- Email: [email protected]
Effect of HMG-CoA reductase inhibitors on chronic allograft rejection
Kidney International, Vol. 56, Suppl. 71 (1999), pp. S-117–S-121
Effect of HMG-CoA reductase inhibitors on chronic allograft rejection STEVEN KATZNELSON Department of Transplantation, California Pacific Medical Center, San Francisco, California, USA
Effect of HMG-CoA reductase inhibitors on chronic allograft rejection. Background. Although chronic rejection is the most important cause of late allograft loss, none of the currently available immunosuppressive agents successfully target this problem. Clinical and laboratory studies suggest that 3-hydroxy-3-methylglutaryl co-enzyme A (HMG-CoA) reductase inhibitors (HRIs) may decrease the incidence of and pathophysiologic factors leading to chronic rejection. Methods. A number of clinical and laboratory investigations have been designed to evaluate the effect of HRIs on chronic rejection. Results. Clinical trials in heart transplant patients suggest that HRIs decrease the incidence of chronic rejection in a manner that may be independent of lipid lowering. Subsequent studies in animal transplant models confirm that HRIs reduce chronic rejection. In further studies to elucidate the possible mechanisms of this effect, it has been observed that HRIs have an inhibitory effect on an number of lymphoid cell lines and vascular smooth muscle cells. HRIs may also prevent chronic rejection by protecting the endothelium from injury and dysfunction, perhaps by up-regulating nitric oxide synthesis. Conclusions. HRIs may be the first agents to be effective in preventing chronic rejection. Although the mechanism behind this protective effect is unclear, it seems likely that HRIs may affect multiple factors that could lead to chronic rejection.
Chronic rejection is the most important cause of longterm allograft loss. This process bears much of the responsibility for the fact that only 20% of cadaveric kidneys continue to function at 10 years post-transplant. This allograft attrition rate is virtually unchanged throughout the transplant experience [1], despite the use of new immunosuppressive medications and a subsequent decrease in the incidence of acute rejection episodes. The great advances in immunosuppression thus have not significantly affected the incidence of chronic rejections. The initial injury to the allograft leading to chronic rejection appears to be damage to the vascular endothelium mediated by a number of different alloantigenKey words: pravastatin, simvastatin, nitric oxide, B cell, T cell, transplant coronary vasculopathy.
1999 by the International Society of Nephrology
dependent and alloantigen-independent factors [1]. The alloantigen-dependent factors include B- and T-cell activation and the effects of a variety of different cytokines and growth factors. Alloantigen-independent factors include the deleterious effects of reperfusion injury, hypertension, cytomegalovirus (CMV) infection and hyperlipidemia. Once sustaining initial injury, the vascular endothelium releases various cytokines and chemoattractants including platelet-derived growth factor, thromboxane, platelet activating factor and tumor necrosis factor. Circulating monocytes, macrophages, lymphocytes, neutrophils and natural killer cells are attracted into the area of injury and subsequently produce various additional injurious cytokines and growth factors. The end product of this process of cell migration and the elaboration of these cytokines and growth factors is smooth muscle cell proliferation and migration into the intima with resulting initimal thickening, vascular luminal obliteration, tubular atrophy and a number of glomerular changes that define the histological lesions of chronic rejection [2]. Our group and others have recently shown that 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (HRIs) decrease the incidence and progression of transplant coronary vasculopathy in human heart transplant recipients [3, 4]. Transplant coronary vasculopathy (TCV) is a term used for the chronic rejection analogue in heart transplantation. We have also shown that pravastatin, one of the HRIs, decreases the incidence of acute rejection after human kidney transplantation [5]. Acute rejection has been considered a risk factor for the development of chronic rejection [6]. A number of laboratory studies now suggest that HRIs may affect a number of factors, both alloantigen-dependent and alloantigen-independent, in a way that may lead to a decrease in the incidence of chronic rejection. The purpose of this article is to summarize the data published to date suggesting the beneficial effect of HRIs in combating chronic rejection. CLINICAL TRIALS We first reported the antirejection effects of pravastatin in a prospective randomized trial in heart trans-
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plant recipients [3]. Patients receiving daily pravastatin therapy (40 mg) had a significantly increased one-year survival rate (94% vs. 78%, P 5 0.02) that coincided with a significant decrease in the incidence of clinically severe acute rejection episodes (3 of 47 patients in the pravastatin-treated group and 14 of 50 patients in the control group, P 5 0.005). All patients in this study were treated with prednisone, azathioprine and cyclosporine as their baseline immunosuppressive regimen. There were no differences in baseline demographic and clinical parameters between groups. This study cohort was also followed to assess the effect of pravastatin on the incidence and progression TCV. The total number of patients found to have TCV by angiography or autopsy was less in the pravastatin-treated group than the control group (3 patients in the pravastatin group vs. 10 patients in the control group, P 5 0.05). In a subgroup of 48 patients (27 in the pravastatin group and 21 in the control group), intracoronary ultrasound was performed immediately after transplantation and at one-year post-transplant. It was noted that the pravastatin treated patients had a decrease in the rate of progression of TCV as compared with the control group, as evidenced by reductions in both the maximal intimal thickness (0.11 mm vs. 23 mm, P 5 0.002) and the intimal index (0.05 vs. 0.10, P 5 0.03). Similar findings were also recently reported with simvastatin, another HRI. Wenke et al reported results of a four-year prospective randomized study of simvastatin in heart transplant recipients [4]. In the course of four years after transplantation, the simvastatin treated group had significantly better long-term survival (88.6% vs. 70.3%, P 5 0.05), and a lower incidence of TCV in the coronary angiographic findings (16.6% vs. 42.3%, P 5 0.045). The incidence of acute allograft rejections did not differ between the two groups, although there was a trend toward a lower number of serious rejections in the simvastatin group (2.8% vs. 13.5%, P 5 0.1). In addition, intracoronary ultrasound performed after four years in a subgroup of 27 patients (simvastatin, N 5 10; control, N 5 17) showed a lower intimal index, that is, a measure of intimal area (13.8 6 7.1% vs. 27.9 6 12.1%, P 5 0.04). In both heart transplant trials, although the HRIs were successful in decreasing lipid levels there was no clear correlation between lipid levels and the incidence of acute or chronic rejection. We also noted an apparent immunomodulatory effect of pravastatin in kidney transplant recipients. In a prospective, randomized, open-labeled pilot study, we studied the effects of early pravastatin use after cadaveric kidney transplantation [5]. Forty-eight patients were randomized to a group taking pravastatin at 20 mg/day or to a control group. All patients in this study were maintained on a double immunosuppressive regimen with prednisone and cyclosporine. The two groups were not
different with respect to baseline clinical or demographic characteristics. The results of this study demonstrate that pravastatin treated patients had decreases in the incidence of acute rejection episodes (64% vs. 25% in the control group, P , 0.01) and decreases in the use of both OKT3 (0.04 course per patient vs. 0.08 course per patient in the control group, P 5 0.02) and solumedrol (0.33 course per patient vs. 0.88 course per patient in the control group, P 5 0.01). As acute rejection may be the most significant clinical factor leading to chronic rejection [6], pravastatin may affect the incidence of chronic rejection in these patients. Follow-up data are currently being collected. ANIMAL MODELS OF CHRONIC REJECTION A number of studies utilizing animal models of chronic rejection (or, TCV) have been published suggesting that HRIs may be helpful in preventing or treating this disease. Mesier et al described a rat heterotopic heart transplant model wherein simvastatin treatment significantly reduced the incidence of TCV [7]. This effect was independent of lipid lowering. In another rat heterotopic heart transplant model, Maggard et al described that pravastatin inhibited the development of transplant coronary vasculopathy. They also noted an inhibition of the synthesis and subsequent degradation of extracellular matrix proteins as well as a decrease in the number of graft-infiltrating macrophages in the pravastatin-treated animals [8]. In a rat orthotopic liver transplant model, Kakkis et al showed that pravastatin therapy improved rat survival and decreased the incidence of chronic rejection [9]. Finally, in a rat model of chronic rejection utilizing orthotopic femoral artery allografts, we showed that rats pretreated with pravastatin and cyclosporine three days prior to engraftment demonstrated a reduction in the development of chronic rejection as measured by a decrease in intimal proliferation as compared with control animals treated with cyclosporine alone [10]. EFFECT OF HMG-COA REDUCTASE INHIBITORS ON VARIOUS CELL LINES T lymphocytes We have shown that pravastatin and cyclosporine are synergistic in their ability to decrease cytotoxic T lymphocyte (CTL) activity [11]. In a one-way mixed lymphocyte reaction, we demonstrated that the combination of pravastatin and cyclosporine decreases CTL cytotoxicity from 41.4% kill of target cells to 20.3% kill (P , 0.01). Cutts et al and others have shown that HRIs decrease the proliferation of mitogen-stimulated T-lymphocytes in vitro [12].
Katznelson: HMG-CoA reductase inhibitors and chronic rejection
B lymphocytes B-cell activity also seems to be inhibited by HRIs. In their rat heteroptopic heart transplant model, Maggard et al demonstrate that IgG allo-antibody production is diminished in the same pravastatin-treated rats that also demonstrate an inhibition of transplant vasculopathy [8]. This is suggestive of an inhibitory effect of HRIs on B cell activity. In addition, Rudich et al have shown that both pravastatin and simvastatin diminishes B-lymphocyte activation [13]. Monocytes HMG-CoA reductase inhibitors also affect monocytes. Kreuzer, Bader and Jahn have demonstrated that pravastatin decreases monocyte chemotaxis [14]. In addition, fluvastatin has been shown to decrease monocyte expression of LFA-1 and ICAM-1 [15], thus decreasing their ability to interact with endothelial cells and other lymphoid cell types. This effect was reversed by adding mevalonate back to the experimental system. Lovastatin decreases monocyte expression of the CD11b/CD18 complex [16], thus also decreasing their ability to adhere to and inflict injury on vascular endothelial cells. Natural killer cells Some of the earliest studies were performed by Cutts and Bankhurst, who showed that HRIs decrease natural killer cell (NKC) cytotoxicity in vitro [17]. The addition of mevalonate back to the culture medium restored NKC cytotoxicity, suggesting that it was truly a decreased production of a downstream metabolite of the cholesterol synthesis pathway that was responsible for the inhibition of NKC cytotoxicity. In this assay, cellular activity was also restored after the introduction of small amounts of interleukin-2 to the culture medium. In an ex vivo study, we demonstrated that NKC cytotoxicity was inhibited by more than 50% in heart and kidney transplant patients taking pravastatin [3, 5]. In these studies, there was no direct correlation between the degree of NKC inhibition and the occurrence of acute rejection episodes in any given patient. The inhibition of NKC activity in this study was not associated with an increased risk of infectious episodes or de novo malignancies. Vascular smooth muscle cells As mentioned earlier in this article, vascular smooth muscle cell (VSMC) proliferation is a vital pathophysiological event in the development of chronic rejection. Negre-Aminou et al studied the antiproliferative effects of all of the clinically available HRIs on human VSMCs [18], and found that all of these agents have antiproliferative activity, though each to a different degree. Of the agents tested, cerivastatin was the most potent antiproliferative drug. Simvastatin was also noted to inhibit the
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post-translational processing of newly synthesized p21 ras protein, probably by inhibiting its farnesylation. In a parallel study, we have shown that pravastatin inhibits ras activity [19], again probably by decreasing the production of the farnesyl moiety (a metabolite of the cholesterol synthesis pathway), thus subsequently decreasing the production of farnesylated ras, its activatable form. This was associated with a decrease in VSMC proliferation. EFFECT OF HRIs ON LIPID LEVELS AND ENDOTHELIAL CELL RESPONSE— AN IMPACT ON CHRONIC REJECTION? Hyperlipidemia is a prevalent complication of solid organ transplantation affecting up to 80% of recipients [20]. HRIs have been demonstrated to be safe and efficacious in the treatment of post-transplant hyperlipidemia [20]. Hyperlipidemia may be an independent risk factor for the development of chronic rejection. Some, but not all, authors have correlated post-transplant hyperlipidemia with the development of TCV after heart transplantation [21]. Hyperlipidemia may also be a risk factor for the development of chronic rejection after kidney transplantation [22]. The pathophysiology of any effect of elevated lipid levels on the development of chronic rejection is unclear. Some authors have suggested that elevated levels of oxidized low density lipoprotein (LDL) in the allograft may lead to endothelial cell injury and subsequently TCV. Although lipid levels were lower in the heart transplant patients treated with pravastatin [3, 5] and simvastatin [4] discussed earlier in this article, there was no direct correlation between the absolute lipid levels or changes in lipid levels and the development of either acute or chronic rejection. Thus, although control of post-transplant hyperlipidemia may be important with respect to helping address the problem of the high incidence of cardiovascular ischemic events in the post-transplant setting, conclusive evidence suggesting that lowering lipid levels with HRIs helps to prevent or treat chronic rejection has not yet been obtained. In fact, in one rat model of chronic rejection, simvastatin was successful in protecting against the development of chronic rejection without any effect on lipid levels [7]. As previously mentioned, the response of the allograft endothelium to injury is the initial process in the development of chronic rejection. It has been hypothesized that HRIs may protect the endothelium by stabilizing these cells even in the presence of injurious substances or events. Although this argument was mostly conjecture until recently in the transplant literature, there is some new evidence suggesting that HRIs may protect the allograft vascular endothelium by interactions with nitric oxide (NO). NO has a number of properties critical to
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the normal behavior of blood vessels. As a partial list, NO induces local vasodilation and inhibits platelet aggregation, monocyte adhesion and vascular smooth muscle cell proliferation [23]. Inhibition of NO leads to vasoconstriction, vascular thrombosis, endothelial inflammation and interstitial monocyte and macrophage infiltration. Inhibition of NO can lead to many of the same early endothelial cell changes that initiate the processes that begets chronic rejection. In fact, it has been demonstrated that down-regulation of inducible nitric oxide synthase (iNOS) has been associated with the development of chronic rejection in a variety of experimental transplant models [24]. LDL-cholesterol itself may be one of the factors that can down-regulate iNOS expression. Conversely, lovastatin and simvastatin have been shown to reverse the LDL-induced down-regulation of iNOS expression [25]. Thus, HRIs may stabilize allograft vascular endothelium, even in the presence of potentially injurious events, by re-establishing the production of NO either directly or through reducing the concentration of LDL-cholesterol.
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CONCLUSIONS HMG-CoA reductase inhibitors (HRIs) have been demonstrated to decrease the incidence and progression of chronic rejection in animal models and in human studies. HRIs inhibit normal T-cell, B-cell and monocyte activity. Various properties of these lymphoid cells are critical to the development of chronic rejection. HRIs also reduce smooth muscle cell migration and proliferation in vivo and in vitro. This action could also have a pronounced inhibitory effect on chronic rejection. HRIs reduce lipid levels in the post-transplant setting, but it is unclear as to whether it is the lipid reduction or another property of HRIs that is responsible for any effect on chronic rejection. Although no conclusive data exist proving that HRIs directly inhibit the development of chronic rejection, sufficient evidence exists for transplant clinicians to consider the use of these agents in the posttransplant setting both for their antihyperlipidemic effects and their possible effects on both acute and chronic rejection.
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Reprint requests to Steven Katznelson, M.D., Department of Transplantation, California Pacific Medical Center, 2340 Clay Street, Suite 251, San Francisco, California 94115, USA. E-mail: [email protected]
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REFERENCES
19.
1. Tullius SG, Tilney NL: Both alloantigen-dependent and -independent factors influence chronic allograft rejection. Transplantation 59:313–318, 1995 2. Paul LC, Fellstrom B: Chronic vascular rejection of the heart and the kidney—Have rational treatment options emerged? Transplantation 53:1169–1179, 1992 3. Kobashigawa JA, Katznelson S, Laks H, Johnson JA, Yeatman L, Wang XM, Chia D, Terasaki PI, Sabad A, Cogert GA, Tro-
20. 21. 22.
sian K, Hamilton MA, Hage A, Drinkwater D, Stevenson LW: Impact of pravastatin on cholesterol lowering, rejection, survival and the development of transplant coronary vasculopathy. N Engl J Med 333:621–627, 1995 Wenke K, Meiser B, Thiery J, Nagel D, von scheidt W, Steinbeck G, Seidel D, Reichart B: Simvastatin reduces graft vessel disease and mortality after heart transplantation: A four-year randomized trial. Circulation 96:1398–1404, 1997 Katznelson S, Wilkinson AH, Kobashigawa JA, Wang WM, Chia D, Ozawa M, Zhong HP, Hirata M, Cohen AH, Terasaki PI, Danovitch GM: The effect of pravastatin on acute rejection after kidney transplantation—A pilot study. Transplantation 61: 1469–1474, 1996 Almond PS, Matas A, Gillingham K, Dunn DL, Payne WD, Gores P, Gruessner R, Najarian JS: Risk factors for chronic rejection in renal allograft recipients. Transplantation 55:752–757, 1993 Meiser BM, Wenke K, Thiery J, Wolf S, Devens C, Seidel D, Hammer C, Billingham ME, Reichart B: Simvastatin decreases accelerated graft vessel disease after heart transplantation in an animal model. Transplant Proc 25:2077–2079, 1993 Magard M, Ke B, Wang T, Kaldas F, Seu P, Busuttil RW, Imagawa D: The effects of pravastatin on chronic rejection of rat cardiac allografts. Transplantation 65:149–155, 1998 Kakkis JL, Ke B, Dawson S, Chai W, Kaldas F, Shau H, Seu P, Sauri H, Busuittil RW, Imagawa DK: Pravastatin increases survival and inhibits natural killer cell enhancement factor in liver transplanted rats. J Surg Res 69:393–398, 1997 Katznelson S, Berryman e, Griffey S, Perez rv, Gregory C: Combined therapy with pravastatin and cyclosporin decreases arterial intimal thickening in a model of severe vascular immune injury. Transplantation 65(Suppl):S38, 1998 Katznelson S, Wang X, Chia D, Ozawa M, Zhong H, Hirata M, Terasaki PI, Kobashigawa JA: The inhibitory effects of pravastatin on natural killer cell activity in vivo and cytotoxic T lymphocyte acitivity in vitro. J Heart Lung Transplant 17:335–340, 1998 Cutts JL, Bankurst AD: Suppression of lymphoid cell function in vitro by inhibition of 3-hydroxy-3-methylglutaryl enzyme A reductase by lovastatin. Int J Immunopharm 11:863–869, 1989 Rudich SM, Mongini PKA, Perez RV, Katznelson S: HMGCoA reductase inhibitors pravastatin and simvastatin inhibit human B lymphocyte activation. Transplant Proc 30:992–995, 1998 Kreuzer J, Bader J, Jahn L: Chemotaxis of the monocyte line U937: Dependence on cholesterol and early mevalonate pathway products. Atherosclerosis 90:203–209, 1991 Niwa S, Totsuka T, Hyashi S: Inhibitory effect of fluvastatin, an HMG-CoA reductase inhibitor, on the expression of adhesion molecules on human monocyte line. Int J Immunopharmac 18:669– 675, 1996 Weber C, Erl W, Weber K, Weber P: HMG-CoA reductase inhibitors decrease CD11b expression and CD11b-dependent adhesion of monocytes to endothelium and reduce increased adhesiveness of monocytes isolated from patients with hypercholesterolemia. J Am Coll Cardiol 30:1212–1217, 1997 Cutts JL, Bankhurst AD: Reversal of lovastatin-mediated inhibition of natural killer cell cytotoxicity by interleukin 2. J Cell Physiol 145:244, 1990 Negre-Aminou P, van Vliet AK, van Erck M, van Thiel GC, van Leeuen RE, Cohen LH: Inhibition of proliferation of human smooth muscle cells by various HMG-CoA reductase inhibitors: Comparison with other human cell types. Biochim Biophys Acta 1345:259–268, 1997 Katznelson S, Ramirez A, Perez RV, Weiss R: Pravastatin and cyclosporine inhibit PDGF-stimulated vascular smooth muscle cell mitogenesis—An investigation of mechanism. Transplant Proc 30: 998–999, 1998 Kobashigawa JA, Kasiske BL: Hyperlipidemia in solid organ transplantation. Transplantation 63:331–338, 1997 Johnson MR: Transplant coronary disease: Nonimmunologic risk factors. J Heart Lung Transplant 11(Suppl):S124–S132, 1992 Dimeny E, Tufveson G, Larsson E, Lithell H, Siegbahn A, Fellstrom B: The influence of posttransplant lipoprotein abnor-
Katznelson: HMG-CoA reductase inhibitors and chronic rejection malities on the early results of renal transplantation. Eur J Clin Invest 23:572–579, 1993 23. Cooke JP: Therapeutic interventions in endothelial dysfunction: Endothelium as a target organ. Clin Cardiol 11(Suppl 2):II-45– II-51, 1997 24. Shears LL, Kawaharada N, Tzeng E, Billiar TR, Watkins SC,
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Kovesdi I, Lizonova A, Pham SM: Inducible nitric oxide synthase suppresses the development of allograft arteriosclerosis. J Clin Invest 100:2035–2042, 1997 25. Laufs U, La Fata V, Plutzky J, Liao JK: Upregulation of endothelial nitric oxide synthase by HMG-CoA reductase inhibitors. Circ 97:1129–1135, 1998
Effect of HMG-CoA reductase inhibitors on chronic allograft rejection STEVEN KATZNELSON Department of Transplantation, California Pacific Medical Center, San Francisco, California, USA
Effect of HMG-CoA reductase inhibitors on chronic allograft rejection. Background. Although chronic rejection is the most important cause of late allograft loss, none of the currently available immunosuppressive agents successfully target this problem. Clinical and laboratory studies suggest that 3-hydroxy-3-methylglutaryl co-enzyme A (HMG-CoA) reductase inhibitors (HRIs) may decrease the incidence of and pathophysiologic factors leading to chronic rejection. Methods. A number of clinical and laboratory investigations have been designed to evaluate the effect of HRIs on chronic rejection. Results. Clinical trials in heart transplant patients suggest that HRIs decrease the incidence of chronic rejection in a manner that may be independent of lipid lowering. Subsequent studies in animal transplant models confirm that HRIs reduce chronic rejection. In further studies to elucidate the possible mechanisms of this effect, it has been observed that HRIs have an inhibitory effect on an number of lymphoid cell lines and vascular smooth muscle cells. HRIs may also prevent chronic rejection by protecting the endothelium from injury and dysfunction, perhaps by up-regulating nitric oxide synthesis. Conclusions. HRIs may be the first agents to be effective in preventing chronic rejection. Although the mechanism behind this protective effect is unclear, it seems likely that HRIs may affect multiple factors that could lead to chronic rejection.
Chronic rejection is the most important cause of longterm allograft loss. This process bears much of the responsibility for the fact that only 20% of cadaveric kidneys continue to function at 10 years post-transplant. This allograft attrition rate is virtually unchanged throughout the transplant experience [1], despite the use of new immunosuppressive medications and a subsequent decrease in the incidence of acute rejection episodes. The great advances in immunosuppression thus have not significantly affected the incidence of chronic rejections. The initial injury to the allograft leading to chronic rejection appears to be damage to the vascular endothelium mediated by a number of different alloantigenKey words: pravastatin, simvastatin, nitric oxide, B cell, T cell, transplant coronary vasculopathy.
1999 by the International Society of Nephrology
dependent and alloantigen-independent factors [1]. The alloantigen-dependent factors include B- and T-cell activation and the effects of a variety of different cytokines and growth factors. Alloantigen-independent factors include the deleterious effects of reperfusion injury, hypertension, cytomegalovirus (CMV) infection and hyperlipidemia. Once sustaining initial injury, the vascular endothelium releases various cytokines and chemoattractants including platelet-derived growth factor, thromboxane, platelet activating factor and tumor necrosis factor. Circulating monocytes, macrophages, lymphocytes, neutrophils and natural killer cells are attracted into the area of injury and subsequently produce various additional injurious cytokines and growth factors. The end product of this process of cell migration and the elaboration of these cytokines and growth factors is smooth muscle cell proliferation and migration into the intima with resulting initimal thickening, vascular luminal obliteration, tubular atrophy and a number of glomerular changes that define the histological lesions of chronic rejection [2]. Our group and others have recently shown that 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (HRIs) decrease the incidence and progression of transplant coronary vasculopathy in human heart transplant recipients [3, 4]. Transplant coronary vasculopathy (TCV) is a term used for the chronic rejection analogue in heart transplantation. We have also shown that pravastatin, one of the HRIs, decreases the incidence of acute rejection after human kidney transplantation [5]. Acute rejection has been considered a risk factor for the development of chronic rejection [6]. A number of laboratory studies now suggest that HRIs may affect a number of factors, both alloantigen-dependent and alloantigen-independent, in a way that may lead to a decrease in the incidence of chronic rejection. The purpose of this article is to summarize the data published to date suggesting the beneficial effect of HRIs in combating chronic rejection. CLINICAL TRIALS We first reported the antirejection effects of pravastatin in a prospective randomized trial in heart trans-
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plant recipients [3]. Patients receiving daily pravastatin therapy (40 mg) had a significantly increased one-year survival rate (94% vs. 78%, P 5 0.02) that coincided with a significant decrease in the incidence of clinically severe acute rejection episodes (3 of 47 patients in the pravastatin-treated group and 14 of 50 patients in the control group, P 5 0.005). All patients in this study were treated with prednisone, azathioprine and cyclosporine as their baseline immunosuppressive regimen. There were no differences in baseline demographic and clinical parameters between groups. This study cohort was also followed to assess the effect of pravastatin on the incidence and progression TCV. The total number of patients found to have TCV by angiography or autopsy was less in the pravastatin-treated group than the control group (3 patients in the pravastatin group vs. 10 patients in the control group, P 5 0.05). In a subgroup of 48 patients (27 in the pravastatin group and 21 in the control group), intracoronary ultrasound was performed immediately after transplantation and at one-year post-transplant. It was noted that the pravastatin treated patients had a decrease in the rate of progression of TCV as compared with the control group, as evidenced by reductions in both the maximal intimal thickness (0.11 mm vs. 23 mm, P 5 0.002) and the intimal index (0.05 vs. 0.10, P 5 0.03). Similar findings were also recently reported with simvastatin, another HRI. Wenke et al reported results of a four-year prospective randomized study of simvastatin in heart transplant recipients [4]. In the course of four years after transplantation, the simvastatin treated group had significantly better long-term survival (88.6% vs. 70.3%, P 5 0.05), and a lower incidence of TCV in the coronary angiographic findings (16.6% vs. 42.3%, P 5 0.045). The incidence of acute allograft rejections did not differ between the two groups, although there was a trend toward a lower number of serious rejections in the simvastatin group (2.8% vs. 13.5%, P 5 0.1). In addition, intracoronary ultrasound performed after four years in a subgroup of 27 patients (simvastatin, N 5 10; control, N 5 17) showed a lower intimal index, that is, a measure of intimal area (13.8 6 7.1% vs. 27.9 6 12.1%, P 5 0.04). In both heart transplant trials, although the HRIs were successful in decreasing lipid levels there was no clear correlation between lipid levels and the incidence of acute or chronic rejection. We also noted an apparent immunomodulatory effect of pravastatin in kidney transplant recipients. In a prospective, randomized, open-labeled pilot study, we studied the effects of early pravastatin use after cadaveric kidney transplantation [5]. Forty-eight patients were randomized to a group taking pravastatin at 20 mg/day or to a control group. All patients in this study were maintained on a double immunosuppressive regimen with prednisone and cyclosporine. The two groups were not
different with respect to baseline clinical or demographic characteristics. The results of this study demonstrate that pravastatin treated patients had decreases in the incidence of acute rejection episodes (64% vs. 25% in the control group, P , 0.01) and decreases in the use of both OKT3 (0.04 course per patient vs. 0.08 course per patient in the control group, P 5 0.02) and solumedrol (0.33 course per patient vs. 0.88 course per patient in the control group, P 5 0.01). As acute rejection may be the most significant clinical factor leading to chronic rejection [6], pravastatin may affect the incidence of chronic rejection in these patients. Follow-up data are currently being collected. ANIMAL MODELS OF CHRONIC REJECTION A number of studies utilizing animal models of chronic rejection (or, TCV) have been published suggesting that HRIs may be helpful in preventing or treating this disease. Mesier et al described a rat heterotopic heart transplant model wherein simvastatin treatment significantly reduced the incidence of TCV [7]. This effect was independent of lipid lowering. In another rat heterotopic heart transplant model, Maggard et al described that pravastatin inhibited the development of transplant coronary vasculopathy. They also noted an inhibition of the synthesis and subsequent degradation of extracellular matrix proteins as well as a decrease in the number of graft-infiltrating macrophages in the pravastatin-treated animals [8]. In a rat orthotopic liver transplant model, Kakkis et al showed that pravastatin therapy improved rat survival and decreased the incidence of chronic rejection [9]. Finally, in a rat model of chronic rejection utilizing orthotopic femoral artery allografts, we showed that rats pretreated with pravastatin and cyclosporine three days prior to engraftment demonstrated a reduction in the development of chronic rejection as measured by a decrease in intimal proliferation as compared with control animals treated with cyclosporine alone [10]. EFFECT OF HMG-COA REDUCTASE INHIBITORS ON VARIOUS CELL LINES T lymphocytes We have shown that pravastatin and cyclosporine are synergistic in their ability to decrease cytotoxic T lymphocyte (CTL) activity [11]. In a one-way mixed lymphocyte reaction, we demonstrated that the combination of pravastatin and cyclosporine decreases CTL cytotoxicity from 41.4% kill of target cells to 20.3% kill (P , 0.01). Cutts et al and others have shown that HRIs decrease the proliferation of mitogen-stimulated T-lymphocytes in vitro [12].
Katznelson: HMG-CoA reductase inhibitors and chronic rejection
B lymphocytes B-cell activity also seems to be inhibited by HRIs. In their rat heteroptopic heart transplant model, Maggard et al demonstrate that IgG allo-antibody production is diminished in the same pravastatin-treated rats that also demonstrate an inhibition of transplant vasculopathy [8]. This is suggestive of an inhibitory effect of HRIs on B cell activity. In addition, Rudich et al have shown that both pravastatin and simvastatin diminishes B-lymphocyte activation [13]. Monocytes HMG-CoA reductase inhibitors also affect monocytes. Kreuzer, Bader and Jahn have demonstrated that pravastatin decreases monocyte chemotaxis [14]. In addition, fluvastatin has been shown to decrease monocyte expression of LFA-1 and ICAM-1 [15], thus decreasing their ability to interact with endothelial cells and other lymphoid cell types. This effect was reversed by adding mevalonate back to the experimental system. Lovastatin decreases monocyte expression of the CD11b/CD18 complex [16], thus also decreasing their ability to adhere to and inflict injury on vascular endothelial cells. Natural killer cells Some of the earliest studies were performed by Cutts and Bankhurst, who showed that HRIs decrease natural killer cell (NKC) cytotoxicity in vitro [17]. The addition of mevalonate back to the culture medium restored NKC cytotoxicity, suggesting that it was truly a decreased production of a downstream metabolite of the cholesterol synthesis pathway that was responsible for the inhibition of NKC cytotoxicity. In this assay, cellular activity was also restored after the introduction of small amounts of interleukin-2 to the culture medium. In an ex vivo study, we demonstrated that NKC cytotoxicity was inhibited by more than 50% in heart and kidney transplant patients taking pravastatin [3, 5]. In these studies, there was no direct correlation between the degree of NKC inhibition and the occurrence of acute rejection episodes in any given patient. The inhibition of NKC activity in this study was not associated with an increased risk of infectious episodes or de novo malignancies. Vascular smooth muscle cells As mentioned earlier in this article, vascular smooth muscle cell (VSMC) proliferation is a vital pathophysiological event in the development of chronic rejection. Negre-Aminou et al studied the antiproliferative effects of all of the clinically available HRIs on human VSMCs [18], and found that all of these agents have antiproliferative activity, though each to a different degree. Of the agents tested, cerivastatin was the most potent antiproliferative drug. Simvastatin was also noted to inhibit the
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post-translational processing of newly synthesized p21 ras protein, probably by inhibiting its farnesylation. In a parallel study, we have shown that pravastatin inhibits ras activity [19], again probably by decreasing the production of the farnesyl moiety (a metabolite of the cholesterol synthesis pathway), thus subsequently decreasing the production of farnesylated ras, its activatable form. This was associated with a decrease in VSMC proliferation. EFFECT OF HRIs ON LIPID LEVELS AND ENDOTHELIAL CELL RESPONSE— AN IMPACT ON CHRONIC REJECTION? Hyperlipidemia is a prevalent complication of solid organ transplantation affecting up to 80% of recipients [20]. HRIs have been demonstrated to be safe and efficacious in the treatment of post-transplant hyperlipidemia [20]. Hyperlipidemia may be an independent risk factor for the development of chronic rejection. Some, but not all, authors have correlated post-transplant hyperlipidemia with the development of TCV after heart transplantation [21]. Hyperlipidemia may also be a risk factor for the development of chronic rejection after kidney transplantation [22]. The pathophysiology of any effect of elevated lipid levels on the development of chronic rejection is unclear. Some authors have suggested that elevated levels of oxidized low density lipoprotein (LDL) in the allograft may lead to endothelial cell injury and subsequently TCV. Although lipid levels were lower in the heart transplant patients treated with pravastatin [3, 5] and simvastatin [4] discussed earlier in this article, there was no direct correlation between the absolute lipid levels or changes in lipid levels and the development of either acute or chronic rejection. Thus, although control of post-transplant hyperlipidemia may be important with respect to helping address the problem of the high incidence of cardiovascular ischemic events in the post-transplant setting, conclusive evidence suggesting that lowering lipid levels with HRIs helps to prevent or treat chronic rejection has not yet been obtained. In fact, in one rat model of chronic rejection, simvastatin was successful in protecting against the development of chronic rejection without any effect on lipid levels [7]. As previously mentioned, the response of the allograft endothelium to injury is the initial process in the development of chronic rejection. It has been hypothesized that HRIs may protect the endothelium by stabilizing these cells even in the presence of injurious substances or events. Although this argument was mostly conjecture until recently in the transplant literature, there is some new evidence suggesting that HRIs may protect the allograft vascular endothelium by interactions with nitric oxide (NO). NO has a number of properties critical to
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the normal behavior of blood vessels. As a partial list, NO induces local vasodilation and inhibits platelet aggregation, monocyte adhesion and vascular smooth muscle cell proliferation [23]. Inhibition of NO leads to vasoconstriction, vascular thrombosis, endothelial inflammation and interstitial monocyte and macrophage infiltration. Inhibition of NO can lead to many of the same early endothelial cell changes that initiate the processes that begets chronic rejection. In fact, it has been demonstrated that down-regulation of inducible nitric oxide synthase (iNOS) has been associated with the development of chronic rejection in a variety of experimental transplant models [24]. LDL-cholesterol itself may be one of the factors that can down-regulate iNOS expression. Conversely, lovastatin and simvastatin have been shown to reverse the LDL-induced down-regulation of iNOS expression [25]. Thus, HRIs may stabilize allograft vascular endothelium, even in the presence of potentially injurious events, by re-establishing the production of NO either directly or through reducing the concentration of LDL-cholesterol.
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CONCLUSIONS HMG-CoA reductase inhibitors (HRIs) have been demonstrated to decrease the incidence and progression of chronic rejection in animal models and in human studies. HRIs inhibit normal T-cell, B-cell and monocyte activity. Various properties of these lymphoid cells are critical to the development of chronic rejection. HRIs also reduce smooth muscle cell migration and proliferation in vivo and in vitro. This action could also have a pronounced inhibitory effect on chronic rejection. HRIs reduce lipid levels in the post-transplant setting, but it is unclear as to whether it is the lipid reduction or another property of HRIs that is responsible for any effect on chronic rejection. Although no conclusive data exist proving that HRIs directly inhibit the development of chronic rejection, sufficient evidence exists for transplant clinicians to consider the use of these agents in the posttransplant setting both for their antihyperlipidemic effects and their possible effects on both acute and chronic rejection.
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Reprint requests to Steven Katznelson, M.D., Department of Transplantation, California Pacific Medical Center, 2340 Clay Street, Suite 251, San Francisco, California 94115, USA. E-mail: [email protected]
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