The data so far establish FerroTACs act as degraders through ternary complex formation, with ferrocene exhibiting high cellular tolerance. We next turned our attention to the hypothesis that the rotational flexibility of ferrocene incorporated into FerroTACsinitially evaluated by NMR studiescan increase the chameleonic behaviour, thus allowing for improved cellular permeability. To this end, we assessed cellular permeability by evaluating VHL target engagement using the Promega NanoBRET assay in live and permeabilised cells. The best-performing degraders for each system (AS2, AS4 and AS6) were selected and studied in comparison to the reference Fc-free PROTACs (CM11, 14a and MZ1, respectively). Specifically in this assay, compounds with lower intracellular availability show a greater right-shift in potency in live-cell mode compared to permeabilised-cell mode. In contrast, highly permeable compounds exhibit similar potency across both conditions. The ratio of potencies between live-cell and permeabilised-cell modes, referred to as the relativebinding affinity (RBA) value increases as intracellular availability decreases and is thus a metric for assessing a PROTAC`s cellular availability. To facilitate this assessment, the permeable control compound VH298 was included to standardise the RBA value, generating an availability index (AI) for each PROTAC. compounds and fitted accordingly. Data points represent the mean ± SEM from three independent experiments. B) Tabulated data of IC50 (live and permeabilised), relative binding affinity (RBA), and availability index (AI). C) Graphical representation of the IC50 live/permeabilised ratio for FerroTACs (empty symbols) and reference compounds (solid symbols), with the diagonal line indicating AI = 1.