Suicide gene therapy with adenoviral delivery of HSV-tK gene for patients with local recurrence of prostate cancer after hormonal therapy

Targeting of prostate cancer cells by a cytotoxic lentiviral vector containing a prostate stem cell antigen (PSCA) promoter

BACKGROUND

The efficacy of prostate cancer gene therapy is limited by the inefficiency of prostate-specific promoters as compared to ubiquitous viral promoters. The purpose of this investigation was to evaluate the specificity and efficacy of a lentiviral vector driven by a PSCA promoter.

METHODS

Prostate cancer (LNCap, C42-B, and LAPC-4) and non-prostate cancer (HeLa, MB231, and MCF-7) cells were transduced with a lentiviral vector expressing either the luciferase or the HSV-TK suicide gene and driven by a short PSCA promoter. Specificity and efficacy were evaluated in vitro and in vivo.

RESULTS

Luciferase expression was only detected in prostate cancer cells and was comparable to the universal CMV promoter. Luciferase expression in prostate cancer cells cultured with androgen was higher than that in cells cultured without androgen. In subsequent cytotoxicity experiments in which the luciferase marker gene was replaced with the HSV-TK gene, the lentiviral vector harboring the PSCA promoter induced cytotoxicity in prostate cancer cell lines while demonstrating a minimal effect on non-prostate cells. Cellular toxicity was correlated to increasing concentrations of the prodrug ganciclovir. Androgen had a positive effect on the cytotoxicity of this lentiviral construct. Intratumoral injection of prostate cancer xenografts with the lentiviral construct induced tumor growth inhibition versus saline controls.

CONCLUSION

Our results indicate that a lentiviral gene therapy vector driven by a short PSCA promoter can induce prostate-specific cellular toxicity in vivo and in vitro and may provide a strategy to selectively treat local and advanced metastatic prostate cancer. Prostate 69: 1422-1434, 2009. (c) 2009 Wiley-Liss, Inc.

Gene therapy of benign gynecological diseases

Gene therapy is the introduction of genetic material into patient's cells to achieve therapeutic benefit. Advances in molecular biology techniques and better understanding of disease pathogenesis have validated the use of a variety of genes as potential molecular targets for gene therapy based approaches. Gene therapy strategies include: mutation compensation of dysregulated genes; replacement of defective tumor-suppressor genes; inactivation of oncogenes; introduction of suicide genes; immunogenic therapy and antiangiogenesis based approaches. Preclinical studies of gene therapy for various gynecological disorders have not only shown to be feasible, but also showed promising results in diseases such as uterine leiomyomas and endometriosis. In recent years, significant improvement in gene transfer technology has led to the development of targetable vectors, which have fewer side-effects without compromising their efficacy. This review provides an update on developing gene therapy approaches to treat common gynecological diseases such as uterine leiomyoma and endometriosis.

Biosafety in ex vivo gene therapy and conditional ablation of lentivirally transduced hepatocytes in nonhuman primates

Ex vivo gene therapy is an interesting alternative to orthotopic liver transplantation (OLT) for treating metabolic liver diseases. In this study, we investigated its efficacy and biosafety in nonhuman primates. Hepatocytes isolated from liver lobectomy were transduced in suspension with a bicistronic liver-specific lentiviral vector and immediately autotransplanted (SLIT) into three cynomolgus monkeys. The vector encoded cynomolgus erythropoietin (EPO) and the conditional suicide gene herpes simplex virus-thymidine kinase (HSV-TK). Survival of transduced hepatocytes and vector dissemination were evaluated by detecting transgene expression and vector DNA. SLIT was safely performed within a day in all three subjects. Serum EPO and hematocrit rapidly increased post-SLIT and their values returned to baseline within about 1 month. Isoforms of EPO detected in monkeys' sera differed from the physiological renal EPO. In liver biopsies at months 8 and 15, we detected EPO protein, vector mRNA and DNA, demonstrating long-term survival and functionality of transplanted lentivirally transduced hepatocytes. Valganciclovir administration resulted in complete ablation of the transduced hepatocytes. We demonstrated the feasibility and biosafety of SLIT, and the long term (>1 year) functionality of lentivirally transduced hepatocytes in nonhuman primates. The HSV-TK/valganciclovir suicide strategy can increase the biosafety of liver gene therapy protocols by safely and completely ablating transduced hepatocytes on demand.

Effectiveness of HSV-tk suicide gene therapy driven by the Grp78 stress-inducible promoter in esophagogastric junction and gastric adenocarcinomas

BACKGROUND

The thymidine kinase gene of the herpes simplex virus (HSV-tk) is a suicide gene when administrated with the prodrug ganciclovir (GCV). This study investigated the effectiveness of HSV-tk activation as gene therapy for gastroesophageal junction and gastric adenocarcinomas using either the stress-inducible Grp78 promoter or the murine leukemia virus long-terminal repeat (LTR) promoter.

METHODS

The HSV-tk gene, controlled by either the Grp78 promoter or the LTR promoter, was transduced into the gastroesophageal junction adenocarcinoma cell line SK-GT-5 and the gastric adenocarcinoma cell line MKN-74. Cell viability after exposure to varying concentrations of GCV was compared. The same cell lines were used to develop a nude mouse model for studies of the HSV-tk/GCV effect in vivo. The effect of intraperitoneal GCV injection on growth of the subcutaneous tumors was measured. HSV-TK expression was measured by Western blot and reverse transcription polymerase chain reaction.

RESULTS

Cell viability in vitro was significantly lower in the HSV-tk expressing (HSV-tk+) cells compared to control (no HSV-tk) cells after exposure to GCV. MKN-74tk+ cells were more sensitive to GCV killing than SK-GT-5tk+ cells. After culture with 1 microg/ml GCV for 10 days, MKN-74/tk cells were totally killed, whereas most SK-GT-5/tk cells survived. Cell viability was significantly lower under glucose starvation conditions when HSV-tk expression was regulated by the Grp78 promoter compared with the LTR promoter. MKN-74 tumors formed with HSV-tk+ cells in nude mice were eliminated after administration of GCV for 3 weeks, but GCV had no effect on tumors formed from HSV-tk- cells. Eradication of tumor formed with Grp78-tk cells was faster than that with LTR-tk cells. HSV-TK protein and mRNA were expressed in the transduced, but not the non-transduced tumors.

CONCLUSION

HSV-tk xwith ganciclovir suicide gene therapy results in significant cell killing in gastroesophageal junction and gastric adenocarcinoma cells both in vitro and in vivo, but complete tumor elimination only occurred with the gastric adenocarcinoma cell tumors. The most effective approach in this study used the Grp78 promoter in glucose-starvation stress conditions.

Combination effect of oncolytic adenovirus therapy and herpes simplex virus thymidine kinase/ganciclovir in hepatic carcinoma animal models

AIM

Oncolytic adenovirus, also called conditionally replicating adenovirus (CRAD), can selectively propagate in tumor cells and cause cell lysis. The released viral progeny can infect neighboring cancer cells, initiating a cascade that can lead to the ultimate destruction of the tumor. Suicide gene therapy using herpes simplex virus thymidine kinase (HSV-TK) and ganciclovir (GCV) offers a potential treatment strategy for cancer and is undergoing preclinical trials for a variety of tumors. We hypothesized that HSV-TK gene therapy combined with oncolytic adenoviral therapy would have an enhanced effect compared with the individual effects of the therapies and is a potential novel therapeutic strategy to treat liver cancer.

METHODS

To address our hypothesis, a novel CRAD was created, which consisted of a telomerase-dependent oncolytic adenovirus engineered to express E1A and HSV-TK genes (Ad-ETK). The combined effect of Ad-ETK and GCV was assessed both in vitro and in vivo in nude mice bearing HepG2 cell-derived tumors. Expression of the therapeutic genes by the transduced tumor cells was analyzed by RT-PCR and Western blotting.

RESULTS

We confirmed that Ad-ETK had antitumorigenic effects on human hepatocellular carcinoma (HCC) both in vitro and in vivo, and the TK/GCV system enhanced oncolytic adenoviral therapy. We confirmed that both E1A and HSV-TK genes were expressed in vivo.

CONCLUSION

The Ad-ETK construct should provide a relatively safe and selective approach to killing cancer cells and should be investigated as an adjuvant therapy for hepatocellular carcinoma.

Titration of variant HSV1-tk gene expression to determine the sensitivity of 18F-FHBG PET imaging in a prostate tumor

Because of its high selectivity and specificity for the imaging reporter probe 9-(4-(18)F-fluoro-3-[hydroxymethyl]butyl)guanine ((18)F-FHBG), the herpes simplex virus type 1 thymidine kinase (HSV1-tk) variant sr39tk is actively being studied as a PET reporter gene. We recently demonstrated the capability of using a prostate-specific transcriptional amplification PET reporter vector, AdTSTA-sr39tk, to target prostate cancer lymph node metastasis. However, one area that warrants further study is the examination of the sensitivity of PET by determining the minimum percentage of cells expressing the sr39tk transgene needed for detection. Addressing this question could determine the sensitivity of vector-mediated sr39tk PET in cancer-targeting strategies.

METHODS

DU-145, PC-3, and CWR22Rv.1 prostate cancer cell lines (a total of 1 x 10(6) cells) were studied, of which 7%, 10%, 25%, 50%, or 70% were transduced with the lentiviral vector constitutively expressing HSV1-sr39tk-IRES-enhanced green fluorescent protein (EGFP). Cells were subcutaneously implanted into the left shoulder of severe combined immunodeficient mice and evaluated. Tumor cells comparably transduced with an EGFP control vector were implanted on the right shoulder. Mice were imaged using PET with (18)F-FHBG at 8, 15, and 22 d after tumor implant. On day 23, tumors were isolated and analyzed for sr39tk transgene expression by quantitative reverse-transcriptase polymerase chain reaction (RT-PCR), Western blotting, immunohistochemistry, and flow cytometry for EGFP expression.

RESULTS

Results showed a linear relationship between the level of sr39tk expression and the quantity of tracer accrual in DU-145, with the minimal value for PET detection at 10%. The magnitude of tracer retention in sr39tk-expressing cells was amplified over time as the tumor grew. Protein levels in the stepwise titration increased with the percentage of sr39tk-transduced cells.

CONCLUSION

The stepwise titration of prostate cancer cells transduced with the lenti-CMV-sr39tk-IRES-EGFP determined the minimum number of sr39tk-expressing tumor cells necessary to be detected by PET using the (18)F-FHBG reporter probe. Furthermore, PET signal correlated well with traditional methods of protein evaluation such as flow cytometry, quantitative RT-PCR, Western blotting, and immunohistochemistry. Unlike the traditional methods, however, the use of PET is noninvasive and will be more advantageous in clinical situations.

Phase I and biodistribution study of recombinant adenovirus vector-mediated herpes simplex virus thymidine kinase gene and ganciclovir administration in patients with head and neck cancer and other malignant tumors

In this study, we investigated the safety and efficacy in cancer patients of a single intra-tumor injection of recombinant adenovirus vector-mediated herpes simplex virus thymidine kinase gene (AdV/TK) followed by systemic administration of ganciclovir (GCV). In 18 patients with malignant tumors refractory to standard treatment, AdV/TK was injected on day 1 with dose escalation from 2.5 x 10(11) to 1 x 10(12) virus particles (VP), and GCV (5 mg kg(-1)) was delivered intravenously every 12 h from days 2 to 15. The most common treatment-related toxicities were transient fever (10/18) and local injection site reaction (10/18), and most adverse events were WHO grade I/II. Anti-adenovirus antibody levels increased continuously during treatment, but anti-HSV antibody levels remained stable. One patient had a PR at the injection site but PD was found in the primary site (lung cancer), one patient with fibrosarcoma of the neck had an MR, five patients had SD, and 10 patients had PD. In conclusion, AdV/TK followed by GCV can be administered safely to Chinese cancer patients, and achieved a local response with few environmental effects. Because the response was localized, single regional tumor relapse, especially after radiation, may be an indication for this suicide gene therapy.

Towards fibroid gene therapy: adenovirus-mediated delivery of herpes simplex virus 1 thymidine kinase gene/ganciclovir shrinks uterine leiomyoma in the Eker rat model

BACKGROUND/AIMS

The objective of this study was to assess in vivo gene therapy of uterine leiomyomas in the Eker rat model using adenovirus (Ad)-mediated delivery of herpes simplex virus 1 thymidine kinase gene (HSV1TK) followed by ganciclovir (GCV) treatment.

METHODS

We randomized 27 female Eker rats with MRI-confirmed uterine leiomyomas to a single treatment with direct intra-tumor injection of Ad-HSV1TK/GCV, Ad-LacZ/GCV, or medium alone. Samples were collected from tumors, other body organs, and blood at 10, 20, and 30 days after treatment to assess the safety and efficacy of the treatment.

RESULTS

Ad-HSV1TK/GCV treatment significantly decreased uterine fibroid volume by 75 +/- 16, 58.7 +/- 6.3, and 67.5 +/- 27.5%, of the pretreatment volume at days 10, 20, and 30, respectively. Ad-HSV1TK/GCV increased caspase-3 activity, Bax expression, and TUNEL apoptosis marker, and it decreased cyclin D1, PCNA, Bcl2, and PARP protein expressions. Ad transfection induced local CD4+ and CD8+ infiltration and serum anti-Ad antibodies. Additionally, Ad transfection was tumor-localized and safe to non-target tissues.

CONCLUSION

These studies demonstrate a marked efficiency and high safety for the Ad-HSV1TK/GCV therapeutic approach in the context of Eker rat uterine leiomyomas and provide essential preclinical data for the development of Ad-HSV1TK/GCV gene therapy for uterine fibroids.

Molecular markers in prostate cancer. Part II: potential roles in management

Predicting treatment responses in advanced prostate cancer (PCa) currently centres on prostate-specific antigen (PSA) kinetics and on being able to visualize measurable changes in imaging modalities. New molecular markers have emerged as potential diagnostic and prognostic indicators; these were summarized in Part I of this review in the Asian Journal of Andrology. A number of molecular markers are now being used to enhance PCa imaging and staging. However, management options for advanced and hormone-resistant PCa (HRPC) are limited and additional therapeutic options are needed. Molecular markers have been proposed as potential therapeutic targets using gene therapy and immunomodulation. Additionally, markers identified in early PCa and precursor lesions may offer novel targets for chemoprevention and vaccine development. This review summarizes the current advances regarding the roles of these markers in the management of PCa.

The anti-tumor effect of human monocyte-derived dendritic cells loaded with HSV-TK/GCV induced dying cells

Herpes simplex virus thymidine kinase (HSV-TK) gene and dendritic cells (DC) have been used as the pioneering in cancer therapy. HSV-TK gene can induce apoptosis and necrosis in tumor cells in the presence of the non-toxic prodrug ganciclovir (GCV). We investigated the anti-tumor effect of DC vaccination by introducing dying cells from HSV-TK gene treatment as an adjuvant. HepG(2)-TK cell line was established by transfecting human hepatoma cell line HepG(2) (HLA-A(2) positive) with HSV-TK gene. Dying tumor cells were generated by culturing HepG(2)-TK cells with GCV. After engulfed dying cells efficiently, immature DCs (imDC) derived from human monocytes were fully matured and elicited marked proliferation and cytotoxicity against HLA matched HepG(2) cells in autologous peripheral blood mononuclear cells (PBMC). It also implied that HepG(2) specific CTLs played an important role in the cytotoxicity which was primarily depended on Th1 responses. Given the feasibility of inducing dying cells by HSV-TK/GCV in vivo, our results suggest an effective method in clinical human hepatocellular carcinoma (HCC) treatment by an in vitro model of applying HSV-TK gene modified human tumor cells integrated with DC vaccination.

miR-122a-regulated expression of a suicide gene prevents hepatotoxicity without altering antitumor effects in suicide gene therapy

The combined use of adenovirus (Ad) vectors expressing herpes simplex virus thymidine kinase (HSVtk) and ganciclovir (GCV) offers a potential therapeutic strategy against cancer. However, intratumorally injected Ad vectors are disseminated into the systemic circulation and efficiently transduce the liver, resulting in severe hepatotoxicity. In order to overcome this problem, an Ad vector carrying a microRNA (miRNA)-regulated expression system was developed by inserting into the 3'-untranslated region (3'-UTR) of the expression cassette four tandem copies of sequences with perfect complementarity to miR-122a, which exhibits liver-specific expression. Transgene expression from the Ad vector carrying the miR-122a target sequences was 7- to 70-fold lower in cells with high miR-122a expression as compared to expression from a conventional Ad vector. Intratumoral injection of the Ad vector containing the miR-122a target sequences resulted in a 130- to 1,500-fold reduction in hepatic transgene products (without affecting the transgene expression in the tumor) when compared with those from a conventional Ad vector. In suicide gene therapy, the inclusion of the miR-122a target sequences in the HSVtk expression cassette achieved not only significant antitumor effects, but also a dramatic reduction in HSVtk/GCV-induced hepatotoxicity. These results indicate that Ad vectors that mediate miR-122a-regulated HSVtk expression provide a safe and efficient suicide gene therapy strategy.

REIC/Dkk-3 overexpression downregulates P-glycoprotein in multidrug-resistant MCF7/ADR cells and induces apoptosis in breast cancer

The overexpression of reduced expression in immortalized cells (REIC)/Dickkopf-3 (Dkk-3), a tumor suppressor gene, induced apoptosis in human prostatic and testicular cancer cells. The aim of this study is to examine the potential of REIC/Dkk-3 as a therapeutic target against breast cancer. First, the in vitro apoptotic effect of Ad-REIC treatment was investigated in breast cancer cell lines and the adenovirus-mediated overexpression of REIC/Dkk-3 was thus found to lead to apoptotic cell death in a c-Jun-NH(2)-kinase (JNK) phosphorylaion-dependent manner. Moreover, an in vivo apoptotic effect and MCF/Wt tumor growth inhibition were observed in the mouse model after intratumoral Ad-REIC injection. As multidrug resistance (MDR) is a major problem in the chemotherapy of progressive breast cancer, the in vitro effects of Ad-REIC treatment were investigated in terms of the sensitivity of multidrug-resistant MCF7/ADR cells to doxorubicin and of the P-glycoprotein expression. Ad-REIC treatment in MCF7/ADR cells also downregulated P-glycoprotein expresssion through JNK activation, and sensitized its drug resistance against doxorubicin. Therefore, not only apoptosis induction but also the reversal of anticancer drug resistance was achieved using Ad-REIC. We suggest that REIC/Dkk-3 is a novel target for breast cancer treatment and that Ad-REIC might be an attractive agent against drug-resistant cancer in combination with conventional antineoplastic agents.

Direct and distant antitumor effects of a telomerase-selective oncolytic adenoviral agent, OBP-301, in a mouse prostate cancer model

We previously constructed OBP-301 (Telomelysin, a telomerase-specific replication-competent adenovirus with human telomerase reverse transcriptase (hTERT) promoter), which showed a strong anticancer effect by inducing cell lysis of human non-small cell lung cancer and colorectal cancer cells. To investigate the utility of OBP-301 for prostate cancer treatment, we herein evaluate the cell killing and antitumor effects. First, in vitro hTERT-specific adenovirus transduction in human prostate cancer cells (LNCaP, PC3, DU145) was confirmed using OBP-401 (Telomelysin-green fluorescent protein (GFP)). There was no detectable GFP transduction in the human prostate normal cells (PrEC, PrSC). Consistently, the cell-killing effect of OBP-301 was observed only in the cancer cells. Second, using an in vivo subcutaneous LNCaP tumor model in nude mice, we demonstrated that three intratumoral OBP-301 injections (10(7) PFU per tumor x 3 days) were sufficient to eradicate the detectable LNCaP prostate tumor. We also demonstrated that the ispilateral treatment with OBP-301 significantly suppressed contralateral LNCaP tumor growth in both sides of the tumor model. Histological and immunohistochemical analyses revealed diffuse oncolytic degeneration and adenoviral E1A protein expression in both sides of the tumors. Therefore, in situ OBP-301 administration could be a promising therapeutic strategy against prostate cancer and its metastatic lesions.

Long-term outcome of phase I/II clinical trial of Ad-OC-TK/VAL gene therapy for hormone-refractory metastatic prostate cancer

We evaluated the long-term safety and efficacy of Ad-OC-TK (recombinant adenoviral vector carrying an osteocalcin promoter-driven herpes simplex virus thymidine kinase gene) plus VAL (valacyclovir) gene therapy for hormone-refractory prostate cancer. Ad-OC-TK/VAL therapy is the first in vivo adenovirus-mediated gene therapy to be used to treat metastatic prostate cancer, including bone metastasis. Six patients were enrolled in this trial, and two doses of Ad-OC-TK (2.5 x 10(9) or 2.5 x 10(10) plaque-forming units) were injected into locally recurrent tumor or bone metastasis on day 1 and day 8. Patients were also given VAL (3 g/day) for 21 days. Safety and efficacy were evaluated for at least 8 months in each patient. All patients tolerated this therapy with no serious adverse events. One prostate-specific antigen (PSA) response (from 318.3 to 4.9 ng/ml) was observed with a time to PSA progression (TTP) of 12 months. Docetaxel (30 mg/m2 per week) and estramustine (560 mg/day) combination chemotherapy (DE) was given to three docetaxel-naive patients on PSA failure after gene therapy. All three patients had a PSA response to DE therapy with 21, 7, and 4 months of TTP. These results suggest that additional trials are warranted.

ATRA enhances bystander effect of suicide gene therapy in the treatment of prostate cancer

OBJECTIVES

All-trans retinoic acid (ATRA) has been shown to inhibit the growth of many malignancies by altering gap junctional intercellular communication (GJIC) and the expression of connexin (Cx) 43. Here, we report that the alteration of GJIC by ATRA may directly enhance the bystander effect (BE) of suicide gene therapy against prostate cancer in vitro and in vivo.

METHODS

PC-3 cells were exposed to different concentrations of ATRA for varying lengths of time in culture. Flow cytometry was performed to measure Cx43-positive cells and the GJIC function of the cells was examined with the scrape-loading dye transfer assay. Cells were treated with ATRA in combination with an adenovirus/ganciclovir (Ad-TK/GCV) system encoding herpes simplex virus-thymidine kinase, and the BE was assessed in the treatment of androgen-independent prostate cancer both in vitro and in vivo. Semiquantitative reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry were performed to assess the expression of Cx43 mRNA and protein in tumor tissues.

RESULTS

ATRA significantly increased the amount of Cx43-positive cells in a time- and dose-dependent manner (P < 0.05). GJIC functions were enhanced 3- to 5-fold in the presence of ATRA, although ATRA did not augment GCV toxicity of PC-3 cells. In the mixing assay, ATRA significantly increased cell killing when the ratio of TK-positive cells in the coculture ranged from 30% to 60% compared with ATRA-untreated cell (P < 0.05), and attained 50% cell killing cells when the ratio of TK-positive cell was 30%, but the same result did not appear until the ratio of TK-positive cell was up to 60% in the ATRA-untreated cell. Mice treated with a combination of ATRA and GCV had significantly smaller Ad-TK infected tumors than those treated with GCV or ATRA alone after 3-weeks of therapy (P < 0.05). However, from the fourth-week of therapy, there was no difference in tumor growth inhibition between GCV treatment and GCV + ATRA treatment (P > 0.05), as two tumors in the latter group started to grow more quickly than tumors in the control group. This phenomenon was not found in other groups.

CONCLUSIONS

ATRA could enhance the efficiency of cell killing in suicide gene therapy against prostate cancer by strengthening the BE in vitro and in vivo. Induction of Cxs and GJIC by ATRA might provide an element of selectivity to suicide gene therapy. Future studies should focus on safety and tailoring this cooperative therapy to the patient.

Current status of gene therapy in Asia

Asian countries, in particular China, Japan, and Korea, have been aggressively researching and developing gene medicines over the last 15 years or so. In China, an adenovirus expressing p53 was approved for commercial use in the year 2003, and has been on the actual market since then, becoming the world's first commercial gene-based drug. In Japan and Korea, many interesting scientific discoveries have been made, and industrially valuable technologies have been developed. It is particularly noteworthy to see that in these countries, gene therapy has been very keenly nurtured in relation with industrial and financial sectors. Despite remarkable progresses made in Asia, however, their activities have not been visibly noticed by many scientists in the US and European countries. This article briefly reviews key features of the past achievements and recent progresses made in three Asian countries.

Carboxypeptidase-G2-based gene-directed enzyme-prodrug therapy: a new weapon in the GDEPT armoury

Gene-directed enzyme-prodrug therapy (GDEPT) aims to improve the therapeutic ratio (benefit versus toxic side-effects) of cancer chemotherapy. A gene encoding a 'suicide' enzyme is introduced into the tumour to convert a subsequently administered non-toxic prodrug into an active drug selectively in the tumour, but not in normal tissues. Significant effects can now be achieved in vitro and in targeted experimental models, and GDEPT therapies are entering the clinic. Our group has developed a GDEPT system that uses the bacterial enzyme carboxypeptidase G2 to convert nitrogen mustard prodrugs into potent DNA crosslinking agents, and a clinical trial of this system is pending.