Size estimation of chemical space: how big is it?

OBJECTIVES: To estimate the size of organic chemical space and its sub-regions, i.e. drug-like chemical space and known drug space (KDS). METHODS: Analysis of the growth of organic compounds as a function of their carbon atoms based on a power function (f(x)=A×B, C=x) and an exponential function (f(x)=AeBx). Also, the statistical distribution of KDS and drug-like chemical space (drugs with good oral-bioavailability) based on their carbon atom count was used to deduce their size. KEY FINDINGS: The power function (f(x)=A×B, C=x) gives a superior fit to the growth of organic compounds leading to an estimate of 3.4×109 populating chemical space. KDS is predicted to be 2.0×106 molecules and drug-like chemical space is calculated to be 1.1×106 compounds. CONCLUSIONS: The values here are much smaller than previously reported. However, the numbers are large but not astronomical. A clear rationale on how we reach these numbers is given, which hopefully will lead to more refined predictions.