Influence of changes in protein binding on the central activity of antidepressants

Influence of plasma-protein binding on analgesic effect of methadone in rats with spontaneous withdrawal

The effect of spontaneous withdrawal on alpha 1-acid glycoprotein (AAG) levels and methadone protein binding has been studied in the rat. Animals were made physically dependent on morphine by providing morphine HCl in drinking water for three weeks. The natural opiate withdrawal was induced in rats by substituting the morphine solution with drinking water. The severity of the abstinence syndrome was assessed at various time intervals. After 12 h of withdrawal, the animals showing abstinence signs and low morphine levels were injected with intravenous methadone (0.35 mg kg-1) and the analgesic effect was measured by the tail-flick method and compared with animals receiving water. The oral administration of morphine produced an increase in AAG levels from 0.64 +/- 0.05 g L-1 in control animals to 1.47 +/- 0.92 g L-1 in experimental animals at the point of withdrawal and 1.21 +/- 0.09 g L-1 24 h after withdrawal. The percentage of methadone unbound was significantly lower in morphine-treated than in control animals. A significant correlation between AAG levels and percentage of methadone bound was observed. A parallel analgesic effect after intravenous methadone, as measured by AUC in the tail-flick test, was less in abstinence animals than in control (287.6 +/- 24.8 compared with 401.0 +/- 37.06s min). We suggest that in the withdrawal syndrome an adjustment of methadone dose may be necessary because of changes in protein binding.

Resistance to atracurium in rats with experimental inflammation: role of protein binding

The influence of altered protein binding on the neuromuscular effect of atracurium has been studied in rats with experimental inflammation induced by subcutaneous injection of turpentine oil. Doses of atracurium ranging from 0.45 to 1.5 mg.kg-1 were administered to control (n = 30) and to experimental inflammation induced rats (n = 30). Neuromuscular transmission was monitored by recording the twitch tension of the tibialis-anterior muscle elicited by stimulation of the sciatic nerve. Three effect parameters were recorded: (i) intensity of the effect, measured as percentage depression of baseline twitch tension, (ii) duration of drug action (min) and (iii) recovery time (min). The dose-intensity of the effect relationship was modelled using a sigmoid Emax model. The ED50 (effective dose eliciting 50% of the maximum effect) was significantly increased (P < 0.01) in the inflammation group as compared to the control group (0.94 vs. 0.68 mg.kg-1). This change was reflected in a shift of the dose-response curve to the right in the pretreated rats. For equipotent doses ED95 (defined as the effective dose eliciting 95% of maximum effect), no differences were found in recovery time and duration of action between the two groups of rats. Mucoproteins levels (index of alpha 1-acid glycoprotein (AAG) and protein binding were significantly increased in rats with experimental inflammation as compared to control rats. Based on these results, altered serum protein binding of atracurium appears to be responsible, at least in part, for the resistance to atracurium.

Altered methadone analgesia due to changes in plasma protein binding: role of the route of administration

1. The effect of experimental inflammation on methadone analgesia was evaluated in rats, by the tail-flick test, after single intravenous (0.35 mg/kg) and subcutaneous (3 mg/kg) doses. 2. After i.v. administration a significant decrease (P < 0.05) in the area under the methadone time-response curve was seen in rats with experimental inflammation, when compared with control. However, no differences in the analgesic response to methadone were detected between control rats and rats with inflammation when the drug was administered by s.c. injection. 3. Plasma mucoprotein levels were significantly increased (P < 0.001) and methadone free fraction was significantly decreased in rats with inflammation (P < 0.05). In addition, after i.v. methadone a decrease in brain uptake in rats with inflammation was detected. A significant correlation between brain uptake index and plasma free fraction was also observed. 4. These results suggest that a decreased immediate response to i.v. methadone may occur in circumstances in which there is an increase in alpha 1 acid glycoprotein, but that this is not likely to be observed when the absorption is not instantaneous.