From ONYX-015 to armed vaccinia viruses: the education and evolution of oncolytic virus development

The current field of oncolytic virus development has evolved from, and been educated by, the route adenoviruses have taken to Phase III development in the United States (Onyx-015) and commercial approval in China (H101). Clinical development of these E1B-deleted viruses showed that a staged approach, from single-agent intratumoral injections to trials testing intravenous delivery and trials in combination with approved therapies is judicious and can be successful. Additional oncolytic products are in development, including andenovirus plus other promising platforms such as herpes simplex virus, Newcastle disease virus, reovirus and vaccinia virus. These second-generation products seek to expand clinical utility beyond the modest local efficacy of Onyx-015/H101 to potent systemic delivery and efficacy. Improvement of efficacy in metastatic cancer will depend not only on enhanced killing of tumor cells, but also on achieving intravenous delivery and better intratumoral dissemination. Many viruses inherently replicate preferentially in tumors, and engineering can increase this therapeutic index by targeting genetic features of cancers. However, both viruses and cancer cells have complex biologies. Therefore, research may reveal that there is not a single predictive factor for tumor specificity. For example, the Onyx-015 mechanism-of-selectivity has proved to be complex. Further research regarding pathway dependence for other oncolytic viruses may also reveal multiple influences on their tumor tropism.     

Virotherapy--cancer targeted pharmacology

Building on their success in vaccination, many groups are now exploring the use of viruses as anticancer agents. In general, viral therapeutics provide the possibility to express anticancer proteins directly at the tumour site, decreasing exposure to normal tissue during delivery and maximising therapeutic index. Some viruses are also 'oncolytic', either naturally or by design, and these agents function to kill cancer cells selectively before spreading to infect adjacent cells and repeat the process. This whole field of cancer 'virotherapy' is moving forward rapidly at the moment, with notable clinical successes demonstrated with a range of oncolytic agents developed as directly oncolytic and also as oncolytic cancer vaccines. Given the versatility of oncolytic viruses to express therapeutic proteins we anticipate this approach will provide the platform for useful application of a broad range of innovative biological therapies.     

Radiovirotherapy: principles and prospects in oncology

Radiovirotherapy is defined as the use of viruses to deliver radioisotopic treatment into infected cells. Oncolytic viruses are able to selectively target and kill cancer cells. The combination of oncolytic viruses and radiation therapies can have synergistic antitumour properties. Viruses may act as radiosensitizers, and radiations can increase viral oncolytic properties. The combination of oncolytic viruses with a virally-directed radioisotope therapy is an innovative method to combine viruses and radiation therapy, selectively within the tumour cells. The sodium/iodide symporter (NIS) is the main transgene that has been studied for this approach. NIS can mediate the uptake of isotopes of iodine and technetium 99m for in vivo gene expression imaging and therapy. This review highlights the principles of radiovirotherapy, and its recent progress. Better understanding of the regulation of NIS opens up pathways by which to potentiate the functional expression of NIS. In terms of the therapeutic isotope, Iodine-131 has been most frequently studied but other isotopes (astatine- 211, rhenium-188) are of growing interest. Oncolytic viruses are able to infect selectively and replicate in cancer cells and promising early phase clinical trials have been recently published. Their development allows a better selectivity of viral infection and adds a virus-specific cytotoxicity to the therapeutic approach. Active research into strategies such as immunosuppressive treatment and cell-based carrier systems is seeking to circumvent the host antiviral immune response and, thus, increase the potential for systemic delivery. Finally, other anticancer therapies such as chemotherapy and external beam radiotherapy may have a synergistic effect with radiovirotherapy and such combinatorial approaches offering the prospect of accelerated translation into clinical studies.     

Targeted and armed oncolytic poxviruses for cancer: the lead example of JX-594

Oncolytic viruses (OVs) are designed to replicate in, and subsequently lyse cancer cells. Numerous oncolytic virus platforms are currently in development. Here we review preclinical and clinical experience with JX-594, the lead candidate from the targeted and armed oncolytic poxvirus class. JX-594 is derived from a vaccinia vaccine strain that has been engineered for 1) enhanced cancer targeting and 2) has been "armed" with the therapeutic transgene granulocytemacrophage colony stimulating factor (GM-CSF) to stimulate anti-tumoral immunity. Poxviruses have many ideal features for use as oncolytic agents. The development of oncolytic vaccinia viruses is supported by a large safety database accumulated in the smallpox eradication program. In addition, poxviruses have evolved unique capabilities for systemic spread through the blood that can be harnessed for the treatment of metastatic disease. JX-594 demonstrates a high degree of cancer selectivity and systemic efficacy by multiple mechanisms-of-action (MOAs) in preclinical testing. Data from Phase 1 and 2 clinical trials has confirmed that these features result in potent and systemic efficacy in patients with treatment refractory metastatic cancers.     

Lonafarnib in cancer therapy

Farnesyl transferase inhibitors (FTIs) are anticancer agents that were designed to block the post-translational attachment of the prenyl moiety to C-terminal cysteine residue of Ras and thus inactivate it. Because Ras plays an important role in tumour progression and the ras mutation is one of the most frequent aberrations in cancer, FTIs have been expected to exert excellent therapeutic activities. Phase I and II clinical trials confirmed relevant antitumour activity and low toxicity; however, no improvement in overall survival has been reported in Phase III trials. The exact mechanism of action of this class of agents is currently unknown. Increasing lines of evidence indicate that the cytotoxic actions of FTIs are not due to the inhibition of Ras proteins exclusively, but to the modulation of other targets, including RhoB, the centromere-binding proteins and other proteins that have not yet been identified. This review describes the pharmacological and clinical data as well as mechanisms of action of FTIs, especially lonafarnib (SCH-66336), a non-peptidomimetic inhibitor that has shown anticancer activity.     

Farnesyltransferase inhibitors: mechanism and applications

Farnesyltransferase (FT) inhibitors (FTIs) are among the first wave of signal transduction inhibitors to be clinically tested for antitumour properties. FTIs were designed to attack Ras oncoproteins, the function of which depends upon post-translational modification by farnesyl isoprenoid. Extensive preclinical studies have demonstrated that FTIs compromise neoplastic transformation and tumour growth. In preclinical models, FTIs display limited effects on normal cell physiology and in Phase I human trials FTIs have been largely well tolerated. Exactly how FTIs selectively target cancer cells has emerged as an important question, one which has become more pressing with the somewhat disappointing results from initial Phase II efficacy trials. Although FTI development was predicated on Ras inhibition, it has become clear that the drugs’ antineoplastic properties are based to a large degree on altering the prenylation and function of proteins other than Ras. One key candidate that has emerged is RhoB, an endosomal protein that has been implicated in selective growth inhibition and apoptosis in neoplastic cells. On the basis of mechanistic studies and other recent developments, we propose that FTIs may be useful to treat a unique spectrum of diseases including not only inflammatory breast cancer and melanoma but also non-neoplastic diseases such as diabetic retinopathy and macular degeneration.     

Protein farnesyltransferase inhibitors

Current systemic cytotoxic therapies for cancer are limited by their nonspecific mechanism of action, unwanted toxicities on normal tissues and short-term efficacy due to the emergence of drug resistance. However, identification of the molecular abnormalities in cancer, in particular the key proteins involved in abnormal cell growth, has resulted in various signal transduction inhibitor drugs being developed as new treatment strategies against the disease. Protein farnesyltransferase inhibitors (FTIs) were originally designed to target the Ras signal transduction pathway, although it is now clear that several other intracellular proteins are dependent on post-translational farnesylation (addition of a 15-carbon farnesyl moiety) for their function. Preclinical data revealed that although FTIs inhibit the growth of ras-transformed cells, they are also potent inhibitors of a wide range of cancer cell lines, many of which contain wild type ras. While understanding the mechanism of action of FTIs remains an important research goal, three different FTIs have entered clinical development. Several Phase I trials with each drug have explored different schedules for prolonged administration, and dose-limiting toxicities (DLTs) have varied from myelosuppression, gastrointestinal toxicity and neuropathy. Evidence for anticancer efficacy has come from a number of Phase II studies, not necessarily in tumour types containing ras mutations, which were the initial target for these drugs. Perhaps the most promising use for FTIs will be in combination with conventional cytotoxic drugs, based on preclinical data suggesting synergy, particularly with the taxanes. Clinical combination studies are in progress, and larger Phase II/III clinical trials are planned to see if FTIs can add to the efficacy of conventional therapies.     

Farnesyltransferase and geranylgeranyltransferase I inhibitors in cancer therapy: important mechanistic and bench to bedside issues

The fact that proteins such as Ras, Rac and RhoA require farnesylation or geranylgeranylation to induce malignant transformation prompted many investigators to develop farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I) inhibitors (FTIs and GGTIs, respectively) as novel anticancer drugs. Although FTIs have been shown to antagonise oncogenic signalling, reverse malignant transformation, inhibit human tumour growth in nude mice and induce tumour regression in transgenic mice without any signs of toxicity, their mechanism of action is not known. This review will focus on important mechanistic issues as well as bench to bedside translational issues. These will include the relevance to cancer therapy of the alternative geranylgeranylation of K-Ras when FTase is inhibited; a thorough discussion about evidence for and against the involvement of inhibition of prenylation of Ras and RhoB in the mechanism of FTIs’ antitumour activity as well as effects of FTIs and GGTIs on the cell cycle machinery and the dynamics of bipolar spindle formation and chromosome alignment during mitosis. Bench to bedside issues relating to the design of hypothesis-driven clinical trials with biochemical correlates for proof-of-concept in man will also be discussed. This will include Phase I issues such as determining maximally tolerated dose (MTD) versus effective biological dose (EBD), as well as whether Phase II trials are still needed for clinical evaluations of anti-signalling agents. Other questions that will be addressed include: what levels of inhibition of FTase activity are required for tumour response in Phase II clinical evaluations? What FTase substrates are most relevant as biochemical correlates? Are signalling pathways such as H-Ras/PI3K/Akt and K-Ras/Raf/MEK/Erk significant biological readouts? Does Ras mutation status predict response? What are appropriate clinical end-points for FTI Phase II trials? For this latter important question, time to tumour progression, median survival, percentage of patients that progress, clinical benefits and improvement in quality of life will all be discussed.     

Farnesyltransferase and geranylgeranyltransferase I inhibitors in cancer therapy: important mechanistic and bench to bedside issues

The fact that proteins such as Ras, Rac and RhoA require farnesylation or geranylgeranylation to induce malignant transformation prompted many investigators to develop farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I) inhibitors (FTIs and GGTIs, respectively) as novel anticancer drugs. Although FTIs have been shown to antagonise oncogenic signalling, reverse malignant transformation, inhibit human tumour growth in nude mice and induce tumour regression in transgenic mice without any signs of toxicity, their mechanism of action is not known. This review will focus on important mechanistic issues as well as bench to bedside translational issues. These will include the relevance to cancer therapy of the alternative geranylgeranylation of K-Ras when FTase is inhibited; a thorough discussion about evidence for and against the involvement of inhibition of prenylation of Ras and RhoB in the mechanism of FTIs' antitumour activity as well as effects of FTIs and GGTIs on the cell cycle machinery and the dynamics of bipolar spindle formation and chromosome alignment during mitosis. Bench to bedside issues relating to the design of hypothesis-driven clinical trials with biochemical correlates for proof-of-concept in man will also be discussed. This will include Phase I issues such as determining maximally tolerated dose (MTD) versus effective biological dose (EBD), as well as whether Phase II trials are still needed for clinical evaluations of anti-signalling agents. Other questions that will be addressed include: what levels of inhibition of FTase activity are required for tumour response in Phase II clinical evaluations? What FTase substrates are most relevant as biochemical correlates? Are signalling pathways such as H-Ras/PI3K/Akt and K-Ras/Raf/MEK/Erk significant biological readouts? Does Ras mutation status predict response? What are appropriate clinical end-points for FTI Phase II trials? For this latter important question, time to tumour progression, median survival, percentage of patients that progress, clinical benefits and improvement in quality of life will all be discussed.     

Pro-oncogenic cell signaling machinery as a target for oncolytic viruses

Viruses function in close harmony with the signaling machinery of their host. Upon exposure to the cell, a battery of viral products become engaged in boosting friendly signaling elements of the host or suppressing harmful ones. The efficiency of viral replication is indeed the biological outcome of this interaction between cellular and host signaling molecules. Oncolytic viruses, natural or man-made, follow the same set of rules of engagement. Pro-oncogenic cell signaling machinery, therefore, is undoubtedly the most important area influencing the development of the next generation of effective, specific and rationally designed oncolytic viruses. Ras signaling, with its central role in what is known today as molecular oncology, is an attractive topic for studying the behavior of viruses versus cancer cells and to develop strategies to target cancer cells on the basis of such platform. This work reviews the development of oncolytic herpes viruses capable of targeting Ras signaling pathway along with a few other examples of viruses which are developed to contain specificity for certain pro-oncogenic characteristics of their host cells.     

Farnesyl transferase inhibitors: a major breakthrough in anticancer therapy? Naples, 12 April 2002

An international meeting focused on farnesyl transferase inhibitors (FTIs) was held in Naples on 12 April 2002 and represented an excellent occasion to gather most of the clinicians who are involved in clinical trials with this class of new compounds. Oncogene mutations of the gene occur in approximately 30% of all human cancers and may have prognostic significance. Ras protein is normally synthesized as pro-Ras, which undergoes a number of post-translational modifications, among which farnesylation. Processed Ras proteins localize to the inner surface of the plasma membrane, and function as a molecular switch that cycles between an inactive and an active form. When in its active form, either because of the binding of an external ligand or because of its constitutive activation, Ras activates several downstream effectors, such as Raf-1, Rac, Rho and phospahtidylinositol-3 kinase, which mediate important cellular functions, such as proliferation, cytoskeletal organization and others. Interruption of the Ras signaling pathway can be basically achieved in three ways, i.e. inhibition of Ras protein expression through antisense oligonucleotides, prevention of Ras membrane localization and inhibition of Ras downstream effectors. SCH 66336 (lonafarnib; Sarasar), a tricyclic orally active FTI, has been the first of these compounds to undergo clinical development. The toxicity profile observed in all completed phase I/II trials has been fairly similar, since gastrointestinal tract toxicity (nausea, vomiting and diarrhea) and fatigue have generally qualified as dose-limiting toxicity (DLT). One objective response in a patient with pretreated non-small cell lung cancer (NSCLC) was observed. Based on preclinical evidence of synergism between lonafarnib and other anticancer agents, combination studies have been started. In particular, lonafarnib has been combined both with gemcitabine and with paclitaxel in phase I studies. Nausea, vomiting, diarrhea and myelosuppression represented DLTs in these studies, in which an encouraging clinical activity was observed, in particular in pancreatic carcinoma (lonafarnib plus gemcitabine) and in NSCLC (lonafarnib plus paclitaxel). R115777 (Zarnestra) is another novel orally active FT competitive inhibitor in clinical development. Single-agent phase I/II studies have shown that myelotoxicity and neurotoxicity are DLTs, intermittent schedule is probably better tolerated and antitumor activity is observed particularly in breast cancer. A number of combination studies with R115777 have been carried out; taken as a whole, they show that the drug can be easily combined with several anticancer agents and phase III trials exploring the potential benefit from incorporation of R115777 into active chemotherapy regimens are indicated. Two other FTIs are in an earlier stage of clinical development. BMS-214662 has the main advantage of being cytotoxic in nature, rather than cytostatic; in particular, potent antitumor activity in human tumor xenografts of different histologies has been reported. A major drawback for BMS-214662 is its severe gastrointestinal and liver toxicities, which prevent the achievement of adequate systemic exposures following the oral route. L-778,123 has been stopped in its clinical development due to its severe and unexpected toxicity, i.e. grade 4 thrombocytopenia and significant Q-T prolongation.     

Farnesyl transferase inhibitors in clinical development

Farnesyl transferase (FT) inhibitors block the main post-translational modification of the Ras protein, thus interfering with its localisation to the inner surface of the plasma membrane and subsequent activation of downstream effectors. Although initially developed as a strategy to target Ras in cancer, FT inhibitors have subsequently been acknowledged as acting by additional and more complex mechanisms that may extend beyond Ras, involving RhoB, centromere-binding proteins and probably other farnesylated proteins. SCH66336 (lonafarnib, Sarasar?, Schering-Plough), a tricyclic orally active FT inhibitor, was the first of these compounds to undergo clinical development. Gastrointestinal tract toxicities and fatigue have qualified as dose-limiting toxicities in all Phase I/II studies. Evidence of clinical activity has been reported. Lonafarnib combination studies with both gemcitabine and paclitaxel have been carried out. No unexpected toxicities were observed in these Phase I studies, while encouraging clinical activity was observed mainly in pancreatic cancer and non-small cell lung cancer. Further combination studies are ongoing. R115777 (Zarnestra?, Janssen Pharmaceutica) is another orally active FT competitive inhibitor in clinical development. Single-agent Phase I/II studies have shown that myelotoxicity and neurotoxicity are dose-limiting toxicities; intermittent schedule is probably better tolerated; antitumour activity is observed particularly in breast cancer and haematological malignancies. A number of combination studies with R115777 have been carried out. A recently completed, large Phase III trial comparing gemcitabine plus R115777 versus gemcitabine plus placebo in advanced pancreatic cancer has failed to demonstrate any survival benefit in the R115777 arm. BMS-214662 is the third FT inhibitor in clinical development. It has the main advantage of being cytotoxic in nature, rather than cytostatic, and potent in vivo antitumour activity has been reported. A major drawback for BMS-214662 is its severe gastrointestinal and liver toxicities, which prevent the achievement of adequate systemic exposures following the oral route. Alternative ways of interference with the ras oncogene pathway, in particular inhibition of ras downstream effectors, are discussed and early clinical data are presented.     

Advances in oncolytic viral therapy

Viral oncolytic therapy is under intense investigation as a novel anticancer strategy. Both alone and in combination with other conventional treatment modalities, viral oncolytics exploit the natural cytotoxicity of viruses to directly kill tumor cells. Results from preclinical studies demonstrating the intricate interaction between oncolytic viruses, the targeted tumors and their hosts, has resulted in new strategies being developed to overcome the challenges of maximizing oncolytic viral efficacy while ensuring safety. Advances in genetic engineering and the understanding of tumor biology targeting every step of the inherent host-virus interaction have allowed the design of novel viruses that selectively target, preferentially replicate in, and destroy cancer cells. A number of these oncolytic viruses have the potential to become powerful anticancer agents, especially against advanced solid tumors. The theory behind their design, the accomplishments to date and the most promising areas of research for this class of novel antineoplastic therapies are summarized in this review, with a particular focus on completed or ongoing clinical trials.     

Oncolytic viruses for the treatment of cancer: current strategies and clinical trials

Tumor-selective replicating viruses offer appealing advantages over conventional cancer therapy and are a promising new approach for the treatment of human cancer. The development of virotherapeutics is based on several strategies that each provides a different foundation for tumor-selective targeting and replication. Results emerging from clinical trials with oncolytic viruses demonstrate the safety and feasibility of a virotherapeutic approach and provide early indications of efficacy. Strategies to overcome potential obstacles and challenges to virotherapy are currently being explored and are discussed here. Importantly, the successful development of systemic administration of oncolytic viruses will extend the range of tumors that can be treated using this novel treatment modality.     

Regulatory aspects of oncolytic virus products

Many types of oncolytic viruses, wild-type virus, attenuated viruses and genetically-modified viruses, have been developed as an innovative cancer therapy. The strategies, nature, and technologies of oncolytic virus products are different from the conventional gene therapy products or cancer therapy products. From the regulatory aspects to ensure the safety, efficacy and quality of oncolytic viruses, there are several major points during the development, manufacturing, characterization, non-clinical study and clinical study of oncolytic viruses. The major issues include 1) virus design (wild-type, attenuated, and genetically engineered strains), 2) poof of concept in development of oncolytic virus products, 3) selectivity of oncolytic virus replication and targeting to cancer cells, 4) relevant animal models in non-clinical studies, 5) clinical safety, 6) evaluation of virus shedding. Until now, the accumulation of the information about oncolytic viruses is not enough, it may require the unique approach to ensure the safety and the development of new technology to characterize oncolytic viruses.     

Farnesyl transferase inhibitors in clinical development

Farnesyl transferase (FT) inhibitors block the main post-translational modification of the Ras protein, thus interfering with its localisation to the inner surface of the plasma membrane and subsequent activation of downstream effectors. Although initially developed as a strategy to target Ras in cancer, FT inhibitors have subsequently been acknowledged as acting by additional and more complex mechanisms that may extend beyond Ras, involving RhoB, centromere-binding proteins and probably other farnesylated proteins. SCH66336 (lonafarnib, Sarasar( trade mark ); Schering-Plough), a tricyclic orally active FT inhibitor, was the first of these compounds to undergo clinical development. Gastrointestinal tract toxicities and fatigue have qualified as dose-limiting toxicities in all Phase I/II studies. Evidence of clinical activity has been reported. Lonafarnib combination studies with both gemcitabine and paclitaxel have been carried out. No unexpected toxicities were observed in these Phase I studies, while encouraging clinical activity was observed mainly in pancreatic cancer and non-small cell lung cancer. Further combination studies are ongoing. R115777 (Zarnestra( trade mark ); Janssen Pharmaceutica) is another orally active FT competitive inhibitor in clinical development. Single-agent Phase I/II studies have shown that myelotoxicity and neurotoxicity are dose-limiting toxicities; intermittent schedule is probably better tolerated; antitumour activity is observed particularly in breast cancer and haematological malignancies. A number of combination studies with R115777 have been carried out. A recently completed, large Phase III trial comparing gemcitabine plus R115777 versus gemcitabine plus placebo in advanced pancreatic cancer has failed to demonstrate any survival benefit in the R115777 arm. BMS-214662 is the third FT inhibitor in clinical development. It has the main advantage of being cytotoxic in nature, rather than cytostatic, and potent in vivo antitumour activity has been reported. A major drawback for BMS-214662 is its severe gastrointestinal and liver toxicities, which prevent the achievement of adequate systemic exposures following the oral route. Alternative ways of interference with the ras oncogene pathway, in particular inhibition of ras downstream effectors, are discussed and early clinical data are presented.     

Topoisomerase I inhibition with topotecan: pharmacologic and clinical issues

Topoisomerase I (topo-I) inhibitors are a new class of anticancer agents with a mechanism of action aimed at interrupting DNA replication in cancer cells, the result of which is cell death. Most, if not all, topo-I inhibitors are derivatives of the plant extract camptothecin. Topotecan is a derivative of camptothecin which has been structurally modified to increase water solubility. The pharmacokinetic profile of topotecan is usually characterised by a two-compartment model and is linear in the dose range of 0.5 - 3.5 mg/m(2). Current clinical trials suggest antitumour activity against a variety of human tumour types, including ovarian cancer, non-small cell lung cancer (NSCLC) and non-lymphocytic haematologic malignancies. The main dose-limiting toxicity (DLT) is non-cumulative myelosuppression. Non-haematologic toxicities are usually mild. Based on several Phase I studies, the recommended Phase II dose was 1.5 mg/m(2)/day iv. for 5 days. Current Phase I and Phase II trials are evaluating the combination of topotecan with other chemotherapeutic agents to increase the therapeutic benefits of topotecan. The DLT in these trials is mainly myelosuppression.     

Topoisomerase I inhibition with topotecan: pharmacologic and clinical issues

Topoisomerase I (topo-I) inhibitors are a new class of anticancer agents with a mechanism of action aimed at interrupting DNA replication in cancer cells, the result of which is cell death. Most, if not all, topo-I inhibitors are derivatives of the plant extract camptothecin. Topotecan is a derivative of camptothecin which has been structurally modified to increase water solubility. The pharmacokinetic profile of topotecan is usually characterised by a two-compartment model and is linear in the dose range of 0.5 - 3.5 mg/m². Current clinical trials suggest antitumour activity against a variety of human tumour types, including ovarian cancer, non-small cell lung cancer (NSCLC) and non-lymphocytic haematologic malignancies. The main dose-limiting toxicity (DLT) is non-cumulative myelosuppression. Non-haematologic toxicities are usually mild. Based on several Phase I studies, the recommended Phase II dose was 1.5 mg/m²/day iv. for 5 days. Current Phase I and Phase II trials are evaluating the combination of topotecan with other chemotherapeutic agents to increase the therapeutic benefits of topotecan. The DLT in these trials is mainly myelosuppression.     

Oncolytic viruses as novel anticancer agents: turning one scourge against another

Although the use of viruses as oncolytic agents is an historic concept, the use of genetically modified viruses to selectively target tumour cells is relatively novel and recent. The ability of viruses to efficiently infect and lyse cells, combined with the potential augmentation of this effect by progeny viruses throughout the tumour provide justification for exploitation of these agents in cancer therapy. Before application to humans, though, issues related to tumour cell selectivity, lack of toxicity to normal tissues and the effect of the antiviral immune response, will have to be clarified. The more commonly used oncolytic viruses are based on mutant strains of herpes simplex virus, adenovirus and reovirus. The tumour selectivity of each of these strains is discussed, particularly the complementation of the viral defect by cellular pathways involved in tumourigenesis. The combination of oncolytic viruses with radiation, chemotherapy and gene therapy is also reviewed. Further study of the interaction of viral proteins with cellular pathways involved in cell cycle control will provide the rationale for viral mutants with increased selectivity for tumour cells.     

Farnesyltransferase inhibitors: potential role in the treatment of cancer

New targets for drug discovery have been identified rapidly as a result of the many recent and rapid advances in the understanding of signal transduction pathways that contribute to oncogenesis. In particular, oncogenic Ras proteins have been seen as an important target for novel anti-cancer drugs. Since the decade-old identification and cloning of farnesyltransferase (FTase), a critical enzyme that post-translationally modifies Ras and other farnesylated proteins, FTase inhibitors (FTIs) have been under intense investigation designed to bring them to clinical practice for cancer therapy. FTIs can inhibit the growth of tumour cells in culture and in animal models, and are now in clinical trials. Interestingly, their mechanism of action is not as simple as originally envisioned, and Ras is probably not the most important farnesylated protein whose modification is inhibited as a result of FTI treatment. Although K-Ras can escape inhibition of processing by FTIs, tumours with oncogenically mutated K-Ras proteins can still be inhibited by FTI treatment. Indeed, Ras mutation status does not correlate with FTI sensitivity or resistance. Instead, it now appears likely that inhibition of the processing of other farnesylated proteins such as RhoB and the centromere-binding proteins CENP-E and CENP-F can explain the ability of FTIs to cause cell cycle arrest and apoptosis in preclinical studies, and even to cause regression in animal tumour models. Preclinical studies suggest the likelihood that FTIs will be useful in combination therapies with conventional treatment modalities including cytotoxics (especially paclitaxel) and radiation. Phase I combination trials are underway, and early phase II/III trials using FTIs as monotherapy are open for patients with a wide variety of cancers. Early preclinical results also suggest the possibility of using FTIs as chemopreventive agents. Studies to be completed over the next 2 or 3 years should define the appropriate patient populations, administration and scheduling necessary to optimise the use of these novel anticancer agents.