Synthesis and pharmacological evaluation of substituted naphth[1,2,3-de]isoquinolines (dinapsoline analogues) as D1 and D2 dopamine receptor ligands

Dinapsoline ((2); (±)-dihydroxy-2,3,7,11b-tetrahydro-1H-naphth[1,2,3-de]isoquinoline) is a full D1 dopamine agonist that also has significant D2 receptor affinity. Based on a similar pharmacophore, dinapsoline has pharmacological similarities to dihydrexidine ((1); (±)-trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine), the first high affinity full D1 agonist. Small alkyl substitutions on the dihydrexidine backbone are known to alter markedly the D1:D2 selectivity of dihydrexidine, and it was of interest to determine whether similar SAR exists within the dinapsoline series. This report describes the synthesis and pharmacological evaluation of six analogues of dinapsoline: N-allyl-(3);N-n-propyl- (4); 6-methyl- (5); 4-methyl- (6); 4-methyl-N-allyl- (7); and 4-methyl-N-n-propyl-dinapsoline (8). As expected from earlier studies with the dihydrexidine backbone, N-allyl (3) or N-n-propyl (4) analogues had markedly decreased D1 affinity. Unexpectedly, and unlike the dihydrexidine series, these same substituents did not markedly increase D2 affinity. The addition of a methyl group to position 6 (5) increased D1:D2 selectivity, but less markedly than did the analogous 2-methyl substituent added to 1. Unlike the analogous 4-methyl substituent of 1, the addition of a 4-methyl-group (6) actually decreased D1 affinity without affecting D2 affinity. These data demonstrate that the dinapsoline (2) backbone can be modified to produce dopamine agonists with novel properties. Moreover, as rigid ligands in which small substituents can cause significant changes in selectivity, they are important tools for deriving ‘differential’ SARs of the dopamine receptor isoforms.