Plasma Concentrations Of Radioactivity Research Articles

Blood–Brain Barrier Penetration of Zolmitriptan—Modelling of Positron Emission Tomography Data

Positron emission tomography (PET) with the drug radiolabelled allows a direct measurement of brain or other organ kinetics, information which can be essential in drug development. Usually, however, a PET-tracer is administered intravenously (i.v.), whereas the therapeutic drug is mostly given orally or by a different route to the PET-tracer. In such cases, a recalculation is needed to make the PET data representative for the alternative administration route. To investigate the blood-brain barrier penetration of a drug (zolmitriptan) using dynamic PET and by PK modelling quantify the brain concentration of the drug after the nasal administration of a therapeutic dose. [11C]Zolmitriptan at tracer dose was administered as a short i.v. infusion and the brain tissue and venous blood kinetics of [11C]zolmitriptan was measured by PET in 7 healthy volunteers. One PET study was performed before and one 30 min after the administration of 5 mg zolmitriptan as nasal spray. At each of the instances, the brain radioactivity concentration after subtraction of the vascular component was determined up to 90 min after administration and compared to venous plasma radioactivity concentration after correction for radiolabelled metabolites. Convolution methods were used to describe the relationship between arterial and venous tracer concentrations, respectively between brain and arterial tracer concentration. Finally, the impulse response functions derived from the PET studies were applied on plasma PK data to estimate the brain zolmitriptan concentration after a nasal administration of a therapeutic dose. The studies shows that the PET data on brain kinetics could well be described as the convolution of venous tracer kinetics with an impulse response including terms for arterial-to-venous plasma and arterial-to-brain impulse responses. Application of the PET derived impulse responses on the plasma PK from nasal administration demonstrated that brain PK of zolmitriptan increased with time, achieving about 0.5 mg/ml at 30 min and close to a maximum of 1.5 mg/ml after 2 hr. A significant brain concentration was observed already after 5 min. The data support the notation of a rapid brain availability of zolmitriptan after nasal administration.

Rapid Uptake, Metabolism, and Elimination of Inhaled Sulfuryl Fluoride Fumigant by Rats

Sulfuryl fluoride (SO(2)F(2)) is a structural fumigant gas used to control drywood termites and wood-boring beetles. The pharmacokinetics and metabolism of inhaled SO(2)F(2) were evaluated in male Fischer-344 rats exposed to 30 or 300 ppm (35)S-labeled SO(2)F(2) for 4 h. Blood, urine and feces were collected during and after the exposures and analyzed for radioactivity, (35)S-labeled fluorosulfate and sulfate, and fluoride (urine and feces only). Selected tissues were collected 7 days post-exposure and analyzed for radioactivity. During and after unlabeled SO(2)F(2) exposures, blood, brain, and kidney were collected and analyzed for fluoride ion. SO(2)F(2) was rapidly absorbed, achieving maximum concentrations of radioactivity in both plasma and red blood cells (RBC) near the end of the 4-h exposure period. Radioactivity was rapidly excreted, mostly via the urine. Seven days post-exposure, small amounts of radioactivity were distributed among several tissues, with the highest concentration detected in respiratory tissues. Radioactivity associated with the RBC remained elevated 7 days post-exposure, and highly perfused tissues had higher levels of radioactivity than other non-respiratory tissues. Radioactivity cleared from plasma and RBC with initial half-lives of 2.5 h after 30 ppm and 1-2.5 h after 300 ppm exposures. The terminal half-life of radioactivity was 2.5-fold longer in RBC than plasma. Based on the radiochemical profiles, there was no evidence of parent (35)SO(2)F(2) in blood. Identification of fluorosulfate and sulfate in blood and urine suggests that SO(2)F(2) is hydrolyzed to fluorosulfate, with release of fluoride, followed by further hydrolysis to sulfate and release of the remaining fluoride.

INCREASED BIOACCUMULATION OF URETHANE IN CYP2E1-/- VERSUS CYP2E1+/+ MICE

Urethane is a fermentation by-product and a potent animal carcinogen. Human exposure to urethane occurs through consumption of alcoholic beverages and fermented foods. Recently, CYP2E1 was identified as the primary enzyme responsible for the metabolism of [(14)C]carbonyl-labeled urethane. Subsequently, attenuation of urethane-induced cell proliferation and genotoxicity in CYP2E1-/- mice was reported. The present work compares the metabolism of single versus multiple exposures of CYP2E1-/- and CYP2E1+/+ mice to (14)C-ethyl-labeled urethane. Urethane was administered as a single 10 or 100 mg/kg gavage dose or at 100 mg/kg/day for 5 consecutive days. CYP2E1+/+ mice administered single or multiple doses exhaled 78 to 88% of dose as (14)CO(2)/day. CYP2E1-/- mice eliminated 30 to 38% of a single dose as (14)CO(2) in 24 h and plateaued after day 3 at approximately 52% of dose/day. The concentrations of urethane-derived radioactivity in plasma and tissues were dose-dependent, increased as a function of the number of doses administered, and were significantly higher in CYP2E1-/- versus CYP2E1+/+ mice. Whereas urethane was the main chemical found in the plasma and tissues of CYP2E1-/- mice, it was not detectable in CYP2E1+/+ mice. In conclusion, multiple dosing led to considerable bioaccumulation of urethane in mice of both genotypes; however, greater retention occurred in CYP2E1-/- versus CYP2E1+/+ mice. Furthermore, greater bioaccumulation of (14)C-ethyl-labeled than [(14)C]carbonyl-labeled urethane was observed in mice. Comparison of the metabolism of ethyl-versus carbonyl-labeled urethane was necessary for tracing the source of CO(2) and led us to propose for the first time that C-hydroxylation is a likely pathway of urethane metabolism.

METABOLISM, PHARMACOKINETICS, AND PROTEIN COVALENT BINDING OF RADIOLABELED MAXIPOST (BMS-204352) IN HUMANS

MaxiPost [(3S)-(+)-(5-chloro-2-methoxyphenyl)-1,3-dihydro-3-fluoro-6-(trifluoromethyl)-2H-indole-2-one); BMS-204352] is an investigational maxi-K channel opener to treat ischemic stroke. This study reports the disposition, metabolism, pharmacokinetics, and protein covalent binding of (14)C-labeled MaxiPost in healthy male volunteers as well as in dogs and rats. After each human subject received a single dose of 10 mg (14)C-labeled BMS-204352 (50 microCi) as a 5-ml intravenous infusion lasting 5 min, the plasma radioactivity concentrations showed a unique profile, wherein the concentration appeared to increase initially, followed by a terminal decline. The mean terminal t(1/2) of plasma radioactivity (259 h) was prolonged compared with that of unchanged parent (37 h). Furthermore, the extractability of radioactivity in plasma decreased over time, reaching approximately 20% at 4 h after dosing. The unextractable radioactivity was covalently bound to plasma proteins through a des-fluoro-des-methyl BMS-204352 lysine adduct. Unchanged BMS-204352 and minor metabolites were identified in plasma extract following protein precipitation. The recovery of the radioactive dose in urine and feces was nearly complete in 14-day collections (approximately 37% in urine and 60% in feces). The N-glucuronide of the parent was the prominent metabolite in urine (16.5% of dose), whereas the parent was a major drug-related component in feces (11% of dose). Similar disposition, metabolism, pharmacokinetic, and protein covalent binding properties of (14)C-labeled BMS-204352 were observed in humans, dogs, and rats.

Pharmacokinetics of gefitinib, an epidermal growth factor receptor tyrosine kinase inhibitor, in rat and dog

1. The pharmacokinetics of gefitinib and its metabolites in rat and dog were investigated in preclinical studies conducted to support the safety evaluation and clinical development of gefitinib, the first EGFR tyrosine kinase inhibitor approved for the treatment of non-small-cell lung cancer.2. Following intravenous dosing (5 mg kg−1), gefitinib plasma half-life was 3–6 h in rats and dogs, although studies using a more sensitive HPLC-MS assay produced longer estimates of half-life (7–14 h).3. In these studies, plasma clearance was high (male rat: 25 ml min−1 kg−1; female rat: 16 ml min−1 kg−1; male dog: 16 ml min−1 kg−1), as was the volume of distribution (8.0–10.4 l kg−1 in rat; 6.3 l kg−1 in dog), and exposure in female rats was double that in males.4. Following administration of [14C]-gefitinib, concentrations of radioactivity in plasma exceeded gefitinib throughout the profile, indicating the presence of circulating metabolites in both rat and dog.5. An HPLC-MS assay was developed to measure concentrations of gefitinib and five potential metabolites in plasma. All five metabolites were detected in the rat, but at levels much lower than gefitinib. In the dog, exposure to gefitinib and M523595 was similar, with much lower concentrations of M537194 and only trace levels of the other metabolites. This profile of metabolites is similar to that observed in man.

Bioavailability and half-life of decabromodiphenyl ether (BDE-209) in rat

1. The absorption of the flame retardant decabromodiphenyl ether (BDE-209) has been shown by its detection in human plasma, but reported experimental data on its absorption in rat are inconsistent. The bioavailability and half-life were therefore determined in rat, and qualitative analyses by GC/MS of metabolites in plasma were performed. The relative amount of phenolic metabolites was determined in a rat plasma sample obtained after administration of radiolabelled BDE-209.2. The bioavailability of parent BDE-209 was calculated to be about 26% in rat. The concentrations of phenolic radioactivity in plasma 3 and 7 days after dosing were four times higher than those of the neutral compounds, i.e. parent compound, indicating that total absorption was higher than 26%.3. Thirteen phenolic metabolites were determined in the plasma and the major phenolic metabolites were characterized as a hydroxy-octaBDE, a hydroxy-nonaBDE and a hydroxy-methoxy-hexaBDE (guaiacol-type). The exposure to the phenolic metabolites seemed higher than the parent compound, BDE-209.4. The initial elimination phase in plasma t1/2α for BDE-209 was 2 h, implying a rapid distribution of BDE-209 to well-perfused tissues. The distribution volume at steady state was 1.4 l kg−1, implying a low tendency for distribution to adipose tissue. The terminal t1/2 for BDE-209 in the intravenously dosed rat was calculated as 2.5 days (58 h).

The disposition and metabolism of rofecoxib, a potent and selective cyclooxygenase-2 inhibitor, in human subjects.

The disposition and metabolism of rofecoxib, a selective cyclooxygenase-2 inhibitor, were examined in healthy human subjects and in cholecystectomy patients. After oral administration of [(14)C]rofecoxib (125 mg, 100 micro Ci) to healthy subjects, the mean concentrations of total radioactivity and rofecoxib in plasma as a function of time indicated that the t(max) was achieved at 9 h postdose. After t(max), levels of both radioactivity and rofecoxib decreased in a parallel, exponential fashion (effective t(1/2) approximately equal 17 h). A similar result was obtained after oral administration of [(14)C]rofecoxib (142 mg, 100 micro Ci) to cholecystectomy patients equipped with an L-tube. In healthy subjects, radioactivity was recovered predominantly from the urine (71.5% of dose), with a small amount excreted in feces (14.2%). In patients with an L-tube, half the radioactive dose was recovered in feces, with a lesser amount excreted in urine (28.8%) and a negligible fraction in bile (1.8%). Rofecoxib underwent extensive metabolism in humans, and very little parent drug was recovered unchanged in urine (<1%). Products resulting from both oxidative and reductive pathways were identified by a combination of (1)H NMR and liquid chromatography-tandem mass spectrometry analyses, and included rofecoxib-3',4'-trans-dihydrodiol, 4'-hydroxyrofecoxib-O-beta-D-glucuronide, diastereomeric 5-hydroxyrofecoxib-O-beta-D-glucuronide conjugates, 5-hydroxyrofecoxib, rofecoxib-erythro-3,4-dihydrohydroxy acid, and rofecoxib-threo-3,4-dihydrohydroxy acid. Interconversion of rofecoxib and 5-hydroxyrofecoxib appeared not to be a quantitatively important pathway of rofecoxib disposition in human subjects, in contrast to previous findings in rats.

Sucralose metabolism and pharmacokinetics in man

The metabolic and pharmacokinetic profile of sucralose was studied in human volunteers. Following a single oral dose of 14C-sucralose (1 mg/kg, 100 μCi) to eight male subjects, a mean of 14.5% (range 8.9 to 21.8%) of the radioactivity was excreted in urine and 78.3% (range 69.4 to 89.6%) in the faeces, within 5 days. The total recovery of radioactivity averaged 92.8%. Plasma concentrations of radioactivity were maximal at about 2 hours after dosing. The mean residence time (MRT) for sucralose was 18.8 hr, while the effective half-life for the decline of plasma radioactivity was 13 hr. Two volunteers given a higher oral dose (10 mg/kg, 22.7 μCi) excreted a mean of 11.2% (9.6 and 12.7%) of the radioactivity in urine, and 85.5% (84.1 and 86.8%) in faeces over 5 days. The total recovery of radioactivity was 96.7%. The radiolabelled material present in faeces was essentially unchanged sucralose. Sucralose was the principal component in the urine together with two more polar components which accounted for only 2.6% of the administered dose (range 1.5 to 5.1% of dose); both metabolites possessed characteristics of glucuronide conjugates of sucralose.

Pharmacokinetics of [14C]Genistein in the Rat: Gender-Related Differences, Potential Mechanisms of Biological Action, and Implications for Human Health

Mass balance, plasma pharmacokinetics, tissue distribution, and metabolism of [14C]genistein were investigated in male and female rats (n = 5) following an oral dose of [14C]genistein (4 mg kg−1) to determine potential sites and mechanisms of biological action. Mean total excretion of radioactivity in urine and feces for both sexes was 66 and 33% of the dose respectively at 166 h after administration. Mean and maximal concentrations of radioactivity in plasma were significantly (p < 0.02) higher in male than female rats, with half-lives of 12.4 and 8.5 h, respectively. The concentration of radioactivity was significantly (p < 0.002) higher in liver from females than males and in reproductive (vagina, uterus, ovary, and prostate) compared with other peripheral organs. Analysis of plasma extracts by radio-HPLC-MS indicated that the predominant metabolites were genistein glucuronides, 4-hydroxyphenyl-2-propionic acid, and trace amounts of parent compound (<5%). Radioactive residues in uterus and prostate were predominantly parent compound and 4-hydroxyphenyl-2-propionic acid, respectively. Significantly (p < 0.05) increased retention of [14C]genistein or metabolites was associated with reproductive organs, such as vagina, uterus, ovary, and prostate, likely to contain relatively high concentrations of estrogen receptors or binding proteins compared with other peripheral tissues. Factors liable to influence bioavailability of biologically active genistein or metabolites, such as dietary intake, warrant further investigation to determine the risks or benefits for different consumer groups of phytoestrogen-containing foods.

Effect of diet and gavage on the dose- and dose-mode-dependent absorption and metabolism of bidisomide in rat

1. The metabolism of bidisomide was investigated to examine how dose and mode of drug administration (i.e. diet admixture versus oral solution) affect the absorption and metabolism of bidisomide in the toxicity studies. 2. After dietary admixture, bidisomide was more absorbed and less metabolized at the higher doses. Reduced metabolism at the high doses resulted from saturation of stereospecific formation of the N-desisopropyl-arylhydroxy bidisomide (NDABD) metabolite. 3. The rat-specific NDABD metabolite was formed only from (-)-bidisomide on incubation with rat liver microsomes. 4. After oral solution dosing, absorption was increased and metabolism reduced compared with the dietary admixture. 5. After 24-h infusion, plasma concentrations of radioactivity were approximately dose-proportional. However, the concentrations in the liver were similar at the 200 and 400 mg/kg doses due to saturation of liver uptake of the NDABD metabolite.

Percutaneous absorption of [3H]tretinoin and systemic exposure to mequinol after dermal application of 2% mequinol/0.01% [3H]tretinoin (Solagé) solution in healthy volunteers.

Solagé is a combination product composed of 2% mequinol (4-hydroxyanisole) and 0.01% tretinoin (all-trans-retinoic acid) in an ethanolic solution, which is being studied for its safety and efficacy as a topical treatment for disorders of skin hyperpigmentation. The purpose of this study was to evaluate the extent of percutaneous absorption of [3H]tretinoin and to estimate the systemic exposure to mequinol from this combination product when topically applied to the backs of healthy subjects. Eight subjects received bid topical applications of nonradiolabelled 2% mequinol/0.01% tretinoin solution on a 400 cm2 area of the back for 14 days. The subjects then received a single topical application of 2% mequinol/0.01% [3H]tretinoin solution. After 12 h, the radiolabelled dose was removed and bid treatment with nonradiolabelled 2% mequinol/0.01% tretinoin solution was continued for 7 days. Plasma, urine and faecal samples were analysed for total radioactivity and plasma was analysed for both mequinol and tretinoin by GC/MS procedure. Mean percutaneous absorption of [3H]tretinoin based on the cumulative recoveries of radioactivity in the urine and faeces was about 4.5% (median 2.18%). Tretinoin concentrations in plasma did not increase above endogenous levels. This was consistent with the concentrations of radioactivity in plasma, which showed an average Cmax of 91 pg-eq/mL (median 26 ng/mL). Average Cmax and AUC(0-12 h) values for mequinol were 10 ng/mL and 33 ng h/mL, respectively. Based on the results of this study, systemic toxicity from topical application of tretinoin in this formulation is unlikely, because percutaneous absorption of tretinoin is minimal and because endogenous levels of tretinoin are not increased following bid dosing with this combination formulation. The safety of mequinol in this combination formulation is supported by the low systemic exposures of the subjects in this study compared with the systemic exposures at the highest doses in the dermal toxicity studies in mice (16.6-fold) and rats (34.6-fold).

Effects of renal insufficiency and aging on the pharmacokinetics of a phenethylamine class α 1A-adrenoceptor agonist NS-49

Effects of renal insufficiency and of aging on the pharmacokinetics of NS-49, a newly developed phenethylamine class α 1A-adrenoceptor agonist eliminated mainly by renal excretion, were investigated in rats after a single administration of 14C-NS-49. The gromerular filtration rate (GFR) in the partially nephrectomized rats was about half that in the sham-operated rats, and the plasma creatinine concentration in the former was well above the normal limit. Plasma concentrations of radioactivity after intravenous or oral administration of 14C-NS-49 were much higher in the nephrectomized rats than in the intact and sham-operated rats. As a result, the areas under the plasma concentration-time curve (AUC 0–∞) after intravenous and oral administrations respectively increased 5-fold and 7-fold after partial nephrectomy. The elimination half-life ( t 1/2,β) was increased about 2-fold by partial nephrectomy. The systemic availability for the partially nephrectomized rats remained unchanged, indicative that partial nephrectomy does not affect the absorption of NS-49. Plasma concentrations of radioactivity after intravenous or oral administration of 14C-NS-49 to 88-week-old rats were higher than in 7-week-old rats, the AUC 0–∞ value for the aged rats being about two times higher. The aged rats, unlike the nephrectomized rats, showed no marked difference in the t 1/2,β, value, whereas their V ss value was about half that for the young rats. These findings are considered to be caused by physiologic age-related changes; decrease in renal function and loss of body water. Systemic availability in the aged rats did not differ from that in the young, indicative that aging has no effect on the extent of absorption of this drug.

125I-labelled human chorionic gonadotrophin (hCG) as an elimination marker in the evaluation of hCG decline during chemotherapy in patients with testicular cancer.

The rate of reduction in the concentration of serum human chorionic gonadotrophin (hCG) following chemotherapy for germ cell tumours may follow a complex pattern, with longer apparent half-life during later stages of chemotherapy, even in patients treated successfully. The commonly used half-life of less than 3 days for hCG to monitor the effect of chemotherapy in patients with germ cell tumours of the testis may represent too simple a model. 125I-labelled hCG was injected intravenously in 27 patients with germ cell tumours and elevated hCG during chemotherapy. The plasma radioactivity and hCG concentrations were followed. During chemotherapy, the plasma disappearance of hCG showed a biphasic pattern, with an initial fast and a later slow component in all patients. Using the steep part of the hCG plasma disappearance curve, five patients who achieved long-term remission had half-lives longer than 3 days (3.6–6.8 days), whereas four out of five patients not achieving long-term remission had half-lives shorter than 3 days. After the third treatment cycle, eight patients who achieved long-term remission had hCG half-lives longer than 3 days (7.4–17.0 days). In these patients, the plasma disappearance of [125I]hCG was equivalent to that of hCG. Thus, the slow decline of hCG represented a slow plasma disappearance rather than a hCG production from vital tumour cells and could, consequently, not be used to select patients for additional or intensified chemotherapy. The concept of a fixed half-life for plasma hCG during treatment of hCG-producing germ cell tumours is inappropriate and should be revised. Difficulties in interpreting a slow decline of hCG may be overcome by comparing the plasma disappearance of total hCG with the plasma disappearance of [125I]hCG. © 1999 Cancer Research Campaign

I-131 orthoiodohippurate assessment of renal function after heart transplantation.

The authors studied the relation between cardiac output (CO) and effective renal plasma flow (ERPF) and compared values from control patients with various cardiac problems with those in a group of study patients who had undergone heart transplantation. The experimental group was divided into three subgroups according to the interval between the time of surgery and the time of the CO-ERPF studies. Group 1 consisted of patients studied fewer than 10 days after surgery; group 2 consisted of patients studied 10 to 20 days after operation; and group 3 consisted of patients studied more than 20 days after operation. Effective renal plasma flow was determined by the single-injection, single plasma sample method, where 50 microCi I-131 orthoiodohippurate was injected intravenously in a single dose and plasma concentrations of radioactivity were determined. The quotient of injected dose radioactivity divided by plasma radioactivity is highly predictive of global ERPF. Cardiac output was measured by thermodilution. In the control group, a positive linear correlation was found between CO and ERPF; however, the CO:ERPF ratio was elevated, and after heart transplantation, a lag time was observed for as long as 3 weeks in some patients before CO:ERPF ratios returned to control group levels. The regression equation and standard error for the control group was CO = 1.85 + 0.0065 ERPF (+/-0.62) l/min versus 1.433 + 0.0068 ERPF (+/-0.64) l/min for group 3. The correlation coefficients comparing CO with ERPF were r = 0.88, 0.23, 0.51, and 0.85, for the control group and groups 1, 2, and 3, respectively. A localized release of catecholamines from the adrenal gland is proposed to cause ERPF damping after abrupt increases in CO.

Metabolism and excretion of citalopram in man: identification of O-acyl- and N-glucuronides

1. The antidepressant citalopram (CT), a selective serotonin uptake inhibitor, was given in its labelled form, [14C]-CT, as a single oral dose in 50 ml aqueous solution (0.1 mmol/30 μCi/1.1 MBq) to four healthy male volunteers. 2. Concentrations of radioactivity in whole blood and plasma were similar. The respective pharmacokinetic parameters were: Cmax=214±41 and 246±69 nmol eq./litre, Tmax=3 and 2 h, AUC=18289±2959 and 14537±2883 nmol eq.h/litre, and t1/2=90.2±22.5 and 79.5±14.9 h respectively. A mean of 85.2±10.4% of the radioactive dose was recovered after 17 days of collection of excreta. The majority of radioactivity was excreted in urine (74.7±8.9%) and the remaining part in faeces (10.5±2.3%). 3. The HPLC profile of urinary components showed that besides the known metabolites of citalopram, three glucuronides were present. The relative amounts of CT and its metabolites in urine collected for 7 days were: CT (26%), N-demethyl-CT (DCT, 19%), N,N-didemethyl-CT (DDCT, 9%), the N-oxide (7%), the quaternary ammonium glucuronide of CT (CT-GLN, 14%), the N-glucuronide of DDCT (DDCT-GLN, 6%), and the glucuronide of the acid metabolite (CT-acid-GLN, 12%) formed by N,N- dimethyl deamination of CT. CT-GLN was isolated using preparative chromatography and identified by LC-MS-MS and NMR. DDCT-GLN and CT-acid-GLN were identified by LC-MS. 4. This study shows that protracted renal excretion represents the major route of elimination, with a small fraction voided with faeces. A considerable portion of the urinary excreted dose consists of N-glucuronides of CT and DDCT together with the O-acyl glucuronide of CT-acid.

Brain uptake of dihydroergotamine after intravenous and nasal administration in the rat.

This study was conducted to determine the uptake of dihydroergotamine (DHE) into the brain after intravenous and intranasal administration in rats. Eight anesthetized rats received either an intravenous (i.v.) or two successive intranasal (i.n.) doses of tritium labeled dihydroergotamine (3H-DHE) with 14C-inulin as a non-BBB (blood-brain barrier) permeable marker. Radioactivity concentrations in plasma were determined at designated times within 30 min postdose, and in blood and seven brain regions (olfactory bulb, frontal cortex, parietal cortex, occipital cortex, cerebellum, mid-brain areas, and brain stem) at 30 min. The plasma-to-brain permeability*area product (PeA) following an i.v. dose was calculated based on the 30-min brain tissue concentration and the area under the plasma concentration-time curve (AUC0-30 min, i.v.) assuming unidirectional transport from plasma to brain. Direct transport from nasal cavity to brain was assessed based on the amount of radioactivity in brain determined experimentally and predicted based on plasma AUC0-30 min, i.n. and PeA obtained from i.v. data. Following an i.v. dose, DHE distributed into the brain with a brain-to-plasma concentration ratio of approximately 5% at 30 min postdose. The PeA value of DHE ranged from 8.6 x 10(-4) to 37.5 x 10(-4) mL min(-1) g(-1) in different brain regions. Following i.n. doses the experimentally determined concentration in olfactory bulb was approximately 51 times, and in other regions three to seven times, greater than predicted values based only on PeA and plasma AUC, suggesting a direct transport pathway from the nasal cavity to the brain. As a result, the brain tissue concentrations at 30 min were similar to (0.31-1.04 times) those following an i.v. dose except for the olfactory bulb, in which the concentration was approximately four times greater than that following an i.v. dose. In conclusion, 3H-DHE penetrated the BBB following intravenous administration. Following i.n. doses, 3H-DHE was able to enter the brain directly from the nasal cavity, with the olfactory bulb being a part of the direct passage from nasal cavity to brain.

Absorption, distribution, metabolism and excretion of 14C-labelled enantiomers of the calcium channel blocker benidipine after oral administration to rat

1. Each of the 14C-labelled optical isomers of benidipine, a new calcium antagonist, was separately administered orally to the male rat at a dose of 0·5 or 1 mg/kg. The absorption, distribution, metabolism and excretion of the 14C-labelled optical isomers were investigated. 2. Plasma concentrations of radioactivity after administration of the (−)-α isomer were higher than those after administration of the (+)-α isomer. 3. The highest radioactivity was found in liver and high levels of radioactivity were found in the kidney, adrenal gland and lung after administration of the (+)- or (−)-α isomers of benidipine. Up to 72 h, the tissue concentration of radioactivity fell from 1·4 to 9·2% of the highest level in each tissue for the (+)-α isomer and from 1·8 to 13·0% for the (−)-α isomer. 4. The ratios of the area under the time-curve of each tissue concentration to that of the corresponding plasma concentration were almost equal after the separate administrations of both isomers. 5. The dominant urinary and biliary metabolic pathways of the (+)-isomer were the hydrolysis of 1-benzyl 3-piperidylester followed by the oxidation of the dihydropyridine ring and N-dealkylation followed by hydrolysis of the methylester. Those of the (−)isomer were the hydrolysis of 1-benzyl 3-piperidylester followed by the oxidation of dihydropyridine ring and of the oxidation methyl group, N-dealkylation followed by hydrolysis of the methylester,decarboxylation and glucuronidation of the piperidyl moiety after the oxidation of the dihydropyridine ring. 6. The cumulative excretion of radioactivity in urine and faeces up to 72 h after administration of the (+)-α isomer was 8·8 and 90·7% of the dose respectively. The corresponding values of the (+)-α isomer were 19·7 and 72·9% of the dose respectively. 7. The excretion of radioactivity in bile up to 48 h after administration of the (+)- and (−)-α isomer was 42·1 and 46·7% of the dose respectively.

Perturbation of Epidermal Growth Factor Clearance after Radioiodination and Its Implications

The clearance of human epidermal growth factor (hEGF1–53) has been thought to be mediated mainly by a high-capacity receptor system, yet relatively low in vivo clearance rates (<10mL/min/kg) and long terminal elimination half-lives (>120 min) have been observed in rats receiving the peptide that was iodinated by the oxidative chloramine-T (CT) method. We investigated if a mild, less oxidative iodination by the lactoperoxidase (Enzymobeads, EB) method, which is known to yield an iodinated peptide with receptor-binding equivalence, could produce a labeled peptide that behaves pharmacokinetically similar to the native material. For comparison, a parallel study was also conducted with EB-125I-hEGF1–48, which in its native form has a much reduced receptor binding activity due to the loss of the C-terminal pentapeptide. Plasma radioactivity concentrations were determined by trichloroacetic acid (TCA) precipitation and immunoprecipitation. Rats cleared unlabeled hEGF1–53 and hEGF1–48 markedly faster (CLtot>120mL/min/kg) than their radiolabeled counterparts. Approximately 96% of the hEGF1–53 dose was cleared during the initial phase (0–4 min), as opposed to only 5–14% for the iodinated peptide. Similar change was also observed for EB-125I-hEGF1–48 and CT-125I-hEGF1–53. The pharmacokinetic behavior of EB-125I-hEGF1–53 was, in fact, comparable to that of CT-125I-hEGF1–53. These observations indicate that receptor-binding equivalence does not have direct relationship with in vivo EGF clearance. Both iodination methods (oxidative CT and less oxidative EB) might have perturbed one or more steps in the cascade of ligand–receptor internalization and intracellular procession, which in turn modified the disposition of the peptides. In addition, the two independent precipitation techniques for the same peptide generated different kinetic outcomes. The overall experimental results suggest that it is unacceptable to use an iodinated form to characterize the disposition of peptides/proteins like EGF with a specific receptor system mediating its clearance.

Absorption, metabolism and excretion after oral administration of a new Ca antagonist, 14C-benidipine hydrochloride to man

1. In healthy male volunteers, the absorption, metabolite profiles and excretion of Cbenidipine hydrochloride, a new Ca antagonist, were investigated after oral administration at a dose of 8 mg. 2. C-benidipine hydrochloride was rapidly absorbed, and the plasma concentration of radioactivity and unchanged drug reached a maximum of 71 2 ng eq. ml at 1 1 h and 2 56 ng ml at 0 6 h respectively, and then declined bi-exponentially. The half-life in the elimination phase was 14 7 and 5 3 h respectively. AUC of unchanged drug was low, about 1% of that of radioactivity. 3. Five days after administration,36 4% of the administered radioactivity was excreted in urine and 58 9% in faeces. 4. The metabolite profiles in plasma, urine and faeces were analysed by hplc. At 1 h after administration the predominant metabolites in plasma were M9 and M2, which accounted for 13 8 and 8 2% of the radioactivity respectively, whereas unchanged drug represented 1 2%. Predominant metabolites in urine 12 h after administration were M3 andM8,whichaccountedfor2 22and2 21%oftheadministeredradioactivityrespectively. Metabolites excreted in faeces 120 h after administration were very complex and poorly separated by hplc and could not be characterized: unchanged drug was not detected in the faeces.

Pharmacokinetics, disposition and biotransformation of [14C]-radiolabelled valsartan in healthy male volunteers after a single oral dose

1. The disposition of valsartan, a potent angiotensin II receptor antagonist, was investigated in six healthy male volunteers. They each received a single oral dose of80 mg of a C-labelled preparation as a neutral buffered solution. 2. Peak concentrations of radioactivity and valsartan in plasma measured 1 h after dosing showed rapid onset of absorption. The results of this study combined with other available data indicate that at least 51% of the dose was absorbed. 3. Valsartan was the predominant radioactive compound in plasma. Elimination of valsartan and radioactivity was fast and multiexponential. beta -Half-lives of 6 1 h were observed. In aterminal elimination phase, low radioactivity levels decreased with ahalf-life of81 33 h.Aminor, pharmacologically inactive metabolite (valeryl-4-hydroxy-valsartan; M1) was detected in the plasma at time points later than 2 h after dosing, representing approximately 11% of the AUCh of plasma radioactivity. 4. The bulk of the dose was excreted within 4 days. The totalexcretion within 7 days amounted to 99 1% of dose. Faecal excretion was predominant (86 5% of dose). Valsartan was largely excreted unchanged (81 5% of the dose in the excreta). The predominant clearance mechanism appeared to be direct elimination via bile. 5. An inactive metabolite, M1, was formed by oxidative biotransformation and accounted for 9 3% of the dose in the excreta.