  Modeling the interactions between drugs, targets, and diseases is paramount
in drug discovery and has significant implications for precision medicine and
personalized treatments. Current approaches frequently consider drug-target or
drug-disease interactions individually, ignoring the interdependencies among
all three entities. Within human metabolic systems, drugs interact with protein
targets in cells, influencing target activities and subsequently impacting
biological pathways to promote healthy functions and treat diseases. Moving
beyond binary relationships and exploring tighter triple relationships is
essential to understanding drugs' mechanism of action (MoAs). Moreover,
identifying the heterogeneity of drugs, targets, and diseases, along with their
distinct characteristics, is critical to model these complex interactions
appropriately. To address these challenges, we effectively model the
interconnectedness of all entities in a heterogeneous graph and develop a novel
Heterogeneous Graph Triplet Attention Network (\texttt{HeTriNet}).
\texttt{HeTriNet} introduces a novel triplet attention mechanism within this
heterogeneous graph structure. Beyond pairwise attention as the importance of
an entity for the other one, we define triplet attention to model the
importance of pairs for entities in the drug-target-disease triplet prediction
problem. Experimental results on real-world datasets show that
\texttt{HeTriNet} outperforms several baselines, demonstrating its remarkable
proficiency in uncovering novel drug-target-disease relationships.
