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: Massive urinary protein excretion has been observed after conversion from calcineurin inhibitors to mammalian target of rapamycin ( mToR ) inhibitors , especially sirolimus , in renal transplant recipients with chronic allograft nephropathy .
### Output Text:
Massive urinary protein excretion has been observed after conversion from calcineurin inhibitors to mammalian target of <span class="chemical">rapamycin</span> ( mToR ) inhibitors , especially <span class="chemical">sirolimus</span> , in renal transplant recipients with chronic allograft nephropathy . 