Comparative study on pharmacokinetics and in vivo alpha1-adrenoceptor binding of [3H]tamsulosin and [3H]prazosin in rats

Biol Pharm Bull. 1999 Apr;22(4):412-7. doi: 10.1248/bpb.22.412.

Abstract

The plasma concentration, total radioactivity and in vivo alpha1-adrenoceptor binding in rat tissues after intravenous (i.v.) injection of [3H]tamsulosin were measured and they were compared with those obtained after the injection of [3H]prazosin. The plasma concentration of [3H]tamsulosin was consistently higher than that of [3H]prazosin, with 1.4 times greater areas under the curve (AUC(0-infinity)) of plasma concentration. As there was a significantly lower value of apparent volume of central compartment (Vd(c)) and distribution volume at steady state (Vd(ss)) for [3H]tamsulosin than [3H]prazosin with little difference in elimination rate constant (beta), the higher concentration of [3H]tamsulosin in plasma might be associated mainly with the smaller volume of distribution. The ratio of total radioactivity in tissues to the plasma unbound concentration of [3H]tamsulosin after i.v. injection of the ligand was consistently lower than that of [3H]prazosin. These observations suggest that [3H]tamsulosin is distributed in rat tissues in a more limited manner than [3H]prazosin. A significantly lower level of in vivo specific binding of [3H]tamsulosin than [3H]prazosin was observed in the spleen, heart and liver. Further, the apparent dissociation constant (Kd) and maximal number of binding sites (Bmax) for in vivo specific [3H]tamsulosin binding were considerably lower than those for [3H]prazosin binding. Therefore, these findings suggest that [3H]tamsulosin labels preferentially a subpopulation of the alpha1-adrenoceptor sites in rat tissues labeled by [3H]prazosin. In conclusion, the present study has shown that there is a significant difference in the pharmacokinetics and in vivo alpha1-adrenoceptor binding characteristics between tamsulosin and prazosin.

Publication types

  • Comparative Study

MeSH terms

  • Adrenergic alpha-Antagonists / metabolism
  • Adrenergic alpha-Antagonists / pharmacokinetics*
  • Animals
  • Area Under Curve
  • Male
  • Prazosin / metabolism
  • Prazosin / pharmacokinetics*
  • Radioligand Assay
  • Rats
  • Rats, Sprague-Dawley
  • Receptors, Adrenergic, alpha-1 / metabolism*
  • Sulfonamides / metabolism
  • Sulfonamides / pharmacokinetics*
  • Tamsulosin
  • Tritium

Substances

  • Adrenergic alpha-Antagonists
  • Receptors, Adrenergic, alpha-1
  • Sulfonamides
  • Tritium
  • Tamsulosin
  • Prazosin