Radioreceptor assay analysis of tamsulosin and terazosin pharmacokinetics

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1 Br J Clin Pharmacol 998; 45: Radioreceptor assay analysis of tamsulosin and terazosin pharmacokinetics Katsunari Taguchi, Rafael F. Schäfers, & Martin C. Michel Department of Medicine, University of Essen, 45 Essen, Germany Aims A radioreceptor assay has been developed for a -adrenoceptor subtypes and applied to a pharmacokinetic analysis of tamsulosin and terazosin. Methods Young, male, healthy volunteers received.4 mg tamsulosin (as OmnicA modified release capsules) or 5 mg terazosin (as FlotrinA tablets) in a randomized, cross-over design. Before and after, 3, 5, 7, and 3.5 h plasma was analyzed by radioreceptor assay using cloned human a A -, a B - and a D -adrenoceptors and specific h.p.l.c. analysis. Results Following ingestion of tamsulosin median peak plasma levels of 6 ng ml were reached after 5 h and declined to ng ml at 3.5 h. The time course in the radioreceptor assay was similar, and at most time points binding to a A -adrenoceptors was significantly greater than to a B -anda D -adrenoceptors. Following ingestion of terazosin median peak plasma levels of 9 ng ml were reached after h and declined to ng ml at 3.5 h. In the radioreceptor assay binding also peaked at h and declined thereafter, but even after 3.5 h considerable binding activity remained detectable at all three subtypes. At most time points binding to the a A - and a D -adrenoceptor was significantly greater than to the a B -adrenoceptor. Conclusions We conclude that a -adrenoceptor antagonist pharmacokinetics can be monitored by radioreceptor assays in a subtype-selective manner. Tamsulosin and terazosin exhibit subtype selective receptor binding ex vivo. The discordance between terazosin blood levels as determined by h.p.l.c. and radioreceptor assay at late time points indicates the possible involvement of metabolites in in vivo terazosin effects. Keywords: radioreceptor assay, tamsulosin, terazosin, a -adrenoceptor subtypes Introduction state of the a A -adrenoceptor []. Thus, the overall evidence suggests that indeed the a A -adrenoceptor may be a -Adrenoceptor antagonists have long been used in the the main mediator of human prostatic contraction. treatment of arterial hypertension [, ]. Recently, they Most clinically used a -adrenoceptor antagonists, e.g. have also been introduced for the symptomatic treatment of terazosin, were originally developed for the treatment of benign prostatic hyperplasia [3]. Receptor cloning and arterial hypertension and do not discriminate a -adrenoceptor pharmacological analysis have identified the existence of at subtypes [6, 4]. In contrast, the novel antagonist, least three a -adrenoceptor subtypes which are designated tamsulosin, was specifically developed for the symptomatic a A (formerly a c ), a B and a D (formerly a a/d ) [4]. Since treatment of benign prostatic hyperplasia and has about human prostate predominantly expresses a A -adrenoceptors 5-fold selectivity for a A - over a B -adrenoceptors with at the mrna [5] and protein level [6], it has been postulated intermediate affinity for a D -adrenoceptors [6, 8, 3 6]. that selective binding to a A -adrenoceptors may be sufficient Interestingly placebo-controlled clinical trials with terazosin to yield full therapeutic efficacy in benign prostatic [7 9] have reported larger incidences of unwanted side hyperplasia with perhaps less side effects related to vaso- effects than trials with tamsulosin [, ] but a direct dilation [7]. On the other hand, it has been reported that comparison has not been performed. some a A -selective antagonists only poorly inhibit contrac- The present study was designed to gain further insight tion of human prostate in vitro; it was proposed that an into a potential role of a -adrenoceptor subtype selectivity additional subtype designated a L may be involved in in the action of tamsulosin and terazosin. This was done by prostatic contraction [8, 9]. More recent studies, however, a pharmacokinetic analysis of receptor binding following a have found that several compounds which have higher single, oral dose of tamsulosin (.4 mg) and terazosin (5 mg) affinity for a A - than for the putative a L -adrenoceptors in in a placebo-controlled, single-blind, randomized, three-way standard binding assays using membrane preparations at cross-over study. For this purpose we have developed a new room temperature also have low affinity for a A -adrenoceptors radioreceptor assay [] using cloned human a -adrenoceptor when assayed in intact cells at 37 C; they have subtypes stably expressed in rat- fibroblasts [3]. With this suggested that the a L subtype may be a conformational assay we have assessed binding to each of the three subtypes at, 3, 5, 7, and 3.5 h following drug intake. Specific Correspondence: Dr Martin C. Michel, Nephrol. Lab. IG, Klinikum Essen, 45 analysis of tamsulosin and terazosin plasma concentrations Essen, Germany. by specific h.p.l.c. analysis was performed in comparison. 998 Blackwell Science Ltd 49

2 K. Taguchi et al. Methods Subjects was by fluorescence monitoring with excitation at 5 nm. The limit of accurate measurement of the assay was 5ngml. The study protocol was approved by the ethics committee Radioreceptor assays at the University of Essen Medical School. Ten healthy male subjects (median age: 6.5 years, range: 36 years) Radioreceptor assays were performed using membrane participated after having given informed written consent. preparations from rat- fibroblasts which had been transfected Each subject completed 3 study days during which they with cdnas encoding the human a A -, a B - or a D - received in a single-blinded, randomized cross-over manner adrenoceptors [3]. These cell lines had kindly been provided tablet of 5 mg terazosin (purchased as FlotrinA from a by Dr Geoff Johnston at Pfizer Central Research (Sandwich, German pharmacy), capsule of.4 mg tamsulosin modified Kent, UK). They were cultured in Dulbecco s modified release formulation (OmnicA, provided by Yamanouchi Eagle s medium supplemented with 4.5 g l glucose, mm Europe B.V., Leiderdorp, Netherlands), or capsule of l-glutamine and 5% heat-inactivated fetal calf serum. A placebo matching the tamsulosin capsule. Study days were membrane preparation from these cells was prepared as at least 7 days apart to secure sufficient drug washout. previously described [4]. On each study day the subjects reported to the laboratory Radioligand binding assays were performed in a total at 7. h after an overnight fast. An indwelling catheter volume of 3 ml consisting of ml plasma sample and was placed into a forearm vein for blood withdrawals. The mlbuffer (5 mm Tris,.5 mm EDTA, ph 7.5) including subjects remained in the supine position until after the last [ 3 H]-prazosin and aliquots of the membrane preparation blood withdrawal 4 h after drug ingestion. Blood samples from the transfected rat- cells (#5 35, 3 6 and 4 8 mg were taken h before and, 3, 5, 7, and 3.5 h after protein for a A -, a B -anda D -adrenoceptors, respectively, drug intake. The blood samples were collected into EDTA- depending on the respective receptor expression densities). coated tubes to prevent coagulation. They were stored on The mixtures were incubated for 6 min at 5 C. ice immediately. Plasma samples were generated by centrifu- Incubations were terminated by rapid vacuum filtration over gation and stored at C until analysis, i.e. up to 4 Whatman GF/C filters using a Brandell cell harvester. Non- months. Each blood sample was used for h.p.l.c. and specific binding was defined as binding in the presence of radioreceptor analysis. A light snack was allowed after the 5 mm phentolamine. Samples from each time point of the 7 h blood withdrawal and a pizza or pasta dish after the h tamsulosin and terazosin study days were assayed in withdrawal. While no other food was given until after the quadruplicate in competition binding assays. Samples from last blood withdrawal, drinking of water was allowed various time points of the placebo study day were pooled ad libitum. to perform a [ 3 H]-prazosin saturation binding experiment using eight radioligand concentrations; the results of this H.p.l.c. analysis saturation experiment were used to calculate apparent K d values for [ 3 H]-prazosin for the plasma of each subject. For H.p.l.c. analysis for tamsulosin content was performed by each a -adrenoceptor subtype and subject samples from the the biopharmaceutical department of Yamanouchi Europe placebo day (saturation binding) and the tamsulosin and B.V. (Leiderdorp, Netherlands). The plasma samples were terazosin day (competition binding) were run side-by-side analysed by reversed phase h.p.l.c. with fluorescence in the same assay. While interference by endogenously detection. Tamsulosin and the internal standard AB89 were released catecholamines cannot fully be excluded, this is extracted from plasma by liquid-liquid extraction. The quite unlikely because the subjects remained resting in the extracts were redissolved into the mobile phase (.85 g supine position throughout the study and because the drugs KH PO 4,.4 g Na HPO 4, 54 g H O, 56 g acetonitrile, had no major effects on resting supine blood pressure. 8 ml ethyl acetate) and injected on a 5 4 mm C8 column. Detection was performed fluorimetrically at an Data analysis excitation wavelength of 75 nm with emission being read at 35 nm. This method is suitable for quantification of Analysis of the radioreceptor assay data was performed tamsulosin in human plasma between.5 and 5 ng ml. according to procedures published for a b-adrenoceptor H.p.l.c. analysis for terazosin content was performed by subtype radioreceptor assay []. Active drug concentration the National Poisons Unit, Guy s and St Thomas NHS i was calculated as multiples of apparent affinity (K i ) Trust (London, UK). The samples were analysed by h.p.l.c. equivalents at the a -adrenoceptor subtypes, i.e. as with fluorescence detection. The internal standard solution, according to equation [] aqueous dimethothiazine (5 mg l,ml), NaOH solution i = AA B maxl L B (4 m, 5 ml) and saturated NaCl solution ( ml) were added to plasma (5 ml). Terazosin and the internal standard K i Bl nsb K B.5 d were extracted into methyl tertiary-butyl ether ( ml) by In this equation L is the concentration of the radioligand mixing for 3 s and centrifugation at rev min for [ 3 H]-prazosin, K d is the apparent affinity of the radioligand 4 min. A portion (4 ml) of the extract was injected onto when assayed in the presence of plasma as determined for the h.p.l.c. column (mobile phase: 4 mm ammonium each subject and a -adrenoceptor subtype in the saturation perchlorate in methanol5deionised water (9+) adjusted to binding experiments, Bl is the amount of the radioligand ph 6.8 with % (v/v) methanolic perchloric acid). Detection bound in the presence of plasma at the radioligand Blackwell Science Ltd Br J Clin Pharmacol, 45, 49 55

3 Tamsulosin and terazosin pharmacokinetics concentration L, and nsb is the amount of non-specific coefficient of variation was smaller in the h.p.l.c. assay (5% binding in the presence of plasma at the radioligand and 35%, respectively) than in the radioreceptor assays for concentration L; the factor.5 is introduced to correct for a A -adrenoceptors (6% and 4%, respectively), a B -adrenoceptors dilution of the ml sample in the 3 ml assay volume. (4% and 74%, respectively) or a D -adrenoceptors Similar data were obtained when the pooled K d of all (3% and 9%, respectively) when assayed on parallel subjects (a A : 4349 [363, 77] pm, a B : 335 [66, 394] samples. Similarly, the interquartile ranges in the radiorecep- pm, a D : 698 [39, 858] pm; n= each for median values tor assays for all three subtypes were consistently greater with upper and lower quartile in squared brackets) rather than those for the h.p.l.c. measurements (Figures ). than the individually determined K d values were used in the Thus, the radioreceptor assay appears to yield greater data calculations (data not shown). B max is calculated individually scatter than the specific h.p.l.c. methods. for each data set based upon the law of mass action according Following tamsulosin ingestion, binding to a A -, a B -and to equation [] a D -adrenoceptors in the radioreceptor assay peaked after 5 h and was.7 (.; 3.33),.43 (.6;.97) and.69 B max = (L+K d)(bl nsb) (.46;.4) times K i, respectively (Figure ). From there L blood levels declined gradually and were no longer Analysis of saturation binding experiments was performed significantly different from after 3.5 h for all three by fitting rectangular hyperbolic functions to the experimenadrenoceptors was significantly greater than that to a B - subtypes (Figure ). At most time points binding to a A - tal data using non-linear regression analysis with the Inplot programme (Graphpad Software, San Diego, CA). Analysis adrenoceptors, and after 5 and 7 h also significantly greater of the competition binding data was performed with a than binding to a D -adrenoceptors (Figure 3). spreadsheet to implement the above function using the The behaviour of terazosin in the radioreceptor assay was Microsoft Excel programme. more complex. Binding to the a A -adrenoceptor (similar to All data are given as median with upper and lower the h.p.l.c. data) peaked after h and was.56 (.93;.75) quartiles of subjects. Statistical significance of differences times K i ; from there it declined gradually to.7 (.3; in drug binding to the a.7) times K i after 3.5 h (Figure ). Binding to the a B - -adrenoceptor subtypes was assessed by Wilcoxon s signed rank test with P<.5 being and a D - adrenoceptors did not exhibit a clear time- considered significant. dependency between and h after terazosin intake. At the h time point binding to the a B -anda D -adrenoceptor Results was.7 (.4;.37) and.4 (.6; 3.57) times K i, respectively. Even after 3.5 h considerable binding was H.p.l.c. analysis maintained, i.e..45 (.34;.69) and.7 (.9;.49) at the a B -and and a D -adrenoceptor, respectively (Figure ), Median plasma concentrations of tamsulosin peaked 5 h which corresponds to 64% and 33% of peak values, following drug intake at 6. (.4; 7.4) ng ml ; thereafter respectively. Binding to a A -anda D -adrenoceptors at most they gradually declined to. (.9;.8) ng ml, i.e. 3% time points was significantly greater than that to a B - of peak values after 3.5 h (Figure ). Plasma concentrations adrenoceptors (Figure 3). of terazosin had peaked h following drug intake and median C max was 9 (7; 97) ng ml declining gradually Discussion to (; 4) ng ml, i.e. % of peak values, after 3.5 h (Figure ). Classical pharmacokinetic analysis is based on quantification of drug levels in blood or plasma by specific h.p.l.c. or Ex vivo radioreceptor assay antibody-based techniques, e.g. radioimmunoassay. Using these techniques, the pharmacokinetic profiles of tamsulosin The median intraassay coefficient of variation of the binding and terazosin have been documented. Thus, tamsulosin in assay was calculated from the quadruplicate determination its modified release formulation reaches its maximum plasma of the h time point in each experiment and was 4.3% concentration at 5 6 h and has a half-life (t / ) of 3 h (.7; 5.8), 3.9% (.9; 4.7) and 5.9% (4.; 7.6) for the assay [5]. While terazosin similarly has a t / of h, it is with a A -, a B -anda D -adrenoceptors, respectively (n=4 absorbed more quickly and reaches its maximum concen- each). Our study design did not allow a formal calculation trations after h [5, 6]. In the present study maximum of the interassay coefficient of variation of the radioreceptor plasma concentrations for tamsulosin and terazosin in plasma assay. However, the interassay variance can be estimated occured at time points which are consistent with the from the replicate determination of the h time point previously reported values. Reliable formal calculations of from the tamsulosin and terazosin study days; the mean plasma half-life and t max values were not feasible from the deviation of each replicate from the mean of both present data, particularly for tamsulosin, since the number measurements was expressed as percentage of the mean and and spacing of time points in the present study were not was 5.3% (4.; 8.9), 3.% (.; 5.8) and 3.% (.; 5.4) for designed for such analysis. Nevertheless our h.p.l.c. data are the assay with a A -, a B -and a D -adrenoceptors, respectively compatible with the above estimates of plasma half-life (n= each). The intersubject coefficient of variation was values for tamsulosin and terazosin. determined at the time of peak plasma concentrations of While such specific measurements yield sensitive and parent compound, i.e. after 5 h for tamsulosin and after h reliable estimates of drug levels in blood or plasma, they also for terazosin. For both compounds the median intersubject have several disadvantages. Firstly, measurement of drug 998 Blackwell Science Ltd Br J Clin Pharmacol, 45,

4 K. Taguchi et al. Plasma concentration (ngml ) H.p.l.c a B 4 3 a A a D Figure Pharmacokinetic profile of a single oral dose of tamsulosin (.4 mg) in h.p.l.c. and radioreceptor analysis. Ten young, healthy, male subjects received a single oral dose of.4 mg tamsulosin. Before and, 3, 5, 7, and 3.5 h after drug intake a blood sample was taken. Plasma derived from those samples was analyzed by h.p.l.c. and used in the radioreceptor assay. Drug levels in the radioreceptor assay are expressed as multiples of the concentration which occupies 5% of the human a A -, a B -ora D -adrenoceptor ( )as calculated according to equation (). Data are median values from subjects (filled circles) with upper and lower quartiles (dashed lines). plasma levels does not account for a possible contribution this was achieved using ml of plasma in a total assay of drug metabolites to the observed effects, particularly volume of 3 ml. When [ 3 H]-prazosin saturation binding when such metabolites have not yet been fully identified. experiments were performed in the presence of plasma, the Secondly, active drug levels are frequently lower than plasma apparent K d values were higher than those typically observed concentrations due to plasma protein binding, which may at cloned subtypes [8, 4]. While this may be partially be different for a parent compound and its metabolites. explained by the known plasma protein binding of prazosin, Thirdly, drug metabolites may have a pharmacodynamic it was noted that the apparent affinity at the a A -adrenoceptor profile which is distinct from that of their parent compound, was much lower than with the other subtypes. While we e.g. with regard to receptor subtype affinity and specificity. have no good explanation for this aberrant behaviour of the These disadvantages become important when a comparison a A -adrenoceptor in the presence of plasma, it should be of pharmacokinetic and pharmacodynamic parameters is noted that the shift from membrane binding at 5 Cto intended, particularly when subtype-selective drugs are whole cell binding at 37 C also reduces the apparent affinity involved. of [ 3 H]-prazosin at a A - but not at a B -ora D -adrenoceptors To circumvent these problems radioreceptor assays have []. been introduced into the quantitative analysis of drug effects Radioreceptor assay techniques have originally been used in man []. They determine the ability of plasma of a for studies on b-adrenoceptor antagonists []. Thereafter, drug-treated subject to compete for radioligand binding to they have also been applied to studies of muscarinic the receptor of interest relative to the ability of plasma from acetylcholine receptors [7] and a -adrenoceptors [8]. the non-treated subject. This technique is effectively When the assay is carried out in parallel on subtypes of a independent of knowledge of the chemical identity of drug given receptor, e.g. on b -andb -adrenoceptors [, 9] or metabolites and evaluates the parent compound and all of its M and M muscarinic acetylcholine receptors [7], the metabolites according to their relative contribution to assay allows a receptor subtype-specific analysis of receptor receptor binding. The presence of plasma in the radioreceptor binding. A simultaneous radioreceptor assay for subtypes of assay automatically corrects for plasma protein binding. To b-adrenoceptors [9] or muscarinic acetylcholine receptors secure autocorrection for plasma protein binding, it is key [7] has allowed to assign distinct functions to a specific that a major fraction of the assay volume is plasma in order subtype in vivo in man. to maintain plasma protein binding and not to dilute the In a radioreceptor assay it is possible to quantitate blood drugs present in the sample too much. In the present study concentrations relative to known standards, which have also Blackwell Science Ltd Br J Clin Pharmacol, 45, 49 55

5 Tamsulosin and terazosin pharmacokinetics Plasma concentration (ngml ) H.p.l.c a A a B 6 a D Figure Pharmacokinetic profile of a single oral dose of terazosin (5 mg) in h.p.l.c. and radioreceptor analysis. Ten young, healthy, male subjects received a single oral dose of 5 mg terazosin. Before and, 3, 5, 7, and 3.5 h after drug intake a blood sample was taken. Plasma derived from those samples was analyzed by h.p.l.c. and used in the radioreceptor assay. Drug levels are expressed as multiples of the concentration which occupies 5% of the human a A -, a B -ora D -adrenoceptor ( ) as calculated according to equation (). Data are median values from subjects (filled circles) with upper and lower quartiles (dashed lines). been evaluated in the presence of plasma [8]. However, a corresponds well to various previous in vitro studies quantitative analysis of this type implies that drug metabolites demonstrating an order of potency for tamsulosin of behave very similar to the parent compound. An alternative a A a D >a B [6, 8, 3 6]. Taken together these data method of analysis of radioreceptor assay data has been validate our approach of the radioreceptor assay. developed by Wellstein et al. []. This procedure does not The behaviour of terazosin in the radioreceptor assay was require knowledge of the absolute affinities of all chemical more complex. Binding to the a A -adrenoceptor and entities participating in receptor occupancy. Therefore, drug terazosin concentrations in the h.p.l.c. analysis peaked after concentrations are not given in molar units but rather as h and declined to % and %, respectively, of peak multiples of the concentration occupying 5% of the levels after 3.5 h. In contrast binding to a D - and a B - receptor, i.e. equivalents. This procedure was used in adrenoceptors did not exhibit a clear peak or a clear time the present study. Thus, the aim of the present study was dependency between and h following terazosin intake. to establish a radioreceptor assay for human a -adrenoceptor Moreover, after 3.5 h median binding to a B -anda D - subtypes and use it to test whether tamsulosin and terazosin adrenoceptors was still at 64% and 33%, respectively, of the behave as subtype-selective drugs in man in vivo. median h values while concentrations of parent compound The time course of tamsulosin levels following single oral in the h.p.l.c. analysis were only % of h values. In administration as determined in the radioreceptor assay with contrast to the situation with tamsulosin, this cannot be a A -adrenoceptors was similar to that observed by h.p.l.c. explained by small signal/noise ratios. Moreover, our data analysis of the parent compound. Thus, maximum tamsulosin suggest that terazosin may be somewhat selective for a A - concentrations in the h.p.l.c. assay and maximum binding and a D -adrenoceptors relative to a B -adrenoceptors in vivo to the a A -adrenoceptor were seen 5 h following drug intake whereas terazosin has repeatedly been demonstrated to have and from there plasma concentrations gradually declined to similar affinity for all a -adrenoceptor subtypes in vitro [6, levels of 3% (h.p.l.c.) or even less (radioreceptor assays) of 4]. The considerable binding activity in plasma of the peak concentrations after 3.5 h. The time dependency terazosin-treated subjects after 3.5 h and the apparent of binding to the a B -anda D -adrenoceptor was less clear subtype-selectivity in vivo indicate the possibility that than for the a A -adrenoceptor. The most likely explanation metabolites may contribute to a -adrenoceptor binding of this difference is the smaller signal/noise ratios since the activity in terazosin-treated subjects, particularly at late time binding to the a B - and a D -adrenoceptors was less than points. Indeed terazosin has been demonstrated to undergo with a A -adrenoceptors at most time points. The order of extensive metabolism in humans [6]. The a -adrenoceptor binding in the radioreceptor assay, i.e. a A >a D #a B, subtype-selectivity of terazosin metabolites is not known. 998 Blackwell Science Ltd Br J Clin Pharmacol, 45,

6 K. Taguchi et al. a In summary our study demonstrates that the radioreceptor. assay technique can be applied to human a -adrenoceptor subtypes. Our data with tamsulosin suggest that the.5. radioreceptor assay technique yields data which are compatible with the pharmacokinetic profile according to h.p.l.c. analysis and with known in vitro data regarding subtypeselectivity..5 Our data with terazosin, which does not discriminate a -adrenoceptor subtypes in vitro, surprisingly. indicate a possible in vivo a -adrenoceptor subtype-selectivity This unexpected feature and the discrepancies between the pharmacokinetic profile in the radioreceptor assay and of the parent compound in the h.p.l.c. assay indicate that 3 b metabolites may contribute to the in vivo profile of terazosin with regard to duration of action and a -adrenoceptor subtype-selectivity. Thus, radioreceptor assays based on plasma may not fully reflect receptor occupancies at tissue sites of interest, but seem to provide considerable additional information relative to classical h.p.l.c. analysis. The application of radioreceptor assays based on a -adrenoceptor subtypes may allow the association of distinct physiological effects with specific subtypes. However, such applications may be limited by the fact that data scatter is larger in the radioreceptor assay than with h.p.l.c. analysis. This study was funded in part by a grant from Boehringer Figure 3 Analysis of a -adrenoceptor subtype-selectivity of Ingelheim (Ingelheim, Germany). We thank Dr H. plasma obtained following tamsulosin (.4 mg) and terazosin Schumacher (Department of Biometrics, Boehringer (5 mg) intake. Data for tamsulosin (a) and terazosin (b) are taken Ingelheim) for help with the statistical analysis, Dr J. Schloos from Figures and, respectively, and replotted to allow a direct comparison of plasma binding to the a (Beiersdorf-Lilly GmbH, Hamburg, Germany) for advice on -adrenoceptor subtypes (# a A ; & a B ; 6 a D ). * and +: P<.5 vs a B -anda - the principles of the radioreceptor assay, Dr G. Johnston adrenoceptors, respectively, in a Wilcoxon s signed rank test. (Pfizer, Sandwich, Kent, UK) for providing the cell lines, and Dr R. J. Flanagan (National Poisons Unit, London, Median i / K i However, it is noteworthy that two of the three major terazosin metabolites are 6-O- and 7-O-demethyl-terazosin [6]. Demethylation of the corresponding moiety in the tamsulosin molecule (tamsulosin metabolite M4) interestingly yields compounds with selectivity for a D -anda A - UK) and Dr W. J. J. Krauwinkel (Yamanouchi Europe, Leiderdorp, The Netherlands) for performing the h.p.l.c. analysis. relative to a B -adrenoceptors [6], similar to what we References observed in vivo with terazosin. Evaluation of this possibility Schäfers RF, Reid JL. Alpha Blockers. In New Therapeutic appears intriguing, but unfortunately the terazosin metab- Strategies in Hypertension, Edited by Kaplan NM, Brenner BM, olites were not available to us for investigation. Thus, Laragh JH, 989; New York, Raven Press. confirmation of an involvement of metabolites in functional Khoury AF, Kaplan NM. a-blocker therapy of hypertension. in vivo effects of terazosin has to await further studies. An unfulfilled promise. J Am Med Ass 99; 66: Oesterling JE. Benign prostatic hyperplasia. Medical and In contrast after 3.5 h, when tamsulosin levels in the minimally invasive treatment options. New Engl J Med 995; h.p.l.c. assay had declined to 3% of peak values, a - 33: adrenoceptor binding activity in plasma of tamsulosin-treated 4 Michel MC, Kenny BA, Schwinn DA. Classification of a - subjects was no longer significantly different from with all adrenoceptor subtypes. Naunyn-Schmiedeberg s Arch Pharmacol three subtypes; moreover, the observed profile of a - 995; 35:. adrenoceptor subtype-selectivity ex vivo was similar to that 5 Price DT, Schwinn DA, Lomasney JW, Allen LF, Caron reported in vitro. These data support the view that metabolites MG, Lefkowitz RJ. Identification, quantification, and do not play a major role in the therapeutic effects of localization of mrna for three distinct alpha adrenergic tamsulosin. Thus, tamsulosin appears to undergo less receptor subtypes in human prostate. J Urol 993; 5: metabolism in man [3] than in rats or dogs [3], and the metabolites being formed appear to be rapidly eliminated 6 Michel MC, Grübbel B, Taguchi K, Verfürth F, Otto T, Kröpfl D. Drugs for treatment of benign prostatic hyperplasia: from plasma and thus contribute only little to overall plasma affinity comparison at cloned a -adrenoceptor subtypes and in levels [3]. Moreover, the limited contribution of metabolites human prostate. J Auton Pharmacol 996; 6: 8. is not likely to alter the qualitative pharmacological profile 7 Chapple CR. a-adrenergic blocking drugs in bladder outflow of tamsulosin in vivo since we have previously demonstrated obstruction: what potential has a -adrenoceptor selectivity? Br that most of the tamsulosin metabolites which do occur J Urol 995; 76 Suppl. : have an affinity and a -adrenoceptor subtype-selectivity 8 Ford APDW, Arredondo NF, Blue DR, et al. RS-753 similar to tamsulosin itself [6]. (N-[-cyclopropylmethoxy-phenoxy)ethyl]-5-chloro-a,a Blackwell Science Ltd Br J Clin Pharmacol, 45, 49 55

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