In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Leucovorin rescue was initiated 12 hours after the end of the infusion with a loading dose of 200 mg / m2 followed by 12 mg / m2 every three hours for six doses and then every six hours until the plasma methotrexate level decreased to less than 1 X 10 ( - 7 ) mol / L .
### Output Text:
<span class="chemical">Leucovorin</span> rescue was initiated 12 hours after the end of the infusion with a loading dose of 200 mg / m2 followed by 12 mg / m2 every three hours for six doses and then every six hours until the plasma <span class="chemical">methotrexate</span> level decreased to less than 1 X 10 ( - 7 ) mol / L . 