Database for drug-target relation
There are quite a lot drug databases. I think DGIbd is a good place to start (see below). Further there are a lot of databases which describe drugs and their targets, take a look at these (and the links at the bottom) hopefully you can use several of these in your implementation. 

SIDER


  SIDER contains information on marketed medicines and their recorded
  adverse drug reactions. The information is extracted from public
  documents and package inserts. The available information include side
  effect frequency, drug and side effect classifications as well as
  links to further information, for example drug–target relations.




DrugCentral


  DrugCentral provides information on active ingredients chemical
  entities, pharmaceutical products, drug mode of action, indications,
  pharmacologic action. We monitor FDA, EMA, and PMDA for new drug
  approval on regular basis to ensure currency of the resource. Limited
  information on discontinued and drugs approved outside US is also
  available however regulatory approval information can't be verified.




Therapeutic Target Database


  Therapeutic Target Database (TTD) is a database to provide information
  about the known and explored therapeutic protein and nucleic acid
  targets, the targeted disease, pathway information and the
  corresponding drugs directed at each of these targets. Also included
  in this database are links to relevant databases containing
  information about target function, sequence, 3D structure, ligand
  binding properties, enzyme nomenclature and drug structure,
  therapeutic class, clinical development status. All information
  provided are fully referenced.
  




Potential drug target database (PDTD)
NOTE: I think this one is coupled to DrugBank however you can take a look here


  PDTD is a dual function database that associates an informatics
  database to a structural database of known and potential drug targets.
  PDTD is a comprehensive, web-accessible database of drug targets, and
  focuses on those drug targets with known 3D-structures.




PharmGKB


  The PharmGKB is a pharmacogenomics knowledge resource that encompasses
  clinical information including dosing guidelines and drug labels,
  potentially clinically actionable gene-drug associations and
  genotype-phenotype relationships. PharmGKB collects, curates and
  disseminates knowledge about the impact of human genetic variation on
  drug responses through the following activities: Annotate genetic
  variants and gene-drug-disease relationships via literature reviews
  Summarize important pharmacogenomic genes, associations between
  genetic variants and drugs, and drug pathways Curate FDA drug labels
  containing pharmacogenomic information Enable consortia examining
  important questions in pharmacogenomics Curate and participate in
  writing pharmacogenomic-based drug dosing guidelines Contribute to
  clinical implementation projects for pharmacogenomics through
  collaborations Publish pharmacogenomic-based drug dosing guidelines,
  very important pharmacogene summaries and drug-centered pathways
  Display all information on the website and provide comprehensive
  downloads




STITCH


  To facilitate access to this data, STITCH (‘search tool for
  interactions of chemicals’) integrates information about interactions
  from metabolic pathways, crystal structures, binding experiments and
  drug–target relationships. Inferred information from phenotypic
  effects, text mining and chemical structure similarity is used to
  predict relations between chemicals. STITCH further allows exploring
  the network of chemical relations, also in the context of associated
  binding proteins. Each proposed interaction can be traced back to the
  original data sources. Our database contains interaction information
  for over 68 000 different chemicals, including 2200 drugs, and
  connects them to 1.5 million genes across 373 genomes and their
  interactions contained in the STRING database.




SuperTarget and Metador


  An easy-to-use query interface enables the user to pose complex
  queries, for example to find drugs that target a certain pathway,
  interacting drugs that are metabolized by the same cytochrome P450 or
  drugs that target the same protein but are metabolized by different
  enzymes. Furthermore, we provide tools for 2D drug screening and
  sequence comparison of the targets. The database contains more than
  2500 target proteins, which are annotated with about 7300 relations to
  1500 drugs; the vast majority of entries have pointers to the
  respective literature source. A subset of these drugs has been
  annotated with additional binding information and indirect
  interactions and is available as a separate resource called Matador.




Pharos


  Pharos is the user interface to the Knowledge Management Center (KMC)
  for the Illuminating the Druggable Genome (IDG) program funded by the
  National Institutes of Health (NIH) Common Fund. (Grant No.
  5U54CA189205-02). The goal of KMC is to develop a comprehensive,
  integrated knowledge-base for the Druggable Genome (DG) to illuminate
  the uncharacterized and/or poorly annotated portion of the DG,
  focusing on four of the most commonly drug-targeted protein families:
   G-protein-coupled receptors (GPCRs); nuclear receptors (NRs); ion
  channels (ICs); and kinases. For more information on opportunities in
  the druggable human genome see this poster




KEGG drug


  KEGG DRUG is a comprehensive drug information resource for approved
  drugs in Japan, USA, and Europe unified based on the chemical
  structures and/or the chemical components, and associated with target,
  metabolizing enzyme, and other molecular interaction network
  information. All the marketed drugs in Japan, not only the
  prescription drugs but also the OTC drugs, are fully represented in
  KEGG DRUG and integrated with the package insert information (labels
  information). These include crude drugs and TCM (Tradictional Chinese
  Medicine) drugs.




The IUPHAR/BPS Guide to PHARMACOLOGY (database)


  The information in the database is presented at two levels: the
  initial view or landing pages for each target family provide
  expert-curated overviews of the key properties and selective ligands
  and tool compounds available. For selected targets more detailed
  introductory chapters for each family are available along with curated
  information on the pharmacological, physiological, structural, genetic
  and pathophysiogical properties of each target. The database is
  enhanced with hyperlinks to additional information in other databases
  including Ensembl, UniProt, PubChem, ChEMBL and DrugBank, as well as
  curated chemical information and literature citations in PubMed.




DGIbd


  The druggable genome can be defined as the genes or gene products that
  are known or predicted to interact with drugs, ideally with a
  therapeutic benefit to the patient. Such genes are of particular
  interest to large-scale cancer profiling efforts such as TCGA, ICGC
  and others that identify lists of potential cancer driver genes from
  high-throughput sequence and other genome-wide data. In cancer
  therapy, the increasing number of targeted drugs--those designed to
  inactivate proteins carrying activating amino acid changes as
  determined by mutational analyses--make more compelling the need for a
  searchable database of drug-gene interactions, available here.




There are a lot more databases, maybe these links will help you:


Drug to Target/Gene/Protein Interaction Databases
Question: Database Of Drugs Targeting Genes
Drug Databases