All compounds were initially analyzed using CHI-IAM chromatography and a Caco-2 assay to evaluate the passive membrane permeability. A good correlation between these two assays was observed and some correlation with the cLogP values of the compounds as well. The two most permeable compounds in the test set (according to Caco-2) were the smallest macrocycles with either a pyridine (SA-B1) or a benzene (SA-B3) incorporated into the backbone. The increased polarity of the N-oxide (SA-B2) clearly interfered with membrane permeability reducing both the CHI-IAM and Caco-2 values. From the CHI-IAM and Caco-2 data it also becomes apparent that backbone flexibility is a major factor in membrane permeability. Extension of the macrocycle with a single carbon atom on either side of the aromatic group completely disrupted the permeability, irrespective of the nature of the aromatic group. The correlation between increased size and reduced permeability has been observed by others for various model systems. Similar effects were previously observed for peptides with an aromatic group in their backbones used as NV 3CL protease inhibitors. Here, the largest oxadiazole modified macrocycle was most active in a biochemical assay, but less active than analogues with smaller macrocycle sizes in cellular activity assays. The actual cell permeability was not evaluated, making it somewhat hard to draw an accurate conclusion.