Application of pharmacokinetics in drug safety evaluation.

The discipline of pharmacokinetics plays an important role in safety evaluation of drugs. In this presentation various kinds of pharmacokinetic studies have been discussed with specific examples of the studies that should be conducted in support of safety evaluation of new drugs. The design and evaluation of toxicologic and pathologic studies in animals, as well as of safety and efficacy studies in man, should take into consideration the pharmacokinetic characteristics of drugs.

Ototoxicity of subcutaneously administered aztreonam in neonatal rats.

Aztreonam (SQ 26,776) was given subcutaneously to three groups of neonatal rats at daily doses of 150, 600, and 2400 mg/kg from postnatal Days 10 through 16. Two similar groups given 400 mg of streptomycin kg-1 day-1 or 12 ml of 0.9% sodium chloride kg-1 day-1 on the same schedule served as a positive and negative control group, respectively. On postnatal Days 28 and 56, the neonates were evaluated for level of spontaneous activity and auditory and vestibular function. Half of the neonates in each group were necropsied on postnatal Day 29, and the other half on postnatal Day 57. The inner ears of all neonates were evaluated for histopathologic evidence of ototoxicity. No functional or histopathologic evidence of ototoxicity was found in any neonatal rat dosed with aztreonam or saline. However, neonates given streptomycin were hyperactive, had severely impaired hearing and vestibular function, and had morphologic changes in the sensory nerve endings of the semicircular canals, utriculus, sacculus, and cochlea. The histopathologic evidence of the ototoxic effects of streptomycin correlated highly with the functional data. Thus, under these conditions, aztreonam demonstrated no ototoxic effects in neonatal rats.

Distribution of aztreonam into fetuses and milk of rats.

Subcutaneous administration of [14C]aztreonam (150 mg/kg) to pregnant rats was followed by the appearance of radioactive moieties in fetuses and amniotic fluid. Concentrations of both total radioactivity and unchanged aztreonam in maternal serum declined more rapidly than those in fetuses and amniotic fluid. [14C]aztreonam (150 mg/kg) was also administered subcutaneously to lactating rats. Radioactivity and unchanged aztreonam were found in the suckling pups and in the serum and milk of the dam. Results obtained by whole-body autoradiography were generally consistent with these obtained by measuring radioactivity present in the excised tissues. Autoradiographs of the pups showed detectable amounts of radioactivity in the brain; since no radioactivity was detectable in the brain of the dam, it appears that the blood-brain barrier was not fully developed in 7-day-old pups.

Preclinical safety evaluation of the nadolol/bendroflumethiazide combination in mice, rats, and dogs.

Nadolol, a beta-adrenergic antagonist, and bendroflumethiazide, a thiazide diuretic, were administered orally alone and in combination to animals in acute and 6-month toxicity studies and in a rat teratology study. The two drugs in combination showed no evidence of potentiation of acute toxicity in mice. Nadolol and/or bendroflumethiazide were administered orally to rats at daily doses of 1000 or 160 mg/kg of nadolol and 125 or 20 mg/kg of bendroflumethiazide and to dogs at daily doses of 160 or 40 mg/kg of nadolol and 20 or 5 mg/kg of bendroflumethiazide for 6 months. The two drugs, alone and in combination, caused only minor changes in clinical-laboratory tests and no major gross or histopathologic changes. Many of the changes noted were expected pharmacologic effects of the individual agents. The drugs, alone or in combination, produced no evidence of embryotoxicity, fetotoxicity, or teratogenicity in rats. The results of these studies indicate that nadolol and bendroflumethiazide have a low order of toxicity individually, and when given in combination show no additional or potentiated toxicity.

Induction of hepatic drug-metabolizing enzymes in rats, dogs, and monkeys after repeated administration of an anti-inflammatory hexahydroindazole.

The effect of repeated administration of an anti-inflammatory hexahydroindazole, (+/-)-3,3 alpha,4,5,6,7-hexahydro-2-[3-(4-morpholinyl)-propyl]-3-phenyl-7-(phenylmethylene)-2H-indazole (HMPPI), on hepatic microsomal drug-metabolizing enzymes in rats, dogs and monkeys was studied. Dose-dependent increases in the ratio of liver weight to body weight were observed in all three species. Microsomal protein concentration increased in rats and monkeys but not in dogs. Cytochrome P-450 concentration and aminopyrine N-demethylase activity increased in all species. Aniline hydroxylase activity increased only in dogs. In all three species, the largest increase in all parameters, except for the liver weight/body weight ratio, was observed in animals on the intermediate dose (50 mg/kg per d) of the hexahydroindazole, probably of the toxicity of the hexahydroindazole at the high dose (250 mg/kg per d).

Hyperplasia of juxtaglomerular cells and renin localization in kidney of normotensive animals given captopril. Electron microscopic and immunohistochemical studies.

Captopril, a competitive inhibitor of angiotensin I-converting enzyme (ACE), is an orally potent antihypertensive agent. Light and electron microscopic studies of th kidneys of mice, rats, and monkeys given large oral doses of captopril for long duration were conducted. All mice and some rats and monkeys developed hyperplasia of the renin-secreting cells which appeared in several layers surrounding the vascular wall of the afferent arterioles. In the electron microscope, these epithelioid cells appeared heavily loaded with aggregates of homogeneous electron dense, osmiophilic amorphous granules filling distended spaces of the endoplasmic reticulum. The Golgi cisterns often included small, sharply outlined triangular or rhomboid osmiophilic granules. The use of specific renin antibodies and the application of the "three-layer bridge technique" for peroxidase-antiperoxidase defined and verified the accumulation of renin in the juxtaglomerular cells. After cessation of dosing, hyperplasia of the juxtaglomerular cells markedly regressed, and there was a significant reduction in the number and size of the renin granules in such cells.

Pharmacokinetics of captopril in dogs and monkeys.

[14C]Captopril was given as a priming dose, followed by constant intravenous infusion for 4 or 6 hr, to three anesthetized dogs and three anesthetized monkeys. Blood, urine, and bile samples were collected during and after drug infusion. Pharmacokinetic evaluations were carried out exclusively on data obtained for unchanged captopril. The average total body clearance (ClT) and the renal clearance (ClR) of captopril, in milliliters per kilogram per hour, were 605 and 341 in the dog and 1135 and 944 in the monkey. The biliary clearance of captopril was negligible in both species. The greater difference between the ClR and ClT values in the dog compared to that in the monkey was the result of more extensive metabolism of captopril by the dog. Since almost all of the radioactive dose was recovered in urine in both species, captopril and its metabolites were almost exclusively eliminated by the kidneys. One primary reason that body and renal clearance values of captopril were much greater in the monkey than in the dog was that the net tubular secretion fo captopril was about three times greater in the monkey (82%) than in the dog (28%). The volume of distribution of captopril was higher in the monkey (3.6 liters/kg) than in the dog (2.6 liters/kg); the volume of the central compartment was about the same (0.5 liter/kg) for both species. The terminal half-life value was slightly higher in the dog (2.8 hr) than in the monkey (2.2 hr).

Absorption and bioavailability of captopril in mice and rats after administration by gavage and in the diet.

The absorption of captopril (I), a new antihypertensive agent, was studied in mice and rats at doses (50 and 1350 mg/kg) administered in the diet in chronic toxicological studies. 3H- or 35S-Labeled I was administered by gavage and in the diet to male and female animals in a two-way crossover study. Animals received daily doses of nonradiolabeled I in the diet for 25 days, except on Days 15 and 22 when radiolabeled I was administered either by gavage or in the diet. Absorption of the total radioactivity in 2-month-old mice averaged 49 and 48%, respectively, of the 50- and 1350-mg/kg doses given in the diet and 57 and 65%, respectively, of the doses given by gavage. The bioavailability of I in 2-month-old mice averaged 48 and 39% (diet) and 44 and 59% (gavage) of the 50- and 1350-mg/kg doses, respectively. In 2-month-old rats, absorption of the total radioactivity averaged 41% of the 50-mg/kg dose given in the diet. In 2- and 15-month-old rats, minimum absorption of the 1350-mg/kg dose averaged 36 and 45% (diet) and 51 and 71% (gavage), respectively; the minimum bioavailability averaged 20 and 29% (diet) and 39 and 44% (gavage), respectively. These studies demonstrate adequate absorption and bioavailability of I over a wide range of doses from the drug-diet mixtures and by young and old animals and also illustrate a useful experimental design for the estimation of relative oral absorption of a drug administered continuously in the diet over several days.

Comparative toxicological studies of amphotericin B methyl ester and amphotericin B in mice, rats, and dogs.

In acute and subacute toxicological studies, amphotericin B methyl ester was shown to be much less toxic than the parent antibiotic. As a single intravenous dose in mice, the methyl ester was approximately 20 times less toxic than amphotericin B. Also, the acute toxicity of the methyl ester in mice was not enhanced by the presence of chemically induced hepatic or renal damage or by the concurrent administration of amphotericin B or flucytosine. In a 1-month intraperitoneal study in rats, the methyl ester was about one-fourth as nephrotoxic as amphotericin B. In a 1-month intravenous study in dogs, the methyl ester was about one-eighth as nephrotoxic and one-fourth to one-half as hepatotoxic as the parent compound. In addition, the methyl ester, unlike amphotericin B, produced minimal renal effects, which did not increase in severity with increasing dosage. Based on the results of these studies, it is concluded that amphotericin B methyl ester has the potential for an improved therapeutic ratio in the treatment of systemic mycoses.

Amphotericin B methyl ester hydrochloride and amphotericin B: comparative acute toxicity.

In single-dose parenteral studies with mice and dogs, the methyl ester hydrochloride of amphotericin B proved to be significantly less toxic than the parent compound, especially to the kidney.