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: Furthermore , our data suggest that TR ( - ) rats are an interesting tool to study consequences of overexpression of Pgp in the BBB on access of drugs in the brain , without the need of inducing seizures or other Pgp - enhancing events for this purpose .
### Output Text:
Furthermore , our data suggest that TR ( - ) rats are an interesting tool to study consequences of overexpression of Pgp in the BBB on access of drugs in the brain , without the need of inducing seizures or other Pgp - enhancing events for this purpose . 