Expression and function of β-glucuronidase in pancreatic cancer: potential role in drug targeting
Lack of tumor selectivity is a severe limitation of cancer chemotherapy. Consequently, reducing dose-limiting organ toxicities such as the cardiac toxicity of doxorubicin (Dox) is of major clinical relevance. Approaches that would facilitate a more tumor-selective anticancer therapy by using nontoxic prodrugs that are converted to active anticancer agents at the tumor site have been the subject of intensive research. One potential method to overcome the cardiac toxicity of Dox is to apply a nontoxic, glucuronide prodrug (HMR 1826) from which Dox is released by the action of beta-glucuronidase, an enzyme present at high levels in many tumors. Using a recently developed, isolated, perfused human lung model, we compared the uptake of Dox into normal lung and lung tumors after a 2.5-h lung perfusion with doxorubicin (n = 8) and with the novel doxorubicin glucuronide prodrug (n = 8). Dox showed a poor uptake into lung tumors as compared with normal lung [mean Dox concentration at the end of perfusion, 1.78 +/- 3.11 (median, 0.66) microg/g versus 22.03 +/- 10.4 (median, 18.5) microg/g; P < 0.001]. However, after perfusion with HMR 1826, the level of Dox in tumor tissue was about 7-fold higher than after perfusion with Dox itself [14.04 +/- 12.9 (median, 12.9) microg/g versus 1.78 +/- 3.11 (median, 0.66) microg/g, P < 0.05, n = 8]. In vitro experiments showed a significantly higher beta-glucuronidase expression and activity in the tumors. The extent of in vitro cleavage of HMR 1826 by homogenized lung tissue was closely related to the content of beta-glucuronidase (r = 0.9834, P < 0.0001). When D-saccharolactone, a specific inhibitor of beta-glucuronidase, was added to the perfusate containing HMR 1826, no accumulation of Dox in lung tissue was seen. These data indicate that the high Dox levels achieved in the tumors with HMR 1826 resulted from cleavage of the prodrug by beta-glucuronidase at the tumor site. Thus, the problem of poor Dox uptake into lung tumors could be circumvented by applying the doxorubicin glucuronide prodrug. Several lines of evidence based on both ex vivo and in vitro results indicate that the approach described using a glucuronide prodrug may be useful in facilitating more selective delivery of chemotherapy to tumors in humans.