-stranded DNA. Furthermore, blocking the polymerase active site due to either binding of Foscarnet or phosphonoacetic acid to the E-DNA complex or polymerization of acyclovir onto the DNA also had a minimal effect on exonuclease activity. The inability of the exonuclease to excise acyclovir from the primer 3'-terminus results from the altered sugar structure directly impeding phosphodiester bond hydrolysis as opposed to inhibiting binding, unwinding of the DNA by the exonuclease, or transfer of the DNA from the polymerase to the exonuclease. Removing the 3'-hydroxyl or the 2'-carbon from the nucleotide at the 3'-terminus of the primer strongly inhibited exonuclease activity, although addition of a 2'-hydroxyl did not affect exonuclease activity. The biological consequences of these results are twofold. First, the ability of acyclovir and Foscarnet to block dNTP polymerization without impacting exonuclease activity raises the possibility that their effects on herpes replication may involve both direct inhibition of dNTP polymerization and exonuclease-mediated destruction of herpes DNA. Second, the ability of the exonuclease to rapidly remove a ribonucleotide at the primer 3'-terminus in combination with the polymerase not efficiently adding dNTPs onto this primer provides a novel mechanism by which the herpes replication machinery can prevent incorporation of ribonucleotides into newly synthesized DNA.

Structural Analysis of Substrate, Reaction Intermediate, and Product Binding in Haemophilus influenzae Biotin Carboxylase.

Acetyl-CoA carboxylase catalyzes the first and regulated step in fatty acid synthesis. In most Gram-negative and Gram-positive bacteria, the enzyme is composed of three proteins: biotin carboxylase, a biotin carboxyl carrier protein (BCCP), and carboxyltransferase. The reaction mechanism involves two half-reactions with biotin carboxylase catalyzing the ATP-dependent carboxylation of biotin-BCCP in the first reaction. In the second reaction, carboxyltransferase catalyzes the transfer of the carboxyl group from biotin-BCCP to acetyl-CoA to form malonyl-CoA. In this report, high-resolution crystal structures of biotin carboxylase from Haemophilus influenzae were determined with bicarbonate, the ATP analogue AMPPCP; the carboxyphosphate intermediate analogues, phosphonoacetamide and phosphonoformate; the products ADP and phosphate; and the carboxybiotin analogue N1'-methoxycarbonyl biotin methyl ester. The structures have a common theme in that bicarbonate, phosphate, and the methyl ester of the carboxyl group of N1'-methoxycarbonyl biotin methyl ester all bound in the same pocket in the active site of biotin carboxylase and as such utilize the same set of amino acids for binding. This finding suggests a catalytic mechanism for biotin carboxylase in which the binding pocket that binds tetrahedral phosphate also accommodates and stabilizes a tetrahedral dianionic transition state resulting from direct transfer of CO2 from the carboxyphosphate intermediate to biotin.

Practical agar-based disk potentiation test for detection of fosfomycin-nonsusceptible Escherichia coli clinical isolates producing glutathione S-transferases.

The number of reports concerning Escherichia coli clinical isolates that produce glutathione S-transferases responsible for fosfomycin resistance (FR-GSTs) has been increasing. We have developed a disk-based potentiation test in which FR-GST producers expand the growth inhibition zone around a Kirby-Bauer disk containing fosfomycin in combination with sodium phosphonoformate (PPF). PPF, an analog of fosfomycin, is a transition-state inhibitor of FosA(PA), a type of FR-GST from Pseudomonas aeruginosa. Considering its mechanism of action, PPF was expected to inhibit a variety of FR-GSTs. In the presence of PPF, zone enlargement around the disk containing fosfomycin was observed for FosA3-, FosA4-, and FosC2-producing E. coli clinical isolates. Moreover, the growth inhibition zone was remarkably enlarged when the Mueller-Hinton (MH) agar plate contained 25 µg/ml glucose-6-phosphate (G6P). When we retrospectively tested 12 fosfomycin-resistant (MIC, ≥256 µg/ml) E. coli clinical isolates from our hospital with the potentiation test, 6 FR-GST producers were positive phenotypically by potentiation disk and were positive for FR-GST genes: 5 harbored fosA3 and 1 harbored fosA4. To identify the production of FR-GSTs, we set the provisional cutoff value, 5-mm enlargement, by adding PPF to a fosfomycin disk on the MH agar plates containing G6P. Our disk-based potentiation test reliably identifies FR-GST producers and can be performed easily; therefore, it will be advantageous in epidemiological surveys and infection control of fosfomycin-resistant bacteria in clinical settings.

Inhibition of phosphate transporters ameliorates the inflammatory and necrotic side effects of the nitrogen-containing bisphosphonate zoledronate in mice.

Bisphosphonates (BPs) are pyrophosphate analogs. They are widely used against enhanced bone-resorption in various diseases. Nitrogen-containing BPs (N-BPs) exhibit strong anti-bone-resorptive effects but have inflammatory and necrotic side effects. The non-nitrogen-containing BPs (non-N-BPs) etidronate and clodronate lack such side effects, but their anti-bone-resorptive effects are weak. In mice, etidronate and clodronate reduce the inflammatory/necrotic effects of N-BPs, even those of zoledronate, the N-BP with the strongest anti-bone-resorptive effect yet reported and the highest risk of inflammation/necrosis. Here, to explore the mechanisms underlying this protection, we used a mouse model in which a single reagent or a mixture of two reagents was injected subcutaneously into ear-pinnas. These reagents included zoledronate, four non-N-BPs, pyrophosphate, and inhibitors of various organic-anion-transporters. Pyrophosphate and two of the four non-N-BPs (not etidronate or clodronate) had inflammatory/necrotic effects. These effects were reduced by etidronate and clodronate, but not by phosphonoformate, an inhibitor of two of the three known phosphate-transporter families. Phosphonoformate reduced the inflammatory/necrotic effects of zoledronate, but not those of pyrophosphate or of non-N-BPs. Conversely, pyrophosphate, at non-inflammatory/necrotic concentrations, reduced the inflammatory/necrotic effects of non-N-BPs, but not those of zoledronate. The efficacies of the protective effects against the inflammatory/necrotic effects of zoledronate were clodronate > etidronate > phosphonoformate. These findings suggest that (i) the N-BP zoledronate may enter soft-tissue cells via phosphonoformate-inhibitable phosphate-transporters, (ii) other phosphate-transporters may carry pyrophosphate and inflammatory/necrotic non-N-BPs into such cells, and (iii) etidronate and clodronate inhibit all these transporters, and they ameliorate the side effects of zoledronate by inhibiting phosphonoformate-inhibitable phosphate-transporters.

Region-dependent absorption of faropenem shared with foscarnet, a phosphate transporter substrate, in the rat small intestine.

Faropenem, a penem antibiotic, is orally active despite its hydrophilic nature. However, its intestinal absorption has not yet been characterised in detail. This study was undertaken to determine the factors regulating faropenem absorption using intestinal loops prepared in the rat duodenum, jejunum and terminal ileum. Faropenem disappearance was much greater than that of cefotaxime and meropenem, and faropenem disappeared more extensively from the terminal ileum than from the jejunum or duodenum. In contrast to faropenem, the disappearance of ceftibuten was much greater from the duodenum and jejunum than from the terminal ileum. As the accumulation and enzymatic degradation of faropenem was minimal in the intestinal mucosa, faropenem was considered to enter the portal vein smoothly after its disappearance from the intestinal loops. Faropenem disappearance was not significantly influenced by the presence of monocarboxylic acids, amino acids or bile acid. Dipeptides such as L-carnosine and glycylglycine slightly but significantly lowered faropenem disappearance from the terminal ileum. On the other hand, foscarnet exerted a marked inhibitory effect on faropenem disappearance, but the antiviral agent did not modulate ceftibuten absorption. The present results suggest that faropenem is in part absorbed via a phosphate transporter present in the rat small intestine.

Mechanistic basis of zidovudine hypersusceptibility and lamivudine resistance conferred by the deletion of codon 69 in the HIV-1 reverse transcriptase coding region.

Deletions in the beta 3-beta 4 hairpin loop of human immunodeficiency virus type 1 reverse transcriptase (RT) are associated with the emergence of multidrug resistance. Common mutational patterns involve the deletion of Asp67 (Delta 67) and mutations such as K70R and T215F or T215Y, or the deletion of Thr69 (Delta 69) and mutations of the Q151M complex. Human immunodeficiency virus type 1 clones containing Delta 69 in a multidrug-resistant sequence background, including the Q151M complex and substitutions K103N, Y181C, M184V, and G190A, showed high-level resistance to all tested nucleoside RT inhibitors. In a multidrug-resistant sequence context, the deletion increases viral replication capacity. By itself, Delta 69 conferred increased susceptibility to beta-d-(+)-3'-azido-3'-deoxythymidine (AZT) and beta-l-(-)-2',3'-dideoxy-3'-thiacytidine resistance. Here, we use transient kinetics to show that, in a wild-type sequence background, Delta 69 does not affect the discrimination between AZT triphosphate and 2'-deoxythymidine 5'-triphosphate, but decreases the catalytic efficiency of the incorporation of beta-l-(-)-2',3'-dideoxy-3'-thiacytidine triphosphate relative to 2'-deoxycytidine 5'-triphosphate. In comparison with the wild-type RT, the Delta 69 mutant showed decreased ability to excise primers terminated with AZT monophosphate in the presence of ATP or pyrophosphate (PPi). These data support the role of the excision mechanism in mediating AZT hypersusceptibility. In addition, we demonstrate that the deletion has no effect on resistance to foscarnet (a PPi analogue) on phenotypic and nucleotide incorporation assays carried out with viral clones and recombinant enzymes, respectively. The results of molecular modeling studies suggest that the side chains of Lys65, Asp67, and Lys219 could play an important role in AZT hypersusceptibility mediated by Delta 69, whereas in the absence of Thr69, local structural rearrangements affecting the beta 3-beta 4 and beta 11a-beta 12 loops of the 66-kDa subunit of the RT could reduce the accessibility of the PPi donor to the terminating nucleotide at the 3' end of the primer.

Stable complexes formed by HIV-1 reverse transcriptase at distinct positions on the primer-template controlled by binding deoxynucleoside triphosphates or foscarnet.

Binding of the next complementary dNTP by the binary complex containing HIV-1 reverse transcriptase (RT) and primer-template induces conformational changes that have been implicated in catalytic function of RT. We have used DNase I footprinting, gel electrophoretic mobility shift, and exonuclease protection assays to characterize the interactions between HIV-1 RT and chain-terminated primer-template in the absence and presence of various ligands. Distinguishable stable complexes were formed in the presence of foscarnet (an analog of pyrophosphate), the dNTP complementary to the first (+1) templating nucleotide or the dNTP complementary to the second (+2) templating nucleotide. The position of HIV-1 RT on the primer-template in each of these complexes is different. RT is located upstream in the foscarnet complex, relative to the +1 complex, and downstream in the +2 complex. These results suggest that HIV-1 RT can translocate along the primer-template in the absence of phosphodiester bond formation. The ability to form a specific foscarnet complex might explain the inhibitory properties of this compound. The ability to recognize the second templating nucleotide has implications for nucleotide misincorporation.

Selective excision of chain-terminating nucleotides by HIV-1 reverse transcriptase with phosphonoformate as substrate.

A major mechanism for human immunodeficiency virus 1 (HIV-1) reverse transcriptase (RT) resistance to nucleoside analogs involves the phosphorolytical removal of the chain-terminating nucleotide from the 3'-end of the primer. In this work, we analyzed the effect of phosphonoformate (PFA) and other pyrophosphate (PP(i)) analogs on PP(i)- and ATP-dependent phosphorolysis catalyzed by HIV-1 RT. Our experimental data demonstrated that PFA did not behave as a linear inhibitor but as an alternative substrate, allowing RT to remove AZT from a terminated primer through a PFA-dependent mechanism. Interestingly, in non-terminated primers, PFA was not a substrate for this reaction and competitively inhibited PP(i)- and ATP-dependent phosphorolysis. In fact, binding of PFA to the RT.template/primer complex was hindered by the presence of a chain terminator at the 3'-end of the primer. Other pyrophosphate analogs, such as phosphonoacetate, were substrates for the excision reaction with both terminated and nonterminated primers, whereas pamidronate, a bisphosphonate that prevents bone resorption, was not a substrate for these reactions and competitively inhibited the phosphorolytic activity of RT. As expected from their mechanisms of action, pamidronate (but not PFA) synergistically inhibits HIV-1 RT in combination with AZT-triphosphate in the presence of PP(i) or ATP. These results provide new clues about the mechanism of action of PFA and demonstrate that only certain pyrophosphate analogs can enhance the effect of nucleosidic inhibitors by blocking the excision of chain-terminating nucleotides catalyzed by HIV-1 RT. The relevance of these findings in combined chemotherapy is discussed.

Elevated inorganic phosphate stimulates Akt-ERK1/2-Mnk1 signaling in human lung cells.

Inorganic phosphate (Pi) plays a critical role in diverse cellular functions. Among three classes of sodium/phosphate co-transporters (NPTs), two types have been identified in mammalian lung. The potential importance of Pi as a novel signaling molecule and pulmonary expression of NPTs with poor prognosis of diverse lung diseases including cancer have prompted us to begin to define the pathways by which Pi regulates nontumorigenic human bronchial epithelial cells. Pi activates Akt phosphorylation on Thr308 specifically, and activated signal transmits on the Raf/MEK/ERK signaling. Here, we report that Pi controls cell growth by activating ERK cascades and by facilitating the translocation of Mnk1 from cytosol into nucleus through an Akt-mediated MEK pathway. Sequentially, translocated Mnk1 increases eIF4E-BP1 phosphorylation. As a result, Pi stimulates cap-dependent protein translation. Such Akt-mediated signaling of inorganic phosphate may provide critical clues for treatment as well as prevention of diverse lung diseases.

An in vitro study of the design and development of a novel doughnut-shaped minitablet for intraocular implantation.

A novel doughnut-shaped minitablet (DSMT) was developed and evaluated as a biodegradable intraocular drug delivery system for rate-modulated delivery of antiviral bioactives. The DSMT device was manufactured using a special set of punches fitted with a central-rod in a Manesty tableting press. The DSMT device released the antiretrovirals foscarnet and ganciclovir at a first-order rate. The erosion kinetics was assessed by gravimetric analysis and scanning electron microscopy. The device gradually eroded when immersed in simulated vitreous humor (SVH) (pH 7.4, 37 degrees C) and released bioactives in a sustained manner. The novel geometric design and veracity of the DSMT device was retained even after 24 weeks of erosion. When considering the duration of the bioactive released from the DSMT device, it was found that by the careful selection of the type and concentration of polymer employed in formulating the DSMT device, it was possible to produce a device that could release drug for any period up to 12 months.

Role of helix P of the human cytomegalovirus DNA polymerase in resistance and hypersusceptibility to the antiviral drug foscarnet.

Mutations in the human cytomegalovirus DNA polymerase (UL54) can not only decrease but also increase susceptibility to the pyrophosphate (PP(i)) analogue foscarnet. The proximity of L802M, which confers resistance, and K805Q, which confers hypersusceptibility, suggests a possible unifying mechanism that affects drug susceptibility in one direction or the other. We found that the polymerase activities of L802M- and K805Q-containing mutant enzymes were literally indistinguishable from that of wild-type UL54; however, susceptibility to foscarnet was decreased or increased, respectively. A comparison with the crystal structure model of the related RB69 polymerase suggests that L802 and K805 are located in the conserved alpha-helix P that is implicated in nucleotide binding. Although L802 and K805 do not appear to make direct contacts with the incoming nucleotide, it is conceivable that changes at these residues could exert their effects through the adjacent, highly conserved amino acids Q807 and/or K811. Our data show that a K811A substitution in UL54 causes reductions in rates of nucleotide incorporation. The activity of the Q807A mutant is only marginally affected, while this enzyme shows relatively high levels of resistance to foscarnet. Based on these data, we suggest that L802M exerts its effects through subtle structural changes in alpha-helix P that affect the precise positioning of Q807 and, in turn, its presumptive involvement in binding of foscarnet. In contrast, the removal of a positive charge associated with the K805Q change may facilitate access or increase affinity to the adjacent Q807.

Carbonic anhydrase inhibitors. Interaction of isozymes I, II, IV, V, and IX with phosphates, carbamoyl phosphate, and the phosphonate antiviral drug foscarnet.

A detailed inhibition study of five carbonic anhydrase (CA, EC 4.2.1.1) isozymes with inorganic phosphates, carbamoyl phosphate, the antiviral phosphonate foscarnet as well as formate is reported. The cytosolic isozyme hCA I was weakly inhibited by neutral phosphate, strongly inhibited by carbamoyl phosphate (K(I) of 9.4 microM), and activated by hydrogen- and dihydrogenphosphate, foscarnet and formate (best activator foscarnet, K(A)=12 microM). The cytosolic isozyme hCA II was weakly inhibited by all the investigated anions, with carbamoyl phosphate showing a K(I) of 0.31 mM. The membrane-associated isozyme hCA IV was the most sensitive to inhibition by phosphates/phosphonates, showing a K(I) of 84 nM for PO(4)(3-), of 9.8 microM for HPO(4)(2-), and of 9.9 microM for carbamoyl phosphate. Foscarnet was the best inhibitor of this isozyme (K(I) of 0.82 mM) highly abundant in the kidneys, which may explain some of the renal side effects of the drug. The mitochondrial isozyme hCA V was weakly inhibited by all phosphates/phosphonates, except carbamoyl phosphate, which showed a K(I) of 8.5 microM. Thus, CA V cannot be the isozyme involved in the carbamoyl phosphate synthetase I biosynthetic reaction, as hypothesized earlier. Furthermore, the relative resistance of CA V to inhibition by inorganic phosphates suggests an evolutionary adaptation of this mitochondrial isozyme to the presence of high concentrations of such anions in these energy-converting organelles, where high amounts of ATP are produced by ATP synthetase, from ADP and inorganic phosphates. The transmembrane, tumor-associated isozyme hCA IX was on the other hand slightly inhibited by all these anions.

Paracellular drug transport across intestinal epithelia: influence of charge and induced water flux.

The influence of drug charge and transepithelial water flux on passive paracellular drug transport was investigated in Caco-2 cell monolayers and rat ileal mucosa in vitro. Three small hydrophilic compounds with different net charges (creatinine, erythritol and foscarnet) were used as model drugs. A hypotonic glucose-rich solution was applied apically to induce epithelial absorption of water. In the Caco-2 monolayers, permeability to creatinine (positively charged) was 25-fold greater than to foscarnet (negatively charged), indicating a pronounced cation selective paracellular permeability. During apical exposure to the hypotonic glucose-rich solution, transport of all model drugs increased in both the absorptive and secretory directions. This enhanced transport coincided with a decrease in transepithelial resistance. Further, fluorescence and transmission electron microscopy indicated dilatations of the paracellular spaces but no damage to the cell membranes. These findings suggested that the enhancement in drug transport was attributable to increased paracellular tight junction permeability rather than to "solvent drag". In the ileal segments, mucosal exposure to the hypotonic glucose-rich solution had no effect on transepithelial resistance and only a marginal increase in drug transport was observed. Taken together, the modest absorption enhancement demonstrated in the in vitro models agrees with results obtained in vivo, supporting the conclusion that a more pronounced disruption of the tight junction barrier than that obtained through stimulation of epithelial absorption of water is required for efficient enhancement of paracellular intestinal drug absorption.

Flow cytometric analysis of herpes simplex virus type 1 susceptibility to acyclovir, ganciclovir, and foscarnet.

We established a quantitative flow cytometric method for determination of herpes simplex virus type 1 (HSV-1) susceptibility to acyclovir (ACV), ganciclovir, and foscarnet in vitro. Susceptibility was defined in terms of the drug concentration which reduced the number of cells expressing HSV-1 glycoprotein C (gpC) with a fluorescence intensity of > or =10(2) by 50% (IC50). Flow cytometry allowed us to use a high (1.0) as well as a low (0.005) multiplicity of infection, and determination of the IC50 was possible after one or more viral replicative cycles. IC50s were dependent on virus input and on time postinfection. In mixture experiments, 1 to 2% resistant viruses added to a sensitive strain could be detected. The results obtained by flow cytometry showed a good qualitative correlation with those achieved by cytopathic effect inhibitory assay. However, flow cytometry might detect more quantitative differences in drug susceptibility, especially among resistant strains, as confirmed also by determination of intracellular drug phosphorylation. The mean IC50s for ACV-sensitive strains were 0.45 to 1.47 microM, and those for ACV-resistant strains were between 140 and 3,134 microM. Flow cytometric analysis was fast and accurate, automatizable, and highly reproducible. Flow cytometry may be a more powerful tool than standard cytopathic effect-based assays and could have advantages for the detection of low levels of drug resistance or mixtures of sensitive and resistant virus strains.

Carrier-mediated transport mechanism of foscarnet (trisodium phosphonoformate hexahydrate) in rat intestinal tissue.

New findings are presented on the specific transport mechanisms of foscarnet (trisodium phosphonoformate hexahydrate) in rat small intestinal tissue and proof for the partial participation of the Na(+)-phosphate co-transport system in foscarnet transport. The transport of the free acid form of foscarnet, phosphonoformic acid (PFA), was studied in rat small intestine by applying Ussing chambers. Transport studies in both mucosal (m)-to-serosal (s) and s-to-m directions revealed polarization of PFA transport. In m-to-s studies, nonlinear concentration-dependent transport was observed and described by the following transport parameters (estimate +/- asymptotic standard error): 0.84 +/- 0.13 mumol/h.cm2, 1.13 +/- 0.29 mM and 0.22 +/- 0.05 cm/h for the maximal transport rate (Jmax), the half-maximal transport concentration (Kt) and the passive membrane permeability constant (Pm), respectively. PFA transport (1.0 mM) was reduced to 72% and to 56% in the presence of the structural analogs phosphate and arsenate (10 mM), respectively. Bidirectional transport studies of PFA at 38 degrees C and 4 degrees C revealed a higher decrease in transport rate for the m-to-s studies than for the s-to-m studies. The combined results of the experiments described in this study demonstrate that PFA transport across rat small intestine is partly passive, using both the paracellular and transcellular pathways, and partly carrier-mediated, involving the phosphate co-transport system.

Enhanced bioavailability of phosphonoformic acid by dietary phosphorus restriction.

Phosphonoformic acid (PFA, foscarnet) is a potent inhibitor of Na(+)-P(i) cotransport in intestinal and renal brush border membranes (BBM). We have studied the effect of dietary phosphorus restriction on intestinal PFA absorption and bioavailability. Rats were placed on low (0.04% P(i), LPD) or normal (0.95% P(i), NPD) phosphorus diets for 5 days, followed by administration of an oral bolus of [14C]PFA (100 mg/kg). Of the oral PFA dose, 60 +/- 4% was absorbed in LPD rats, compared with 43 +/- 3% in NPD rats (P < 0.05, N = 5). This was associated with higher plasma PFA concentrations in LPD compared with NPD rats (44.2 +/- 2.0 and 17.9 +/- 4.3 micrograms/mL, respectively). [14C]PFA uptake, determined in intestinal BBM vesicles (BBMV), was Na+ gradient (Na+out > Na+in) dependent. Dietary phosphorus restriction resulted in a 39.8% increase in the initial (1 min) Na(+)-dependent [14C]PFA uptake by intestinal BBMV. We conclude that PFA absorption is enhanced by dietary phosphorus restriction.