In vivo evaluation of a new sustained-release formulation of morphine.

The pharmacokinetics of a novel sustained-release oral formulation of morphine have been evaluated. The formulation consisted of tablets containing a morphine-EudragitL complex (MEC) which had shown good sustained-release properties in previous in vitro dissolution studies. MEC tablets were administered orally to beagle dogs and the morphine plasma levels and pharmacokinetic parameters obtained were compared with those obtained with MST Continus, a commercially available sustained release form of morphine. Blood samples were withdrawn up to 12 h after dosing and plasma morphine concentrations were determined by HPLC with electrochemical detection. Both formulations presented a relatively rapid absorption of morphine with similar values of Cmax (MST: 53 ng/ml; MEC: 50 ng/ml) and Tmax (MST: 86 min; MEC: 88 min), and prolonged morphine plasma levels. Mean plasma morphine concentrations were higher for the MEC tablets than for MST tablets during the terminal phase of the corresponding curves and the mean AUC(0-12h) for MEC tablets was 138% of that obtained with MST tablets. Our findings indicate that MEC tablets can produce prolonged plasma levels of morphine and could be an alternative to commercially available morphine sustained-release forms.

Determination of busulfan in human plasma using high-performance liquid chromatography with pre-column derivatization and fluorescence detection.

A rapid, sensitive and reproducible high-performance liquid chromatographic assay for busulfan in human plasma was developed. After extraction of plasma samples with acetonitrile and methylene chloride, busulfan and the internal standard [1,5-bis(methanesulfonyloxy)pentane] were derivatized with 8-mercaptoquinoline to yield fluorescent compounds which were detected with a fluorescence detector equipped with filters of 360 nm (excitation) and 425 nm (emission). Calibration graphs showed a linear correlation (r>0.9990) over the concentration range of 20-2000 ng/ml. The recovery of busulfan from plasma standards was 70+/-5%. The detection and quantification limits for busulfan in plasma samples were established at 9 ng/ml and 20 ng/ml, respectively. The intra- and inter-assay variations were lower than 8% and 10%, respectively. The applicability of the method was verified by analyzing the plasma concentrations of busulfan in a patient to whom it was administered orally on two different days.

Distribution of ceftazidime in rat tissues.

The pharmacokinetics and tissue distribution of ceftazidime (CFT), a third generation cephalosporin antibiotic commonly used in clinical practice, were investigated in rats after intravenous administration of the antibiotic. Studies using intravenous bolus administration were carried out at two dose levels (5 and 20 mg) of the antibiotic. Results of these studies showed a linear disposition of CFT, no differences between the arterial and venous plasma compartments, and linear distribution in all of the tissues assayed. Experiments carried out under steady-state conditions after constant infusion of the antibiotic showed that CFT distribution was restricted to the extracellular water of the rat tissues, as deduced from the tissue-to-plasma partition coefficients lower than 1 obtained in these experiments. Concentration of CFT in the extracellular water appears to be equal to that of plasma at the same sampling time.

Zinc uptake in five sectors of the rat gastrointestinal tract: kinetic study in the whole colon.

PURPOSE: The uptake of zinc as acexamic acid salt in the rat gastrointestinal tract, using an in situ static technique, was studied. Our aim was to investigate an absorption window for zinc and the uptake kinetics in the colon. METHODS: To detect selectivity phenomena in zinc absorption, buffered saline solutions of zinc (50 micrograms/ ml) were perfused in stomach, whole colon and three 33-cm fractions of the small intestine (proximal, middle and distal segments). To characterize zinc uptake kinetics in whole colon, five different zinc concentrations (5, 25, 50, 150 y 250 micrograms/ml) were assayed. Zinc secreted into the gastrointestinal tract during the experiments was deducted from the uptake. RESULTS: Zinc secretion was characterized as an apparent zero-order process for all the studied segments (mean secretion rate = 0.10 +/- 0.03 microgram/(ml x min)). The stomach exhibited little ability to absorb zinc (apparent first order rate constant = 0.17 +/- 0.07 h-1), whereas the highest transport rates were found in the last two thirds of the small intestine and colon (first order constants: 0.66 +/- 0.13 h-1, 1.00 +/- 0.06 h-1, 0.97 +/- 0.14 h-1, 0.96 +/- 0.19 h-1 for proximal, middle, distal and colon segments, respectively). Zinc uptake in the colon was characterized by means of a Michaelis-Menten and first-order combined kinetics, with the following parameters: Vm = 0.36 +/- 0.02 microgram/(ml x min), Km = 18.01 +/- 0.40 microgram /ml and Ka = 0.40 +/- 0.01 h-1. CONCLUSIONS: Zinc is preferably absorbed in the middle and distal parts of the rat gastrointestinal tract. In the colon a saturable mechanism may be involved in apparent absorption.

Use of rats chronically cannulated in the jugular vein and the duodenum in pharmacokinetic studies. Effect of ether anesthesia on absorption of amoxicillin.

In order to be able to use unanesthetized rats in pharmacokinetic studies it is necessary to find methods of drug administration and repeated blood sampling that do not stress the animals and therefore avoid possible alterations in the pharmacokinetic parameters caused by stress. Two surgical procedures that permit chronic cannulation of the jugular vein and duodenum of the rat are described in this paper, and the influence of ether (CAS 60-29-7) anesthesia on the intestinal absorption of amoxicillin (CAS 26787-78-0) is evaluated. Amoxicillin (2.2 mg) was administered to the rats by the oral and intraduodenal routes. Intraduodenal administration was performed in conscious rats, whereas oral administration was performed under light and heavy ether anesthesia. In each animal, 10 blood samples were collected through the jugular cannula at fixed times after drug administration and the plasma antibiotic concentration was determined by a microbiological diffusion procedure, with Micrococcus luteus as the test organism. Plasma concentration versus time curves of amoxicillin after intraduodenal and oral administration under light anesthesia were similar, with a slight delay in the tmax value for the oral administration. However, after oral administration of amoxicillin to rats under heavy anesthesia, antibiotic absorption was very irregular, with a decreased Cmax value and an increased tmax value as compared with the other two administration methods. Furthermore, the amount of amoxicillin absorbed in rats given the drug under heavy anesthesia was smaller than in the other two groups. These results suggest that drugs can be administered to rats under light ether anaesthesia without altering their pharmacokinetic features, but heavy anaesthesia can significantly alter their absorption.

Studies on the renal excretion mechanisms of cefadroxil.

The mechanisms of renal excretion of cefadroxil were investigated in conscious rats. The drug was intravenously infused at several infusion rates (0.27, 1.08, 5.40, 12.00, and 31.35 mg/hr), and the total and renal clearances were determined after the steady-state was reached. Renal clearance accounted for approximately 91% of total clearance. Renal clearance of cefadroxil increased from 2.51 +/- 0.39 to 3.57 +/- 0.43 ml/min as the steady-state cefadroxil plasma concentration increased from 1.7 +/- 0.3 to 24.4 +/- 3.8 micrograms/ml, and this has been attributed to a saturable renal tubular reabsorption of the antibiotic. The ratio of unbound cefadroxil renal clearance to glomerular filtration rate was larger than unity, which indicates that the antibiotic also undergoes active renal tubular secretion. When cefadroxil was administered together with cephalexin, an increase in the renal clearance of cefadroxil was observed, which has been attributed to a competitive inhibition of the tubular reabsorption of cefadroxil by cephalexin. A pharmacokinetic model for the renal excretion of cefadroxil was developed, and mathematical expressions showing the relationship between renal clearance and steady-state plasma concentration were deduced.

Low bioavailability of amoxicillin in rats as a consequence of presystemic degradation in the intestine.

Several studies have been carried out to elucidate the causes of the low oral bioavailability of amoxicillin in rats. The hepatic first-pass effect of the antibiotic was estimated by comparing the area under the plasma drug concentration-versus-time curve from time zero to infinity (AUC0-infinity) obtained after injecting the drug into a mesenteric vein with the AUC0-infinity value obtained after injecting the drug into the jugular vein of conscious rats. No hepatic first-pass effect was detected. The bioavailability of amoxicillin after intraduodenal administration was only 51%, and the fraction of the dose remaining in the intestine at the end of the experiment was 4.5%. This was far less than the fraction that did not reach systemic circulation, which indicates a presystemic loss of drug, probably at the intestine. In vitro studies corroborated the fact that amoxicillin is subjected to presystemic degradation by intestinal juices and intestinal tissues. The greatest loss of drug occurred in the complete intestine (45% of the initial amount), and it was mainly due to the action of intestinal tissues (28% of the initial amount) but was also due to the action of intestinal juices (15% of the initial amount). The absorption of amoxicillin in three parts of the intestine (upper, middle, and lower) was also evaluated. The largest AUC0-infinity value and the highest plasma drug levels were obtained when amoxicillin absorption took place in the middle intestine. The smallest AUC0-infinity value and the lowest plasma drug levels corresponded to absorption from the upper intestine.

Nonlinear pharmacokinetics of cefadroxil in the rat.

The pharmacokinetics and bioavailability of cefadroxil in the rat were examined after intravenous and oral administration at three doses (2.5, 10, and 15 mg). Cefadroxil disposition kinetics was clearly nonlinear, with an increase in plasma clearance as the dose increased (2.65 +/- 0.55, 3.17 +/- 0.48, and 3.82 +/- 0.39 ml/min for the three doses assayed). This phenomenon was attributed to a saturable renal tubular reabsorption of the antibiotic. After oral administration, the normalized Cmax was lower for the largest dose (4.6 +/- 0.7 micrograms/ml) than for the other two doses (5.5 +/- 0.5 and 5.4 +/- 0.7 micrograms/ml). Renal excretion of cefadroxil in the rat after intravenous and oral administration was investigated at two level doses (2.5 and 15 mg). No significant differences were found between doses or administration routes, and the mean percentage of dose recovered in the urine for the intravenous and oral routes was 80.7 +/- 6.1% and 76.4 +/- 3.7%, respectively. Cefadroxil bioavailability estimated from plasma levels or from the amounts of drug excreted in the urine was high and ranged from 90% to 100%.

Influence of permanent cannulation of the jugular vein on pharmacokinetics of amoxycillin and antipyrine in the rat.

The effect of chronic cannulation of the rat jugular vein on the pharmacokinetics of amoxycillin and antipyrine administered by the i.v. and oral routes has been evaluated. Animals that received the i.v. dose of amoxycillin on the eighth day after jugular vein cannulation showed decreased clearance (4.0 +/- 0.3 ml/min) and steady-state volume of distribution (105 +/- 8 ml) compared to animals that received the i.v. dose on the fourth day (5.5 +/- 1.1 ml/min and 155 +/- 17 ml, respectively). Rats first dosed by the i.v. route showed an oral bioavailability of 54 +/- 12%, whereas for those first dosed by the oral route the calculated bioavailability was 31 +/- 6%. Antipyrine was administered to rats by the i.v. and oral routes on the first and fourth days after jugular vein cannulation. Animals intravenously dosed on the fourth day showed a decreased clearance (1.9 +/- 0.3 ml/min) compared to rats intravenously dosed on the 1st day (2.7 +/- 0.6 ml/min). Antipyrine bioavailability was larger in animals first dosed by the i.v. route than in animals first dosed by the oral route (173 +/- 43 and 74 +/- 15%, respectively). These results argue against the use of crossover studies in rats with permanently implanted cannulas since kinetic changes induced by cannulation can be larger than previously proposed.

General treatment of the enterohepatic recirculation of drugs and its influence on the area under the plasma level curves, bioavailability, and clearance.

A general treatment of enterohepatic recirculation of drugs has been developed based on the fraction of drug in systemic circulation that is excreted in the bile and the fraction of drug reabsorbed from the gut that reaches systemic circulation in each enterohepatic cycle. The deduced equations make it possible to establish mathematical relationships between the areas under the blood level curves (AUC) of a drug when administered to normal and bile duct-cannulated animals and to predict the effect of enterohepatic recycling on bioavailability and clearance. The results were compared with those obtained by other authors using different approaches to enterohepatic recirculation, and some discrepancies were found in the equations describing the effect of enterohepatic recycling on AUC and bioavailability of drugs. The cause of such discrepancies and the problems associated with the prediction of hepatic extraction ratio from in vitro studies are discussed.

Nonlinearities in amoxycillin pharmacokinetics. I. Disposition studies in the rat.

Several features of amoxycillin pharmacokinetics in man are not well known in spite of the extensive clinical use of the antibiotic. In this paper it is demonstrated that amoxycillin disposition kinetics in rats is clearly nonlinear, and that this may be due mainly to its elimination mechanisms. At different intravenous bolus dose levels, and in steady-state perfusion studies, the most striking feature is an increased renal clearance as dose increases (from 3.5 to 7.0 mg kg-1 for intravenous bolus, and from 4.6 to 20.0 micrograms min-1 for intravenous perfusions). This phenomenon has been attributed to a saturation of the active renal tubular reabsorption of the antibiotic. When the intravenous dose is substantially increased (28.0 mg kg-1 bolus), plasma clearance tends to stabilize, probably because saturation of the active tubular secretion of amoxycillin takes place at these doses. Extrarenal clearance seems to remain linear throughout the entire dose range. On the basis of these observations and a review of selected bibliography, an interpretation of the kinetic disposition behaviour of amoxycillin in man is attempted.

Nonlinearities in amoxycillin pharmacokinetics. II. Absorption studies in the rat.

Most factors influencing amoxycillin oral absorption are, even today, unknown. Since many dosage schedules have been shown to lead to incomplete absorption, it would be desirable to find a suitable animal model where these factors could be studied in depth. In this paper, it is shown that, in the rat, plasma level curves obtained after oral doses of 7 and 28 mg kg-1 are poorly fitted using first-order absorption kinetics and that the best fit is obtained through the use of an input equation combining zero and first-order kinetics. In contrast, plasma level curves found after intraduodenal administration of amoxycillin solutions (7 mg kg-1) are well fitted by first-order input kinetics. It would seem that precipitation of some dose fraction of the orally administered antibiotic may occur in proximal gastrointestinal areas; this plays an important role in absorption profiles and prevents any possible saturation phenomena associated with active intestinal transport of the antibiotic. A comparative study of available human oral data revealed close similarities with those found in rats. As a result, the plasma level curve fitting was greatly improved by the use of the input function described here for the rat. Although oral bioavailability (as assessed from urinary excretion data) is lower in this latter species, the use of suitable correction factors led to superimposable plasma level curves in the two species, as occurred in previously reported disposition studies.

Pharmacokinetics and bioavailability of diclofenac in the rat.

Diclofenac sodium is a widely used drug with interesting absorption and disposition features when administered to laboratory animals. The present study was undertaken to assess the pharmacokinetics of the drug after iv and gastrointestinal dosing to rats. Renal excretion of unchanged drug was negligible, but biliary excretion of the drug (unchanged and conjugated) was detected in bile duct-cannulated rats; it accounted for 27.2 and 31.2% of the total dose following iv and intraduodenal administration, respectively. Most of the drug excreted in the bile was conjugated diclofenac; unchanged drug accounted for only 4.7 and 5.4% of total diclofenac excreted in the bile after iv and intraduodenal dosing, respectively. In normal animals, intestinal absorption of the drug excreted in the bile resulted in higher drug concentrations in plasma than those obtained in bile duct-cannulated rats, but only after 60 min of dosing. When administered directly into the duodenum, diclofenac absorption was extremely fast and the maximum plasma diclofenac concentration was reached within 2 min. After oral dosing, an early peak was also observed, but it was lower than that obtained after intraduodenal dosing: 71% diclofenac bioavailability was found in bile duct-cannulated rats intraduodenally dosed, whereas in normal animals dosed by mouth a bioavailability of 79% was obtained. In normal animals intraduodenally dosed, an apparent bioavailability of 106% was observed. All of these features, particularly the influence of enterohepatic circulation on drug bioavailability, are discussed.

Kinetics of the intestinal uptake of zinc acexamate in normal and zinc-depleted rats.

The uptake of zinc as acexamic acid salt in the small intestine of the anaesthetized rat was shown to be a two-phase process in normal animals. The first phase is rapid mucosal binding which satisfies the Freundlich isotherm equation and which involves about 30 per cent of the initially perfused zinc. The second phase was characterized as an apparent absorption step which obeys Michaelis-Menten and first-order combined kinetics, with the following parameters: Vm = 6.51 mg h-1; Km = 2.96 mg; ka = 0.306 h-1. In largely non-saturated conditions, an apparent global rate constant of about 2.50 h-1 was calculated. No significant interference due to endogenous zinc excretion into the small intestine was observed during the absorption period. In zinc-deficient animals, the two phases were not so well characterized. Binding was non-linear and apparent absorption efficiency was much greater at high zinc concentrations, so no evidence of saturable kinetics was found, thus confirming the hypothesis of a homeostatic zinc regulation mechanism.

Evidence of a specialized transport mechanism for the intestinal absorption of baclofen.

Absorption of the spasmolytic drug baclofen in three selected intestinal segments of living anaesthetized rats in situ, is shown to be a specialized transport mechanism obeying Michaelis-Menten kinetics. Equation parameters were calculated through different procedures, whose features are discussed. A computer method based on the integrated form of Michaelis-Menten equation which reproduces the entire time course of drug absorption from the data found in three intestinal perfusion series at different initial concentrations, yielded Vm and Km values of 12.0 mg h-1 and 8.0 mg, respectively, in the mean segment of the small intestine, a rather selective absorption site for baclofen. Lesser but comparable absorption rates were found in the proximal and distal segments of the small intestine, whereas in colon, drug absorption was negligible. Baclofen transport was significantly reduced in the presence of the enzymatic inhibitor sodium azide. If these results were extrapolated to humans, they would explain the excellent bioavailability profiles reported for baclofen at normal doses in spite of its physicochemical properties, which do not favour passive diffusion. Based on the same principle, the administration of usual doses at shorter time intervals could be recommended, instead of high, when higher plasma levels at steady-state are needed. On the other hand, more than 8-h sustained-release preparations of baclofen should, probably, be avoided.

Non-linear intestinal absorption kinetics of cefadroxil in the rat.

Absorption of cefadroxil in a selective intestinal absorption area (the proximal third of the small intestine) of the anaesthetized rat, at seven initial perfusion concentrations, ranging from 0.01 to 10.0 mg mL-1, is shown to be a non-linear transport mechanism. With the aid of computer-fitting procedures based on differential and integrated forms of Michaelis-Menten equation, Vm and Km values of 36.7-37.3 mg h-1 and 12.0-13.0 mg, respectively, were found. The statistical parameters were better than those obtained both for first-order and for combined Michaelis-Menten and first-order kinetics. There is no evidence for substantial passive diffusion processes. The results reported here, together with allometric considerations and literature data analysis, may help to explain some particular non-linear features of plasma level curves associated with the administration of fairly high oral doses of cefadroxil to humans.