Drug and chemical metabolites in clinical toxicology investigations: the importance of ethylene glycol,methanol and cannabinoid metabolite analyses

Metabolic pathways in humans have been elucidated for most therapeutic drugs, drugs of abuse, and various chemical/solvents. In most drug overdose cases and chemical exposures, laboratory analysis is directed toward identification and quantitation of the unchanged drug or chemical in a biologic fluid such as serum or whole blood. Specifically, most clinical laboratories routinely screen and quantitate unchanged methanol and/or ethylene glycol in suspected poisonings without toxic metabolite analysis. Martin-Amat established in 1978 that methanol associated toxicity to the optic nerve in human poisonings was due to the toxic metabolite formic acid found in methanol poisonings and not due to the direct action by unchanged methanol. Jacobsen reported in 1981 that ethylene glycol central nervous system and renal toxicity were primarily due to one acidic metabolite (glycolic acid) and not due to unchanged ethylene glycol. The first objective of this review is to describe clinical experience with formic acid and glycolic acid analysis in methanol and ethylene glycol human poisonings. Drug metabolite analysis also provides useful information in the assessment and monitoring of drug use in psychiatry and substance abusing populations. Drug analysis in substance abuse monitoring is focused on urine analysis of one or more major metabolites, and less frequently on the unchanged drug(s). Serial monitoring of the major urinary cannabinoid metabolite (delta(9)-THC-COOH) to creatinine ratios in paired urine specimens (collected at least 24 h apart) could differentiate new marijuana or hashish use from residual cannabinoid metabolite excretion in urine after drug use according to Huestis. The second objective is to demonstrate that creatinine corrected urine specimens positive for cannabinoids may help differentiate new marijuana use from the excretion of residual delta(9) -THC-COOH in chronic users of marijuana or hashish. Analysis of toxic chemical metabolites are helpful in the assessment and treatment of chemical poisoning whereas serial monitoring of urinary cannabinoid metabolites are predictive of illicit drug use in the substance abusing population.     

Discovery of methoxyacetylcarbamide in the urine of normal adults and phenylketonuric children

In order to study metabolic distinctions in phenylketonuria, urinary metabolites in the form of trimethylsilyl derivatives have been characterized by high resolution gas chromatography and mass spectrometry. A previously unknown metabolite has been found in the urine of some untreated phenylketonuric infants between 2 and 5 yr of age. The metabolite was absent in healthy children of the same age. The metabolite appeared to be present in the urine of apparently healthy adults (25-32 yr old). The metabolite was identified as methoxyacetylcarbamide on the basis of mass fragmentation analysis and compared with synthetic methoxyacetylcarbamide. Their retention times and mass spectra coincided.     

Excretion of endogenous digoxin-like immunoreactive factors in human urine is a function of urine flow rate

We studied the effect of varying water and salt intake on the renal excretion of endogenous digoxin-like immunoreactive factors (DLIF). DLIF were measured in human urine and serum by competitive displacement of 125I-labeled digoxin from anti-digoxin antibodies. Diuresis was selectively induced in normal healthy humans by acute water ingestion, and natriuresis was preferentially induced by acute saline ingestion. We found the amount of endogenous immunoreactivity excreted in urine to be correlated with urine flow rate but not with urinary sodium excretion. Urinary excretion of DLIF, normalized to creatinine, was 3.6-fold greater at a urine flow rate of 5.5 mL/min than at 0.5 mL/min. On the other hand, saline intake increased urine flow rate 1.9-fold and increased sodium excretion threefold, but did not affect urinary excretion of DLIF. Fractional excretion of DLIF was linearly related to fractional excretion of water. This study demonstrates that normalization of DLIF values to urinary creatinine does not make DLIF excretion independent of urine flow rate and underscores the need for information on urine flow rate when DLIF measurements in urine are being interpreted.     

Effect of phenylalanine and its metabolites on the proliferation and viability of neuronal and astroglial cells: possible relevance in maternal phenylketonuria

Phenylketonuria is a genetic defect that, without strict dietary control, results in the accumulation of phenylalanine (Phe) in body fluids. If a low-Phe diet is not maintained during pregnancy, the offspring of phenylketonuric women are born with mental retardation and microcephaly. Primary cultures of rat cerebellar granule cells, rat cortical astrocytes, human fetal astrocytes, and human neuroblastoma (SY5Y) cells and human astrocytoma (1321N1) cells were used to test the hypothesis that the microencephaly may be a result of neuronal cell death and reduced astrocyte proliferation. Exposure to Phe or to six Phe metabolites [phenylacetic acid (PAA), phenyllactic acid, hydroxyphenylacetic acid, phenylpyruvic acid, phenylethylamine (PEA), and mandelic acid] did not result in astroglial or neuronal cell cytotoxicity. Treatment of 1321N1 cells, human fetal astrocytes, or rat astrocytes with 5 mM Phe for 24 h decreased DNA synthesis 19 +/- 4, 30 +/- 4, and 60 +/- 6%, respectively. This effect was concentration dependent, and flow cytometry revealed that Phe treatment resulted in the accumulation of cells in the G(0)/G(1) phase of the cell cycle. In addition, in 1321N1 cells, exposure to 5 mM PAA, and in rat astrocytes, exposure to 0.5 mM PEA inhibited cell proliferation 42 +/- 4 and 55 +/- 4%, respectively. These metabolites also resulted in the accumulation of cells in the G(0)/G(1) phase of the cell cycle. In human fetal astrocytes, 0.5 mM PEA and 0.5 mM PAA resulted in a 41 +/- 12 and 52 +/- 11% reduction proliferation, respectively.     

alpha-Bromoisovalerylurea as model substrate for studies on pharmacokinetics of glutathione conjugation in the rat. II. Pharmacokinetics and stereoselectivity of metabolism and excretion in vivo and in the perfused liver

The hypnotic drug alpha-bromoisovalerylurea (BIU) has been studied in the rat with respect to its potential use as model substrate to investigate the pharmacokinetics of glutathione conjugation in vivo. The major metabolites of racemic BIU are the diastereomeric glutathione conjugates (bile) and mercapturates (urine). BIU was metabolized mainly by glutathione conjugation: after i.v. administration of [14C]BIU to freely moving rats, 89% of the dose was recovered in urine within 24 hr, mostly as mercapturates. The rate-limiting step in the clearance of BIU from blood most likely is glutathione conjugation as it was shown that rate-limitation is not due to flow-limited clearance in the liver (the initial extraction ratio of BIU in the perfused liver preparation was low: hepatic extraction ratio = 0.23), protein binding (60% was unbound in plasma) or enzyme saturation (linear pharmacokinetics in the dose range studied: 22-270 mumol/kg). Water solubility of BIU was sufficient to allow its i.v. administration, whereas the absence of toxic effects enables animal as well as human studies. Thus, BIU is a promising model substrate for studies of glutathione conjugation in vivo. In pentobarbital-anesthetized rats with a bile duct catheter, equal amounts of metabolites were excreted in bile (almost exclusively as the two diastereomeric BIU glutathione conjugates) and urine (mostly as the two diastereomeric mercapturates). Based on similar experiments with bile duct-ligated rats, it was concluded that the appearance of the mercapturates in urine could also occur without biliary excretion and subsequent gut metabolism of the BIU glutathione conjugates. The ability of the liver to metabolize BIU was studied in a hemoglobin-free, recirculating liver perfusion system. Of the recovered radioactivity 40% was excreted in bile within 2 hr, almost exclusively in the form of the two BIU glutathione conjugates. Also, glutathione conjugates were found in the perfusate (16% of the radioactivity present in the perfusate after 2 hr). A distinct stereoselectivity was observed in the metabolite excretion rates. The excretion half-lives of the two diastereomeric glutathione conjugates in bile differed 2- to 3-fold, both in anesthetized rats and in the perfused liver preparation. A similar difference in excretion half-lives was found for the urinary excretion of the diastereomeric mercapturates. Thus, BIU can be used to investigate in vivo the stereoselectivity of glutathione conjugation.     

Urinary excretion of homocysteine-thiolactone in humans

BACKGROUND: A metabolite of homocysteine (Hcy), the thioester Hcy-thiolactone, has been implicated in coronary heart disease in humans. Because inadvertent reactions of Hcy-thiolactone with proteins can lead to cell and tissue damage, the ability to detoxify or eliminate Hcy-thiolactone is essential for biological integrity. We examined the hypothesis that the human body eliminates Hcy-thiolactone by urinary excretion. METHODS: We used a sensitive HPLC method with postcolumn derivatization and fluorescence detection to examine Hcy-thiolactone concentrations in human urine and plasma. RESULTS: We discovered a previously unknown pool of Hcy-thiolactone in human urine. Urinary concentrations of Hcy-thiolactone (11-485 nmol/L; n = 19) were approximately 100-fold higher than those in plasma (<0.1-22.6 nmol/L; n = 20). Urinary Hcy-thiolactone accounted for 2.5-28.3% of urinary total Hcy, whereas plasma Hcy-thiolactone accounted for <0.002-0.29% of plasma total Hcy. Urinary concentrations of Hcy-thiolactone, but not of total Hcy, were negatively correlated with urinary pH. Clearance of Hcy-thiolactone, relative to creatinine, was 0.21-6.96. In contrast, relative clearance of Hcy was 0.001-0.003. CONCLUSIONS: The analytical methods described here can be used to quantify Hcy-thiolactone in biological fluids. Using these methods we showed that the human body eliminates Hcy-thiolactone by urinary excretion. Our data also suggest that the protonation status of its amino group affects Hcy-thiolactone excretion.     

Relationship between plasma atriopeptin concentration and function in the conscious primate

The renal actions of atriopeptins (APs) 24, 21 and 28 were examined in the conscious primate, macaca fascicularis. AP-24 increased urine flow rate and sodium excretion 20- and 100-fold, respectively. The circulating form of the atriopeptins, AP-28, had similar, even slightly greater (25%) effects when compared to AP-24. AP-21 on the other hand had dramatically reduced effects, less than 20%, when compared to either AP-24 or AP-28. Infusion of AP-24 resulted in marked increases in plasma immunoreactive AP and in renal function. There were direct, significant linear relations between plasma levels and arterial pressure, heart rate, glomerular flow rate, urine flow rate, sodium and potassium excretion. However, the threshold for these effects was generally higher than expected, i.e., greater than 100 pg/ml. Interestingly, there was a 4-fold greater slope for sodium excretion when compared to other renal functions implying a distinctly different mechanism of action. Whereas, the plasma half-life of the peptide was 2 to 3 min, the biological half-life varied from 6 min for sodium excretion to 10 min for urine flow and potassium excretion. The increased slope for the relationship between sodium excretion and plasma AP concentration and the short half-life for sodium excretion indicate that the change in renal sodium handling is independent of urine flow rate and glomerular filtration rate. There is a direct and linear relationship between plasma peptides and renal function which may imply a cause and effect relationship. This extrapolation may, however, be valid only when plasma peptide levels are elevated markedly.     

Effect of inhibition of gamma-glutamyltranspeptidase on biliary and urinary excretion of glutathione-derived thiols and methylmercury

Acivicin (AT-125; 6.25-200 mumol/kg i.v.) inhibited hepatic, biliary and renal gamma-glutamyltranspeptidase (GGT) activity up to 88, 99 and 97%, respectively, in 4-week-old rats. This inhibition of GGT by acivicin resulted in a 10- to 12-fold increase in the biliary excretion of reduced (GSH) and oxidized glutathione. Because the biliary excretion of cysteinylglycine (Cys-Gly), Cys-Gly disulfide, cysteine (Cys) and cystine concomitantly decreased (63-99%), the biliary excretion rate of total glutathione-derived thiols and disulfides did not change. In contrast, acivicin treatment dramatically elevated the urinary excretion rate of glutathione-derived thiols in a dose-dependent fashion, resulting in a 390-fold increase at the highest dosage. This mainly originated from enhancement of urinary excretion of GSH (up to 7200-fold), although the excretion of Cys and Cys-Gly into urine was also increased. Acivicin treatment did not affect hepatic and renal levels of GSH but, at high dosages, reduced the concentration of Cys in these organs. GSH and oxidized glutathione concentrations in serum were increased, whereas cystine was diminished in acivicin-treated rats. Inhibition of GGT by acivicin (100 mumol/kg i.v.) failed to influence the biliary excretion of methylmercury but increased urinary excretion 34-fold. Even though the urinary thiol excretion was much higher than the biliary thiol excretion in the acivicin-treated rats, methylmercury was preferentially excreted into bile rather than urine, indicating the importance of the liver as an excretory organ for methylmercury.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adenine metabolism in man. 1. After intravenous and peroral administration

The basal plasma concentration of adenine and its renal excretion was studied in two men. For its analysis partly new chromatographic techniques were developed. The plasma concentration varied around 70 nmol/l; the renal excretion rate was, as reported earlier by other investigators, around 10 nmol/min. Loadings, intravenously during about 3 h and orally, both with about 30 nmol adenine per kg body mass revealed that most of the adenine was metabolized to nucleotide form. In the experiments with intravenous administration of adenine only about 2% of the given dose appeared in the urine as adenine and somewhat less as the easily precipitable metabolite 2,8-dihydroxyadenine. In the peroral loads, with higher plasma adenine concentrations, the ratios of the renally excreted two compounds were one to a few per cent higher.     

Kinetics and pharmacodynamics of atrial natriuretic peptide and lithium clearance in the isolated perfused rat kidney

We used the kinetics of atrial natriuretic peptide (ANP) and fractional lithium excretion to test the hypothesis that ANP-induced natriuresis is related directly to the ANP perfusate concentration and is mediated by a decrease in proximal tubular sodium reabsorption. Wistar rat kidneys were perfused for 90 min after administration of 1 microgram of human ANP. Comparisons were made to control perfusions. ANP had a short half-life (12.4 min) and a renal clearance 6-fold greater than the creatinine clearance. ANP treatment resulted in a significant peak increase in sodium (5.6-fold), lithium (2.1-fold), potassium (2.3-fold) and water (5.1-fold) excretion. The natriuretic response to ANP was delayed 25 min. Exogenous creatinine clearance, perfusion pressure, perfusion flow and renal vascular resistance were not affected. We conclude that the kidney eliminates ANP rapidly by mechanisms that appear to be located primarily in the peritubular circulation. The initial renal response to ANP did not follow perfusate concentrations. Rather, natriuresis increased as ANP concentrations decreased. The relationship was defined by a counterclockwise hysteresis loop. Natriuresis was not mediated by an increased delivery of sodium to the tubule, but may have been due to either a direct or an indirect action of ANP on the kidney.     

Pharmacokinetics and metabolism of 14C-tinidazole in humans

Following intravenous infusion of 800 mg of 14C-tinidazole during 30 min to two human subjects, a mean of 44% of the dose was excreted in urine during the first 24 h, increasing to 63% of the dose during five days: 12% of the dose was excreted in the faeces, indicating the possible involvement of biliary excretion and other secretory processes in the disposition of tinidazole. At 6 min after the end of the infusion, the mean plasma tinidazole concentration was 12 mg/l. Tinidazole was a major component in 0-120 h urine (about 32% of urinary 14C): the major metabolite in the 0-12 h urine examined was ethyl 2-(5-hydroxy-2-methyl-4-nitro-1-imidazolyl)ethyl sulphone (about 30% urinary 14C), the product of hydroxylation and nitro-group migration. These compounds were also present in the faeces. A minor urinary metabolite was 2-hydroxymethyltinidazole (about 9% urinary 14C), which was also present in plasma. The mean pharmacokinetic parameters obtained for tinidazole were similar to those reported in the literature; total clearance 51 ml/min, renal clearance 10 ml/min, volume of distribution 501 and half-life 11.6 h.     

The effect of polyethylene glycol by gavage on electrolyte and water excretion in the rat

The effects of polyethylene glycol (PEG) 200 administered by gavage on electrolyte and water excretion were investigated in the rat. PEG 200 led, in intact rats, to dose-related increased drinking and to diuresis. In the first 2 h after PEG 200 administration, water consumption in intact rats exceeded urine output. PEG 200 enhanced the excretion of both sodium and potassium, but the sodium excretion was proportionately greater, resulting in an elevation of the urinary sodium/potassium ratio. Bilateral nephrectomy was not accompanied by increased drinking in PEG 200-treated rats, although raised serum osmolality was seen. Thus, given by gavage, PEG 200 is not an inert vehicle for drug administration.     

Complexing activity of 2,3-dimercapto-1-propanesulfonate and its disulfide auto-oxidation product in rat kidney

The complexing activity of 2,3-dimercapto-1-propanesulfonate (DMPS) was examined in vivo and in the isolated perfused rat kidney. Intravenous infusions of the sulfhydryl form of DMPS into rats pretreated with HgCl2 resulted in a dose-dependent increase in the urinary excretion of mercury and a decrease in the retention of mercury in kidney. A 120-min infusion of 2 mg sulfhydryl DMPS.min-1.kg body wt-1 resulted in excretion of 76% of the mercury body burden of which greater than 90% was removed from kidney. The disulfide auto-oxidation product of DMPS significantly increased the urinary excretion of mercury in the rat, but its effect was less than that of an equivalent dose of sulfhydryl DMPS. The presence of disulfide DMPS or 3:1 mixtures of sulfhydryl DMPS and disulfide DMPS in the perfusate increased the excretion of mercury and induced a loss of mercury from the cortex and outer medulla in the isolated perfused rat kidney. Thus both the disulfide and sulfhydryl forms of DMPS can act directly on the kidney to accelerate the excretion of mercury. This is consistent with previous observations of the ability of the kidney to reduce disulfide DMPS to sulfhydryl DMPS.     

Urinary pH as a determinant of prostaglandin E2 excretion by the conscious rat

The urinary excretion of prostaglandin E2 (PGE2) was measured in conscious rats under conditions which produced either acid or alkaline urine, but a similar change in fluid and solute excretion. Oral isotonic saline increased both urine flow and sodium excretion but did not alter urinary PGE2 output (which remained constant at 80 pmol/3 h per rat) or urine pH (6.2). When the urine was made alkaline (pH 7.8) by oral sodium bicarbonate or carbonate, urinary PGE2 was approximately 3-fold greater (P less than 0.001) than the control (pH 6.2). The urine flow and sodium output were also increased. When the urine was made acidic (pH 5.7) by oral ammonium chloride, urinary PGE2 excretion was reduced (P less than 0.01) to approximately half the control output. The urine flow and sodium output increased. Within a group of 12 rats receiving oral isotonic saline a positive linear correlation coefficient (P less than 0.002) was established between urine pH and PGE2 excretion. The results indicate that urine pH may be a determinant of PGE2 excretion in unrestrained, conscious rats. It seems likely that this effect of pH is mediated by a change in the passive reabsorption of PGE2 in the distal nephron, although alternative explanations such as altered tubular secretion or synthesis cannot be categorically excluded.     

Contribution of the human kidney to the metabolic clearance of drugs.

OBJECTIVE: To demonstrate that the human kidney is capable not only of filtering and secreting drugs and their metabolites, but also of carrying out conjugation reactions such as acyl glucuronidation, N-glucuronidation, and glycination. DATA SOURCES: Plasma concentrations and renal excretion rates of drugs are measured and renal clearance is calculated in a series of selected pharmacokinetic studies in healthy human volunteers (some studies were conducted in the authors' laboratory and others were reported in the literature). BACKGROUND THEORY: It is generally agreed that the liver plays the dominant role in drug metabolism, and that the function of the kidneys is limited to excretion of parent drug and metabolites. This can be easily understood when a metabolite is present in both plasma and urine. When the metabolite is present in urine but is not measurable in plasma, then the possibility exists that the metabolite is formed by the kidneys. RESULTS: "Simple" excretion by the kidneys is demonstrated for sulfatroxazole/sulfamethoxazole. Ether glucuronides of codeine are formed in the liver, and the resulting glucuronide is excreted by the kidneys. Possible formation of N1- and N2-glucuronides by the kidneys is demonstrated for sulfadimethoxine, sulfametomidine, and sulfaphenazole. Acyl glucuronidation of probenecid and nalidixic acid is carried out by the kidneys. The acyl glucuronidation of probenecid shows a capacity-limited formation/excretion rate of 46 mg/h, which is subject dependent. During this process, the acyl glucuronidation of co-administered nalidixic acid is reduced from 53 to 16 percent compared with that of nalidixic acid alone. Probenecid and its acyl glucuronidation do not inhibit the ether glucuronidation of codeine in the liver, but only interfere with the active tubular secretion process. The acyl glucuronidation of the nonsteroidal antiinflammatory drug naproxen and its metabolite, O-desmethylnaproxen, may be carried out by the liver and kidneys. Glycination of benzoic acid and salicylic acid is carried out in both the liver and kidneys. CONCLUSIONS: It is difficult to recognize renal drug metabolism in the intact human body (in vivo); the glucuronides or conjugates must be measured via direct HPLC analysis. In cases where the metabolite is present in high concentrations in urine but not in blood, there may be an indication that the kidneys are responsible for the formation of the metabolite. Impaired kidney function not only affects renal excretion but may also affect renal metabolism.     

The uri nary excretion of 2-phenylethylamine in phenylketonuria

The urinary concentrations of free and conjugated 2-phenylethylamine were determined in phenylketonuric patients and normal subjects by solvent extraction and gas chromatography. Free 2-phenylethylamine excretion was found to be significantly elevated above normal in phenylketonuric adults and children receiving a normal or a slightly restricted intake of phenylalanine. Urinary 2-phenylethylamine was also significantly increased in phenylketonuric children receiving low phenylalaine dietary therapy. Conjugated 2-phenylethylamine excretion was found not to be increased above normal. In the light of these results, a relationship between blood phenylalanine concentration and 2-phenylethylamine excretion is proposed, and the possible role of this amine in phenylketonuria is discussed.     

Biochemical and neuropsychological effects of elevated plasma phenylalanine in patients with treated phenylketonuria. A model for the study of phenylalanine and brain function in man.

Phenylketonuria provides a human model for the study of the effect of phenylalanine on brain function. Although irreversible mental retardation is preventable through newborn diagnosis and dietary phenylalanine restriction, controversy exists regarding the effects of increased concentrations of phenylalanine in older patients. We have studied ten older, treated, phenylketonuric patients using a triple-blind, multiple trials, crossover design. Each patient was tested at the end of each of three 1-wk periods of high or low phenylalanine intakes. Tests included a repeatable battery of neuropsychological tests, analysis of plasma amino acids, and measurement of urine amino acids, phenyl organic acids, dopamine, and serotonin. In all 10 patients plasma phenylalanine rose (900-4,000 microM). In 9 of 10 patients there was an inverse relationship between plasma phenylalanine and urine dopamine excretion. When blood phenylalanine was elevated, these patients had prolonged performance times on neuropsychological tests of higher but not lower integrative function. Urinary serotonin fell during phenylalanine loading in six patients. The concentration of phenylacids in the urine was not proportional to the plasma phenylalanine at concentrations below 1.5 mM. In one patient, neither performance time nor dopamine excretion varied as blood phenylalanine rose or fell. We interpret these data as follows: blood phenylalanine above 1.3 mM impairs performance on neuropsychological tests of higher integrative function, this effect is reversible, and one mechanism may involve impaired biogenic amine synthesis.     

Isoproterenol excretion and metabolism in the isolated perfused rat kidney.

[3H]isoproterenol excretion and metabolism were studied in the isolated perfused rat kidney using a one-pass, non-recirculating perfusion system with constant infusion rates of [3H]isoproterenol. The [3H]isoproterenol (U/P) to inulin (U/P) ratio was approximately 15 indicating extensive tubular secretion. A major renal metabolite, 3-O-methylisoproterenol, appeared in the urine and renal vein perfusate and also accumulated in the renal tissue. The fractional excretion of isoproterenol decreased with time while fractional excretion of p-aminohippurate remained stable. The observed decreasing urinary clearance and percent extraction of isoproterenol with time may be due to the progressive intrarenal accumulation of 3-O-methylisoproterenol.     

Pharmacokinetic studies in humans with the oral iron chelator 1,2-dimethyl-3-hydroxypyrid-4-one

Pharmacokinetic studies have been carried out with the oral iron chelator 1,2-dimethyl-3-hydroxypyrid-4-one (L1). HPLC analysis of serum of a normal volunteer and seven transfusional iron loaded patients who ingested a 3 gm dose of L1 revealed that L1 was most probably absorbed from the stomach and was transferred to the blood with a half-life of 0.7 to 32 minutes. L1 reached maximum concentration in the serum 12 to 120 minutes after administration with 85% to 90% elimination within the first 5 to 6 hours, with a half-life of 47 to 134 minutes. L1 and its glucuronide metabolite were identified in serum and urine but not in feces. In most cases hydrolysis of 24-hour urine samples with use of beta-glucuronidase resulted in almost complete recovery of the administered dose. Urinary iron excretion was proportional to the iron load but not to the serum or urine concentration of L1. The therapeutic efficiency of L1 can therefore be improved by repeated administration of 2 to 3 gm doses at least every 6 hours.     

Caffeine metabolism in the newborn.

The concentrations of caffeine and metabolites in urine have been examined as a function of age to explore the remarkably slow elimination of caffeine by human infants. Urine samples were obtained from 3 adults and 10 infants aged 8 days to 8 months during therapeutic treatment with caffeine. A high-performance liquid chromatographic (HPLC) procedure involving reversed-phase partition chromatography was developed to separate caffeine and 13 of its metabolites. During the first month of life, caffeine accounted for more than 85% of the identifiable products in urine. Caffeine remained the predominant component for the first 3 months, but its percentage decreased gradually to the adult value of less than 2% by the age of 7 to 9 months. This change reflected increasing metabolite production, not decreasing urinary caffeine concentration. The adult metabolite pattern of partially demethylated xanthines and urates was attained by 7 to 9 months. The data indicate that the 4-day plasma t1/2 of caffeine characteristic of the newborn depends in large part on slow urinary excretion of unchanged drug since there is little or no metabolism. Subsequent decrease in the t1/2 to about 4 hr by the age of 8 months correlates closely with the rise in metabolite production.