药物的肝外代谢

根据国内外在肝外药物代谢酶的种类、分布及生物学作用，药物肝外代谢的主要部位和代谢特点，影响药物肝外代谢的因素等方面的有关报道进行综述。肝外代谢在某些药物的代谢中发挥了重要的作用，其作用不容忽视。     

Meeting of Drug Metabolism Group Extrahepatic Metabolism

1. Four non-acidic primary metabolites of N-(5-pyrrolidinopent-3-ynyl)succinimide (BL 14) were identified and quantified using g.l.c. and mass spectrometry. The metabolites are α-hydroxy-N-(5-pyrrolidinopent-3-ynyl)succinimide (A), N-(5-(2-oxopyrrolidino)-pent-3-ynyl)succinimide (B), N-(2-hydroxy-5-pyrrolidinopent-3-ynyl)succinimide (C) and N-(5-pyrrolidinopent-3-ynyl)succinimide N'-oxide (E), the latter analysed after reduction to the parent amine.

2. In rat liver preparations, all metabolites are formed by microsomal, NADPH-dependent enzyme systems, but with different characteristics. The response to inhibitors such as CO and SKF 525A indicates participation of cytochrome P-450 enzymes in the formation of all metabolites. Phenobarbital pretreatment markedly enhances propynylic hydroxylation (C) but has little or no effect on the other metabolic pathways. Succinimide hydroxylation (A) exhibits a pH optimum at 7±0, while the formation of metabolites B and C increases at pH values between 6±4 and 7±7.

3. Kinetic studies on the formation of metabolites A-C revealed differences in the Michaelis constant, while the V_max values were similar. Succinimide hydroxylation (A) is most efficient with a K_m of 3±7 × 10^?5 M, compared with a K_m of 1±7 × 10^?3 M for propynylic hydroxylation (C).

4. The formation of metabolites B and E conforms to the corresponding mechanisms for lactam and N-oxide formation for other xenobiotics. The formation of metabolites A and C represents two extremities, reflected in their different responses to phenobarbital pretreatment, pH changes and in their different K_m values. Although little can be discerned about the mechanisms from the literature, the enzymes cataly sing both reactions appear to be cytochromes.     

In-vivo Clearance Study of Vancomycin in Rats

The renal handling of vancomycin in rats and the effects of various drugs (probenecid, cimetidine and quinidine) on urinary excretion of the antibiotic were investigated by in-vivo clearance. The vancomycin-to-inulin excretion ratio (ER) was greater than unity at various infusion rates of vancomycin. Quinidine, co-administered with vancomycin, significantly decreased the total, renal and net secretory clearance of the antibiotic. Cimetidine also decreased, though not significantly, the secretory clearance of vancomycin by about 20%, but probenecid did not. These results suggested that vancomycin is secreted in renal tubules in rats, and that quinidine decreases the total clearance of vancomycin partly by inhibiting its tubular secretion in the kidney.     

Effects of Intestinal and Hepatic Metabolism on the Bioavailability of Tacrolimus in Rats

Purpose. Tacrolimus, an immunosuppressive agent, has poor and variable bioavailability following oral administration in clinical use. We investigated the contribution of intestinal metabolism to the first pass effect of tacrolimus in rats. Methods. Tacrolimus was administered intravenously, intraportally or intraintestinally to rats. Blood samples were collected over a 240-min period, and blood tacrolimus concentrations were measured. The extraction ratios of tacrolimus in the intestine and liver were investigated. In addition, the metabolism of tacrolimus in the everted sacs of the small intestine was examined. Results. The rate of absorption of tacrolimus in the intestine was rapid, and tacrolimus was almost completely absorbed after intestinal administration. The bioavailability of tacrolimus was about 40% and 25% after intraportal and intraintestinal administration, respectively, indicating that tacrolimus is metabolized in both the intestine and the liver. In addition, tacrolimus was significantly metabolized in the everted sacs of the rat intestine. Conclusions. The present study suggested that the metabolism of tacrolimus in the intestine contributes to its extensive and variable first pass metabolism following the oral administration.     

Bioavailability Study of Glycyrrhetic Acid after Oral Administration of Glycyrrhizin in Rats; Relevance to the Intestinal Bacterial Hydrolysis

To clarify the metabolic fate of glycyrrhizin when orally ingested, we investigated the bioavailability of glycyrrhetic acid, the aglycone of glycyrrhizin, after intravenous or oral administration of glycyrrhetic acid (5.7 mg kg^?1, equimolar to glycyrrhizin) or glycyrrhizin (10 mg kg^?1) at a therapeutic dose in rat. Plasma concentration of glycyrrhetic acid rapidly decreased after its intravenous administration, with AUC of 9200 ± 1050 ng h mL^?1 and MRT of 1.1 ±0.2 h. The AUC and MRT values after oral administration were 10600± 1090 ng h mL^?1 and 9.3 ±0.6 h, respectively. After oral administration of glycyrrhizin, the parent compound was not detectable in plasma at any time, but glycyrrhetic acid was detected at a considerable concentration with AUC of 11700 ± 1580 ng h mL^?1 and MRT of 19.9 ± 1.3 h, while glycyrrhetic acid was not detected in plasma of germ-free rats at 12 h after oral administration of glycyrrhizin. The AUC value of glycyrrhetic acid after oral administration of glycyrrhizin was comparable with those after intravenous and oral administration of glycyrrhetic acid, indicating a complete biotransformation of glycyrrhizin to glycyrrhetic acid by intestinal bacteria and a complete absorption of the resulting glycyrrhetic acid from intestine. Plasma glycyrrhizin rapidly decreased and disappeared in 2 h after intravenous administration. AUC and MRT values were 2410 ± 125 μg min mL^?1 and 29.8 ± 0.5 min, respectively. Plasma concentration of glycyrrhetic acid showed two peaks, a small peak at 30 min and a large peak at 11.4 h, after intravenous administration of glycyrrhizin, with an AUC of 15400±2620 ng h L^?1 and an MRT of 18.8 ± 1.0 h. The plasma concentration profile of the latter large peak was similar to that of glycyrrhetic acid after oral administration of glycyrrhizin, which slowly appeared and declined. The difference of MRT values (19.9 and 9.3 h) for plasma glycyrrhetic acid after oral administration of glycyrrhizin and glycyrrhetic acid suggests the slow conversion of glycyrrhizin into glycyrrhetic acid in the intestine.     

First-Pass Metabolism of Diltiazem in Anesthetized Rabbits: Role of Extrahepatic Organs

Pharmaceutical Research - Purpose. The aim of this study was to assess in vivo which organs contribute to the first-pass metabolism of diltiazem. Methods. Anaesthetized rabbits received diltiazem...     

Absorption,Distribution and Metabolism of Sulphafurazole and Chloramphenicol in Rats with Intestinal Occlusion

Abstract The rat small intestine was occluded at the ileocaecal junction, while control rats underwent sham–operation. 42–46 hours later sulphafurazole 50 mg/kg or chloramphenicol 250 mg/kg was given by gavage. The drug concentrations in the whole blood, small intestine, large intestine, liver, kidneys, lungs and heart at respectively 30 and 60 min. after sulphafurazole and at 60 min. after chloramphenicol administration were assayed chemically. Occlusion increased the absolute liver and small intestinal weights. The total sulphafurazole levels reached in blood and other tissues except the large intestine at 30 min. were about similar at 60 min. both in sham–operated and occlusion rats, but occlusion increased total sulphafurazole in the large intestine at 60 min. Occlusion increased acetyl sulphafurazole in the liver, kidneys and lungs at 60 min. and in the large intestine acetyl sulphafurazole was higher at 60 min. (about 60 %) than at 30 min. (about 30 %) in both sham–operated and occlusion rats. Occlusion did not significantly modify total chloramphenicol in any tissue or blood. Occlusion increased free and NO₂–reduced chloramphenicol in the large intestine. It seems that occlusion does not alter the absorption or distribution of sulphafurazole and chloramphenicol, but modifies their inactivation by increasing metabolism and probably by altering their excretion through the large intestine.     

Microdosing Assessment to Evaluate Pharmacokinetics and Drug Metabolism in Rats Using Liquid Chromatography-Tandem Mass Spectrometry

PURPOSE: To evaluate the sensitivity requirement for LC-MS/MS as an analytical tool to support human microdosing study with sub-pharmacological dose, investigate proportionality of pharmacokinetics from the microdose to therapeutic human equivalent doses in rats and characterize circulating metabolites in rats administered with the microdose. MATERIALS AND METHODS: Five drugs of antipyrine, metoprolol, carbamazepine, digoxin and atenolol were administered orally to male Sprague-Dawley rats at 0.167, 1.67, 16.7, 167 and 1,670 microg/kg doses. Plasma samples were extracted using either solid phase extraction or liquid-liquid extraction, and analyzed using LC-MS/MS. RESULTS: Using 100 microl of plasma sample, the lower limit of quantitation for antipyrine (10 pg/ml), carbamazepine (1 pg/ml), metoprolol (5 pg/ml), atenolol (20 pg/ml), and digoxin (5 pg/ml) were achieved using an API 5000. Proportional pharmacokinetics were observed from 0.167 microg/kg to 1,670 microg/kg for antipyrine and carbamazepine and from 1.67 to 1,670 microg/kg for atenolol and digoxin, while metoprolol exhibited a non-proportional pharmacokinetics relationship. Several metabolites of carbamazepine were characterized in plasma from rats dosed at 1.67 mug/kg using LC-MS/MS. CONCLUSIONS: This study has shown the promise of sensitive LC-MS/MS method to support microdose pharmacokinetics and drug metabolism studies in human.     

Skin Structure and Metabolism: Relevance to the Design of Cutaneous Therapeutics

The outer layer of the epidermis or stratum corneum is the major barrier to percutaneous absorption. It has been shown that there are numerous enzyme systems beneath the stratum corneum in the viable epidermis capable of metabolizing drugs. A number of prodrug and soft drug topical therapeutic agents have been designed. After these agents penetrate the stratum corneum, they are metabolized by the cutaneous esterase systems to the desired metabolites.     

Metabolism of Diltiazem in Hepatic and Extrahepatic Tissues of Rabbits: In Vitro Studies

Diltiazem (DTZ) is a calcium channel blocker widely used in the treatment of angina and hypertension. DTZ undergoes extensive metabolism yielding several metabolites, some of which are active like N-desmethyldiltiazem (MA), desacetyldiltiazem (M1) and N-desmethyl,desacetyldiltiazem (M2). Due to the nature of its biotransformation, several organs should have the ability to metabolize DTZ, however it is still assumed that the liver is the only organ implicated in its elimination. In this study, the fate of DTZ, MA and M1 was assessed in several organs that could contribute to their biotransformation. To this purpose, DTZ (48.2 µM) was incubated in the 10,000 × g supernatant of homogenates of rabbit tissues for 60 min at 37°C. Multiple samples were withdrawn, and DTZ and its metabolites were assayed by HPLC. The elimination rate constant of DTZ in 10,000 × g supernatants varied between the organs: liver 334 ± 45, proximal small intestine 69 ± 11, distal small intestine 25 ± 3, lungs 15 ± 6 and kidneys 8 ± 6 (10^–4 min^–1). The metabolism of DTZ in the liver generated large amounts of MA but no M1, and in the small intestine, modest amounts of both metabolites. When MA (50.0 µM) or M1 (53.7 µM) were incubated in liver homogenates, the estimated elimination rate constant were 166 ± 23 and 468 ± 53 (10^–4 min^–1), respectively. The rate of degradation of the metabolites in the small intestine was much slower. These results demonstrate that, in vitro, DTZ is metabolized by several organs, the liver accounting for 75% of the total activity, and that MA is the major metabolite generated.     

左旋紫草素体内代谢研究

本文对紫草素在大鼠体内的生物转化进行了研究,对经苯巴比妥预处理（i.p.60mg.kg^-1.day,3天）的Wistar种健康雄性大鼠腹腔注射紫草素（20mg.kg^-1),收集到的胆汁和血样分别经β－葡萄糖醛酸酶水解后,用乙酸乙酯提取,建立PP－HPLC／DAD方法对紫草素及其代谢物大大鼠胆汁和尿样中的分布进行了初步考察,根据色谱峰的保留时间和紫外光谱分析,大鼠的胆汁和尿样中分别有10个代谢物。     

天麻素在大鼠体内的代谢

大鼠肾、肝、脑、血体外温孵实验表明:天麻素在肾中水解最强,其次为肝、脑。对大鼠的尿液进行提取、分离,得两种主要代谢物。通过光谱分析,鉴定为对羟基苯甲醇和对羧基苯一吡喃葡萄糖醛酸甙。本文根据以上实验结果,推测出天麻素在大鼠体内的代谢过程。     

Determining Intestinal Metabolism and Permeability for Several Compounds in Rats. Implications on Regional Bioavailability in Humans

Purpose. To investigate the regional differences in small intestinal (SI) metabolism and permeability for several compounds and to ascertain the potential significance of these differences on the reported reductions in regional bioavailability in humans. Methods. The regional SI metabolism and permeability of captopril, didanosine (ddl), mannitol, ofloxacin and zidovudine (ZDV) were investigated in rats using a Single Pass Intestinal Perfusion (SPIP) procedure or intestinal homogenates. Results. ddI was metabolized to a greater extent in the upper SI whereas captopril was metabolized to a greater extent in the lower SI. Relatively low homogenate concentrations resulted in significant degradation of captopril in the upper and lower SI. All other compounds were stable and changes in the buffer system or the initial concentration did not affect the results. The SI permeabilities of all compounds, with the exception of mannitol, decreased linearly with respect to SI location and the slopes of the corresponding normalized regression lines were not significantly different. Conclusions. It has been reported that captopril and ddl demonstrate regional intestinal bioavailability in several species including humans. The current results suggest that the reported reduction in the lower SI bioavailability of captopril may be a result of a reduction in permeability and an increase in intestinal metabolism whereas for ddl, the reduction in the lower SI bioavailability appears to be attributable to a reduction in intestinal permeability. Other factors such as luminal metabolism may also significantly effect regional differences in the intestinal bioavailability of ddl or captopril. Based on these results, a strong possibility exists that ofloxacin and ZDV may also demonstrate regional differences in intestinal bioavailability.     

Do 5-HT4 Receptors Mediate the Intestinal Secretory Response to 5-HT in Rat In-vivo?

The involvement of the recently characterized 5-HT₄ receptor in the actions of 5-hydroxytryptamine (5-HT) on jejunal, ileal and colonic electrogenic ion secretion was investigated in the rat in-vivo. 5-HT and the 5-HT₁-, 5-HT₂- and 5-HT₄-receptor agonist 5-methoxytryptamine (5-MeOT), induced a rise in transintestinal PD in all regions of the gut. However, the 5-HT₄-receptor agonists renzapride and cisapride had no effect. Furthermore, the 5-HT₄-receptor antagonists SDZ 205–557 (2-diethylaminoethyl-[2-methoxy-4-amino-5-chloro] benzoate), tropisetron and SB 204070 ([1-butyl-4-piperidinylmethyl]-8-amino-7-chloro-1,4-benzodioxan-5-carboxylate hydrochloride) did not affect the secretory response to either 5-HT or 5-MeOT in the jejunum, but did cause a small inhibition in the ileum and colon. It is concluded that 5-HT₄ receptors do not make a contribution to the electrically monitored 5-HT intestinal secretory response in the rat jejunum in-vivo, but may play a small role in the ileum and colon.     

Assessment of the Developmental Toxicity, Metabolism, and Placental Transfer of Di-n-butyl Phthalate Administered to Pregnant Rats

The developmental toxicity and placental transfer of di-n-butylphthalate(DBP) were evaluated in Sprague-Dawley rats given a single oraldose of DBP on Gestational Day 14. In the developmental toxicitystudy, dams were dosed with 0, 0.5, 1, 1.5, or 2 g DBP/kg andwere necropsied on GD21. Increased incidence of resorptionsand reduced fetal body weight were observed at 1.5 and 2 g/kg.Higher incidences of skeletal variations were found at doses     

Intestinal First-Pass Metabolism of Eperisone in the Rat

Purpose. The purpose of this study was to clarify quantitatively the contribution of the intestine to the first-pass metabolism of eperisone in rats. Methods. The systemic availabilities of eperisone were estimated by administering the drug into the duodenum, portal vein, and femoral vein in rats in vivo. The first-pass metabolism of eperisone was confirmed in the perfused rat small intestine in situ. Metabolism of eperisone to an -1-hydroxylated metabolite (HMO), the first step of eperisone metabolism, was studied using rat intestinal microsomes in vitro. Results. The bioavailabilities in the intestine were 0.176 and 0.0879 at administration rates of 100 and 25 mg/h/kg, respectively, whereas those in the liver were 0.532 and 0.486, respectively. In the intestinal perfusion experiment, the appearance clearance to the portal vein from the intestinal lumen was much lower than the elimination clearance from the intestinal lumen, resulting in high metabolic clearance of eperisone in the small intestine. Eperisone was biotransformed to HMO by rat intestinal microsomes, and this was inhibited by -naphthoflavone and an anti-rat CYP1A antibody. Conclusions. Those data strongly suggest that eperisone may be metabolized to HMO by CYP1A in rat intestinal microsomes during the first-pass through the epithelium of the small intestine.     

An Improved Model for Studies on Transdermal Drug Absorption In-vivo in Rats

Abstract— In rats, transdermal drug absorption can be studied under physiological conditions by cannulating the peripheral skin vein, draining the area of the skin which is used for drug application, and collecting the blood. This method leads to decreased blood volume, which causes a reduction in skin blood flow and limits the maximal duration of the experiment. We improved the model by replacing the collected blood with blood from donor animals, so enabling the measurement of transdermal absorption over a period of 5 h under near constant conditions of blood pressure, haematocrit and skin blood flow. The model was applied to the transdermal absorption of [³H]prazosin and [³H]scopolamine and their permeability coefficients, fluxes and lag-times were determined. The model is suitable for measurements of transdermal drug absorption under in-vivo conditions, both for comparison of absorption profiles of different drugs and of the same drug in different formulations.     

Intestinal 5-Fluorouracil Absorption: Use of Ussing Chambers to Assess Transport and Metabolism

We have employed an in vitro system to study transport and metabolism of organic molecules by gastrointestinal tissues. Such a system would aid in the evaluation of the potential for oral delivery of organic molecules. Transport and metabolism of 5-fluorouracil (5-FU) were studied using rabbit intestinal preparations. Unidirectional fluxes and metabolism were measured in vitro in Ussing chambers under short-circuit conditions. Results from these studies reveal that in ileum, proximal, and distal colon, steady-state fluxes of 5-FU (10 µM added to both bathing solutions) are established after 30 min and remain constant for at least 110 min. Transport of 5-FU under sink conditions with 10 µM 5-FU present in the mucosal or serosal bathing solution alone demonstrated similar rates of transport as under nonsink conditions. The concentration dependence of 5-FU fluxes indicates that the mucosal (m)-to-serosal (s) flux is composed of both a saturable and a linear component over the range of 1–100 µM in the ileum, whereas the s-to-m flux in the ileum and both fluxes in the colon are linear functions of concentration. Over the concentration range employed and the time course of these studies, 5-FU had no effect on the electrical properties of the ileum or colon. In the ileum, the m-to-s but not the s-to-m flux of 5-FU was reduced by (1) serosal ouabain (0.1 mM); (2) reduction of the bathing solution Na concentration; and (3) addition of uracil, thy mine, thymidine, uridine, 2-deoxyuridine, or uridine-5-monophosphate. These results indicate that 5-FU absorption in the ileum occurs by a Na-dependent mechanism that is inhibited by uracil and structurally related compounds. In distal colon, no evidence for an active transport mechanism was obtained. High-performance liquid chromatography (HPLC) analysis reveals that both ileum and distal colon metabolize 5-FU to more polar compounds. Metabolism in ileum is quantitatively greater than in distal colon. Metabolites are found predominantly on the side to which transport has occurred, suggesting that metabolism occurs concomitantly with transport. Since the intestinal cells metabolize 5-FU to more polar compounds and active absorption is inhibited in a competitive manner by related compounds, these results may provide an explanation for the variable oral activity reported for 5-FU.     

Intestinal Uptake of Particulate Material by Dexamethasonetreated Rats: Use of a Novel Technique to Avoid Intestinal Mucosal Contamination

The aim of this study was to investigate the effect of immune suppression on the uptake of particles across the wall of the intestine and the dissemination of the particles to systemic organs. Normal and dexamethasone-immunosuppressed rats were dosed orally with 0.5 mL distilled water or fluorescent polystyrene latex particle suspension containing 2.33 times 10⁹ 2-μm diameter particles. One hour after particle dosing, the animals were killed by CO₂ asphyxiation. The intestinal tissues and systemic organs were sampled for particle quantitation. To avoid contamination by particles adherent to intestinal mucosa the epithelium of intestinal tissue samples was removed before quantification. The number of fluorescent particles in tissues was determined by fluorescence microscopy of digests of selected samples. The uptake of particulate material across the intestinal wall was significantly (P < 0.05) increased in rats treated with dexamethasone but the number of particles transferred to systemic organs did not differ from values found for control animals. The results suggest that although dexamethasone increased intestinal permeability the apparatus or mechanisms involved in particle transport to distal sites were not affected during immune suppression.