In search of selective P2 receptor ligands: interaction of dihydropyridine derivatives at recombinant rat P2X2 receptors

1,4-Dihydropyridines are regarded as privileged structures for drug design, i.e. they tend to bind to a wide variety of receptor sites. We have shown that upon appropriate manipulation of the substituent groups on a 1,4-dihydropyridine template, high affinity and selectivity for the A₃ subtype of adenosine receptors (‘P1 receptors’) may be attained. In the present study we have begun to extend this approach to P2 receptors which are activated by ATP and other nucleotides. Nicardipine, a representative dihydropyridine, used otherwise as an L-type calcium channel blocker, was shown to be an antagonist at recombinant rat P2X₂ (IC₅₀=25 μM) and P2X₄ (IC₅₀ ∼220 μM) receptors expressed in Xenopus oocytes. Thus, this class of compounds represents a suitable lead for enhancement of affinity through chemical synthesis. In an attempt to modify the 1,4-dihydropyridine structure with a predicted P2 receptor recognition moiety, we have replaced one of the ester groups with a negatively charged phosphonate group. Several 4-phenyl-5-phosphonato-1,4-dihydropyridine derivatives, MRS 2154 (2,6-dimethyl), MRS 2155 (6-methyl-2-phenyl), and MRS 2156 (2-methyl-6-phenyl), were synthesized through three component condensation reactions. These derivatives were not pure antagonists of the effects of ATP at P2X₂ receptors, rather were either inactive (MRS 2156) or potentiated the effects of ATP in a concentration-dependent manner (MRS 2154 in the 0.3–10 μM range and MRS 2155 at >1 μM). Antagonism of the effects of ATP at P2X₂ receptor superimposed on the potentiation was also observed at >10 μM (MRS 2154) or 0.3–1 μM (MRS 2155). Thus, while a conventional dihydropyridine, nicardipine, was found to antagonize rat P2X₂ receptors ninefold more potently than P2X₄ receptors, the effects of novel, anionic 5-phosphonate analogues at the receptor were more complex.