Key Starting Materials in Pharmaceutical Manufacturing: Regulatory Boundaries, Impurity Control, and Supply-Chain Resilience

Key starting materials (KSMs) sit at an awkward and consequential point in drug-substance manufacturing. They are usually bought as ordinary chemical intermediates, yet they mark the place where full good manufacturing practice (GMP) begins, they carry their own impurities into the active pharmaceutical ingredient (API) process, and they often depend on a handful of upstream suppliers that the API manufacturer has never audited. This review brings these three threads together. We summarise how ICH Q7 and Q11 define and bound the starting material, and why regulators question designations made too close to the final molecule. We then discuss impurity fate and purge, with attention to mutagenic, nitrosamine, azido, elemental and solvent-related risks, and outline a proportionate analytical control strategy illustrated with methods developed in our own laboratories. The supply-chain sections examine hidden multi-tier dependency, the limits of "China-plus-one" sourcing that stops at the API stage, petrochemical and logistics exposure, and recent policy responses in India, Europe and the United States. We close with practical measures (risk-based supplier qualification, dual sourcing with real technical comparability, strategic inventory, backward integration, continuous-flow manufacturing and digital visibility) and a five-stage governance framework that treats KSM management as a shared quality, regulatory, technical and supply-chain responsibility rather than a purchasing task.