Regulatory and scientific roles for biodistribution studies in aquatic species.

The advantage of focusing on biliary excretion of drugs and other xenobiotics is emphasized in this discussion of the regulatory and scientific roles for biodistribution studies in aquatic species. The role of the hepatobiliary system in the overall disposition of absorbed chemicals is briefly reviewed. The present survey includes 18 drugs, 6 petroleum hydrocarbons, and 16 different fish species. The fact that aquatic species concentrate most xenobiotics and their metabolites into bile to as much as several orders of magnitude underscores the advantage of using bile rather than tissue to determine whether aquatic species are being exposed to given chemicals. Until these advantages are recognized, analysis of edible muscle tissue remains the method of choice for assessment of human exposure to chemicals.

Disposition and bioavailability of 3H-tetracycline in the channel catfish (Ictalurus punctatus).

1. The tissue disposition and bioavailability of 3H-tetracycline were examined in the channel catfish (Ictalurus punctatus) after intravascular (4 mg/kg body weight) and per os (4 and 80 mg/kg) dosing. 2. The pharmacokinetics of the intravascularly administered drug using a two-compartment open model revealed plasma half-lives of 1.3 and 16.5 h for the distribution and elimination phases, respectively. 3. The drug was highly concentrated in both hepatobiliary and urinary compartments. The concentrations in the edible flesh were the lowest of any compartment examined. 4. The apparent bioavailability of the per os-administered drug was less than 5% and was influenced by the presence of feed material and the dosage. 5. The drug was 72% bound to plasma protein at both 4 and 24 h after intravascular administration.

Principles of drug absorption and recent studies of bioavailability in aquatic species.

An overview is presented on the absorption of drugs and other xenobiotics in mammals and fish. Species differences in gastrointestinal tract physiology as well as other factors that influence bioavailability of these compounds are considered. Results of recent studies on the bioavailability of drugs in aquatic animal species are discussed.

Aquatic versus mammalian toxicology: applications of the comparative approach.

The large body of literature and techniques generated by mammalian toxicity studies provides a conceptual and technical framework within which the absorption, fate, and disposition of xenobiotics in aquatic organisms can be studied. This review emphasizes the similarities and differences between mammalian and aquatic systems, e.g., lung vs. gill as site of absorption and toxicity. These must be taken into consideration when designing aquatic toxicity studies. Studies of phenol red in dogfish shark as an example show physiologic-based pharmacokinetic modeling to be a useful tool for investigating and eventually predicting species differences in xenobiotic disposition and drug differences within the same species. This discussion demonstrates that both laboratory and modeling procedures are now available to carry out sophisticated studies of xenobiotic fate and disposition in fish. Such studies are needed to pinpoint sites and mechanisms of pollutant toxicity in aquatic organisms.

In vivo metabolism and disposition of drugs by aquatic species.

The model system developed in the dogfish shark was reviewed to determine the kinds of studies that can be done on in vivo metabolism and disposition of drugs. Biliary and urinary sampling provided a compilation on the distribution and pharmacokinetics of 16 drugs and model compounds. The major transport and excretory and metabolic parameters in this fish were similar to those found in mammals. The use of such studies for scientific and regulatory purposes is considered.

Metabolism, disposition, and toxicity of drugs and other xenobiotics in aquatic species.

An overview is given of the absorption, distribution, excretion, and in vivo metabolism of drugs in fish. Several methods for chemical disposition and metabolism in fish are outlined, and a few well-characterized studies of the relationships between xenobiotic metabolism and toxicity in aquatic species are highlighted. For more than 40 drugs and other xenobiotics in several species of fish, concentrations of chemicals in bile were usually considerably higher than values in other body compartments or in the surrounding water. This was shown to be true particularly for parent substances or their biotransformed products with molecular weights greater than 400. The use of bile analysis is recommended as a regulatory tool to detect contamination of fish from drug treatment or environmental exposure.

Effects of environmental toxicants on development of a teleost embryo.

Embryos of the teleost Fundulus heteroclitus are shown to be useful model systems for monitoring the effects of xenobiotic compounds on development. Fourteen different substances were tested: malathion, aroclor, aldrin, diquat, parathion, pentachlorophenol, sevin, toxaphene, lindane, 2,4-D, DDT, paraquat, 2,4,5-T, and aminotriazole. Concentrations used for each of these was from 0.01 to 10.0 ppm in the incubation dishes. The variety of effects on development observed depended on the compound and its concentration. These effects included inhibition of gastrulation, abnormal axis formation, diminished pigmentation, slowed rate of development, reduced frequency of hatching, loss of neuromuscular control, and reduction or inhibition of heart beat. Possible modes of action of some of these compounds are discussed. It is also shown that embryogenesis is not always the most susceptible part of the organism's life cycle.

The utility of changes in cardiac weight as an index of drug-induced cardiotoxicity.

The effects of toxic doses of various drugs and of food or water deprivation upon heart weights of mice were evaluated over a four day period to test the validity of the hypothesis that changes in cardiac weights are indicators of cardiotoxicity. Drugs included in the study were actinomycin-D, methotrexate, 5-fluorouracil, adriamycin, daunomycin, N-dimethyladriamycin, N-trifluoroacetyladriamycin-14-valerate, isoproterenol, atropine, and acetylsalicylic acid. Additional groups of mice served as vehicle controls, or were deprived of food or water for the duration of the experiment to control for the anorexia and dehydration accompanying treatment with antineoplastic drugs. Body weights were taken at the start of the experiment (day 0), day 2, and day 4 (just prior to sacrifice). Heart ventricle wet weights were determined immediately, and dry weights after thorough desiccation of the samples. Statistical evaluation of the weights revealed that there were no ventricular weight changes unique to any particular drug, and that decreases in heart weights correlated well with decreases in body weights, thereby reflecting the general toxicities of the drugs, including inanition, and not any specific cardiotoxicities.

Concentration-dependent disappearance of fluorouracil from peritoneal fluid in the rat: experimental observations and distributed modeling.

The rate of disappearance of fluorouracil from peritoneal fluid has been experimentally measured and mathematically modeled. The experimental data were obtained following the instillation of 50 ml of dialysis fluid which contained an initial fluorouracil concentration ranging from 24 microM to 12 mM. The rate of disappearance was strongly dependent upon concentration. A distributed model has been formulated which incorporates concepts of diffusion with saturable metabolism and nonsaturable capillary uptake in the tissue surrounding the peritoneal fluid. This model successfully describes the experimental observations and also suggests that the effective penetration depth into tissue is highly dependent upon concentration.

Local and systemic toxicity resulting from large-volume ip administration of doxorubicin in the rat.

Doxorubicin was administered to rats in a simulated "belly bath" protocol. Fifty milliliters of various concentrations of drug solution was administered ip and was allowed to remain in situ for either 4 or 36 hours prior to removal. Animals were analyzed at 2, 14, and 60 days after treatment. Doses ranged from lethal (75 and 150 micrograms/ml for 4 hours; 12 and 24 micrograms/ml for 36 hours) to nontoxic (5 micrograms/ml for 4 hours). The most common lesion in surviving animals was chronic fibrosing peritonitis. Grossly, there were large volumes of peritoneal fluid in animals exposed to low concentrations (12 and 24 micrograms/ml) for 36 hours, but peritoneal adhesions were the most commonly observed finding when higher concentrations (20-150 micrograms/ml) were used for 4 hours. Commonly observed systemic toxic effects (bone marrow, gastrointestinal tract, and heart) were not seen in this study. Vehicle-treated control animals were negative for all histologic lesions and gross observations.

Bile secretory function: a determinant of adriamycin disposition.

Adriamycin (ADR) is rapidly cleared from plasma and enters tissues, while it is extensively eliminated in bile and moderately in urine following i.v. injection of 20 mg/kg to anesthetized rats. In bile duct- and bladder-cannulated rats with physiologic bile and urine production, 26.6% of the injected ADR is excreted in bile as total ADR equivalents during a 3 hr period and 4.4% in urine. When the elimination of the drug in urine is prevented by ligation of the kidneys, no significant differences are observed in the disposition of the total drug equivalents. Conversely, when bile flow is inhibited by the administration of sodium taurolithocholate, the biliary excretion of the total ADR equivalents declines significantly and in a fashion related to the degree of bile flow reduction. In parallel with the diminished biliary elimination, the urinary excretion increases significantly and is responsible for most of the drug eliminated when severe cholestasis is produced. However, despite the increased urinary excretion, the overall elimination of the total ADR equivalents in cholestatic rats is significantly reduced and both plasma and tissue levels of the total drug equivalents rise significantly and in a fashion closely related to the degree of the induced cholestasis.

Biliary and urinary excretion of adriamycin in anesthetized rats.

Adriamycin (ADR), an antibiotic widely used in cancer chemotherapy, is rapidly cleared from plasma, extensively excreted in bile and only moderately in urine during the first few hours after its intravenous injection to anesthetized rats. When bile and urine are collected in bile duct- and bladder-cannulated rats, about 33-35% of the injected doses, 5, 20 or 40 mg/kg ADR, is excreted in bile as total drug equivalents during a 10-hour collection period, while 4-8% is eliminated in urine. Within 60 min from the injection, as much as 15-17% of the dose is excreted in bile and 1-3% in urine. The biliary excretion of ADR, within the dose range of 5-40 mg/kg, is not saturable, is linearly related to the dose administered, and occurs in absence of choleretic or cholestatic manifestations. Conversely, the urinary excretion of the drug is a dose-limited process; when ADR is injected at 40 mg/kg, a significantly lower percentage is eliminated in urine. This decline is associated with a severe, although transient, antidiuretic effect thus suggesting an intrinsic toxicity of the drug at this high dose on renal function.

Disposition and metabolism of adriamycin in the rat.

Adriamycin (ADR) is extensively excreted in bile and moderately in urine following its administration to anesthetized rats. At intravenous doses ranging from 5 to 20 mg/kg, approximately 34% of the injected ADR is excreted in bile as total drug equivalents and 6-8% in urine over a 10-hour period. Thin-layer chromatography of bile, urine and tissue extracts revealed the presence of three major metabolic products in addition to the parent drug. In either bile or urine, unchanged ADR was the predominant form excreted and accounted for about 70% of the total drug equivalents. Adriamycinol and ADR conjugates were the major metabolites, were excreted at comparable rates and together accounted for most of the remaining fluorescence or radioactivity in either body fluid. ADR itself was the main form found in all tissues examined. ADR conjugates were not detected in any tissue whereas adriamycinol was observed in kidney, heart and spleen, but not in the liver. ADR aglycones could not be detected in the heart. They appeared preferentially in the liver where, 3 h after ADR was injected, they accounted for about 40% of total tissue fluorescence or radioactivity. The low rate of ADR conversion observed in the present studies supports the hypothesis of species difference in the metabolism of the drug.