Chronokinetics of liposomal encapsulated ampicillin in fasting and nonfasting rats

Chronobiology and chronotoxicology of antibiotics and aminoglycosides

Few investigators have examined the circadian variation in the symptom intensity of infectious diseases. Seasonal patterns in a variety of infectious are well know. Less appreciated are the circadian patterns in the symptom expression of infections. Studies indicate that fever which accompanies the common cold peaks at 4 p.m., and this is in agreement with other studies indicating that the elevation of body temperature, fever, due to bacterial infections is higher in the evening while that due to viral infections is more likely in the morning. Animal and human studies reveal also administration-time-dependent differences in the pharmacokinetics and toxicity of antimicrobial agents. This is particularly true for the aminoglycosides, as their nephrotoxicity is greatest when administered during the resting period of laboratory animals and human beings. Food intake and low urinary pH has been found to be protective of the toxicity of aminoglycosides at this time of the day. Knowledge of the administration-time-dependence of aminoglycosides and the underlying mechanisms can be used to develop once-a-day formulations that are significantly less toxic, in particular to the kidney, in patients who require around-the-clock antimicrobial therapy.

Dosing-time-dependent variation in biliary excretion of flomoxef in rats

We previously reported that the biliary excretion of flomoxef, an oxacephem antibiotic, was greater after dosing at 21:00 than at 09:00 h in diurnally active human subjects. The present study was undertaken to examine whether the biliary excretion of flomoxef is also dependent on its dosing time in rats. Adult male Wistar rats were housed under light on at 07:00 h and off at 19:00 h. Bile fluid was completely drained through a polyethylene catheter from conscious animals. Flomoxef (20 mg/kg) was injected into the tail vein at 09:00 or 21:00 h by a cross-over design, and drained bile fluid was collected for 8 h after each dosing. The maximum concentration of biliary flomoxef was significantly greater and its total excretion tended to be greater after dosing at 09:00 than 21:00 h. These results suggest the biliary excretion of flomoxef is enhanced after dosing at the beginning of the rest period in rats, as it is in humans.

Chronokinetics of active biliary ampicillin secretion in rats

The existence of temporal variation in biliary excretion has been demonstrated for dibromosulfophthalein and ampicillin (AMP). This study was performed to investigate if the 24 h rhythm of active AMP biliary secretion could be attributed to circadian rhythms in the capacity and/or binding affinity of the active secretion mechanism. In this study, 12 Sprague-Dawley rats, housed under a 12 h light/12 h dark environment, were used. Each rat received four 1 h infusions of incremental doses of AMP during either the active (24:00 group) or rest phase (12:00 group) under pentobarbital anesthesia. High doses of AMP were administered to saturate the biliary secretion of AMP via the anion carrier system. Bile and plasma were collected at steady state for each infusion and analyzed by a microbiological assay. The systemic clearance of AMP was increased approximately twofold during the active phase (24:00 group) compared to the resting phase (12:00 group). Plots of bile excretion rate versus plasma concentration indicated saturation of the anion carrier system. Analysis of the data using the Michaelis-Menten model revealed no significant difference in the binding affinity (1/Km) of the biliary anion carrier system between the 12:00 and 24:00 groups. However, the maximum AMP excretion rate attained in the bile (maximum transport or Vmax) showed a 50% increase during the active phase, thus implicating a day-night variation in transport capacity of the anionic pathway. Therefore, temporal variation in the capacity of the secretory mechanisms is a determinant contributor to the proposed circadian rhythm observed in the biliary elimination of AMP.