Control of the structural and functional consequences of vein graft intimal hyperplasia with a 21-aminosteroid—U74389G

Abstract

Following angioplasty and vein bypass grafting, there is endothelial cell injury, infiltration of leukocytes and smooth muscle cell (SMC) proliferation leading to intimal hyperplasia which may result in stenosis and can lead to eventual occlusion. This study examines the effect of the 21-aminosteroid U74389G (Upjohn Company), on the formation of vein graft intimal hyperplasia in vivo and on SMC DNA synthesis and proliferation in vitro. Twenty New Zealand White rabbits had a right carotid interposition bypass graft using the ipsilateral external jugular vein. Ten animals received chronic oral therapy with U74389G (25 mg/kg/day; begun 5 days before surgery and continued until harvest) and 10 control animals received vehicle only. All animals were sacrificed on the 28th postoperative day. Vein grafts were harvested either for histology/videomorphometry (n = 6 per group) or for in vitro isometric tension studies (n = 4; four 5mm rings per graft). The incorporation of [³H]thymidine into the cellular DNA of serum-stimulated rabbit aortic SMC (passage 6th to 12th) was assessed in the presence of increasing concentrations of U74389G (10⁻⁹ to 10⁻⁴M). The effect of U74389G on in vitro cell proliferation was also assessed. Treatment with U74389G produced a 44% decrease in overall mean intimal thickness from 82 ± 1 μM (mean ± s.e.m.) in the controls to 57 ± 10 μM in the U74389G treated vein grafts (p = 0.003). Furthermore, there was a 40% increase in overall luminal areas of the treated vein grafts compared to controls (19.4 ± 2.9 vs. 13.9 ± 2.0 mm²; p = 0.13; mean ± s.e.m.) while there was no statistical differences in the medial thicknesses of the control and treated vein grafts. The vasomotor function of the vein grafts was not altered by U74389G. Incubation with U74389G inhibited in vitro [³H]thymidine incorporation of serum-stimulated rabbit SMC with an IC₅₀ of 6.9 μM (4.9 μg/ml) and a maximal inhibition of 67 ± 3% (mean ± s.e.m.) at 10μM. In addition, the presence of U74389G produced a concentration dependent inhibition of in vitro cell proliferation. This study shows that a 21-aminosteroid, U74389G, significantly reduced intimal hyperplasia in experimental vein grafts, but did not modulate the increased vasoconstrictive properties of the grafts. In addition, U74389G inhibited SMC DNA synthesis in vitro. The in vivo reduction in intimal hyperplasia together with an increased luminal area would mitigate against the development of vein graft stenosis. However, the absence of vasomotor changes suggests that the tendency towards vasospasm remains in the treated grafts. Furthermore, the inhibition of SMC proliferation by U74389G suggests that it may have properties unrelated to its antioxidant activity and thus, may have additional benefits in controlling the development of intimal hyperplasia.