Transport mechanisms for the antidepressant citalopram in brain microvessel endothelium

Progress and limitations in the use of in vitro cell cultures to serve as a permeability screen for the blood-brain barrier

A relatively simple, widely applicable, and robust in vitro method of predicting blood-brain barrier (BBB) permeability to central nervous system-acting drugs is an increasing need. A cell-based model offers the potential to account for transcellular and paracellular drug diffusional processes, metabolism, and active transport processes, as well as nondefined interactions between a drug and cellular material that may impact upon a membrane's overall permeability profile. Any in vitro BBB cell model to be utilized for the transendothelial BBB permeability screening of potential central nervous system drugs must display reproducible solute permeability, and a number of other general criteria including: a restrictive paracellular barrier; a physiologically realistic cell architecture; the functional expression of key transporter mechanisms; and allow ease of culture to meet the technical and time constraints of a screening program. This article reviews the range of in vitro cell-based BBB models available, including the primary/low passage bovine and porcine brain endothelial cultures as well as the spectrum of immortalized brain endothelial cell lines that have been established. The article further discusses the benefits and limitations of exploiting such systems as in vitro BBB permeability screens.

Review of pharmacokinetic and pharmacodynamic interaction studies with citalopram

Citalopram is a selective serotonin reuptake inhibitor that is N-demethylated to N-desmethylcitalopram partially by CYP2C19 and partially by CYP3A4 and N-desmethylcitalopram is further N-demethylated by CYP2D6 to the likewise inactive metabolite di-desmethylcitalopram. The two metabolites are not active. The fact that citalopram is metabolised by more than one CYP means that inhibition of its biotransformation by other drugs is less likely. Besides citalopram has a wide margin of safety, so even if there was a considerable change in serum concentration then this would most likely not be of clinical importance. In vitro citalopram does not inhibit CYP or does so only very moderately. A number of studies in healthy subjects and patients have confirmed, that this also holds true in vivo. Thus no change in pharmacokinetics or only very small changes were observed when citalopram was given with CYP1A2 substrates (clozapine and therophylline), CYP2C9 (warfarin), CYP2C19 (imipramine and mephenytoin), CYP2D6 (sparteine, imipramine and amitriptyline) and CYP3A4 (carbamazepine and triazolam). At the pharmacodynamic level there have been a few documented cases of serotonin syndrome with citalopram and moclobemide and buspirone. It is concluded that citalopram is neither the source nor the cause of clinically important drug-drug interactions.

In vivo steady-state pharmacokinetic outcome following clinical and toxic doses of racemic citalopram to rats

The thymoleptic drug citalopram (CIT) belongs to the selective serotonin reuptake inhibitors (SSRIs) and is today extensively used in psychiatry. Further clarification of the enantiomer-selective distribution of racemic CIT in both clinical and toxic doses is highly warranted. By a steady-state in vivo paradigm, rats underwent chronic systemic exposure for 10 days by using osmotic pumps and the total as well as the individual distributions of the S- and R-enantiomers of CIT, and its metabolites in serum and two different brain regions, were analysed. In serum, the S/R ratios in the groups treated with 10, 20, or 100 mg kg(-1) day(-1) were 0.94, 0.83, and 0.34, respectively. The ratios were almost the same in the brain regions. In the group treated with 100 mg kg(-1) day(-1), the serum and brain total CIT levels were found to be 20 times and 6 - 8 times higher than in the rats treated with 10 or 20 mg kg(-1) day(-1), respectively. In all groups, the CIT levels were higher in brain tissue as compared to serum. In a spontaneous open-field behavioural test, a correlation between clinical and toxic drug concentrations was observed. In conclusion, the R-enantiomer was present in an increased proportion compared with the S-enantiomer when higher steady-state CIT concentration was prevailing. This is of particular interest, since the S-enantiomer is responsible for the inhibition of serotonin reuptake in vitro. The present data may be of importance, as full understanding on where different racemic or enantiomeric drug effects of CIT and its main metabolites are unravelled.