Validated RP-HPLC method for quantification of doxazosin in human plasma: Application in a bioequivalence study.

OBJECTIVES: The aim of the present study was to validate a simple, sensitive, HPLC method of analysis of doxazosin in human plasma with fluorescence detection. METHODS: The validated method employed one-step direct protein precipitation with acetonitrile. Chromatographic separation was attained using a reverse-phase 250mm×4.6mm 5µ Hypersil® BDS C 18 column and the mobile phase consisted of 10mm sodium dihydrogen phosphate dihydrate (pH=3.0) and acetonitrile at a ratio of (65:35 v/v). The method was evaluated in terms of linearity, precision, accuracy, selectivity and stability as per standard guidelines. The total run time was about 4.5min which make this method suitable for high throughput analyses. This method was applied to the bioequivalence study of two doxazosin tablets in healthy human volunteers. RESULTS: Good linear response was achieved over the range of 5.0-200ng/mL. The observed within- and between-day assay precision ranged from 0.64% to 14.73%; accuracy varied between 94.11% and 105%. The 90% confidence intervals for the ratio Cmax, and AUC 0-∞ of the test product over those of reference were within the acceptable range (0.8-1.25) for bioequivalence. CONCLUSION: The developed method was simple and could be applied to therapeutic drug monitoring of doxazosin.

Doxazosin XL reduces symptoms of posttraumatic stress disorder in veterans with PTSD: a pilot clinical trial.

BACKGROUND: Serotonin and norepinephrine reuptake inhibitors are effective first-line agents for the treatment of posttraumatic stress disorder (PTSD), but treatment is associated with a range of side effects that limit treatment adherence. Prazosin, an α1-noradrenergic antagonist with a half-life of roughly 2-3 hours, has shown promise in the treatment of sleep disturbance and nightmares. Doxazosin extended release (XL) is also an α1-noradrenergic antagonist but with a half-life of approximately 15-19 hours. METHODS: We conducted a double-blind, placebo-controlled, within-subjects trial to characterize the impact of doxazosin XL on PTSD symptoms. Participants (N = 8) were diagnosed using DSM-IV criteria. They completed the study twice, once during treatment with doxazosin XL and once during treatment with matched placebo, with a 2-week washout separating the 2 episodes. Doxazosin XL was titrated from 4 mg/d to 16 mg/d over 12 days. After 4 days of treatment at 16 mg/d or the equivalent number of placebo capsules, PTSD symptoms were assessed using the Clinician-Administered PTSD Scale (CAPS17) and the PTSD Checklist-Military version (PCL-M). Repeated measures analysis of variance were used to evaluate effects of treatment, time, and treatment × time. This study was run from November 20, 2013, to June 31, 2014. RESULTS: Doxazosin XL treatment was associated with a nonsignificant treatment × time reduction in ratings on the CAPS hyperarousal subscale (P < .10) (but not on the CAPS Total score) and with significant treatment × time reductions in PCL-M ratings (P = .002). CONCLUSIONS: Doxazosin XL may be an effective alternative to prazosin for the treatment of some PTSD symptoms. TRIAL REGISTRATION: ClinicalTrials.gov Identifier:NCT02308202.

Enantioselective pharmacokinetics of doxazosin and pharmacokinetic interaction between the isomers in rats.

In this study, the stereoselective pharmacokinetics of doxazosin enantiomers and their pharmacokinetic interaction were studied in rats. Enantiomer concentrations in plasma were measured using chiral high-pressure liquid chromatography (HPLC) with fluorescence detection after oral or intravenous administration of (-)-(R)-doxazosin 3.0 mg/kg, (+)-(S)-doxazosin 3.0 mg/kg, and rac-doxazosin 6.0 mg/kg. AUC values of (+)-(S)-doxazosin were always larger than those of (-)-(R)-doxazosin, regardless of oral or intravenous administration. The maximum plasma concentration (Cmax ) value of (-)-(R)-doxazosin after oral administration was significantly higher when given alone (110.5 ± 46.4 ng/mL) versus in racemate (53.2 ± 19.7 ng/mL), whereas the Cmax value of (+)-(S)-doxazosin did not change significantly. The area under the curve (AUC) and Cmax values for (+)-(S)-doxazosin after intravenous administration were significantly lower, and its Cl value significantly higher, when given alone versus in racemate. We speculate that (-)-(R)-doxazosin increases (+)-(S)-doxazosin exposure probably by inhibiting the elimination of (+)-(S)-doxazosin, and the enantiomers may be competitively absorbed from the gastrointestinal tract. In conclusion, doxazosin pharmacokinetics are substantially stereospecific and enantiomer-enantiomer interaction occurs after rac-administration.

The truth about the lower plasma concentration of the (-)-isomer after racemic doxazosin administration in rats: Stereoselective inhibition of the (-)-isomer by the (+)-isomer at CYP3A.

Doxazosin (DOX), a long-lasting α1-adrenoceptor antagonist, is used clinically as a racemate that consists of two optical isomers. In humans and rats, following oral administration of racemic DOX [(±)-DOX], the plasma concentration of the (-)-isomer is lower than that of the (+)-isomer, but the mechanism for this interaction is not known. In this study, a chiral HPLC with fluorescence detection was used to measure the drug concentrations for analysis of the stereoselective metabolism of DOX in in vivo and in vitro experiments. We found that the plasma levels of the (-)-isomer were significantly lower than those of the (+)-enantiomer following i.v. administration of (±)-DOX to the rats and that the depletion rate constant (kdep) of (-)-DOX (0.0107±0.0007L/min) was significantly larger than that of (+)-DOX (kdep 0.0088±0.0005L/min) (p<0.05) when (±)-DOX was incubated with rat liver microsomes (RLMs). However, (-)-DOX was not depleted faster than (+)-DOX following their separate incubation with RLMs. The metabolism of (-)- or (+)-isomer in RLMs was catalysed by CYP3A because the depletion of the compounds was inhibited by ketoconazole (a potent CYP3A-selective inhibitor) similarly. More importantly, the kdep of (+)-DOX in the 1.0/2.0 and 0.5/2.5 (+)-DOX/(-)-DOX mixtures was significantly lower than that of (-)-DOX in the 1.0/2.0 and 0.5/2.5 (-)-DOX/(+)-DOX mixtures (p<0.05). In conclusion, although (-)-DOX is not depleted faster than (+)-DOX when only a single isomer of DOX is incubated with rat liver microsomes, it is depleted much faster than (+)-DOX when a mixture of the two isomers was used, suggesting a prominent and stereoselective inhibition of the (-)-isomer over the (+)-isomer at the CYP3A enzyme.

Development of a doxazosin and finasteride transdermal system for combination therapy of benign prostatic hyperplasia.

The treatment of benign prostatic hyperplasia can be accomplished by the use of different drugs including, doxazosin, an α-1 adrenergic antagonist, and finasteride (FIN), a 5-α reductase inhibitor. Traditionally, treatments using these drugs have been administered as either a mono or combination therapy by the oral route. A transdermal delivery system optimized for doxazosin and FIN combination therapy would provide increased patient adherence and facilitate dose adjustment. Doxazosin base (DB) was prepared from doxazosin mesylate and characterized together with FIN, by X-ray powder diffraction (XRD), differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR). The permeation enhancers, azone and lauric acid, and the gelling agents, hydroxypropyl cellulose (HPC) and Poloxamer 407 (P407), were evaluated to determine their ability to promote in vitro permeation of drugs through the pig ear epidermis. Successful preparation of DB was confirmed by evaluating the XRD, DSC, and NMR patterns and in vitro studies revealed that 3% (w/w) azone was the best permeation enhancer. When P407 gel was compared with HPC gel, it showed reduced lag time and promoted higher permeation of both drugs. This may be because of the interactions of the former with the stratum corneum, which disorganizes the lipid structure and consequently promotes higher drug permeation.

Transfer of doxazosin into breast milk.

To the best of our knowledge, there have been no published studies of doxazosin transfer into human milk. In rats, milk concentrations twentyfold higher than in plasma have been reported. Based on these animal data, some references advise to avoid breastfeeding during doxazosin therapy. However, the physicochemical properties of doxazosin suggest low transfer into human milk. A 37-year-old breastfeeding woman who was administered doxazosin 4 mg daily for 2 doses was studied. Doxazosin concentrations in milk and plasma were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The milk/plasma area under the concentration-time curve (AUC0-18 hours) ratio was 0.1. This finding is consistent with what could be predicted based on the physicochemical properties of doxazosin. The average and maximum milk concentrations were 2.9 and 4.2 µg/L. These values correspond to estimated relative infant doses of 0.06% and 0.09%, respectively, assuming standard infant milk intake. These values are well below the generally accepted cutoff of 10% for predicting safety during breastfeeding. A low relative infant dose of < 0.1% suggests that maternal doxazosin therapy may be compatible with breastfeeding after careful individual risk-benefit analysis.

Chiral recognition of doxazosin enantiomers in 3 targets for therapy as well as adverse drug reactions in animal experiments.

Doxazosin used in benign prostatic hyperplasia has the side effects of causing hypotension and the risk of heart failure. The 3 targets of α(1A)-adrenoceptors (in the prostate), α(1D)-adrenoceptors (in the aorta), and an unknown mechanism (in the heart) are involved, respectively. We hypothesized that there is a chiral recognition of doxazosin enantiomers in the 3 targets. Using isolated rat aorta (α(1D)-adrenoceptors) and rabbit prostate (α(1A)-adrenoceptors), we examined pA(2) and pK(B) values of doxazosin enantiomers. We observed chronotropic and inotropic effects of doxazosin enantiomers in isolated rat and rabbit heart tissues. (-)Doxazosin and (+)doxazosin produced a shift to the right of concentration-contraction curves for noradrenalin (aorta) and phenylephrine (prostate smooth muscle). The pA(2) value of (-)doxazosin (8.625 ± 0.053) was smaller than (+)doxazosin (9.503 ± 0.051) in rat aorta, but their pK(B) values in rabbit prostate were the same. In rat and rabbit heart tissues, (+)doxazosin (3-30 µmol·L(-1)) significantly decreased atrial rate, and produced negative inotropic effects; however, (-)doxazosin did not affect the atrial rate, and produced positive inotropic effects in the atria. Thus, the chiral carbon atom of doxazosin does not affect its activity at the therapeutic target of α(1A)-adrenoceptors in the prostate, but significantly changes its blocking activity against α(1D)-adrenoceptors in the aorta, and produces opposite inotropic effects in the atria via an α(1)-adrenoceptor-independent mechanism.

In vitro vs. canine data for assessing early exposure of doxazosin base and its mesylate salt.

In this study, we evaluated the usefulness of biorelevant in vitro data and of canine data in forecasting early exposure after the administration of two phases of a BCS Class II compound, i.e., doxazosin base (DB) and its mesylate salt (DM). DB, DM, and doxazosin hydrochloride (DH) were prepared and characterized. In vitro data were collected in various media, including human aspirates. Solubilities of DB and DM in human gastric fluid were forecasted by data in fasted state simulating gastric fluid containing physiological components (FaSSGF-V2) but not by data in HCl(pH 1.8). Unlike data in FaSSGF-V2, dissolution of DB and DM tablets in HCl(pH 1.6) is rapid. Dissolution of DB tablet in FaSSGF-V2 is incomplete and conversion to DH seems to occur. Differences between DB and DM in dissolution in the small intestine are overestimated in the absence of physiological solubilizers. Using the in vitro data and previously described modeling procedures, the cumulative doxazosin profile in plasma was simulated and the 0-2h profile was used for evaluating early exposure. Individual cumulative doxazosin profiles in plasma, after single DM tablet administrations to 24 adults, were constructed from corresponding actual plasma profiles. Compared with in vitro DM data in pure aqueous buffers, DM data in biorelevant media led to better prediction of early exposure. Based on intersubject variability in early exposure after DM administration and simulated profiles, the administered phase, DB or DM, does not have a significant impact on early exposure. Partial AUCs were used for evaluating early exposure after DB and DM administration in 4 dogs. Early exposure was significantly higher after administration of DM to dogs. Dogs are not appropriate for evaluating differences in early exposure after DB and DM administrations.

Enantioselective determination of doxazosin in human plasma by liquid chromatography-tandem mass spectrometry using ovomucoid chiral stationary phase.

An enantioselective and sensitive method was developed and validated for determination of doxazosin enantiomers in human plasma by liquid chromatography-tandem mass spectrometry. The enantiomers of doxazosin were extracted from plasma using ethyl ether/dichloromethane (3/2, v/v) under alkaline conditions. Baseline chiral separation was obtained within 9 min on an ovomucoid column using an isocratic mobile phase of methanol/5mM ammonium acetate/formic acid (20/80/0.016, v/v/v) at a flow rate of 0.60 mL/min. Acquisition of mass spectrometric data was performed in multiple reaction monitoring mode, using the transitions of m/z 452-->344 for doxazosin enantiomers, and m/z 384-->247 for prazosin (internal standard). The method was linear in the concentration range of 0.100-50.0 ng/mL for each enantiomer using 200 microL of plasma. The lower limit of quantification (LLOQ) for each enantiomer was 0.100 ng/mL. The intra- and inter-assay precision was 5.0-11.1% and 5.7-7.6% for R-(-)-doxazosin and S-(+)-doxazosin, respectively. The accuracy was 97.4-99.5% for R-(-)-doxazosin and 96.8-102.8% for S-(+)-doxazosin. No chiral inversion was observed during the plasma storage, preparation and analysis. The method proved adequate for enantioselective pharmacokinetic studies of doxazosin after oral administration of therapeutic doses of racemic doxazosin.

A LC-MS-MS method for determination of low doxazosin concentrations in plasma after oral administration to dogs.

A rapid and sensitive reversed phase liquid chromatography- tandem mass spectrometry (LC-MS-MS) method is developed for the determination of doxazosin in canine plasma. The samples are prepared by precipitation of proteins using a mixture of methanol and acetonitrile, followed by freezing and evaporation of the organic solvent. The remaining dry residue is redissolved in mobile phase and analyzed by LC-MS-MS with positive electrospray ionization using the selected reactions monitoring mode. An XTerra MS C(18) column, a mobile phase composed of acetonitrile and 2mM ammonium acetate with gradient elution, and a flow rate of 400 microL/min are employed. The elution times for prazosin (internal standard) and doxazosin are approximately 8 and 10 min, respectively. Calibration curves are linear in the 1-20 ng/mL concentration range. Limits of detection and quantification are 0.4 ng/mL and 1.2 ng/mL, respectively. Recovery is higher than 94%. Intra- and inter-day relative standard deviations are below 7% and 8%, respectively. The method is applied for the determination of doxazosin plasma levels following a single administration of doxazosin base and doxazosin mesylate tablets (2 mg dose) to dogs in the fed state. The results indicate possible superiority of the mesylate salt on the plasma input rates of doxazosin.

Combination of doxazosin and sildenafil exerts an additive relaxing effect compared with each compound alone on human cavernosal and prostatic tissue.

INTRODUCTION: Phosphodiesterase 5 inhibitors (PDE5) such as sildenafil are first-line treatment for erectile dysfunction (ED). Alpha1 (alpha1)-adrenoceptor antagonists such as doxazosin are indicated for the treatment of patients with lower urinary tract symptoms (LUTS)/benign prostatic hyperplasia (BPH). ED and LUTS/BPH are conditions that are often associated. Accordingly, alpha1-adrenoceptor antagonists and PDE5 inhibitors will be often prescribed in real life setting together. AIM: To evaluate the effects of the combination of sildenafil and doxazosin on human cavernosal and prostatic tissue. METHODS: Prostatic and erectile tissues were obtained from nine to 12 patients, respectively. Patients underwent cystoprostatectomy for infiltrating bladder cancer or penile surgery for penile implant, congenital curvature or Peyronie's disease. MAIN OUTCOME MEASURES: In organ baths, prostatic and cavernosal strips were submitted to either concentration-response curves (CRC) to phenylephrine (Phe) or norepinephrine (NE), respectively, in presence of vehicle, sildenafil (10(-6) M, 10(-5) M), doxazosin (10(-8) M, 3.10(-8) M, or 10(-7) M), or a combination of both. Continuous electrical field stimulation (EFS; 32 Hz, 5 ms, 5 seconds, 300 mA) was performed on prostatic strips which were incubated with sildenafil 10(-6) M or vehicle before the successive addition of doxazosin (10(-7) M, 10(-6) M) or vehicle. Cavernosal strips were pre-incubated with doxazosin (10(-9) M, 10(-8) M) or vehicle, then CRC to sildenafil were constructed on NE (3.10(-6) M) precontracted cavernosal strips. RESULTS: Combination of sildenafil and doxazosin exerted a greater relaxing effect on CRC to Phe or NE compared with each compound alone in both tissues. Sildenafil significantly enhanced the relaxing effect of doxazosin on EFS-induced contractions in prostatic strips. Doxazosin significantly increased the ability of sildenafil to inhibit NE-induced contractions in cavernosal strips. CONCLUSIONS: Sildenafil and doxazosin reduced adrenergic tone of prostatic and cavernosal smooth muscle and their combination provided a significant benefit when targeting relaxation of both tissues. These experiments provide support for further clinical evaluation of the sildenafil and doxazosin combination in ED patients with LUTS/BPH.

Release characteristics and in vitro-in vivo correlation of pulsatile pattern for a pulsatile drug delivery system activated by membrane rupture via osmotic pressure and swelling.

This study attempted to characterize the influence of core and coating formulations on the release profiles to establish in vitro/in vivo correlations of pulsatile pattern for a pulsatile drug delivery system activated by membrane rupture based on three core tablet formulations (A-core: HPMC 50+4000 cps, B-core: E10M, and C-core: K100M) coated with various thicknesses of a semipermeable ethylcellulose membrane plasticized with HPMC 606 (Pharmacoat 606) at different ratios with/without adding various amounts of water to dissolve it in the coating solution. Drug release behaviors were investigated using apparatus II in four media of pH 1.2 solution, pH 6.8 buffer, deionized water, and a NaCl solution rotated at 75, 100, and 150 rpm. Pilot studies of the in vivo pharmacokinetics were conducted as well for comparison with the in vitro results. Results demonstrated that drug release from the three kinds of core tablets in deionized water increased with an increasing stirring rate, and decreased with an increasing viscosity grade of HPMC used in the core formulations. A significant promotion of drug release from core tablets was observed for the three levels of NaCl media in comparison with that in deionized water. Results further demonstrated that a slightly slower release rate in pH 1.2 solution and a faster release rate in pH 6.8 buffer than that in deionized water were observed for the A-core and B-core tablets, with the former being slower than the latter. However, similar release rates in the three kinds of media were observed for C-core tablets, but they were slower than those for the A- and B-core tablets. Dissolution of coated tablets showed that the controlling membrane was ruptured by osmotic pressure and swelling which activated drug release with a lag time. The lag time was not influenced by the pH value of the release medium or by the rotation speeds. The lag time increased with a higher coating level, but decreased with the addition of the hydrophilic plasticizer, Pharmacoat 606, and of the water amount in the coating solution. The lag time also increased with a higher concentration of NaCl in the medium. The release rate after the lag time was determined by the extent of retardation of gelation of HPMC in the core tablet based on the ionic strength of the medium. Results of the three pilot crossover studies for the exemplified pulsatile systems indicated that the lag time for the in vivo plasma profile was well correlated with that determined from the in vitro release profile in pH 1.2 solution and the in vivo release rate was better reflected by that performed in pH 6.8 buffer.

Quantification of doxazosin in human plasma using hydrophilic interaction liquid chromatography with tandem mass spectrometry.

Hydrophilic interaction LC with MS/MS (HILIC-MS/MS) was described as a rapid, sensitive, and selective method for the quantification of doxazosin in human plasma. Doxazosin and cisapride (internal standard) were extracted from human plasma with ethyl acetate at alkaline pH and analyzed on an Atlantis HILIC Silica column with the mobile phase of ACN/ammonium formate (100 mM, pH 4.5) (93:7 v/v). The analytes were detected using an ESI MS/MS in the selective-reaction-monitoring mode. The standard curve was linear (r = 0.9994) over the concentration range of 0.2-50 ng/mL. The LOQ for doxazosin was 0.2 ng/mL using 100 microL plasma sample. The CV and relative error for intra- and interassay at four QC levels were 3.7-8.7% and 0.0-9.8%, respectively. The matrix effect for doxazosin and cisapride were practically absent. The recoveries of doxazosin and cisapride were 67.4 and 61.7%, respectively. This method was successfully applied to the pharmacokinetic study of doxazosin in humans.

Pharmacokinetics of doxazosin gastrointestinal therapeutic system after multiple administration in Korean healthy volunteers.

Doxazosin mesylate is a selective alpha-adrenoreceptor antagonist for the treatment of hypertension and benign prostatic hyperplasia. A simple high performance liquid chromatographic method has been developed and validated for the quantitative determination of doxazosin in plasma. A reversed phase C18 column was used for the separation of doxazosin and prazosin (internal standard) with a mobile phase composed of water, acetonitrile, triethylamine (68:32:0.2 v/v, pH 5.0) at a flow rate of 1.2 mL/min. The fluorescence detector was operated at 246 (excitation) and 389 nm (emission). Intra- and inter-day precision and accuracy were acceptable for all quality control samples including the lower limit of quantification of 1 ng/mL. Recovery of doxazosin from human plasma was greater than 93.4%. Doxazosin was stable in human plasma under various storage conditions. This method was used successfully for a pharmacokinetic study in plasma after oral administration of multiple 4-mg dose of doxazosin gastrointestinal therapeutic system formulation to 16 healthy volunteers. At steady state the mean area under the curve for a dosing interval and elimination half-life were calculated to be 367.0 +/- 63.5 ng x hr/mL and 29.2 +/- 4.5 hr, respectively. There was no difference in pharmacokinetic parameters between male and female.

Assessment of alpha1-adrenoceptor antagonists in benign prostatic hyperplasia based on the receptor occupancy theory.

AIMS: To assess the mechanistic relationship between doxazosin (alpha(1)-receptor antagonist) and receptor occupancy and a measure of pharmacological effect (Q(max), the maximum urinary flow rate) and to compare the mean receptor occupancy ratio at clinical doses of doxazosin, tamsulosin, terazosin and prazosin in benign prostatic hyperplasia (BPH). METHODS: A ternary complex model, which described the mechanism of alpha(1)-receptor antagonists, was fitted to the pharmacological effects and receptor occupancy ratio data for doxazosin (standard tablet). In addition, mean receptor occupancy was calculated for other alpha(1)-receptor antagonists and the optimal receptor occupancy was evaluated. The clinical pharmacological effects of the controlled release formulation of doxazosin (doxazosin GITS) were estimated based on the receptor occupancy. RESULTS: The mechanistic based model was able to describe the pharmacological effects of doxazosin. Regardless of the plasma concentrations or clinical dose of each drug, the results suggest that receptor occupancy is useful to assess quantitatively and compare the pharmacological effects of drugs with similar mechanisms of action. The clinical dosage for doxazosin GITS was estimated to be at least 8 mg and the stable pharmacological effect is expected based on the estimated receptor occupancy. CONCLUSIONS: A model for Q(max) improvement in BPH based on the receptor occupancy theory was able to describe the clinical effects of the alpha(1)-receptor antagonists. Receptor occupancy is a useful index for predicting the clinical effects of alpha(1)-receptor antagonists.

LC-MS determination and relative bioavailability of doxazosin mesylate tablets in healthy Chinese male volunteers.

This study aims to develop a standard protocol for the relative bioavailability testing of doxazosin mesylate tablets. For this purpose, a simple rapid and selective LC-MS method using a single quadrupole mass spectrometer was developed and validated to determine the concentration of doxazosin mesylate in human plasma. Using this method, we carried out a study of relative bioavailability. N-Hexylane-tertiary butyl methyl ether (1:1, v/v) was used to extract doxazosin mesylate and terazosin (internal standard, I.S.) from an alkaline plasma sample. LC separation was performed on a Thermo Hypersil-Hypurity C18 (5 microm, 150 mm x 2.1mm) using aqueous solution (20 mmol/l ammonium acetate, pH 4.28), methanol and acetonitrile (55:10:35, v/v/v) as the mobile phase. The retention time of doxazosin and the internal standard was 2.7 and 1.8 min, respectively. Quadrupole MS detection was done by monitoring at m/z 388 (M+1) corresponding to doxazosin mesylate and at m/z 452 (M+1) for I.S. The assay method described above showed acceptable precision, accuracy, linearity, stability, and specificity. The bioavailability of doxazosin mesylate was evaluated in 12 healthy Chinese male volunteers. The following pharmacokinetic parameters were elucidated after administering a single dose of 4 mg doxazosin. The area under the plasma concentration versus time curve from time 0 to 72 h (AUC(0-72 h)) 743.4+/-149.5 ngh/ml; peak plasma concentration (C(max)) 47.66 ng/ml; time to C(max) (T(max)) 3.0+/-1.0 h; and elimination half-life (t(1/2)) 18-20 h. The method was successfully used to determine the relative bioavailability of doxazosin mesylate.

Validation and pharmacokinetic application of a method for determination of doxazosin in human plasma by high-performance liquid chromatography.

A simple high-performance liquid chromatographic method for the determination of doxazosin in human plasma was developed and validated. Prazosin was used as internal standard. After extraction twice with ethyl acetate, chromatographic separation of doxazosin in human plasma was carried out using a reversed-phase Apollo C18 column (250 x 4.6 mm, 5 microm) with mobile phase of methanol-acetonitrile-0.04 m disodium hydrogen orthophosphate (22:22:56, v/v/v) adjusted to pH 4.9 with 0.9 m phosphoric acid and quantified by fluorescence detection operated with an excitation wavelength of 246 nm and an emission wavelength of 389 nm. The lower limit of quantification (LLOQ) of this assay was 1 ng/mL using 500 microL human plasma. Linearity was established over the range 1-25 ng/mL (r2 > 0.9994). The intra- and inter-day accuracy ranged from 90.5 to 104.4% and the coefficient of variation were not more than 8.6% for both intra- and inter-day precision, over the range of the calibration curve. The absolute recoveries of doxazosin and prazosin from human plasma were more than 91%. Doxazosin demonstrated acceptable short-term, long-term and freeze-thaw stability in human plasma. The assay has been successfully applied to plasma sample ana-lysis for pharmacokinetic study.

Bioequivalence evaluation of two formulations of doxazosin tablet in healthy thai male volunteers.

The bioequivalence of two doxazosin 2 mg tablets was determined in 24 healthy Thai male volunteers after one single dose in a randomized cross-over study with a one week washout period. The study was conducted at Faculty of Pharmaceutical Sciences and Health Sciences Research Institute, Naresuan University, Phitsanulok, Thailand. Reference (Cardura, Heinrich Mack Nachf. GmbH & Co. GK, Illertissen, Germany) and test (Dozozin-2, Umeda Co., Ltd., Bangkok Thailand) were administered to volunteers after overnight fasting. Blood samples were collected at specified time intervals and plasma was separated. The validated HPLC method with fluorescence detection was used for quantification of doxazosin in plasma samples. The pharmacokinetic parameters, T(max), C(max), AUC(t), AUC(infinity), T(1/2), lambda(z), Cl and V(d), were determined from plasma concentration time profile of both formulations by using non-compartment analysis. The calculated pharmacokinetic parameters were compared statistically to evaluate bioequivalence between the two brands. The analysis of variance (ANOVA) using log-transformed C(max), AUC(t), and AUC(infinity) did not show any significant difference between two formulations. The point estimates and 90% confidence intervals for C(max), AUC(t) and AUC(infinity) were within the acceptance range (0.80-1.25), satisfying the bioequivalence criteria of the Thailand Food and Drug Administration Guidelines. These results indicate that Dozozin-2 is bioequivalent to Cardura and, thus, may be prescribed interchangeably.

Doxazosin gastrointestinal therapeutic system: a review of its use in benign prostatic hyperplasia.

Doxazosin, a well established treatment in patients with bothersome lower urinary tract symptoms from benign prostatic hyperplasia (BPH), is available in a new controlled-release formulation, doxazosin gastrointestinal therapeutic system (GITS). Doxazosin GITS (Cardura XL, Cardular PP Uro, Diblocin PP Uro) has an altered pharmacokinetic profile, which allows a higher initial dosage to be used than with the standard formulation and less titration steps to reach a clinically effective dosage. In two large, double-blind, randomised studies (one was placebo-controlled) in patients with BPH, doxazosin GITS (4-8 mg once daily) was as effective as the standard formulation (2-8 mg once daily), and both were more effective than placebo, after 13 weeks' treatment in improving symptom scores, health-related quality of life (HR-QOL), and maximum urinary flow rate (Qmax). Doxazosin GITS was at least as well tolerated as the standard formulation of doxazosin in clinical studies in patients with BPH.