Characterization of binding of [3H]PD 128907, A selective Dopamine D3 receptor agonist ligand,to CHO-K1 cells

PD 128907 4a R, 10 b R-(+)-trans- 3, 4, 4a, 10 b - tetrahydro - 4- n-propy12 H,5H-1] benzopyrano4,3-b]1,4-oxazin-9-ol.], a selective dopamine (DA) D3 receptor agonist ligand exhibits about a 1000-fold selectivity for human D3 receptors (K_i, 1 nM) versus human D₂ receptors (K_i, 1183 nM) and a 10000-fold selectivity versus human D₄ receptors (K_i, 7000 nM) using ³H]spiperone as the radioligand in CHO-K1-cells. Studies with ³H]PD 128907, showed saturable, high affinity binding to human D₃ receptors expressed in CHO-K1 cells (CHO-K1-D₃) with an equilibrium dissociation constant (K_d) of 0.99 nM and a binding density (B_max) of 475 fmol/mg protein. Under the same conditions, there was no significant specific binding in CHO-K1-cells expressing human D₂ receptors (CHO-K1-D₂). The rank order of potency for inhibition of ³H]PD 128907 binding with reference DA agents was consistent with reported values for D₃ receptors. These results indicate that ³H]PD 128907 is a new, highly selective D₃ receptor ligand with high specific activity, high specific binding and low non-specific binding and therefore should be useful for further characterizing the DA D₃ receptors.