A. Van Hecken

In vitro activity and human pharmacokinetics of teicoplanin.

The in vitro activity of teicoplanin, a new antibiotic related to vancomycin, was determined against 456 gram-positive cocci. The activity of teicoplanin in comparison with that of vancomycin was similar against staphylococci but 4 to 40 times higher against enterococci and beta-hemolytic and viridans streptococci. The single-dose pharmacokinetics of teicoplanin were studied in six healthy volunteers after administration of 3 and 6 mg/kg intravenously and of 3 mg/kg intramuscularly. The kinetic parameters after both intravenous doses were very similar. The curves for concentration in plasma for the 3- and 6-mg/kg intravenous doses showed a triexponential decline with elimination half-lives of 47.3 and 44.1 h, respectively. The percentages of the doses recovered in urine (0 to 102 h) were 43.2 and 44.1%, respectively. The areas under the plasma curves were dose related: 256.5 and 520.9 micrograms/h per ml, respectively. The bioavailability of teicoplanin after injection of 3 mg/kg intramuscularly was 90%, and the peak level was 7.1 micrograms/ml. The mean levels in plasma 24 h after the 3-mg/kg doses were 2.1 and 2.3 micrograms/ml, respectively, and the mean level in plasma 24 h after the 6-mg/kg intravenous dose was 4.2 micrograms/ml.

Pharmacokinetics, COX-2 specificity, and tolerability of supratherapeutic doses of rofecoxib in humans.

AbstractObjective: Prostaglandin synthesis is catalyzed by a constitutive cyclo-oxygenase isoform (COX-1) and an inducible isoform (COX-2). It is hypothesized that the analgesic and anti-inflammatory effects of nonsteroidal anti-inflammatory drugs (nonspecific COX-1/COX-2 inhibitors) such as ibuprofen principally derive from COX-2 inhibition. The purpose of this study was to evaluate steady-state pharmacokinetics, biochemical selectivity and tolerability of rofecoxib (VioxxTM), characterized in vitro as a COX-2 inhibitor. Methods: Four panels of healthy men (n=8 per panel) were administered rofecoxib (n=6) (25, 100, 250, 375 mg) or placebo (n=2) once daily on day 1 and days 3–14. Blood samples for assays of rofecoxib plasma concentration and COX isoform activity were obtained pre-dose and at specified time points post-dose. Results: Rofecoxib pharmacokinetics were found to be complex and nonlinear. Elimination half-life ranged from 9.9 h to 17.5 h after multiple dosing with an accumulation ratio close to 2 for all doses. COX-2 inhibitory activity as assessed by average inhibition of whole blood lipopolysaccharide-stimulated prostaglandin E2 over the 8-h post-dose period on day 14 was 0.3, 67, 96, 92 and 96% for the placebo and the 25-, 100-, 250- and 375-mg treatment groups, respectively. No treatment group showed significant inhibition of COX-1 as assessed by thromboxane B2 generation in clotting whole blood. Side effects were mild and transient. Conclusion: The results indicate that rofecoxib is a potent and specific inhibitor of COX-2 in humans even at doses more than tenfold higher than those associated with efficacy in patients with osteoarthritis.

Kinetics of cotinine after oral and intravenous administration to man

SummaryCotinine, the main metabolite of nicotine, was administered intravenously to healthy male non-smoking volunteers in doses of 5, 10 and 20 mg, and orally in doses of 10 and 20 mg.Intravenous administration was characterized by a dose-independent half-life of 12.2 h, mean residence time of 15.9 h, total clearance of 3.64 l h−1 and a volume of distribution of 56.5 l. Renal clearance was 0.46 l h−1 and approximately 12.0% of the dose was excreted unchanged in the urine. The mean absorption time after oral dosing ranged between 1 and 3 h, the peak concentration was reached within 45 min and the mean elimination half-lives were 12.9 and 11.7 h, respectively, after the 10 and 20 mg doses. Systemic bioavailability ranged between 0.84 and 1.11 following 10 mg and between 0.97 and 1.03 following the 20 mg dose. Mean urinary recovery and renal clearance were almost identical with the values after iv administration.

Pharmacokinetics, Pharmacodynamics, and Safety of a Prostaglandin D2 Receptor Antagonist

Laropiprant is a selective antagonist of the prostaglandin D2 (PGD2) receptor subtype 1 (DP1). Three double‐blind, randomized, placebo‐controlled studies evaluated the safety, tolerability, pharmacokinetics, and pharmacodynamics of single and multiple oral doses of laropiprant in healthy male volunteers. Single doses up to 900 mg and multiple doses up to 450 mg were generally well tolerated. Laropiprant exhibited dose‐proportional pharmacokinetics. Oral absorption is rapid (Tmax=0.8–2.0 h) and the terminal half‐life is approximately 12–18 h. The pharmacokinetics of laropiprant was not affected by food. Single doses of 6 mg and higher were effective in suppressing PGD2‐induced cyclic AMP accumulation in platelets, demonstrating laropiprant target engagement with DP1. Laropiprant has detectable off‐target antagonist effects at the thromboxane A2 receptor but no clinically significant effect on collagen‐induced platelet aggregation or bleeding times with multiple doses up to 200 mg.

MRK-409 (MK-0343), a GABAA receptor subtype-selective partial agonist, is a non-sedating anxiolytic in preclinical species but causes sedation in humans.

MRK-409 binds to α1-, α2-, α3- and α5-containing human recombinant GABAA receptors with comparable high affinity (0.21–0.40 nM). However, MRK-409 has greater agonist efficacy at the α3 compared with α1 subtypes (respective efficacies relative to the full agonist chlordiazepoxide of 0.45 and 0.18). This compound readily penetrates the brain in rats and occupies the benzodiazepine site of GABAA receptors, measured using an in vivo [3H]flumazenil binding assay, with an Occ50 of 2.2 mg/kg p.o. and a corresponding plasma EC50 of 115 ng/mL. Behaviourally, the α3-preferring agonist efficacy profile of MRK-409 produced anxiolytic-like activity in rodent and primate unconditioned and conditioned models of anxiety with minimum effective doses corresponding to occupancies, depending on the particular model, ranging from ∼35% to 65% yet there were minimal overt signs of sedation at occupancies greater than 90%. In humans, however, safety and tolerability studies showed that there was pronounced sedation at a dose of 2 mg, resulting in a maximal tolerated dose of 1 mg. This 2 mg dose corresponded to a Cmax plasma concentration of 28 ng/mL, which, based on the rodent plasma EC50 for occupancy of 115 ng/mL, suggested that sedation in humans occurs at low levels of occupancy. This was confirmed in human positron emission tomography studies, in which [11C]flumazenil uptake following a single dose of 1 mg MRK-409 was comparable to that of placebo, indicating that occupancy of GABAA receptor benzodiazepine binding sites by MRK-409 was below the limits of detection (i.e. <10%). Taken together, these data show that MRK-409 causes sedation in humans at a dose (2 mg) corresponding to levels of occupancy considerably less than those predicted from rodent models to be required for anxiolytic efficacy (∼35–65%). Thus, the preclinical non-sedating anxiolytic profile of MRK-409 did not translate into humans and further development of this compound was halted.

Preclinical and clinical pharmacology of TPA023B, a GABAA receptor α2/α3 subtype-selective partial agonist.

In the accompanying paper we describe how MRK-409 unexpectedly produced sedation in man at relatively low levels of GABAA receptor occupancy (∼10%). Since it was not clear whether this sedation was mediated via the α2/α3 or α1 GABAA subtype(s), we characterized the properties of TPA023B, a high-affinity imidazotriazine which, like MRK-409, has partial agonist efficacy at the α2 and α3 subtype but is an antagonist at the α1 subtype, at which MRK-409 has weak partial agonism. TPA023B gave dose- and time-dependent occupancy of rat brain GABAA receptors as measured using an in vivo [3H]flumazenil binding assay, with 50% occupancy corresponding to a respective dose and plasma drug concentration of 0.09 mg/kg and 19 ng/mL, the latter of which was similar to that observed in mice (25 ng/mL) and comparable to values obtained in baboon and man using [11C]flumazenil PET (10 and 5.8 ng/mL, respectively). TPA023B was anxiolytic in rodent and primate (squirrel monkey) models of anxiety (elevated plus maze, fear-potentiated startle, conditioned suppression of drinking, conditioned emotional response) yet had no significant effects in rodent or primate assays of ataxia and/or myorelaxation (rotarod, chain-pulling, lever pressing), up to doses (10 mg/kg) corresponding to occupancy of greater than 99%. In man, TPA023B was well tolerated at a dose (1.5 mg) that produced occupancy of >50%, suggesting that the sedation previously seen with MRK-409 is due to the partial agonist efficacy of that compound at the α1 subtype, and highlighting the importance of antagonist efficacy at this particular GABAA receptor population for avoiding sedation in man.

Calcitonin Gene-Related Peptide8-37 Antagonizes Capsaicin-Induced Vasodilation in the Skin: Evaluation of a Human in Vivo Pharmacodynamic Model

The purpose of this study was to identify the mediators involved in capsaicin-induced vasodilation in the human skin and to evaluate a pharmacodynamic model for the early clinical evaluation of calcitonin gene-related peptide (CGRP) receptor antagonists. Dermal blood flow (DBF) response of the forearm skin to topically applied capsaicin was measured using laser Doppler perfusion imaging in 22 subjects. The effect of intra-arterially administered CGRP8-37 (1200 ng · min–1 · dl–1 forearm), indomethacin (5 μg · min–1 · dl–1 forearm), and NG-monomethyl-l-arginine (l-NMMA; 0.2 mg · min–1 dl–1 forearm), and orally administered aprepitant (375 mg) on capsaicin-induced dermal vasodilation was assessed. Furthermore, the diurnal variation of the DBF response to capsaicin was studied. CGRP8-37 inhibited the capsaicin-induced DBF increase: 217(145, 290)% in infused versus 370 (254, 486)% in the noninfused arm [mean (95% CI); p = 0.004]. In contrast, indomethacin, l-NMMA, aprepitant, and the time of assessment did not affect the DBF response to capsaicin. Thus, capsaicin-induced vasodilation in the human forearm skin is largely mediated by CGRP, but not by vasodilating prostaglandins, nitric oxide, or substance P. The response to capsaicin does not display a circadian rhythm. A pharmacodynamic model is proposed to evaluate CGRP receptor antagonists in humans in vivo.

Effect of clopidogrel on naproxen-induced gastrointestinal blood loss in healthy volunteers.

The effect of clopidogrel, a potent inhibitor of platelet aggregation, on naproxen-induced faecal blood loss was investigated in 30 healthy volunteers in a randomized, double-blind, placebo-controlled, two parallel treatment groups study. All subjects first received naproxen 250 mg b.i.d. during 7 days, after which they were randomly allocated to additionally receive either clopidogrel 75 mg once daily (n = 15) or matching placebo (n = 15) for 11 days. Faecal blood loss was measured by the 51Cr-labelled erythrocyte method during the last four days of each of the four study periods, i.e. baseline, treatment with naproxen alone, combined treatment and wash-out. Mean daily faecal blood loss was below 0.5 ml/day during the baseline period in both treatment groups and increased during treatment with naproxen alone to (mean +/- SD) 1.14 +/- 0.58 ml/day in the naproxen + placebo group and to 1.93 +/- 1.51 ml/day in the naproxen + clopidogrel group. Addition of clopidogrel to naproxen treatment was associated with an increase of the mean daily blood loss to 6.83 +/- 9.32 ml/day, which was statistically significantly higher than 1.75 +/- 1.40 ml/day observed during treatment with naproxen + placebo. During the wash-out period, mean daily blood loss decreased to 0.98 +/- 0.51 ml/day in the naproxen + placebo group and to 1.07 +/- 0.46 ml/day in the naproxen + clopidogrel group. Based on these results, it can be concluded that clopidogrel increases naproxen-induced gastrointestinal blood loss in healthy volunteers. Caution should therefore be called for when these drugs are coadministered.

Comparative pharmacokinetics of ceftazidime and moxalactam.

The pharmacokinetics of ceftazidime and moxalactam were compared after intravenous and intramuscular administration of single 1-g doses to eight healthy volunteers in a crossover study. The bioavailability of the antibiotics after administration by either route was almost complete. Both drugs had similar areas under the serum curves. Significant differences between ceftazidime and moxalactam were observed with respect to the apparent volume of distribution (18.4 and 24.1 liters, respectively), to the terminal half-life (1.6 versus 2.0 h), and to urinary recovery of the active compound (96 versus 79%). Ceftazidime was almost completely eliminated by renal excretion (greater than 96%), whereas about 20% of the moxalactam was eliminated by nonrenal mechanisms. The concentrations of ceftazidime and moxalactam in serum after a 1-g dose exceeded the concentrations required to inhibit 90% of the Enterobacteriaceae for about 8 and 10 h, respectively. The levels of ceftazidime and moxalactam in serum exceeded the 90% minimal inhibitory concentration of Pseudomonas aeruginosa for about 6 and 1 h, respectively.

Effects of enteric-coated, low-dose aspirin on parameters of platelet function

Background : Aspirin is widely used as an anti‐thrombotic drug; however, it has been suggested that enteric‐coated formulations of aspirin may be less bioavailable and less effective as anti‐thrombotic agents.