Here is an explanation:

"The reason for this is interesting and is related to the absorption of tacrolimus in the normal gut. Tacrolimus has a low oral bioavailability with only approximately 20% reaching the bloodstream. There are 3 reasons why this happens. The first is that it undergoes significant first-pass metabolism in the liver by the cytochrome p450 system. The second two specifically relate to the gut:

1. P-glycoprotein: This is a membrane associated protein that transports many substances across intracellular and extracellular membranes. It is thought to be important in the development of multidrug resistance to antineoplastic agents. In the gut, it transports drugs back into the intestinal lumen from enterocytes thus reducing bioavailability

2. CYP3A4: This enzyme is the most important member of the cytochrome p450 family and is primarily expressed in the liver. However, there is also significant expression of this enzyme in the small intestine making this an important site of metabolism of some drugs including tacrolimus

In the setting of an inflammatory diarrhea, both of these processes are downregulated in enterocytes. This leads to both increased absorption and decreased gut metabolism of tacrolimus thus increasing drug levels despite the faster gut transit times. Once the gut mucosa heals, these processes begin to function normally again and the drug levels fall."

Source : http://renalfellow.blogspot.in/2011/09/diarrhea-and-tacrolimus.html

Thank u sir.