Local enzymatic hydrolysis of an endogenously generated metabolite can enhance CPT-11 anticancer efficacy

Abstract

Irinotecan (CPT-11) is a clinically important anticancer prodrug that requires enzymatic hydrolysis by carboxyesterase to generate the active metabolite SN-38. However, SN-38 is further metabolized to inactive SN-38 glucuronide (SN-38G), thus diminishing the levels of active SN-38. Although exogenously administered glucuronide drugs are being investigated for cancer therapy, it is unknown if endogenously generated camptothecin glucuronide metabolites can be used for tumor therapy. Here, we tested the hypothesis that tumor-located hydrolysis of endogenously generated SN-38G can enhance the antitumor efficacy of CPT-11 therapy. EJ human bladder carcinoma cells expressing membrane-tethered β-glucuronidase (EJ/mβG cells) were used to selectively hydrolyze SN-38G to SN-38. Parental EJ and EJ/mβG cells displayed similar in vitro and in vivo growth rates and sensitivities to CPT-11 and SN-38. By contrast, EJ/mβG cells were more than 30 times more sensitive than EJ cells to SN-38G, showing that SN-38 could be generated from SN-38G in vitro. Systemic administration of CPT-11 resulted in tumor-located hydrolysis of SN-38G and accumulation of SN-38 in EJ/ mβG subcutaneous tumors. Importantly, systemic administration of CPT-11, which itself is not a substrate for β-glucuronidase, dramatically delayed the growth of EJ/ mβG xenografts without increased systemic toxicity. Thus, the anticancer activity of CPT-11 can be significantly enhanced by converting the relatively high levels of endogenously generated SN-38G to SN-38 in tumors. The high concentrations of SN-38G found in the serum of patients treated with CPT-11 suggest that clinical response to CPT-11 may be improved by elevating β-glucuronidase activity in tumors. [Mol Cancer Ther 2009;8(4):940-6]

Intestinal glucuronidation protects against chemotherapy-induced toxicity by irinotecan (CPT-11)

“…However, the FDA's recommendation has been questioned because Gilbert's patients receiving CPT-11 therapy have been shown to experience toxicity to a lesser degree than previously anticipated (16,17). After its excretion into the bile, SN-38G reaches the gastrointestinal tract, where it is subjected to bacterial β-glucuronidase β-linkage cleavage that produces a free SN-38 aglycone (18)(19)(20)(21). Recent findings, using specific bacterial β-glucuronidase inhibitors to protect mice from CPT-11-induced late diarrhea, strongly suggest that the enterohepatic circulation of SN-38 plays an essential role in the delayed diarrhea (18,22).…”

Enhancement of CPT-11 antitumor activity by adenovirus-mediated expression of β–glucuronidase in tumors

“…23 In a recent study, we demonstrated that stable expression of a membrane-tethered form of bG on EJ bladder cancer cells increased their sensitivity to SN-38G and enhanced the therapeutic efficacy of CPT-11 in vivo. 24 We transduced the tumor cells ex vivo with a retroviral vector to achieve bG expression. Because this approach is difficult to use clinically, in the present study we used an adenoviral (Ad) vector for gene transfer into tumors.…”

Potential Repurposing of Known Drugs as Potent Bacterial β-Glucuronidase Inhibitors

“…Recent evidence suggests that mammalian GUS may convert SN-38G back to SN-38 within the tumor and thus increase the concentration of active drug (SN-38) in the tumor. [18][19][20] Therefore, any inhibitor of bacterial GUS used therapeutically should not inhibit the mammalian GUS because this may decrease the efficacy of irinotecan at the site of the tumor. Therefore, we tested the three most potent drugs from the screen-nialamide, isocarboxazid, and amoxapinein enzyme assays identical to the E. coli GUS enzyme assay, except for the use of mammalian GUS purified from B. taurus liver.…”