Synovial fluid concentration is considered to be an important determinant of clinical response to non-steroidal anti-inflammatory drugs (NSAIDs). Trans-synovial transport of these drugs is a process of limited diffusion, governed partly by the pharmacological characteristics of NSAIDs and partly by the properties of the joint and joint space themselves. The studies which report simultaneous pharmacokinetics of NSAIDs in both plasma and synovial fluid compartments are of 2 types: (1) some compare the concurrent concentrations of drugs in plasma and joint fluid after a single administration. These provide pharmacokinetic information: (2) others, which conform more closely to the therapeutic conditions, look at synovial fluid and plasma concentrations after repeated administration of the drug. Recent findings on the pharmacokinetics of NSAIDs in synovial fluid are reviewed. These studies reveal 2 types of NSAIDs, according to their pharmacokinetic behaviour. First, there are NSAIDs with a short or intermediate plasma elimination half-life. These drugs equilibrate rapidly relative to their elimination; their peak synovial fluid concentrations occur later and are lower than those in plasma. Several hours after administration there is crossover of the concentration curves, and beyond this point, concentrations in synovial fluid may exceed those in plasma. During prolonged treatment, the synovial fluid concentrations of these NSAIDs fluctuate to a much lesser extent than plasma concentrations. Secondly, there are NSAIDs with a long plasma elimination half-life; their peak concentration in synovial fluid is also lower and later than that in plasma. At steady-state their concentrations (total and free) in synovial fluid are about half those in plasma. Numerous variables must be taken into account in attempts to correlate synovial fluid NSAIDs concentrations with clinical response, including protein binding and determination of both active metabolites and (eventually) the enantiomers.
A novel liquid chromatographic method for the determination of thiamine in plasma has been developed and has been used to study plasma thiamine concentrations after multiple dosage regimens for 11 days. The method involves purification, concentration and analytical separation of thiochrome on-line, using a switching column system. Ten healthy men were given 500 mg thiamine i.m. once a day (Group 1) and ten were given 250 mg p.o. every 12 h (Group 2). The times to reach steady state (7 and 5.6 days for Groups 1 and 2, respectively) were not different (P greater than 0.05). The mean elimination half-life was 1.8 days. The mean minimum steady-state concentration after the oral regimen (23 micrograms.l-1) was 78% of that after the intramuscular regime (29 micrograms.l-1).
The effect of food intake on the pharmacokinetics of DEPAKINE CHRONO 500 mg (Sanofi, France), a sustained release formulation containing 333 mg sodium valproate and 145 mg valproic acid, was studied in 12 young healthy female volunteers. Relative to fasting conditions (F), when the tablet was given at the midpoint of the breakfast (NF), the maximum concentration (F: 34.6 +/- 8.9 micrograms ml-1 and NF: 40.9 +/- 7.3 micrograms ml-1; p = 0.014) and the mean cumulative amount absorbed up to time 6 h (F: 76.3 +/- 11.8% and NF: 90 +/- 10.4%; p = 0.0099) were significantly increased. Nevertheless, the extent of absorption (F: 46.7 +/- 9.9 mg l-1; NF: 48.7 +/- 7 mg l-1) was not significantly affected. There was no change in the area under the curve (1129 micrograms.h ml-1), in the mean residence time (28 h), or in the elimination half-life (16 h). On the basis of this study, the question as to whether DEPAKINE CHRONO should be administered to subjects in the fasting or non-fasting state would not appear to be a major consideration when deciding on the regimen.
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