Expression and function of β-glucuronidase in pancreatic cancer: potential role in drug targeting

Abstract

Improvement of non-surgical strategies is a pivotal task in the treatment of pancreatic cancer. Response to treatment with most anticancer agents has been very poor, probably due to insufficient drug concentration in tumor tissue. Increased response rates during chemotherapy might be achieved by dose escalation; however, this approach is often hampered by severe side effects. One strategy to overcome these adverse effects is application of nontoxic glucuronide prodrugs from which the active moiety is released by beta-glucuronidase within or near the tumor. The use of glucuronide prodrugs in pancreatic cancer requires increased expression of the enzyme in the diseased tissue, a problem that has not been addressed so far. We therefore investigated function and expression of beta-glucuronidase in tissue samples from human healthy pancreas (n=7) and pancreatic adenocarcinoma (n=8), respectively. Comparing the ability of tissue homogenates to cleave the standard substrate 4-methylumbelliferyl-beta-D-glucuronide, we found a significantly increased specific beta-glucuronidase activity (P<0.05) in pancreatic cancer (median: 133; 75% percentile: 286; 25% percentile: 111 nmol/mg per h) as compared to healthy pancreas (median: 74; 75% percentile: 113; 25% percentile: 71 nmol/mg per h). Enzyme kinetic experiments with the model prodrug N-[4-beta-glucuronyl-3-nitrobenzyloxycarbonyl] doxorubicin (HMR 1826) demonstrated bioactivation of HMR 1826 by pancreatic beta-glucuronidase. Enzymatic activity was found to be closely related to enzyme contents (r=0.87) as assessed by Western blot analysis. Our data indicate that increased beta-glucuronidase activity in pancreatic cancer seems to be due to an elevated steady-state level of the protein. This may be the basis for new therapeutic strategies in treatment of pancreatic carcinoma by using glucuronide prodrugs of anticancer agents.

PET Imaging of β-Glucuronidase Activity by an Activity-Based 124I-Trapping Probe for the Personalized Glucuronide Prodrug Targeted Therapy

“…bG is considered as a tumor marker (14,15,20) and a prodrug-activating enzyme for cancer prodrug therapies (35,36). Connors and Whisson demonstrated that high bG levels present in plasmacytomas were correlated with higher drug sensitivity of the tumors (37).…”

“…Indeed, we also found that Colo205 xenografts exhibited high levels of endogenous bG activity based on the imaging results using TrapG probes. Meanwhile, several studies have found that bG accumulates in the necrotic areas of human cancers (14,18,38,39). In addition, glucuronide conjugates usually exhibit enhanced solubility and decreased cell permeability (3) that greatly increases the utility and specificity of prodrugs or proprobes relying on selective hydrolysis by the bG enzyme.…”

“…For instance, Albin and colleagues reported that the level of bG human breast tumors was up to 6-times greater than that in normal tissues (17). Sperker and colleagues also reported that pancreatic adenocarcinoma exhibited higher bG levels than did normal pancreatic tissue (14). Furthermore, human tumor xenografts also showed elevated levels of bG, including human breast (MCF-7, BT20, and HS578T), colon (HT29 and SW480), and small-cell lung cancer (OH3 and SW2) cell lines (18).…”

“…9-11) and gene-directed enzyme prodrug therapy (GDEPT; refs. 6, 12, 13) as well as directly in prodrug monotherapy, which relies on the elevated levels of bG found in the tumor microenvironment (14)(15)(16) to selectively convert prodrug into active drug. For instance, Albin and colleagues reported that the level of bG human breast tumors was up to 6-times greater than that in normal tissues (17).…”

“…Elevated levels of bG present in the tumor environment are believed to be due to tumor overexpression (14) and release from necrotic tumor tissues (19) or tumor-infiltrating immune cells (15). Because bG has been considered as a tumor marker (14,15,20), it would be very useful to image bG activity in vivo to personalize glucuronide prodrug treatment and greatly improve exogenous or endogenous bG-based targeted therapy.…”