Major role of the renin angiotensin system in the neointima formation after vascular injury in guinea pigs

Oral renin inhibitors

Use of drugs that inhibit the renin-angiotensin system is an effective way to intervene in the pathogenesis of cardiovascular and renal disorders. The idea of blocking the renin system at its origin by inhibition of renin has existed for more than 30 years. Renin inhibition suppresses the generation of the active peptide angiotensin II. The first generation of orally active renin inhibitors were never used clinically because of low bioavailability and weak blood-pressure-lowering activity. At present, aliskiren is the first non-peptide orally active renin inhibitor to progress to phase-III clinical trials. It might become the first renin inhibitor with indications for the treatment of hypertension and cardiovascular and renal disorders. Novel compounds with improved oral bioavailability, specificity, and efficacy are now in preclinical development. This Review summarises the development of oral renin inhibitors and their pharmacokinetic and pharmacodynamic properties, with a focus on aliskiren.

Effects of enalaprilat on neointimal growth of cultured rabbit aorta following balloon injury

Our objective was to determine if the ability of an angiotensin-converting enzyme (ACE) inhibitor to attenuate neointima formation in balloon-damaged vessel is expressed in an isolated organ culture model of neointimal growth. In vivo balloon angioplasty in combination with in vitro organ culture was used to produce a unique model of vascular neointima formation. Aortic segments were cultured in medium containing a broad concentration range of the ACE inhibitor enalaprilat (0-100 µM). Cell proliferative indices and neointima:media thickness ratios were determined from vessel segments after 1, 4, and 7 days in culture. We observed no significant effect on either parameter at any dose of enalaprilat. Linear regression analysis on the rate of increase in intima to media thickness ratios during the 7 days of culture also showed no effect of enalaprilat at any concentration. We conclude that enalaprilat has no effect on neointimal growth or cell proliferation in this vascular organ culture model, and it is suggested that ACE inhibitors may act by mechanisms other than local converting enzyme inhibition to attenuate neointimal growth in rabbits following vascular ballooning in vivo.Key words: angiotensin, restenosis, angiotensin-converting enzyme inhibitors, organ culture, aorta.

Inhibitor of angiotensin-converting enzyme modifies myointimal origin in an arterial autograft model

Pharmacologic modulation by an inhibitor of angiotensin-converting enzyme (IACE: cilazapril) of vascular proliferative response to a full-thickness arterial injury (autograft) was studied in rats. An arterial autograft 5 mm long was made in the right common iliac artery of 50 female Sprague-Dawley rats (weight 250-300 g) by microsurgical techniques. The animals were divided into two study groups: group I (controls), 20 animals that underwent arterial autograft but received no other treatment; and group II (cilazapril-treated), 20 rats that underwent arterial autograft and received cilazapril (Roche), 10 mg/day orally (p.o.) in an excipient of 2% arabic gum, for 4 days before operation. Animals were killed on postoperative days 7, 14, 21, 30, and 50, and grafts were studied by light microscopy, scanning and transmission electron microscopy, and morphometry. In the control group, the hyperplasic response had begun by postoperative day 14 and was established by postoperative day 50. In the medial layer, the muscle cells changed in phenotype from contractile to secretory cells. The adventitia had a highly proliferative appearance. In the cilazapril-treated group, fibrin deposits and platelets formed a layer on the internal elastic lamina. This layer appeared to evolve toward an intimal hyperplasia that became quantifiable by postoperative day 21. The medial layer was clearly thinned and showed intense accumulation of lipid microvacuoles, elastic degeneration, and vacuolized cells. Our results suggest that the use of an inhibitor of ACE modified the origin of the intimal hyperplasia in the arterial autograft model. Enhancement of the thrombogenicity of the luminal surface favors myointimal development by thrombus reorganization.

Prevention of neointima formation by mibefradil after vascular injury in rats: comparison with ACE inhibition

Cilazapril, an angiotensin-converting enzyme inhibitor, and mibefradil, a selective T-type voltage-operated calcium channel blocker, have been shown to prevent neointima formation after vascular injury. The goal of the present study was to evaluate the mechanism of action of both drugs. For this purpose, the influence of the renin angiotensin system on the effects of mibefradil (30 mg/kg po) and cilazapril (10 mg/kg po) on neointima formation after carotid injury were evaluated in normotensive rats (normal renin angiotensin system) and DOCA hypertensive rats (suppressed renin angiotensin system). In addition, in order to differentiate an effect on cell migration or cell proliferation, both drugs were given either before or after the smooth muscle migration phase. Finally, cilazapril and mibefradil were given in combination. In normotensive rats, mibefradil and cilazapril decreased neointima formation, resulting in neointima/media ratios of 38% (p < 0.05) and 53% (p < 0.01), respectively. However, in DOCA hypertensive rats, mibefradil was active, with a reduction of the neointima/media ratio by 63% (p < 0.001), whereas cilazapril reduced it only slightly (19%) and not significantly. In addition, cilazapril was active only when treatment started before the migration phase (63%, reduction in neointima/media ratio, p < 0.001) but not when started thereafter (13% reduction in neointima/media ratio, n.s.). In contrast, treatment with mibefradil was also active when started after the migration phase (51% reduction in neointima/ media ratio, p < 0.001 when treatment started 1 day before balloon injury and 41%, p < 0.01 when treatment started 5 days after balloon injury). The combination of both drugs was additive (67% reduction in neointima/media ratio, p < 0.001 vs. control). These experiments clearly show that mibefradil and cilazapril have a different mechanism of action after vascular injury. Mibefradil most likely prevents the proliferation of smooth muscle cells. In contrast, cilazapril most likely inhibits the migration of smooth muscle cells. These two different mechanisms of action explain why the effects of both drugs are additive.