Matrix metalloproteinase activity in thoracic aortic aneurysms associated with bicuspid and tricuspid aortic valves

Boyum J, Fellinger EK, Schmoker JD, Trombley L, McPartland K, Ittleman FP, Howard AB

Objective: Matrix metalloproteinases are endopeptidases that function in cell matrix turnover. Abnormal matrix metalloproteinase activity has been implicated in the formation of atherosclerotic abdominal aortic aneurysms. Recent studies suggest that abnormal matrix metalloproteinase activity may also be associated with the formation of atherosclerotic and nonatherosclerotic thoracic aortic aneurysms. Bicuspid aortic valves are associated with an intrinsic aortic pathology that predisposes to formation of proximal thoracic aneurysms while tricuspid aortic valves are not. The objective of this study was to compare the activities of matrix metalloproteinases and levels of their inhibitors in thoracic aneurysms of patients with bicuspid and tricuspid aortic valves.

Methods: Endogenous and total activity of matrix metalloproteinase-2 and matrix metalloproteinase-9 were measured in proximal nonatherosclerotic thoracic aortic aneurysms of 16 patients with bicuspid aortic valves and 12 patients with tricuspid aortic valves. Levels of tissue inhibitor metalloproteinase-1 and -2 were also measured. Results were standardized to total protein (mg).

Results: Total matrix metalloproteinase-2 activity was greater in aneurysms associated with bicuspid valves when compared with those from tricuspid valves (43 ± 11 ng/mg vs 14 ± 2 ng/mg, P = .02). Total matrix metalloproteinase-9 activity was also greater in aneurysms associated with bicuspid aortic valves (4.0 ± 0.9 vs 1.5 ± 0.3, P = .02). There was no meaningful difference between groups in levels of tissue inhibitor-1 and -2.

Conclusion: The increased activity of matrix metalloproteinases in the walls of aneurysms associated with bicuspid aortic valves may partly explain the predilection to aneurysm formation in these patients.

Journal of Thoracic and Cardiovascular Surgery 2004; 127: 686-691

Paper selected by IKT

Comment by mdt: The authors make the unfortunate assumption that when atherosclerosis is detected as a component of a thoracic aneurysm, atherosclerosis has caused the aneurysmal disease. There is minimal, if any, direct evidence for this assumption. As well known to peripheral vascular surgeons, aneurysms of diverse etiologies (for example, an initial post-stenotic dilatation from a subclavian rib), will exhibit features of atherosclerotic degeneration in due course. Accordingly, they exclude all specimens with either gross or microscopic atherosclerotic change. It would be interesting if they had reported how many aneurysms were excluded on this basis; as an "atherosclerosis-free" aorta would be rare, in human beings after childhood. The authors also miss out on the opportunity to report on whether MMP activity is increased in association with bicuspid versus normal valve in the "atherosclerotic" subset of patients.

As part of the rationale for excluding aneurysms with "any" evidence of atherosclerosis, they argue that they wanted to rule out any possible spurious findings that might be related to MMP activity in the inflammatory component of the atherosclerotric plaque. Another way around this problem would have been to study the adventitia and outer media in all specimens. The inflammatory changes of atherosclerosis are subendothelial.

In order to report a statistically significant increase in MMP activity in the bicuspid valve subset, the authors had to include latent MMP activity. There was no significant difference in what they measured as "endogenous" activity.

Finally, the authors report a p value = 0.11 for the difference of 50% incidence of aortic stenosis in the bicuspid valve group versus 17% incidence in the normal valve group. If twice as many patients in each group had been studied and the percentages remained the same, this finding would have been statistically significant at a one-tailed p < 0.01, by Fisher's Exact Test for 2x2 Tables. Since aortic stenosis is associated with post-stenotic dilatation, the elevations of MMP detected in the bicuspid valve group may have been secondary to this feature of aortic remodeling. The authors might have provided data relevant to that issue, by doing a regression analysis of enzyme activity versus calculated valve diameter.

For all of these reasons, mdt doubts that the present paper will be the last word on the subject of why patients with bicuspid aortic valves are more susceptible to aneurysmal degeneration of the ascending aorta.

From the Western Vascular Society

Continuous periaortic infusion improves doxycycline efficacy in experimental aortic aneurysms

Sho E, Chu J, Sho M, Fernandes B, Judd D, Ganesan P, Kimura H, Dalman RL

Objective: We created a novel continuous infusion system to evaluate the efficacy of juxta-aortic doxycycline delivery as a transitional step toward developing hybrid drug/device treatment strategies for abdominal aortic aneurysm (AAA) disease.

Methods: Controlled comparison of treatment outcomes was studied in animal models with molecular and morphologic tissue analysis in a collaboration between university and corporate research laboratories. Rat AAAs were created via porcine pancreatic elastase (PPE) infusion and grouped and analyzed by subsequent treatment status (either doxycycline in vehicle or vehicle alone) and drug delivery method (continuous infusion via periaortic delivery system [PDS] or twice-daily subcutaneous injection). The main outcome measures were AAA diameter via direct measurement, medial elastin lamellar preservation via light microscopy, mural smooth muscle cell (SMC) proliferation and SMC and macrophage density via immunostaining and counting, expression of matrix metalloproteinases 2, 9, and 14 and tissue inhibitors of metalloproteinases 1 and 2 via real-time reverse transcriptase–polymerase chain reaction, and enzymatic activity via substrate zymography. Serum drug levels were analyzed via liquid chromatography/mass spectroscopy.

Results: PDS (1.5 mg/kg/day) and subcutaneous (60 mg/kg/day) delivery methods caused comparable reductions in AAA diameter during the period of 14 days after PPE infusion. PDS rats gained more weight during the postoperative period (P < .001), possibly as a result of reduced serum drug levels and systemic toxicity. Doxycycline treatment reduced AAA macrophage infiltration and SMC proliferation significantly. Despite reduced diameter, circumferential elastic lamellar preservation was not apparent in doxycycline-treated AAAs.

Conclusions: Continuous periaortic infusion lowers the effective doxycycline dose for experimental AAA limitation. Alternative biologic inhibition strategies might also be amenable to direct intra-aortic or juxta-aortic delivery. Periaortic infusion might improve the clinical outcome of minimally invasive AAA treatment strategies.

Clinical Relevance: Aneurysm remodeling may continue after successful endovascular AAA exclusion. Continued proteolytic activity within the aneurysm wall potentiates late graft migration and failure. The doxycycline infusion system developed in these experiments may serve as a prototype for adjuvant treatment modalities that complement endovascular AAA exclusion. Local delivery of doxycycline or other agents active in AAA disease, either continuously or at selected intervals after graft implantation, may stabilize the wall and aid in maintaining aneurysm exclusion. Alternative delivery methods could include passive diffusion from either the graft material itself or treatment reservoirs incorporated into endografts. Given the recognized limitations of current technologies, adjuvant biologic therapies have the potential to improve long-term patient outcome significantly after endovascular exclusion.

Journal of Vascular Surgery 2004; 39: 1312-1321

Paper selected by IKT

Comment by IKT: The most widely studied drug for AAA inhibition is doxycycline, however its inhibitory effect in experimental models is demonstrated in dosages nearly 10-fold higher than the typical antimicrobial dose. In this study the authors developed a periaortic drug delivery system for continuous infusion of doxycycline (1.5mg/kg/day); and they demonstrated MMP-9 inhibition and a reduced the rate of aneurysm progression, while the recorded serum levels of doxycycline were dramatically reduced. This approach is an innovative and important contribution, as it might limit systemic side effects while maintaining therapeutic efficacy. The authors mention in Discussion that a similar delivery system might also be used to deliver other agents that may inhibit aneurysmal dilation after endovascular procedures.