Abstract
Discovering (or planning) a new drug candidate involves many parameters, which makes this process slow, costly, and leading to failures at the end in some cases. In the last decades, we have witnessed a revolution in the computational area (hardware, software, large-scale computing, etc.), as well as an explosion in data generation (big data), which raises the need for more sophisticated algorithms to analyze this myriad of data. In this scenario, we can highlight the potentialities of artificial intelligence (AI) or computational intelligence (CI) as a powerful tool to analyze medicinal chemistry data. According to IEEE, computational intelligence involves the theory, the design, the application, and the development of biologically and linguistically motivated computational paradigms. In addition, CI encompasses three main methodologies: neural networks (NN), fuzzy systems, and evolutionary computation. In particular, artificial neural networks have been successfully applied in medicinal chemistry studies. A branch of the NN area that has attracted a lot of attention refers to deep learning (DL) due to its generalization power and ability to extract features from data. Therefore, in this mini-review we will briefly outline the present scope, advances, and challenges related to the use of DL in drug design and discovery, describing successful studies involving quantitative structure-activity relationships (QSAR) and virtual screening (VS) of databases containing thousands of compounds.
Artificial Intelligence and Machine Learning in Drug Discovery and Design
In the last decades, we have experienced a revolution in data science in terms of the huge amount of data to be analyzed (era of big data) and the availability of high-performance processors, in particular graphics processing unit (GPU) computing. In drug discovery, this scenario is not different: the large volume of data (chemical, biological, etc.) along with the automation of techniques have generated a fertile ground for the use of artificial (or computational) intelligence.
In medicinal chemistry, molecular features, such as physicochemical properties and other molecular descriptors can be related to the bioactivity level as molecular target affinity and to other pharmacokinetics properties (absorption, distribution, metabolism, and excretion) by using several computational methods. The results obtained by these techniques can assist the discovery and design of new drug candidates, if a suitable technique is chosen (Duch et al., ; Maltarollo et al., , ).
The autonomous knowledge acquisition from the molecular properties of compounds requires the use of machine learning (ML) techniques, such as k-nearest neighbors (kNN), partial least squares (PLS), and artificial neural networks (ANN) (Gertrudes et al., ; Lavecchia, ; Lima et al., ). ML is a branch of artificial intelligence (AI), focusing on giving computers the capability of learning from data, which is of great importance in drug discovery protocols nowadays. From IEEE, computational intelligence (CI) involves the theory, the design, the application, and the development of biologically and linguistically motivated computational paradigms. In addition, CI encompasses three main methodologies: neural networks (NN), fuzzy systems, and evolutionary computation. In particular, artificial neural networks have been successfully applied in medicinal chemistry. Among the methodologies comprised by CI, deep learning (DL) has attracted a lot of attention in several areas due to its generalization power and ability to extract features from data (Gawehn et al., ; Sharma and Sharma, ).
Deep Learning
Deep learning methods can be described as a class of representation-learning techniques that are able to discover, from the raw data, multiple-level of representations of increasing complexity by composing non-linear models. In this structure, each module in a level transform its input into a higher, more abstract representation. In this context, the term “deep” is associated to the number of layers in the network—the more layers, the deeper the network (Goodfellow et al., ). Two popular deep architectures are Deep Belief Networks (DBNs) (Hinton et al., ) and Convolutional Neural Networks (CNN) (LeCun et al., ). Multi-task learning for deep architectures has also been of great interest in many domain applications due to its ability to generalize predictive models to new scenarios (Zhang et al., ). Such approaches will be briefly described as follows.
Restricted Boltzmann Machines (RBMs) and Deep Belief Networks (DBNs)
A restricted Boltzmann machine (Smolensky, ) is a two-layer, undirected, bipartite graphical model where the first layer (or visible layer) consists of the input data, and the second layer (or hidden layer) consists of latent variables. In this architecture, there are no intra-layer connections within either the visible or the hidden layer and the connections between the layer nodes are bi-directional, so given an input vector we can obtain its latent feature representation and vice versa. In this sense, RBM is considered a generative model, which learns the probability distribution of input data and generates new data points.
DBNs (Hinton et al., ) are neural networks consisted by RBMs stacked where each layer encodes statistical dependencies among the units in the layer below it. In such architecture, an RBM uses the previous layer's activations as inputs. The training procedure aims at maximizing the likelihood of the training data and involves two consecutive processes: the training of the individual layers, which is done in an unsupervised manner and the final fine-tuning process that is accomplished by a linear classifier. An illustration of DBN structure is displayed in Figure 1.
Figure 1
Convolutional Neural Networks (CNNs)
Widely applied in computing vision tasks, CNNs (LeCun et al.,
Figure 2

Illustration of the structure of a standard CNN. In drug design, the input data could be molecular structures or atom distances from molecular graphs (adapted from Rawat and Wang,
Multi-Task Learning
Multi-task learning (Caruana,
DL in Medicinal Chemistry
In the drug discovery, DL has proved to be effective in handling large chemical libraries to provide predictive computational models, as well as modeling various properties of drug candidates, showing to be a highly relevant tool in virtual screening and quantitative structure-activity relationships (Pereira et al.,
DL has also been used to predict several different endpoints related to medicinal chemistry. From literature, we have found various studies on models to predict protein-ligand interactions, to score docking poses and to perform virtual screenings (Wallach et al.,
Ligand-Based Studies
Ligand-based approaches involve methodologies that used only information from the samples; in this case, chemicals with biological activity or other physicochemical property of interest (to be predicted). In general, molecular descriptors are used as input to construct models that relate chemical structure and biological activity. However, when DL is used for this purpose, graph representations of molecules (as graph convolution neural networks) are broadly employed.
An example of ligand-based study is performed by Lusci et al. (
Unterthiner et al. (
Xu et al. (
Kearnes et al. (
Altae-Tran et al. (
Ohue et al. (
Structure-Based Studies
Different from the previous approaches, structure-based techniques consider information from the samples and its molecular targets (for example, receptors, enzymes, and other structural or functional proteins). In this case, structure-based approaches can use interaction fingerprints (atom pairs from ligand-target complexes) and/or machine learning-based scoring functions to rank poses and hits (or classify the samples as active/inactive).
An example of this approach was performed by Wallach et al. (
Pereira et al. (
Ligand-target Interaction Prediction
Finally, in this last category of studies the main objective is to predict the binding affinity of samples (ligands) with a specific molecular target. For example, Tian et al. (
Wen et al. (
Rifaioglu et al. (
Tsubaki et al. (
As proposed by Lee et al. (
Conclusions
Due to the fast development of computing power and the generation of enormous amount of chemical and biological data, projects involving drug discovery have been benefited from artificial intelligence. Particularly, in the last decades, we have observed a significant increase in the number of studies using deep learning. Some applications of DL involve studies of quantitative structure-activity relationships (QSAR), virtual screening, drug repositioning and in silico prediction of pharmacokinectic properties (absorption, distribution, metabolism, and excretion–ADME) and toxicity. It is important to highlight that traditional techniques have been outperformed by DL in some applications related to drug design due some intrinsic characteristics of biological and chemical data, such as complexity, uncertainty, diversity, and high dimensionality. The main advantages of DL refer to the scale and the complexity of the neural networks used to build robust and predictive models, as well the flexibility in their architecture, allowing for adaptations to specific problems. Some drawbacks of applying DL include the limited number of data in some areas of study and the difficult interpretation of the chemical and biological mechanisms involved in the DL models. In summary, from the many applications of DL in drug design we can conclude that many advances have been observed in this area and new applications and methodologies have been developed every day, making this technique a reliable tool in the arsenal available for the discovery of new drug candidates.
Statements
Author contributions
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.
Acknowledgments
The authors would like to thank FAPESP-IBM (2016/18840-3), FAPESP (2016/24524-7), Pró-Reitoria de Pesquisa–Universidade de São Paulo (USP), CNPq, and CAPES for funding.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
artificial intelligence, deep learning, medicinal chemistry, drug design, drug discovery
Citation
Lipinski CF, Maltarollo VG, Oliveira PR, da Silva ABF and Honorio KM (2019) Advances and Perspectives in Applying Deep Learning for Drug Design and Discovery. Front. Robot. AI 6:108. doi: 10.3389/frobt.2019.00108
Received
06 May 2019
Accepted
11 October 2019
Published
05 November 2019
Volume
6 - 2019
Edited by
Akihiro Kishimoto, IBM Research, Ireland
Reviewed by
Patricia A. Vargas, Heriot-Watt University, United Kingdom; Masahito Ohue, Tokyo Institute of Technology, Japan
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© 2019 Lipinski, Maltarollo, Oliveira, da Silva and Honorio.
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*Correspondence: Kathia Maria Honorio kmhonorio@usp.br
This article was submitted to Computational Intelligence, a section of the journal Frontiers in Robotics and AI
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