Brain-derived neurotrophic factor Val66Met genotype and ovarian steroids interactively modulate working memory-related hippocampal function in women: a multimodal neuroimaging study.

Preclinical evidence suggests that the actions of ovarian steroid hormones and brain-derived neurotrophic factor (BDNF) are highly convergent on brain function. Studies in humanized mice document an interaction between estrus cycle-related changes in estradiol secretion and BDNF Val66Met genotype on measures of hippocampal function and anxiety-like behavior. We believe our multimodal imaging data provide the first demonstration in women that the effects of the BDNF Val/Met polymorphism on hippocampal function are selectively modulated by estradiol. In a 6-month pharmacological hormone manipulation protocol, healthy, regularly menstruating, asymptomatic women completed positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) scans while performing the n-back working memory task during three hormone conditions: ovarian suppression induced by the gonadotropin-releasing hormone agonist, leuprolide acetate; leuprolide plus estradiol; and leuprolide plus progesterone. For each of the three hormone conditions, a discovery data set was obtained with oxygen-15 water regional cerebral blood flow PET in 39 healthy women genotyped for BDNF Val66Met, and a confirmatory data set was obtained with fMRI in 27 women. Our results, in close agreement across the two imaging platforms, demonstrate an ovarian hormone-by-BDNF interaction on working memory-related hippocampal function (PET: F2,37=9.11, P=0.00026 uncorrected, P=0.05, familywise error corrected with small volume correction; fMRI: F2,25=5.43, P=0.01, uncorrected) that reflects differential hippocampal recruitment in Met carriers but only in the presence of estradiol. These findings have clinical relevance for understanding the neurobiological basis of individual differences in the cognitive and behavioral effects of ovarian steroids in women, and may provide a neurogenetic framework for understanding neuropsychiatric disorders related to reproductive hormones as well as illnesses with sex differences in disease expression and course.

Acute changes in arginine vasopressin, sweat, urine and serum sodium concentrations in exercising humans: does a coordinated homeostatic relationship exist?

The parallel response of sweat rate and urine production to changes in plasma osmolality and volume support a role for arginine vasopressin (AVP) as the main endocrine regulator of both excretions. A maximal test to exhaustion and a steady-state run on a motorised treadmill were both completed by 10 moderately trained runners, 1 week apart. Sweat, urine and serum sodium concentrations ([Na+]) were evaluated in association with the plasma concentrations of cytokines, neurohypophyseal and natriuretic peptides, and adrenal steroid hormones. When data from both the high-intensity and steady-state runs were combined, significant linear correlations were noted between: sweat [Na+] versus postexercise urine [Na+] (r=0.80; p<0.001), postexercise serum [Na+] versus both postexercise urine [Na+] (r=0.56; p<0.05) and sweat [Na+] (r=0.64; p<0.01) and postexercise urine [Na+] versus postexercise plasma arginine vasopressin concentration ([AVP](P)) (r=0.48; p<0.05). A significant positive correlation was noted between postexercise [AVP](P) and sweat [Na+] during the steady-state condition only (r=0.66; p<0.05). These correlations suggest that changes in serum [Na+] during exercise may evoke corresponding changes in sweat and urine [Na+], which are likely regulated coordinately by changes in [AVP](P) to preserve body fluid homeostasis.

Diurnal variation of steroid hormones and their reference intervals using mass spectrometric analysis.

OBJECTIVE: Accurate measurement of steroid hormones remains challenging. Mass spectrometry affords a reliable means for quantitating steroid profiles accurately. Our objective was to establish and define (1) the extent of diurnal fluctuations in steroid concentrations that potentially necessitate strict adherence to time of sample acquisition and (2) time-dependent steroid reference intervals. DESIGN: Nine steroid markers were examined in couplets in males and females. METHODS: Using isotope dilution high-performance liquid chromatography-tandem mass spectrometric (LC-MS/MS) analysis, we developed a multi-steroid profile requiring only a minimal volume of serum (0.1 mL). Couplet (AM and PM) measurements of steroid hormones for 120 healthy females (F) and 62 healthy males (M) were obtained. Patients were recruited from several participating centers. RESULTS: The following diurnal values were noted to be significantly different in both females and males: cortisone, cortisol, corticosterone, 11 deoxycortisol (11 DOC), androstenedione, 17a-hydroxyprogesterone (17 OHP) and dehydroepiandrosterone (DHEA). Testosterone was only found to have significant diurnal variance in males. Progesterone showed no significant difference in AM and PM values for either groups and thus may provide an internal control. CONCLUSIONS: When diagnosing endocrine disorders, it is imperative to acknowledge the 24-h diurnal variation of the biochemical steroid markers. We highlight the importance of standardization of collection times and appropriate implementation of reference intervals. PRECIS: We identify diurnal fluctuations in steroid concentrations with time of day and emphasize the importance of adhering to firm time of sample acquisition.

Does TSH Reliably Detect Hypothyroid Patients?

OBJECTIVES: To evaluate the reliability of normal Thyroid Stimulating Hormone (TSH) as a thyroid function test and assess the effect of Adrenocorticotropic Hormone (ACTH) on serum TSH concentration. DESIGN AND METHODS: Patients presenting to the National Institutes of Health Department of Endocrinology outpatient clinic with symptoms consistent with hypothyroidism were identified. Thyroid hormone concentrations were measured by liquid chromatography/tandem mass spectrometry and immunoassay. Patients with normal TSH concentrations were assessed for both clinical and biochemical hypothyroidism.We evaluated the effect of ACTH stimulation (performed on patients for assessment of adrenal function) on TSH concentration. RESULTS: Patients with symptoms consistent with hypothyroidism but with normal TSH values in the range of 1-4 IU/mL and normal free T4 (FT4) values by immunoassay measurements were confirmed to be biochemically hypothyroid following measurements of thyroid hormones by mass spectrometry. We present case studies of two patients, a 76-year-old male and a 58-year-old female. Improvement in the male patient's hypothyroid symptoms, including afternoon fatigue, constipation, alopecia, dry skin and high cholesterol, was documented after initiating thyroid hormone replacement.ACTH stimulation resulted in an average decrease of 17% in TSH between time 0 and 60 minutes post stimulation. CONCLUSION: Although measurement of TSH is a convenient screen for thyroid function, it is influenced by many factors which may affect its overall reliability. We believe thyroid function should be assessed by more than a single test. We recommend measurement of thyroid hormone concentrations by mass spectrometry if the patient's clinical presentation is discordant with their TSH levels.

Age-dependent pharmacokinetics of lamivudine in HIV-infected children.

The recommended dose of lamivudine in children is higher when compared with adults: 4 mg/kg vs approximately 2 mg/kg (150 mg) and administered twice a day. Limited data are available to demonstrate that this increased dose results in adequate exposure to lamivudine in children with human immunodeficiency virus (HIV) infection. Data were selected from children who were using lamivudine for at least 2 weeks before a full pharmacokinetic (PK) study was conducted. Lamivudine PK parameters were significantly related to age. The age of 6 years appeared to be a cutoff for a change in PK parameters of lamivudine, with children <6 years of age (n=17) having a median area under the curve 43% lower and a median peak plasma concentration 47% lower (both P<0.001) than older children (n=34). In conclusion, further investigation of the relationship between decreased lamivudine exposure and treatment outcome and long-term resistance development in younger children with HIV infection is warranted.

Biopterin synthesis defect. Treatment with L-dopa and 5-hydroxytryptophan compared with therapy with a tetrahydropterin.

We have identified a generalized deficiency of monoamine neurotransmitters in a patient with a defect in biopterin synthesis. Neurotransmitter precursors (L-3,4-dihydroxyphenylalanine [L-dopa]; 5-hydroxytryptophan [5-HTP] and a tetrahydropterin [6-methyltetrahydropterin (6MPH4)] were investigated for their ability to normalize monoamine neurotransmitter metabolism. Before treatment, the concentrations of dopamine (DA), norepinephrine, epinephrine, and six monoamine metabolites were very low or undetectable in plasma, cerebrospinal fluid, or urine. L-Dopa and 5-HTP replacement was begun at age 7 mo. This therapy generally corrected the deficiency of monoamines and their metabolites, and improved neurological development until the age of 25 mo. Despite these benefits, the intermittent administration of L-dopa could not produce a stable improvement of acute neurological function or DA metabolism. In the 3 h after L-dopa administration, plasma DA and the motor activity and alertness of the patient rose and fell in parallel. Doses of L-dopa that were clinically optimal produced normal plasma levels of norepinephrine and epinephrine, but excessive concentrations of DA and its metabolites. Furthermore, the clinical and biochemical effects of L-dopa were inhibited by phenylalanine and 5-HTP, respectively, demonstrating that these amino acids have antagonistic pharmacological effects. Physiological correction of the monoamine deficit and the hyperphenylalaninemia of this disorder was attempted at age 35 mo using high doses (8-38 mg/kg per d) of 6MPH4. 6MPH4, a synthetic analogue of tetrahydrobiopterin, controlled the hyperphenylalaninemia. Significant concentrations of 6MPH4 were obtained in the cerebrospinal fluid; no neurological improvement or stimulation of monoamine synthesis in the central nervous system was detected. These findings indicate the complexity in replacement therapy with L-dopa and 5-HTP, but suggest that this treatment may be partially effective in biopterin-deficient patients who are unresponsive to high doses of tetrahydropterins.

Significance of the endogenous digoxin-like substance in infants and mothers.

Digoxin serum concentrations were measured by a routine radioimmunoassay in 30 neonates not receiving digoxin; nonetheless, digoxin levels were between 0.17 nM and 1.64nM (means = 0.64nM +/- 0.27 nM). There was a negative correlation between gestational age and concentration of an endogenous digoxin-like substance (EDLS). Neonates less than or equal to 32 wk gestational age had higher levels of EDLS than neonates greater than 32 wk old. EDLS concentrations were compared in 22 mothers and their 24 offspring and were higher in all newborn infants (0.34nM +/- 0.09nM and 0.15nM +/- 0.08nM). EDLS was shown to inhibit Na+-K+-adenosinetriphosphatase activity by measurement of 86Rb uptake in erythrocytes exposed to sera samples from 30 infants in the study. EDLS levels greater than 0.6 ng/ml were associated with lesser 86Rb uptake. Simulation kinetics suggest that the presence of 0.6nM EDLS would lengthen the digoxin t1/2 by 64%, reduce the volume of distribution by 23%, and lower clearance by 53% if the peak "true" digoxin level were 2 ng/ml. EDLS concentrations of 1.5 ng/ml would increase the t1/2 by 207% while reducing the volume of distribution by 43% and clearance by 81%. These considerations cast serious doubts on the validity of currently accepted digoxin kinetics and dosing in preterm infants.

Rapid development of enhanced clearance after high-dose cyclophosphamide.

We have studied the disposition of cyclophosphamide, its major cytotoxic metabolite phosphoramide mustard, and the synthetic glucocorticoid dexamethasone in nine patients receiving high-dose cyclophosphamide daily for 2 days before bone marrow transplantation. The total body clearance of cyclophosphamide was observed to increase from 93 +/- 27 ml/min on the first day to 178 +/- 83 ml/min on the second day. This was associated with an increase in the clearance of dexamethasone from 369 +/- 104 ml/min to 526 +/- 123 ml/min. An increased rate of formation of phosphoramide mustard with higher peak concentrations was also seen. Simulation studies show that these changes are most likely the result of an increase in the hepatic metabolism of cyclophosphamide. These results show that high-dose cyclophosphamide causes an increase in its own clearance and that of dexamethasone through an apparent induction of hepatic-metabolizing enzymes detectable 24 hours after initial exposure to cyclophosphamide.

Ranitidine kinetics and dynamics. I. Oral dose studies.

Ranitidine, an H2-receptor antagonist, has been shown to reduce pentagastrin-stimulated gastric secretion. We examined the relationship between inhibition of gastric secretion and ranitidine serum concentration. Twelve normal male subjects received 20, 40, or 80 mg of ranitidine orally 90 min before starting a 3-hr continuous infusion of pentagastrin, 2 micrograms/kg/hr. Ranitidine, 20, 40, and 80 mg, reduced hydrogen ion output by 29%, 50%, and 70% and secretion volume by 21%, 37%, and 47%. Pepsin activity was reduced by 8%, 50%, and 49% by the same doses. Peak serum concentration was correlated positively with percent reduction in hydrogen ion output (r = 0.81, P less than 0.001) and volume (r = 0.71, P less than 0.01) over a 2-hr period. A 50% inhibition of hydrogen ion output was associated with a peak ranitidine serum concentration of 165 micrograms/l and subjects reached peak serum concentration 60 to 120 min after oral dosing. An appropriate therapeutic effect should be achieved with 8 hourly doses of 80 mg ranitidine. No clinically significant subjective or toxic biochemical effect of ranitidine was seen after single doses. White blood cell count was reduced in 11 of 12 subjects 7 days after ranitidine, an observation which calls for further investigation.

Pharmacokinetic determinants of 6-mercaptopurine myelotoxicity and therapeutic failure in children with acute lymphoblastic leukemia.

The pharmacokinetics of oral 6-mercaptopurine (6MP) was assessed in 20 children with acute lymphoblastic leukemia during maintenance therapy. The AUC was between 0 and 6 X 10 ng X min/ml, and AUC normalized to 1 mg/m2 of 6MP was between 0 and 815 ng X min/ml. Good correlation existed between peak concentrations and AUC (r = 0.866; P less than 0.001). In more than half of the cases there was evidence of prolonged elimination t1/2 or rebound of a serum concentration during the elimination phase corresponding to either an additional compartment or enterohepatic circulation of 6MP. One child did not achieve detectable concentrations on 2 different study days and was switched to a different protocol. The two children who had severe myelotoxicity achieved the largest AUC values per milligram per square meter of 6MP. Our results indicate that pharmacokinetic variability may contribute to either severe myelotoxicity or therapeutic failures. This suggests that monitoring of this drug in children with acute lymphoblastic leukemia may be helpful.

Ranitidine kinetics and dynamics. II. Intravenous dose studies and comparison with cimetidine.

Intravenous ranitidine has been shown to reduce pentagastrin-stimulated gastric secretion. Eight normal men received, in randomized order, 60 mg ranitidine or 300 mg cimetidine intravenously over 2 min. Both ranitidine and cimetidine induced decreases in volume hydrogen ion content and pepsin activity of stimulated gastric juice. Ranitidine half-life (t1/2) was 2.1 +/- 0.1 hr and cimetidine (t1/2) was 1.5 +/- 0.1 hr. Ranitidine volume of distribution was 1.6 +/- 0.1 l/kg and that of cimetidine was 1.12 +/- 0.12 l/kg. The clearance of ranitidine was 0.54 +/- 0.04 l/kg hr-1 and that of cimetidine was 0.5 +/- 0.05 l/kg hr-1. It is suggested that the intravenous loading dose of ranitidine necessary to attain a serum concentration of 200 micrograms/l (which would achieve a 50% inhibition of gastric acid) is 0.3 mg/kg, followed by an infusion rate of 0.11 mg/kg hr-1.

Aminoglycoside-related nephrotoxicity in the premature newborn.

The nephrotoxicity of gentamicin and amikacin was compared during presumed sepsis in 107 premature neonates. To examine the possibility that nephrotoxicity was directly associated with the clinical conditions of "sepsis," a control group of 26 chloramphenicol-treated newborns was also studied. Two markers of proximal renal tubular injury, N-acetyl-beta-glucosaminidase (NAG) and beta 2-microglobulin, were measured in 6-hr aliquots of urine. Because urine creatinine excretion increased with postconception age, markers were expressed in terms of excretion rate rather than per milligram of creatinine. The NAG excretion rate was significantly higher in gentamicin-treated patients (138 +/- 10 U/min, mean +/- SE) than in amikacin-treated patients (85 +/- 7 U/min) but did not differ between patients treated with amikacin and those treated with chloramphenicol (81 +/- 11 U/min). Excretion of beta 2-microglobulin did not differ among the three patient groups. We conclude that amikacin may be less nephrotoxic than gentamicin in the premature newborn.

Neurologic manifestations of cocaine exposure in childhood.

OBJECTIVE: To describe the neurologic manifestations of cocaine exposure in children and adolescents as the neurologic effects of cocaine have been described in adults and neonates. METHODS: During 1-year period, 41 children between the ages of 2 months and 18 years who had been exposed to cocaine, were examined in the emergency department at the Children's National Medical Center. Cocaine exposure was documented on urine samples; all were confirmed by urine gas chromatographic/mass spectrometric analysis. RESULTS: Nineteen (46%) of 41 had neurologic abnormalities, including seizures (7), obtundation (6), delirium (4), dizziness (1), drooling (1), and ataxia (1). In 14 others, the neurologic effects of cocaine were difficult to determine because of other concomitant medical conditions, including head injuries and severe abdominal or chest trauma. Two major age-related patterns were seen: (a) in each child < 5 years of age, seizures and obtundation; and (b) in 11 older children, delirium (3), dizziness (1), drooling (2), and lethargy (4). Seizures, occurring at ages 12 months to 8 years, were focal with secondary generalization in three and generalized in four. They were associated with fever in two children. Six children had no further seizures, and one developed a mixed-seizure disorder. Passive intoxication while being in a room in which "crack" was smoked was the most likely cause of exposure for young victims. Multiple drug abuse was not documented in any child with neurologic impairment. CONCLUSIONS: 1) Cocaine exposure is common in children in our urban setting; 2) neurologic manifestations frequently occur; 3) in children 8 years of age or younger, "passive" ingestion/inhalation is associated with focal and generalized seizures without evidence of structural brain injury; 4) cocaine may lower seizure threshold in children predisposed to seizures; 5) in children > 8 years of age, manifestations are similar to those in adults; 6) trauma and motor vehicle accidents were seen in the adolescent age group exposed to cocaine; and 7) urine toxicological study in cocaine exposure is recommended in all first-time seizures as well as first-time febrile seizures.

High-versus low-dose, frequently administered, nebulized albuterol in children with severe, acute asthma.

Thirty-two 5- to 17-year-old children who had severe, acute asthma were randomly assigned to receive either high doses (0.15 mg/kg of body weight per dose) or low doses (0.05 mg/kg of body weight per dose) of nebulized albuterol every 20 minutes for six doses. Compared with the low-dose regimen, the high-dose regimen resulted in significantly greater improvement in forced expiratory volume in 1 second, forced vital capacity, and wheeze score and a lower hospitalization rate. The changes in heart rate, respiratory rate, blood pressure, white blood cell count, and serum potassium concentration did not differ significantly between the groups. The incidence of side effects, which included tremor, hyperactivity, and vomiting, was not significantly different in the two populations. Serum albuterol levels varied widely, but there was no correlation between the levels and the increase in heart rate or other side effects. high-dose, frequently administered, nebulized albuterol appears both safe and effective in treating severe, acute asthma in children.

Methylphenidate hydrochloride given with or before breakfast: II. Effects on plasma concentration of methylphenidate and ritalinic acid.

Methylphenidate HCl (Ritalin) is often prescribed for the treatment of hyperactivity and is usually administered orally 30 minutes to 1 hour before meals, based on an assumption that meals may interfere with the absorption or metabolism of the drug. Seven boys who were taking methylphenidate regularly for the treatment of hyperactivity were hospitalized and given their established dose of the drug intravenously or orally, either with breakfast or in a fasted state. Blood samples were taken to determine the pharmacokinetics of the drug in each condition. Few differences between the "fed" and "fasted" states were noted, but the statistically significant differences indicated that meals accelerate rather than impede the absorption of methylphenidate.

Pemoline pharmacokinetics and long term therapy in children with attention deficit disorder and hyperactivity.

The pharmacokinetic behaviour of pemoline was studied in 28 children, aged 5 to 12 years, diagnosed as having the attention deficit disorder with hyperactivity. The mean elimination half-life of pemoline in these children was approximately 7 hours, which is considerably shorter than the half-life of 11 to 13 hours previously reported in adults. The tendency of the half-life to increase with age may be explained by the statistically significant decrease in total body clearance with age. The increasing half-life of pemoline with age should be considered during long term drug therapy. In this study no tolerance to the beneficial effects of pemoline was observed over 6 months. The apparent therapeutic serum concentration range for these children was attained after doses of 37.5 to 131.25 mg pemoline daily. Since the optimum serum concentration shows wide variation, the dosing regimen must be determined individually. Routine monitoring of the pemoline serum concentrations is not useful because of this apparent variation in optimum serum concentration and because of the linear relationship between dose and concentration.

An immunophilin-binding assay for sirolimus.

OBJECTIVE: This review examines the performance of 4 assays for sirolimus in terms of their ability to meet 6 guidelines determined by a panel of experts. BACKGROUND: Four methods have been described to date for the analysis of sirolimus concentrations in whole blood: high-performance liquid chromatography-mass spectrometry (HPLC-MS); microparticle enzyme immunoassay (MEIA); p70 S6 kinase inhibition; and an immunophilin-binding assay (IBA). METHODS: A MEDLINE search of the literature was performed to identify relevant studies. RESULTS: The HPLC methods suffer from precision problems because of the substantial specimen preparation required, and HPLC-MS methods are not practical for clinical use. Initial studies of the MEIA have found overestimation of sirolimus concentrations that may be caused by antibody cross-reactivity with sirolimus metabolites. Monitoring of sirolimus effects by p70 S6 kinase inhibition is as yet possible only theoretically, and the assay itself is not yet optimal. With the IBA, use of a T-cell protein that binds to sirolimus and that may be the intracellular target of the drug as the assay binding protein allows the assay to measure sirolimus selectively, even in the presence of structurally similar metabolites. CONCLUSION: More than 200 clinical samples have been analyzed by the IBA, and correlation with HPLC values has been good, with a regression line slope near 1.0. In addition, the assay is easier to perform and more precise than HPLC, and has the potential to be automated. Thus, the IBA appears to have certain clear advantages over the other assays.

Disposition of total and free cisplatin on two consecutive treatment cycles in patients with ovarian cancer.

The disposition of total and ultrafilterable cisplatin was determined in 12 women with ovarian carcinoma receiving cyclophosphamide 500 mg/m2, adriamycin 50 mg/m2 and cisplatin 50 mg/m2 during their first and second course. Plasma samples were obtained over 96 h following the completion of the cisplatin infusion and assayed for total platinum by atomic absorption spectroscopy. Plasma samples obtained up to 4 h after cisplatin infusion contained measurable ultrafilterable (free) cisplatin. The mean disposition of free cisplatin conformed to a two-compartment model with a mean terminal half-life (+/- SD) of 46.2 +/- 20.2 min during the first course and 37.8 +/- 18.0 min during the second course of therapy. The mean disposition of total cisplatin conformed to a three-compartment model with a mean terminal half-life (+/- SD) of 57.8 +/- 19.3 h during the first course and 86.6 +/- 33.3 h during the second course of therapy. We found that the mean total cisplatin levels were significantly higher during the second course than the first course and the total body clearance of total platinum decreased from the first to the second course. Divided urine collections were obtained over 24 h after completion of cisplatin infusion, but cisplatin was not always detectable at all time intervals. The total fraction recovered was 0.14 and 0.12 of administered dose after the first and the second course, respectively. Renal clearance was 0.61 +/- 0.32 l/h/m2 and 0.45 +/- 0.16 l/h/m2 for the first and the second course, respectively. We conclude that: urinary platinum excretion is variable between patients and with time; a trend to decreased renal clearance of platinum from first to second course may be due to a decrease in renal excretion of cisplatin; and the body's elimination pathways clear less platinum upon repeat administration.

Variability in the pharmacokinetics of cyclophosphamide, methotrexate and 5-fluorouracil in women receiving adjuvant treatment for breast cancer.

A total of 23 women with stage II breast cancer receiving adjuvant cyclophosphamide, methotrexate and 5-fluorouracil had detailed pharmacokinetic monitoring performed on the first and third courses of therapy. The area under the concentration time curve (AUC) of each of these three drugs varied by a factor of 3-4 among patients. No systematic change in pharmacokinetics between the first and third courses was seen for cyclophosphamide, methotrexate or 5-fluorouracil, and the mean AUC for each of the three drugs did not change. However, significant intrapatient variability in drug pharmacokinetics was observed for all three drugs such that the AUC, clearance and half-life in an individual on the third course could not be reliably predicted from data generated on the first course. On the basis of these results, cyclophosphamide, methotrexate, and 5-fluorouracil pharmacokinetic data from one treatment would not be useful information from which the doses of subsequent courses could be determined.

Receptor assays in the clinical laboratory.

OBJECTIVE: To review the relative advantages/disadvantages of receptor assays versus immunoassays. REVIEW OF CURRENT LITERATURE RESULTS: The history of immunoassays is evaluated. Current shortcomings are emphasized. The present and future role of receptor assays is assessed. CONCLUSION: The author predicts a shift away from immunoassays to receptor assays for certain analytes such as Vitamin B12, folic acid and drugs that undergo extensive metabolism.