
C. Drug-Drug Interactions
DrugCombDB [12] is a database containing drug-drug in-
teractions obtained from various sources, including external
databases, manual curations from PubMed literature and ex-
perimental results. We take the drug-drug interactions that have
been classified as exhibiting synergistic or antagonistic effects
from DrugCombDB
2
. For the 86 drugs used for suggestion,
we extract 97 drug pairs classified as having synergistic effects
and 243 drug pairs classified as having antagonistic effects
from the DrugCombDB database. Based on these drug-drug
interactions, the proposed decision support system obtains
better effectiveness by avoiding pairs with antagonistic effects
and promoting pairs with synergistic effects in medication
suggestions.
III. PROBLEM FORMULATION
In this paper, we design three modules for the proposed
decision support system: Drug-Drug Interaction (DDI) mod-
ule, Medical Decision (MD) module and Medical Support
(MS) module. In this section, we will first introduce the
generalized decision support system, then define the DDI
graph constructed in the DDI module, and give the definitions
of medical decision and medical support.
Definition 1 (Decision Support System): Given a set of drug
candidates denoted as V = {D
1
, D
2
, · · · , D
|V |
}, the decision
support system is designed to suggest a list of appropriate
drugs from V to a patient S
i
based on the patient features x
i
.
Definition 2 (Drug-Drug Interaction (DDI) Graph): We
define the drug-drug interaction (DDI) graph as G = (V, E),
where the node set V = {D
1
, D
2
, · · · , D
|V |
} denotes the
drugs and the edge set E denotes the synergistic or antagonis-
tic effects between drugs. An edge e
uv
= 1 in E represents
a synergistic effect between drugs D
u
and D
v
, and an edge
e
uv
= −1 in E represents an antagonistic effect.
Definition 3 (Medical Decision): The target of medical
decision is to identify the most effective combination of drugs
for the patients from a set of drug candidates. By representing
the relationship between patients and drugs as a bipartite
graph, we formulate the medical decision problem based on
the results of link prediction and further use the drug-drug
interactions as constraints to identify the appropriate drugs.
Specifically, suppose that we are given a set of observed
patients’ data denoted as O = {x
i
, y
i
}
m
i=1
, y
i
is a vector with
y
iv
= 1 if patient S
i
is taking drug D
v
and y
iv
= 0 otherwise,
m is the number of observed patients. The set of unobserved
patients’ data is denoted as U = {x
j
, y
j
}
n
j=m+1
, where n
is the number of all patients. Given the patient features x
j
,
the target of medical decision is first to predict the score
of each drug and then to suggest the most k reliable drugs
Q = {D
q 1
, D
q 2
, · · · , D
q k
} to the unobserved patient S
j
based
on drug-drug interactions.
Definition 4 (Medical Support): The target of medical
support is to find explainable factors through drug-drug inter-
2
http://drugcombdb.denglab.org/download/
actions for the k suggested drugs Q = {D
q 1
, D
q 2
, · · · , D
q k
}.
Specifically, given Q and the DDI graph G, we can find
a subgraph G
sub
of G containing all drug-drug interactions
associated with the suggested drugs, and thus can act as
medical support for the Medical Decision module.
IV. THE PROPOSED DSSDDI
Fig. 4 depicts the overall architecture of DSSDDI which
consists of three modules: Drug-Drug Interaction, Medical
Decision and Medical Support. In Drug-Drug Interaction, we
learn the drug relation representations. In Medical Decision,
we capture the causal relationship between DDI and medi-
cation use. In Medical Support, we generate the explanation
of the suggested drugs. In this section, we elaborate on each
module.
A. Drug-Drug Interaction Module
In the Drug-Drug Interaction (DDI) module, we first de-
velop a model, DDIGCN, to learn the drug representations.
The main idea is to learn drug relation features through
synergistic or antagonistic effects between drugs. In the fol-
lowing, we first describe how to construct the DDI graph, then
illustrate how to update the drug representations by DDIGCN,
and finally describe the model training process.
1) DDI Graph Construction: As described in Definition 2,
we construct the DDI graph G = (V, E) based on the data
collected from DrugCombDB [12] with Drug ID embedding
vectors z
v
for D
v
∈ V . To better capture the relation features
among drugs, we use one-hot ID embeddings instead of
pre-trained embeddings as the original features in this DDI
module. Besides synergistic and antagonistic effects, we add
the third type of edges between drugs in the DDI graph to
explicitly indicate that they have no interactions. Specifically,
we randomly sample drug pairs, denoted as D
u
and D
v
, from
V with no synergistic or antagonistic effect, and create an
edge e
uv
= 0 to represent the lack of interactions between
them. In this manner, DDIGCN can capture synergistic and
antagonistic drug-drug interactions as well as no interactions
in drug embeddings.
2) DDIGCN: In this step, we design a DDIGCN to update
the drug representations. We use Graph Isomorphism Network
(GIN) [13] as the backbone. The graph convolutional operation
is defined as:
z
(t)
v
= f
(t)
Θ
1
(1 +
(t)
) · z
(t−1)
v
+
P
u∈N
v
z
(t−1)
u
|N
v
|
!
, (1)
where z
(t)
v
denotes the updated hidden representation of drug
D
v
after t layers propagation, f
(t)
Θ
1
denotes the multi-layer
perceptrons (MLP) with parameters Θ
1
,
(t)
is a learnable
parameter of the t-th graph convolutional layer, N
v
denotes
the set of drugs that have interactions with drug D
v
.
Besides GIN, we also consider signed graph-based models,
such as SGCN [14], SiGAT [15] and SNEA [16] as alternative
backbones, as there are both positive and negative edges in the
DDI graph G. Take SGCN as an example, we denote B
v
(t) =
{D
u
|e
uv
= 1} and U
v
(t) = {D
u
|e
uv
= −1} for drug D
v
.
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