Techniques for plasma protein binding of demethylchlorimipramine

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Abstract
The cerebrospinal fluid (CSF) and plasma levels of demethylchlorimipramine (DMC1) were determined during treatment of depression or obsessive-compulsive disorders with chlorimipramine. In 18 patients the mean CSF/plasma ratio of DMCI was 2.6% ± 0.7 SD with fourfold variation (1.1% to 4.0%). In spite of this variation, the levels in CSF and plasma at steady state correlated closely (r = 0.91; p < 0.001). With equilibrium dialysis for the determination of the protein binding of DMCI, a much higher free fraction was found in patients (8.0 ± 1.6%) and in control subjects (8.2 ± 1.4%). It was shown that part of the plasma binding capacity was lost during the incubation. Results obtained by ultrafiltration (3.9 ± 1.0% unbound drug) were closer to the in vivo results, but this method also had disadvantages; much of the drug was absorbed on the ultrafiltration dialysis membrane. Our results suggest that there is a need for care in the selection of a technique for studies of drug protein binding.