Optimization of Affibody Affinity to Improve Distribution in Tumor Models.

Poor distribution is often a limiting factor when using affinity targeted treatments. High affinity creates a barrier to transport because proteins cannot diffuse throughout the tissue while bound to the cell surface. Lowering the affinity can help with transport throughout a tumor but can also result in a loss of total delivery and specificity for the target. In this research, we used point mutations to create affibodies with a range of different affinities for epidermal growth factor receptor (EGFR) and measured their penetration through tumor spheroids. Mutants in the range of ∼7-12 nM KD were able to distribute more evenly throughout 3D tumor spheroids without losing total accumulation or receptor specificity. Lower affinity mutants showed both low target specificity and accumulation in 3D tumor spheroids. This work shows the effects of affinity on distribution and how slightly reducing binding affinity can improve transport.