Phase I trial of XR9576 in healthy volunteers demonstrates modulation of P-glycoprotein in CD56+ lymphocytes after oral and intravenous administration.

XR9576 is a novel inhibitor of P-glycoprotein (P-gp) that has been shown to reverse P-gp-dependent multidrug-resistance in tumor cell lines and tumor-bearing animals. Here we report the first i.v. and p.o. administration to healthy volunteers of XR9576 in dose-escalating studies with the aim of investigating its effects on safety, its pharmacokinetics, and a surrogate marker of efficacy. XR9576 was administered as a single dose-upward titration of 0.1, 0.2, 0.5, 1.0, and 2 mg/kg XR9576 i.v. or 50, 100, 200, 500, and 750 mg/volunteer p.o. The surrogate marker for in vivo efficacy examined the accumulation of the P-gp substrate Rhodamine-123 (Rh-123) in P-gp-expressing CD56+ lymphocytes by flow cytometry. Addition of Rh-123 to blood samples from subjects given XR9576 or a placebo demonstrated drug-dependent modulation of P-gp activity. Even at the lowest doses, significant effects were observed on Rh-123 accumulation in CD56+ cells. Maximal effects were seen during the i.v. infusion or 4-6 h after oral administration. As the dose was increased, a concomitant rise in the level and duration of P-gp blockade was observed. A dose of 2.0 mg/kg i.v. and > or = 200 mg/volunteer p.o. gave approximately 100% inhibition of P-gp for in excess of 24 h. All doses of XR9576 were well tolerated. Inhibition increased with XR9576 plasma concentration, and maximal activity was achieved at 150-200 ng/ml XR9576. In conclusion, XR9576 has demonstrated sustained inhibition of P-gp after i.v. and oral administration and, supported by the elimination half-life of about 24 h, XR9576 is being taken into Phase II as a once-daily agent.

The relationship between paroxetine and the sparteine oxidation polymorphism.

The relationship between the selective serotonin reuptake inhibitor paroxetine and the sparteine oxidation polymorphism was investigated in a combined single-dose (30 mg) and steady-state (30 mg/day for 2 weeks) study including a panel of nine extensive metabolizers and eight poor metabolizers of sparteine. The median area under the plasma concentration-time curve (AUC) after the first paroxetine dose was about seven times higher in poor metabolizers than in extensive metabolizers (3910 versus 550 nmol.hr/L), whereas at steady state the median AUCss tau interphenotype difference was only twofold (4410 versus 2550 nmol.hr/L). Plasma half-life and steady-state plasma concentration were significantly longer and higher, respectively, in poor metabolizers than in extensive metabolizers (41 versus 16 hours and 151 versus 81 nmol/L). Paroxetine pharmacokinetics were linear in poor metabolizers and nonlinear only in extensive metabolizers. Sparteine metabolic ratio (MR = 12 hour urinary ratio of sparteine/dehydrosparteine), increased during treatment with paroxetine in subjects who were extensive metabolizers, and after 14 days treatment two extensive metabolizers were phenotyped as poor metabolizers and the remaining extensive metabolizers were changed into extremely slow extensive metabolizers with sparteine MRs of 5.7 to 16.5. The inhibition of sparteine metabolism was rapidly reversed after cessation of paroxetine administration. In the poor metabolizers there were no significant changes in MRs during the study. It is concluded that paroxetine and sparteine metabolism cosegregates, but the interphenotype difference in metabolism was less prominent at steady state than after a single dose, presumably because of saturation of the sparteine oxygenase (CYP2D6) in subjects who were extensive metabolizers. Paroxetine is a potent inhibitor of sparteine oxidation by CYP2D6 in vivo.

A review of the metabolism and pharmacokinetics of paroxetine in man.

Paroxetine is well absorbed from the gastrointestinal tract, and appears to undergo first-pass metabolism which is partially saturable. Consistent with its lipophilic amine character, paroxetine is extensively distributed into tissues. Its plasma protein binding at therapeutically relevant concentrations is about 95%. Paroxetine is eliminated by metabolism involving oxidation, methylation, and conjugation. All of these factors lead to wide interindividual variation in the pharmacokinetics of paroxetine. Renal clearance of the compound is negligible. The major metabolites of paroxetine are conjugates which do not compromise its selectivity nor contribute to the clinical response. Ascending single-dose studies reveal that the pharmacokinetics of paroxetine are non-linear to a limited extent in most subjects and to a marked degree in only a few. Also, steady-state pharmacokinetic parameters are not predictable from single-dose data. In many subjects, daily administration of 20-50 mg of paroxetine leads to little or no disproportionality in plasma levels with dose, although in a few subjects this phenomenon is evident. Steady-state plasma concentrations are generally achieved within 7 to 14 days. The terminal half-life is about one day, although there is a wide intersubject variability (e.g. with 30 mg, a range of 7-65 hours was observed in a group of 28 healthy young subjects). In elderly subjects there is wide interindividual variation in steady-state pharmacokinetic parameters, with statistically significantly higher plasma concentrations and slower elimination than in younger subjects, although there is a large degree of overlap in the ranges of corresponding parameters. In severe renal impairment higher plasma levels of paroxetine are achieved than in healthy individuals after single dose. In moderate hepatic impairment the pharmacokinetics after single doses are similar to those of normal subjects. Paroxetine is not a general inducer or inhibitor of hepatic oxidation processes, and has little or no effect on the pharmacokinetics of other drugs examined. Its metabolism and pharmacokinetics are to some degree affected by the induction or inhibition of drug metabolizing enzyme(s). From a pharmacokinetic standpoint, drug interactions involving paroxetine are considered unlikely to be a frequent occurrence. Data available have failed to reveal any correlation between plasma concentrations of paroxetine and its clinical effects (either efficacy or adverse events).

Safety of ticarcillin disodium/potassium clavulanate.

Clavulanate potentiated ticarcillin contains two components with an established safety record. Ticarcillin used clinically alone and with clavulanate, a component of clavulanate potentiated amoxycillin, given orally or intravenously. No pharmacokinetic interaction occurs between the two components when given together in man or animals in which metabolism is qualitatively similar to man. Safety evaluation studies carried out in the animals established as suitable for studying the toxicology of ticarcillin have shown no unexpected synergistic or antagonistic toxic effects of Timentin not predicted from the toxicological evaluation of clavulanate alone. Reproductive mutagenic, cardiovascular and general pharmacological studies have shown no significant hazard from Timentin. Clinical experience with ticarcillin has been reviewed, and impairment of platelet function, seen at supratherapeutic doses, has been shown in an animal model not to be influenced by clavulanate. The assessment of the safety of ticarcillin and clavulanate for therapeutic use in man is thus comprised of the clinical experience with ticarcillin and parenteral clavulanate in clavulanate potentiated amoxycillin, animal safety evaluation studies, and the clinical experience with Timentin reported in this symposium.

Pseudomonic acid, a new antibiotic for topical therapy.

Pseudomonic acid, a new wide-spectrum antimicrobial agent, was evaluated as a 2% formulation in a cream. Animal studies showed that this formulation was just capable of penetrating the skin. When administered parenterally to animals, pseudomonic acid was converted to inactive metabolites that were quickly eliminated from the body. When pseudomonic acid was applied as a cream to human skin, no sensitization was observed. In an open clinical study, sixty-eight patients with skin infections (mostly superficial conditions such as impetigo, infected eczema, folliculitis, or balanitis) applied pseudomonic acid cream three times a day for 5 days. In fifty patients the infections completely cleared within 2 days of the end of therapy, and considerable clinical improvement was noted in sixteen more. One patient stopped the treatment prematurely due to local burning pain, and one patient could not be evaluated clinically.

Human pharmacokinetics of temocillin (BRL 17421) side chain epimers.

The pharmacokinetics of the side-chain epimers of temocillin were investigated in four healthy male subjects following a single iv dose of temocillin disodium (1 g pure free acid) containing 64.2% R-epimer. Plasma and urinary concentrations of the epimers were determined by hplc methods. The R-epimer was twice as rapidly cleared, had a 23% larger volume of distribution and a 60% shorter beta half-life than the S-epimer. Intermediate values were obtained for total temocillin (from hplc data). The differences in the pharmacokinetic properties of the epimers are most likely the result of different extents of plasma protein binding. In each plasma sample, the free fraction of the R-epimer was higher (up to two-fold) than that of the S-epimer. In a comparison of temocillin pharmacokinetic parameters derived from hplc and microbiological assay data, the values obtained from the latter analyses reflected most closely those for the R-epimer. Further indications that the R-epimer is more microbiologically active against Pseudomonas aeruginosa NCTC 10701 from other assessments of relative antibacterial activity are discussed.

Temocillin. Summary of safety studies.

Temocillin is a novel injectable beta-lactam antibiotic designed for parenteral use. It is active against the majority of Gram-negative bacteria and is stable to a wide range of beta-lactamases. Disposition and metabolic studies on temocillin in animals and man demonstrate that the drug is well distributed throughout the body tissues and will cross the placenta. The major route of elimination is via the kidney (89%) where the drug is excreted unchanged as parent compound, mainly by the glomerular filtration of unbound temocillin. A full toxicological safety evaluation programme has been completed and includes acute, subacute and chronic toxicology in rat and dog, and reproductive and mutagenicity studies. Temocillin was well tolerated in animals at dosages of up to 1000 mg/kg and appears to be without any potential hazard for man in dosages which are well in excess of the proposed therapeutic dose.

Effect of renal function and dialysis on temocillin pharmacokinetics.

Temocillin pharmacokinetics in renal impairment were investigated following an intravenous bolus injection of 15 mg/kg. The 28 patients were divided into 5 groups of varying renal function, from normal to uraemic [including a group being treated with haemodialysis and a group on continuous ambulatory peritoneal dialysis (CAPD)]. The distribution of temocillin into the tissues was not affected by renal dysfunction. Uraemia as compared to normal renal function resulted in a 4.3-fold decrease in temocillin clearance and a 3.1-fold decrease in urinary recovery over 24 hours, as well as a 5- and 3.7-fold increase in the beta half-life and the area under the curve (AUC), respectively. Haemodialysis doubled the serum clearance and halved the beta half-life of temocillin in the uraemic subject, but CAPD over 24 hours eliminated only 8% of the temocillin dose, resulting in a minimal change in pharmacokinetics. Temocillin dosage adjustments in renal failure are proposed.

Human pharmacokinetics and antimicrobial activities of the temocillin epimers.

The pharmacokinetics of the epimers of temocillin were investigated in 4 healthy male subjects following intravenous administration of 1g of temocillin disodium (free acid) which contains a R : S epimer ratio of approximately 65 : 35. The R epimer had a 2-fold greater total plasma clearance, a 23% larger volume of distribution and a shorter beta half-life than the S epimer. Intermediate values were obtained for total temocillin (R + S) from high pressure liquid chromatography (HPLC) data. In each plasma sample, the unbound fraction of the R epimer was generally 2-fold higher than that of the S epimer, which is suggested as the reason for the differences in the pharmacokinetic properties of the epimers. The temocillin pharmacokinetic parameters obtained from the microbiological assay data reflect most closely those for the R epimer derived from HPLC data. The resolved R epimer exhibited twice the potency of the S epimer against the microbiological assay organism Pseudomonas aeruginosa NCTC 10701. However, in tests for antibacterial susceptibility, for instance minimum inhibitory concentration (MIC) determinations involving prolonged incubation, there was little difference in the inhibitory activities of the resolved R and S epimers compared with temocillin (R + S), presumably as a consequence of the epimerization of the individual epimers. In contrast, in rapid tests for bactericidal activity, which minimise the effect of epimerization, the R epimer exhibited greater bactericidal activity than the S epimer.

The pharmacokinetics of temocillin in patients with normal and impaired renal function.

The pharmacokinetics of temocillin was studied in 16 subjects with varying degrees of renal functional impairment. The subjects were divided into three groups, depending on their creatinine clearance: Group A greater than 70 ml/min/1.73 m2; Group B 20-70ml/min/1.73 m2 and Group C less than 20 ml/min/1.72 m2. Following intravenous administration the total serum clearance of temocillin was reduced in patients with renal insufficiency. The distribution of temocillin into the tissues was not affected by renal dysfunction, apparent distribution volumes being 15.11, 16.01 and 13.81 in Groups A, B and C respectively. The change in clearance was reflected in an increased area under the serum concentration versus time curve from 418 mg h/l in Group A to 1301 mg h/l and 1553 mg h/l in Groups B and C respectively. The beta elimination half-life was also prolonged from 3.5 h in Group A 12.7 h in Group B and 17.5 h in Group C. The urinary excretion of temocillin was both delayed and reduced according to the extent of kidney damage. The reduction in temocillin clearance was proportional to the extent of reduction from normal of the clearance of creatinine. This relationship has been used to suggest dosage schedules for temocillin in patients with renal insufficiency.

Interaction of pseudomonic acid A with Escherichia coli B isoleucyl-tRNA synthetase.

Sodium pseudomonate was shown to be a powerful competitive inhibitor of Escherichia coli B isoleucyl-tRNA synthetase (Ile-tRNA synthetase). The antibiotic competitively inhibits (Ki 6 nM; cf. Km 6.3 microM), with respect top isoleucine, the formation of the enzyme . Ile approximately AMP complex as measured by the pyrophosphate-exchange reaction, and has no effect on the transfer of [14C]isoleucine from the enzyme . [14C]Ile approximately AMP complex to tRNAIle. The inhibitory constant for the pyrophosphate-exchange reaction was of the same order as that determined for the inhibition of the overall aminoacylation reaction (Ki 2.5 nM; cf. Km 11.1 microM). Sodium [9'-3H]pseudomonate forms a stable complex with Ile-tRNA synthetase. Gel-filtration and gel-electrophoresis studies showed that the antibiotic is only fully released from the complex by 5 M-urea treatment or boiling in 0.1% sodium dodecyl sulphate. The molar binding ratio of sodium [9'-3H]pseudomonate to Ile-tRNA synthetase was found to be 0.85:1 by equilibrium dialysis. Aminoacylation of yeast tRNAIle by rat liver Ile-tRNA synthetase was also competitively inhibited with respect to isoleucine, Ki 20 microM (cf. Km 5.4 microM). The Km values for the rat liver and E. coli B enzymes were of the same order, but the Ki for the rat liver enzyme was 8000 times the Ki for the E. coli B enzyme. This presumably explains the low toxicity of the antibiotic in mammals.

Inhibition of isoleucyl-transfer ribonucleic acid synthetase in Escherichia coli by pseudomonic acid.

The mode of action of the antibiotic pseudomonic acid has been studied in Escherichia coli. Pseudomonic acid strongly inhibits protein and RNA synthesis in vivo. The antibiotic had no effect on highly purified DNA-dependent RNA polymerase and showed only a weak inhibitory effect on a poly(U)-directed polyphenylalanine-forming ribosomal preparation. Chloramphenicol reversed inhibition of RNA synthesis in vivo. Pseudomonic acid had little effect on RNA synthesis in a regulatory mutant, E. coli B AS19 RC(rel), whereas protein synthesis was strongly inhibited. In pseudomonic acid-treated cells, increased concentrations of ppGpp, pppGpp and ATP were observed, but the GTP pool size decreased, suggesting that inhibition of RNA synthesis is a consequence of the stringent control mechanism imposed by pseudomonic acid-induced deprivation of an amino acid. Of the 20 common amino acids, only isoleucine reversed the inhibitory effect in vivo. The antibiotic was found to be a powerful inhibitor of isoleucyl-tRNA synthetase both in vivo and in vitro. Of seven other tRNA synthetases assayed, only a weak inhibitory effect on phenylalanyl-tRNA synthetase was observed; this presumably accounted for the weak effect on polyphenylalanine formation in a ribosomal preparation. Pseudomonic acid also significantly de-repressed threonine deaminase and transaminase B activity, but not dihydroxyacid dehydratase (isoleucine-biosynthetic enzymes) by decreasing the supply of aminoacylated tRNA(Ile). Pseudomonic acid is the second naturally occurring inhibitor of bacterial isoleucyl-tRNA synthetase to be discovered, furanomycin being the first.

On the mode of action of pseudomonic acid: inhibition of protein synthesis in Staphylococcus aureus.

The effect of the antibiotic, pseudomonic acid, on the major metabolic processes in Staphylococcus aureus was studied. The primary effect of low concentrations of the antibiotic, leading to bacteriostasis, is inhibition of protein synthesis. Pseudomonic acid also severely inhibits RNA synthesis which can be prevented by chloramphenicol treatment. DNA and cell wall peptidoglycan synthesis are inhibited to a lesser extent and interference with these processes is considered to be a secondary effect. Oxidative phosphorylation as measured by ATP levels was not inhibited.