Synthesis of a new photoreactive derivative of dipyridamole and its use in the manufacture of artificial surfaces with low thrombogenicity

Photoimmobilization of dipyridamole (Persantin) was accomplished through the use of a new synthetic conjugate molecule, 1. Persantin is a powerful inhibitor of platelet activation and aggregation and is widely used as a vasodilator. Conjugate 1 consists of triply protected dipyridamole [three of the four hydroxyl groups carry a tert-butyldimethylsilyl (TBDMS) protective group) and the photoreactive 4-azidobenzoyl group. A short hydrophilic spacer chain, derived from triethylene glycol, separates the protected dipyridamole system and the photoreactive group. Compound 1 was immobilized on polyurethane sheets (Pellethane D-55) through irradiation with ultraviolet (UV) light, and the protective groups were removed afterward. The resulting modified polyurethane surfaces were characterized by different physicochemical techniques: UV extinction, contact angle measurements (captive bubble technique), and X-ray photoelectron spectroscopy (XPS). The UV extinction measurements showed the presence of 13 +/- 1 nmol of immobilized dipyridamole/cm(2). The contact angle measurements revealed that the modified surface was markedly more hydrophilic than the control (i.e. unmodified polyurethane). XPS measurements clearly established the presence of immobilized dipyridamole in the outermost layers of the modified surface. This was especially clear from the XPS spectra recorded at a low take-off angle (similar to 6 degrees). Furthermore, the XPS spectra showed that the TBDMS protective groups had been quantitatively removed during the deprotection/washing treatment. The in vitro blood compatibility of the modified surface was studied with the thrombin generation assay as developed in our group, as well as with scanning electron microscopy. The thrombin generation test produced a lag time of 1275 s for the modified surface, as opposed to 569 s for the control. Scanning electron microscopy showed that far fewer platelets adhere to the modified surface (approximately 7 x 10(3)/mm(2)) as compared to the control (approximately 6 x 10(2)/mm(2)). Taken together, the experimental data reveal that the modified surface has excellent blood compatibility in vitro. It is discussed that the use of conjugate 1 leads to simultaneous exposure of dipyridamole at the modified surface and to a marked increase of the surface hydrophilicity, which is likely to hamper adsorption of plasma proteins. The combination of these effects is uniquely related to the molecular buildup of 1. Conjugate 1 will be used in future work that is aimed at preparing small-caliber polyurethane vascular grafts with a blood compatible lumenal surface.