Isoform number I--a new tool to evaluate the quality of erythropoietin

The overall isoform number I, calculated from capillary zone electrophoresis (CZE) data, is introduced as a new parameter that enables the assessment of the quality of erythropoietin (EPO) in a simple, yet efficient manner. I is defined as the sum of the products of the individual CZE peak area percent shares (pn) of the EPO isoforms (n = 1-8) and the corresponding individual isoform numbers (n): I=p1x1+p2x2+p3x3+ p4x4+p5x5+p6x6+p7x7+p8x8 The results from an internal collaborative study were used to calculate I-numbers for 2 successive batches of EPO Biological Reference Preparations (BRPs) established by the European Pharmacopoeia (Ph. Eur.) Commission. Based on the results of 6 participating laboratories each, the I-numbers calculated for batch 1 (BRP1) and candidate batch 2 (cBRP2) were I = 518.7 +/- 3.5 (coefficient of variation (CV) = 0.7 %) and I = 542.4 +/- 2.2 (CV = 0.4 %) respectively. Thus, the I-numbers of the 2 EPO preparations clearly indicated the difference between BRP1 and cBRP2 described in the literature, but in a much more apparent and simple manner. Notably, one of the 7 laboratories participating in the study with cBRP2 yielded an outlier value of I = 525.6, pointing towards a suboptimal CZE analysis, and was therefore excluded from the cBRP2 mean value calculation. It is suggested that the overall isoform number I of erythropoietin provides a new and highly valid quality control measure that makes it possible to ensure the batch-to-batch consistency of the active pharmaceutical ingredient of EPO market products.