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 two of them , the MPO - ANCA titres transiently increased to 12 . 8 and 15 . 0 U / ml , respectively , despite continued PTU therapy , but no vasculitic disorders developed .
### Output Text:
In two of them , the MPO - ANCA titres transiently increased to 12 . 8 and 15 . 0 U / ml , respectively , despite continued <span class="chemical">PTU</span> therapy , but no vasculitic disorders developed . 