If some anti-cancer drugs are taken with grapefruit juice, the potential cost savings could be about 80%, since the drugs interact with CYP3A4. The powerful compounds in the grapefruit called furanocoumarins obliterate the CYP3A4 enzyme in the intestines and liver. The result is that more of the drug gets into the bloodstream. Individuals have different levels of CYP3A4 that breaks down drugs before they even have the chance to get into the bloodstream. Patients with very active CYP3A4 get lower amounts of drugs into their systems than those with low levels of the enzyme. Some patients may have naturally low levels of the CYP3A4 enzyme and thus wouldn't need it. Certain drugs have a hard time reaching optimal blood levels at prescribed doses. Some doctors are interested in intentionally boosting the effects with grapefruit. In addition to Tykerb, Zytiga (abiraterone acetate) is also better absorbed with food, though the label says empty stomach. Cell-based assayists have been able to reverse Zytiga resistance by having patients take it with a fatty meal.
http://jco.ascopubs.org/content/25/23/3397.full.pdf