Balance/excretion of 3H- and 14C-tyloxapol in the male rabbit after intratracheal administration.

1. Tyloxapol, trace-labelled (50-100 microCi/animal) with 3H or 14C, was administered intratracheally in a surfactant formulation (EXOSURF Neonatal) to the male rabbit in a total tyloxapol dose of 5 mg/kg. Urine, faeces, expired air, and blood were collected for up to 10 days following tyloxapol administration. 2. Over 5 days, 3H-tyloxapol-related radioactivity in the urine (13.4%) and faeces (27.4%) accounted for a major fraction of the labelled dose. However, urine also contained an additional 13% of the dose as tritiated water. Expired air accounted for only 4.2% of the dose. At the end of the study, an additional 35.6% of the radioactive dose was found in tissues and the carcass, mainly in the lung (27.4%) and to a lesser extent in the liver (2.8%) and kidney (0.4%). Levels of radioactivity in other tissues, including whole blood, were low. 3. Over a separate 10-day study, faecal (30.4%) and renal (9.7%) elimination of 14C-tyloxapol accounted for 40% of the radioactive dose, with expired air accounting for much less (2.7%). At the end of the study, additional radioactivity was recovered from the lung (43.9%) and to a lesser extent from the liver (3.8%) and kidney (0.3%). The half-life for the elimination of total radioactivity from the lung was estimated to be 10-12 days. 4. These data indicate that, following intratracheal administration, tyloxapol and metabolites were retained by the lung, released slowly into the systemic circulation, and eliminated through faecal and renal excretion.

Metabolism of synthetic surfactants.

This article provided a brief overview of synthetic surfactants and DPPC metabolism, and summarized disposition data on the most widely used synthetic surfactant, CPHT (colfosceril palmitate, hexadecanol, and tyloxapol; Exosurf Neonatal, Burroughs Wellcome Co.). In separate experiments, young male rabbits were given intratracheal administrations of CPHT trace-labeled with either hexadecanol-3-[14C], choline-labeled [14C]DPPC, or [3H]tyloxapol. In mass balance and tissue distribution studies, radioactivity was quantitated in excreta and selected tissues over 5 days. Hexadecanol entered the pathways of intermediary lipid metabolism, via oxidation to palmitic acid, which was then utilized in the synthesis of phospholipids. After 5 days, most of the radiolabeled dose (56%) was distributed throughout the body, with renal (4%) and fecal (2%) excretion being minor routes of elimination compared with expired air (28%). The active component, DPPC, was still retained by the body (72%) after 5 days, having entered the pathways of lipid metabolism to become tissue associated. At this time, the lung and liver each contained approximately 10% of the labeled dose, whereas elimination in excreta (8%) was minimal compared with that in expired air (17%). Tyloxapol and metabolites were retained by the lung, released slowly (t1/2 = 5 to 6 days) into the systemic circulation, and eliminated through fecal (27%) and renal (26%) excretion.

Triprolidine radioimmunoassay: disposition in animals and humans.

A hapten derivative of triprolidine, bearing an acrylic acid side chain ortho to the pyridine ring nitrogen atom, was synthesized and coupled to bovine serum albumin. Immunization of New Zealand White rabbits with the resulting drug-protein conjugate resulted in the production of antisera capable of binding a radioiodinated tyramine conjugate of the triprolidine hapten derivative at high antiserum dilutions (1:70,000-1:150,000). These antisera were used to develop a radioimmunoassay (RIA) for triprolidine in human plasma with a sensitivity limit of 0.1 ng/mL (0.01 ng of actual mass). The known hydroxymethyl and carboxyl metabolites of triprolidine cross-reacted weakly (less than 2 and less than 0.05%, respectively) with this antiserum. The RIA could be used for the direct analysis of triprolidine in human and rabbit plasma, but not for rat or dog plasma, presumably due to the presence of other interfering substances (possibly metabolites). The validity of the RIA procedure in human plasma was demonstrated by comparative analysis of a number of samples by quantitative TLC (r = 0.985, slope = 1.076). The assay was employed to describe the pharmacokinetics of triprolidine in the rabbit (t 1/2, beta = 1.7 h). The assay had adequate sensitivity to detect low circulating drug concentrations in humans after therapeutic oral doses and also substantiated previous disposition experiments with triprolidine in humans (t 1/2, beta = 2.27 h). TLC analysis demonstrated that the absolute oral bioavailability of triprolidine (1-mg/kg dose) in the dog was low (4%). A comparison of triprolidine pharmacokinetic parameters in dogs, rabbits, rats, and humans revealed considerable similarity in elimination characteristics in these species.

Analgesic drugs in breast milk and plasma.

The disposition of salicylic acid, phenacetin, caffeine, and codeine, and two metabolites, acetaminophen and morphine, was studied in breast milk and plasma of two lactating mothers after single oral doses of a compound analgesic. Salicylic acid penetrated poorly into milk, with peak levels of only 1.12 to 1.60 micrograms/ml, whereas peak plasma levels were 33 to 43.4 micrograms/ml. The drug was also eliminated more slowly from milk than plasma. In contrast, caffeine and phenacetin kinetics in breast milk and plasma were similar, but milk levels were somewhat lower than plasma levels in both subjects. Metabolically produced acetaminophen levels in both fluids were much higher than those of the parent drug, phenacetin, in one subject, but early plasma and milk phenacetin levels exceeded those of acetaminophen in the other subject, thereafter dropping sharply to assume the pattern of the first subject. Elimination of the metabolite, acetaminophen, from milk was slower than from plasma (subject 1, half-life (t1/2) of drug in milk, 4.7 hr; t1/2 in plasma, 2.9 hr). In both subjects codeine concentrations in milk were 1.5 to 2.4 times as high as in plasma at the same times after drug. Metabolically produced morphine levels in milk from both mothers were low but exceeded those in plasma after 1 hr. Calculations based on average milk concentrations over the 12 hr after drug in subject 1 revealed milk excretion of 0.7% or less of the ingested dose of each drug. Similar calculations based on predicted steady-state milk drug concentrations in subject 2 indicated maximum milk excretion of 2.7% of the dose. In each case caffeine was excreted in the milk in the greatest amount.

Quantitation of the anticonvulsant cinromide (3-bromo-N-ethylcinnamamide) and its major plasma metabolites by thin-layer chromatography.

A quantitative thin-layer chromatography (TLC) procedure is described for the analysis of cinromide (3-bromo-N-ethylcinnamamide) and its two major metabolites, 3-bromocinnamamide and 3-bromocinnamic acid in plasma of the dog. These compounds were recovered from acidified plasma by extraction into benzene with a recovery of 95 +/- 5%. All three compounds were quantitated directly on a TLC plate by ultraviolet absorbance densitometry at 270 nm. The linear dynamic range for the quantitation of the compounds on a TLC plate ranged between 10 and 1000 ng. The complete procedure is useful in the working range of 50 ng/ml to 100 microgram/ml of plasma with a coefficient of variability of about 10%. Specificity of the method for parent drug and each of its plasma metabolites was confirmed by high-performance liquid chromatography. The method was used to determine the pharmacokinetics of cinromide and its two major plasma metabolites in dogs following a single oral dose of the drug.

TLC determination of griseofulvin in plasma and 6-demethylgriseofulvin in urine.

Fluorometric TLC procedures are described for the determination of griseofulvin in human plasma and 6-demethylgriseofulvin in human urine. Griseofulvin is extracted from plasma with ether, and its metabolite, 6-demthylgriseofulvin, is extracted from urine with benzene. Both compounds are subjected to TLC on silica gel plates. The plates are scanned using the fluorescent mode of a spectrodensitometer. For griseofulvin, the quantitation limit is 20 ng/ml of plasma and the recovery is 100%; for 6-demethylgriseofulvin, the limit is 1 microgram/ml of urine and the recovery is 90%. The methods were used to determine the plasma levels of griseofulvin and the amount of 6-demethylgriseofulvin excreted in the urine of human volunteers after a single oral dose of griseofulvin.

Lipid-soluble inhibitors of dihydrofolate reductase. I. Kinetics, tissue distribution, and extent of metabolism of pyrimethamine, metoprine, and etoprine in the rat, dog, and man.

With the aim of developing anticancer compounds which overcome some of the clinical limitations of the polar dihydrofolate reductase inhibitor, methotrexate, the physicochemical properties, kinetics, and metabolism of a series of lipid-soluble 2,4-diamino-5-phenylpyrimidine folate antagonists have been studied. Metoprine and etoprine, potent inhibitors of mammalian dihydrofolate reductase, were compared with pyrimethamine, a widely used antimalarial drug. The development of assay procedures in our laboratory and the synthesis of radiolabeled compounds have enabled a comparison of the kinetic characteristics and tissue distribution of these compounds in several species. The relative lipophilicities as indicated by the octanol/water partition coefficient are: etoprine (log P = 3.19) greater than metoprine (log P = 2.82) greater than pyrimethamine (log P = 2.69). Etoprine has the greatest affinity for plasma proteins, but all three compounds are bound to human plasma protein by 87% or more at therapeutic concentrations. Pharmacokinetic studies in the mouse, rat, dog, and man indicate that metoprine has the longest plasma half-life in all four species. The mean plasma half-lives in man are: pyrimethamine, 85 hr; metoprine, 216 hr; etoprine, 176 hr.

Determination of triprolidine in human plasma by quantitative TLC.

A chromatographic thin-layer fluorescence procedure, with a sensitivity limit of 0.8 ng/ml, is described for the quantitative analysis of triprolidine in human and rat plasma. Following the intravenous administration of 1 mg/kg of triprolidine to rats, the drug distributed rapidly into tissues and was eliminated from plasma with a half-life of 53 min. The method was used to determine the plasma triprolidine levels in 16 normal human volunteers following oral administration of 3.75 mg of triprolidine hydrochloride in 15 ml of a syrup. The drug obtained a mean peak plasma level of 8.2 ng/ml in 2 hr and was eliminated from the plasma with a half-life of 5 hr. Considerable individual variation was observed in the area under the plasma triprolidine level-time curve; values ranged from 19 to 163 ng hr/ml with a mean value of 75 ng hr/ml.

Effect of 3-methylcholanthrene pretreatment on the bioavailability of phenacetin in the rat.

A thin-layer chromatographic procedure is described for the quantitative determination of phenacetin and acetaminophen in rat plasma. The method was used to determine the effect of 3-methylcholanthrene (3-MC) on the disposition and bioavailability of phenacetin following its oral and iv administration to rats. Pretreatment with 3-MC decreased the plasma half-life of phenacetin, after iv administration, from 28 min to 4.5 min and reduced the systemic bioavailability of phenacetin, after oral administration, from 45% in control rats to 6% in 3-MC-treated rats. By comparing the plasma levels of phenacetin in the portal circulation with those in the peripheral circulation, following the oral administration of phenacetin, it was concluded that the 7-fold reduction in the bioavailability of phenacetin observed in 3-MC treated rats was caused by a marked increase in the metabolism of phenacetin during its first pass through the liver.

Elimination of antipyrine from saliva as a measure of metabolism in man.

The salivary half-life of antipyrine was used as a convenient procedure for estimating the relative rates of drug metabolism in man. The concentration ratio of antipyrine in plasma and saliva was one over a 24-hr period following the oral or parenteral administration of the drug to man and rat. Phenobarbital, a known stimulator of drug metabolism in animals and man, increased markedly the elimination of antipyrine from saliva of rats, while SKF-525A, a potent inhibitor of drug metabolism, prolonged the elimination of antipyrine from rat saliva. In addition, the known sex difference in the metabolism of drugs in the rat was detected by measuring the elimination rate of antipyrine from saliva of male and female rats. The clinical application of the procedure indicated that a group of epileptic patients treated with anticonvulsants for more than 2 mo had a mean antipyrine salivary half-life of 4 hr, whereas a mean half-life of 13 hr was found in a group of normal volunteers. The results show that the elimination rate of antipyrine from saliva is a useful index of drug metabolism in animals and man.

Quantitative thin-layer chromatography of pyrimethamine and related diaminopyrimidines in body fluids and tissues.

Several 2,4-diaminopyrimidines which inhibit the enzyme dihydrofolate reductase are quantitated following extraction and separation on silica gel thin-layer chromatographic plates. These compounds are candidates for the treatment of brain tumors and meningeal leukemia, because they have the ability to cross the blood-brain barrier. The ultraviolet absorption of the pyrimidine ring at 275 nm is utilized to quantitate these compounds on thin-layer chromatographic plates with a scanning instrument. This method offers the advantages of speed, specificity, versatility and sensitivity, and has proven to be satisfactory for the measurement of as little as 10 ng/ml of these compounds in biological fluids.