=Paper=
{{Paper
|id=Vol-2692/paper6
|storemode=property
|title=ChemoVerse: Manifold Traversal of Latent Spaces for Novel Molecule Discovery
|pdfUrl=https://ceur-ws.org/Vol-2692/paper6.pdf
|volume=Vol-2692
|authors=Harshdeep Singh, Nicholas McCarthy, Qurrat Ul Ain, Jer Hayes
|dblpUrl=https://dblp.org/rec/conf/ecai/SinghMAH20
}}
==ChemoVerse: Manifold Traversal of Latent Spaces for Novel Molecule Discovery==
https://ceur-ws.org/Vol-2692/paper6.pdf
ChemoVerse: Manifold traversal of latent spaces for novel molecule discovery
Harshdeep Singh1∗ , Nicholas McCarthy2 , Qurrat Ul Ain3 , Jeremiah Hayes2
1
Swiss Federal Institute of Technology, Lausanne, 2 Accenture Labs, Dublin 3 AI Innovation Lab, DSAI,
Novartis
harshdeep.harshdeep@epfl.ch, {nicholas.mccarthy,jer.hayes}@accenture.com,
qurrat.ul ain@novartis.com
Abstract embed the chemical properties of such into a vector space. By
decoding from this ’latent’ space of chemical structure we can
In order to design a more potent and effective generate new, previously unidentified chemical compounds.
chemical entity, it is essential to identify molecu- In the domain of computational chemistry, Simplified
lar structures with the desired chemical properties. molecular-input line-entry system or SMILES are a com-
Recent advances in generative models using neural mon string textual method for encoding and representation of
networks and machine learning are being widely molecular structures [Anderson et al., 1987]. This facilitates
used by many emerging startups and researchers the use of models more commonly used in natural language
in this domain to design virtual libraries of drug- processing. Therefore, SMILES strings have been used as
like compounds. Although these models can help raw input strings to generative models, which are given the
a scientist to produce novel molecular structures task of encoding and decoding the SMILES string directly
rapidly, the challenge still exists in the intelligent [Anderson et al., 1987]. Advances were made by employing
exploration of the latent spaces of generative mod- variational autoencoders (VAE), a neural network comprised
els, thereby reducing the randomness in the gen- of an encoder that transforms a compound’s representation
erative procedure. In this work we present a mani- into a compressed latent space, and a decoder that gener-
fold traversal with heuristic search to explore the la- ates compounds from the latent space [Kingma and Welling,
tent chemical space. Different heuristics and scores 2013]. Although, directed search of the resulting latent space
such as the Tanimoto coefficient, synthetic accessi- is difficult. To counter this, conditional variational autoen-
bility, binding activity, and QED drug-likeness can coders (CVAE)[Kang and Cho, 2018] were used in order
be incorporated to increase the validity and proxim- to facilitate the generation of new molecules with specified
ity for desired molecular properties of the generated molecular properties. This is achieved by incorporating the
molecules. For evaluating the manifold traversal molecular properties of a compound into the encoder layer
exploration, we produce the latent chemical space and helping in the generation of more drug-like molecules
using various generative models such as grammar [Lim et al., 2018]. Generative adversarial networks (GANs)
variational autoencoders (with and without atten- have also been applied in the same manner [Maziarka et al.,
tion) as they deal with the randomized generation 2020], and have been recently combined with reinforcement
and validity of compounds. With this novel traver- learning and graph representation of molecules to optimize
sal method, we are able to find more unseen com- the generation of molecules with specified molecular proper-
pounds and more specific regions to mine in the la- ties [De Cao and Kipf, 2018].
tent space. Finally, these components are brought
together in a simple platform allowing users to per- Discovery in the latent space generated by these models is
form search, visualization and selection of novel often performed using random sampling and linear interpola-
generated compounds. tion, primarily due to the ease of the implementation of these
methods. However, this is not suitable for most generative
models as their latent spaces are generally high dimensional
1 Introduction and sparse. While doing traversal, we will traverse regions
where the data is not very well represented. In other words,
Designing a new chemical entity is a time consuming, expen- this could lead to a ’dead zone’ as the space of molecular
sive, and error-prone task. Pharmaceutical companies invest samples in the training dataset are present only on a subset of
billions of dollars into screening vast libraries of chemical the latent space [White, 2016]. Hence, decoding a point from
compounds for hit and lead identification [Fleming, 2018]. the latent space will end up returning noisy or invalid results.
The past few years has seen the rise of deep generative models It can also be challenging to incorporate contextual domain
that can operate over large spaces of molecular structures and information during search, and as a result discovery of com-
∗
This work was performed during an internship programme at pounds with specific properties is often very inconsistent.
Accenture Labs, Dublin, Ireland. In this work we implemented various flavours of auto-
Copyright ©2020 for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0)
Figure 1: Visualization of the latent space as a K-dimensional tree, while traversing the latent manifold
of compounds used in the treatment of diabetes and lung cancer.
encoders as the generative model for producing sets of latent ther nine modified in order to represent the more complex
spaces, and in particular show that our novel implementation ChEMBL dataset.
of Grammar VAE [Kusner et al., 2017] with an additional at-
tention mechanism [Vaswani et al., 2017] is highly performa- 2.2 Latent Space Generation
tive, with a low rate of invalid molecules generated. We also Three models are implemented in our system: a VAE
introduce a novel manifold interpolation method employing [Kingma and Welling, 2013], a Grammar VAE [Kusner et al.,
the Riemannian metric [Arvanitidis et al., 2017] in conjunc- 2017], and a Grammar VAE with self-attention [Vaswani et
tion with a set of molecular property heuristics to perform al., 2017]. As our search algorithm is model agnostic, latent
directed search and interpolation of these latent spaces in or- spaces can be substituted with minimal effort. However, re-
der to design novel molecules with desired properties. This sults will vary depending on the underlying data, model archi-
combination of generation and exploration of latent space has tecture, and training parameters used to generate each latent
enabled us to not only design molecules which have not been space.
seen before but also to explore new regions of latent chemical The ChEMBL dataset is less standardized and contains
space where more potent chemical compounds may exist. more complex molecules, therefore we perform transfer
learning by initially training each model on the ZINC dataset
2 System Architecture for 50 epochs, then switching to the ChEMBL dataset for 50
epochs. Training, validation and test sets of 85%, 10%, and
In this section we describe the various components of our 5% respectively was used, with the test set comprised entirely
system architecture: generation of latent spaces and our al- of ChEMBL molecules. We use the Adam optimizer with
gorithm for manifold traversal. a learning rate scheduler that is instantiated after 15 epochs
with a factor of 0.1 (initialized at 0.001).
2.1 Data The encoder is comprised of three 1D convolutional layers
We used a dataset of 250,000 molecules drawn from the ZINC with filters of size 9, 10 and 11 respectively, while the de-
dataset [Irwin et al., 2012], and an additional 100,000 drawn coder is comprised of 3 gated recurrent units (GRU) of 501
from the ChEMBL dataset [Gaulton et al., 2017]. These units [Kusner et al., 2017]. The structural validity of the gen-
two datasets are comprised of commercially available drug erated compounds are checked using the open-source RDKit
molecules and have been used in related work using models library [Landrum, ]. Examples of test set compounds that
like variational autoencoders (VAEs) [Gómez-Bombarelli et have been encoded and decoded are shown in table 1.
al., 2018]. Molecules are represented in canonical SMILES As noted in the original Grammar VAE work [Kusner et
string format, and are further processed into 1) a one-hot char- al., 2017] the vanilla VAE architectures encoding SMILES
acter encoding and 2) a set of context-free grammar (CFG) strings directly generally produce a very low valid decode
rules. Grammar rules are obtained from the OpenSMILES rate on larger molecular datasets - just 17% using a condi-
specification [James and Dalke, 2016], which denotes how tional VAE under their bayesian optimization search method-
the SMILES representation was formed based on the rules. ology. By instead using a Grammar VAE to generate produc-
This context free grammar (CFG) consists of 76 production tion rules of a grammar instead of SMILES strings directly a
rules, to which an additional seven are added, and a fur- much higher rate of valid compounds was attained. However,
Figure 2: Visualization of the path in the latent space along which our algorithm is interpolating. On the
left are generated compounds not present in the training data. On the right are histograms of different
molecular properties, including synthetic accessibility, activity scores.
Algorithm 1 Interpolation with manifold traversal sume that the latent space is Euclidean and flattened out, and
Input: latent space L, source and destination points s and d, generally produce noisier results [Arvanitidis et al., 2017].
points of interest R, path length m In our algorithm, source and destination points are selected
Parameter: k heuristics and associated edge weights W k in the latent space; these can either be latent space encod-
Output: a path of m equidistant points from s to d ing of single molecules, or cluster centroids of molecules la-
for each reference point i in R do beled with a desired property. Points of interest are the set
Find n nearest neighbors of i → Ni of known molecules with desired properties, for example all
for each (i, j), where j ∈ Ni do molecules used in treatment of a specific condition. The goal
1. Calculate Jacobian matrix Jij of decoder output (dec)
is to define a path from source to destination points in the
w.r.t to latent space (L) ∂dec(L
∂Lij
i j)
latent space through regions of interest, factoring in any addi-
k
2. Obtain k heuristic distances Hij between the 2 points tional user-specified heuristics such as synthetic accessibility,
3. Determine path edge weight based on Jacobian and binding activity, or drug-likeness that will augment generated
PN P k k
heuristic: Pij ← ij Jij + k Hij · W and store in a molecules.
k-d tree. Interpolation is performed by first calculating the Jacobian
end for distances for all points of interest. This helps us to understand
Apply a path finding algorithm such as Yen’s algorithm (with A*) how much each latent space point differs from another based
to determine a path from s to d on the k-d tree. on the representation learnt by the model, and understand the
Segment the path using m equidistant points.
stretching and rotational transformations of the local neigh-
return P ← (s, p1 , .., pm−2 , d)
borhood of each point with respect to other points.
A k-dimensional tree is built using the resulting Jacobian
as OpenSMILES is a context-free instead of regular gram- distances as edge weights between compounds in the points
mar it is still unable to model certain subtle characteristics of of interest. Therefore, placing compounds with greater struc-
the SMILES grammar such as paired ring bonds; for example tural similarity in closer proximity on the tree. A k-d tree is
the SMILES string ’c1ccccc1C2CCCC2’ would be decoded chosen since it divides the domain of search into half at each
as ’c1ccccc1C2CCCC’, incorrectly dropping the final paired level. Hence search for a node in the tree can be done in log-
digit. By incorporating a self-attention layer in the Grammar arithmic time and makes the data structure run time efficient.
VAE architecture this effect was mitigated, and increased the Edges can also be weighted by user-specified domain heuris-
validity of decoded test set molecules from 61% to 70%. tics, adding a weighted cost to augment the paths produced to
generate molecules more relevant for a specific target. These
2.3 Manifold Traversal of Latent Space heuristics are listed below:
Once a latent space has been generated we can apply our man- • Fingerprint Similarity: A fingerprint is a series of bi-
ifold traversal algorithm to generate interpolative paths in the nary digits (bits) that represent the presence or absence
latent space in order to decode molecules with desired prop- of particular substructures in the molecule. The simi-
erties. A common approach here is to use linear or spheri- larity can be tested using cosine or Tanimoto distance
cal interpolation [White, 2016], however both approaches as- metrics. The Tanimoto similarity takes into account the
Table 1: Input and corresponding decoded SMILES strings of test set compounds generated by the model. Tanimoto similarity is used to
measure the the structural similarity. An observation that can be made here is that small changes in the generated structure w.r.t to actual
structure might lead to disproportionate changes in the similarity.
Actual Compound Generated compound Similarity
OC(=O)CSc1oc2ccccc2n1 OC(=O)CCS1cc2ccccs2n1 0.141
CC(=O)C(=Cc1ccccc1)c2ccccc2 O.CCCN(Cc1ccccc1c2)cccnn2 0.171
Cc1occc1C(=O)NCc2onc(n2)c3ccccc3 Nc1nnc1NC(=O)NCSc2nn(s2)c3ccccc3 0.221
Cc1ccc(SCC(=O)NNC(=O)c2cncc(Br)c2)cc1 Clc1ccc(SCC(=O)NC(=O)c2cncc(Br)c2cc1)CF 0.373
CCc1ccc(cc1)N(CC(=O)N2CCOCC2)S(=O)(=O)C CSc1ccc(cc1N(C(=O)Oc2CCCCC2S(=O)(=O)))ON 0.235
Table 2: Samples of the compounds generated while interpolating on the latent space path as shown in Figure 2. These compounds are not
present in the train/test datasets, and we assign a potential target label based on labels of their nearest neighbours in the test set. SAS score
indicates if a molecule is difficult to synthesize. According to the Lipinski’s rule of five [Benet et al., 2016], the molecular mass for an active
drug should be less than 500 daltons. Activity range demonstrates the binding activity of a compound: the greater the range more active it is.
Generated compound Activity range SAS Molecular Potential Label
Score weight
CS1(C2)CC(C(C3C(C))C)[C@]1n1C2n1C3C4CC4C Less than 5 6.186 299.89 DIABETES
CCC1(C)C(C)CCCCS1C(C1(O([C@+2](C1))))S Between 5 and 7 6.251 274.49 DIABETES
CSCCCC2(C=C(O))S(C=O)C2C1[C@@](C1(C))CC Between 5 and 7 5.990 301.49 DIABETES
CC1(N(S))NN1CCCNc1ncnc2CCn2c1CC Greater than 7 5.233 299.89 LUNG CANCER
CS1(C2)CC(C(C3C(C))C)[C@]1n1C2n1C3C4CC4Cl Less than 5 6.186 309.44 LUNG CANCER
CCCC1CCC(S)N(C1)C(O)NCC Less than 5 4.355 232.93 DIABETES
structural properties whereas cosine does not. decoded along the path of centroids generated just 3 com-
pounds with valid structures. On the contrary, applying man-
• Synthetic Accessibility (SA Score): A molecule syn-
ifold traversal with fingerprint similarity and Yen’s algorithm
thetic accessibility is a score which is between 1 (easy to
as heuristic and perturbing the source and destination points
produce) and 10 (very difficult to produce). This is cal-
produced 4 different paths. These four paths generated a total
culated based on fragment contributions and molecule
of 156 valid, novel compounds along the interpolated mani-
complexity. The absolute difference between two
fold between the latent regions of diabetes and lung cancer
molecules is taken into account.
labeled molecules. Samples of these generated compounds
• Drug-likeliness: This is a score which takes into account can be seen in Table 2. Specific regions to mine within the
if the molecule is ’drug-like’. This is evaluated using latent space can be found by plotting different paths, bound
several parameters such as molecular weight, solubility them and finding the overlap region where compounds with
in water or lipophilic efficiency. The absolute difference the right structure and specific characteristics can be found.
between two molecules is taken into account.
• Binding activity: This demonstrates the potency to a tar- 3 Conclusions and Future Work
get for a potential drug compound; less than 5 is consid- In this work we presented a model-agnostic platform for per-
ered inactive, 5-7 of intermediate activity, greater than 7 forming manifold traversal on latent spaces with user speci-
active. fied domain heuristics. This interpolation method allows us
Yen’s algorithm [Yen, 1971] in combination with the A* to add more context and direction to search and discovery of
algorithm is then applied on the k-d tree to find the short- molecules in the latent space. Methods for exploration of la-
est path from source to destination given the user constraints. tent spaces generated from datasets of millions of molecules
Once the shortest path is found, we interpolate along this path provide an extremely valuable tool for virtual drug screening,
equidistantly and decode the points on the latent space using and an ability to facilitate rapid drug discovery.
the generator to generate compounds. Multiple paths can be Some avenues for future work in this domain include: im-
found by taking into account the shortest path and either per- plementation of alternative models to produce latent-spaces
turbing it or by changing the number of interpolation points of various characteristics; more sophisticated methods for
between the source and the destination, and intuitively this curve fitting in high dimensional spaces such as Bézier curves
increases the overall number of novel generated compounds. and Gaussian regression; alternative search methods such as
using evolutionary and genetic algorithms on the latent space.
Manifold traversal is inherently more useful than linear or
Future work would also focus on implementing latent space
spherical interpolation as it gives users greater flexibility in
evaluation metrics using this interpolation method to under-
path exploration under various conditions, and empirically
stand the underlying aspects of these spaces.
demonstrates a much higher rate of valid decoded molecules.
For example, when considering the diabetes and lung can-
cer centroids; linear interpolation with 100 equidistant points
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