International comparative field study of N8 evaluating factor VIII assay performance

Discrepancies between the one-stage clotting assay and the chromogenic method, and also among different variations of each method, have been a significant challenge for one B-domain deleted FVIII product. N8 is a B-domain truncated FVIII product developed by Novo Nordisk. The comparison of N8 and Advate (R) was performed in an international, multicentre, randomized and blinded field study of simulated postinfusion samples. Overall, Advate (R) and N8 performed similarly in the one-stage assay. In the one-stage clotting assay, the measured mean FVIII levels of Advate (R) vs. N8 were 0.046/0.047, 0.24/0.24, 0.58/0.60 and 0.82/0.83 IU mL(-1) for the target values of 0.03, 0.2, 0.6 and 0.9 IU mL(-1), respectively. In the chromogenic assays, the concentration estimates showed a tendency towards higher N8 values as compared with Advate (R); the measured FVIII levels of Advate (R) vs. N8 were 0.030/0.032, 0.22/0.24, 0.65/0.74 and 0.98/1.08 IU mL(-1) for the target values of 0.03, 0.2, 0.6 and 0.9 IU mL(-1), respectively. In the one-stage assays, the measured values were above 150% of target at the lowest concentration, decreasing to around 90% of target at the highest concentration. In contrast, the chromogenic assays were close to target at the lowest concentration and consistently above target at the three highest concentrations. Therefore, the ratio of chromogenic/one-stage potencies was concentration dependent, ranging from 0.66 to 1.30. The SSC plasma standard was similar in both. Assay variability was similar for both compounds. The results show that N8 can be reliably measured in plasma without the need for a separate N8 standard.