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

Effect of Cellular Location of Human Carboxylesterase 2 on CPT-11 Hydrolysis and Anticancer Activity

CPT-11 is an anticancer prodrug that is clinically used for the treatment of metastatic colorectal cancer. Hydrolysis of CPT-11 by human carboxylesterase 2 (CE2) generates SN-38, a topoisomerase I inhibitor that is the active anti-tumor agent. Expression of CE2 in cancer cells is under investigation for the tumor-localized activation of CPT-11. CE2 is normally expressed in the endoplasmic reticulum of cells but can be engineered to direct expression of active enzyme on the plasma membrane or as a secreted form. Although previous studies have investigated different locations of CE2 expression in cancer cells, it remains unclear if CE2 cellular location affects CPT-11 anticancer activity. In the present study, we directly compared the influence of CE2 cellular location on substrate hydrolysis and CPT-11 cytotoxicity. We linked expression of CE2 and enhanced green fluorescence protein (eGFP) via a foot-and-mouth disease virus 2A (F2A) peptide to facilitate fluorescence-activated cell sorting to achieve similar expression levels of ER-located, secreted or membrane-anchored CE2. Soluble CE2 was detected in the medium of cells that expressed secreted and membrane-anchored CE2, but not in cells that expressed ER-retained CE2. Cancer cells that expressed all three forms of CE2 were more sensitive to CPT-11 as compared to unmodified cancer cells, but the membrane-anchored and ER-retained forms of CE2 were consistently more effective than secreted CE2. We conclude that expression of CE2 in the ER or on the membrane of cancer cells is suitable for enhancing CPT-11 anticancer activity.

Impediments to enhancement of CPT-11 anticancer activity by E. coli directed beta-glucuronidase therapy

CPT-11 is a camptothecin analog used for the clinical treatment of colorectal adenocarcinoma. CPT-11 is converted into the therapeutic anti-cancer agent SN-38 by liver enzymes and can be further metabolized to a non-toxic glucuronide SN-38G, resulting in low SN-38 but high SN-38G concentrations in the circulation. We previously demonstrated that adenoviral expression of membrane-anchored beta-glucuronidase could promote conversion of SN-38G to SN-38 in tumors and increase the anticancer activity of CPT-11. Here, we identified impediments to effective tumor therapy with E. coli that were engineered to constitutively express highly active E. coli beta-glucuronidase intracellularly to enhance the anticancer activity of CPT-11. The engineered bacteria, E. coli (lux/βG), could hydrolyze SN-38G to SN-38, increased the sensitivity of cultured tumor cells to SN-38G by about 100 fold and selectively accumulated in tumors. However, E. coli (lux/βG) did not more effectively increase CPT-11 anticancer activity in human tumor xenografts as compared to non-engineered E. coli. SN-38G conversion to SN-38 by E. coli (lux/βG) appeared to be limited by slow uptake into bacteria as well as by segregation of E. coli in necrotic regions of tumors that may be relatively inaccessible to systemically-administered drug molecules. Studies using a fluorescent glucuronide probe showed that significantly greater glucuronide hydrolysis could be achieved in mice pretreated with E. coli (lux/βG) by direct intratumoral injection of the glucuronide probe or by intratumoral lysis of bacteria to release intracellular beta-glucuronidase. Our study suggests that the distribution of beta-glucuronidase, and possibly other therapeutic proteins, in the tumor microenvironment might be an important barrier for effective bacterial-based tumor therapy. Expression of secreted therapeutic proteins or induction of therapeutic protein release from bacteria might therefore be a promising strategy to enhance anti-tumor activity.

PET imaging of β-glucuronidase activity by an activity-based 124I-trapping probe for the personalized glucuronide prodrug targeted therapy

Beta-glucuronidase (βG) is a potential biomarker for cancer diagnosis and prodrug therapy. The ability to image βG activity in patients would assist in personalized glucuronide prodrug cancer therapy. However, whole-body imaging of βG activity for medical usage is not yet available. Here, we developed a radioactive βG activity-based trapping probe for positron emission tomography (PET). We generated a (124)I-tyramine-conjugated difluoromethylphenol beta-glucuronide probe (TrapG) to form (124)I-TrapG that could be selectively activated by βG for subsequent attachment of (124)I-tyramine to nucleophilic moieties near βG-expressing sites. We estimated the specificity of a fluorescent FITC-TrapG, the cytotoxicity of tyramine-TrapG, and the serum half-life of (124)I-TrapG. βG targeting of (124)I-TrapG in vivo was examined by micro-PET. The biodistribution of (131)I-TrapG was investigated in different organs. Finally, we imaged the endogenous βG activity and assessed its correlation with therapeutic efficacy of 9-aminocamptothecin glucuronide (9ACG) prodrug in native tumors. FITC-TrapG showed specific trapping at βG-expressing CT26 (CT26/mβG) cells but not in CT26 cells. The native TrapG probe possessed low cytotoxicity. (124)I-TrapG preferentially accumulated in CT26/mβG but not CT26 cells. Meanwhile, micro-PET and whole-body autoradiography results demonstrated that (124)I-TrapG signals in CT26/mβG tumors were 141.4-fold greater than in CT26 tumors. Importantly, Colo205 xenografts in nude mice that express elevated endogenous βG can be monitored by using infrared glucuronide trapping probes (NIR-TrapG) and suppressed by 9ACG prodrug treatment. (124)I-TrapG exhibited low cytotoxicity allowing long-term monitoring of βG activity in vivo to aid in the optimization of prodrug targeted therapy.

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

Camptothecin (CPT)-11 (irinotecan) has been used widely for cancer treatment, particularly metastatic colorectal cancer. However, up to 40% of treated patients suffer from severe late diarrhea, which prevents CPT-11 dose intensification and efficacy. CPT-11 is a prodrug that is hydrolyzed by hepatic and intestinal carboxylesterase to form SN-38, which in turn is detoxified primarily through UDP-glucuronosyltransferase 1A1 (UGT1A1)-catalyzed glucuronidation. To better understand the mechanism associated with toxicity, we generated tissue-specific Ugt1 locus conditional knockout mouse models and examined the role of glucuronidation in protecting against irinotecan-induced toxicity. We targeted the deletion of the Ugt1 locus and the Ugt1a1 gene specifically in the liver (Ugt1(ΔHep)) and the intestine (Ugt1(ΔGI)). Control (Ugt1(F/F)), Ugt1(ΔHep), and Ugt1(ΔGI) adult male mice were treated with different concentrations of CPT-11 daily for four consecutive days. Toxicities were evaluated with regard to tissue glucuronidation potential. CPT-11-treated Ugt1(ΔHep) mice showed a similar lethality rate to the CPT-11-treated Ugt1(F/F) mice. However, Ugt1(ΔGI) mice were highly susceptible to CPT-11-induced diarrhea, developing severe and lethal mucositis at much lower CPT-11 doses, a result of the proliferative cell loss and inflammation in the intestinal tract. Comparative expression levels of UGT1A1 in intestinal tumors and normal surrounding tissue are dramatically different, providing for the opportunity to improve therapy by differential gene regulation. Intestinal expression of the UGT1A proteins is critical toward the detoxification of SN-38, whereas induction of the UGT1A1 gene may serve to limit toxicity and improve the efficacy associated with CPT-11 treatment.

ECSTASY, an adjustable membrane-tethered/soluble protein expression system for the directed evolution of mammalian proteins

We describe an adjustable membrane-tethered/soluble protein screening methodology termed ECSTASY (enzyme cleavable surface tethered all-purpose screening system) which combines the power of high-throughput fluorescence-activated cell sorting of membrane-tethered proteins with the flexibility of soluble assays for isolation of improved mammalian recombinant proteins. In this approach, retroviral transduction is employed to stably tether a library of protein variants on the surface of mammalian cells via a glycosyl phosphatidylinositol anchor. High-throughput fluorescence-activated cell sorting is used to array cells expressing properly folded and/or active protein variants on their surface into microtiter culture plates. After culture to expand individual clones, treatment of cells with phosphatidylinositol-phospholipase C releases soluble protein variants for multiplex measurement of protein concentration, activity and/or function. We utilized ECSTASY to rapidly generate human β-glucuronidase variants for cancer therapy by antibody-directed enzyme prodrug therapy with up to 30-fold greater potency to catalyze the hydrolysis of the clinically relevant camptothecin anti-cancer prodrug as compared with wild-type human β-glucuronidase. A variety of recombinant proteins could be adjustably displayed on fibroblasts, suggesting that ECSTASY represents a general, simple and versatile methodology for high-throughput screening to accelerate sequence activity-based evolution of mammalian proteins.

Potential repurposing of known drugs as potent bacterial β-glucuronidase inhibitors

The active metabolite of the chemotherapeutic irinotecan, SN-38, is detoxified through glucuronidation and then excreted into the gastrointestinal tract. Intestinal bacteria convert the glucuronidated metabolite back to the toxic SN-38 using β-glucuronidase (GUS), resulting in debilitating diarrhea. Inhibiting GUS activity may relieve this side effect of irinotecan. In this study, we sought to determine whether any known drugs have GUS inhibitory activity. We screened a library of Food and Drug Administration-approved drugs with a cell-free biochemical enzyme assay using purified bacterial GUS. After triage, five drugs were confirmed to inhibit purified bacterial GUS. Three of these were the monoamine oxidase inhibitors nialamide, isocarboxazid, and phenelzine with average IC(50) values for inhibiting GUS of 71, 128, and 2300 nM, respectively. The tricyclic antidepressant amoxapine (IC(50) = 388 nM) and the antimalarial mefloquine (IC(50) = 1.2 µM) also had activity. Nialamide, isocarboxazid, and amoxapine had no significant activity against purified mammalian GUS but showed potent activity for inhibiting endogenous GUS activity in a cell-based assay using living intact Escherichia coli with average IC(50) values of 17, 336, and 119 nM, respectively. Thus, nialamide, isocarboxazid, and amoxapine have potential to be repurposed as therapeutics to reduce diarrhea associated with irinotecan chemotherapy and warrant further investigation for this use.

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

CPT-11 is a clinically important prodrug that requires conversion into the active metabolite SN-38, a potent topoisomerase I poison, for antitumor activity. However, SN-38 is rapidly metabolized to the inactive SN-38 glucuronide (SN-38G) in the liver, which reduces the amount of SN-38 available for killing cancer cells. Here, we investigated if local expression of β-glucuronidase (βG) on cancer cells to catalytically convert SN38G to SN38 could enhance the antitumor activity of CPT-11. βG was tethered on the plasma membrane of three different human cancer cell lines: human colon carcinoma (LS174T), lung adenocarcinoma (CL1-5) and bladder carcinoma (EJ). Surface β-glucuronidase-expressing cells were 20 to 80-fold more sensitive to SN-38G than the parental cells. Intravenous CPT-11 produced significantly greater suppression of CL1-5 and LS174 T tumors that expressed βG as compared with unmodified tumors. Furthermore, an adenoviral vector expressing membrane-tethered βG (Ad.βG) increased the sensitivity of cancer cells to SN-38G even at multiplicity of infections as low as 0.16, indicating bystander killing of non-transduced cancer cells. Importantly, intratumoral injection of Ad.βG significantly enhanced the in vivo antitumor activity of CPT-11 as compared with treatment with CPT-11 or Ad vectors alone. This study shows that Ad.βG has potential to boost the therapeutic index of CPT-11.