Virus-directed enzyme/prodrug therapy (VDEPT). Selectively engineering drug sensitivity into tumors

Structural characterization of an isocytosine-specific deaminase VCZ reveals its application potential in the anti-cancer therapy

Non-natural nucleobase isocytosine (IC) is the isomer of cytosine; its chemical derivate 5-fluoroisocytosine (5-FIC) together with the isocytosine-specific deaminase (ICD) VCZ was suggested to be potential practical enzyme/prodrug pair for cancer therapy through gene-directed enzyme-prodrug therapy (GDEPT) method. In this study, we have determined the crystal structures of apo-VCZ and its complex with 5-FU. We identified the critical residues for substrate binding and catalytic reaction. We also captured the substrate-induced conformational changes of VCZ, then proposed the conjectural reaction procedures of VCZ for converting the IC into the uracil. Moreover, we evaluated the therapeutic effect of wildtype or the mutated VCZ protein in the colorectal cancer cell lines. Our studies will shed light on optimizing the ICD/5-FIC pairs by modifying either the enzyme or the prodrug based on the structural observations, thereby improving the possibility of applying the ICD/5-FIC pair in clinical trials.

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.

Cell-mediated enzyme prodrug cancer therapies

Cell-directed gene therapy is a promising new frontier for the field of targeted cancer therapies. Here we discuss the current pre-clinical and clinical use of cell-mediated enzyme prodrug therapy (EPT) directed against solid tumors and avenues for further development. We also discuss some of the challenges encountered upon translating these therapies to clinical trials. Upon sufficient development, cell-mediated enzyme prodrug therapy has the potential to maximize the distribution of therapeutic enzymes within the tumor environment, localizing conversion of prodrug to active drug at the tumor sites thereby decreasing off-target toxicities. New combinatorial possibilities are also promising. For example, when combined with viral gene-delivery vehicles, this may result in new hybrid vehicles that attain heretofore unmatched levels of therapeutic gene expression within the tumor.

Advancements in the development of HIF-1α-activated protein switches for use in enzyme prodrug therapy

While gene-directed enzyme prodrug therapy has shown potential as a cancer therapeutic in animal and clinical trials, concerns over the efficacy, selectivity, and safety of gene delivery vehicles have restricted its advance. In an attempt to relieve some of the demands on targeted gene delivery vehicles and achieve the full potential of enzyme prodrug therapy, cancer-targeted activity can be engineered into the enzyme itself. We previously engineered a switchable prodrug-activating enzyme that selectively kills human cancer cells accumulating the cancer marker hypoxia-inducible factor-1α (HIF-1α). This HIF-1α-activated protein switch (Haps59) is designed to increase its ability to convert the prodrug 5-fluorocytosine into the chemotherapeutic 5-fluorouracil in a HIF-1α-dependent manner. However, in cancer cell lines expressing Haps59 the 5FC sensitivity difference between the presence and absence of HIF-1α was not as large as desired. In this work, we aimed to improve the cancer specificity of this switch via a directed evolution approach utilizing random mutagenesis, linker mutagenesis, and random insertion and circular permutation. We identified improved HIF-1α-activated protein switches that confer E. coli with modest increases in HIF-1α-dependent 5FC toxicity. Additionally, the current bottleneck in the development of improved HIF-1α-activated protein switches is screening switch candidates in mammalian cells. To accommodate higher throughput and reduce experimental variability, we explored the use of Flp recombinase-mediated isogenic integration in 293 cells. These experiments raised the possibility that Haps59 can be activated by other interactors of the CH1 domain, and experiments in E. coli indicated that CITED2 can also activate Haps59. Although many CH1 binding partners are also oncogenes, CH1's promiscuous binding and subsequent off-target activation of Haps59 needs to be examined under normal physiological conditions to identify off-target activators. With aberrant activating molecules identified, further directed evolution can be performed to improve the cancer specificity of HIF-1α-activated protein switches.

Enforced effect of tk-MCP-1 fusion gene in ovarian cancer

OBJECTIVE

The efficiency of HSV-tk/GCV system is not high because of insufficient gene transfer and incompletely initiative of host antineoplastic potency. The present study was designed to assess the antitumor efficacy of tk-MCP-1 on ovarian cancer in vitro and vivo.

METHODS

A novel bicistronic expression system can help to improve the expression level of a gene in a stable manner. pLXSN/tk-MCP-1 co-expressing tk and MCP-1 genes was constructed using a pLXSN retroviral vector and an internal ribosome entry site sequence by restriction enzyme. Western blot was performed to determine tk and MCP-1 expression in the infected SKOV3. The GCV-sensitively tumoricidal activities of SKOV3/tk-MCP-1 with or without monocytes were compared to those of SKOV3 expressing HSV-tk or MCP-1. We investigated the growth of subcutaneous tumors in SCID mice immuno-reconstituted, and evaluated the antitumor effect of MCP-1 in conjunction with suicide gene.

RESULTS

The significant GCV-sensitively tumoricidal activity of pLXSN/tk-MCP-1 was observed when compared with those of pLXSN/tk, pLXSN/MCP-1 and pLXSN/neo, especially when monocytes were added. The growth of subcutaneous tumors in SCID mice immuno-reconstituted was markedly suppressed by co-delivery of HSV-tk and MCP-1 genes, and the enhanced antitumor effect was associated with the recruitment of monocytes.

CONCLUSION

These results demonstrated pLXSN/tk-MCP-1 presented an enhanced antitumor effects on ovarian cancer by orchestration of immune responses.

Annexin V-targeted enzyme prodrug therapy using cytosine deaminase in combination with 5-fluorocytosine

A fusion protein, consisting of cytosine deaminase (CD) linked to human annexin V, was created for use in an enzyme prodrug therapy targeted to the tumor vasculature and associated cancer cells in the primary tumor and distant metastases. The major finding of this study is that the CD-annexin V fusion protein in combination with the prodrug 5-fluorocytosine has significant cytotoxic activity against endothelial cells and two breast cancer cells lines in vitro that expose phosphatidylserine on their surface. The cytotoxicity experiments verified this novel enzyme prodrug system has the ability to produce therapeutic levels of 5-fluorouracil and thus appears promising.

Complex interactions between the replicating oncolytic effect and the enzyme/prodrug effect of vaccinia-mediated tumor regression

Replicating viruses for cancer gene therapy have beneficial antitumor effects, however, in the setting of an enzyme/prodrug system, the interactions between these viruses and the activated agents are complex. A replicating vaccinia virus expressing the cytosine deaminase gene (VVCD), which converts the prodrug 5-FC into 5-FU, was characterized in vitro and in vivo for its antitumor effects and pathogenicity. Replicating VVCD (+/-5-FC) at various MOIs was used to infect MC38 murine colon adenocarcinoma cells. At high MOIs (>0.1) virus alone was able to kill the majority (65-90%) of cells by day 5 with no additional benefit from prodrug. At low MOIs only the effect of prodrug is seen. Cell lysates demonstrated 300-fold reduced viral recovery from cells treated with both VVCD and 5-FC compared with controls treated with virus alone. Nude mice bearing subcutaneous MC38 tumors were injected with VVCD (or control) and treated with 5FC or control. Mice injected with VVCD (with or without 5FC treatment) had smaller tumors than the controls, suggesting that replicating vaccinia alone is cytotoxic to tumors in vivo. The addition of 5-FC improved the antitumor response when a low dose of virus was injected into tumors. Also, compared with mice that received virus alone, those that received VVCD and 5FC had significantly prolonged survival from virus-mediated death. In conclusion, the addition of an enzyme/prodrug system to a replicating virus can improve the antitumor response and decrease viral pathogenicity. Gene Therapy (2000) 7, 1217-1223.

Targeted prodrug design to optimize drug delivery

Classical prodrug design often represents a nonspecific chemical approach to mask undesirable drug properties such as limited bioavailability, lack of site specificity, and chemical instability. On the other hand, targeted prodrug design represents a new strategy for directed and efficient drug delivery. Particularly, targeting the prodrugs to a specific enzyme or a specific membrane transporter, or both, has potential as a selective drug delivery system in cancer chemotherapy or as an efficient oral drug delivery system. Site-selective targeting with prodrugs can be further enhanced by the simultaneous use of gene delivery to express the requisite enzymes or transporters. This review highlights evolving strategies in targeted prodrug design, including antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and peptide transporter-associated prodrug therapy.

The role of the toxicologic pathologist in the preclinical safety evaluation of biotechnology-derived pharmaceuticals

Biotechnology-derived pharmaceuticals, or biopharmaceuticals, represent a special class of complex, high molecular weight products, such as monoclonal antibodies, recombinant proteins, and nucleic acids. With these compounds, it is not appropriate to follow conventional safety testing programs, and the preclinical "package" for each biopharmaceutical needs to be individually designed. In addition to standard histopathology, the use of molecular pathology techniques is often required either in conventional animal studies or in in vitro tests. In this review, the safety evaluation of biopharmaceuticals is discussed from the perspective of the toxicologic pathologist, and appropriate examples are given of the use of molecular pathology procedures. Examples include the use of in situ hybridization to localize gene therapy vectors, the assessment of vector integration into genomic DNA by the polymerase chain reaction (PCR), and the use of immunohistochemistry to evaluate the potential cross-reactivity of monoclonal antibodies. In situ PCR techniques may allow for confirmation of the germ cell localization of nucleic acids and may therefore facilitate the risk assessment of germline transmission. Increased involvement with biopharmaceuticals will present challenging opportunities for the toxicologic pathologist and will allow for much greater use of molecular techniques, which have a critical role in the preclinical development of these compounds.

Regional versus systemic delivery of recombinant vaccinia virus as suicide gene therapy for murine liver metastases

OBJECTIVE

Specific and efficient tumor-targeted gene delivery is the major goal for successful cancer gene therapy.

SUMMARY BACKGROUND DATA

A recombinant thymidine kinase-deleted vaccinia virus (vv) encoding the firefly luciferase (luc) reporter gene or the prodrug converter gene cytosine deaminase (CD) was constructed. The authors compared the extent, duration, and pattern of transgene (luc) expression in vivo after portal venous, intraperitoneal, or intravenous virus administration and survival after treatment with the vv containing CD followed by the prodrug 5-fluorocytosine (5-FC) in a murine model of disseminated liver metastases from colon cancer.

METHODS

Recombinant vv containing the luc transgene within the thymidine kinase locus was administered to mice with isolated liver metastases from an MC38 adenocarcinoma. Transgene expression was determined in tumor and organs at various time points. Tumor-bearing mice were treated with recombinant vv containing CD and 5-FC or with appropriate controls and followed for survival.

RESULTS

Tumor-specific gene delivery was achieved irrespective of administration route, with gene expression in tumors increased by up to 100,000-fold compared with normal tissues. There was significantly increased transgene expression in tumor after portal venous or intraperitoneal virus administration (p = 0.001 vs. systemic). Treatment using a CD-expressing vv and systemic 5-FC resulted in a significant survival benefit in all treatment groups compared with controls (p < 0.007); there was no additional benefit for portal venous or intraperitoneal virus administration.

CONCLUSIONS

Suicide gene therapy using vv with the CD/5-FC system leads to tumor-specific gene expression and improved survival and can result in cure of established liver metastases.

Gene directed enzyme prodrug therapy for cancer

One strategy for gene therapy in malignant disease is gene directed enzyme prodrug therapy (GDEPT). An exogenous enzyme gene is delivered to tumour cells. The enzyme, when expressed, can convert a non-toxic prodrug into a cytotoxic species that is capable of killing the cell in which it has been produced. The most frequently used systems are HSV thymidine kinase with ganciclovir and E. coli cytosine deaminase with 5-fluorocytosine. The bystander effect is of key importance to GDEPT: This describes the local spread of active species from cells that express the enzyme to kill adjacent, untransduced cells. The ultimate success of GDEPT will depend on the ability to achieve efficient gene delivery to and expression in target cells, whilst minimising expression in other tissues. A variety of techniques exist to achieve this goal, including loco-regional administration, manipulation of tumour blood supply and use of tumour-specific promoters to drive enzyme gene expression.

Converting cancer genes into killer genes

Over the past decade, it has become clear that tumorigenesis is driven by alterations in genes that control cell growth or cell death. Theoretically, the proteins encoded by these genes provide excellent targets for new therapeutic agents. Here, we describe a gene therapy approach to specifically kill tumor cells expressing such oncoproteins. In outline, the target oncoprotein binds to exogenously introduced gene products, resulting in transcriptional activation of a toxic gene. As an example, we show that this approach can be used to specifically kill cells overexpressing a mutant p53 gene in cell culture. The strategy may be generally applicable to neoplastic diseases in which the underlying patterns of genetic alterations or abnormal gene expression are known.

Prodrugs in cancer chemotherapy

At present, chemotherapy is not very effective against common solid cancers, especially once they have metastasized. However, laboratory experiments and studies on dose intensification in humans have indicated that some anticancer agents might be curative, but only if the dose given was very much higher than that attainable clinically. Prodrugs activated by enzymes expressed at a high level in tumors can deliver at least 50-fold the normal dose and can cure animals with tumors normally resistant to chemotherapy. The approach is not practicable clinically because of the rarity of human tumors expressing a high level of an activating enzyme. However, new therapies have been proposed that overcome this limitation of prodrug therapy. Enzymes that activate prodrugs can be directed to human tumor xenografts by conjugating them to tumor-associated antibodies. After allowing for the conjugate to clear from the blood a prodrug is administered which is normally inert, but which is activated by the enzyme delivered to the tumor. This procedure is referred to as ADEPT (antibody-directed enzyme prodrug therapy). Using different combinations of antibody, enzyme and prodrug, many classes of human tumor xenograft have been shown to be very sensitive to this procedure although in most cases they are quite resistant to conventional chemotherapy. Early clinical trials are promising and indicate that ADEPT may become an effective treatment for all solid cancers for which tumor-associated or tumor-specific antibodies are known. Tumors have also been targeted with the genes encoding for prodrug activating enzymes. This approach has been called virus-directed enzyme prodrug therapy (VDEPT) or more generally GDEPT (gene-directed enzyme prodrug therapy) and has shown good results in laboratory systems. These new therapies may finally realize the potential of prodrugs in cancer chemotherapy.