RESUMEN
We investigate the impact of displaced heavy-quark matching scales in a global fit. The heavy-quark matching scale µ m determines at which energy scale µ the QCD theory transitions from N F to N F + 1 in the variable flavor number scheme (VFNS) for the evolution of the parton distribution functions (PDFs) and strong coupling α S ( µ ) . We study the variation of the matching scales, and their impact on a global PDF fit of the combined HERA data. As the choice of the matching scale µ m effectively is a choice of scheme, this represents a theoretical uncertainty; ideally, we would like to see minimal dependence on this parameter. For the transition across the charm quark (from N F = 3 to 4), we find a large µ m = µ c dependence of the global fit χ 2 at NLO, but this is significantly reduced at NNLO. For the transition across the bottom quark (from N F = 4 to 5), we have a reduced µ m = µ b dependence of the χ 2 at both NLO and NNLO as compared to the charm. This feature is now implemented in xFitter 2.0.0, an open source QCD fit framework.
RESUMEN
Pharmacokinetic studies of therapeutic monoclonal antibodies necessitate the measurement of their biologically active fraction. The aim of this work was to develop an enzyme-linked immunosorbent assay (ELISA) for rituximab, a chimeric anti-CD20 monoclonal antibody, based on its binding to a 20-mer peptide (P20) derived from the extracellular loop of human CD20 (residues 165-184). Derivatives of P20 were prepared by conjugation to bovine serum albumin (BSA-P20ACM) or biotin (Biot-P20ACM). Interactions of P20 and its derived peptides with rituximab were analyzed by surface plasmon resonance (SPR) and by ELISA. A monoclonal anti-idiotype antibody (MB2A4) was used as the reference in each case. SPR analysis showed that P20 (conjugated or unconjugated) had a lower affinity for rituximab than MB2A4. ELISA methods based on P20 or MB2A4 were both highly accurate and reproducible for rituximab measurement in spiked samples, but the MB2A4-based assay had a lower limit of quantification. Interestingly, discrepant results were obtained with the two ELISA methods when analyzing pharmacokinetic samples, with the rituximab concentrations obtained with the MB2A4-based method being systematically higher than those determined by the P20-based method. Possible interference of circulating CD20 with the P20-based method was supported by competition experiments. Rituximab aggregation in the bloodstream may also account for the bias observed in samples from healthy mice. The P20-based ELISA is far less sensitive than the MB2A-based ELISA, thus limiting its utility for pharmacokinetic studies. However, the discrepancy observed between two different approaches for rituximab measurement indicates that data from different studies should be interpreted with care.