Dopamine D(2) activity of R-(-)-apomorphine and selected analogs: a microdialysis study

In the present study, R-(-)-apomorphine and three of its analogs were studied for their potency in decreasing the release of dopamine in the striatum after subcutaneous administration and for their oral bioavailability using the microdialysis technique in freely moving rats. The analogs R-(-)-N-n-propylnorapomorphine and R-(-)-11-hydroxy-N-n-propylnoraporphine displayed a higher potency than R-(-)-apomorphine in decreasing the release of dopamine in the striatum. A high dose of R-(-)-11-hydroxyaporphine, a dopamine D(2) receptor partial agonist, had a small effect on the release of dopamine in the striatum. The catechols R-(-)-N-n-propylnorapomorphine and R-(-)-apomorphine displayed a comparable oral bioavailability (1%), while the mono-hydroxy analog R-(-)-11-hydroxy-N-n-propylnoraporphine displayed a slightly higher oral bioavailability (3%). In conclusion, R-(-)-N-n-propylnorapomorphine and R-(-)-11-hydroxy-N-n-propylnoraporphine did not show a substantial improvement in bioavailability. However, due to the clear difference in their efficacy in decreasing dopamine release, in spite of the similar agonist binding affinities to the dopamine D(2) receptor of the two analogs compared to R-(-)-apomorphine, they could be useful alternatives for apomorphine in the treatment of Parkinson's disease.