Pharmaceutical composition not containing antioxidant and preparation method therefor

A method of preparing an antioxidant-free pharmaceutical composition, comprising:

• (a) preparing an antioxidant-free solution with dissolved oxygen comprising pemetrexed or a pharmaceutically acceptable salt thereof, and an aqueous solvent in a container;
• (b) freezing the antioxidant-free solution in the container to produce a frozen product;
• and
• (c) degassing the frozen product under a pressure of 2,000 mTorr or lower to obtain a degassed and frozen product,
• wherein the degassed and frozen product comprises 95 to 100 parts by weight of the solvent per 100 parts by weight of the solvent contained in the solution of step (a), and
• the steps of (b) and (c) are performed at a temperature of −20° C. or tower in a sealed chamber.