  The discovery of functional molecules is an expensive and time-consuming
process, exemplified by the rising costs of small molecule therapeutic
discovery. One class of techniques of growing interest for early-stage drug
discovery is de novo molecular generation and optimization, catalyzed by the
development of new deep learning approaches. These techniques can suggest novel
molecular structures intended to maximize a multi-objective function, e.g.,
suitability as a therapeutic against a particular target, without relying on
brute-force exploration of a chemical space. However, the utility of these
approaches is stymied by ignorance of synthesizability. To highlight the
severity of this issue, we use a data-driven computer-aided synthesis planning
program to quantify how often molecules proposed by state-of-the-art generative
models cannot be readily synthesized. Our analysis demonstrates that there are
several tasks for which these models generate unrealistic molecular structures
despite performing well on popular quantitative benchmarks. Synthetic
complexity heuristics can successfully bias generation toward
synthetically-tractable chemical space, although doing so necessarily detracts
from the primary objective. This analysis suggests that to improve the utility
of these models in real discovery workflows, new algorithm development is
warranted.
