A pyridoxine cyclic phosphate and its 6-azoaryl derivative selectively potentiate and antagonize activation of P2X1 receptors

Analogues of the P2 receptor antagonists pyridoxal-5'-phosphate and the 6-azophenyl-2',4'-disulfonate derivative (PPADS), in which the phosphate group was cyclized by esterification to a CH2OH group at the 4-position, were synthesized. The cyclic pyridoxine-alpha(4,5)-monophosphate, compound 2 (MRS 2219), was found to be a selective potentiator of ATP-evoked responses at rat P2X(1) receptors with an EC50 value of 5.9 +/- 1.8 mu M, while the corresponding 6-azophenyl-2',5'-disulfonate derivative, compound 3 (MRS 2220), was a selective antagonist. The potency of compound 3 at the recombinant P2X(1) receptor (IC50 10.2 +/- 2.6 mu M) was lower than PPADS (IC50 98.5 +/- 5.5 nM) or iso-PPADS (IC50 42.5 +/- 17.5 nM), although unlike PPADS its effect was reversible with washout and surmountable. Compound 3 showed weak antagonistic activity at the rat P2X(3) receptor (IC50 58.3 +/- 0.1 mu M), while at recombinant rat P2X(2) and P2X(4) receptors no enhancing or antagonistic properties were evident. Compounds 2 and 3 were found to be inactive as either agonists or antagonists at the phospholipase C-coupled P2Y(1) receptor of turkey erythrocytes, at recombinant human P2Y(2) and P2Y(4) receptors, and at recombinant rat P2Y(6) receptors. Similarly, compounds 2 and 3 did not have measurable affinity at adenosine Al, A(2A), Or A(3) receptors. The lack of an aldehyde group in these derivatives indicates that Schiffs base formation with the P2X1 receptor is not necessarily required for recognition of pyridoxal phosphate derivatives. Thus, compounds 2 and 3 are relatively selective pharmacological probes of P2X(1) receptors, filling a long-standing need ill the P2 receptor field, and are also important lead compounds for future studies.