Sequential group trial to determine gastrointestinal site of absorption and systemic exposure of azathioprine.

Azathioprine (AZA) is used in the treatment of patients with refractory inflammatory bowel disease; however, its use is limited because of systemic toxicity associated with long-term use. Ileocecal delivery of AZA might be advantageous if local intestinal therapeutic effects could be provided with decreased systemic side effects. Decreased cecal systemic absorption would allow higher dosages of AZA to be administered. A two-phase study was performed to compare the systemic exposure of AZA and 6-mercaptopurine (6-MP) following administration of AZA into the stomach, jejunum, and cecum and to compare the systemic exposure to AZA and 6-MP following administration of three different dosages of AZA into the cecum. In phase I, six healthy male volunteers received three 50 mg sequential doses of AZA via an oral tube directly placed into the stomach, jejunum, and cecum, respectively. In phase II, six healthy male volunteers received three different dosages (50, 300, 600 mg of AZA) into the cecum. Plasma concentrations of AZA and 6-MP at various times were quantified and area under the plasma concentration-time curve (AUC) and mean residence time (MRT) were determined. No significant differences in the AUC of AZA were seen at the different sites. The AUC of 6-MP following administration of AZA into the jejunum (67.0 +/- 30.1 ng x hr/ml) was higher compared to the stomach (39.9 +/- 38.1 ng/hr/ml) and cecum (29.2 +/- 10.9 ng x hr/ml). Jejunal absorption was 68% higher than absorption from the stomach and 129% higher than that of the cecum. Gastric absorption was 27% higher than that of the cecum. Increased dosages given into the cecum resulted in increased AUCs of AZA and 6-MP. The AUCs of AZA following 50, 300, and 600 mg dosages were 16.9 +/- 7.4, 52.3 +/- 67.2, and 132 +/- 151 ng x hr/ml, respectively, and the AUCs of 6-MP were 22.2 +/- 14.9, 63.4 +/- 50.6, and 104 +/- 115 ng x hr/ml, respectively. Systemic exposure to 6-MP is reduced following administration of AZA into the cecum, most likely secondary to reduced absorption of 6-MP from the colon. Higher dosages of AZA presented to the cecum do result in increased systemic absorption, but may still allow more drug to be administered with less toxicity than the same dose received orally.

Disposition and protein binding of valproic acid in the developing rat.

The disposition of valproic acid (VPA) in serum and brain tissue was examined in developing rats (5, 10, 20, and 60 days postpartum) following both single and multiple intraperitoneal doses of VPA. The binding of VPA to proteins in serum was determined ex vivo by ultrafiltration for each age group in pooled serum at various time points following VPA administration, as well as after in vitro addition of VPA (8-2400 micrograms/ml) to pooled rat serum from naive animals of each age. Concentration-time data for VPA in serum and brain tissue were fit simultaneously, assuming first-order absorption from the peritoneal injection site and first-order transfer of drug between serum and brain tissue. Kinetic analysis revealed that total clearance increased with postnatal age, whereas the volume of distribution and brain-to-serum partitioning of VPA decreased during development. Furthermore, enterohepatic recirculation, a well-described facet of VPA disposition in adult rats, was not evident from examination of the serum concentration-time profile in animals prior to the time of weaning. A progressive increase in the binding of VPA to proteins in serum was observed during postnatal development. The bound fraction determined ex vivo was less than that determined in vitro for all age groups, suggesting the possibility of competition for VPA binding sites by metabolite(s) formed in vivo.

Central nervous system uptake kinetics of pentylenetetrazol in the developing rat.

The present investigation was undertaken to examine the brain uptake kinetics of the central nervous system (CNS) stimulant pentylenetetrazol (PTZ) during postnatal development. This study represents part of an ongoing effort to develop an appropriate seizure model for investigations of anticonvulsant action in the developing rat. The systemic pharmacokinetics of PTZ were examined in male and female adult animals following both intravenous (IV) and subcutaneous (SC) injection. PTZ administered SC was absorbed rapidly and was completely bioavailable in both genders. No statistically significant gender-dependent differences in the disposition of PTZ were identified. To examine the CNS uptake kinetics of PTZ, animals of four different age groups (5, 10, 20, and 60 days postnatal) received timed SC infusions of PTZ. Significant age-related differences in the rate of uptake of PTZ into the CNS were observed. These differences paralleled the previously reported age-dependent changes in the dose of PTZ required to elicit seizure activity. The results of this investigation indicate that the apparent age-related change in sensitivity to the convulsant effects of PTZ is due in part to age-dependent uptake of the convulsant into brain tissue.

Chronic stress impairs oxidative metabolism and hepatic excretion of model xenobiotic substrates in the rat.

Traumatic injury to both hard and soft tissue has been associated with a decrease in the rate of hepatic drug metabolism. The mechanism(s) underlying this phenomenon have yet to be determined, but may involve substances released from damaged tissues or activation of the adrenocortical axis secondary to stress. To determine whether a generalized stress response is involved in the trauma-induced perturbations of xenobiotic metabolism, rats were exposed to atraumatic stress for a period of 21 days prior to determining the disposition of antipyrine (an in vivo marker for the hepatic mixed-function oxidase system) and indocyanine green (a tricarbocyanine dye often used as an in vivo marker of active hepatic uptake). Exposure to stress resulted in a significant decrease in the systemic clearance of antipyrine, suggesting a stress-induced inhibition of hepatic oxidation. In addition, the stressed animals evidenced a decreased rate of uptake of indocyanine green by the liver, an apparent decrease in the storage of the dye within the liver, and a decreased hepatic clearance of indocyanine green (presumably due to a decrease in the KM for biliary transport). These observations suggest that atraumatic stress affects several processes involved in the hepatobiliary disposition of xenobiotics.