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: In contrast , monkeys with long - term MPTP exposure , slow symptom progression and / or long symptom duration prior to initiation of levodopa therapy were more resistant to developing LIDs ( e . g . , dyskinesia developed no sooner than 146 days of chronic levodopa administration ) .
### Output Text:
In contrast , monkeys with long - term <span class="chemical">MPTP</span> exposure , slow symptom progression and / or long symptom duration prior to initiation of <span class="chemical">levodopa</span> therapy were more resistant to developing LIDs ( e . g . , dyskinesia developed no sooner than 146 days of chronic <span class="chemical">levodopa</span> administration ) . 