Amphotropic murine leukemia retrovirus is not an acute pathogen for primates

Efficient gene transfer in primitive CD34+/CD38lo human bone marrow cells reselected after long-term exposure to GALV-pseudotyped retroviral vector

Successful retroviral gene transfer into human hematopoietic stem cells was demonstrated in preliminary clinical trials at low efficiency. We have shown previously that gene transfer into committed hematopoietic progenitor cells is more efficient using a gibbon ape leukemia virus (GALV)-pseudotyped retroviral vector instead of an amphotropic retroviral vector. Here, we have conducted a systematic study of human hematopoietic progenitor cells after extended transduction with a GALV-pseudotyped retroviral vector. CD34+/CD38lo Cells were transduced for 5 days and reselected according to phenotype after culture and analyzed for cell cycle status, long-term culture-initiating cell (LTC-IC) activity, and gene transfer. Reselection of rare, very primitive progenitor cells was successful. Equal to fresh CD34+/CD38lo cells, >90% of reselected CD34+/CD38lo cells were in G0/G1. CD34+/CD38lo reselection enriched for LTC-IC (10-fold), as compared to freshly isolated CD34+/CD38lo cells with excellent specificity (82.7% of total LTC-IC were recovered in the reselected CD34+/CD38lo population) and recovery (62% of initial LTC-IC number in CD34+/CD38lo cells were recovered in the reselected fraction after transduction). Gene transfer into primitive progenitor cells was efficient with 50.5% G418-resistant LTC-IC colonies and more than 40 copies of vector provirus detectable per 100 nuclei of CD34+/CD38lo cells. To our knowledge, this is the first systematic analysis of phenotype, function, and cell cycle demonstrating retroviral gene transfer into rare, very primitive human hematopoietic progenitor cells. The chosen strategy should be of considerable value for analyzing and improving gene therapy of the hematopoietic system.

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.

Gene transfer into human haematopoietic stem cells

This review of gene transfer to the human haematopoietic system (1) describes the different vectors used to transduce genes into stem cells, emphasizing retroviruses that have already shown their efficiency and innocuousness; (2) analyses which human cells should be targeted to ensure long-lasting engraftment; (3) indicates the different means of infecting these targets ex vivo, underscoring the role of cytokines and stromal cells; (4) recollects the methods used to evaluate transduction efficiency; and (5) gathers the results of clinical trials recently performed using human stem cells. The major conclusions are that good practice can ensure safe gene delivery to human beings and that long-lasting, multilineal precursors can be transduced using retroviral vectors of marker genes or genes of therapeutic interest. However, transduction rates appear to remain relatively low, which should stimulate ongoing research on both vector design and means of ex vivo gene transfer.

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.

Gene therapy of metastatic pancreas cancer with intraperitoneal injections of concentrated retroviral herpes simplex thymidine kinase vector supernatant and ganciclovir

OBJECTIVE

The objective of this study was to determine the efficacy of intraperitoneal (IP) injections of a new concentrated herpes simplex thymidine kinase (HS-tk) retroviral vector and ganciclovir (GCV) for peritoneal metastases from pancreas cancer.

SUMMARY BACKGROUND DATA

Metastatic pancreas cancer is fatal. Gene therapy may provide a novel approach for this disease. Gene therapy with adeno- or retroviral-mediated transfer of the HS-tk gene into tumor cells renders the cells susceptible to GCV. Intratumoral or intracavity injections of retroviral vectors have been ineffective in previous studies.

METHODS

Pancreatic cancer B x PC3 cells (3 x 10(7)) were injected into the tail of pancreas in nude mice. Mice received IP injections of a concentrated HS-tk vector (5 x 10(7)) cfu/mliters) or a control vector (G1Na) without the tk gene for 10 days and GCV (100 mg/kg) for 14 days. To determine whether the vector would survive in the milieu of the peritoneal cavity, the authors examined the effects of ascitic fluid on the vector. Pancreas cancer cells were transduced in vitro with HS-tk vector in presence of media or ascitic fluid and treated with GCV.

RESULTS

Highly significant reductions in the mass of metastatic peritoneal tumor deposits were found in HS-tk-treated group (124 +/- 27 mg; n = 11) compared with G1Na vector controls (910 +/- 168 mg; n = 8; p < 0.0001). Results of polymerase chain reaction analysis demonstrated integration of the vector in the tumors, and on immunohistochemistry, expression of the TK protein was seen in the number of surviving colonies (representing nontransduced cells) were similar in both groups, suggesting that the vector effectively transduced tumor cells bathed in the ascitic fluid.

CONCLUSIONS

Results demonstrate that IP administration of concentrated retroviral HS-tk vectors is effective treatment for pancreas cancer metastatic to the peritoneal cavity; furthermore, the vector is active in the presence of ascitic fluid. Intraperitoneal retroviral HS-tk may provide a novel approach to treatment of metastatic pancreas cancer.

Myoblast gene therapy in canine mucopolysaccharidosis. I: Abrogation by an immune response to alpha-L-iduronidase

Three dogs with deficiency of the lysosomal enzyme alpha-L-iduronidase were treated by gene replacement therapy targeted at muscle. Direct intramuscular injections of plasmid encoding the alpha-L-iduronidase gene cDNA resulted in no detectable enzyme production, but may have resulted in immunologic sensitization to iduronidase protein, which the dogs lack totally. Myoblasts were grown from skeletal muscle biopsies and transduced with a retroviral vector containing the canine gene under control of the muscle creatine kinase enhancer. Several hundred-fold overexpression of enzyme production occurred in cultured cells; however, following reintroduction of the cultured cells into dogs, enzyme production declined rapidly. Concurrent with the falling enzyme levels, there was production of specific immunoglobulin G (IgG) antibody against iduronidase that was further associated with cellular infiltration of the myoblast injection sites. Most inflammatory cells were lymphocytes and plasma cells, suggesting local humoral and cellular immune responses to the enzyme-producing muscle cells. PCR analysis of tissues collected 2-22 weeks after the final treatment showed the persistence of Neo and canine alpha-L-iduronidase sequences in a progressively decreasing percentage of myoblasts. Results from this study in a canine model of mucopolysaccharidosis I underscore the fact that immunologic reactions to cells producing desirable, normal, but foreign, proteins may be as much an impediment to gene therapy as reactions to the viral vectors used to introduce the foreign gene.

Retroviral gene transfer in autologous bone marrow transplantation for adult acute leukemia

To evaluate whether marrow contributes to relapse after autologous bone marrow transplantation (AuBMT) for acute leukemia, transplanted marrow was marked with the G1N retroviral vector (Genetic Therapy Inc.) containing the neomycin phosphotransferase gene (neo). Between April 1992 and August 1993, 4 patients were transplanted for acute myeloid leukemia (AML) in second complete remission (CR) and 1 patient for acute lymphoid leukemia in first CR. An average of 12.4% (range 5-19%) of transplanted marrow mononuclear cells were exposed to G1N vector for 4 hr. In the vector-treated portion of the marrow, 4.9% of GM-CFU and 3.6% of erythroid burst-forming units (BFU-E) were resistant to G418 in vitro. In the 5 patients, the polymerase chain reaction (PCR) detected the neo sequence on only two occasions after AuBMT. Of 4 patients surviving 1 year after transplantation, only 1 had evidence of gene marked cells by PCR. Two AML patients have relapsed, one of whom had evidence of neo sequences in the bone marrow at day 100 but not at relapse 11 months after AuBMT. The second patient relapsed 18 months after AuBMT but never had PCR evidence of neo sequences before or after relapse. Our results indicate vector-transduced autologous bone marrow from heavily pretreated adults with acute leukemia mark with low efficiency, although vector sequences have been detected in bone marrow and peripheral blood up to 1 year after transplant. Of the 2 relapsed patients, no evidence of vector-marked leukemic blasts have been detected.

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.

Retroviral vector producer cell killing in human serum is mediated by natural antibody and complement: strategies for evading the humoral immune response

The introduction of retroviral vector producer cells (VPC) into tumors as a means of increasing transduction efficiency has recently been employed in human gene therapy trials. However, the fate of these xenogeneic cells in humans is not well understood. In the present study, we used an in vitro model to examine the survival of commonly used VPC lines in serum from humans and various other species. VPC derived from the murine NIH-3T3 cell line, including PA317, Psi CRIP, and GP + E-86, were effectively killed in sera from Old World primates, including human and baboon. Conversely, the same murine cell lines survived exposure to sera from dog, rabbit, rat, and mouse. This pattern of serum killing parallels the occurrence of the anti-alpha-galactosyl natural antibody (Ab) found exclusively in Old World primates. The anti-alpha-galactosyl Ab targets the terminal glycosidic structure Gal alpha 1-3Gal beta 1-4GlcNAc-R (alpha-galactosyl epitope) found on the surface of mammalian cells, excluding Old World primates. All murine-derived VPC tested expressed high levels of the alpha-galactosyl epitope as determined by FACS analysis. VPC killing was complement-mediated, because preincubation of human serum with a functionally blocking anti-C5 mAb completely abolished cell lysis. Furthermore, addition of soluble galactose(alpha 1-3)galactose (Gal alpha 1-3Gal) to human serum or down-regulation of the alpha-galactosyl epitope on the surface of VPC effectively reduced VPC killing, indicating that complement activation by these cells is primarily initiated by natural antibody recognition of the alpha-galactosyl epitope. Finally, VPC incubated with human serum for 8 hr in the presence of complement inhibition continued to produce viable retroviral particles, thus demonstrating a correlation between VPC and particle survival. Taken together, these data suggest that elimination of the alpha-galactosyl epitope or complement blockade may provide a strategy to prolong the survival of VPC and the particles that they produce in vivo.

A rapid and quantitative assay to estimate gene transfer into retrovirally transduced hematopoietic stem/progenitor cells using a 96-well format PCR and fluorescent detection system universal for MMLV-based proviruses

The polymerase chain reaction (PCR) is an extremely sensitive assay that has many uses in retroviral-mediated gene transfer protocols. Because the majority of retroviral vectors used in current gene transfer protocols are based on the Moloney-murine leukemia virus (MMLV), we have designed primers which amplify a region of the psi packaging sequence from all MMLV retroviruses tested. This assay detects gene transfer by all MMLV-based vectors and is especially useful for the laboratory that routinely screens a number of different retroviruses for their gene transfer efficiency. Furthermore, we present here a novel technique for harvesting single colonies derived from hematopoietic stem/progenitor cells growing in methylcellulose medium that expedites and substantially improves the resulting quantitative estimates of retroviral transduction frequencies. This technique utilizes a conventional 96-well format and, when coupled with a fluorescence-based post-PCR detection system, makes it unnecessary to run agarose gels to visualize the PCR product. This system of PCR product detection, which uses the 5'-->3' exonuclease activity of Taq DNA polymerase to cleave a fluorescently labeled probe during each round of PCR amplification, is fast, convenient, and at least as sensitive as an ethidium bromide-based detection system when used in conjunction with our universal PCR assay.

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.

Adenovirus-mediated gene transfer of wild-type p53 results in melanoma cell apoptosis in vitro and in vivo

Gene transfer techniques may provide efficient treatment for a variety of malignant neoplasms. A replication-deficient adenovirus (Ad) vector which carries the cDNA for wild-type p53 (AdCMV.p53) was tested for its in vitro and in vivo effects on the growth of murine melanoma cell line B16-G3.26 and human melanoma cell line SK-MEL-24. The growth of B16-G3.26 cells infected with AdCMV.p53 was inhibited when compared to the uninfected cells or cells infected with the control vector AdCMV.NLS beta gal. Similarly, the growth of SK-MEL-24 cells infected with AdCMV.p53 was also below that of AdCMV.NLS beta gal-infected and uninfected controls. DNA laddering using agarose gel electrophoresis and in situ labeling of DNA fragmentation (TUNEL) showed that AdCMV.p53-infected murine and human melanoma cells underwent apoptosis. Nude mice injected s.c. either with B16-G3.26 cells or with SK-MEL-24 cells developed localized tumors. These tumors were subsequently infiltrated with either AdCMV.p53, AdCMV.NLS beta gal or saline alone. One week after infection, B16-G3.26 tumors exposed to AdCMV.p53 were 2.5 times smaller than control tumors and exhibited DNA fragmentation. A similar growth-inhibitory effect of AdCMV.p53 was observed with SK-MEL-24 tumors. Thus, Ad-mediated wild-type p53 overexpression resulted in melanoma cell apoptosis and inhibition of melanoma growth in vitro and in vivo. These gene therapy approaches may be useful in targeting rapidly growing, malignant melanomas in a clinical setting.

A novel mechanism of retrovirus inactivation in human serum mediated by anti-alpha-galactosyl natural antibody

Type C retroviruses endogenous to various nonprimate species can infect human cells in vitro, yet the transmission of these viruses to humans is restricted. This has been attributed to direct binding of the complement component C1q to the viral envelope protein p15E, which leads to classical pathway-mediated virolysis in human serum. Here we report a novel mechanism of complement-mediated type C retrovirus inactivation that is initiated by the binding of "natural antibody" [Ab] (anti-alpha-galactosyl Ab) to the carbohydrate epitope Gal alpha 1-3Gal beta 1-4GlcNAc-R expressed on the retroviral envelope. Complement-mediated inactivation of amphotropic retroviral particles was found to be restricted to human and other Old World primate sera, which parallels the presence of anti-alpha-galactosyl natural Ab. Blockade or depletion of anti-alpha-galactosyl Ab in human serum prevented inactivation of both amphotropic and ecotropic murine retroviruses. Similarly, retrovirus was not killed by New World primate serum except in the presence of exogenous anti-alpha-galactosyl Ab. Enzyme-linked immunosorbent assays revealed that the alpha-galactosyl epitope was expressed on the surface of amphotropic and ecotropic retroviruses, and Western blot analysis further localized this epitope to the retroviral envelope glycoprotein gp70. Finally, down-regulation of this epitope on the surface of murine retroviral particle producer cells rendered them, as well as the particles liberated from these cells, resistant to inactivation by human serum complement. Our data suggest that anti-alpha-galactosyl Ab may provide a barrier for the horizontal transmission of retrovirus from species that express the alpha-galactosyl epitope to humans and to other Old World primates. Further, these data provide a mechanism for the generation of complement-resistant retroviral vectors for in vivo gene therapy applications where exposure to human complement is unavoidable.

Gene therapy for head and neck cancer

BACKGROUND

Molecular or gene therapy involves the introduction of genetic material (DNA) into host cells to induce the expression of the therapeutic product of that gene. This ability to transfer genetic material provides a novel approach for the investigation and potential treatment of a variety of both inherited and acquired diseases.

METHODS

This review summarizes the principles of molecular therapy and potential strategies for its use in the treatment of solid malignancies. Some molecular therapy strategies permanently integrate the gene into the targeted cells, whereas others induce only transient expression of a therapeutic gene product. Initial clinical studies in gene therapy are being closely regulated for public safety by both the National Institutes of Health Recombinant DNA Advisory Committee and the U.S. Food and Drug Administration.

CONCLUSIONS

These studies will provide insight into further applications of gene therapy and novel molecular interventions that may alter our management of solid malignancies; they may also reveal mechanisms for preventing or reversing the carcinogenic process.

Protection of retroviral vector particles in human blood through complement inhibition

The rapid inactivation of murine-derived retroviral vectors in human or nonhuman primate sera is largely attributed to the activity of complement mediated through the classical pathway. In this study, we have further investigated the relationship between the human complement cascade and retrovirus inactivation. Preincubation in normal human serum effectively inactivated LXSN retroviral vector particles, whereas the vector maintained the ability to transduce cells following incubation in sera deficient in either the C1, C2, C3, C5, C6, C8, or C9 human complement proteins. Preincubation of serum with monoclonal antibodies (mAbs) that functionally block specific complement components, including C5, C6, C8, and C9, successfully protected the LXSN vector from complement-mediated inactivation. Treatment of serum with cobra venom factor, which consumes terminal complement, also effectively protected the vector from inactivation. LXSN vector survival in serum corresponded inversely to the level of complement activity following treatment of serum with anti-C5 mAb as assessed in an erythrocyte hemolytic assay. Additionally, pretreatment of human whole blood with anti-C5 mAb effectively inhibited inactivation of the LXSN vector. Taken together, these data demonstrate that formation of the membrane attack complex (MAC, C5b-9) is required for the inactivation of the murine-based LXSN retroviral vector in human blood and that this process can be abrogated with the use of soluble complement inhibitors.

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.

Stereotactic delivery of a recombinant adenovirus into a C6 glioma cell line in a rat brain tumor model

The dismal results of conventional therapy for primary malignant brain tumors has justified exploring gene therapy approaches for this disease. Transduction of animal brain tumor models in vivo has been reported previously with retroviruses and herpes viruses. Because adenoviruses have the advantage of transducing quiescent and actively dividing tumor cells, they may prove to be more effective in such therapy. We used a replication-deficient recombinant adenovirus bearing the Escherichia coli beta-galactosidase gene in a rat C6 glioma tumor model. Transduced cells were detected by X-5-bromo-4-chloro-3-indolyl beta-D-galactoside staining to reveal beta-galactosidase activity. Initial experiments in vitro showed 50% and 90% transduction at vector titers of approximately 10(7) and 10(8) plaque-forming units/ml, respectively. Although no cytopathic effects were seen at 10(7) plaque-forming units/ml, more than 50% reduction in tumor cell growth was noted at 10(8) plaque-forming units/ml both in vitro and in vivo. Stereotactic delivery of the recombinant adenovirus into the frontal lobe of normal rat brains resulted in intense staining of all cell types, that is, neurons, astrocytes, and ependymal cells. Stereotactic injection into C6 glioma brain tumors in rats stained 25 to 30% of the tumor cells. We conclude that adenovirus vectors can be used to transfer genes to central nervous system tumors in vivo. Using stereotactic delivery, adenovirus vectors can transfer genes into the central nervous system intended for tumor therapy.

Survival and toxicity of xenogeneic murine retroviral vector producer cells in liver

Murine retroviral vector producer cells (VPC) can selectively transfer genes stably into proliferating cells. We injected LacZ gene producing VPC directly into normal rat liver. There was no measurable gene transfer into the surrounding hepatic parenchyma with X-GAL staining. Rejection of the xenogeneic murine VPC occurred 7-14 days after injection. Toxicity of this delivery method was evaluated with the herpes simplex-thymidine kinase (HS-tk) gene, which confers sensitivity to the antiherpes drug, ganciclovir (GCV). HS-tk VPC were injected and allowed to grow in normal liver for 7 days before GCV treatment. There was no clinical or histologic evidence of toxicity with GCV treatment. These findings suggest that the direct injection of murine VPC into xenogeneic human tumors may survive sufficiently long without immunosuppression to transfer genes to tumor cells in situ without attendant toxicity.

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.

Retroviral mediated gene transfer in chronic myelogenous leukaemia

Gene therapy for chronic myelogenous leukaemia (CML) may provide a therapeutic option for patients who are ineligible for bone marrow transplantation. To determine the feasibility of such an approach we evaluated the transduction efficiency of CML progenitor colonies from seven patients in chronic phase. Vector transduction was optimized using the CML-derived K562 cell line and applied to CML mononuclear cells. After vector exposure, optimal gene transfer was noted when CML mononuclear cell cultures contained stem cell factor, IL-3, GM-CSF and erythropoietin. The addition of IL-6 to this combination decreased transduction efficiency. Using these conditions, 20.4% +/- 2.4 (SE) of erythroid colonies (CFU-GEMM and BFU-E) and 20.2% +/- 4.7 of CFU-GM colonies were G418 resistant. This compares with a transduction efficiency of 5.9% +/- 1.1 and 6.4% +/- 1.5, respectively, for erythroid and CFU-GM colonies using marrow obtained from normal donors. Only a modest increase in gene transfer was noted when CML cells were stimulated with cytokines for the 24 h preceding vector exposure. Vector DNA in colonies expressing the BCR/ABL transcript was documented by performing PCR analysis on individual colonies. The relatively high gene transfer rate in CML suggests that this disease might be very suitable for gene therapy.

Long-term survivors in chronic granulocytic leukaemia: a study by the International CGL Prognosis Study Group. Italian Cooperative CML Study Group

The purpose of the present work were to identify the initial characteristics associated with long-term survival in chronic granulocytic leukaemia (CGL) and to analyse the accuracy of prognostic models in identifying long-term survivors. 813 Philadelphia (Ph) chromosome-positive, nonblastic CGL patients from six American and European institutions, the majority treated conventionally, with a minimum follow-up > 10 years, were studied. Stepwise logistic regression was performed to ascertain the association between the initial clinicohaematological variables and survival > or = 8 years, and a prognostic index was derived. The usefulness of both Sokal's and the new prognostic index to identify long-term survivors was assessed by calculating their positive and negative predictive accuracies, sensitivity and specificity. Median survival of the series was 45 months (range 1-255), with 784 patients (96.4%) having died and 109 (13.4%) surviving 8 years or longer. Younger age, smaller spleen, platelets < or = 600 x 10(9)/l, and lower blood blast percentage were associated with survival > or = 8 years; platelets < or = 600 x 10(9)/l and lower blood blast percentage were the predictive factors in patients 50 years old or younger. Two-thirds of long survivors belonged to Sokal's low-risk group, but the positive predictive accuracy and specificity for prolonged survival of Sokal's index were very low. This was also the case for the new predictive index.(ABSTRACT TRUNCATED AT 250 WORDS)

Clonal dominance detected in metastases but not primary tumors of retrovirally marked human breast carcinoma injected into nude mice

Human breast cancer cell lines which grow in athymic (nude) mice provide a model of tumor cell growth and metastasis. Marking transplanted tumor cell populations with retroviral vectors provides a means of studying the dynamics of tumor cell growth in vivo. We evaluated three human breast cancer cell lines, MDA-MB-435, MDA-MB-231 and MCF-7, and found the cells were highly susceptible to retroviral gene transfer after a single 2-h exposure (90.9%, 62.7% and 72.3%, respectively). MDA-MB-435 cells (5 x 10(5)) marked with a retroviral vector containing the beta-galactosidase gene (approximately 10(4) uniquely marked clones) were injected into the mammary fat pad of athymic mice to study clonal dominance. Primary tumors resected 10 weeks after injection expressed beta-galactosidase, demonstrating persistent vector expression in vivo. Southern blot analysis did not reveal clonal dominance in the primary tumors of the five mice studied. In contrast, pulmonary metastases in each animal were monoclonal or biclonal. These results demonstrate clonal dominance in pulmonary metastases but not primary tumors of retrovirally marked MDA-MB-435 cells. Our findings suggest that this model may also be used to introduce retroviral vectors expressing oncogenes, and anti-sense oncogenes, to determine their effect on tumor cell proliferation and metastasis in vivo.

Circumvention of chemotherapy-induced myelosuppression by transfer of the mdr1 gene

Drug-induced myelosuppression is a frequent reason for curtailing chemotherapy in cancer patients. 'Rescue' of myelosuppressed patients with autologous marrow transplants is reasonably advanced and permits an increase in the dose of anticancer drugs. Despite this improvement, patients often relapse with drug resistance disease. The human multidrug resistance (mdr1) gene might make it possible to render hemopoietic stem cells resistant to anticancer drugs after transfer of this gene. By introducing resistant stem cells into patients it might be possible to treat these patients repeatedly with otherwise ablative therapy. This review explores the feasibility of mdr1 gene therapy.

Toxicity studies of retroviral-mediated gene transfer for the treatment of brain tumors

Retroviral-mediated transfer of the herpes simplex virus thymidine kinase (HSVtk) gene into malignant tumors confers drug susceptibility to the antiviral drug ganciclovir. The authors have recently shown that in situ transduction of the rat 9L brain tumor following HSVtk-producer cell implantation led to tumor regression after ganciclovir administration in treated rats. A wide spectrum of potential adverse effects may, however, be associated with the application of this approach to treat brain tumors, including dissemination of the retroviral vector to nontumoral tissues within or outside the central nervous system, proliferation of the injected murine vector-producer cells at the injection site, immune-mediated responses to the implantation of xenogeneic cells, and damage to the brain from toxic by-products of the HSVtk-ganciclovir interaction. These possibilities were investigated using intracerebral and systemic injections of retroviral vector-producer cells carrying the HSVtk or the lacZ gene in mice, rats, and nonhuman primates. Using the lacZ gene as a reporter gene, no evidence of beta-galactosidase activity consistent with vector transduction was detected in any major body organ in the treated mice or rats. Similarly, the HSVtk gene transfer did not result in toxicity, with or without ganciclovir administration. In studies using rat and monkey models, no proliferation of the vector-producer cells occurred after intracerebral injection. Vector-producer cell survival was limited to 7 to 14 days. High-dose steroid therapy did not appear to extend the survival of these xenogeneic cells in rats. No significant inflammatory response was observed in the meninges or brain parenchyma. Endothelial cells were occasionally transduced in brain capillaries adjacent to the injected site of the vector-producer cells. Injection of producer cells into brain tissue elicited mild edema and reactive gliosis surrounding the injection site, which were probably the cause of a transient toxic response arising 4 to 5 days following injection of the producer cells; short-term administration of dexamethasone eliminated that response. No neurological deficits were observed in the rats or primates treated with the HSVtk vector-producer cells, with or without ganciclovir. In primates injected with producer cells, magnetic resonance imaging before, during, and after ganciclovir administration showed minimal and localized breakdown of the blood-brain barrier without significant edema or mass effect. Similarly, histological examination of the monkeys' brains showed no damage to neurons, astroglia, or myelin. Long-term clinical (> 9 months) and radiological (3 months) assessment of the primates has revealed no evidence of toxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

High-efficiency gene transfer to primary monkey airway epithelial cells with retrovirus vectors using the gibbon ape leukemia virus receptor

The efficiency of retrovirus-mediated gene transfer to primary airway epithelial cells from rhesus monkeys was evaluated. We compared the use of murine amphotropic retrovirus vectors to the use of murine retrovirus vectors containing the envelope (Env) glycoproteins from gibbon ape leukemia virus (GALV). These vectors use distinct receptors to gain entry into host cells. We found that vectors with the GALV Env glycoproteins are up to 10-fold more efficient at transducing genes into primary monkey airway epithelial cells than vectors with the amphotropic Env glycoproteins. Under optimal conditions, up to about 80% of primary monkey airway epithelial cells could be transduced with the vector containing the GALV Env glycoproteins. In addition, we found that delivery of retrovirus vectors to the apical side of polarized airway epithelial cultures was significantly more efficient than delivery to the basal side. These results suggest the feasibility of luminal delivery of retrovirus vectors to the lung.

Gene therapy of cancer

Retroviral-mediated gene transfer has permitted the development of clinical protocols for the study and treatment of cancer. These protocols can be divided into gene-labeling and gene therapy proposals. Labeling studies include the tracking of tumor infiltrating lymphocytes (TIL) following the administration of those cells, and the detection, at the time of relapse, of tumor cells from transplanted autologous bone marrow. Most gene therapy protocols are designed to induce an immune attack against the tumor by inserting genes into tumor cells themselves. Although uncertainty about the safety of the procedure still exists, gene therapy of cancer holds much promise as an effective treatment modality.

Human gene therapy comes of age

Advances in the understanding of molecular biology of human disease and the development of efficient gene transfer techniques have resulted in practical approaches to human gene therapy, with new techniques being developed at an increasing rate. The first trials have now begun in humans and initial results are positive.

Gene transfer of adenosine deaminase into primitive human hematopoietic progenitor cells

The inherited deficiency in adenosine deaminase (ADA), which results in severe combined immunodeficiency, is generally regarded as an optimal model for the development of human somatic gene therapy. The ideal target for the correction of ADA deficiency and other lympho-hematopoietic disorders would be the hematopoietic stem cell. We have used a combination of recombinant human interleukins-3 and -6 to stimulate the proliferation of primitive human hematopoietic progenitor cells during a period of co-cultivation with irradiated cells producing high titers of an ADA-transducing retroviral vector packaged in amphotropic particles. In a series of nine experiments, an average of 83% of the clonogenic progenitors (CFU-E and CFU-GM) were found to have acquired the transferred sequence as determined by polymerase chain reaction analysis. In addition, in two experiments, 24-44% of the clonogenic progenitors derived from long-term myeloid cultures 9 weeks post-transduction were found to contain vector sequence. The latter cells are derived from so-called "long-term culture-initiating cells" (LTC-IC), which are primitive cells probably related to hematopoietic stem cells. Moreover, the transduced ADA enzyme was found to be expressed in both normal and ADA-deficient erythroid colonies, and in the nonadherent cells of long-term bone marrow culture for at least 2 weeks at levels that approximate the endogenous ADA levels of normal erythroid cells. These results indicate that the ADA coding sequence can efficiently be introduced by retroviral gene transfer into both committed and primitive human hematopoietic progenitor cells, and that this will result in adequate expression of the transduced enzyme in the progeny of committed hematopoietic progenitors.

Regulatory concerns in human gene therapy

Gene therapy in humans is now being undertaken in an investigational setting. Such therapy involves the administration of biological products to human patients. A document entitled, "Points to Consider in Human Somatic Cell Therapy and Gene Therapy" has been prepared by the Center for Biologics Evaluation and Research (CBER) of the Food and Drug Administration (FDA) and is published elsewhere in this issue. This paper provides explanatory material about the CBER regulatory process and the scientific and regulatory basis for the requests for data in that document.

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

Four monkeys were exposed to a retroviral vector and replication-competent murine amphotropic retrovirus in a bone marrow transplantation/gene transfer protocol (Kantoff et al., 1987). We have studied these animals 2 and 3 years post-transplantation and did not detect replicating virus in serum, peripheral blood mononuclear cells, or bone marrow cells. Amphotropic envelope sequences could not be detected in blood or bone marrow cells by Southern blotting or the polymerase chain reaction. Antibodies directed against the p30 and gp70 viral antigens were detected by Western blot and immunoprecipitation. The animals remain alive and well. Our findings suggest that primates can clear murine amphotropic retroviruses even when exposure occurs during a time of severe immunosuppression.