PREVENTION OF GRAFT VESSEL DISEASE BY COMBINED FTY720/CYCLOSPORINE A TREATMENT IN A RAT CAROTID ARTERY TRANSPLANTATION MODEL

Background. Graft vessel disease (GVD) is an important problem often responsible for late graft loss. The effects of FTY720, an immunomodulator with a novel mechanism of action were investigated in combination with cyclosporine A (CsA) in a carotid artery allograft model. Methods. A segment of the carotid artery of Lewis rats was replaced by a DA allograft. Seven groups of eight rats were treated for 8 weeks with vehicle (P), CsA 0.3 (C0.3), 1 (C1) or 3 (C3) mgzkg 21 .day 21 or a combination of CsA 1 with FTY 0.01 (C1F0.01), 0.03 (C1F0.03), and 0.1 (C1F0.1) mgzkg 21 .day 21 . Lumen area was estimated by magnetic resonance imaging, peripheral lymphocyte count and drug concentrations were determined at 1 and 8 weeks. Neointima, media, and lumen area were measured morphometrically. Intimal and adventitial infiltration of mononuclear cells, and medial smooth muscle cells number was assessed using a score. Results. FTY720 did not influence CsA blood concentrations. FTY720 but not CsA decreased the PLC dose dependently. Magnetic resonance imaging revealed that treatment groups have larger lumen size than group P. Histological and morphometric evaluation showed that all aspects of GVD were dose dependently suppressed by treatment and lumen narrowing was prevented. Conclusions. CsA, at clinically relevant blood levels, suppressed GVD only partly. The addition of FTY720 was well tolerated and completely suppressed GVD development. In vivo lumen size did not correlate with the histologically estimated neointimal thickness. Graft vessel disease (GVD) remains a major obstacle to long-term graft survival. Perivascular inflammation and concentric neointima formation, combined with medial thinning and necrosis, are observed in 40 ‐ 60% of grafts within the first 5 years of transplantation, frequently resulting in late graft loss (1, 2). Transplanted arteries, like vessels in solid organ grafts, develop concentric and generalized intimal thickening thus differing from classical atherosclerosis (3). Transplantation of blood vessels, especially of the aorta (3‐ 6), can be used to model GVD (7, 8). For mechanistic studies vascular grafts are preferred to solid organ transplants for investigating the specificity of the immunological vessel wall response without the complications of an immune response to parenchymal tissue (8) or parenchymal ischemia and reperfusion damage. We chose carotid artery to represent a small caliber artery reflecting the architecture and fate of smallersized arteries in solid organ transplants that develop intimal thickening, to investigate the suppression of immunologically induced GVD. Our previous investigations have shown that immunosuppression stronger than needed to maintain solid organ grafts, can prevent allograft vasculopathy (7). The main aim of our study was to investigate the contribution of FTY720 added to a low-dose CsA regimen. No results on the effects of FTY720, an agent with a completely new mechanism of action, on GVD have previously been reported. FTY720 [2-amino-2-(2-{4-octylphenyl}ethyl)-1,3-propanediol hydrochloride], is a new compound with a unique mechanism of action characterized by a reduction of the number of circulating lymphocytes. FTY720 accelerates the homing of lymphocytes to peripheral lymph nodes, mesenteric lymph nodes, and Peyer’s patches (9, 10). FTY720 has been shown to extend the survival of rat skin, heart, liver, and small bowel allografts (9, 11‐13), and in combination with CsA also allotransplants in rat, canine, and cynomolgus monkey models (11, 12, 14, 15). The purpose of the study was to analyze the effect of FTY720 in combination with a “reasonable” CsA dose on lesion development in transplanted carotid arteries. We based this approach on our previous experience with studies on lumen size after balloon-catheter injury (16, 17) and after simultaneous allogeneic and autologous transplantation of rat carotid arteries (7). We had observed that histological neointimal thickness did not correlate with in vivo lumen size measured by MRI, which may be functionally more relevant.