Why is cytosine a critical upstream material for lamivudine and other antiviral APIs?

Why is cytosine a critical upstream material for lamivudine and other antiviral APIs?
Cytosine is a key starting material (KSM) for lamivudine, an antiviral drug, and its supply chain is heavily concentrated in China. Indian manufacturers rarely produce cytosine themselves, forcing them to import from China, which gives Chinese producers significant pricing power in the global market. The cost advantage is stark: producing a drug in China is typically 20% cheaper than in India, largely due to lower raw material costs, which can account for up to two-thirds of total production expenses. This dynamic has made cytosine a strategic intermediate in the antiviral API supply chain, with China's dominance reinforced by large-scale production capacity and regulatory support for the API industry. Any disruption in Chinese cytosine output, such as a plant shutdown, can trigger global price spikes for downstream drugs like lamivudine.