Testing the Limits of SMILES-based De Novo Molecular Generation with Curriculum and Deep Reinforcement Learning

1Deep reinforcement learning methods have been shown to be potentially powerful tools for de novo design. Recurrent neural network (RNN)-based techniques are the most widely used methods in this space. In this work, we examine the behaviour of RNN-based methods when there are few (or no) examples of molecules with the desired properties in the training data. We find that targeted molecular generation is often possible, but the diversity of generated molecules is often reduced, and it is not possible to control the composition of generated molecular sets. To help overcome these issues, we propose a new curriculum learning-inspired, recurrent Iterative Optimisation Procedure that enables the optimisation of generated molecules for seen and unseen molecular profiles and allows the user to control whether a molecular profile is explored or exploited. Using our method, we generate specific and diverse sets of molecules with up to 18 times more scaffolds than standard methods for the same sample size. However, our results also point to significant limitations of one-dimensional molecular representations as used in this space. We find that the success or failure of a given molecular optimisation problem depends on the choice of SMILES.