No retroviremia or pathology in long-term follow-up of monkeys exposed to a murine amphotropic retrovirus

Clinical development of retroviral replicating vector Toca 511 for gene therapy of cancer

INTRODUCTION

The use of tumor-selectively replicating viruses is a rapidly expanding field that is showing considerable promise for cancer treatment. Retroviral replicating vectors (RRV) are unique among the various replication-competent viruses currently being investigated for potential clinical utility, because they permanently integrate into the cancer cell genome and are capable of long-term persistence within tumors. RRV can mediate efficient tumor-specific delivery of prodrug activator genes, and subsequent prodrug treatment leads to synchronized cell killing of infected cancer cells, as well as activation of antitumor immune responses.

AREAS COVERED

Here we review preclinical studies supporting bench-to-bedside translation of Toca 511, an optimized RRV for prodrug activator gene therapy, the results from Phase I through III clinical trials to date, and potential future directions for this therapy as well as other clinical candidate RRV.

EXPERT OPINION

Toca 511 has shown highly promising results in early-stage clinical trials. This vector progressed to a registrational Phase III trial, but the results announced in late 2019 appeared negative overall. However, the median prodrug dosing schedule was not optimal, and promising possible efficacy was observed in some prespecified subgroups. Further clinical investigation, as well as development of RRV with other transgene payloads, is merited.

MicroRNA 142-3p attenuates spread of replicating retroviral vector in hematopoietic lineage-derived cells while maintaining an antiviral immune response

We are developing a retroviral replicating vector (RRV) encoding cytosine deaminase as an anticancer agent for gliomas. Despite its demonstrated natural selectivity for tumors, and other safety features, such a virus could potentially cause off-target effects by productively infecting healthy tissues. Here, we investigated whether incorporation of a hematopoietic lineage-specific microRNA target sequence in RRV further restricts replication in hematopoietic lineage-derived human cells in vitro and in murine lymphoid tissues in vivo. One or four copies of a sequence perfectly complementary to the guide strand of microRNA 142-3p were inserted into the 3' untranslated region of the RRV genome expressing the transgene encoding green fluorescent protein (GFP). Viral spread and GFP expression of these vectors in hematopoietic lineage cells in vitro and in vivo were measured by qPCR, qRT-PCR, and flow cytometry. In hematopoietic lineage-derived human cell lines and primary human stimulated peripheral blood mononuclear cells, vectors carrying the 142-3pT sequence showed a remarkable decrease in GFP expression relative to the parental vector, and viral spread was not observed over time. In a syngeneic subcutaneous mouse tumor model, RRVs with and without the 142-3pT sequences spread equally well in tumor cells; were strongly repressed in blood, bone marrow, and spleen; and generated antiviral immune responses. In an immune-deficient mouse model, RRVs with 142-3pT sequences were strongly repressed in blood, bone marrow, and spleen compared with unmodified RRV. Tissue-specific microRNA-based selective attenuation of RRV replication can maintain antiviral immunity, and if needed, provide an additional safeguard to this delivery platform for gene therapy applications.

No evidence of xenotropic murine leukemia virus-related virus transmission by blood transfusion from infected rhesus macaques

The discovery of xenotropic murine leukemia virus-related virus (XMRV) in human tissue samples has been shown to be due to virus contamination with a recombinant murine retrovirus. However, due to the unknown pathogenicity of this novel retrovirus and its broad host range, including human cell lines, it is important to understand the modes of virus transmission and develop mitigation and management strategies to reduce the risk of human exposure and infection. XMRV transmission was evaluated by whole-blood transfusion in rhesus macaques. Monkeys were infected with XMRV to serve as donor monkeys for blood transfers at weeks 1, 2, and 3 into naïve animals. The donor and recipient monkeys were evaluated for XMRV infection by nested PCR assays with nucleotide sequence confirmation, Western blot assays for development of virus-specific antibodies, and coculture of monkey peripheral blood mononuclear cells (PBMCs) with a sensitive target cell line for virus isolation. XMRV infection was demonstrated in the virus-injected donor monkeys, but there was no evidence of virus transmission by whole-blood transfusion to naïve monkeys based upon PCR analysis of PBMCs using XMRV-specific gag and env primers, Western blot analysis of monkey plasma up to 31 to 32 weeks after transfusion, and coculture studies using monkey PBMCs from various times after transfusion. The study demonstrates the lack of XMRV transmission by whole-blood transfusion during the acute phase of infection. Furthermore, analysis of PBMC viral DNA showed extensive APOBEC-mediated G-to-A hypermutation in a donor animal at week 9, corroborating previous results using macaques and supporting the possible restriction of XMRV replication in humans by a similar mechanism.

Non-human viruses developed as therapeutic agent for use in humans

Viruses usually infect a restricted set of host species, and only in rare cases does productive infection occur outside the natural host range. Infection of a new host species can manifest as a distinct disease. In this respect, the use of non‐human viruses in clinical therapy may be a cause for concern. It could provide the opportunity for the viruses to adapt to the new host and be transferred to the recipient's relatives or medical caretakers, or even to the normal host species. Such environmental impact is evidently undesirable. To forecast future clinical use of non‐human viruses, a literature study was performed to identify the viruses that are being considered for application as therapeutic agents for use in humans. Twenty‐seven non‐human virus species were identified that are in (pre)clinical development, mainly as oncolytic agents. For risk management, it is essential that the potential environmental consequences are assessed before initiating clinical use, even if the virus is not formally classified as a genetically modified organism. To aid such assessment, each of these viruses was classified in one of five relative environmental risk categories, ranging from “Negligible” to “Very High”. Canary pox virus and the Autographa californica baculovirus were assigned a “Negligible” classification, and Seneca Valley virus, murine leukemia virus, and Maraba virus to the “High” category. A complicating factor in the classification is the scarcity of publicly available information on key aspects of virus biology in some species. In such cases the relative environmental risk score was increased as a precaution. Copyright © 2011 John Wiley & Sons, Ltd.

Enhanced efficiency of prodrug activation therapy by tumor-selective replicating retrovirus vectors armed with the Escherichia coli purine nucleoside phosphorylase gene

Gene transfer of the Escherichia coli purine nucleoside phosphorylase (PNP) results in potent cytotoxicity after administration of the prodrug fludarabine phosphate (F-araAMP). Here, we have tested whether application of this strategy in the context of replication-competent retrovirus (RCR) vectors, which can achieve highly efficient tumor-restricted transduction as well as persistent expression of transgenes, would result in effective tumor inhibition, or, alternatively, would adversely affect viral replication. We found that RCR vectors could achieve high levels of PNP expression concomitant with the efficiency of their replicative spread, with significant cell killing activity in vitro and potent therapeutic effects in vivo. In U-87 xenograft models, replicative spread of the vector resulted in progressive transmission of the PNP transgene, as evidenced by increasing PNP enzyme activity with time after vector inoculation. On F-araAMP administration, high efficiency gene transfer of PNP by the RCR vector resulted in significant suppression of tumor growth and extended survival time. As the RCR mediates stable integration of the PNP gene and continuous expression, an additional round of F-araAMP administration resulted in further survival benefit. RCR-mediated PNP suicide gene therapy thus represents a highly efficient form of intracellular chemotherapy, and may achieve effective antitumor activity with less systemic toxicity.

Promoter Choice for Retroviral Vectors: Transcriptional Strength Versus Trans-Activation Potential

Gene expression from retroviral vectors can be driven by either the retroviral long terminal repeat (LTR) promoter or by cellular or viral promoters located internally in an LTR-deleted self-inactivating vector design. Adverse events in a gene therapy clinical trial for X-linked severe combined immune deficiency have led to the realization that the enhancer/promoter elements contained within integrated vectors may also act outside the vector genome to trans-activate host genes. Ideally, the gene expression system chosen for a vector should possess a low probability of trans-activation while still being able to support adequate levels of transgene expression. However, the parameters that define these specific characteristics are unknown. To gain insight into the mechanism of trans-activation, we compared a panel of commonly used retroviral LTRs and cellular and viral promoters for their ability to drive gene expression and to trans-activate a nearby minimal promoter in three different cell lines. These studies identified two elements, the cytomegalovirus enhancer/chicken beta-actin (CAG) and elongation factor (EF)-1alpha promoters, as being of potential value for use in vectors targeting lymphoid cells, as these elements exhibited both high levels of reporter gene expression and relatively low levels of trans-activation in T cells.

Adaptive evolution of a tagged chimeric gammaretrovirus: identification of novel cis-acting elements that modulate splicing

Retroviruses are well known for their ability to incorporate envelope (Env) proteins from other retroviral strains and genera, and even from other virus families. This characteristic has been widely exploited for the generation of replication-defective retroviral vectors, including those derived from murine leukemia virus (MLV), bearing heterologous Env proteins. We investigated the possibility of "genetically pseudotyping" replication-competent MLV by replacing the native env gene in a full-length viral genome with that of another gammaretrovirus. Earlier, we developed replication-competent versions of MLV that stably transmit and express transgenes inserted into the 3' untranslated region of the viral genome. In one such tagged MLV expressing green fluorescent protein, we replaced the native env sequence with that of gibbon ape leukemia virus (GALV). Although the GALV Env protein is commonly used to make high-titer pseudotypes of MLV vectors, we found that the env replacement greatly attenuated viral replication. However, extended cultivation of cells exposed to the chimeric virus resulted in selection of mutants exhibiting rapid replication kinetics and different variants arose in different infections. Two of these variants had acquired mutations at or adjacent to the splice acceptor site, and three others had acquired dual mutations within the long terminal repeat. Analysis of the levels of unspliced and spliced viral RNA produced by the parental and adapted viruses showed that the mutations gained by each of these variants functioned to reverse an imbalance in splicing caused by the env gene substitution. Our results reveal the presence of previously unknown cis-acting sequences in MLV that modulate splicing of the viral transcript and demonstrate that tagging of the retroviral genome with an easily assayed transgene can be combined with in vitro evolution as an approach to efficiently generating and screening for replicating mutants of replication-impaired recombinant viruses.

Tissue- and tumor-specific targeting of murine leukemia virus-based replication-competent retroviral vectors

Replication-competent retrovirus vectors based on murine leukemia virus (MLV) have been shown to effectively transfer therapeutic genes over multiple serial infections in cell culture and through solid tumors in vivo with a high degree of genomic stability. While simple retroviruses possess a natural tumor selectivity in that they can transduce only actively dividing cells, additional tumor-targeting strategies would nevertheless be advantageous, since tumor cells are not the only actively dividing cells. In this study, we used the promiscuous murine cytomegalovirus promoter, a chimeric regulatory sequence consisting of the hepatitis B virus enhancer II and the human alpha1-antitrypsin (EII-Pa1AT) promoter, and a synthetic regulatory sequence consisting of a series of T-cell factor binding sites named the CTP4 promoter to generate replicating MLV vectors, whereby the last two are transcriptionally restricted to liver- and beta-catenin/T-cell factor-deregulated cells, respectively. When the heterologous promoters were used to replace almost the entire MLV U3 region, including the MLV TATA box, vector replication was inefficient since nascent virus particle production from infected cells was greatly decreased. Fusion of the heterologous promoters lacking the TATA box to the MLV TATA box, however, generated vectors which replicated with almost-wild-type kinetics throughout permissive cells while exhibiting low or negligible spread in nonpermissive cells. The genomic stability of the vectors was shown to be comparable to that of a similar vector containing wild-type MLV long terminal repeats, and tropism analysis over repeated infection cycles showed that the targeted vectors retained their original specificity.

Single-shot, multicycle suicide gene therapy by replication-competent retrovirus vectors achieves long-term survival benefit in experimental glioma

Achieving therapeutically efficacious levels of gene transfer in tumors has been a major obstacle for cancer gene therapy using replication-defective virus vectors. Recently, replicating viruses have emerged as attractive tools for cancer therapy, but generally achieve only transitory tumor regression. In contrast to other replicating virus systems, transduction by replication-competent retrovirus (RCR) vectors is efficient, tumor-selective, and persistent. Correlating with its efficient replicative spread, RCR vector expressing the yeast cytosine deaminase suicide gene exhibited remarkably enhanced cytotoxicity in vitro after administration of the prodrug 5-fluorocytosine. In vivo, RCR vectors replicated throughout preestablished primary gliomas without spread to adjacent normal brain, resulting in profound tumor inhibition after a single injection of virus and single cycle of prodrug administration. Furthermore, stable integration of the replicating vector resulted in persistent infection that achieved complete transduction of ectopic glioma foci that had migrated away from the primary tumor site. Thus, efficient and stable integration of suicide genes represents a unique property of the RCR vector that achieved multiple cycles of synchronous cell killing upon repeated prodrug administration, resulting in chronic suppression of tumor growth and prolonged survival.

Human primary T lymphocytes have a low capacity to amplify MLV-based amphotropic RCR and the virions produced are largely noninfectious

The presence of replication-competent retrovirus (RCR) in retroviral-based gene therapy products poses a potential safety risk for patients. Therefore, RCR testing of clinical gene therapy products and monitoring of patients enrolled in gene therapy trials is required to assure viral safety. The requirement to test ex vivo-transduced cells originates from the presumed amplification of adventitious RCR during the transduction procedure. However, data on the capacity of different cell types to do so are lacking. In this study, we sought to analyze the amplification potential of primary human T lymphocytes after infection with amphotropic MLV-based RCR. The total number of viral particles produced after 1 or 2 weeks was measured by a quantitative 4070A env-specific RT-PCR assay. The fraction of infectious replication-competent viral particles was analyzed in the PG-4 S+L- assay. From this study, we conclude that the total number of viral particles RCR produced by T lymphocytes is 2-4 logs lower than the number produced by NIH-3T3 cells. Surprisingly, less than 1% of the viral particles produced by primary T lymphocytes appeared to be infectious, while nearly all virions produced by NIH-3T3 were. We conclude that primary human T lymphocytes are low producers of MLV-based amphotropic RCR.

Highly efficient and tumor-restricted gene transfer to malignant gliomas by replication-competent retroviral vectors

The first large randomized phase III trial in gene therapy demonstrated no improvement in the survival of patients injected with packaging cells that produced conventional replication-defective retroviral vectors carrying the herpes simplex virus thymidine kinase gene, a disappointing result that was attributed to extremely poor levels of transduction efficiency. To circumvent this problem, we have developed a modified replication-competent retrovirus (RCR) that is capable of transducing human glioma cell lines A-172, U-87, T-98G, U-373, and U-138 and rat glioma cell lines C6 and 9L, over multiple infection cycles in vitro, resulting in a tremendous enhancement in transduction efficiency over conventional replication-defective retroviral vectors at the same dose. Whereas the transduction efficiency of conventional retroviral vectors injected into preestablished subcutaneous U-87 tumors at a dose of 1.0 x 10(5) transducing units (TU) was only 0.2% at 6 weeks postinjection, the same dose of RCR vector resulted in up to 97.2% transduction. When RCR vectors at a dose of 1.0 x 10(4) TU were injected into preestablished intracranial U-87 tumors, transduction efficiency at 2 and 3 weeks was 74 and 98.1%, respectively. Notably, however, intracranial injection of RCR vectors did not result in detectable infection of normal brain cells. Furthermore, using a sensitive polymerase chain reaction assay, no detectable RCR signal could be observed in any extracerebral tissues, including lung, liver, kidney, upper gastrointestinal tract (esophagus and stomach), lower gastrointestinal tract (colon and small intestine), skin, spleen, and bone marrow. Treatment of U-87 intracranial gliomas with RCR vectors carrying the yeast cytosine deaminase suicide gene followed by 5-fluorocytosine prodrug administration resulted in 100% survival over a 60-day follow-up period, compared with 0% survival of control groups receiving vector alone or prodrug alone. Our results demonstrate that RCR vectors can achieve therapeutically significant levels of transduction in malignant human gliomas, and that RCR vector spread after intratumoral injection is restricted to the tumor itself.

Pig endogenous retroviruses and xenotransplantation

Xenotransplantation of porcine organs might provide an unlimited source of donor organs to treat endstage organ failure diseases in humans. However, pigs harbour retroviruses with unknown pathogenic potential as an integral part of their genome. While until recently the risk of interspecies transmission of these porcine endogenous retroviruses (PERV) during xenotransplantation has been thought to be negligible, several reports on infection of human cells in vitro and spread of PERV from transplanted porcine islets in murine model systems have somewhat challenged this view. Here, we compile available data on PERV biology and diagnostics, and discuss the significance of the results with regard to the safety of clinical xenotransplantation.

Gene transfer into nonhuman primate hematopoietic stem cells: implications for gene therapy

Hematopoietic stem cells (HSCs) are desirable targets for gene therapy because of their self-renewal and multilineage differentiation abilities. Retroviral vectors are extensively used for HSC gene therapy. However, the initial human trials of HSC gene marking and therapy showed that the gene transfer efficiency into human HSCs with retroviral vectors was very low in contrast to the much higher efficiency observed in murine experiments. The more quiescent nature of human HSCs and the lower density of retroviral receptors on them hindered the efficient gene transfer with retroviral vectors. Since nonhuman primates have marked similarity to humans in all aspects including the HSC biology, their models are considered to be important to evaluate and improve gene transfer into human HSCs. Using these models, clinically relevant levels (around 10% or even more) of gene-modified cells in peripheral blood have recently been achieved after gene transfer into HSCs and their autologous transplantation. This has been made possible by improving ex vivo transduction conditions such as introduction of Flt-3 ligand and specific fibronectin fragment (CH-296) into ex vivo culture during transduction, and the use of retroviral vectors pseudotyped with the gibbon ape leukemia virus or feline endogenous retrovirus envelope. Other strategies including the use of lentiviral vectors and in vivo selective expansion of gene-modified cells with the drug resistance gene or selective amplifier gene (also designated the molecular growth switch) are now being tested to further increase the fraction of gene-modified cells using nonhuman primate models. In addition to the high gene transfer efficiency, high-level and long-term expression of transgenes in human HSCs and their progeny is also required for effective HSC gene therapy. For this purpose, other backbones of retroviral vectors such as the murine stem cell virus and cis-DNA elements, such as the ss-globin locus control region and the chromatin insulator, also need to be tested in nonhuman primate models. Nonhuman primate studies will continue to provide an important framework for human HSC gene therapy. Well-designed nonhuman primate studies will also offer unique insights into the HSCs, immune system, and transplantation biology characteristic of large animals.

A uniquely stable replication-competent retrovirus vector achieves efficient gene delivery in vitro and in solid tumors

A major obstacle in cancer gene therapy is the limited efficiency of in vivo gene transfer by replication-defective retrovirus vectors in current use. One strategy for circumventing this difficulty would be to use vectors capable of replication within tumor tissues. We have developed a replication-competent retrovirus (RCR) vector derived from murine leukemia virus (MuLV). This vector utilizes a unique design strategy in which an internal ribosome entry site-transgene cassette is positioned between the env gene and the 3' long terminal repeat (LTR). The ability of this vector to replicate and transmit a transgene was examined in culture and in a solid tumor model in vivo. The RCR vector exhibited replication kinetics similar to those of wildtype MuLV and mediated efficient delivery of the transgene throughout an entire population of cells in culture after an initial inoculation with 1 plaque-forming unit (PFU) of vector per 2000 cells. After injection of 6 x 10(3) PFU of vector into established subcutaneous tumors, highly efficient spread of the transgene was observed over a period of 7 weeks, in some cases resulting in spread of the transgene throughout the entire tumor. MuLV-based RCR vectors show significant advantages over standard replication-defective vectors in efficiency of gene delivery both in culture and in vivo. This represents the first example of the use of an RCR vector in an adult mammalian host, and their first application to transduction of solid tumors.

Viral vectors for gene therapy: the art of turning infectious agents into vehicles of therapeutics

Considered by some to be among the simpler forms of life, viruses represent highly evolved natural vectors for the transfer of foreign genetic information into cells. This attribute has led to extensive attempts to engineer recombinant viral vectors for the delivery of therapeutic genes into diseased tissues. While substantial progress has been made, and some clinical successes are over the horizon, further vector refinement and/or development is required before gene therapy will become standard care for any individual disorder.

Targeted and stable gene delivery into muscle cells by a two-step transfer system

We developed a muscle-specific gene delivery system based on two-step gene transfer. The first step involved adenovirus-mediated transfer of the ecotropic retrovirus receptor (EcoRec) gene driven by the muscle-specific desmin promoter. Both human primary myoblasts and fibroblasts were efficiently transduced with this adenovirus vector. However, expression of EcoRec was detected only in myoblasts. In the second step, EcoRec-expressing myoblasts could be stably transduced with the ecotropic retroviral vector with the beta-galactosidase gene. Approximately 15% of myoblasts were transduced by this two-step strategy. When the transduced myoblasts were differentiated into myotubes, extensive cell-cell fusion occurred, and the apparent number of beta-galactosidase-positive cells increased to 28%. These results indicate that our two-step gene delivery system could be used for targeted and stable gene transfer into muscle cells.

Xenotransplantation: is the risk of viral infection as great as we thought?

Two major hurdles remain before xenotransplantation can enter the clinic. The first is the more technical issue of being able to overcome the human immune response that leads to rejection of transplanted organs/cells from other species. The second, reviewed here, concerns the potential risk of inadvertent transfer of animal viruses present in the xenotransplant that are able to infect the human recipient. The threat from viruses is a particularly contentious topic because it poses a risk not only to those individuals who receive xenotransplants, but also to healthy individuals who come into contact, either directly or indirectly, with the xenotransplant recipient. In this review, we describe some of the virus types, in addition to the much discussed porcine endogenous retroviruses that might cross the species barrier, and assess the risk of such viruses causing disease in human hosts.

Clinical outcome of a protocol to produce immunosuppression in rhesus monkeys (Macaca mulatta): application to infectious disease and gene therapy studies

Induced immunosuppression is required for a number of studies using rhesus monkeys (Macaca mulatta). This report describes the clinical outcome and safety of a dose-finding experiment that determined doses of cyclophosphamide and prednisone that could be used to induce a state of immunosuppression in rhesus monkeys. After determining the optimum dose of immunosuppressive agents, the protocol was then used on animals participating in infectious disease and gene therapy studies. Splenectomy was performed in some animals to increase the severity of immunosuppression. The onset, duration, and severity of lymphopenia and leukopenia were consistent in all animals. In most animals, physical examination findings and clinical serum chemistry profiles demonstrated only transient abnormalities. With proper clinical monitoring, combination treatment with cyclophosphamide and prednisone is an effective and safe method for inducing immunosuppression in rhesus monkeys.

Sensitive assays for isolation and detection of simian foamy retroviruses

Simian foamy viruses (SFVs) are highly prevalent in a variety of nonhuman primate species ranging from prosimians to apes. SFVs possess a broad host range, and human infections can occur by cross-species transfer (W. Heneine et al., Nat. Med. 4:403-407, 1998). Retrovirus screening of potential sources of infection, such as laboratory research animals and simian-derived biological products, could minimize human exposure to SFVs by reducing the risk of potential retrovirus infection in humans. We describe a variety of sensitive assays for SFV isolation and detection which were developed with a prototype strain of SFV serotype 2. The Mus dunni cell line (M. R. Lander and S. K. Chattopadhyay, J. Virol. 52:695-698, 1984) was found to be highly sensitive for SFV production on the basis of various general and specific retrovirus detection assays such as reverse transcriptase assay, transmission electron microscopy, immunofluorescence assay, and Western blotting. A highly sensitive PCR assay was developed on the basis of the sequences in primary SFV isolates obtained from pig-tailed macaques (Macaca nemestrina) and rhesus macaques (Macaca mulatta). Analysis of naturally occurring SFV infection in macaques indicated that analysis by a combination of assays, including both highly sensitive, specific assays and less sensitive, broadly reactive assays, is important for evaluation of retrovirus infection.

Biosafety monitoring of patients receiving intracerebral injections of murine retroviral vector producer cells

Patients with recurrent malignant brain cancer, who were receiving gene therapy by intracerebral injection of murine retroviral vector producer cells (VPCs), were monitored for the presence of replication-competent retrovirus (RCR). RCR sequences were not detected by polymerase chain reaction (PCR) in any of the 608 peripheral blood leukocyte (PBL) samples analyzed. Vector DNA sequences were detected transiently in PBL samples from a subset of 34 patients. Humoral immune responses to a retroviral core protein p30 and murine VPC were detected in some patients, most frequently in patients receiving repeated administrations of VPC. RCR was not detected in biological assays of PBLs from 41 patients who had either anti-retroviral antibodies in sera and/or vector DNA in PBLs. Our data suggest that in situ generation of RCR was not detected following intracerebral inoculation of VPCs in any of the 128 patients evaluated.

Genetic immunotherapy for cancer

The application of gene therapy to the treatment of human and veterinary diseases offers an innovative addition to the clinician's treatment options. Gene therapy can potentially be used to (1) replace defective or missing genes, (2) treat cancer, and (3) deliver drugs. The focus of this paper is the use of gene therapy in the treatment of cancer. To be effective, genes must be delivered to target cells which can then serve as the factory to produce the gene product. Delivery systems include retroviral vectors, adenoviral vectors, and direct introduction of plasmid DNA into cells. In the case of cancer immunotherapy, introduced genes produce products that enhance tumor immunosurveillance and tumor cell killing by immune mechanisms.

Evaluation of PCR and ELISA assays for screening clinical trial subjects for replication-competent retrovirus

Gene delivery via murine-based recombinant retroviral vectors is currently widely used in gene therapy clinical trials. The vectors are engineered to be replication defective by replacing the structural and nonstructural genes of a cloned infectious retrovirus with a therapeutic gene of interest. The retroviral particles are currently generated in packaging cell lines, which supply all retroviral proteins in trans. Recombination between short homologous regions of the retroviral vector and packaging cell line elements can theoretically generate replication-competent retrovirus (RCR) and hence the Food and Drug Administration (FDA) requires the monitoring of clinical trial subjects for the presence of RCR. Sensitive polymerase chain reaction (PCR) assays have been used for the detection of murine leukemia virus (MLV) nucleotide sequences in peripheral blood mononuclear cells (PBMCs). A novel serological enzyme-linked immunosorbent assay (ELISA) for the detection of anti-MLV specific immunoglobulin (Ig) has been developed to be used as an alternative to the PCR assay. Both assays were used to monitor human immunodeficiency virus (HIV)-positive clinical trial subjects who had received multiple injections of HIV-IT (V), a retroviral vector encoding HIV-1 IIIBenv/rev. Western blot analysis and an in vitro vector neutralization assay were used to characterize further a subset of serum samples tested by ELISA. Results show no evidence of RCR infection in clinical trial subjects. PCR and ELISA assays are discussed in terms of their advantages and limitations as routine screening assays for RCR. The PCR assay is our current choice for monitoring clinical trial subjects receiving direct administration of vector, and the ELISA is our choice for those receiving ex vivo treatment regimens.

Amphotropic murine leukemia viruses induce spongiform encephalomyelopathy

Recombinants of amphotropic murine leukemia virus (A-MuLV) have found widespread use in retroviral vector systems due to their ability to efficiently and stably infect cells of several different species, including human. Previous work has shown that replication-competent recombinants containing the amphotropic env gene, encoding the major SU envelope glycoprotein that determines host tropism, induce lymphomas in vivo. We show here that these viruses also induce a spongiform encephalomyelopathy in mice inoculated perinatally. This fatal central nervous system disease is characterized by noninflammatory spongiform lesions of nerve and glial cells and their processes, and is associated with moderate astro- and microgliosis. The first clinical symptoms are ataxia, tremor, and spasticity, progressing to complete tetraparesis and incontinence, and finally death of the animal. Sequences within the amphotropic env gene are necessary for disease induction. Coinfection of A-MuLV recombinants with nonneuropathogenic ecotropic or polytropic MuLV drastically increases the incidence, degree, and distribution of the neurodegenerative disorder. The consequence of these results in view of the use of A-MuLV recombinants in the clinic is discussed.

Evaluation of recommendations for replication-competent retrovirus testing associated with use of retroviral vectors

With input from the gene therapy community, CBER is actively examining the recommendations for RCR testing during retroviral vector production, production of ex vivo-transduced cells, and in patients who receive such material. Our initial recommendations were made at a time when our experience with RCR detection assays and clinical use of retroviral vectors was limited. As the gene therapy field has matured, there is an increasing amount of data available on RCR detection assays and from monitoring of patients in clinical trials. The cumulative data give assurance that RCR detection assays in use are of sufficient sensitivity to provide a margin of safety to patients: no patients to date have evidence of RCR infection. However, CBER encourages members of the gene therapy community to continue to submit data to the FDA or to publish data that will enhance the cumulative data base on RCR testing assays, experience with different VPC, and patient monitoring. Based on the analysis of data accumulated to data, and ongoing discussions with members of the gene therapy community, CBER is proposing to discuss changes to the current RCR testing recommendations, as summarized below. RCR testing during production of retroviral vector and ex vivo-transduced cells. Development of characterized standards for RCR testing of supernatant and cells should allow comparison of assay sensitivity. One proposal under consideration is to apply statistical methods to determine how much material needs to be tested independent of the size of the production lot. Data and discussion are still needed to define a limit concentration and a value for probability of detection for RCR testing, while maintaining an appropriate margin of safety. These modifications of RCR testing strategies could lead to improvements in assay sensitivity. Additional discussion and data are also needed to evaluate the current recommendations of the testing for ex vivo-transduced cells: should both cells and supernatant be tested in all cases? RCR testing during patient follow-up. The time points required for RCR testing during patient follow-up need examination. One proposal under consideration is to sample and assay at three time points during the first year of treatment (e.g., 4-6 weeks, 3 months, and 1 year post-treatment). Further discussion is needed to define appropriate additional follow-up. Choice of assays to detect surrogate markers for RCR infection (i.e., serologic or PCR-based assays) should consider mode of vector administration and the patient population. Positive results with such assays should be pursued by direct culture assay to obtain and characterize the infectious viral isolate. These proposals will be the focal point for the discussion at the Retroviral Vector Breakout Session at the 1997 FDA/NIH Gene Therapy Conference. After the 1997 FDA/NIH Gene Therapy Conference, CBR plans to propose revised recommendations for RCR testing for public comment.

Retroviral stem cell gene therapy

Long-term in vivo gene transfer studies in mice have shown that recombinant murine retroviruses are able to infect murine hemopoietic stem cells with high efficiency. Taken together the results indicated that the proviral structure was present at high frequency in circulating hemopoietic cells resulting in significant expression levels. Because of the success of these murine studies, it was believed that gene therapy would soon be applicable to treat a wide variety of congenital or acquired human diseases associated with the hemopoietic system. However, results from gene transfer studies in nonhuman primates and first human clinical trails have indicated that murine retrovirus infection of primate hemopoietic stem cells is inefficient. Although there are essential differences between the murine and primate gene therapy studies with respect to the recombinant viruses and transduction protocols used, these differences cannot solely account for the differences observed in infection efficiency. Therefore, in recent years effort has been spent on the identification of factors limiting retroviral transduction of primate hemopoietic stem cells. Increasing knowledge concerning hemopoiesis and retroviral infection has helped in identifying a number of limiting factors. Novel transduction strategies and tools have been generated which attempt to circumvent these limiting factors. These factors as well as the strategies that showed increased retroviral infection of primate hemopoietic stem cells will be discussed.

Pharmaceutical approach to somatic gene therapy

The pharmaceutical approach to somatic gene therapy is based on consideration of a gene as a chemical entity with specific physical, chemical and colloidal properties. The genes that are required for gene therapy are large molecules (> 1 x 10(6) Daltons, > 100 nm diameter) with a net negative charge that prevents diffusion through biological barriers such as an intact endothelium, the plasma membrane or the nuclear membrane. New methods for gene therapy are based on increasing knowledge of the pathways by which DNA may be internalized into cells and traffic to the nucleus, pharmaceutical experience with particulate drug delivery systems, and the ability to control gene expression with recombined genetic elements. This article reviews two themes in the development of gene therapies: first, the current approaches involving the administration of cells, viruses and plasmid DNA; second, the emerging pharmaceutical approach to gene therapy based on the pharmaceutical characteristics of DNA itself and methods for advanced drug delivery.

Gene transfer into hematopoietic stem cells of nonhuman primates

Nonhuman primates provide an appropriate preclinical large-animal model to test the efficacy of bone marrow gene therapy procedures. Successful retroviral vector-mediated gene transfer into monkey pluripotent hematopoietic stem cells (PHSC) has closed the gap between gene transfer experiments in mouse models and clinical application of bone marrow gene therapy. After initial bone marrow transplant failures, ex vivo bone marrow culture conditions were found that sufficiently supported maintenance of the long-term repopulating ability of genetically modified autologous monkey grafts. The efficiency of gene transfer into primate PHSC has, however, remained at least one order of magnitude lower than has been achieved in mice. Similar gene transfer efficiencies have been obtained with total bone marrow grafts, CD34+ bone marrow grafts, and mobilized peripheral blood progenitor cell grafts; however, various attempts to increase the transduction efficiency have been without significant success. Primate PHSC seem to require quite different culture conditions for their maintenance and transduction than mouse PHSC, in particular regarding hematopoietic growth factor addition. In contrast to observations in other species, some form of conditioning appeared essential for engraftment of transduced PHSC in monkeys. Although it has been shown that mouse retroviruses can replicate in monkeys and are capable of inducing neoplasms, experiments in monkeys have sufficiently confirmed the safety of current gene transfer procedures to allow their clinical application.

Keratinocyte gene transfer and gene therapy

Gene therapy has moved beyond the pre-clinical stage to the treatment of a variety of inherited and acquired diseases. For such therapy to be successful, genes must be efficiently delivered to target cells and gene products must be expressed for prolonged periods of time without toxic effects to the host. This may be achieved by means of an in vivo strategy where genes are transferred directly into a host cell, or by means of an ex vivo approach through which cells are removed, cultured, targeted for gene delivery, and grafted back to the host. Several obstacles continue to delay safe and effective clinical application of gene therapy in a variety of target cells. The limited survival of transplanted cells, transient expression of transferred genes, and difficulties in targeting stem cells are technical issues requiring further investigation. Epidermal and oral keratinocytes are potential vehicles for gene therapy. Several features of these tissues can be utilized to achieve delivery of therapeutic gene products for local or systemic delivery. These qualities include: (1) the presence of stem cells; (2) the cell-, strata-, and site-specific regulation of keratinocyte gene expression; (3) tissue accessibility; and (4) secretory capacity. Such features can be exploited by the use of gene therapy strategies to facilitate: (1) identification, enrichment, and targeting of stem cells to ensure the continued presence of the transferred gene; (2) high-level and persistent transgene expression using keratinocyte-specific promoters; (3) tissue access needed for culture and grafting for ex vivo therapy and direct in vivo gene transfer; (4) secretion of transgene product for local or systemic delivery; and (5) monitoring of genetically modified tissue and removal if treatment termination is required. Optimal gene therapy strategies are being tested in a variety of tissues to treat dominant and recessive genetic disorders as well as acquired diseases such as neoplasia and infectious disease. This experience provides a basis for the application of such clinical studies to a spectrum of diseases effecting epidermal and oral keratinocytes. Gene therapy is in an early stage yet holds great promise for its ultimate clinical application.

Retroviral vectors. From laboratory tools to molecular medicine

The majority of clinical trials for gene therapy currently employ retroviral-mediated gene delivery. This is because the life cycle of the retrovirus is well understood and can be effectively manipulated to generate vectors that can be efficiently and safely packaged. Here, we review the molecular technology behind the generation of recombinant retroviral vectors. We also highlight the problems associated with the use of these viruses as gene therapy vehicles and discuss future developments that will be necessary to maintain retroviral vectors at the forefront of gene transfer technology.

An array of murine leukemia virus-related elements is transmitted and expressed in a primate recipient of retroviral gene transfer

Direct RNA-PCR analyses of T-cell lymphomas that developed in rhesus macaques during a gene transfer experiment revealed the presence of several different recombinant murine leukemia viruses (MuLV). Most prominent was the expected MuLV recombinant, designated MoLTRAmphoenv in which the amphotropic env of the helper packaging virus was joined to the long terminal repeat (LTR) of the Moloney MuLV-derived vector. This retrovirus does not exist in nature. An additional copy of the core enhancer acquired from the vector LTR may have augmented the replicative properties of MoLTRAmphoenv MuLV in several different rhesus cell types compared with the prototype amphotropic MuLV4070A. Unexpectedly, at least two types of mink cell focus-forming MuLV elements, arising from endogenous retroviral sequences expressed in the murine packaging cell line, were also transmitted and highly expressed in one of the macaques. Furthermore, murine virus-like VL-30 sequences were detected in the rhesus lymphomas, but these were not transcribed into RNA. The unanticipated presence of an array of MuLV-related structures in a primate gene transfer recipient demands ever-vigilant scrutiny for the existence of transmissible retroviral elements and replication-competent viruses possessing altered tropic or growth properties in packaging cells producing retroviral vectors.

Prospects for gene therapy in sports medicine

For the orthopedic sports medicine physician soft tissue injuries often present the greatest clinical problems. Not only do many of the most frequently injured tissues, such as the cruciate ligaments and articular cartilage, have very limited capabilities for spontaneous repair, but they also respond poorly to surgical or nonsurgical intervention. In this article we try to define the role of growth factors in these conditions and to outline concepts for future treatment based upon modulation of the native repair response. We suggest that gene transfer could improve the management of such injuries, particularly when used as vehicles for the targeted delivery of growth factors. The concept of gene therapy in orthopedic sports medicine can be extended to include disorders that present as laxity or mechanical weakness of ligaments. We speculate that subtle genetic differences between individuals may account for those who appear to be injury prone. In these cases it is likely that genes encoding the structural macromolecules of the matrix are defective. Local gene supplementation in such cases could be useful in the future.

Gene therapy on renal-cell carcinoma: magic bullet or tragic insanity?

Correction of the aberrant genetic code as a means of rational therapy has been a challenge since the first discoveries of an abnormal genetic link to expression of certain disorders. Our growing understanding of the molecular basis of cancer has also led us into a new era in cancer therapy. The possibility of gene therapy represents one of the biggest potential returns on the investment in molecular biology research over the past several years. As a massive gene therapy attack mounts against many forms of malignancy employing various techniques, strategies, and concepts, there appears to be reason to be optimistic, with expectations thus far decidedly outweighing results. Scientists and clinicians have joined together to target directly the molecular basis of tumorigenesis through the restoration of tumor-suppressor gene function or inhibition of oncogene expression. In addition, scientists mapping the human genome have supplied us with a number of genes that can be used to destroy cancer cells selectively [e.g., the herpes simplex-thymidine kinase (HS-tk) gene], induce a potent antitumor immune response (e.g., interleukin 2), and afford protection to normal tissues from the toxic effects of standard chemotherapy [e.g., multidrug resistance gene type 1 (mdr 1)]. These new anticancer tools provide new opportunities for more specific tumor cell destruction in vivo without the common regional and systemic side effects related to conventional forms of chemotherapy, immunotherapy, radiation, and surgery. Hence, over the next 5-10 years, gene therapy is likely to become a realistic treatment option for certain cancers.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of human serum and cerebrospinal fluid on retroviral vectors and packaging cell lines

Human serum is known to inactivate many retroviruses, including murine leukemia viruses (MLV). Exposure of vectors based on MLV to human serum components would presumably decrease the efficiency of gene transfer in vivo. Human serum also lyses xenogeneic cells, which would affect the survival of retroviral vector packaging cells in vivo. The effects of other body fluids, such as cerebrospinal fluid (CSF), on MLV vectors and packaging lines have not been studied. We have found that retroviral vectors packaged in ecotropic, amphotropic, and gibbon ape leukemia virus (GALV) envelope proteins were all inactivated by human sera, and human sera also lysed mouse NIH-3T3 cells and the retroviral vector packaging cells derived from them. Human fibroblasts producing amphotropic vector particles were resistant to lysis, but the particles produced by them were inactivated. In contrast, CSF did not inactivate MLV vectors, nor did it lyse murine retrovirus packaging cells. Our results suggest that exposure to human serum may prevent in vivo gene transfer by MLV vectors and xenogeneic packaging lines, but gene transfer within the central nervous system should be more successful.

Regulatory review of cellular and gene therapies: an overview of the process

Cell and gene therapies, using several different approaches, have been proposed for a variety of genetic diseases, cancer and AIDS. The major regulatory review process in the US consists of an institutional review board, the recombinant DNA advisory committee (RAC) and the Food and Drug Administration (FDA). Within the Center for Biologics Evaluation and Research, the Division of Cellular and Gene Therapies has been formed to primarily review investigational new drug applications (INDs) for cellular and gene therapies. Several appropriate "points to consider" documents have been prepared and the RAC has approved over 40 clinical protocols. Advances in biotechnology and the scientific basis for these advances are changing rapidly. Although a flexible, case-by-case approach is necessitated by these rapid changes, regulatory concerns common to all biologicals administered to human subjects remain unchanged. These include safety, efficacy, purity, potency, quality control and assessment, and reproducibility of individual lots. The goal of the review process is a prompt, complete and meticulous review. The emphasis of a pre-IND meeting is toward a working relationship between the sponsor and the FDA prior to the phase I, II and III clinical trials. A timely and ongoing evaluation of pre-clinical testing cannot be overemphasized in this rapidly growing and changing field. The development of a working relationship at this stage will ensure a seamless integration of the IND process with the product and establishment license applications. Because replication-competent retrovirus (RCR) represents a potential for pathogenicity, the FDA is recommending a conservative approach to RCR testing.

Epidermis: an attractive target tissue for gene therapy

Important advances have been made within the past several years in understanding diseases at the molecular and cellular levels, which may enable the application of somatic gene therapy to a wide variety of genetic and acquired diseases. The initial clinical trials involving somatic gene therapy have demonstrated that gene transfer into human subjects can be performed safely and with public acceptance. This review focuses on use of the epidermis as a target tissue for gene therapy and assesses various delivery systems for both ex vivo and in vivo approaches. In addition, we discuss candidate diseases that may be amenable to epidermal gene therapy and the advantages of employing transgenic mouse model systems to test the efficacy of a given gene therapy prior to clinical trials.

Characterization of replication-competent retroviruses from nonhuman primates with virus-induced T-cell lymphomas and observations regarding the mechanism of oncogenesis

Rapidly progressive T-cell lymphomas were observed in 3 of 10 rhesus monkeys several months after autologous transplantation of enriched bone marrow stem cells that had been transduced with a retroviral vector preparation containing replication-competent virus (R. E. Donahue, S. W. Kessler, D. Bodice, K. McDonagh, C. Dunbar, S. Goodman, B. Agricola, E. Byrne, M. Raffeld, R. Moen, J. Bacher, K. M. Zsebo, and A. W. Nienhuis, J. Exp. Med. 176:1124-1135, 1992). The animals with lymphoma appeared to be tolerant to retroviral antigens in that their sera lacked antibodies reactive with viral proteins and contained 10(4) to 10(5) infectious virus particles per ml. By molecular cloning and DNA sequencing, we have now demonstrated that the serum from one of the monkeys contained a replication-competent retrovirus that arose by recombination between vector and packaging encoding sequences (vector/helper [V/H] recombinant) in the producer clone used for transduction of bone marrow stem cells. Southern blot analysis demonstrated 14 or 25 copies of this genome per cell where present in two animals. The genome of a second replication-competent virus was also recovered by molecular cloning; it arose by recombination involving the genome of the V/H recombinant and endogenous murine retroviral genomes in the producer clone. Twelve copies of this amphotropic virus/mink cell focus-forming virus genome were present in tumor DNA of one animal, but it was not found in tumor DNA of the other two animals with lymphoma. Southern blot analysis of DNA from various tissues demonstrated common insertion site bands in several samples of tumor DNA from one animal, suggesting clonal origin of the lymphoma. Our data are most consistent with a pathogenic mechanism in which chronic productive retroviral infection allowed insertional mutagenesis of critical growth control genes, leading to cell transformation and clonal tumor evolution.

Improved methods of retroviral vector transduction and production for gene therapy

To facilitate clinical applications of retroviral-mediated human gene transfer, retroviral vectors must be of high titer and free of detectable replication-competent retroviruses. The purpose of this study was to optimize methods of retroviral vector production and transduction. Studies were conducted using 22 retroviral vector producer cell lines. Inactivation of retroviral vectors was greater at 37 degrees C than at 32 degrees C. A 5- to 15-fold increase of vectors was produced at 32 degrees C compared to 37 degrees C; the vector increase at 34 degrees C was intermediate. For example, PA317/G1Na.40 grew to a titer of 1.8 x 10(7) cfu/ml at 32 degrees C, compared to 5.0 x 10(5) cfu/ml at 37 degrees C. The production of retroviral vectors was scalable achieving similar results in flasks, roller bottles, or a CellCube Bioreactor. Retroviral vectors were concentrated 15-24 times with vector recovery ranging from 91 to 96% in a Pellicon tangential flow filtration system. Retroviral supernatants were successfully lyophilized. The combination of glucose or sorbitol with gelatin resulted in recovery rates of 64-83%. In studies on transduction by retroviral vectors, centrifugation of vector supernatants onto target cells significantly increased transduction efficiency as measured by vector titration for G418 resistance, fluorescence-activated cell sorting (FACS), and polymerase chain reaction (PCR) analyses. The combination of the above methods has significantly increased the growth and transduction by this vector system.

Infection of human cells with murine amphotropic replication-competent retroviruses

Replication of the murine wild-type 4070A amphotropic retrovirus and a recombinant amphotropic replication-competent retrovirus arising from the PA12 packaging cell line varied considerably among the primate cell types tested. Medium from infected primate fibroblasts and endothelial cells contained the highest viral titers [10(4)-10(5) focus-forming units (ffu)/ml], while most hematopoietic cell lines, such as K562 and MOLT4, were associated with viral titers in the range of 10(3)-10(4) ffu/ml. Interestingly, HTLV-1-transformed T cell lines (TJF-2 and HM) and primary tumor infiltrating lymphocytes (TIL) had very low viral titer (0-10(1) ffu/ml). The low production of virus was not due to low infectivity and, in contrast to the virus, retroviral vectors were expressed without difficulty. Because screening for replication-competent retrovirus (RCR) is an important component of human retroviral-mediated gene therapy clinical protocols, a variety of assays were tested for their ability to detect RCR in virus-exposed cell lines. A biologic assay (3T3 amplification) and polymerase chain reaction (PCR) for the 4070A viral envelope are effective screening methods for RCR, even in cell lines associated with low virus production.