Ilka Steiner

Influence of functional haplotypes in the drug transporter gene ABCB1 on central nervous system drug distribution in humans

Single nucleotide polymorphisms in the human multidrug‐resistance gene ABCB1 have been reported to be associated with altered expression and function of P‐glycoprotein, an efflux transporter, expressed at the blood‐brain barrier. To test whether certain ABCB1 haplotypes contribute to interindividual differences in central nervous system drug distribution, brain distribution of a model P‐glycoprotein substrate, the calcium channel inhibitor verapamil, was measured by positron emission tomography (PET) in 2 groups of healthy volunteers.

Pharmacokinetics and Pharmacodynamics of Cefpirome in Subcutaneous Adipose Tissue of Septic Patients

ABSTRACT The objective of the present study was to evaluate whether cefpirome, a member of the latest class of broad-spectrum cephalosporins, sufficiently penetrates subcutaneous adipose tissue in septic patients. After the administration of the drug at 2 g, tissue cefpirome concentrations in septic patients (n = 11) and healthy controls (n = 7) were determined over a period of 4 h by means of microdialysis. To assess the antibacterial effect of cefpirome at the target site, the measured pharmacokinetic profiles were simulated in vitro with select strains of Staphylococcus aureus and Pseudomonas aeruginosa. The tissue penetration of cefpirome was significantly impaired in septic patients compared with that in healthy subjects. For subcutaneous adipose tissue, the area under the concentration-versus-time curve values from 0 to 240 min were 13.11 ± 5.20 g · min/liter in healthy subjects and 6.90 ± 2.56 g · min/liter in septic patients (P < 0.05). Effective bacterial growth inhibition was observed in all in vitro simulations. This was attributed to the significantly prolonged half-life in tissue (P < 0.05), which kept the tissue cefpirome levels above the MICs for relevant pathogens for extended periods in the septic group. By consideration of a dosing interval of 8 h, the values for the time above MIC (T > MIC) in tissue were greater than 60% for pathogens for which the MIC was ≤4 mg/liter in all septic patients. The present data indicate that cefpirome is an appropriate agent for the treatment of soft tissue infections in septic patients. However, due to the high interindividual variability of the pharmacokinetics of cefpirome in tissue, dosing intervals of not more than 8 h should be preferred to ensure that susceptible bacterial strains are killed in each patient.

The High-Affinity Binding Site for Tricyclic Antidepressants Resides in the Outer Vestibule of the Serotonin Transporter

The structure of the bacterial leucine transporter from Aquifex aeolicus (LeuTAa) has been used as a model for mammalian Na+/Cl−-dependent transporters, in particular the serotonin transporter (SERT). The crystal structure of LeuTAa liganded to tricyclic antidepressants predicts simultaneous binding of inhibitor and substrate. This is incompatible with the mutually competitive inhibition of substrates and inhibitors of SERT. We explored the binding modes of tricyclic antidepressants by homology modeling and docking studies. Two approaches were used subsequently to differentiate between three clusters of potential docking poses: 1) a diagnostic SERTY95F mutation, which greatly reduced the affinity for [3H]imipramine but did not affect substrate binding; 2) competition binding experiments in the presence and absence of carbamazepine (i.e., a tricyclic imipramine analog with a short side chain that competes with [3H]imipramine binding to SERT). Binding of releasers (para-chloroamphetamine, methylene-dioxy-methamphetamine/ecstasy) and of carbamazepine were mutually exclusive, but Dixon plots generated in the presence of carbamazepine yielded intersecting lines for serotonin, MPP+, paroxetine, and ibogaine. These observations are consistent with a model, in which 1) the tricyclic ring is docked into the outer vestibule and the dimethyl-aminopropyl side chain points to the substrate binding site; 2) binding of amphetamines creates a structural change in the inner and outer vestibule that precludes docking of the tricyclic ring; 3) simultaneous binding of ibogaine (which binds to the inward-facing conformation) and of carbamazepine is indicative of a second binding site in the inner vestibule, consistent with the pseudosymmetric fold of monoamine transporters. This may be the second low-affinity binding site for antidepressants.

Immunomodulatory Effects of Fosfomycin in Experimental Human Endotoxemia

ABSTRACT To evaluate the effect of fosfomycin on proinflammatory cytokines, a bolus of 2 ng of bacterial lipopolysaccharide/kg of body weight was injected intravenously into healthy volunteers. After 2 h, subjects received 8 g of fosfomycin or placebo in a randomized crossover study design. The resulting concentrations of tumor necrosis factor alpha, interleukin-1β (IL-1β), and IL-6 expressed as protein and mRNA levels were almost identical with and without fosfomycin.

A Bioequivalence Study of Two Oral Desmopressin Tablet Formulations

The present study was carried out to test bioequivalence between two different oral desmopressin formulations. Sixty healthy volunteers were enrolled in the study and were randomly assigned to receive the test (T) and reference (R) drug in a two-period two-sequence, crossover, analyst-blinded study design. Subjects received an oral dose of 400 µg of desmopressin acetate separated by a wash-out period of at least 7 days. The area under the concentration-time curve (AUC) over 12 h in plasma and the maximum concentration (Cmax) were compared by analysis of variance (ANOVA) after log transformation. The mean ratios of the T to R drug were within the bioequivalence boundaries with mean values of 1.00 (90% CI: 0.87–1.14) and 1.03 (90% CI: 0.92–1.15) for AUC0–t and AUC0–inf, respectively. For the Cmax, the mean ratio of the T to R drug was 0.97 (90% CI: 0.87–1.08). The rate and the extent of oral desmopressin absorption were identical for both formulations. Hence, the desmopressin test tablet met all bioequivalence criteria of the marketed reference desmopressin tablet.

Do the binding sites of substrates and tricyclic antidepressants overlap on the serotonin transporter

Based on the crystal structure of LeuTAa, a bacterial homologue to the Na+/Cl--dependent neurotransmitter transporters (SLC6 family) from Aquifex aeolicus, it was proposed that inhibitors of the serotonin and norepinephrine transporters (i.e., tricyclic antidepressants, TCAs) bind in the outer vestibule of the transporter and thus occlude access of the substrate to the substrate binding site. This model predicts that inhibitors and substrates are bound to different sites and is inconsistent with the competitive inhibition of TCA binding by substrate. We surmised that the binding sites overlapped and tested this conjecture by using carbamazepine as a ligand for the human serotonin transporter (hSERT). The underlying rationale is the fact that carbamazepine an analogue of imipramine is devoid of the aliphatic side chain. As expected, carbamazepine competes for binding of [3H]imipramine (and blocks uptake of serotonin mediated by hSERT). However, in the presence of carbamazepine, hSERT is still capable of binding substrates such as serotonin and MPP+ (1-methyl-4-phenylpyridinium); this was demonstrated in experiments with [3H]imipramine as the radioligand, where the slope of the Dixon plot for serotonin and MMP+ was increased upon addition of carbamazepine. In contrast, addition of carbamazepine to compounds capable of inducing transport reversal (p-chloro-amphetamine = PCA, 3,4-methylene-dioxymethamphetamine = MDMA) invariably gave parallel shift in the Dixon plot. Based on these observations, we conclude that (i) the ring of tricyclic antidepressants does not occupy the substrate binding site, and that (ii) releasers induce a conformation, which precludes their simultaneous binding together with of transport inhibitors, resulting in competitive inhibition.

Daptomycin elimination by continuous venovenous hemofiltration: in vitro evaluation of factors influencing sieving and membrane adsorption

The present in vitro study set out to determine for the first time the continuous venovenous hemofiltration (CVVH) clearance of the novel lipopetide antibiotic, daptomycin, from human whole blood. Factors influencing daptomycin sieving and membrane adsorption were investigated in an in vitro setting. A recirculation model was established and daptomycin was added to the simulated blood circuit at different concentrations and in different solvent systems. The concentration of daptomycin over time in the modelled blood compartment and in ultrafiltrate was measured by HPLC. Mean sieving coefficients (SCs) of daptomycin over time were 1 ± 0.05, 0.3 ± 0.02 and 0.4 ± 0.03 in Ringer lactate, Ringer lactate containing human albumin and plasma, respectively. The CVVH clearance of daptomycin from whole blood exceeded the physiological clearance in an individual with normal renal function. Adsorption of daptomycin to synthetic surfaces proved moderate, resulting in loss of around 20% of the initial dose at 1 hour after the start of CVVH. Since mean SCs of daptomycin in protein-containing media were higher than the free fraction in plasma, our results suggest that besides protein binding, unknown factors such as intracellular drug partitioning influence sieving of daptomycin during hemofiltration. Due to the high in vitro CVVH clearance of daptomycin we determined from human whole blood, we recommend monitoring of daptomycin concentrations in patients undergoing hemofiltration. from 13th Scientific Symposium of the Austrian Pharmacological Society (APHAR). Joint Meeting with the Austrian Society of Toxicology (ASTOX) and the Hungarian Society for Experimental and Clinical Pharmacology (MFT) Vienna, Austria. 22–24 November 2007