SYNTHESIS AND BIOLOGICAL EVALUATION OF A SERIES OF SUBSTITUTED BENZO[A]PHENANTHRIDINES AS AGONISTS AT D-1 AND D-2 DOPAMINE-RECEPTORS

Dihydrexidine [4; (+/-)-trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo [a]phenanthridine (DHX)], the first high-affinity full D-1 agonist, also is known to have significant D-2 activity. The present work reports the synthesis and pharmacological activity of a series of analogs substituted in the pendent phenyl ring (i.e., 2-, 3-, or 4-position). (+/-)-trans-2-Methyl-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine (5) was a high-affinity D-1 agonist, having approximately 4-fold greater D-1 vs D-2 selectivity than DHX itself. All of the analogs containing a methyl or ethyl (but not a phenyl) substituent at the 2-, 3-, or 4-position had a pharmacological profile similar to that of the lead compound DHX (4). Each analog was found to be a high-affinity full agonist with moderate selectivity for the D-1 receptor, It is apparent from these results that the D-1 receptor can tolerate small substituents at the 2-, 3-, and 4-positions of the pendent phenyl ring. On the basis of earlier studies showing that N-alkylation increases D-2 selectivity, the 3-methyl N-n-propyl and 4-methyl N-n-propyl compounds 11 and 13 were synthesized. While these analogs exhibited much higher affinity for the D-2 receptor, surprisingly 4-methyl-N-propyl-DHX (13) exhibited high affinity for both the D-1 and D-2 receptors. It was subsequently established that this compound is a selective D-3 ligand (110-fold selectivity for the D-3 over D-2 receptor). The results from these studies demonstrate that several of the hexahydrobenzo[a]phenanthridine derivatives are agonists with high intrinsic activity that may serve as powerful tools to explore the structural features that determine affinity and selectivity (relative to the D-2 receptor) of drugs for D-1 receptors.