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: Experiments with systemic administration of the Pgp substrate phenobarbital and the selective Pgp inhibitor tariquidar in TR ( - ) rats substantiated that Pgp is functional and compensates for the lack of MRP2 in the BBB .
### Output Text:
Experiments with systemic administration of the Pgp substrate <span class="chemical">phenobarbital</span> and the selective Pgp inhibitor <span class="chemical">tariquidar</span> in TR ( - ) rats substantiated that Pgp is functional and compensates for the lack of MRP2 in the BBB . 