Enantiomers of 3-(3,4-dihydroxyphenyl)- and 3-(3-hydroxyphenyl)-N-n-propylpiperidine: central pre- and postsynaptic dopaminergic effects and pharmacokinetics.

This study emphasizes the importance of the metabolic conversion of the enantiomers of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) into their catechol analogues, the enantiomers of 3-(3,4-dihydroxyphenyl)-N-n-propylpiperidine. These isomers are both shown to be excellent substrates for COMT, with a slight preference for the S-(-) enantiomer. Assessment of the dopaminergic activity of these catechols and the results from the determination of brain levels of the enantiomers of 3-PPP and their metabolites indicate that the metabolites probably do not alter the pharmacological profiles established for (R)-(+)- and (S)-(-)-3-PPP. The conversion of the monophenols into catecholic metabolites is only 1-5%, and the further conversion of these catecholic metabolites into methoxylated analogues is very rapid. However, the very interesting observation was made that, when inhibiting COMT by means of tropolone and subsequently treating the rats with high doses of (S)-(-)-3-PPP (ip), postsynaptic dopaminergic activity was elicited. This has never been seen for (S)-(-)-3-PPP without tropolone pretreatment and might indicate that, in this special case, the catecholic metabolite affects the in vivo pharmacological profile of (S)-(-)-3-PPP.

HPLC-assay with electrochemical detection for the neurotoxin MPTP, its metabolite MPP+ and MPTP-analogues in biological samples after purification over Sephadex G10.

A sensitive and selective method for assaying the neurotoxin MPTP and some MPTP-analogues in mouse brain and serum is described. The method is based on isolation of the compounds from biological samples on small Sephadex G10 columns followed by reverse phase HPLC with amperometric detection. HPLC separation was performed at pH 3, after which the pH was increased to 6.8 by mixing the column effluent with 0.5 M phosphate pH 9, to provide the conditions required for electrochemical detection. A metabolite of MPTP, MPP+, was determined as MPTP after reduction with NaBH4. This assay allows the determination of brain and serum concentrations in the pmol/g range of administered MPTP and MPTP-analogues and the effects of these substances on dopamine and its metabolites in the same tissue sample.

In-vitro and in-vivo metabolism of the presynaptic dopamine agonist 3-PPP to a catecholic analogue in rats.

The dopamine agonist 3-PPP and its enantiomers are hydroxylated in-vitro by rat liver microsomes to the catecholamine 3-(3,4-dihydroxyphenyl)-N-n-propylpiperidine (4-OH-3-PPP) with Km and Vmax values of about 1 microM and 2 nmol (mg protein)-1 min-1 respectively. As the catecholamine formed appears to be a good substrate for catechol-O-methyltransferase, in-vivo catecholamine formation in rats from 3-PPP was only detectable after inhibition of COMT by tropolone. The resulting brain levels of 4-OH-3-PPP, as measured by HPLC with electrochemical detection 45 min after administration, were about 350 pmol g-1 after i.p., and about 100 pmol g-1 after s.c. injection of 45 mumol kg-1 3-PPP, with no significant difference between racemic, ( + ) or (-) 3-PPP. It was estimated that these catecholamine levels represent about 1-5% of the 3-PPP levels after i.p., and about 0.2-0.5% after s.c. administration of 3-PPP. The relevance of this metabolic conversion of 3-PPP for its pharmacological profile is discussed.