The design (Figure ) is based on Enamine's in-stock fragment library in a process similar to the development of the F2X-Library. As all biophysical screening methods require a certain degree of solubility, the logS based on the dominant protonation state at pH 7.4 was calculated (MOE . A logS of minus three, corresponding to 1 mM/L, was set as the minimum, and all compounds below that threshold were removed. Next, the fragments were analysed regarding their "Rule of Three" compliance. As this rule is the FBDD analogue to Lipinski's famous "Rule of Five", the same guidelines and limitations apply. Thereby, the RO3 was taken as an estimate of developability, and all fragments with more than one violation were removed. All fragments not passing the PAINS , BRENK , NIH or ZINC filters were sorted out to prevent artefacts and false positive results during screening and to ease further development.