code: def _get_drug_target_interactions(manager: Optional['bio2bel_drugbank.manager'] = None) -> Mapping[str, List[str]]:
      
      if manager is None:
          import bio2bel_drugbank
          manager = bio2bel_drugbank.Manager()
  
      if not manager.is_populated():
          manager.populate()
  
      return manager.get_drug_to_hgnc_symbols()
docs: Get a mapping from drugs to their list of gene.