INVESTIGATION OF THE ANTI-COMPLEMENT AGENTS, FUT-175 AND K76COOH, IN DISCORDANT XENOTRANSPLANTATION

We examined whether hyperacute rejection (HAR) of a discordant xenogra ft in a nonhuman primate model could be inhibited by the anticomplemen t agents, FUT-175 (FUT) and K76COOH (K76). The inhibitory effect of FU T and K76 on baboon sera was studied in vitro by i) complement-mediate d hemolysis of sheep erythrocytes (by measuring serum CH50) and ii) cy totoxicity to cultured pig kidney (PK15) cells. The in vivo administra tion of FUT (at 0.2-25 mg/kg/h i.v. continuously) and R76 (50 mg/kg i. v. bolus) allowed evaluation of the serum levels of these drugs. Both FUT and K76 inhibited serum CH50 in a concentration-dependent manner. An enhanced effect was obtained by combining K76 with FUT therapy. Hig h concentrations of FUT (>10(-4) M) and K76 (>10(3) mu g/ml) were nece ssary to suppress serum CH50 to <5% of the normal level. However, PK15 cytotoxicity remained at >50% in the presence of i) 10(-4) M of FUT, ii) 10(3) mu g/ml of K76, and iii) 10(-6) M of FUT + 10(3) mu g/ml of K76. Pig heart transplantation (HTX) was performed in two baboons rece iving FUT (1 mg/kg/h i.v. continuously) and K76 (at 200 mg/kg x1 or 40 0 mg/kg + 200 mg/kg x2 i.v, respectively). Cytotoxicity of the serum t o PK15 cells at the time of HTX showed 39% and 1% cell death, respecti vely, in these two baboons, and the CH50 level was 1% (of control leve l) and 0%, respectively. Graft survival was 4.5 hours and 10 hours (wi th death of the baboon), respectively (compared with a mean of 29 minu tes in control experiments). Both excised grafts showed typical featur es of hyperacute rejection. Immunopathological studies revealed deposi tion of C1q, C3d, C6, properdin, and Factor B, demonstrating that comp lement activation was not fully inhibited by FUT and K76. We conclude that i) FUT and K76 are indeed potent complement inhibitors, ii) the d osages of FUT and K76 necessary to suppress complement-mediated injury cannot be extrapolated from previously reported data obtained from se rum CH50 levels, and iii) higher (possibly toxic) dosages will be requ ired to inhibit complement activation completely. It seems unlikely th at HAR will be prevented by these drugs alone, although they may be be neficial when combined with other forms of therapy.