A Humanized Immunoenzyme with Enhanced Activity for Glucuronide Prodrug Activation in the Tumor Microenvironment

Engineering stable and non-immunogenic immunoenzymes for cancer therapy via in situ generated prodrugs

Engineering human enzymes for therapeutic applications is attractive but introducing new amino acids may adversely affect enzyme stability and immunogenicity. Here we used a mammalian membrane-tethered screening system (ECSTASY) to evolve human lysosomal beta-glucuronidase (hBG) to hydrolyze a glucuronide metabolite (SN-38G) of the anticancer drug irinotecan (CPT-11). Three human beta-glucuronidase variants (hBG3, hBG10 and hBG19) with 3, 10 and 19 amino acid substitutions were identified that display up to 40-fold enhanced enzymatic activity, higher stability than E. coli beta-glucuronidase in human serum, and similar pharmacokinetics in mice as wild-type hBG. The hBG variants were two to three orders of magnitude less immunogenic than E. coli beta-glucuronidase in hBG transgenic mice. Intravenous administration of an immunoenzyme (hcc49-hBG10) targeting a sialyl-Tn tumor-associated antigen to mice bearing human colon xenografts significantly enhanced the anticancer activity of CPT-11 as measured by tumor suppression and mouse survival. Our results suggest that genetically-modified human enzymes represent a good alternative to microbially-derived enzymes for therapeutic applications.

Glucuronides: From biological waste to bio-nanomedical applications

Long considered as no more than biological waste meant to be eliminated in urine, glucuronides have recently contributed to tremendous developments in the biomedical field, particularly against cancer. While glucuronide prodrugs monotherapy and antibody-directed enzyme prodrug therapy have been around for some time, new facets have emerged that combine the unique properties of glucuronides notably in the fields of antibody-drug conjugates and nanomedicine. In both cases, glucuronides are utilized as a vector to improve pharmacokinetics and confer localized activation of potent drugs at tumor sites while also decreasing systemic toxicity. Here we will discuss some of the most promising strategies using glucuronides to promote successful anti-tumor therapeutic treatments.

Prodrugs in medicinal chemistry and enzyme prodrug therapies

Prodrugs are cunning derivatives of therapeutic agents designed to improve the pharmacokinetics profile of the drug. Within a prodrug, pharmacological activity of the drug is masked and is recovered within the human body upon bioconversion of the prodrug, a process that is typically mediated by enzymes. This concept is highly successful and a significant fraction of marketed therapeutic formulations is based on prodrugs. An advanced subset of prodrugs can be engineered such as to achieve site-specific bioconversion of the prodrug - to comprise the highly advantageous "enzyme prodrug therapy", EPT. Design of prodrugs for EPT is similar to the prodrugs in general medicinal use in that the pharmacological activity of the drug is masked, but differs significantly in that site-specific bioconversion is a prime consideration, and the enzymes typically used for EPT are non-mammalian and/or with low systemic abundance in the human body. This review focuses on the design of prodrugs for EPT in terms of the choice of an enzyme and the corresponding prodrug for bioconversion. We also discuss the recent success of "self immolative linkers" which significantly empower and diversify the prodrug design, and present methodologies for the design of prodrugs with extended blood residence time. The review aims to be of specific interest for medicinal chemists, biomedical engineers, and pharmaceutical scientists.

Annexin-directed β-glucuronidase for the targeted treatment of solid tumors

Enzyme prodrug therapy has the potential to remedy the lack of selectivity associated with the systemic administration of chemotherapy. However, most current systems are immunogenic and constrained to a monotherapeutic approach. We developed a new class of fusion proteins centered about the human enzyme β-glucuronidase (βG), capable of converting several innocuous prodrugs into chemotherapeutics. We targeted βG to phosphatidylserine on tumor cells, tumor vasculature and metastases via annexin A1/A5. Phosphatidylserine shows promise as a universal marker for solid tumors and allows for tumor type-independent targeting. To create fusion proteins, human annexin A1/A5 was genetically fused to the activity-enhancing 16a3 mutant of human βG, expressed in chemically defined, fed-batch suspension culture, and chromatographically purified. All fusion constructs achieved >95% purity with yields up to 740 μg/l. Fusion proteins displayed cancer selective cell-surface binding with cell line-dependent binding stability. One fusion protein in combination with the prodrug SN-38 glucuronide was as effective as the drug SN-38 on Panc-1 pancreatic cancer cells and HAAE-1 endothelial cells, and demonstrated efficacy against MCF-7 breast cancer cells. βG fusion proteins effectively enable localized combination therapy that can be tailored to each patient via prodrug selection, with promising clinical potential based on their near fully human design.

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.

Bioorthogonal prodrug activation driven by a strain-promoted 1,3-dipolar cycloaddition

Bioorthogonal prodrug activation controlled by the reaction of a trans-cyclooctene with an azide-functionalized prodrug is presented.

Due to the formation of hydrolysis-susceptible adducts, the 1,3-dipolar cycloaddition between an azide and strained trans-cyclooctene (TCO) has been disregarded in the field of bioorthogonal chemistry. We report a method which uses the instability of the adducts to our advantage in a prodrug activation strategy. The reaction of trans-cyclooctenol (TCO-OH) with a model prodrug resulted in a rapid 1,3-dipolar cycloaddition with second-order rates of 0.017 M^–1 s^–1 and 0.027 M^–1 s^–1 for the equatorial and axial isomers, respectively, resulting in release of the active compound. ¹H NMR studies showed that activation proceeded via a triazoline and imine, both of which are rapidly hydrolyzed to release the model drug. Cytotoxicity of a doxorubicin prodrug was restored in vitro upon activation with TCO-OH, while with cis-cyclooctenol (CCO-OH) no activation was observed. The data also demonstrates the potential of this reaction in organic synthesis as a mild orthogonal protecting group strategy for amino and hydroxyl groups.

β-Glucuronidase-responsive prodrugs for selective cancer chemotherapy: an update

The design of novel antitumor agents allowing the destruction of malignant cells while sparing healthy tissues is one of the major challenges in medicinal chemistry. In this context, the use of non-toxic prodrugs programmed to be selectively activated by beta-glucuronidase present at high concentration in the microenvironment of most solid tumors has attracted considerable attention. This review summarizes the major progresses that have been realized in this field over the past ten years. This includes the new prodrugs that have been designed to target a wide variety of anticancer drugs, the prodrugs employed in the course of a combined therapy, the dendritic glucuronide prodrugs and the concept of β-glucuronidase-responsive albumin binding prodrugs.

The Escherichia coli glucuronylsynthase promoted synthesis of steroid glucuronides: improved practicality and broader scope

Steroid glucuronides can be quickly and conveniently prepared on the milligram scale using theE. coliglucuronylsynthase enzyme followed by purification with solid-phase extraction.

Selective cancer therapy by extracellular activation of a highly potent glycosidic duocarmycin analogue

Conventional cancer chemotherapy is limited by systemic toxicity and poor selectivity. Tumor-selective activation of glucuronide prodrugs by beta-glucuronidase in the tumor microenvironment in a monotherapeutic approach is one promising way to increase cancer selectivity. Here we examined the cellular requirement for enzymatic activation as well as the in vivo toxicity and antitumor activity of a glucuronide prodrug of a potent duocarmycin analogue that is active at low picomolar concentrations. Prodrug activation by intracellular and extracellular beta-glucuronidase was investigated by measuring prodrug 2 cytotoxicity against human cancer cell lines that displayed different endogenous levels of beta-glucuronidase, as well as against beta-glucuronidase-deficient fibroblasts and newly established beta-glucuronidase knockdown cancer lines. In all cases, glucuronide prodrug 2 was 1000-5000 times less cytotoxic than the parent duocarmycin analogue regardless of intracellular levels of beta-glucuronidase. By contrast, cancer cells that displayed tethered beta-glucuronidase on their plasma membrane were 80-fold more sensitive to glucuronide prodrug 2, demonstrating that prodrug activation depended primarily on extracellular rather than intracellular beta-glucuronidase activity. Glucuronide prodrug 2 (2.5 mg/kg) displayed greater antitumor activity and less systemic toxicity in vivo than the clinically used drug carboplatin (50 mg/kg) to mice bearing human lung cancer xenografts. Intratumoral injection of an adenoviral vector expressing membrane-tethered beta-glucuronidase dramatically enhanced the in vivo antitumor activity of prodrug 2. Our data provide evidence that increasing extracellular beta-glucuronidase activity in the tumor microenvironment can boost the therapeutic index of a highly potent glucuronide prodrug.

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.

Recent advances in the application of antibodies as therapeutics

The application of antibodies as therapeutic agents in the treatment of cancer now represents a significant proportion of the oncology drug arena. Despite this success, the ability to engineer and exploit antibodies in many different formats is ensuring that new avenues for their therapeutic application are constantly being examined. This review examines a selection of novel antibody-based therapeutic strategies that are currently in late preclinical and clinical evaluation.

Bacterial glucuronidase as general marker for oncolytic virotherapy or other biological therapies

Background

Oncolytic viral tumor therapy is an emerging field in the fight against cancer with rising numbers of clinical trials and the first clinically approved product (Adenovirus for the treatment of Head and Neck Cancer in China) in this field. Yet, until recently no general (bio)marker or reporter gene was described that could be used to evaluate successful tumor colonization and/or transgene expression in other biological therapies.

Methods

Here, a bacterial glucuronidase (GusA) encoded by biological therapeutics (e.g. oncolytic viruses) was used as reporter system.

Results

Using fluorogenic probes that were specifically activated by glucuronidase we could show 1) preferential activation in tumors, 2) renal excretion of the activated fluorescent compounds and 3) reproducible detection of GusA in the serum of oncolytic vaccinia virus treated, tumor bearing mice in several tumor models. Time course studies revealed that reliable differentiation between tumor bearing and healthy mice can be done as early as 9 days post injection of the virus. Regarding the sensitivity of the newly developed assay system, we could show that a single infected tumor cell could be reliably detected in this assay.

Conclusion

GusA therefore has the potential to be used as a general marker in the preclinical and clinical evaluation of (novel) biological therapies as well as being useful for the detection of rare cells such as circulating tumor cells.