Characterization of non-nitrocatechol pan and isoform specific catechol-O-methyltransferase inhibitors and substrates.

Schizophrenia is a chronic, debilitating disease estimated to occur in 1% of the population.1 Effects of the disease are manifested in patients as any one or combination of three symptom domains; positive symptoms, negative symptoms and cognitive deficit.2 The negative symptoms and cognitive impairment present in individuals with schizophrenia represent an unmet medical need.3,4 Reduced dopamine neurotransmission in the prefrontal cortex (PFC) has been implicated as causal for these symptoms.5−7 Catechol-O-methyltransferase (COMT) is the preferential mechanism for catabolism of dopamine in cortical areas.8−11 The enzyme exists as two major isoforms where the membrane (MB) form is more predominant in human brain and the soluble (S) form is more predominant in peripheral tissues.12−15 Preferential expression of MB versus S-COMT in human brain is an evolutionary change and in contrast to rat; where S-COMT predominates.16 The only difference between human MB and S-COMT is the inclusion of an extra 50 hydrophobic amino acids (43 in rat) in MB-COMT. As a critical player in the manipulation of cortical dopamine levels, inhibition of COMT may serve as a promising adjunct therapy for schizophrenia.9,17 There are numerous functional single nucleotide polymorphisms (SNPs) in COMT. The majority of human studies have focused on a SNP common in the COMT gene which leads to the replacement of valine (Val) with methionine (Met) at codon 108 of S-COMT and codon 158 of MB-COMT.18 Val-COMT isolated from human PFC is ∼40% more catalytically active in vitro than enzyme containing the Met allele.19 Various investigations indicate that healthy controls with the Met allele perform better on certain working memory tasks,20−22 but not always.23 Improvements on working memory tasks have been reported in Val 158 homozygote healthy subjects treated with the COMT inhibitor tolcapone.24,25 However, based on a number of haplotypes that influence COMT expression and activity, it seems likely that associated phenotypes result from a multifaceted set of genetic variations.26 The nitrocatechol COMT inhibitors tolcapone and entacapone are successfully utilized as an adjunct therapy for Parkinson’s disease, where inhibition of COMT in the periphery improves the bioavailability of L-dopa in brain. Despite successful clinical use, idiosyncratic toxicity is associated with tolcapone27 and although entacapone is not associated with hepatotoxicity; like tolcapone, its use is sometimes limited by severe diarrhea due to peripheral inhibition.28 Inhibition of S-COMT in the periphery has been proposed to contribute to tolcapone linked hepatotoxicity,17 thus highlighting an advantage of identifying and characterizing MB-COMT specific inhibitors. Due to the idiosyncratic hepatotoxicity associated with tolcapone and adverse effects associated with peripheral inhibition of COMT, we sought to identify a non-nitrocatechol, brain penetrant MB-COMT inhibitor for the treatment of schizophrenia.29 Additionally, to alleviate side-effects associated with peripheral inhibition, the human brain predominant isoform MB-COMT was utilized to conduct an HTS and complete lead optimization efforts to discover novel, non-nitrocatechol COMT inhibitors. Reported here is the identification and characterization of a series of 4-pyridinone compounds, some of which specifically or preferentially inhibit MB-COMT.