The significance of the simultaneous determination of carbamazepine and its 10,11-epoxide from plasma and human erythrocytes.

The distribution of carbamazepine and its 10,11-epoxide derivative into human erythrocytes was studied in child and adult epileptics on long-term therapy. Firstly, it was found that carbamazepine penetrated into erythrocytes, where the mean concentration of the drug was in rational agreement with the unbound fraction in plasma. Secondly, the relationship between the percentage of carbamazepine concentration in red cell and the drug level in plasma resembled the relationship between the percentage of unbound carbamazepine and the drug level in plasma, which relation has been described in literature recently. This similarity favors this: by determining the ratio of carbamazepine in red cells to the drug in plasma, the unbound fraction can be deduced. Thirdly, carbamazepine-10,11-epoxide was not found in red cells.