DN-9693: A PHOSPHODIESTERASE INHIBITOR WITH A PLATELET MEMBRANE EFFECT

We have examined the effect of DN-9693 (piperidinyl - imidazo - quinazoline: Daiichi Seiyaku, Japan) a water soluble phosphodiesterase inhibitor on platelet aggregation, secretion and thromboxane B₂ (TXB₂) production. In platelet rich plasma and at concentrations of 2uM and 5uM the drug significantly inhibited aggregation induced by adenosine diphosphate, collagen and sodium arachidonate. TXB₂ production and release of adenosine tri-phosphate and ¹⁴C 5-hydroxytryptamine were also significantly inhibited by the drug. Cyclic adenosine mono-phosphate accumulation was enhanced. Inhibition of ristocetin induced platelet agglutination was an unexpected finding and further experiments were undertaken to explore this. These suggested no specific effect against a plasma factor (von Willebrand factor) and reduced expression of the platelet membrane glycoprotein Ib was implicated. To investigate this further we examined the effect of DN-9693 on the binding of a monoclonal antibody (McAb) to platelet membrane glycoprotein lb (AN51). This was assessed by a FACS IV flow cytofluorimeter utilising a goat anti-mouse fluorescein isothiocyanate (FITC) labelled secondary antibody. Similar experiments were also performed with McAbs to the membrane glycoprotein complex IIb/IIIa (M148) and also to glycoprotein IIIa (C17). In platelet rich plasma, at concentrations which have been shown to inhibit aggregation, DN-9693 significantly reduced the mean fluorescence intensity of the cells coated with McAb AN51 in a dose related manner. This strongly suggested a drug effect against the glycoprotein Ib receptor site. Also, the drug appeared to enhance the binding of McAb C17 to glycoprotein IIIa. This study indicates that in addition to potent phosphodiesterase inhibitor activity, DN-9693 causes a platelet surface membrane change which is associated with reduced expression of membrane glycoprotein Ib.