Table 2. Rate constants for binding to FcRn.
| ka1 (1/Ms) | kd1 (1/s) | KD1 (M) | ka2 (1/Ms) | kd2 (1/s) | KD2 (M) | |
|---|---|---|---|---|---|---|
| Wild-type IgG4 | 3.11E+04 | 1.14E-02 | 3.67E-07 | 1.54E+05 | 2.56E-06 | 1.66E-11 |
| Monovalent IgG4 | 7.84E+04 | 3.87E-02 | 4.94E-07 | 2.28E+04 | 1.37E-02 | 6.01E-07 |
Biacore data for wild-type IgG4 and monovalent IgG4 binding to mouse FcRn was fit to a heterogeneous ligand model. This assumes there is both a low and high affinity binding site on FcRn to which an antibody can bind. The half-antibody had a greater dissociation rate constant (kd2) compared with the wild-type IgG4 by ~1800 fold and a greater association rate constant (ka2) by 7-fold, resulting in a decreased affinity (KD2) of 601 nM for the half antibody vs. 16.6 pM for the wild-type IgG4.