LC-MS/MS法测定人血浆中西酞普兰及其在制剂生物等效性中的应用

A sensitive and selective LC-MS/MS method for determination of citalopram in human plasma was established to study the bioequivalence of different formulations containing citalopram. The samples were simply pretreated by protein precipitation using acetonitrile, and then analyzed on a Zorbax Extend C₈column. The mobile phase consisted of acetonitrile-water-formic acid (60∶40∶0.2), at a flow-rate of 0.5 mL·min^-1. A Thermo Finnigan TSQ Quantum Ultra tandem mass spectrometer equipped with electrospray ionization source was used as detector and was operated in the positive ion mode. Selected reaction monitoring using the precursor to product ion combinations of m/z325 → m/z109 and m/z265 → m/z167 was performed to quantify citalopram and the internal standard, respectively. The pharmacokinetic parameters of citalopram in different formulations were calculated by non-compartment model. The linear calibration curves were obtained in the concentration range of 0.10-100 μg·L^-1. The lower limit of quantification was 0.10 μg·L^-1. The intra- and inter-day relative standard deviation (RSD) over the entire concentration range was less than 5.2%. Accuracy determined at three concentrations (0.25, 8.00 and 90.0 μg·L^-1 for citalopram) ranged from -4.7% to 1.3%. Each plasma sample was chromatographed within 3.0 min. The method was successfully used in bioequivalence study of citalopram in human plasma after oral administration of 20 mg citalopram. Calculated with AUC_{1-120 h}, the bioavailability of two formulations was (102.1±10.9)%. The method is rapid, selective, robust and is proved to be suitable for bioequivalence evaluation of different formulations containing citalopram.     

Fasting and postprandial absorption of digoxin from a microencapsulated formulation

Summary The absorption of digoxin from a capsule preparation containing a large number of small, enteric-coated granules of the glycoside (Preparation CR) was compared in 10 volunteers with that from a rapidly dissolving tablet (Preparation L). Plasma and urine digoxin concentrations were measured by radioimmunoassay. In the fasting state, after a loading dose of digoxin (0.76 mg), peak plasma concentrations were significantly (p<0.001) lower after CR (2.0±0.5 nmol/l, mean±SD) than L (4.7±1.1 nmol/l). Peak concentrations after CR were significantly (p<0.001) delayed compared to L (3.3±0.6 h vs 1.1±0.4 h). Also, postprandial peak plasma concentrations at steady state, were significantly (p<0.01) lower after CR (1.0±0.3 nmol/l) than L (2.7±0.5 nmol/l), and the peak concentrations occurred later (3.9±1.7 h vs 1.4±0.9 h). The area under the plasma concentration-time curves was smaller (p<0.01) for CR (17.7±5.9 nmol·l⁻¹·h) than for L (22.4±4.1 nmol·l⁻¹·h), and so was the amount of drug excreted in urine (174±25 μg vs 190±31 μg; p<0.005). Thus, the absorption rate of digoxin from the enteric-coated formulation was markedly reduced but at the cost of a variable reduction in the amount absorbed.     

Distribution and elimination of hydroflumethiazide in man

Summary The pharmacokinetics of hydroflumethiazide (HFT) were investigated after intravenous and oral administration to healthy subjects. After intravenous infusion, HFT behaved according to a three-compartment model. Two distribution phases were observed, with mean half-lives of 0.26 and 0.85 h, reflecting distribution to red blood cells and tissues. Mean biological half-life (t_1/2β ) after infusion was 5.2 h. Renal blood and plasma clearance of HFT, as well as the ratio renal blood clearance/renal plasma clearance of HFT, were lower after infusion than during the infusion, due to the distribution characteristics of HFT. After a single oral dose of 2 μmol/kg, t_1/2β was significantly shorter in all subjects than after a single oral dose of 6 μmol/kg, with mean t_1/2β of 8.7 and 17.9 h, respectively. Due to lack of a sufficiently sensitive method for determination of HFT in plasma, it could not be established whether the observed dose-dependent difference in biological half-life of HFT was caused by variation in renal clearance and/or the volume of distribution.     

Pharmacokinetics study of extended release formulations of buspirone hydrochloride in Beagle dogs

Objective To evaluate the pharmacokinetics(PK)properties of extended release formulations of buspirone hydrochloride in Beagle dogs.Methods A randomized,two period,two treatment,two sequence crossover bioequivalence study was designed;six healthy Beagle dogs were randomly divided into two groups,each group was orally given buspirone tablets or buspirone extended capsule containing 15 mg buspirone hydrochloride.Blood samples(about 1 mL)were collected in heparinized tubes before dosing and at 0.33,0.67,1,2,3,4,6,8,10,12,18,24 h after administration,and were then immediately centrifuged at 3000 rpm for 15 min.The pharmacokinetics(PK)properties of the drugs were evaluated using the liquid chromatographic-tandem mass spectrometric(LC-MS/MS)method.Results The mean tmax was 4.7,0.8 h and Cmax values was 1.8,6.9 μg·L-1,respectively for the sustained-release test(capsule)and reference formulation(tablet).When compared to the tablets,the residence time of the sustained capsules was dramatically prolonged and Cmax was reduced(P<0.01).The initial release speed was slow and stable.The bioavailability was similar to the common tablets.Conclusions The sustained capsule had showed good pharmacokinetics property of sustained-release in the Beagle dogs.     

Development of a method for determination of osmotic pump controlled-release tablets of lorcaserin hydrochloride in beagle dog plasma and its application to pharmacokinetic study北大核心CSCD

Aim To develop an UPLC-MS/MS method to determine the concentration of lorcaserin hydrochloride in beagle plasma, and study the pharmacokinetics of osmotic pump controlled-release tablets of lorcaserin hydrochloride. Methods A randomized crossover design was used, carbamazepine as the internal standard(IS), and plasma protein precipitation with acetonitrile. The chromatographic was Phenomenex Polar C18 column(100 mm×2. 1 mm, 3 μm), and acetonitrile - water(containing 10 mmol·L-1 ammonium acetate and 0.1% formic acid)(40:60, V/V)was mobile phase. Multiple reaction monitoring mode and electrospray positive ionization were used to detect lorcaserin hydrochloride. The MS/MS ion transitions were monitored at m/z 196.2→129.2 for lorcaserin hydrochloride and m/z 237→194.1 for carbamazepine, respectively. Results The linear range was 1 to 500 μg·L-1(r=0.999 2), the extraction recovery rate ranged from 87.70% to 89.70%, the precision RSD was 9.7%. The accuracy and matrix effect met the requirements, and the stability of lorcaserin hydrochloride was good in -20 ℃ refrigerator for 45 d, repeated freezing and thawing for three times, placed at room temperature for 24 h, and the disposed samples placed in automatsampler for 6 h were stable. The main pharmacokinetic parameters of the controlled-release tablet and immediate-release tablet were as follows:Tmax was(8.00±1.27)h and(1.00±0.13)h, Cmax was(70.56±3.73)μg·L-1 and(176.33±16.73)μg·L-1, and AUC0-t was(966.33±7.56)μg·h·L-1 and(973.05±69.09)μg·h·L-1, respectively. Conclusions The established UPLC-MS/MS method can be used to study the pharmacokinetics of lorcaserin hydrochloride in the plasma of beagle dogs, and osmotic pump controlled-release tablets has sustained release effect. © 2023 Publication Centre of Anhui Medical University. All rights reserved.     

Influence of CYP2D610B genotype on pharmacokinetics of propafenone enantiomers in Chinese subjects

AIM: To study the relationship between genotype of CYP2D6* 10B and pharmacokinetics of propafenone enantiomers. METHODS: Genotype of 17 healthy Chinese HAN subjects was determined by an allele specific amplification method. The blood samples (0-15 h) of the subjects were taken after oral administration of a single dose (400 mg) of propafenone hydrochloride. Concentrations of propafenone enantiomers in plasma were measured by a reverse-phase HPLC with precolumn derivatization. RESULTS: Seventeen subjects characterized for CYP2D6* 10B genotype included (*1/*1) (n=4), (*1/*10) (n=5) and (*10/*10) (n=8). The metabolic ratios (lg MR) of the three genotypes were-2.68±0.23, -2.2±0.7, and -1.1±0.5, respectively. The AUC of the three groups were (1534±334), (1891±793), (3171±1075) μg·h·L~(-1) for S-enantiomer and (1136±345), (1467±817), (2277±745) μg·h·L~(-1) for R-enantiomer, respectively. The AUC of propafenone enantiomers in *10/*10 is about 1.5-2 times of that of *1/*10 group or *1/*1 group, a     

Dissociation of biochemical and hypotensive effects of debrisoquine in hypertensive patients

Summary The 24 h urinary excretion of adrenaline, noradrenaline, metadrenaline, normetadrenaline and vanillylmandelic acid, plasma renin activity and plasma and urinary debrisoquine were measured before and during chronic treatment with oral debrisoquine in 14 in-patients with essential hypertension. There was a significant fall (mean ±SD) in the 24 h urinary excretion of vanillylmandelic acid (15.3±2.8 to 6.7±1.9 μmol) noradrenaline (199.0±105.8 to 125.2±43.3 nmol) and plasma renin activity (0.71±0.47 to 0.40±0.20 pmol Angio I ml⁻¹ h⁻¹) while the urinary normetadrenaline/noradrenaline ratio increased (10.4±6.1 to 17.1±5.1). No significant change was seen in the output of adrenaline or of O-methylated metabolites. Debrisoquine produces extensive noncompetitive inhibition of platelet monoamine oxidase in vivo at low therapeutic plasma concentrations. These changes support the view that treatment with debrisoquine produces intraneuronal inhibition of monoamine oxidase and post-ganglionic blockage. There was a significant correlation between the change in standing diastolic blood pressure and the daily dose (r_s=−0.52), pre-dose plasma concentration (r_s=−0.85) and mean daily urinary recovery (r_s=−0.80), of debrisoquine. The full extent of the biochemical changes were seen at low dose and low plasma concentration and were not directly correlated with the fall in standing or supine blood pressure.     

二甲基阿米洛利对大鼠油酸性急性肺损伤的保护作用

The effect of Na⁺/H⁺ exchange inhibitor dimethyl amiloride(DMA) on rat acute lung injury induced by oleic acid was studied. Five hours after administration (iv) of oleic acid (10 mL·kg^-1), pulmonary coefficient, lung wet/dry weight ratio and protein content, lactic acid dehydrogenase(LDH) activity, total white blood cell(WBC) and polymorphonuclear leukocyte(PMN) in bronchoalveolar lavage(BAL) of rat were increased significantly. However, after pretreatment with DMA(0.2 mg·kg^-1 iv) for 10 min, the pulmonary coefficient and lung wet/dry weight ratio were decreased from 1.12±0.06 to 0.87±0.05 and 5.8±0.5 to 4.5±0.3, respectively. The protein content, LDH activity, total WBC and PMN in BAL were also reduced from (1.69±0.15) g·L^-1 to (1.05±0.13) g·L^-1, (3.37±0.25) μmol·min^-1·L^-1 to (0.81±0.08) μmol·min^-1·L^-1, (4.3±0.9) ×10⁶·L^-1 to (2.4±0.8) ×10⁶·L^-1 and (2.4±0.4) ×10⁶·L^-1 to (1.6±0.4) ×10⁶·L^-1, respectively. These results suggest that activation of Na⁺/H⁺ exchange be involved in the acute rat lung injury induced by oleic acid and that inhibitor of sodium-hydrogen exchange prevent the rat lungs from this injury.     

高效毛细管电泳法研究盐酸班布特罗国产胶囊与进口片剂的人体生物等效性高效毛细管电泳法研究盐酸班布特罗国产胶囊与进口片剂的人体生物等效性

目的研究国产盐酸班布特罗胶囊和进口片剂的人体生物等效性。方法采用高效毛细管电泳法测定血浆中班布特罗及其代谢物特布他林的浓度。结果单次口服国产班布特罗胶囊和进口班布特罗片剂后班布特罗的药代动力学参数:AUC0-t分别为(71±18)和(72±13) μg·h·L^-1,实测C_max分别为(8.1±1.8)和(9.2±2.3) μg·L^-1,实测t_max分别为(3.6±1.3)和(3.7±1.0) h。特布他林药代动力学参数:AUC0-t分别为(129±33)和(130±34) μg·h·L^-1,实测C_max分别为(7.8±2.3)和(8.5±2.9) μg·L^-1,实测t_max分别为(5.4±0.8)和(5.6±1.1) h,国产班布特罗胶囊单次给药后的相对生物利用度为(100±16)%(班布特罗),(101±13)%(特布他林)。结论经统计学证明两制剂具有生物等效性。     

反相高效液相色谱法测定人血浆中阿昔洛韦浓度

A RP-HPLC method was developed for the determination of aeyclovir (ACV) in human plasma. ACV was extracted from plasma using ODS, obtained from Sep-Pak C₁₈ cartridge (Waters associates) and reinstalled in a self-made glass tube suitable for extraction of solutes from small volume plasma samples. After extraction, and rinsing the ODS extract with water to remove hydrophilic impurities, ACV was recovered with methanol and chromatographed on YWG C₁₈H₃₇ column, and detected at 254 mm. The mobile phase was 5.0% (v/v) methanol/water. The average recovery of ACV was 73. 5%; the minimum detection concentration of ACV in plasma was 20 ng/ml with a plasma volume of 0. 2 ml and S/N value of 2. A good linear relationship between the peak area ratios and ACV concentrations was found at the ACV concentrations ranging from 0. 2 to 1.0 μg/ml, C_ACV=0. 840 A_ACV/A_is+0.002, r=0.9969.     

曲美布汀分散片健康人体内的药代动力学及生物等效性研究曲美布汀分散片健康人体内的药代动力学及生物等效性研究

目的建立人血浆中曲美布汀的HPLC-ESI-MS测定法,研究曲美布汀在正常人体内的药代动力学行为,评价其两种制剂的生物等效性。方法血浆样品经碱化以环己烷提取,进行HPLC-ESI-MS分析,内标为西布曲明,检测离子为_m/z 388(曲美布汀)、_m/z 280(内标),裂解电压为50 V。20名健康志愿者交叉口服供试片和参比片,剂量均为100 mg。结果受试制剂及参比制剂的曲美布汀消除半衰期分别为(9.2±2.3) h和(9.2±2.8) h,达峰时间分别为(0.9±0.4) h和(1.0±0.3) h,峰浓度分别为(41±20) μg·L^-1和(40±20) μg·L^-1。以AUC_0-24h计算的受试制剂的相对生物利用度为(97±13)%。结论本法灵敏、准确、简便。统计学结果表明两种制剂生物等效。     

固相萃取结合HPLC-MS测定人血浆中奥曲肽的浓度及相对生物利用度固相萃取结合HPLC-MS测定人血浆中奥曲肽的浓度及相对生物利用度

目的建立人血浆中奥曲肽浓度的HPLC-MS测定法，研究国产奥曲肽注射剂的人体生物利用度。方法 血浆样品用HPL 1cc固相萃取小柱萃取，经Waters Xetrra C₁₈ MS分离后测定。18名健康志愿受试者采用随机交叉试验设计，分别im奥曲肽试验制剂和参比制剂200 μg，不同时间点采血，比较两者的生物利用度。结果线性范围0.5～40 μg·L^-1，方法回收率为97.1%～100.5%。日内、日间RSD分别为1.1%～1.6%，2.9%～4.8%。单剂量im奥曲肽200 μg后两种制剂的C_max分别为（19±10） μg·L^-1和（19±11） μg·L^-1，t_max分别为（0.50±0.15） h和（0.52±0.20） h；AUC_0～7h分别为（50±25） h·μg·L^-1和（50±25） h·μg·L^-1，t_1/2分别为(1.5±0.8) h 和(1.5±0.8) h。二者之间均无显著性差异，以进口奥曲肽为参比制剂，国产奥曲肽注射液的相对生物利用度为101%±10%。结论该方法灵敏、准确度高，可用于奥曲肽体内过程研究。两注射剂为生物等效性制剂。     

家犬单剂量和多剂量口服阿昔莫司缓释制剂的药代动力学和生物等效性研究

目的研究阿昔莫司缓释片在家犬体内单剂量和多剂量的药代动力学和生物等效性。方法测定6只家犬单剂量和多剂量口服缓释片和普通胶囊后的血药浓度。结果阿昔莫司的药-时曲线符合非隔室模型。单剂量给药后，缓释片和普通胶囊的AUC分别为(158±30)和(147±37) μg·h·mL^-1；T_max分别为(4.3±0.8)和(2.6±1.3) h；C_max分别为(29±6)和(42±10) μg·mL^-1；T_1/2分别为(2.3±0.7)和(1.60±0.10) h；MRT分别为(6.0±0.8)和(3.9±0.7) h；F_r为(108±16)%。多剂量给药后，缓释片和普通胶囊的AUC分别为(209±23)和(195±26) μg·h·mL^-1；T_max分别为(6.3±0.8)和(3.4±1.5) h；C_max分别为(27±4)和(36±5) μg·mL^-1；Cmin分别为(2.2±1.0)和(0.20±0.20) μg·mL^-1；C_av分别为(8.7±1.0)和(8.1±1.1) μg·mL^-1；FI分别为(293±73)%和(448±91)%；F_r为(114±19)%。结论单剂量实验的双单侧检验结果表明：缓释片和普通胶囊生物等效；缓释片具有良好的缓释效果。多剂量实验结果表明：缓释片和普通胶囊生物等效；缓释片的波动系数优于普通胶囊。     

人血浆中O-去甲右美沙芬的测定及药代动力学研究

目的建立直接测定人血浆中O-去甲右美沙芬的方法,并应用于药代动力学研究。方法18名健康受试者单剂量po氢溴酸右美沙芬60 mg后,血浆样品经液-液萃取,通过液相色谱-质谱-质谱联用法测定其活性代谢物O-去甲右美沙芬的浓度,用非室模型计算药代动力学参数。结果O-去甲右美沙芬测定的线性范围为0.2～80 μg·L^-1;其主要药代动力学参数T_max为(2.1±0.7) h,C_max为(14±8) μg·L^-1,T_1/2为(3.8±1.8) h,用梯形法计算,AUC_0-t为(60±37) μg·h·L^-1。结论该法灵敏度高,操作简便,可直接测定活性代谢物,适用于右美沙芬的临床药代动力学研究及制剂的生物等效性评价。     

高灵敏度LC/MS/MS法同时测定人血浆中麻黄碱和氯苯那敏

麻黄碱是存在于草麻黄和木贼麻黄等植物中的生物碱,属拟肾上腺素类药物,包括互为差向异构体的麻黄碱和伪麻黄碱两种活性不同的成分;氯苯那敏为丙胺类组胺H1-受体拮抗剂,作用较强,用量少,中枢抑制作用小,为常用抗过敏药,临床上可用于缓解各种感冒症状,与麻黄碱联合应用,用于缓解支气管哮喘与慢性喘息性支气管炎所致支气管痉挛.     

高效液相色谱-质谱联用测定家兔体内可乐定血药浓度

目的用HPLC-MS测定家兔体内可乐定血药浓度,研究可乐定在家兔体内的药代动力学行为。方法 色谱仪为Waters Alliance 2790,色谱柱为XTerra C18(150 mm×2.1 mm ID,5 μm)。流动相为乙腈-碳酸氢铵水溶液,梯度洗脱;质谱仪为Micromass ZQ-4000仪,电喷雾电离源,选择离子监测(SIM)质荷比(m/z)为230。结果本方法回收率较高,重现性好。线性范围为1～80 μg·L^-1,最低检测浓度为0.05 μg·L^-1。主要药代动力学参数Cmax,AUC0-t和tmax分别为(27±9) μg·L^-1,(5 352±1 121) μg·L^-1,(79±17) h。结论本法灵敏度高,选择性强,具有较高的精密度和准确度。透皮贴剂中的可乐定可以平稳地透过家兔皮肤,稳定地控释达7 d。     

沙丁胺醇气雾剂在健康受试者体内的药物动力学和相对生物利用度

目的:研究沙丁胺醇气雾剂在健康受试者的药物动力学和生物利用度.方法:十名健康男性志愿者单剂量吸入1.2 mg沙丁胺醇气雾剂或口服沙丁胺醇水溶液.用HPLC法测定人血浆中沙丁胺醇浓度.以非房室模型计算药物动力学参数,计算气雾剂相对水溶液的生物利用度.结果:气雾剂和口服溶液的药物动力学参数如下:T_(max)(0.22±0.07)和(1.8±0.6)h,C_(max)(3.4±1.1)和(3.9±1.4)μg·L~(-1),T_(1/2)(4.5±1.5)和(4.6±1.1)h,AUC_(0-20min)(0.9±0.3)和(0.16±0.10)μg·h·L~(-1).两种给药途径的T_(max)和AUC_(0-20 min)之间差异显著(P<0.01).AUC_(0-20min)(nihal)为 AUC_(0-20 min)(po)的 8倍.沙丁胺醇气雾剂相对口服溶液的生物利用度为 57％±24％.结论:沙丁胺醇气雾剂在人体的吸收过程与口服溶液差异有显著性.     

盐酸二甲双胍/格列本脲复方片剂在人体的药代动力学

目的　建立人血浆中格列本脲的HPLC ESI MS测定法 ,研究志愿者口服格列本脲与二甲双胍的复方片剂后的药代动力学行为。方法　人血浆样品中格列本脲的测定方法 :血浆样品以 1mol·L- 1的盐酸酸化后用乙酸乙酯提取 ,进行HPLC ESI MS分析 ,色谱柱为LichrospherC18(dp 5μm ,4 6mmID× 2 5cm ) ,流动相为甲醇 -10mmol·L- 1醋酸铵水溶液 (78∶2 2 ,V/V) ,检测方式为SIM方式 ,检测离子为m/z 492 1(格列本脲 )、m /z 444 1(内标 )。 2 0名健康志愿者交叉口服供试片和参比片 ,剂量均为格列本脲 2 5mg和盐酸二甲双胍 50 0mg。 结果　在 0 3 10～ 413 μg·L- 1范围内格列本脲峰面积与内标峰面积的比值与浓度的线性关系良好。格列本脲受试制剂与参比制剂的T1/2 分别为(5 4± 0 8)h、(5 9± 1 0 )h ,Cmax 分别为 (14 6± 2 2 ) μg·L- 1、(12 3± 16) μg·L- 1,Tmax分别为 (2 7± 0 9)h、(3 0±0 7)h ,AUC0～ 3 6 分别为 (73 0± 14 0 ) μg·h·L- 1、(63 2± 117)μg·h·L- 1;二甲双胍受试制剂与参比制剂的T1/2 分别为(3 0± 0 6)h、(3 0± 0 4)h ,Cmax分别为 (1 61± 0 3 2 )mg·L- 1、(1 62± 0 3 3 )mg·L- 1,Tmax分别为 (1 8± 0 2 )h、(1 7± 0 4)h ,AUC0～ 15 分别为 (7 3 7± 1 3 4 )     

氯雷他定及其活性代谢物去羧乙氧基氯雷他定在中国健康受试者的药动学

目的:研究氯雷他定(LOR)及其活性代谢物去羧乙氧基氯雷他定(DCL)在中国健康受试者体内的药动学。方法:20名健康中国男性受试者口服氯雷他定20mg,采用液相色谱-质谱-质谱联用法测定血浆中LOR和DCL的药物浓度。结果:LOR和DCL的C_(max)分别为(17±14)和(16±9)μg/L;T_(max)分别为1.2和1.5h;AUC_(0-∞)分别为(47±49)和(181±122)μg·h·L~(-1);LOR的T_(1/2)平均为(6±4)h,DCL的T_(1/2)平均为(13.4±2.6)h;AUC_(DCL)/AUC_(LOR)比值的范围从0.36到54.5。结论:中国受试者口服单剂量氯雷他定片后,在体内迅速吸收并转化为DCL,母体药物AUC的个体差异极为显著,而活性代谢物DCL的AUC则只表现出中等强度的个体差异。     

Comparison of the plasma pharmacokinetics of lamivudine during twice and once daily administration in patients with HIV

OBJECTIVE: To compare the plasma pharmacokinetics of lamivudine 150mg twice daily and 300mg once daily in patients with HIV-1 infection. DESIGN: Nonblind, sequential, pharmacokinetic study. PARTICIPANTS: 13 patients with HIV-1 infection (median age 36 years). METHODS: Patients were tested during twice daily and then once daily regimens of lamivudine. In both regimens, the total daily dose of lamivudine was identical (300 mg/day). Blood samples for pharmacokinetic analysis were taken over a 12-hour period after > or =7 days of twice daily administration, and again over a 24-hour period after 7 days of once daily administration,. RESULTS: 12 patients completed the study. Lamivudine pharmacokinetic parameters (mean +/- SD) after administration of 150mg twice daily were: peak plasma concentration (Cmax) 2077+/-816 microg/L; trough plasma concentration (Cmin) 332+/-219 microg/L; elimination half-life (t 1/2beta) 6.1+/-1.9h; time to Cmax (t(max)) 1.6+/-0.7h; average concentration over the dosage interval (Cav) 711+/-269 microg/L; and area under the concentration-time curve (AUC) over 2 dosage intervals (24h) 17085+/-6464 microg x h/L. Corresponding values after administration of 300mg once daily were: Cmax 3461+/-854 microg/L; Cmin 146+/-87 microg/L; t1/2 7.9+/-3.4h; t(max) 2.2+/-1.3h; Cav 705+/-177 microg/L; and AUC over 1 dosage interval (24h) 16644+/-4150 microg x h/L. Statistical analysis showed a significant difference (p < 0.05) between the 2 schedules for Cmax and Cmin values, whereas no significant differences emerged for the other parameters. CONCLUSIONS: Once daily lamivudine leads to a similar exposure in plasma as twice daily administration of the same total daily dose. Since once daily administration may result in improved compliance, these results provide the pharmacokinetic basis for using lamivudine in a once daily regimen. Randomised clinical studies are needed to confirm this pharmacokinetic finding.