Stereoselective sulphate conjugation of fenoterol by human phenolsulphotra nsferases

Abstract

1. The objective of this study was to determine (1) the molecular site(s) of sulphoconjugation of fenoterol; (2) the human phenolsulphotransferase (PST) isoform(s) involved; and (3) the stereochemistry of the enzymatic reaction. 2. Using the human Hep G2 cell line, hplc isolation and FAB/ms/ms, it was determined that fenoterol is sulphated both in the 4'-hydroxyphenyl position and in one of the 3',5'-dihydroxyphenyl positions. 3. Recombinant human M-PST preferentially sulphated the 4'-hydroxyphenyl position. In contrast, recombinant P-PST exclusively sulphated the 3',5'-hydroxyphenyl position. 4. The M-PST-catalysed sulphation of the 4'-hydroxyphenyl position was highly selective for the active RR-enantiomer, whereas the sulphation of the 3',5'-dihydroxyphenyl position was slightly selective for the opposite SS-enantiomer. 5. The P-PST-catalysed sulphation of the 3',5'-hydroxyphenyl position was selective for the inactive SS-enantiomer.

Enantioselective sulfation of ?2-receptor agonists by the human intestine and the recombinant M-form phenolsulfotransferase

“…Similar observations were recently made for fenoterol, although in this case the situation was more complex because of the presence of two sulfate acceptor sites. 12 Further insight into the remarkable difference in stereoselectivity in the sulfonation of isoproterenol and albuterol will require knowledge of the binding site on the M-PST protein. At the present time, the selective binding of these basic substrates to M-PST as opposed to the phenol form of PST may be rationalized based on a few sequences containing acidic amino acids specific for M-PST.…”

“…4,5 Recent human studies have demonstrated in vitro stereoselectivity in the sulfate conjugation pathway for several ␤ 2 -agonist drugs. [6][7][8][9][10][11][12] The stereoselective oral dose pharmacokinetics observed for two of these drugs, i.e., terbutaline 13 and albuterol, 14 is likely due to stereoselective sulfation in the intestine. Overall, these in vitro/in vivo studies indicate that stereoselective metabolism can either enhance or diminish the plasma concentrations of the active enantiomer relative to the inactive form.…”

Pharmacokinetics and metabolism of (R,R)-methoxyfenoterol in rat

“…The Fen molecule contains two potential sites of conjugation, the 1,3-benzenediol and the 4-hydroxyphenyl moieties which result in the previously designated meta - (Fen-MG, Fen-MS) and para - (Fen-PG, Fen-PS) metabolites, respectively (Figure 1) (Koster et al 1986; Wilson et al 1997). Using isolated rat hepatocytes and enterocytes, Koster and co-workers demonstrated that (R,R) -Fen is converted to (R,R) -Fen-PG with Cl int values of 1.48 ± 0.32 and 0.79 ± 0.20 μl min -1 kg -1 , respectively.…”

“…In humans, however, previous studies of the metabolic clearance of Fen has indicated that Fen is conjugated to sulphate (Fen-S) and glucuronidate (Fen-G) and that the Fen-S is the predominate presystemic metabolite (Hildebrandt et al 1994; Wilson et al 1997). In vitro studies with recombinant human phenolsulphotransferase isoforms, M-PST and P-PST demonstrated that the sulphation of Fen was regioselective (Wilson et al 1997). In these studies, the conjugation of the 4-hydroxyphenyl moiety of (R,R) -Fen, that is, the production of (R,R) -Fen-PS, by M-PST was the preferred pathway.…”

HPLC–electrospray mass spectrometric assay for the determination of (R,R)-fenoterol in rat plasma

“…The determination of Fen in plasma requires a sensitive assay as the plasma concentrations of the drug are commonly <10 ng/ml due to the compound’s poor bioavailability and extensive metabolism via phase II pathways [3]. Plasma concentrations of Fen have been measured using a radioimmunoassay [4] and enzyme immunoassay [5] techniques as well as gas chromatography–MS [6,7], LC–APCI–MS [8] and HPLC with fluorescence detection [9].…”