Spontaneous inhibitors of factor VIII: Kinetics of inactivation of human and porcine factor VIII

Autoantibodies to factor VIII (FVIII)(spontaneous inhibitors) often inactivate FVIII in a complex fashion (type II inhibitors) as compared with alloantibodies (hemophilic inhibitors), which usually demonstrate second-order reaction kinetics (type I inhibitors). The infusion of porcine FVIII in patients with spontaneous inhibitors may give rise to anti-porcine FVIII antibodies. The purpose of this study was to determine whether these were type I or type II inhibitors. Plasma from 8 patients with spontaneous inhibitors and 6 patients with hemophilia with inhibitors were studied. Equal volumes of patient plasma and either pooled normal human plasma or porcine Frill (Hyate-C) were incubated at 37°C for 90 minutes. Aliquots were removed immediately after mixing and at 30-minute intervals and assayed for Frill by using a two-stage method. The values for residual FVIII were log-transformed and plotted against the time of incubation, and the resultant curves were analyzed for goodness of fit (coefficient of determination, r²) by using linear and exponential equations. The values were examined by paired t tests; P values were two-tailed. Values are expressed as mean ± SD. The titers of spontaneous inhibitors against human FVIII ranged from 2.6 to 416 Bethesda Units (BU), and those against porcine FVIII ranged from 0.7 to 47 BU. Samples were diluted so that the FVIII levels in the mixtures before incubation were similar: human, 0.44 U/ml; porcine, 0.47 U/ml; P = not significant. Four of the 8 patients with spontaneous inhibitors inactivated human Frill in a fashion consistent with complex kinetics; their r² values with the linear equation were less than 0.90. All r² values improved when the exponential equation was used (linear, 0.90 ± 0.08; exponential, 0.92 ± 0.06; P = .007). In contrast, r² values with porcine FVIII were the same (0.94) with either the linear or the exponential equation (P = not significant). r² for the 6 hemophilic inhibitors showed no significant difference between the linear and exponential equations; the values were 0.99 with human FVIII and 0.95 with porcine FVIII. In non-hemophilic patients, antibodies developing to porcine FVIII have kinetics of inhibition that are second order (type I), even though antibodies to human Frill in these same patients may have complex (type II) kinetics.