Rational design of an AKR1C3-resistant analog of PR-104 for enzyme-prodrug therapy

The Catalysis Mechanism of E. coli Nitroreductase A, a Candidate for Gene-Directed Prodrug Therapy: Potentiometric and Substrate Specificity Studies

E. coli nitroreductase A (NfsA) is a candidate for gene-directed prodrug cancer therapy using bioreductively activated nitroaromatic compounds (ArNO2). In this work, we determined the standard redox potential of FMN of NfsA to be -215 ± 5 mV at pH 7.0. FMN semiquinone was not formed during 5-deazaflavin-sensitized NfsA photoreduction. This determines the two-electron character of the reduction of ArNO2 and quinones (Q). In parallel, we characterized the oxidant specificity of NfsA with an emphasis on its structure. Except for negative outliers nitracrine and SN-36506, the reactivity of ArNO2 increases with their electron affinity (single-electron reduction potential, E17) and is unaffected by their lipophilicity and Van der Waals volume up to 386 Å. The reactivity of quinoidal oxidants is not clearly dependent on E17, but 2-hydroxy-1,4-naphthoquinones were identified as positive outliers and a number of compounds with diverse structures as negative outliers. 2-Hydroxy-1,4-naphthoquinones are characterized by the most positive reaction activation entropy and the negative outlier tetramethyl-1,4-benzoquinone by the most negative. Computer modelling data showed that the formation of H bonds with Arg15, Arg133, and Ser40, plays a major role in the binding of oxidants to reduced NfsA, while the role of the π-π interaction of their aromatic structures is less significant. Typically, the calculated hydride-transfer distances during ArNO2 reduction are smallwer than for Q. This explains the lower reactivity of quinones. Another factor that slows down the reduction is the presence of positively charged aliphatic substituents.

Clostridium Bacteria: Harnessing Tumour Necrosis for Targeted Gene Delivery

Necrosis is a common feature of solid tumours that offers a unique opportunity for targeted cancer therapy as it is absent from normal healthy tissues. Tumour necrosis provides an ideal environment for germination of the anaerobic bacterium Clostridium from endospores, resulting in tumour-specific colonisation. Two main species, Clostridium novyi-NT and Clostridium sporogenes, are at the forefront of this therapy, showing promise in preclinical models. However, anti-tumour activity is modest when used as a single agent, encouraging development of Clostridium as a tumour-selective gene delivery system. Various methods, such as allele-coupled exchange and CRISPR-cas9 technology, can facilitate the genetic modification of Clostridium, allowing chromosomal integration of transgenes to ensure long-term stability of expression. Strains of Clostridium can be engineered to express prodrug-activating enzymes, resulting in the generation of active drug selectively in the tumour microenvironment (a concept termed Clostridium-directed enzyme prodrug therapy). More recently, Clostridium strains have been investigated in the context of cancer immunotherapy, either in combination with immune checkpoint inhibitors or with engineered strains expressing immunomodulatory molecules such as IL-2 and TNF-α. Localised expression of these molecules using tumour-targeting Clostridium strains has the potential to improve delivery and reduce systemic toxicity. In summary, Clostridium species represent a promising platform for cancer therapy, with potential for localised gene delivery and immunomodulation selectively within the tumour microenvironment. The ongoing clinical progress being made with C. novyi-NT, in addition to developments in genetic modification techniques and non-invasive imaging capabilities, are expected to further progress Clostridium as an option for cancer treatment.

Design and Biological Evaluation of Piperazine-Bearing Nitrobenzamide Hypoxia/GDEPT Prodrugs: The Discovery of CP-506

Off-target aerobic activation of PR-104A by human aldo-keto reductase 1C3 (AKR1C3) has confounded the development of this dual hypoxia/gene therapy prodrug. Previous attempts to design prodrugs resistant to AKR1C3 activation have resulted in candidates that require further optimization. Herein we report the evaluation of a lipophilic series of PR-104A analogues in which a piperazine moiety has been introduced to improve drug-like properties. Octanol-water partition coefficients (LogD7.4) spanned >2 orders of magnitude. 2D antiproliferative and 3D multicellular clonogenic assays using isogenic HCT116 and H1299 cells confirmed that all examples were resistant to AKR1C3 metabolism while producing an E. coli NfsA nitroreductase-mediated bystander effect. Prodrugs 16, 17, and 20 demonstrated efficacy in H1299 xenografts where only a minority of tumor cells express NfsA. These prodrugs and their bromo/mesylate counterparts (25-27) were also evaluated for hypoxia-selective cell killing in vitro. These results in conjunction with stability assays recommended prodrug 26 (CP-506) for Phase I/II clinical trial.

Nitroreduction: A Critical Metabolic Pathway for Drugs, Environmental Pollutants, and Explosives

Nitro group containing xenobiotics include drugs, cancer chemotherapeutic agents, carcinogens (e.g., nitroarenes and aristolochic acid) and explosives. The nitro group undergoes a six-electron reduction to form sequentially the nitroso-, N-hydroxylamino- and amino-functional groups. These reactions are catalyzed by nitroreductases which, rather than being enzymes with this sole function, are enzymes hijacked for their propensity to donate electrons to the nitro group either one at a time via a radical mechanism or two at time via the equivalent of a hydride transfer. These enzymes include: NADPH-dependent flavoenzymes (NADPH: P450 oxidoreductase, NAD(P)H-quinone oxidoreductase), P450 enzymes, oxidases (aldehyde oxidase, xanthine oxidase) and aldo-keto reductases. The hydroxylamino group once formed can undergo conjugation reactions with acetate or sulfate catalyzed by N-acetyltransferases or sulfotransferases, respectively, leading to the formation of intermediates containing a good leaving group which in turn can generate a nitrenium or carbenium ion for covalent DNA adduct formation. The intermediates in the reduction sequence are also prone to oxidation and produce reactive oxygen species. As a consequence, many nitro-containing xenobiotics can be genotoxic either by forming stable covalent adducts or by oxidatively damaging DNA. This review will focus on the general chemistry of nitroreduction, the enzymes responsible, the reduction of xenobiotic substrates, the regulation of nitroreductases, the ability of nitrocompounds to form DNA adducts and act as mutagens as well as some future directions.

Bioreductive prodrug PR-104 improves the tumour distribution and titre of the nitroreductase-armed oncolytic adenovirus ONYX-411NTR leading to therapeutic benefit

Advances in the field of cancer immunotherapy have stimulated renewed interest in adenoviruses as oncolytic agents. Clinical experience has shown that oncolytic adenoviruses are safe and well tolerated but possess modest single-agent activity. One approach to improve the potency of oncolytic viruses is to utilise their tumour selectivity to deliver genes encoding prodrug-activating enzymes. These enzymes can convert prodrugs into cytotoxic species within the tumour; however, these cytotoxins can interfere with viral replication and limit utility. In this work, we evaluated the activity of a nitroreductase (NTR)-armed oncolytic adenovirus ONYX-411^NTR in combination with the clinically tested bioreductive prodrug PR-104. Both NTR-expressing cells in vitro and xenografts containing a minor population of NTR-expressing cells were highly sensitive to PR-104. Pharmacologically relevant prodrug exposures did not interfere with ONYX-411^NTR replication in vitro. In vivo, prodrug administration increased virus titre and improved virus distribution within tumour xenografts. Colonisation of tumours with high ONYX-411^NTR titre resulted in NTR expression and prodrug activation. The combination of ONYX-411^NTR with PR-104 was efficacious against HCT116 xenografts, whilst neither prodrug nor virus were active as single agents. This work highlights the potential for future clinical development of NTR-armed oncolytic viruses in combination with bioreductive prodrugs.

A Mansonone Derivative Coupled with Monoclonal Antibody 4D5-Modified Chitosan Inhibit AKR1C3 to Treat Castration-Resistant Prostate Cancer

Purpose

Aldo-ketoreductase (AKR) 1C3 is crucial for testosterone synthesis. Abnormally high expression/activity of AKR1C3 can promote castration-resistant prostate cancer (CRPC). A mansonone derivative and AKR1C3 inhibitor, 6e, was combined with 4D5 (extracellular fragment of the monoclonal antibody of human epidermal growth factor receptor-2)-modified chitosan to achieve a nanodrug-delivery system (CS-4D5/6e) to treat CRPC.

Materials and Methods

Morphologies/properties of CS-4D5/6e were characterized by atomic force microscopy, zeta-potential analysis, and Fourier transform-infrared spectroscopy. CS-4D5/6e uptake was measured by immunofluorescence under confocal laser scanning microscopy. Testosterone in LNCaP cells overexpressing human AKR1C3 (LNCaP-AKR1C3) and cell lysates was measured to reflect AKR1C3 activity. Androgen receptor (AR) and prostate-specific antigen (PSA) expression was measured by Western blotting. CS-4D5/6e-based inhibition of AKR1C3 was evaluated in tumor-xenografted mice.

Results

CS-4D5/6e was oblate, with a particle size of 200-300 nm and thickness of 1-5 nm. Zeta potential was 1.39±0.248 mV. 6e content in CS-4D5/6e was 7.3±1.4% and was 18±3.6% for 4D5. 6e and CS-4D5/6e inhibited testosterone production significantly in a concentration-dependent manner in LNCaP-AKR1C3 cells, and a decrease in expression of AKR1C3, PSA, and AR was noted. Half-maximal inhibitory concentration of CS-4D5/6e on LNCaP-AKR1C3 cells was significantly lower than that in LNCaP cells (P<0.05). CS-4D5/6e significantly reduced growth of 22Rv1 tumor xenografts by 57.00% compared with that in the vehicle group (P<0.01).

Conclusion

We demonstrated the antineoplastic activity of a potent AKR1C3 inhibitor (6e) and its nanodrug-delivery system (CS-4D5/6e). First, CS-4D5/6e targeted HER2-positive CRPC cells. Second, it transferred 6e (an AKR1C3 inhibitor) to achieve a reduction in intratumoral testosterone production. Compared with 6e, CS-4D5/6e showed lower systemic toxicity. CS-4D5/6e inhibited tumor growth effectively in mice implanted with tumor xenografts by downregulating testosterone production mediated by intratumoral AKR1C3. These results showed a promising strategy for treatment of the CRPC that develops invariably in prostate-cancer patients.

Hypoxia-activated nanomedicines for effective cancer therapy

Hypoxia, a common characteristic in solid tumors, is found in phenotypically aggressive cancers that display resistance to typical cancer interventions. Due to its important role in tumor progression, tumor hypoxia has been considered as a primary target for cancer diagnosis and treatment. An advantage of hypoxia-activated nanomedicines is that they are inactive in normoxic cells. In hypoxic tumor tissues and cells, these nanomedicines undergo reduction by activated enzymes (usually through 1 or 2 electron oxidoreductases) to produce cytotoxic substances. In this review, we will focus on approaches to design nanomedicines that take advantage of tumor hypoxia. These approaches include: i) inhibitors of hypoxia-associated signaling pathways; ii) prodrugs activated by hypoxia; iii) nanocarriers responsive to hypoxia, and iv) bacteria mediated hypoxia targeting therapy. These strategies have guided and will continue to guide nanoparticle design in the near future. These strategies have the potential to overcome tumor heterogeneity to improve the efficiency of radiotherapy, chemotherapy and diagnosis.

Engineering Escherichia coli NfsB To Activate a Hypoxia-Resistant Analogue of the PET Probe EF5 To Enable Non-Invasive Imaging during Enzyme Prodrug Therapy

Gene-directed enzyme prodrug therapy (GDEPT) uses tumor-tropic vectors to deliver prodrug-converting enzymes such as nitroreductases specifically to the tumor environment. The nitroreductase NfsB from Escherichia coli (NfsB_Ec) has been a particular focal point for GDEPT and over the past 25 years has been the subject of several engineering studies seeking to improve catalysis of prodrug substrates. To facilitate clinical development, there is also a need to enable effective non-invasive imaging capabilities. SN33623, a 5-nitroimidazole analogue of 2-nitroimidazole hypoxia probe EF5, has potential for PET imaging exogenously delivered nitroreductases without generating confounding background due to tumor hypoxia. However, we show here that SN33623 is a poor substrate for NfsB_Ec. To address this, we used assay-guided sequence and structure analysis to identify two conserved residues that block SN33623 activation in NfsB_Ec and close homologues. Introduction of the rational substitutions F70A and F108Y into NfsB_Ec conferred high levels of SN33623 activity and enabled specific labeling of E. coli expressing the engineered enzyme. Serendipitously, the F70A and F108Y substitutions also substantially improved activity with the anticancer prodrug CB1954 and the 5-nitroimidazole antibiotic prodrug metronidazole, which is a potential biosafety agent for targeted ablation of nitroreductase-expressing vectors.

Characterization of a highly specific monoclonal antibody against human aldo-keto reductase AKR1C3

Human aldo-keto reductase AKR1C3 (type 2 3α-hydroxysteroid dehydrogenase/type 5 17β-hydroxysteroid dehydrogenase) is involved in testosterone and estrogen metabolism. AKR1C3 expression is relatively low in most tissues and high in prostate and mammary glands in regulating androgen and estrogen levels. However, in many cancers, overexpression of AKR1C3 was observed, thus prompting the development of therapeutics targeting AKR1C3. To facilitate the development of AKR1C3 targeting therapeutics, evaluating the expression of AKR1C3 is vital. As AKR1C3 is highly homologous with its family proteins, AKR1C1, AKR1C2, AKR1C4 and other AKR1 proteins, reagents that can unambiguously discriminate these enzymes are needed. In this report, a highly specific monoclonal antibody for AKR1C3, 10B10, was developed and characterized. Compared to other AKR1C3 antibodies, 10B10 is highly specific and sensitive to AKR1C3 in multiple assay formats. Thus, 10B10 will be a valuable tool for the clinical development of AKR1C3 targeting therapeutics and the study of AKR1C3 biology.

Therapeutic Potential of Prodrugs Towards Targeted Drug Delivery

In designing of Prodrugs, targeting can be achieved in two ways: site-specified drug delivery and site-specific drug bioactivation. Prodrugs can be designed to target specific enzymes or carriers by considering enzyme-substrate specificity or carrier-substrate specificity in order to overcome various undesirable drug properties. There are certain techniques which are used for tumor targeting such as Antibody Directed Enzyme Prodrug Therapy [ADEPT] Gene-Directed Enzyme Prodrug Therapy [GDEPT], Virus Directed Enzyme Prodrug Therapy [VDEPT] and Gene Prodrug Activation Therapy [GPAT]. Our review focuses on the Prodrugs used in site-specific drug delivery system specially on tumor targeting.

Evaluating the abilities of diverse nitroaromatic prodrug metabolites to exit a model Gram negative vector for bacterial-directed enzyme-prodrug therapy

Gene-directed enzyme-prodrug therapy (GDEPT) employs tumour-tropic vectors including viruses and bacteria to deliver a genetically-encoded prodrug-converting enzyme to the tumour environment, thereby sensitising the tumour to the prodrug. Nitroreductases, able to activate a range of promising nitroaromatic prodrugs to genotoxic metabolites, are of great interest for GDEPT. The bystander effect (cell-to-cell transfer of activated prodrug metabolites) has been quantified for some nitroaromatic prodrugs in mixed multilayer human cell cultures, however while these provide a good model for viral DEPT (VDEPT) they do not inform on the ability of these prodrug metabolites to exit bacterial vectors (relevant to bacterial-DEPT (BDEPT)). To investigate this we grew two Escherichia coli strains in co-culture; an activator strain expressing the nitroreductase E. coli NfsA and a recipient strain containing an SOS-GFP DNA damage responsive gene construct. In this system, induction of GFP by reduced prodrug metabolites can only occur following their transfer from the activator to the recipient cells. We used this to investigate five clinically relevant prodrugs: metronidazole, CB1954, nitro-CBI-DEI, and two dinitrobenzamide mustard prodrug analogues, PR-104A and SN27686. Consistent with the bystander efficiencies previously measured in human cell multilayers, reduced metronidazole exhibited little bacterial cell-to-cell transfer, whereas nitro-CBI-DEI was passed very efficiently from activator to recipient cells post-reduction. However, in contrast with observations in human cell multilayers, the nitrogen mustard prodrug metabolites were not effectively passed between the two bacterial strains, whereas reduced CB1954 was transferred efficiently. Using nitroreductase enzymes that exhibit different biases for the 2- versus 4-nitro substituents of CB1954, we further showed that the 2-nitro reduction products exhibit substantially higher levels of bacterial cell-to-cell transfer than the 4-nitro reduction products, consistent with their relative bystander efficiencies in human cell culture. Overall, our data suggest that prodrugs may differ in their suitability for VDEPT versus BDEPT applications and emphasise the importance of evaluating an enzyme-prodrug partnership in an appropriate context for the intended vector.

Bystander Effects of Hypoxia-Activated Prodrugs: Agent-Based Modeling Using Three Dimensional Cell Cultures

Intra-tumor heterogeneity represents a major barrier to anti-cancer therapies. One strategy to minimize this limitation relies on bystander effects via diffusion of cytotoxins from targeted cells. Hypoxia-activated prodrugs (HAPs) have the potential to exploit hypoxia in this way, but robust methods for measuring bystander effects are lacking. The objective of this study is to develop experimental models (monolayer, multilayer, and multicellular spheroid co-cultures) comprising 'activator' cells with high expression of prodrug-activating reductases and reductase-deficient 'target' cells, and to couple these with agent-based models (ABMs) that describe diffusion and reaction of prodrugs and their active metabolites, and killing probability for each cell. HCT116 cells were engineered as activators by overexpressing P450 oxidoreductase (POR) and as targets by knockout of POR, with fluorescent protein and antibiotic resistance markers to enable their quantitation in co-cultures. We investigated two HAPs with very different pharmacology: SN30000 is metabolized to DNA-breaking free radicals under hypoxia, while the dinitrobenzamide PR104A generates DNA-crosslinking nitrogen mustard metabolites. In anoxic spheroid co-cultures, increasing the proportion of activator cells decreased killing of both activators and targets by SN30000. An ABM parameterized by measuring SN30000 cytotoxicity in monolayers and diffusion-reaction in multilayers accurately predicted SN30000 activity in spheroids, demonstrating the lack of bystander effects and that rapid metabolic consumption of SN30000 inhibited prodrug penetration. In contrast, killing of targets by PR104A in anoxic spheroids was markedly increased by activators, demonstrating that a bystander effect more than compensates any penetration limitation. However, the ABM based on the well-studied hydroxylamine and amine metabolites of PR104A did not fit the cell survival data, indicating a need to reassess its cellular pharmacology. Characterization of extracellular metabolites of PR104A in anoxic cultures identified more stable, lipophilic, activated dichloro mustards with greater tissue diffusion distances. Including these metabolites explicitly in the ABM provided a good description of activator and target cell killing by PR104A in spheroids. This study represents the most direct demonstration of a hypoxic bystander effect for PR104A to date, and demonstrates the power of combining mathematical modeling of pharmacokinetics/pharmacodynamics with multicellular culture models to dissect bystander effects of targeted drug carriers.