Determination of the catechol-O-methyltransferase inhibitor tolcapone and three of its metabolites in animal and human plasma and urine by reversed-phase high-performance liquid chromatography with ultraviolet detection

Repositioning tolcapone as a potent inhibitor of transthyretin amyloidogenesis and associated cellular toxicity

Transthyretin (TTR) is a plasma homotetrameric protein implicated in fatal systemic amyloidoses. TTR tetramer dissociation precedes pathological TTR aggregation. Native state stabilizers are promising drugs to treat TTR amyloidoses. Here we repurpose tolcapone, an FDA-approved molecule for Parkinson's disease, as a potent TTR aggregation inhibitor. Tolcapone binds specifically to TTR in human plasma, stabilizes the native tetramer in vivo in mice and humans and inhibits TTR cytotoxicity. Crystal structures of tolcapone bound to wild-type TTR and to the V122I cardiomyopathy-associated variant show that it docks better into the TTR T4 pocket than tafamidis, so far the only drug on the market to treat TTR amyloidoses. These data indicate that tolcapone, already in clinical trials for familial amyloid polyneuropathy, is a strong candidate for therapeutic intervention in these diseases, including those affecting the central nervous system, for which no small-molecule therapy exists.

Simultaneous determination of levodopa, carbidopa, entacapone, tolcapone, 3-O-methyldopa and dopamine in human plasma by an HPLC-MS/MS method

BACKGROUND

In this study, we developed and validated a HPLC-MS/MS method capable of simultaneously determining levodopa, carbidopa, entacapone, tolcapone, 3-O-methyldopa and dopamine in human plasma. RESULTS & METHODOLOGY: Chromatographic separation was achieved using a C8 column with a mobile phase consisting of a gradient of water and acetonitrile:methanol (90:10 v/v), both containing 0.1% formic acid. The developed method was selective, sensitive (LD<7.0 ng ml(-1)), linear (r>0.99), precise (RSD<11.3%), accurate (RE<11.8%) and free of residual and matrix effects. The developed method was successfully applied in plasma patients with Parkinson's disease using Stalevo®.

CONCLUSION

The new method can be used for the clinical monitoring of these substances and applied to adjustments in drug dosages.

Metabolism and excretion of tolcapone, a novel inhibitor of catechol-O-methyltransferase

AIMS

To investigate the rate of excretion and routes of metabolism of tolcapone, a novel inhibitor of catechol-O-methyltransferase (COMT).

METHODS

Six healthy male volunteers were given 200 mg [14C]-tolcapone (approximately 50 muCi) orally. To assess excretion balance and to identify metabolites, urine and faeces were collected before administration and until radioactivity fell below 75 d min-1 ml-1 (urine) and 100 d min-1 mg-1 (faeces). Blood samples were collected frequently before and after administration to determine plasma radioactivity, to identify tolcapone metabolites and to measure plasma tolcapone and its methylated derivative 3-O-methyltolcapone (3-OMT).

RESULTS

The mean proportion of the dose excreted in urine was 57.3% and in faeces 40.5%. Excretion was almost complete (more than 95%) in all participants after 9 days. The major early metabolite present in plasma was the 3-O-beta, d-glucuronide conjugate, which was detectable within 2 h after dosing. The major late metabolite in plasma was 3-OMT. The 3-O-beta, d-glucuronide was also the most abundant metabolite in urine and faeces, accounting for 27% and 33%, respectively, of the total radioactivity excreted by these routes and for 26% of the original dose. Reduction of the nitro moiety yields an amine derivative, detected in both urine and faeces, with subsequent modifications, such as acetylation of the amine group and conjugation with glucuronic acid or sulphate, or both. Oxidative reactions due to cytochrome P450 enzymes are of small significance, as is 3-O-methylation by COMT.

CONCLUSIONS

Tolcapone is almost completely metabolized and excreted in urine and faeces (only 0.5% of tolcapone was excreted unchanged); glucuronidation is the most important route of metabolism. The relatively long duration of excretion is caused by the long half-life of 3-OMT.