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 , succimer treatment of rats not previously exposed to Pb produced lasting and pervasive cognitive and affective dysfunction comparable in magnitude to that produced by the higher Pb exposure regimen .
### Output Text:
In contrast , <span class="chemical">succimer</span> treatment of rats not previously exposed to <span class="chemical">Pb</span> produced lasting and pervasive cognitive and affective dysfunction comparable in magnitude to that produced by the higher <span class="chemical">Pb</span> exposure regimen . 