This seems wrong. I use druggable to describe biochemical inhibitors which have the potential to be modified into therapeutic drugs. — This comment was unsigned.
Do the Google book search results back up the part about "...especially by a small molecule drug."? Many genes can be targeted with either a small or large molecule. For example, HER2 with either Trastuzumab (aka. Herceptin) or Lapatinib. EGFR with Erlotinib or Cetuximab. Trastuzumab and Cetuximab are antibodies and therefore, large molecules. Lapatinib and Erlotinib are small molecules. However, there are some targets for which we currently only have large molecules. For example, CTLA4 and PD1 and PDL1 (very hot topics at the moment). The drugs to target these, Ipilumumab, Nivolumab and Pembrolizumab, are all large molecules. I would argue that it is monoclonal antibody technology which makes many things druggable which would not otherwise be. By the way, the ending -mab in a drug name typically stands for monoclonal antibody. While there are reasons that small molecule drugs should, in general, be preferred, in practice large molecules are still very important and popular. For example, Trastuzumab is preferred over Lapatinib because the ALTTO clinical trial showed it to be more effective. Cetuximab can still be used when EGFR mutant cancer becomes resistant to Erlotinib or Osimertinib or both. These alternatives make the target more druggable.