Hemoglobin quantitative models

This example shows that (1) the mechanistic PK/PD model developed based on literature data of rHu-EPO and predinical information of a new ERA is suitable to provide a better quantification and prediction of the drug disposition and the time course of hemoglobin in adult healthy subjects, and (2) this model can be used to optimize the design of the Phase I studies of new ERAs, with respect to key design features (number of dose levels, selection of dose levels, number of subjects per dose level, PK/PD sampling times). In this way, a quantitative risk-benefit assessment can be obtained by determining the probability of success of a Phase I study with new ERA, conditional on a certain experimental design. 

To test the quantitative predictions of the model, we measured the disaggregation of the hemoglobin tetramer by the increase in osmotic pressure with pH (5). When the pressure is compared to that observed in solutions of bovine serum albumin (BSA a protein of comparable molecular weight that does not disaggregate), we can ascribe the difference in pressure to disaggregation. In the titration of both protein solutions, the minimum pressure occurs at the isoelectric point (IEP), which is 6.82 0.06 for hemoglobin at 25.0 °C. 

Addition of substrate, which here is synonymous to the allosteric effector, shifts the equilibrium from the low affinity T-form to the substantially more catalytically active R-form. Since one substrate molecule activates four catalytically active sites, the steep rise in enzyme activity after only a slight increase in substrate concentration is not unexpected. In this model it is important that the RT conformation is not permitted. All subunits must be in the same conformational state at one time to conserve the symmetry of the protomers. The equation given by Hill in 1913, derived from the sigmoidal absorption of oxygen by hemoglobin, is also suitable for a quantitative description of allosteric enzymes with sigmoidal behavior ... 

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