Association and transfer functions for analysis of drug binding to proteins

Solid-phase microextraction with capillary gas-liquid chromatography and nitrogen-phosphorus selective detection for the assay of antidepressant drugs in human plasma

Solid-phase microextraction (SPME) was tested as a sample preparation for the simultaneous assay of ten antidepressant drugs and metabolites (TADs) in human plasma. Aqueous NaOH (0.5 ml, 1 M) and chloramitriptyline (50 microliters, 40 micrograms/ml) as internal standard (I.S.) were added to a 2-ml plasma sample. This mixture was extracted with a 100-micron polydimethylsiloxane SPME fiber (Supelco) for 10 min. After washing in water and methanol (50%) and subsequent drying at room temperature, desorption of the fiber was performed in the injection port of a gas chromatograph at 260 degrees C for 1 min (HP 5890, DB-17 30 m x 0.25 mm I.D., 0.25 micron capillary; 0.7 ml/min nitrogen; nitrogen-phosphorus selective detection). The recovery was found to be very low from plasma (0.3% to 0.8%) but considerably higher from water (about 15%). Therefore, the high protein binding of antidepressants appears to be the main limiting mechanism for a better extraction. However, the analytes were well separated and the calibrations were linear between 125 ng/ml and 2000 ng/ml. The limits of quantification were about 90 ng/ml for imipramine and desipramine, 125 ng/ml for amitriptyline, trimipramine, doxepine, nortriptyline and mianserine and about 200 ng/ml for maprotiline, clomipramine and desmethylclomipramine. The recovery was improved by increasing the extraction time. The influence of the concentrations of the sum of proteins and of alpha-acid glycoprotein on the peak-area ratios ATAD/A I.S. and on absolute peak areas was studied. Peak-area ratios increased with decreasing protein concentration but were found to be independent on alpha-acid glycoprotein. A simple model for the explanation of the effect is presented. Measures for the improvement of sensitivity are discussed. As presented in this paper, which first describes SPME for the analysis of drugs in plasma, SPME with a short extraction time can be of only very limited value for therapeutic drug monitoring. Lower concentrations than the limit of quantification are usually found at therapeutic doses. The method can be useful for toxicological analysis after the accidental or suicidal intake of higher doses. However, an about 10-fold improvement of the sensitivity of the method seems to be possible.

Representation of affinity in the case of co-operativity in protein-ligand binding

We have already proposed a "Global Association Function" to represent the global affinity of proteins to a drug; it was first applied in the case of independent binding sites. In this paper, we show that this same function can also be used to assess interactions between sites by varying the number of interacting sites and their co-operativity level. The resulting curves in two application cases are given together with the corresponding Scatchard plot: i) in a system with one single class of identical and interacting sites, ii) in a system with two classes of sites in which either primary or secondary are interacting; unexpectedly, in this latter case we also observed that sometimes positive co-operativity occasionally resulted in a concave-up Scatchard plot which is unusually admitted. In addition, as described in one example, our function is assumption free; this might be an advantage over usual methods, such as discrete parameter methods, because they require additional and empirical hypotheses on their related binding model.