How is pX calculated in Reaxys?

In the Reaxys heatmap, pX values provide an interactive matrix that summarizes affinity for a large number of compound–target pairings and can be used to explore factors that contribute to affinity or find interesting activity hotspots.

The pX is a standardized measure of affinity calculated by normalization of different metrics, e.g. IC₅₀, K_i % inhibition, % stimulation, etc., which is excerpted from original literature. This normalization makes it possible to search for active compounds regardless of metric reported. To calculate pX value, a single value of concentration of the tested compound is necessary. In case a range of concentration, value cannot be used.

In case the concentration-based parameter is available like Ki or IC₅₀ then the following equations are used to calculate pX:

pX=pKi

pX=pIC₅₀

pX= -log₁₀(IC₅₀)

pX= -log₁₀(EC₅₀)

Notes:

• Non-active values are translated into pX=1.
• If value is described with a modulator like <<, <, and if the value is <5 then the given pX will be 1. Otherwise, pX values <1 or >15 are not provided.

How

In case of percentage metric parameter, we use the Michaelis equation and pX normalization is done in two steps:

1. The reported metric parameter is converted into related concentration-based parameter:
   For example: % inhibition is converted into IC₅₀ or % stimulation, % activity or activity in % (unit) are converted into EC₅₀ using the formula: 
   Equation 1: or
   Depending on whether reported metric is available as a single value or range of values, the reported single value or calculated average value of the metric is used to calculate the concentration-based parameter like IC₅₀ or EC₅₀
   The concentration-based parameter is then normalized into a single unit, i.e. if concentration values are expressed in Weight/volume (like g/l) or in ppm, ppb, ppt, they are converted in molar (M), using molecular weight converting table.
2. The pX value is calculated as the -log₁₀ of the concentration-based parameter using the below equation and assumptions:
   Equation 2: pX= -log₁₀(IC₅₀) or pX= -log₁₀(EC₅₀)

   If % metric (or metricaverage) <25	pX=1
   If 25 ≤ % metric (or metricaverage) <95	pX = calculated using equation 2
   If % metric (or metricaverage) ≥ 95 % metric (or metricaverage) =95	pX = calculated using equation 2

   
   Where metric may be example, % inhibition, % stimulation, % activity or activity in %