Pharmacokinetics of anastrozole in Chinese male volunteers

AIM

To compare pharmacokinetics of domestic and imported tablets of anastrozole.

METHODS

Twenty male Chinese volunteers were enrolled in a randomized crossover study with a single oral dose of 1 mg of the two formulations respectively. The anastrozole in plasma was measured by gas chromatography with electron-captured detector. Area under the drug concentration-time curve was evaluated by variance analysis and two one-side t-test.

RESULTS

A two-compartment model was adopted in anastrozole plasma concentration-time data analysis. The paramaters such as Cmax, Tmax, T1/2beta, and AUC(0-infinity) were (10 +/- 3) and (10.2 +/- 2.5) microg/L, (1.2 +/- 0.5) and (1.3 +/- 0.4) h, (42 +/- 14) and (41 +/- 26) h, (443 +/- 141) and (429 +/- 121) microg . h . L-1, respectively, and there were no significant differences between the two formulations.

CONCLUSION

Domestic and imported anastrozole were of bioequivalence. The relative bioavailability of the domestic tablet was 100 % +/- 9 %.

Precoupling of alpha-2B adrenergic receptors and G-proteins in transfected PC-12 cell membranes: influence of pertussis toxin and a lysine-directed cross-linker

The ability of pertussis toxin (PTX) pretreatment to alter the binding of [3H]rauwolscine (RAU) to alpha-2B adrenergic receptors expressed in PC12 cells was examined. PTX caused a 30% increase in the Bmax for [3H]RAU and reduced its KD, whereas in the added presence of Na+ and Gpp(NH)p binding was increased to 75% above the level in untreated membranes. Because all three agents act to reduce receptor/G-protein affinity, the increased binding may reflect extensive precoupling of the alpha-2B receptor. The affinity of the agonist epinephrine in displacing [3H]RAU was normally reduced by both Na+ and Gpp(NH)p; however, in PTX-treated membranes the effect of Gpp(NH)p was eliminated, and Na+ remained effective. The lysine-directed cross-linking reagent ethyleneglycol bis(succinimidyl)succinate (EGS) was utilized in an attempt to cross-link precoupled receptor and G-protein. Maximal [3H]RAU binding was reduced by EGS in a time- and dose-dependent manner, and this action was reversed by prior incubation with Na+ and Gpp(NH)p, suggesting that EGS did indeed cross-link receptor and G-protein. RAU and epinephrine each provided protection against the effect of EGS. The inclusion of Na+ and Gpp(NH)p during [3H]RAU binding studies was able to restore maximal binding in EGS-treated membranes to the same level as untreated membranes. These results indicate that in the absence of Na+ and Gpp(NH)p at least 40% of the total alpha-2B adrenergic receptors in these membranes exist as a precoupled receptor/G-protein complex which fails to bind [3H]RAU.

Vascular effects of tetramethylpyrazine: direct interaction with smooth muscle alpha-adrenoceptors

The interaction of tetramethylpyrazine, a vasoactive ingredient of a Chinese traditional medicinal plant, with the vascular muscle alpha 1-adrenoceptors was investigated by a direct radioligand binding technique using [3H]prazosin and vascular smooth muscle microsomes isolated from dog aorta and mesenteric artery. Tetramethylpyrazine inhibited the binding of [3H]prazosin to vascular muscle membranes in a concentration-dependent manner at a suboptimal concentration of prazosin. Scatchard analysis of the effect of tetramethylpyrazine on the saturation profile of [3H]prazosin binding to vascular muscle microsomes of either arterial muscle indicated a substantial increase of Kd values (the affinity for prazosin) without a change in Bmax (maximal binding sites for prazosin). Thus, the present results provide supporting evidence that the inhibitory effect of tetramethylpyrazine on the vasoconstriction of dog mesenteric artery induced by phenylephrine in the earlier studies may be, at least, in part due to a direct action at the recognition sites of alpha 1-adrenoceptors. Amiloride and amiloride-related compounds, which shares a common pyrazine ring structure with tetramethylpyrazine and other related derivatives, also inhibits the binding of [3H]prazosin to aortic muscle microsomal membranes. Functional studies of dog saphenous vein also indicated that both tetramethylpyrazine and its ethyl derivatives inhibited the responses induced by phenylephrine and B-HT 920 in the presence and absence of extracellular Ca2+. Together with our earlier findings that amiloride also inhibits [3H]prazosin and [3H]rauwolscine binding to vascular muscle alpha 1- and alpha 2-adrenoceptors, the present radioligand binding study in canine arteries and functional study in saphenous veins suggest that the above compounds containing the pyrazine nucleus indeed interacted at the alpha-adrenoceptor sites.

Alpha-adrenoceptors in vascular smooth muscle: all is not well

Studies of binding interactions and contractile responses of vascular muscles at alpha 1- and alpha 2-adrenoceptors revealed the following. (1) Agonists at alpha 1- and alpha 2-adrenoceptors may achieve selectivity by virtue of different efficacies despite similar affinities at the two receptors as well as by differing affinities. This implies that their potencies in binding studies may not correlate with potencies in response and that an agonist may produce positive or negative interactions by occupying both alpha 1- and alpha 2-receptors. (2) Agonists at alpha 2-adrenoceptors have the ability in some vascular muscles to release internal Ca2+ (implying an inositol triphosphate mechanism) as well as open Ca2+ channels. However, their contractile abilities are not closely related to function of Na+/H+ or Na+/Ca2+ exchange sites. Amiloride derivatives probably inhibit contractile effects of alpha-agonists and K+ elevation by an action at sites distal to the receptor or Ca channels. (3) The failure of alpha 2-agonists to contract arteries in vitro is not related to the absence of these receptors but most likely to their uncoupling from contractile responses, possibly owing to changes related to the in vitro condition (loss of modulating endogenous substances present in vivo such as angiotensin II or endothelins or to changed physical conditions such as may alter function of stretch-activated channels).

Inhibitory effects of amiloride on alpha adrenoceptors in canine vascular smooth muscle

Amiloride inhibits vascular smooth muscle contractions from canine aorta and saphenous vein. The mechanisms were studied using radioligand binding and functional techniques. Amiloride inhibited [3H]prazosin and [3H]rauwolscine binding to alpha-1 and alpha-2 adrenoceptors in a concentration-dependent manner. Amiloride increased Kd values for [3H]rauwolscine without affecting the maximum binding of [3H]prazosin. These results suggest that the drug interacts with the alpha-1 adrenoceptor binding sites in a competitive manner and with the alpha-2 adrenoceptor binding sites in a noncompetitive manner. Amiloride reduced maximal contractile responses to agonists selective for both alpha adrenoceptors and to elevated K+, the EC50 values were increased by about 10-fold in the presence of amiloride. In Ca+(+)-free Krebs' solution, contractions induced in saphenous vein after addition of Ca++ in saphenous vein in the presence of adrenoceptor agonists were inhibited by amiloride. Our results suggest that amiloride reduced alpha-1 and alpha-2 adrenoceptor-mediated responses and inhibited Ca++ influx.

Alpha-adrenoceptors in dog mesenteric vessels--subcellular distribution and number of [3H]prazosin and [3H]rauwolscine binding sites

Binding of the alpha-adrenergic antagonists [3H]prazosin and [3H]rauwolscine to well-characterized subcellular membrane fractions isolated from dog mesenteric arteries and veins was studied. Binding of both ligands was saturable with Kd values of 0.5 +/- 0.1 nM for [3H]prazosin and 5.85 +/- 0.85 nM for [3H]rauwolscine in arteries, and 0.87 +/- 0.4 nM for [3H]prazosin and 6.6 +/- 1.5 nM for [3H]rauwolscine in veins. In veins, the maximum number of binding sites for [3H]rauwolscine was higher than that for [3H]prazosin, whereas in arteries the maximum number of binding sites for each ligand was similar. In microsomes from dog aorta, the maximum number of bindings sites for [3H]prazosin was higher than that for [3H]rauwolscine. Neuronal membrane contamination in these studies was minimized by dissection procedures and evaluated by the comparison of [3H]saxitoxin binding in various preparations. Only mesenteric veins responded functionally to agonists acting on alpha 2 adrenoceptors. This study thus identified two distinct populations of [3H]prazosin and [3H]rauwolscine binding sites in the plasma membranes of dog mesenteric vessels and suggests that a much higher density of alpha 2-compared to alpha 1-adrenoceptor binding sites is required for a contractile response.

Relation between density (maximum binding) of alpha adrenoceptor binding sites and contractile response in four canine vascular tissues

In microsomal fractions from dog aorta, saphenous veins, mesenteric arteries and veins, both [3H]prazosin and [3H]rauwolscine displayed monophasic saturation in binding. The Kd for [3H] rauwolscine binding was similar for all these blood vessels, but the maximum number of [3H]rauwolscine binding sites was 3 to 7 times higher in veins compared to arteries. The Kd for [3H] prazosin was higher in saphenous vein than that in the arteries. The maximum number of binding sites for [3H]prazosin was similar, except for that in aorta, which was 3 times greater. Phenylephrine (alpha-1 adrenoceptor selective agonist) or norepinephrine (nonselective adrenoceptor agonist) produced similar maximal responses in all vessels. The alpha-2 adrenoceptor selective agonist, B-HT 920 (2-amino-6-allyl-3,4,7,8-tetrahydro-6H-thiazolo[5,4-d]-azepine)-induced contraction in veins but not in arteries. Prazosin (10(-6) M) inhibited completely the contractions to norepinephrine (3 x 10(-6) M) in mesenteric arteries and to phenylephrine (3 x 10(-6) M) in arteries and veins. Contractile responses of mesenteric artery were unaffected by rauwolscine. Rauwolscine (10(-7) M) caused a greater parallel rightward shift of the concentration-response curve to norepinephrine than did prazosin (10(-7) M) in saphenous veins, and a further rightward shift of responses to norepinephrine after 10(-7) M prazosin in mesenteric vein and saphenous vein and abolished B-HT 920-induced responses at alpha-2 adrenoceptors. The tissues responding to B-HT-920 correspond to those having the highest alpha-2 receptor density as measured by [3H]rauwolscine binding. The density of such sites required for contraction to be initiated in veins was much higher than with alpha-1 adrenoceptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of alpha-adrenoceptor subtypes by [3H]prazosin and [3H]rauwolscine binding to canine venous smooth muscle membranes

Postsynaptic alpha-adrenoceptor subtypes were studied using [3H]prazosin and [3H]rauwolscine binding to plasmalemma-enriched microsomal fractions isolated from dog saphenous veins and mesenteric veins. Both radioligands showed saturable binding consistent with the presence of a single homogeneous binding site in each case, based on Scatchard analysis. The Kd values of [3H]prazosin and [3H]rauwolscine, calculated from kinetic studies were similar to those from equilibrium binding data in both venous muscle membranes. The microsomal membranes of dog saphenous vein and mesenteric vein contained about a fourfold higher density of the high affinity [3H]rauwolscine binding sites than those for [3H]prazosin binding. In competition studies, IC50 values for displacement of rauwolscine or prazosin suggested that the sites of interaction for the antagonists prazosin and rauwolscine were independent. Phenylephrine, a functionally selective alpha-adrenoceptor agonist, competed with a similar IC50 value for the specific binding sites of [3H]prazosin and [3H]rauwolscine; but B-HT 920, a functionally selective alpha 2-adrenoceptor agonist, competed for [3H]rauwolscine and [3H]prazosin binding with distinctly different IC50 values. Our data show the existence of two populations of alpha-adrenoceptor antagonist binding sites in the plasma membranes of dog saphenous vein and mesenteric vein, and raise the question whether agonist selectively depends on different affinities or on differential efficacies at one or two sites.