FK778 in Experimental Xenotransplantation: A Detailed Analysis of Drug Efficacy Sonja Schrepfer, MD, a,e Tobias Deuse, MD, a Friedrich Koch-Nolte, MD, PhD, b Thorsten Krieger, MD, b Munif Haddad, MD, c Hansjörg Schäfer, MD, PhD, d Marc P. Pelletier, MD, e Robert C. Robbins, MD, e and Hermann Reichenspurner, MD, PhD a Background: This study examines the efficacy of FK778 regimens for prevention of different phases of xenograft rejection. Methods: Antibody and complement tissue depositions were measured by immunofluorescence in a discor- dant ex vivo rat-to-human heart perfusion model of hyperacute rejection with immunosuppressant- enriched human blood. The concordant hamster-to-rat aortic xenotransplantation model was used to assess host cellular (lymphocyte activation, mixed lymphocyte reaction [MLR]) and humoral responsiveness (xenoantibody production) as well as histologic xenograft rejection. Recipients were treated for 14 days with FK778, tacrolimus, sirolimus or combination regimens at varying doses. Results: Antibody binding during hyperacute rejection was unaffected by the immunosuppressive treatment, but complement deposition was reduced in the following order: tacrolimus  FK778  sirolimus. FK778 most effectively reduced complement factor 5 in vitro. In untreated rats with hamster aortic xenografts, a large infiltrative response was observed within the grafts with extensive myocyte necrosis. Tacrolimus  FK778  sirolimus dose-dependently diminished xenograft infiltration and in the same order reduced vessel-wall myocyte necrosis. Tacrolimus  FK778  sirolimus reduced in vivo lymphocyte CD25 expression and tacrolimus  FK778  sirolimus diminished MLR. Xenoreactive IgM and IgG antibody production levels were vigorously upregulated a few days after transplantation, but were significantly reduced by tacrolimus  FK778  sirolimus. Combination regimens revealed no significant benefit when compared with the corresponding monotherapy groups. Conclusions: FK778 mildly interfered with hyperacute rejection and markedly suppressed acute humoral and cellular aortic xenograft rejection. However, T-cell– dependent host responses were most potently suppressed by tacrolimus, and the overall efficacy of FK778 was similar to that of sirolimus. J Heart Lung Transplant 2007;26:70 –7. Copyright © 2007 by the International Society for Heart and Lung Transplantation. Every year, an estimated 400,000 new cases of heart failure requiring treatment occur in the USA. 1 Although the only cure for end-stage heart failure is transplanta- tion, current estimates suggest that 5% of patients receive a heart transplant due to organ shortage. 2 The use of animal hearts for human recipients has the potential to resolve this problem, but the few clinical xenotransplantations performed so far have shown very poor results. 3 The first phase of xenorejection (hyperacute rejec- tion, HAR) is triggered by complement and xenoreac- tive natural antibodies (XNA), mainly directed against the galactose -1,3-galactose epitope (Gal). 4 Studies with recently engineered knockout pigs lacking the galactosyltransferase gene (GalT -/- ) or transgeneic donor animals expressing human complement inhibi- tors have proven, in the majority of cases, to be effective in overcoming HAR. 5–7 In situations where HAR is avoided, acute humoral xenograft rejection (AHXR) becomes apparent, which is characterized by antibody and complement deposition and results in vascular thrombosis. 8 Because there are indications of a From the a University Heart Center, b Institute of Immunology, c Insti- tute of Clinical Chemistry and d Department of Pathology, University Hospital Hamburg–Eppendorf, Hamburg, Germany; and e Department of Cardiothoracic Surgery, Stanford University Medical Center, Stan- ford, California. Submitted April 25, 2006; revised September 16, 2006; accepted October 19, 2006. Supported by a research grant (SCHR992/2-1) from the Deutsche Forschungsgemeinschaft (to S.S.). S.S. and T.D. share first co-authorship. Reprint requests: Tobias Deuse, MD, Department of Cardiac Sur- gery University Heart Center Hamburg, Martinistrasse 52, 20246 Hamburg, Germany Telephone: +49-40-42803-2440. Fax: +49-40- 42803-4931. E-mail: t.deuse@uke.uni-hamburg.de Copyright © 2007 by the International Society for Heart and Lung Transplantation. 1053-2498/07/$–see front matter. doi:10.1016/ j.healun.2006.10.013 70 XENOTRANSPLANTATION