Pharmacokinetics,bioavailability and tissue distribution of geniposide following intravenous and peroral administration to rats

In order to characterize the pharmacokinetics, bioavailability and tissue distribution of geniposide following intravenous and peroral administration to rats, a reliable gradient HPLC‐based method has been developed and validated. After p.o. administration of geniposide, the peak concentration of geniposide in plasma occurred at 1 h and plasma geniposide was eliminated nearly completely within 12 h. The AUC_{0→ ∞} values of geniposide were 6.99 ± 1.27 h · µg/ml and 6.76 ± 1.23 h · µg/ml after i.v. administration of 10 mg/kg and p.o. administration of 100 mg/kg of geniposide, respectively. The absolute oral bioavailability (%F) of geniposide was calculated as 9.67%. After p.o. administration of geniposide, the AUC_0→4h values in tissues were in the order of kidney > spleen > liver > heart > lung > brain. This study improved the understanding of the pharmacokinetics, bioavailability and tissue distribution of geniposide in rats and may provide a meaningful basis for clinical application of such a bioactive compound of herbal medicines. Copyright © 2013 John Wiley & Sons, Ltd.     

Pharmacokinetics,tissue distribution and relative bioavailability of geniposide-solid lipid nanoparticles following oral administration

Abstract

Geniposide has various pharmacological effects; however, low oral bioavailability limits its clinical utility. This study explores the pharmacokinetics, tissue distribution and relative bioavailability of geniposide-solid lipid nanoparticles (SLNs) following oral administration. The geniposide solution and geniposide-SLNs were orally administered to the rats, respectively. The C_max value of geniposide in the geniposide-SLNs was significantly higher than that obtained with geniposide solution. Compared with the geniposide solution, the t_1/2 and MRT were prolonged; the CL and V1/F were increased with geniposide-SLNs. The AUC_0–∞values of geniposide-SLNs were 50 times greater than geniposide solution. The ratios of AUC_{0–8 h} in the liver, spleen, heart, kidney, brain and lung of the geniposide-SLNs to geniposide solution were 25.93, 4.28, 27.91, 10.15, 5.16 and 16.22, respectively. Prepared geniposide-SLNs are very helpful for increasing the bioavailability of geniposide. These data suggest that SLNs are a promising delivery system to enhance the oral bioavailability of geniposide.     

Pharmacokinetics, tissue distribution and relative bioavailability of puerarin solid lipid nanoparticles following oral administration

Puerarin has various pharmacological effects; however, poor water-solubility and low oral bioavailability limit its clinical utility. A delivery system of solid lipid nanoparticles could enhance its oral absorption. The objective of this study was to investigate the pharmacokinetics, tissue distribution and relative bioavailability of puerarin in rats after a single dose intragastric administration of puerarin solid lipid nanoparticles (Pue-SLNs). The puerarin concentrations in plasma and tissues were determined by rapid resolution liquid chromatography electrospray ionization-tandem mass spectrometry. The C(max) value of puerarin after the administration of Pue-SLNs was significantly higher than that obtained with puerarin suspension (0.33±0.05 μg/mL vs. 0.16±0.06 μg/mL, P<0.01). The T(max) value after the administration of the Pue-SLNs was significantly shorter than that after puerarin suspension administration (40±0 min vs. 110±15.49 min, P<0.01). The AUC(0→t) values of puerarin were 0.80±0.23 mg h/L, and 2.48±0.30 mg h/L after administration of the puerarin suspension and Pue-SLNs, respectively. Following administration of the Pue-SLNs, tissue concentrations of puerarin also increased, especially in the target organs such as the heart and brain. These data suggest that SLNs are a promising delivery system to enhance the oral bioavailability of puerarin.     

Gastric microbleeding following single and repeated dosing with naproxen

Background: Adaptation to gastric damage from nonsteroidal anti-inflammatory drugs (NSAID) has been observed during ongoing dosage in rats and humans. However, this does not always occur, and our previous data suggest that NSAID half-life may be a determining factor. Aim: To investigate whether adaptation occurs during 1 week of naproxen administration in humans. Subjects: Thirteen healthy volunteers were studied at baseline, and after me or seven daily doses of naproxen 750 mg. Gastric microbleeding was measured 4 h after naproxen in gastric washings collected during a 30-min period. Serum thromboxane B concentrations were also assayed, as a marker of cyclo-oxygenase inhibition. Results: Mean blood loss after placebo was 0.60, μL/10 min (95% CI: 0.21–0.98). This rose to 2.15 (0.73–3.57) and 1.75 (0.74–2.76) μL/10 min after one and seven daily doses of naproxen, respectively (P < 0.05 vs. baseline: day 1 vs. 7 not significant). Thromboxane B concentrations were < 10% of control at both day 1 and 7 of dosing. Conclusion: In accord with our findings in rats, adaptation to this moderately long acting NSAID in humans was not apparent. We conclude that any adaptation to naproxen is unlikely to be clinically important.     

Pharmacokinetics and relative bioavailability of prajmalium bitartrate after single oral dosing.

Pharmacokinetics and relative bioavailability of the marketed prajmalium bitartrate tablet (Neo-Gilurytmal, CAS 2589-47-1) compared to an oral solution were investigated in an open, randomized, single-dose two-fold crossover study in 20 healthy male volunteers. One subject was identified to be a poor metabolizer. In the study population with normal metabolic status the two oral formulations proved to be bioequivalent with regard to the pharmacokinetic parameters Cmax, AUC(0-Tlast), AUC(0-infinity) and Ae(24h). tmax was prolonged after administration of the tablets. The relative bioavailability of prajmalium bitartrate from the tablet amounted to 112%. The poor metabolizer demonstrated in both oral formulations high plasma concentrations, increased AUCs and prolonged terminal half-lives as well as increased renal excretion of prajmalium bitartrate.     

Pharmacokinetics,bioavailability, tissue distribution and excretion of tangeretin in rat

Tangeretin, 4′,5,6,7,8-pentamethoxyflavone, is one of the major polymethoxyflavones (PMFs) existing in citrus fruits, particularly in the peels of sweet oranges and mandarins. Tangeretin has been reported to possess several beneficial bioactivities including anti-inflammatory, anti-proliferative and neuroprotective effects. To achieve a thorough understanding of the biological actions of tangeretin in vivo, our current study is designed to investigate the pharmacokinetics, bioavailability, distribution and excretion of tangeretin in rats. After oral administration of 50 mg/kg bw tangeretin to rats, the C_max, T_max and t_1/2 were 0.87 ± 0.33 μg/mL, 340.00 ± 48.99 min and 342.43 ± 71.27 min, respectively. Based on the area under the curves (AUC) of oral and intravenous administration of tangeretin, calculated absolute oral bioavailability was 27.11%. During tissue distribution, maximum concentrations of tangeretin in the vital organs occurred at 4 or 8 h after oral administration. The highest accumulation of tangeretin was found in the kidney, lung and liver, followed by spleen and heart. In the gastrointestinal tract, maximum concentrations of tangeretin in the stomach and small intestine were found at 4 h, while in the cecum, colon and rectum, tangeretin reached the maximum concentrations at 12 h. Tangeretin excreted in the urine and feces was recovered within 48 h after oral administration, concentrations were only 0.0026% and 7.54%, respectively. These results suggest that tangeretin was mainly eliminated as metabolites. In conclusion, our study provides useful information regarding absorption, distribution, as well as excretion of tangeretin, which will provide a good base for studying the mechanism of its biological effects.     

Pharmacokinetics of diltiazem and its metabolites after repeated single dosing in healthy volunteers

In a crossover study balanced for sex, treatments, and treatment order, we have investigated the pharmacokinetics of diltiazem after single oral doses in 10 healthy middle-aged volunteers. The diltiazem doses employed were 60 mg and 120 mg and contained 1.85 MBq [14C] diltiazem. The absorption was rapid and did not differ between treatments. The disposition could be described using a two-compartment model with terminal half-lives of 5.68 +/- 2.62 h (mean +/- SD) after 60 mg and 5.5 +/- 2.22 h after 120 mg. The half-life of the metabolite N-demethyldiltiazem (MA) was similar to or slightly longer than that of diltiazem, whereas the half-life of deacetyldiltiazem (M1) was longer: 9.80 +/- 5.27 h and 10.43 +/- 5.38 h (n = 5). The area under the curve (AUC) of diltiazem increased significantly more than twofold after doubling of the dose, indicating an increased bioavailability, probably because of decreased presystemic elimination. The ratio between the AUCs for metabolites and diltiazem were 0.48 +/- 0.12 and 0.45 +/- 0.06 for MA and 0.16 +/- 0.10 and 0.15 +/- 0.10 for M1 after 60 mg and 120 mg diltiazem. The cumulative excretions of radioactivity within 120 h were 86 +/- 9% and 87 +/- 6%. The tracer was mainly excreted in urine (69 +/- 7% and 72 +/- 6%) and the remaining amounts were excreted in feces.     

Flavonoid pharmacokinetics and tissue distribution after repeated dosing of the roots of Scutellaria baicalensis in rats

Scutellariae Radix (root of Scutellaria baicalensis, SR) contains numerous flavonoids such as baicalin, baicalein, and wogonin. This study investigated the pharmacokinetics and tissue distribution of flavonoids and their metabolites in rats after repeated dosing of a SR decoction. Sprague-Dawley rats were orally administered SR at 2 g/kg for seven doses. After the 7th dose, blood samples were withdrawn at specific times and organs, including the liver, kidney, lung, and brain, and collected. The concentrations of baicalein and wogonin in the serum and various tissues were assayed by HPLC before and after hydrolysis with glucuronidase and sulfatase. Baicalein and wogonin were not detected in the serum, and the molecules found were their glucuronides/sulfates. In tissues, the free forms of baicalein and wogonin appeared in the liver, kidney, and lung in addition to their glucuronides/sulfates. Baicalein was the major form in the lung, whereas baicalein glucuronides/sulfates were the major forms in the liver and kidney. Wogonin was the major form in the liver, kidney, lung, and traces of wogonin glucuronides/sulfates were detected in the kidney and liver. Neither baicalein and wogonin nor their glucuronides/sulfates were detected in the brain. In conclusion, the glucuronides/sulfates of baicalein and wogonin were exclusively present in the circulation, whereas their free forms appeared in the lung, liver, and kidney.     

Pharmacokinetics and bioavailability of nicotine in healthy volunteers following single and repeated administration of different doses of transdermal nicotine systems

Healthy nicotine-dependent smokers were applied different doses of transdermal nicotine systems (TNS) during single and repeated administrations. Plasma and urine nicotine and cotinine concentrations were determined by high performance liquid chromatography (HPLC). After single application of TNS, the maximal concentration (Cmax) and area under curve (AUC) of nicotine in plasma as well as the amount of nicotine excreted in urine were linearly related to the dose. The stable urinary cotinine excretion was not influenced by the amount of nicotine delivered by the TNS. The relevant 24 h plasma nicotine concentration reached after TNS application compares well with the plasma nicotine footpoints--not the peaks--observed in moderate to heavy cigarette smokers. A comparison between different nicotine doses from different TNS allowed to conclude to the functionality of the systems as regards pharmacokinetics and bioavailability. One or two hours after removal of the systems, there was a very slow decline of the nicotine concentrations. After repeated application of TNS, there was evidence for only a very limited nicotine accumulation in plasma (+14%) or in urine (+9%) over 10 days. The steady-state of nicotine was reached within 4 days. The continuous delivery of nicotine over 24 h resulted in an early morning plasma concentration which probably decreases or prevents the craving for the first cigarette.     

南强菌素在大鼠体内的药动学、组织分布及排泄研究

目的研究海洋新骨架小分子南强菌素经单次静脉注射后在Wistar大鼠体内的药动学、组织分布及排泄特征。方法 48只Wistar大鼠(雌雄各半)分为8组(每组雌雄各三只),3组分别尾静脉注射南强菌素10、20、30 mg·kg-1,液相色谱-串联质谱法(LC-MS/MS)测定其血药浓度,DAS2.0计算药动学参数;其余5组单次尾静脉给予南强菌素20 mg·kg-1,其中3组大鼠给药后0.5、2、3 h麻醉处死、解剖测定南强菌素组织分布,1组给药后收集0² h、>22 h、>2⁴ h、>44 h、>4⁸ h、>88 h、>8²4 h的尿液粪便评价南强菌素的排泄过程,1组麻醉后插管收取024 h的尿液粪便评价南强菌素的排泄过程,1组麻醉后插管收取0² h、>22 h、>2¹6 h、>1616 h、>16²4 h胆汁评价胆汁排泄。结果大鼠分别静脉单次注射10、20、30 mg·kg-1南强菌素后,t蚝虔z分别为(2.09±0.68)、(2.44±0.35)、(2.57±1.33)h,AUC0-8 h分别为(3 378±544)、(3 492±460)、(4 451±573)μg·h·L-1;单次静脉给药20 mg·kg-1,0.5 h后肾脏组织中的南强菌素含量最高(1 736.8 ng·g-1),肺脏及脾脏次之;给药2 h后,多数组织中已经检测不到或仅能检测到极少量南强菌素;原型药物经尿样和胆汁的排泄率分别为1.87%和0.91%,粪便的排泄率极低,低于总给药量的0.01%。结论单次给予南强菌素,各剂量组的AUC0-8 h、ρmax和t蚝虔z用SPSS进行相关检验,无显著差异(P>0.05),表明在置信度(单双侧)0.05水平上剂量与AUC0-8 h、ρmax和t蚝虔z均不相关;原型药物经粪便和尿样排泄率较低,124 h胆汁评价胆汁排泄。结果大鼠分别静脉单次注射10、20、30 mg·kg-1南强菌素后,t蚝虔z分别为(2.09±0.68)、(2.44±0.35)、(2.57±1.33)h,AUC0-8 h分别为(3 378±544)、(3 492±460)、(4 451±573)μg·h·L-1;单次静脉给药20 mg·kg-1,0.5 h后肾脏组织中的南强菌素含量最高(1 736.8 ng·g-1),肺脏及脾脏次之;给药2 h后,多数组织中已经检测不到或仅能检测到极少量南强菌素;原型药物经尿样和胆汁的排泄率分别为1.87%和0.91%,粪便的排泄率极低,低于总给药量的0.01%。结论单次给予南强菌素,各剂量组的AUC0-8 h、ρmax和t蚝虔z用SPSS进行相关检验,无显著差异(P>0.05),表明在置信度(单双侧)0.05水平上剂量与AUC0-8 h、ρmax和t蚝虔z均不相关;原型药物经粪便和尿样排泄率较低,1²4 h内仅有1.87%从尿液中排出。     

Pharmacokinetics, tissue distribution, and excretion of buagafuran in rats

The pharmacokinetics, tissue distribution, and excretion of buagafuran (BF, 4-butyl-α-agarofuran), a promising antianxiety drug isolated from Gharu-wood (Aquilaria agallocha Roxb), were investigated in rats. BF plasma concentration was determined in rats after oral and intravenous doses by GC-TOF-MS. BF showed nonlinear pharmacokinetics after oral and intravenous administration of 4, 16, and 64 mg/kg. The AUC(0-∞) and C(max) did not increase proportionally with doses, indicating the saturation in absorption kinetics of BF in rats after oral dosage. BF absorption was extremely poor with an absolute bioavailability below 9.5%. After oral administration of (3)H-BF (4 mg/kg) to rats, radioactivity was well distributed to the tissues examined. The highest radioactivity was found in gastrointestinal tract, followed by liver and kidney. Radioactivity in brain, as a target organ, was about 20-40% of that in plasma at all time points. Total mean percent recovery of radioactive dose was about 80% in rats (51.2% in urine; 28.7% in feces). Bile elimination was also the major excretion route of BF, and 45.4% of the radioactive dose was recovered in bile.     

Pharmacokinetics,tissue distribution,and excretion of FGF‐21 following subcutaneous administration in rats

As one of the fibroblast growth factor (FGF) superfamily, FGF‐21 has been extensively investigated for its functions and roles since its discovery. It has been demonstrated to be one of the key regulators for glucose and lipid metabolism, and exhibits beneficial effects on cardiovascular disease. However, studies focusing on its pharmacokinetic behavior in vivo as a novel therapeutic agent have not been reported. In the present study, rapid and sensitive analytical approaches including radioactivity assay and assay after precipitation/separation by high performance liquid chromatography (HPLC) were established to determine the content of FGF‐21 tagged with ¹²⁵I in plasma, tissue, and excrement. The results indicated that FGF‐21 were quickly absorbed into systematic circulation and slowly eliminated; C_max and exposure increased in a dose‐dependent manner, exhibiting a typical linear pharmacokinetic pattern. Tissue distribution also confirmed that the kidney is the primary organ for FGF‐21 to be distributed, even though radioactivity of FGF‐21 was recovered in all tissues examined. In addition, the results also supported that urinary excretion was the critical route for FGF‐21 to be eliminated. The study fully clarifies the pharmacokinetic behavior of FGF‐21 and can provide valuable information and support further safety and toxicology development.     

Pharmacokinetics of 4-acetylaminophenylacetic acid. 2nd communication: tissue accumulation and enzyme induction in rats after repeated administration, and placental and milk transfer after single dosing

The absorption, distribution, metabolism and excretion of 14C-labeled 4-acetylaminophenylacetic acid (MS-932) were studied in male rats after administration of an oral dose of 10 mg/kg once a day for 21 days. Comparison with the single dosing showed no marked alterations in absorption, distribution, metabolism and excretion. There were no significant differences in the activities of hepatic aniline hydroxylase and aminopyrine N-demethylase between the MS-932 treated group (10 mg/kg for 8 days) and the 0.5% aqueous sodium carboxymethyl cellulose control group (p greater than 0.05). Placental transfer of radioactivity was studied after single oral administration of 10 mg/kg of 14C-MS-932 to pregnant rats on the 12-13th and 19-20th days of gestation. Radioactivity concentrations were highest in the maternal plasma and lowest in the amniotic fluid and fetus for both middle and late pregnancies. The concentrations in the amniotic fluid and fetus decreased more slowly than did the concentration in the maternal plasma. Excretion of radioactivity to milk was studied after single oral administration of 10 mg/kg of 14C-MS-932 to lactating rats on the 10th day after parturition. Radioactivity concentrations in the rat milk were maximal at 1 h after dosing and were lower than in the maternal plasma at all the sampling times.     

Pharmacokinetics and tissue distribution of olanzapine in rats

The single dose pharmacokinetics of olanzapine in rats, following an oral dose and its distribution in the brain and other tissues after repeated oral and intra-peritoneal (i.p.) administration, were studied. Olanzapine in plasma, brain, liver, lung, kidney, spleen and fat was assayed at predose, 0.25, 0.5, 1, 2, 5, 12, 24, 36, 48 h postoral dose of 6 mg/kg and after daily oral and i.p. doses of 0.25, 1, 3, and 6 mg/kg/day of olanzapine for 15 consecutive days by a sensitive and specific HPLC method with electrochemical detection. Olanzapine was readily absorbed and distributed in plasma and tissues as the peak concentrations were reached within approximately 45 min after the oral dose. The terminal half-life of olanzapine in plasma was 2.5 h and in tissues it ranged from 3 to 5.2 h. The area under the concentration-time curve (AUC(last)) was lowest in plasma and largest in liver and lung. The AUC(last) of olanzapine was eight times larger in brain and three to 32 times larger in other tissues than that in plasma. After repeated oral doses, the plasma and tissue concentrations of olanzapine were generally higher than those after repeated i.p. doses. The liver and spleen had the highest concentrations after oral and i.p doses, respectively. In both cases, the tissue concentrations were four- to 46-fold higher than that in plasma and correlated with administered doses. Likewise, plasma concentrations strongly correlated with the simultaneous brain and tissue concentrations (r(2)>0.908, p<0.0001). On average, the brain levels were 6.3-13.1 and 5.4-17.6 times higher than the corresponding plasma level after oral and i.p. doses, respectively. The tissue to plasma level ratio of olanzapine was higher in other tissues. The data indicated that olanzapine is rapidly absorbed and widely distributed in the tissues of rats after oral and i.p. administration. The plasma concentration appears to predict the simultaneous concentration in brain and other tissues. There was no marked localized accumulation of olanzapine in any of the regions of the rat brain.     

Pharmacokinetics of dihydroartemisinin in Artekin tablets for single and repeated dosing in Chinese healthy volunteers

Aim. To study the pharmacokinetics of dihydroartemisinin (DHA) in Artekin (compound dihydroartemisinin) tablets in Chinese healthy volunteers. Methods. Eighteen healthy volunteers (9 males, 9 females) received Artekin tablets for oral administration. The plasma samples of DHA were analysed by liquid-liquid extraction and determined by HPLC/ESI/MS. Results. The plasma DHA concentration-time curves of single dose and repeated doses of DHA were fitted to a two-compartment open model. The mean pharmacokinetic parameters of DHA in a single dose were: t(1/2(beta))=1.245 +/- 0.495 h, C(max)=243.6 +/- 56.15 microg/l, AUC(0 --> infinity)=450 +/- 69 h x microg/l, V(d)=5.75 +/- 2.2 l/kg and Cl=3.245 +/- 0.38 l/h/kg, while in repeated doses they were: t(1/2(beta))=1.085 +/- 0.298 h, AUC(0 --> infinity)=444.35 +/- 80.43 h x ng/ml, V(d)=4.62 +/- 1.128 ml/kg, Cl=3.0125 +/- 0.875 ml/h/kg, respectively. Conclusion. The study showed that DHA in Artekin was rapidly absorbed, distributed and eliminated in the healthy subjects. The pharmacokinetic properties of DHA in Artekin were not affected by gender in a single dose. While in repeated doses accumulation of DHA did not appear after repeated doses.     

去铁酮在大鼠体内的药代动力学与组织分布

目的研究去铁酮(DFP)在大鼠体内的药代动力学和组织分布。方法雄性Wistar大鼠ig给予DFP35,70和140mg.kg-1后,于不同时间点收集血液和组织样本。采用高效液相色谱法测定大鼠血浆及组织中的DFP的含量,运用DAS2.0药代动力学智能分析软件拟合房室模型,并进行药代动力学参数计算。结果大鼠ig给予DFP35,70和140mg.kg-1后,体内药代动力学过程符合二室模型,t1/2α分别为23.3,22.2和20.9min,t1/2β分别为53.3,50.9和46.3min,Cl分别为0.017,0.021和0.016L.min-1.kg-1。大鼠ig给予DFP70mg.kg-1后,DFP在胃和肝中浓度较高,60min时肝中DFP含量可达(359.22±31.16)μg.g-1,其他组织含量较低。结论DFP在大鼠体内吸收和消除较迅速,在体内组织分布广。     

Pharmacokinetics, bioavailability, metabolism, tissue distribution and urinary excretion of gamma-L-glutamyl-L-dopa in the rat

gamma-L-Glutamyl-L-dopa (gludopa) is believed to be a dopamine prodrug specific for the kidney. Its pharmacokinetics have been studied in the rat given 50 mg kg-1 intravenously (i.v.) and 60 mg kg-1 intraperitoneally (i.p.). By the i.v. route, elimination followed apparent first order kinetics and was biphasic with a t 1/2 alpha of 7 min and terminal half-life of 67 min. After i.p. administration absorption was rapid (t 1/2 ab 6 min), elimination was monophasic with a terminal half-life almost identical following i.v. dosing (65 min), and bioavailability was 40%. In tissues (liver and kidney) gludopa was biotransformed to four intact catecholic products (L-dopa, dopamine, DOPAC and gamma-L-glutamyl-dopamine) which appeared quickly (peaks at 15 min) and which were almost completely cleared by 4 h. Dopamine was the major kidney metabolite accounting for 69% of total catechol content with an AUC 31 times greater than in liver where it accounted for only 34% of total catechols. In rat urine eight major metabolites (5.7% of the dose) and at least 12 minor metabolites were detected of all of which 85% was dopamine. A higher percentage of the dose was excreted as intact catechols in man (15.7%) but fewer metabolites were detected (L-dopa, dopamine, DOPAC). It is confirmed that gludopa is kidney specific in rat but that the pharmacological effects of dopamine are likely to be short lived due to rapid clearance. Gludopa appears to be less dopamine specific in man.     

Pharmacokinetics and bioavailability of omeprazole after single and repeated oral administration in healthy subjects.

1. Ten healthy subjects were given 20 mg omeprazole EC (enteric coated) granules once daily for 8 days. An i.v. tracer dose of [14C]-omeprazole was given simultaneously with the first and last oral doses and blood sampling was performed thereafter. In order to study the extent of absorption at minimal acid exposure, a single dose of 20 mg omeprazole was also given as a buffered solution, before and after the treatment with EC granules. 2. Kinetic parameters of omeprazole after the i.v. tracer dose were unchanged on repeated dosing while AUC increased by approximately 40% for the solution and 60% for the EC granules. 3. The increased AUC is caused by an increased systemic availability, which may be explained by a decreased first-pass elimination during repeated treatment and/or by a reduced degradation of omeprazole in the stomach secondary to the profound decrease in intragastric acidity caused by the drug. 4. The implication of these findings is that the antisecretory effect of therapeutic doses of omeprazole must be studied during repeated administration and not judged from studies using single doses only.     

Oral bioavailability and disposition of sulphadimethoxine in lobster,Homarus americanus,following single and multiple dosing regimens

1. Single and multiple oral doses of sulphadimethoxine or sodium sulphadimethoxine were administered by gavage to lobster, and sequential samples of haemolymph were taken for analysis of parent sulphadimethoxine. Single doses of sodium sulphadimethoxine were given over a dose range of 14–70mg/kg. Five 42-mg/kg doses of sulphadimethoxine on alternate days were administered for the multiple-dose studies. Some experiments were conducted with radiolabelled (³⁵S or ¹⁴C) sulphadimethoxine, and the tissue distribution of radioactivity was determined at different killing times.

2. Pharmacokinetic parameters were obtained by fitting sulphadimethoxine concentrations in haemolymph to a one-compartment model. Oral bioavailability at the 42-mg/kg dose, calculated from the area under the haemolymph concentration-time curve (AUC) relative to the AUC from intravascular administration, was between 47 and 52% for single or multiple doses of the free drug. The bioavailability of sodium sulphadimethoxine was dose dependent, at 97% for the 14mg/kg dose, and 25% for the 70-mg/kg dose. The low bioavailability at the high dose probably resulted from poor absorption due to the limited solubility of sulphadimethoxine at the low pH of the lobster gastrointestinal tract.

3. Sulphadimethoxine and several polar metabolites were excreted in lobster urine. Polar metabolites were also found in the hepatopancreas and haemolymph. At least 20% of the 42-mg/kg dose was metabolized. The major vertebrate metabolite of sulphadimethoxine, N-acetylsulphadimethoxine, was a very minor metabolite in lobster. The identities of the polar metabolites were not established.

4. Elimination of sulphadimethoxine residues from muscle to >0.1 μg sulphadimethoxine equivalents/g tissue required 40 days after a single dose, or 44 days after the last of multiple doses. Concentrations of sulphadimethoxine residues in all other tissues were always greater than muscle concentrations. Data showed that sulphadimethoxine residues were very persistent in lobster tissues.