The development of the late-phase manufacturing process of the drug candidate ZPL389 and the strategies for the control of impurities are outlined in detail. Selective salt formation at several stages was pivotal to controlling the process impurities. The extensive optimization of the N-methylation of a Boc-protected amine with dimethyl sulfate and of a nucleophilic aromatic substitution without the use of metal catalysts led to a robust, scalable process. The process was demonstrated on a >100 kg scale. Overall, improved drug substance quality, higher yield, and reduction of the process mass intensity were achieved.