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: Whereas patient 1 showed lesions of up to 1 cm readily detectable on magnetic resonance imaging under prolonged co - trimoxazole treatment , therapy of patient 2 was switched early .
### Output Text:
Whereas patient 1 showed lesions of up to 1 cm readily detectable on magnetic resonance imaging under prolonged <span class="chemical">co - trimoxazole</span> treatment , therapy of patient 2 was switched early . 