Protein binding of chlorpromazine, propranolol, meperidine, desipramine, salicylic acid and phenytoin was determined in plasma of 64 healthy volunteers (35 males and 29 females). An attempt was made to identify factors affecting the plasma protein binding of these drugs. Whereas plasma albumin levels decreased as a function of age in both sexes, alpha 1-acid glycoprotein levels increased with age, but the increase was more pronounced in males. The free plasma fraction of the acidic drugs (salicylic acid, phenytoin) and desipramine (a base) showed a significant (p less than 0.005) negative correlation with plasma albumin levels. The free fractions of the other three basic drugs (chlorpromazine, propranolol, meperidine) in plasma showed a significant (p less than 0.005) negative correlation with alpha 1-acid glycoprotein plasma levels. Plasma protein binding of salicylic acid, phenytoin and desipramine decreased as a function of age. Plasma protein binding of chlorpromazine, propranolol and meperidine was virtually unaffected by age or was slightly increased (chlorpromazine). Only in the case of salicylic acid could a statistically significant difference be demonstrated between males and females in the free fraction-age relationship. Stepwise multiple linear regression analysis, including age and blood chemistry values such as hematocrit, bilirubin, cholesterol, triglycerides, creatinine, BUN, albumin and alpha 1-acid glycoprotein as independent variables, identified age as the variable explaining most of the variability in plasma binding of salicylic acid, phenytoin and desipramine. For chlorpromazine, propranolol and meperidine alpha 1-acid glycoprotein was the most important determinant of plasma protein binding.

Differences in expression of CYP1A isoforms (CYP1A1 and CYP1A2) in liver and small intestine of male Wistar rats and their inducibility by 3-methylcholanthrene as well as the effect of different CYP1A1/1A2 expression on caffeine metabolism were investigated. In rat liver, CYP1A2 is the predominant isoform and CYP1A1 protein expression in liver is significantly increased after treatment by 3-methylcholanthrene. In contrast, only CYP1A1 was detected in control and 3-methylcholanthrene induced small intestine microsomes. Treatment with 3-methylcholanthrene (40 mg/kg intraperitoneally daily during 1, 2, 3 or 4 days) demonstrated that liver CYP1A1 is more sensitive for the induction effects than CYP1A2 and also that significant induction of CYP1A1 in rat small intestine only occurred after 3 to 4 days pretreatment. Caffeine metabolism and inhibition studies by furafylline, CYP1A1 antiserum and ketoconazole revealed that the differences in the expression of CYP1A1 and CYP1A2 in the two tissues led to significant changes in the contribution of the various isoenzymes involved in the biotransformation of caffeine. Whereas in liver paraxanthine formation was almost exclusively catalyzed by CYP1A2, in rat proximal intestine it was formed by CYP1A1. In addition, other CYP enzymes (most probably CYP3A) play a significant role in theobromine and theophylline formation from caffeine in rat intestine. Overall, this study shows different expression and inducibility of CYP1A1/1A2 by 3-methylcholanthrene in rat liver and small intestine. Furthermore in rat intestine cytochrome P450 isozymes such as CYP1A1 and CYP3A replace CYP1A2 in the caffeine metabolism.

The objective of this study was to validate subcutaneous (sc) microdialysis sampling to study¯urbiprofen pharmacokinetics and plasma protein binding in the awake freely moving rat. A linear microdialysis probe was manufactured using a Hemophane 1 hollow ®ber which was tested in vitro and in vivo for the recovery of urbiprofen and naproxen used as retrodialysis marker. Flurbiprofen was administered intraperitoneally and intravenously at a dose of 20 mg/kg in rats. In both cases, conventional blood sampling and sc microdialysis sampling were simultaneously performed. The microdialysates were analyzed on-line by high-pressure liquid chromatography. Naproxen, which was shown to have a similar in vivo loss by retrodialysis as¯urbiprofen (71.5 AE 0.9% and 71.0 AE 0.8% respectively, n 3), was used to continuously monitor probe recovery. Concentration-dependent protein binding of urbiprofen was demonstrated in vivo based on experiments with a simultaneous sc microdialysis and blood sampling. Values of unbound fraction were similar to those reported previously by intravenous microdialysis sampling, demonstrating that the sc unbound concentrations are very similar to those in the central compartment. There was no signi®cant difference among pharmacokinetic parameters (AUC, CL, t 1/2z , Vd) for total or unbound¯urbiprofen determined after intraperitoneal and intravenous administration. Subcutaneous microdialysis is a simple yet powerful tool to study the pharmacokinetics and the in vivo plasma protein binding of¯urbiprofen in the awake unrestrained rat. ß

It was demonstrated that rapamycin is metabolized in vitro by pig liver microsomes under the influence of the cytochrome P450‐dependent mixed function oxygenase system to a rapamycin tris‐epoxide metabolite, as demonstrated by electrospray tandem mass spectrometry . The in vitro immunosuppressive activity of this metabolite was found to be lower than that of rapamycin, probably because the rapamycin effector sector was structurally modified. The effector region of rapamycin was recognized to include the conjugated double bonds of this compound and metabolic reactions affecting this region may change the binding affinity of the rapamycin–FKBP binary complex towards another pharmacological receptor bound to the binary complex. Moreover, metabolic modifications in the effector region are probably able to induce a change in the binding affinities of the rapamycin–FKBP binary complex, including the pipecolic acid moiety and the lactone function of the parent drug. Copyright © 1999 John Wiley & Sons, Ltd.

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