DNA repair enzyme, O6-methylguanine DNA methyltransferase, modulates cytotoxicity of camptothecin-derived topoisomerase I inhibitors

MGMT in TMZ-based glioma therapy: Multifaceted insights and clinical trial perspectives

Temozolomide (TMZ) is the most preferred and approved chemotherapeutic drug for either first- or second-line chemotherapy for glioma patients across the globe. In glioma patients, resistance to treatment with alkylating drugs like TMZ is known to be conferred by exalted levels of MGMT gene expression. On the contrary, epigenetic silencing through MGMT gene promoter methylation leading to subsequent reduction in MGMT transcription and protein expression, is predicted to have a response favoring TMZ treatment. Thus, MGMT protein level in cancer cells is a crucial determining factor in indicating and predicting the choice of alkylating agents in chemotherapy or choosing glioma patients directly for a second line of treatment. Thus, in-depth research is necessary to achieve insights into MGMT gene regulation that has recently enticed a fascinating interest in epigenetic, transcriptional, post-transcriptional, and post-translational levels. Furthermore, MGMT promoter methylation, stability of MGMT protein, and related subsequent adaptive responses are also important contributors to strategic developments in glioma therapy. With applications to its identification as a prognostic biomarker, thus predicting response to advanced glioma therapy, this review aims to concentrate on the mechanistic role and regulation of MGMT gene expression at epigenetic, transcriptional, post-transcriptional, and post-translational levels functioning under the control of multiple signaling dynamics.

Role of high ubiquitin‑conjugating enzyme E2 expression as a prognostic factor in nasopharyngeal carcinoma

The incidence of nasopharyngeal carcinoma (NPC) in Southeast Asia and Taiwan is high due to epidemiological factors. Cisplatin-based chemoradiotherapy is an important treatment strategy with excellent outcomes for patients with NPC. However, the outcomes for patients who are refractory to cisplatin-based therapy are poor. Methods for risk stratification of patients with NPC undergoing cisplatin-based chemoradiotherapy require to be investigated. A previous study indicated that ubiquitin-conjugating enzyme E2 B (UBE2B) was able to regulate alkylating drug sensitivity in NPC cells. In the present study, the clinical significance of UBE2B expression in patients with NPC was analyzed. Analysis of the two available NPC datasets containing the UBE2B expression profile (GSE12452 and GSE68799) was performed to evaluate the UBE2B expression levels in NPC tissues compared with nasopharyngeal mucosal epithelial tissues. Furthermore, immunohistochemical staining was performed using anti-UBE2B antibodies on samples from 124 patients with NPC who underwent cisplatin-based chemoradiotherapy. Disease-specific survival (DSS), distant metastatic-free survival (DMeFS) and local recurrence-free survival (LRFS) of patients with high and low UBE2B expression was analyzed. Furthermore, the associations between UBE2B expression and the biological behavior of NPC cells were investigated in vitro. Using public NPC datasets and in vitro studies, it was identified that UBE2B expression levels were increased in NPC tumor tissues compared with those in mucosal epithelial tissues. The cell proliferation ability was decreased in UBE2B-deficient NPC cells as compared with that in UBE2B-proficient cells. Immunohistochemical analysis of 124 NPC tissues from patients who underwent cisplatin-based chemoradiotherapy indicated that high UBE2B expression levels were associated with poor DSS, DMeFS and LRFS. Multivariate regression analysis of factors influencing survival also confirmed that high UBE2B expression levels were a statistically significant independent risk factor for poor clinical outcomes in terms of DSS [hazard ratio (HR), 1.955; 95% CI 1.164-3.282], DMeFS (HR, 2.141; 95% CI 1.206-3.801) and LRFS (HR, 2.557; 95 CI 1.313-4.981). In vitro analysis indicated that O6-methylguanine-DNA methyltransferase attenuated cisplatin sensitivity induced by knockdown of UBE2B in NPC cells. In conclusion, the present study demonstrated that high UBE2B expression is associated with poor clinical outcomes for patients with NPC treated with cisplatin-based chemoradiotherapy.

ABCC4/MRP4 contributes to the aggressiveness of Myc-associated epithelial ovarian cancer

Epithelial ovarian cancer (EOC) is a complex disease comprising discrete histological and molecular subtypes, for which survival rates remain unacceptably low. Tailored approaches for this deadly heterogeneous disease are urgently needed. Efflux pumps belonging to the ATP-binding cassette (ABC) family of transporters are known for roles in both drug resistance and cancer biology and are also highly targetable. Here we have investigated the association of ABCC4/MRP4 expression to clinical outcome and its biological function in endometrioid and serous tumors, common histological subtypes of EOC. We found high expression of ABCC4/MRP4, previously shown to be directly regulated by c-Myc/N-Myc, was associated with poor prognosis in endometrioid EOC (P = .001) as well as in a subset of serous EOC with a "high-MYCN" profile (C5/proliferative; P = .019). Transient siRNA-mediated suppression of MRP4 in EOC cells led to reduced growth, migration and invasion, with the effects being most pronounced in endometrioid and C5-like serous cells compared to non-C5 serous EOC cells. Sustained knockdown of MRP4 also sensitized endometrioid cells to MRP4 substrate drugs. Furthermore, suppression of MRP4 decreased the growth of patient-derived EOC cells in vivo. Together, our findings provide the first evidence that MRP4 plays an important role in the biology of Myc-associated ovarian tumors and highlight this transporter as a potential therapeutic target for EOC.

Cytotoxic synergy between alisertib and carboplatin versus alisertib and irinotecan are inversely dependent on MGMT levels in glioblastoma cells

INTRODUCTION

Glioblastoma remains difficult to treat and patients whose tumors express high levels of O⁶-methylguanine DNA methyltransferase (MGMT) usually respond poorly to standard temozolomide chemotherapy. We have previously shown that the selective AURKA inhibitor alisertib potently inhibits growth of glioblastoma cells.

METHODS

We used colony formation assays, annexin V binding, and western blotting to examine the effects of alisertib on the antiproliferative capabilities of carboplatin and irinotecan in glioblastoma cells.

RESULTS

In colony formation assays, alisertib potentiated the antiproliferative effects of both carboplatin and irinotecan, often synergistically, including against glioblastoma tumor stem-like cells, as demonstrated by Chou-Talalay and Bliss statistical analyses. Western blotting showed that high MGMT expression in cell lines correlated with more pronounced potentiation of carboplatin's growth inhibitory effects by alisertib, while low MGMT expression correlated with stronger potentiation of irinotecan by alisertib. This pattern was also observed when these drug combinations were tested for their ability to induce apoptosis via annexin V binding assays. MGMT knockdown increased apoptosis caused by combined alisertib and irinotecan, while exogenous MGMT overexpression increased apoptosis from alisertib and carboplatin combination treatment.

CONCLUSIONS

These results suggest that tumor MGMT expression levels may be predictive of patient response to these drug combinations, and importantly that the combination of alisertib and carboplatin may be selectively effective in glioblastoma patients with high tumor MGMT who are resistant to standard therapy. Since clinical experience with alisertib, carboplatin and irinotecan as single agents already exists, these findings may provide rationale for the design of clinical trials for their use in combination treatment regimens.

Ubiquitin-conjugating enzyme E2 B regulates the ubiquitination of O6-methylguanine-DNA methyltransferase and BCNU sensitivity in human nasopharyngeal carcinoma cells

O⁶-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme that removes the alkyl groups from the O⁶ position of guanine and is then degraded via ubiquitin-mediated degradation. Previous studies indicated that 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) facilitates the ubiquitination and degradation of MGMT in several types of cancer cells. However, the underlying mechanism of MGMT ubiquitination remains unclear. In this study, we demonstrated for the first time that ubiquitin-conjugating enzyme E2 B (UBE2B) is a novel regulator of MGMT ubiquitination mediated by BCNU in nasopharyngeal carcinoma (NPC) cells. The E3 ubiquitin ligase RAD18, a partner of UBE2B, is also involved in BCNU-mediated MGMT ubiquitination. Overexpression/knockdown of UBE2B enhanced/reduced BCNU-mediated MGMT ubiquitination. Surprisingly, UBE2B knockdown significantly increased BCNU cytotoxicity in NPC cells. Therefore, loss of UBE2B seems to disrupt ubiquitin-mediated degradation of alkylated MGMT. We found that UBE2B knockdown reduced MGMT activity, suggesting that loss of UBE2B leads to the accumulation of deactivated MGMT and suppresses MGMT protein turnover in BCNU-treated cells. These findings indicate that UBE2B modulates sensitivity to BCNU in NPC cells by regulating MGMT ubiquitination.

Relationship Between the Expression of O6-Methylguanine-DNA Methyltransferase (MGMT) and p53, and the Clinical Response in Metastatic Pancreatic Adenocarcinoma Treated with FOLFIRINOX

BACKGROUND

To date, no predictive biomarker for the efficacy of FOLFIRINOX in metastatic pancreatic adenocarcinoma has been demonstrated. Deficiency in O6-methylguanine-DNA methyltransferase (MGMT) has been associated with a therapeutic response in endocrine tumors of the pancreas and the lack of expression of protein 53 (p53) could interfere with the action of MGMT.

OBJECTIVE

The aim of our study was to assess the prevalence of MGMT and p53 in patients with metastatic pancreatic adenocarcinoma treated with FOLFIRINOX as a first-line treatment and to investigate their association with therapeutic response and survival.

PATIENTS AND METHODS

The immunohistochemical expression of MGMT was recorded as present or absent and the expression of p53 was semi-quantitatively scored in 30 patients with metastatic pancreatic adenocarcinoma, at Angers Hospital in France between September 2011 and June 2015. Clinical and radiologic data were collected retrospectively.

RESULTS

The presence or absence of MGMT expression entailed no significant differences in response rate. Median values of progression-free survival (PFS) and overall survival (OS) were lower in patients with MGMT expression, but sample size is too small to conclude that there is a statistically significant difference. No significant relationship for response rate and PFS was observed in relation with p53 expression. By contrast, patients with a strong tumor expression of p53 had a significantly lower OS compared to patients with no or weak expression of the protein (p = 0.027). There was a positive correlation between the expression of p53 and MGMT (p = 0.08).

CONCLUSIONS

These preliminary findings suggest that for patients treated with FOLFIRINOX as a first-line treatment for metastatic pancreatic adenocarcinoma, the immunohistochemical evaluation of MGMT could not predict the clinical outcome; however, the survival was not significant probably because of the under-powered study (due to small sample size). A strong tumor expression of p53 is associated with a poor prognosis of OS.

Oncolytic adenovirus-expressed RNA interference of O6-methylguanine DNA methyltransferase activity may enhance the antitumor effects of temozolomide

Temozolomide (TMZ) is an example of an alkylating agent, which are known to be effective anticancer drugs for the treatment of various solid tumors, including glioma and melanoma. TMZ acts predominantly through the mutagenic product O⁶-methylguanine, a cytotoxic DNA lesion. The DNA repair enzyme, O⁶-methylguanine DNA methyltransferase (MGMT), which functions in the resistance of cancers to TMZ, can repair this damage. RNA interference (RNAi) has been previously shown to be a potent tool for the knockdown of genes, and has potential for use in cancer treatment. Oncolytic adenoviruses not only have the ability to destroy cancer cells, but may also be possible vectors for the expression of therapeutic genes. We therefore hypothesized that the oncolytic virus-mediated RNAi of MGMT activity may enhance the antitumor effect of TMZ and provide a promising method for cancer therapy.

Progression of O⁶-methylguanine-DNA methyltransferase and temozolomide resistance in cancer research

Temozolomide (TMZ) is an alkylating agent that is widely used in chemotherapy for cancer. A key mechanism of resistance to TMZ is the overexpression of O(6)-methylguanine-DNA methyltransferase (MGMT). MGMT specifically repairs the DNA O(6)-methylation damage induced by TMZ and irreversibly inactivates TMZ. Regulation of MGMT expression and research regarding the mechanism of TMZ resistance will help rationalize the clinical use of TMZ. In this review, we provide an overview of recent advances in the field, with particular emphasis on MGMT structure, function, expression regulation, and the association between MGMT and resistance to TMZ.

Small Molecules, Inhibitors of DNA-PK, Targeting DNA Repair, and Beyond

Many current chemotherapies function by damaging genomic DNA in rapidly dividing cells ultimately leading to cell death. This therapeutic approach differentially targets cancer cells that generally display rapid cell division compared to normal tissue cells. However, although these treatments are initially effective in arresting tumor growth and reducing tumor burden, resistance and disease progression eventually occur. A major mechanism underlying this resistance is increased levels of cellular DNA repair. Most cells have complex mechanisms in place to repair DNA damage that occurs due to environmental exposures or normal metabolic processes. These systems, initially overwhelmed when faced with chemotherapy induced DNA damage, become more efficient under constant selective pressure and as a result chemotherapies become less effective. Thus, inhibiting DNA repair pathways using target specific small molecule inhibitors may overcome cellular resistance to DNA damaging chemotherapies. Non-homologous end joining a major mechanism for the repair of double-strand breaks (DSB) in DNA is regulated in part by the serine/threonine kinase, DNA dependent protein kinase (DNA-PK). The DNA-PK holoenzyme acts as a scaffold protein tethering broken DNA ends and recruiting other repair molecules. It also has enzymatic activity that may be involved in DNA damage signaling. Because of its’ central role in repair of DSBs, DNA-PK has been the focus of a number of small molecule studies. In these studies specific DNA-PK inhibitors have shown efficacy in synergizing chemotherapies in vitro. However, compounds currently known to specifically inhibit DNA-PK are limited by poor pharmacokinetics: these compounds have poor solubility and have high metabolic lability in vivo leading to short serum half-lives. Future improvement in DNA-PK inhibition will likely be achieved by designing new molecules based on the recently reported crystallographic structure of DNA-PK. Computer based drug design will not only assist in identifying novel functional moieties to replace the metabolically labile morpholino group but will also facilitate the design of molecules to target the DNA-PKcs/Ku80 interface or one of the autophosphorylation sites.

Optimizing glioblastoma temozolomide chemotherapy employing lentiviral-based anti-MGMT shRNA technology

Despite treatments combining surgery, radiation-, and chemotherapy, patients affected by glioblastoma (GBM) have a limited prognosis. Addition of temozolomide (TMZ) to radiation therapy is the standard therapy in clinical application, but effectiveness of TMZ is limited by the tumor's overexpression of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT). The goal of this study was to use the highly specific and efficient RNA interference (RNAi) pathway to modulate MGMT expression to increase TMZ efficiency in chemotherapy resistant GBM. Using lentiviral-based anti-MGMT small hairpin RNA (shRNA) technology we observed a specific inhibition of the MGMT expression in GBM cell lines as well as in subcutaneous tumors. Tumor growth inhibition was observed following TMZ treatment of xenografts with low MGMT expression in contrast to xenografts with high MGMT expression. Bioluminescence imaging (BLI) measurements indicated that luciferase and shRNA-expressing lentiviruses were able to efficiently transduce the GBM xenografts in vivo. Treatment combining injection of a lentivirus expressing an anti-MGMT shRNA and TMZ induced a reduction of the size of the tumors, in contrast with treatment combining the lentivirus expressing the control shRNA and TMZ. Our data suggest that anti-MGMT shRNA therapy could be used in combination with TMZ chemotherapy in order to improve the treatment of resistant GBM.

DNA-Binding and Topoisomerase-I-Suppressing Activities of Novel Vanadium Compound Van-7

Vanadium compounds were studied during recent years to be considered as a representative of a new class of nonplatinum metal anticancer agents in combination to its low toxicity. Here, we found a vanadium compound Van-7 as an inhibitor of Topo I other than Topo II using topoisomerase-mediated supercoiled DNA relaxation assay. Agarose gel electrophoresis and comet assay showed that Van-7 treatment did not produce cleavable complexes like HCPT, thereby suggesting that Topo I inhibition occurred upstream of the relegation step. Further studies revealed that Van-7 inhibited Topo I DNA binding involved in its intercalating DNA. Van-7 did not affect the catalytic activity of DNase I even up to100 μM. Van-7 significantly suppressed the growth of cancer cell lines with IC(50) at nanomolar concentrations and arrested cell cycle of A549 cells at G2/M phase. All these results indicate that Van-7 is a potential selective Topo I inhibitor with anticancer activities as a kind of Topo I suppressor, not Topo I poison.

Effects of the novel DNA dependent protein kinase inhibitor, IC486241, on the DNA damage response to doxorubicin and cisplatin in breast cancer cells

The purpose of this study was to determine the degree to which the novel DNA-PKcs inhibitor, IC486241 (ICC), synergizes the cytotoxicity of DNA damaging agents in 3 genetically diverse breast cancer cell lines. The sulforhodamine B (SRB) assay was employed as a primary screening method to determine the in-vitro cytotoxicity and the degree of synergy of ICC in combination with the topoisomerase II inhibitor, doxorubicin, or the DNA cross linking agent, cisplatin. Molecular mechanisms underlying drug toxicity were probed using immunostaining and flow cytometry, as well as, the alkaline comet assay to detect DNA damage. In this study, improved cytotoxicity and significant synergy were observed with both anticancer agents in the presence of nontoxic concentrations of ICC. Moreover, ICC decreased doxorubicin-induced DNA-PKcs autophosphorylation on Ser2056 and increased doxorubicin-induced DNA fragmentation. In conclusion, the novel DNA-PKcs inhibitor, ICC, synergistically sensitized 3 breast cancer cell lines to doxorubicin and cisplatin. Enhanced efficacy of doxorubicin was achieved by inhibiting non-homologous end joining resulting in increased accumulation of DNA damage.

Irinotecan and DNA-PKcs inhibitors synergize in killing of colon cancer cells

This study sought to measure the degree of synergy induced by specific small molecule inhibitors of DNA-PK [NU7026 and IC486241 (ICC)], a major component of the non-homologous end-joining (NHEJ) pathway, with SN38 or oxaliplatin. Synergy between the DNA damaging drugs and the DNA-PK inhibitors was assessed using the sulforhodamine-B assay (SRB). Effects of drug combinations on cell cycle and DNA-PK activity were determined using flow cytometry and western blot analysis. DNA damage was assessed via comet assay and quantification of γH2AX. The role of homologous recombination repair (HRR) was determined by nuclear Rad51 protein levels and a GFP reporter recombination assay. Significant reductions in the IC(50) values of SN38 were observed at 5 and 10 μM of DNA-PK inhibitors. Moreover, at 1-2 μM (attainable concentrations with ICC in mice) these DNA-PKcs inhibitors demonstrated synergistic reductions in the IC(50) of SN38. Flow cytometric data indicated that SN38 and SN38 in combination with DNA-PKcs inhibitors showed dramatic G2/M arrest at 24 h. Furthermore, reduced phosphorylation of DNA-PKcs and increased DNA damage were observed at this time point with SN38 in combination with DNA-PKcs inhibitors as compared to cells treated with SN38 alone. SN38 alone and in the presence of ICC increased nuclear Rad51 protein levels. Furthermore, inhibition of DNA-PKcs increased HRR suggesting that NHEJ is a negative regulator of HRR. These data indicate that small molecule inhibitors of DNA-PKcs dramatically enhance the efficacy of SN38 in colon cancer cell lines.

Extended O6-methylguanine methyltransferase promoter hypermethylation following n-butylidenephthalide combined with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) on inhibition of human hepatocellular carcinoma cell growth

Epigenetic alteration of DNA methylation plays an important role in the regulation of gene expression associated with chemosensitivity of human hepatocellular (HCC) carcinoma cells. With the aim of improving the chemotherapeutic efficacy for HCC, the effect of the naturally occurring compound n-butylidenephthalide (BP), which is isolated from a chloroform extract of Angelica sinensis, was investigated. In both HepG2 and J5 HCC cell lines, a synergistic antiproliferative effect was observed when a low dosage of BP was combined with the chemotherapeutic drug 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). BCNU is an alkylating agent, and it prompts us to examine one of DNA repair genes, O(6)-methylguanine methyltransferase (MGMT). It was evident from methylation-specific polymerase chain reaction (PCR) analysis that BP/BCNU combined treatment caused a time- and concentration-dependent enhancement of MGMT promoter methylation. Overexpression of MGMT could abolish BP-induced growth inhibition in the J5 tumor cell line as measured by colony formation assay. When BP was combined with BCNU and administered, it showed significant antitumor effects in both HepG2 and J5 xenograft tumors as compared with the use of only one of these drugs. The BCNU-induced apoptosis and inhibited MGMT protein expression in HCC cells, both in vitro and in vivo, resulting from the combination treatment of BP and BCNU suggest a potential clinical use of this compound for improving the prognosis for HCCs.

Phase 1 trial of temozolomide plus irinotecan plus O6-benzylguanine in adults with recurrent malignant glioma

BACKGROUND

The current study was a phase 1 clinical trial conducted with patients who had recurrent or progressive malignant glioma (MG). The trial was designed to determine the maximum tolerated dose (MTD) and toxicity of irinotecan (CPT-11) when administered with temozolomide (TMZ) and O(6)-benzylguanine (O(6)-BG).

METHODS

All 3 drugs, CPT-11, TMZ, and O(6)-BG, were administered on Day 1 of a 21-day treatment. First, patients were treated with a 1-hour bolus infusion of O(6)-BG at a dose of 120 mg/m(2) followed immediately by a 48-hour continuous infusion of O(6)-BG at a dose of 30 mg/m(2)/d. Second, within 60 minutes of the end of the 1-hour bolus infusion of O(6)-BG, TMZ was administered orally at a dose of 355 mg/m(2). Third, 1 hour after administration of TMZ, CPT-11 was infused over 90 minutes. Patients were accrued to 1 of 2 strata based on CYP3A1- and CYP3A4-inducing antiepileptic drug (EIAED) use; dose escalation was conducted independently within these strata.

RESULTS

Fifty-five patients were enrolled. In both strata, the dose-limiting toxicities were hematologic and included grade 4 neutropenia, febrile neutropenia, leukopenia, and/or thrombocytopenia. For Stratum 1 (EIAEDs), when TMZ was administered at a dose of 355 mg/m(2), the MTD of CPT-11 was determined to be 120 mg/m(2). In contrast, for Stratum 2 (no EIAEDs), when TMZ was administered at a dose of 200 mg/m(2), the MTD of CPT-11 was determined to be 80 mg/m(2).

CONCLUSIONS

The authors believe that the results of the current study provide the foundation for a phase 2 trial of O(6)-BG in combination with CPT-11 and TMZ in patients with MG.

Transcriptional repression of O6-methylguanine DNA methyltransferase gene rendering cells hypersensitive to N,N'-bis(2-chloroethyl)-N-nitrosurea in camptothecin-resistant cells

O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that removes alkyl-adducts from the O(6)-guanine in DNA and is a crucial defense against O(6)-alkylating agent-induced cytotoxicity. We demonstrated here that two camptothecin (CPT)-resistant cell lines (CPT30 and KB100) were more sensitive to N,N'-bis(2-chloroethyl)-N-nitrosurea (BCNU) than their parental cells. Enhanced sensitivity to BCNU in these two CPT-resistant cells involved transcriptional repression of the MGMT gene. The mechanism of MGMT gene down-regulation in CPT-resistant cells was not through gene abnormality, mRNA stability, and CpG island hypermethylation. However, the high level of methyl-CpG-binding protein 2 (MeCP2) and dimethylation of H3K9 in the promoter region were found in CPT30 and KB100 cells. Furthermore, increased MeCP2 binding on MGMT promoter was also found to be correlated with MGMT gene-silencing in short-term CPT treatment; thus, enhanced BCNU sensitivity was found in CPT-treated cells. Taken together, we suggest that CPT is able to suppress the transcription of the MGMT gene through recruiting of MeCP2 and H3K9 dimethylation, thus causing a synergistic interaction with BCNU. These findings provide a possible explanation regarding why the combination of CPT and BCNU results in a better objective response than single-use alone. In addition, this study supports a new indication for treating patients who are receiving refractory CPT derivatives with BCNU.

Targeting DNA repair as a promising approach in cancer therapy

An increased DNA-repair activity in tumour cells has been associated with resistance to treatment to DNA-directed drugs, while defects in DNA repair pathways result in hypersensitivity to these agents. In the past years the unravelling of the molecular basis of these DNA pathways, with a better understanding of the DNA damage caused by different anticancer agents, has provided the rationale for the use of some DNA repair inhibitors to optimise the therapeutic use of DNA-damaging agents currently used in the treatment of tumours. In addition, the possibility to specifically target the differences in DNA repair capacity between normal and tumour cells has recently emerged as an exciting possibility. The present review will mainly cover those approaches that are currently under clinical investigation.