Comparison of the pharmacokinetics of tacrolimus and cyclosporine at equivalent molecular doses.

Even though calcineurin inhibitors, namely Tacrolimus (FK) and Cyclosporine (CsA) share similar physicochemical properties and a common mechanism of action, their pharmacokinetics (pk) are different and unpredictable. Both drugs are metabolized by cytochrome P450-3A4 isoforms in the liver and in the mucosa of the upper gastrointestinal tract. FK in clinical practice is given in doses up to 50-fold lower than those of CsA due to its greater potency. It is often assumed that the diverse dosing contributes to the observed pharmacokinetic differences between the two drugs. The objective of the present study was to compare single-dose pk profiles of the two drugs, following oral and intravenous administration, on the basis of equivalent molecular dosing, thus ruling out the quantitative factor. Five healthy volunteers and 14 dialysis patients (7 hemodialysis, 7 peritoneal dialysis) were included in the study. Comparing the pharmacokinetic parameters obtained from the drugs, it appeared that cyclosporine has an greater primary volume of distribution and clearance rate compared to tacrolimus. No other statistically significant differences were observed regarding bioavailability, absorption rate, or elimination rate. The only significant correlation between the pk values of the drugs was in primary volume of distribution. We conclude that even at equivalent molecular doses the pk of each drug remains unique and unpredictable. Furthermore our data fail to reveal significant correlations between the bioavailability, clearance, absorption, and elimination rates of the two drugs.

Reduction of organochlorine contaminants from fish oil during refining.

Samples of crude fish oil have been refined, and the crude fish oil together with samples taken out after each step of the refining process have been analysed for organochlorine pesticides and PCB (quantified both as CB congeners and total PCB). The levels of organochlorine contaminants in fish oils remain almost constant during the neutralisation and bleaching steps of the refining process. The deodorisation step seems to cause a decrease in the amount of contaminants, especially for the most volatile compounds (alpha-HCH, lindane, HCB) where the levels were reduced to below the detection level. Concentrations of the less volatile organochlorine pesticides (dieldrin, p,p'-DDE and p,p'-DDD) and PCB are reduced to about half the concentration in the crude fish oil.

Plasma concentration of topiramate correlates with cerebrospinal fluid concentration.

The authors examined the ratio between the plasma and the cerebrospinal fluid (CSF) concentration of topiramate in 14 adults with epilepsy. Simultaneous trough samples of venous blood and CSF were collected and analyzed as total and unbound concentrations. Concomitant levels were also analyzed of lamotrigine (n = 5) and the relevant oxcarbazepine metabolite, 10-hydroxycarbazepine (n = 3). There was a close correlation between the plasma and the CSF concentration for both the total and unbound concentration of topiramate. The median CSF/plasma ratio of total topiramate was 0.85. The free topiramate concentration in plasma was not different from the free topiramate concentration in CSF. The CSF/plasma ratios showed little variation and were independent of the plasma level for both the total and the unbound levels. The unbound fraction of topiramate was 84% in plasma and 97% in CSF. The CSF concentrations of lamotrigine and 10-hydroxycarbazepine were 50% and 61% of the plasma concentrations, respectively. For topiramate, there is a close correlation between the plasma concentration and the CSF concentration. There does not seem to be a saturable carrier mechanism restricting topiramate transport across the blood-brain barrier. The concentration of topiramate in CSF is equal to the unbound proportion of topiramate in plasma, implying that the delivery of topiramate to the brain occurs via transfer from the unbound plasma pool. Plasma is thus a relevant matrix for therapeutic drug monitoring of topiramate.