Transferrin (Tf) conjugates of CRM107 are currently being tested in clinical trials for treatment of malignant gliomas. However, the rapid cellular recycling of Tf limits its efficiency as a drug carrier. We have developed a mathematical model of the Tf/TfR trafficking cycle and have identified the Tf iron release rate as a previously unreported factor governing the degree of Tf cellular association. The release of iron from Tf is inhibited by replacing the synergistic carbonate anion with oxalate. Trafficking patterns for oxalate Tf and native Tf are compared by measuring their cellular association with HeLa cells. The amount of Tf associated with the cells is an average of 51% greater for oxalate Tf than for native Tf over a two hour period at Tf concentrations of 0.1 nM and 1 nM. Importantly, diphtheria toxin (DT) conjugates of oxalate Tf are more cytotoxic against HeLa cells than conjugates of native Tf. Conjugate IC 50 values were determined to be 0.06 nM for the oxalate Tf conjugate vs. 0.22 nM for the native Tf conjugate. Thus, we show that inhibition of Tf iron release improves the efficacy of Tf as a drug carrier through increased association with cells expressing TfR.

Fc mutants with increased binding affinity for the neonatal receptor, FcRn, exhibit increased half-lives in vivo, and represent an attractive means for extending the half-lives of therapeutic antibodies. The half-lives of other therapeutic molecules (e.g., proteins) may also be extended by conjugating them to Fc fragments, thus decreasing the frequency of patient injections and allowing the administration of low and potentially nontoxic concentrations of the therapeutics. To investigate the possibility for further increasing the half-life of Fc, a pair of quantitative methods is presented to complement combinatorial screening and in vivo testing. Specifically, a simple molecular modeling procedure was developed to predict relative Gibbs free energies of binding values (DeltaDeltaGbind) between Fc and FcRn across different mutants and species. This procedure was found to reasonably reproduce experimental DeltaDeltaGbind values from our experiments and the literature, and may be used as an initial screen to explore Fc sequence space more fully prior to experimental testing. In addition, a mathematical model of Fc trafficking was formulated and combined with a cell-level pulse-chase assay to obtain a quantitative recycling parameter in human T84 cells. This Fc recycling parameter was found to be correlated with binding affinity, but captures the pH dependent nature of the interaction between Fc and FcRn and may serve as an additional screen following combinatorial experiments.

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Although intracellular trafficking processes can play a central role in the physiological function of a protein, these same processes can also limit the benefit of the protein when it is taken out of its physiological context and used as a protein drug. Therefore, the properties of certain protein drugs may be improved by manipulating their trafficking pathways to suit their therapeutic function. A detailed consideration of the factors that govern how protein traffic is routed among different cellular destinations can be used to ascertain molecular design criteria for engineering a protein drug so as to alter its trafficking pathway in a beneficial manner. In this review, we summarize studies that have applied this approach to achieve the following three improvements in protein drug function: (1) half‐life extension of the Fc fragment of IgG, (2) half‐life extension of granulocyte colony‐stimulating factor, and (3) increase in cellular association of transferrin.

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