The slope of the curve is problematic to interpret in a cell-based assay. In an enzyme assay, a slope of greater than 1 indicates either a cooperative effect of multiple inhibitor molecules binding at the same time to the enzyme, or non-specific inhibition, or time-dependent inhibition, or an IC50 close to the enzyme concentration (a/k/a tight binding), or interference in the detection system, or insolubility of the compound.

In addition to these possibilities, a slope greater than 1 in a cell-based assay could arise because the untreated cells go through multiple cell cycles whereas the treated cells don't. Therefore, it is no simple matter to interpret the slope of the IC50 curve, and it can not necessarily be taken as an indicator of potency of the inhibitor.