High-level human adenosine deaminase expression in dog skin fibroblasts is not sustained following transplantation

Erythropoietin delivery by genetically engineered bone marrow stromal cells for correction of anemia in mice with chronic renal failure

The goal of this research was to develop a strategy to couple stem cell and gene therapy for in vivo delivery of erythropoietin (Epo) for treatment of anemia of ESRD. It was shown previously that autologous bone marrow stromal cells (MSCs) can be genetically engineered to secrete pharmacologic amounts of Epo in normal mice. Therefore, whether anemia in mice with mild to moderate chronic renal failure (CRF) can be improved with Epo gene-modified MSCs (Epo+MSCs) within a subcutaneous implant was examined. A cohort of C57BL/6 mice were rendered anemic by right kidney electrocoagulation and left nephrectomy. In these CRF mice, the hematocrit (Hct) dropped from a prenephrectomy baseline of approximately 55% to 40% after induction of renal failure. MSCs from C57BL/6 donor mice were genetically engineered to secrete murine Epo at a rate of 3 to 4 units of Epo/10(6) cells per 24 h, embedded in a collagen-based matrix, and implanted subcutaneously in anemic CRF mice. It was observed that Hct increased after administration of Epo+MSCs, according to cell dose. Implants of 3 million Epo+MSCs per mouse had no effect on Hct, whereas 10 million led to a supraphysiologic effect. The Hct of CRF mice that received 4.5 or 7.5 million Epo+MSCs rose to a peak 54+/-4.0 or 63+/-5.5%, respectively, at 3 wk after implantation and remained above 48 or 54% for >19 wk. Moreover, mice that had CRF and received Epo+MSCs showed significantly greater swimming exercise capacity. In conclusion, these results demonstrate that subcutaneous implantation of Epo-secreting genetically engineered MSCs can correct anemia that occurs in a murine model of CRF.

Marrow Stromal Cells for Interleukin-2 Delivery in Cancer Immunotherapy

Marrow stromal cells (MSCs) can be easily gene-modified and clonally expanded making them ideal candidates for transgenic cell therapy. However, recent reports suggest that MSCs possess immunosuppressive effects, which may limit their clinical applications. We investigated whether interleukin (IL)-2 gene-modified MSCs can be used to mount an effective immune response against the poorly immunogenic B16 melanoma model. We first show that primary MSCs mixed with B16 cells and injected subcutaneously in syngeneic recipients do not affect tumor growth. On the other hand, IL-2-producing MSCs mixed with B16 cells significantly delayed tumor growth in an IL-2 dose-dependent manner. Furthermore, we observed that matrix-embedded IL-2-producing MSCs injected in the vicinity of preestablished B16 tumors led to absence of tumor growth in 90% of treated mice (p < 0.001). We demonstrated that tumor-bearing mice treated with IL-2-producing MSCs developed CD8-mediated tumor-specific immunity and significantly delayed tumor growth of a B16 cell challenge (p < 0.05). In addition, treatment of cd8-/-, cd4-/- and beige mice revealed that CD8+ and natural killer (NK) cells, but not CD4+ cells, were required to achieve antitumor effect. In conclusion, MSCs can be exploited to deliver IL-2 and generate effective immune responses against melanoma in mice with normal immune systems.

A neovascularized organoid derived from retrovirally engineered bone marrow stroma leads to prolonged in vivo systemic delivery of erythropoietin in nonmyeloablated, immunocompetent mice

Marrow stromal cells (MSCs) are postnatal progenitor cells that can be easily cultured ex vivo to large amounts. This feature is attractive for cell therapy applications where genetically engineered MSCs could serve as an autologous cellular vehicle for the delivery of therapeutic proteins. The usefulness of MSCs in transgenic cell therapy will rely upon their potential to engraft in nonmyeloablated, immunocompetent recipients. Further, the ability to deliver MSCs subcutaneously - as opposed to intravenous or intraperitoneal infusions - would enhance safety by providing an easily accessible, and retrievable, artificial subcutaneous implant in a clinical setting. To test this hypothesis, MSCs were retrovirally engineered to secrete mouse erythropoietin (Epo) and their effect was ascertained in nonmyeloablated syngeneic mice. Epo-secreting MSCs when administered as 'free' cells by subcutaneous or intraperitoneal injection, at the same cell dose, led to a significant - yet temporary - hematocrit increase to over 70% for 55+/-13 days. In contrast, in mice implanted subcutaneously with Matrigel trade mark -embedded MSCs, the hematocrit persisted at levels >80% for over 110 days in four of six mice (P<0.05 logrank). Moreover, Epo-secreting MSCs mixed in Matrigel elicited and directly participated in blood vessel formation de novo reflecting their mesenchymal plasticity. MSCs embedded in human-compatible bovine collagen matrix also led to a hematocrit >70% for 75+/-8.9 days. In conclusion, matrix-embedded MSCs will spontaneously form a neovascularized organoid that supports the release of a soluble plasma protein directly into the bloodstream for a sustained pharmacological effect in nonmyeloablated recipients.

Gene transfer of low levels of beta-glucuronidase corrects hepatic lysosomal storage in a large animal model of mucopolysaccharidosis VII

Gene therapy has been at least partially effective in several mouse disease models, but treatment of large mammals has been more difficult to achieve. One major limitation is that only low levels of expression of the corrective gene are often maintained in vivo. In a mouse model of the lysosomal storage disease mucopolysaccharidosis (MPS) type VII (Sly disease) with a null mutation in beta-glucuronidase, gene transfer experiments have shown that only 1-2% of normal beta-glucuronidase can correct the storage in some major organs. In contrast, MPS VII dogs, cats, and humans that have residual beta-glucuronidase activity levels in this range are affected. Thus, higher levels of transferred gene expression may be needed to achieve a therapeutic effect in large animals and humans. We tested this by examining liver pathology in MPS VII dogs after intraperitoneal transplantation of neo-organs containing retrovirus vector-corrected autologous fibroblasts that expressed low levels of beta-glucuronidase. The enzyme secreted from the neo-organs was taken up by the liver and significantly reduced the substrate content compared with untreated dogs. This suggests that small amounts of normal enzyme, when delivered to target tissues, may be therapeutically effective in human MPS VII patients.

Fibroblasts and dermal gene therapy: a minireview

This review highlights our current understanding of the biology of, survival of, and transgene expression by genetically modified fibroblasts (GMFb) carrying stably integrated transgenes in vivo. Experimental data demonstrate that three elements will enhance expression by and survival of GMFb in vivo: a matrix scaffolding to take the place of the existing dermis, the presence of elements of the extracellular matrix in the construct used to move GMFb to the in vivo setting, and the utilization of immortalized fibroblasts to carry the transgenes. Although moving GMFb to an in vivo setting is an invasive procedure, there are a number of clinical settings where GMFb appear to be the suitable cell for gene therapy.

Efficient gene transfer and expression of biologically active glial cell line-derived neurotrophic factor in rat motoneurons transduced wit lentiviral vectors

Several studies have shown the ability of human immunodeficiency virus type 1 (HIV1)-based lentiviral vectors to infect nondividing brain and retinal neurons with high efficiency and long-term expression of the transduced gene. We show that purified embryonic motoneurons can be efficiently (>95%) transduced in culture using an HIV1-based lentiviral vector encoding LacZ. Expression of beta-galactosidase was observed for at least 9 days in these conditions. Furthermore, motoneurons transduced with a lentiviral vector expressing glial cell line-derived neurotrophic factor survived in the absence of additional trophic support, showing that the overexpressed protein was biologically active. Our results demonstrate the potential of lentiviral vectors in studying the biological effects of proteins expressed in motoneurons and in the development of future gene therapy for motoneuron diseases.

Lentiviral and murine retroviral transduction of T cells for expression of human CD40 ligand

Efficient transduction of primary human T cells is an important goal toward treating a number of genetic defects. Patient T cells could be harvested by leukapheresis, transduced, and returned to the donor. A wide range of secreted or cell surface therapeutic proteins may be delivered in this way. The ability to produce antibodies is the consequence of interactions between T cells and B cells and lack of expression of CD40 ligand (CD40L) on T cells causes X-linked hyper-IgM syndrome (XHIM). We are investigating delivery of a normal CD40 ligand to treat this disorder. We tested promoters driving the expression of either reporter genes such as enhanced green fluorescent protein (eGFP) or human CDC40L. Using murine retroviruses, the best able to drive gene expression in T cells was the cytomegalovirus (CMV) promoter enhancer element; however, transduction efficiency was low. To achieve efficient, high-level gene expression we tested lentiviral gene delivery vectors. At a low multiplicity of infection (MOI) (0.5-2) a large fraction of target cells was transduced by lentiviral vectors (40-93%), and the strength of gene expression was high, as determined by flow cytometric analysis. We monitored the expression of eGFP or human CD40L on T cell lines and untransformed primary human T cells from normal and CD40L-deficient patients. We achieved efficient gene expression without an extended exposure to virus, and without the need for selection. These results are encouraging for efficient lentivirus-mediated transduction of refractory human cells to achieve therapeutic gene delivery.

Sustained gene expression in transplanted skin fibroblasts in rats

Retrovirus-mediated gene transfer into adult skin fibroblasts has provided measurable amounts of therapeutic proteins in animal models. However, the major problem emerging from these experiments was a limited time of vector encoded gene expression once transduced cells were engrafted. We hypothesized that sustained transduced gene expression in quiescent fibroblasts in vivo might be obtained by using a fibronectin (Fn) promoter. Fibronectin plays a key role in cell adhesion, migration and wound healing and is up-regulated in quiescent fibroblasts. Retroviral vectors containing human adenosine deaminase (ADA) cDNA linked to rat fibronectin promoter (LNFnA) or viral LTR promoter (LASN) were compared for their ability to express ADA from transduced primary rat skin fibroblasts in vivo. Skin grafts formed from fibroblasts transduced with LNFnA showed strong human ADA enzyme activity from 1 week to 3 months. In contrast, skin grafts containing LASN-transduced fibroblasts tested positive for human ADA for weeks 1 and 2, were faintly positive at week 3 and showed no human ADA expression at 1, 2 and 3 months. Thus, a fibronectin promoter provided sustained transduced gene expression at high levels for at least 3 months in transplanted rat skin fibroblasts, perhaps permitting the targeting of this tissue for human gene therapy.

Biologic aspects of expression of stably integrated transgenes in cells of the skin in vitro and in vivo

The observation that transgenes can be stably integrated into the genome of fibroblasts using recombinant retroviruses enhanced interest in using these cells as a vector for gene therapy. This enthusiasm has lessened during the past 8 years, not because skin has lost the features that make it attractive for gene therapy, but rather because stable transgene expression in vivo has not been achieved. All investigators who have used genetically modified fibroblasts to study in vivo aspects of gene therapy have shown a decrease in transgene expression with time. This contrasts with transgene expression in similarly transduced fibroblasts in vitro, where expression is not lost or is lost very slowly. We have initiated an approach to bring further understanding to the biology of transgene expression by fibroblasts carrying stably integrated transgenes in an in vivo setting. Experiments described permit the following conclusions. Expression by and survival of genetically modified fibroblasts a) requires a persistent matrix scaffold in in vivo settings; b) is prolonged if the matrix is allowed to mature in vitro; c) is enhanced if the matrix is partially sequestered behind a coating of normal fibroblasts; and d) can be substantively prolonged in vivo by immortalizing the cells. These observations support the notion that prolonged expression of transgenes by fibroblasts can be achieved in vivo and that gene therapy utilizing fibroblasts and other cells of the skin has clinical utility.

Stomach Implant for Long-Term Erythropoietin Expression in Rats

To approach the goal of consistent long-term erythropoietin (Epo) expression in vivo, we developed an implantation procedure in which transduced autologous vascular smooth muscle was introduced into rats in a chamber created from a polytetrafluoroethylene (PTFE) ring placed under the serosa of the stomach. The implant became vascularized and permitted the long-term survival of smooth muscle cells expressing Epo. Hematocrits of treated animals increased rapidly and monitored over 12 months gave a mean value of 56.0 ± 4.0% (P < .001; n = 9), increased from a presurgery mean of 42.3 ± 1.6%. Hemoglobin levels rose from a presurgery mean of 15.2 ± 0.4 g/dL and for 12 months were significantly elevated with a mean value of 19.5 ± 1.3 g/dL (P < .001; n = 9). The hematocrit and hemoglobin levels of control animals receiving human adenosine deaminase (ADA)–expressing cells were not significantly different from baseline (P > .05; n = 5). In response to tissue oxygenation, kidney, and (to a lesser extent) liver are specific organs that synthesize Epo. Treated animals showed downregulation of endogenous Epo mRNA in kidney over a 12-month period. The PTFE implant provides sustained gene delivery, is safe, and is minimally invasive. It allows easy engraftment of transduced cells and may be applied generally to the systemic delivery of therapeutic proteins such as hormones and clotting factors.

© 1998 by The American Society of Hematology.

Stomach Implant for Long-Term Erythropoietin Expression in Rats

To approach the goal of consistent long-term erythropoietin (Epo) expression in vivo, we developed an implantation procedure in which transduced autologous vascular smooth muscle was introduced into rats in a chamber created from a polytetrafluoroethylene (PTFE) ring placed under the serosa of the stomach. The implant became vascularized and permitted the long-term survival of smooth muscle cells expressing Epo. Hematocrits of treated animals increased rapidly and monitored over 12 months gave a mean value of 56.0 ± 4.0% (P &lt; .001; n = 9), increased from a presurgery mean of 42.3 ± 1.6%. Hemoglobin levels rose from a presurgery mean of 15.2 ± 0.4 g/dL and for 12 months were significantly elevated with a mean value of 19.5 ± 1.3 g/dL (P &lt; .001; n = 9). The hematocrit and hemoglobin levels of control animals receiving human adenosine deaminase (ADA)–expressing cells were not significantly different from baseline (P &gt; .05; n = 5). In response to tissue oxygenation, kidney, and (to a lesser extent) liver are specific organs that synthesize Epo. Treated animals showed downregulation of endogenous Epo mRNA in kidney over a 12-month period. The PTFE implant provides sustained gene delivery, is safe, and is minimally invasive. It allows easy engraftment of transduced cells and may be applied generally to the systemic delivery of therapeutic proteins such as hormones and clotting factors.

© 1998 by The American Society of Hematology.

Cloning and sequence analysis of the immediate promoter region and cDNA of porcine granulocyte colony-stimulating factor

Granulocyte colony-stimulating factor (G-CSF) is a cytokine that stimulates the proliferation and differentiation of hematopoietic progenitor cells committed to the neutrophil/granulocyte lineage. Recombinant G-CSF (rG-CSF) is routinely used in the prevention of chemotherapy-induced neutropenia and in the setting of bone marrow transplantation. Chronic idiopathic and congenital neutropenic disorders also show improvement after rG-CSF injections. Applications of either rG-CSF or G-CSF gene transfected cells into mice give rise to leukocytosis, which can be measured easily. This makes G-CSF a versatile tool for studying systemic effects of therapeutic proteins delivered by genetically modified cells in vivo. Although the biological activity of G-CSF is not species-specific, studies on long-term expression would require the use of species-identical proteins in order to avoid host immune reactions against the foreign gene product. Because of the physiological and immunological similarity of pigs and human, the pig has become an important large-animal model for biomedical research. We have therefore cloned porcine G-CSF cDNA from RNA isolated from pig PBLs. Pig G-CSF is a 195-amino-acid polypeptide that shares a high degree of homology to human (78%), murine (71%) as well as rat (68%) G-CSF. In contrast to human and murine, but not to rat G-CSF, a different ATG translation start codon is used, resulting in a shorter, but still functional signal sequence.

Use of cloned genetically modified human fibroblasts to assess long-term survival in vivo

Because human fibroblasts are easily brought to tissue culture conditions and can be stably transduced with retroviral vectors encoding transgenes ex vivo, genetically modified fibroblasts are frequently considered in strategies to correct disease with gene therapy. This enthusiasm has been dampened by studies showing that transgene expression by genetically modified fibroblasts diminishes with time in vivo, but not in vitro, for reasons that are unclear. We elected to study this problem using cloned human fibroblasts that had been cloned by limiting dilution and stably transduced with a retroviral vector encoding lacZ ex vivo. These were seeded onto a nonbiodegradable nylon matrix that was transplanted to nude mice. Transgene expression was followed prospectively by histologic exam. Data show that human fibroblasts can withstand the pressure of cloning by limiting dilution. In addition, they can be passaged from 10 to > 20 times, and > 1 x 10(20) of genetically modified fibroblasts can be generated as progeny of one cell. Loss of transgene expression by the cloned genetically modified fibroblasts in vivo occurs in an orderly and progressive fashion, but is not complete by 4 months. Neither the loss nor the persistence of expression appear to be random. These observations are most compatible with the thesis that a major cause of the loss of transgene expression in vivo is secondary to apoptosis of the genetically modified fibroblast. Loss of expression of transgenes in senescent genetically modified fibroblasts occurs more rapidly than in their presenescent counterparts in the age-neutral, in vivo setting of the nude mouse.

Systemic delivery of human growth hormone or human factor IX in dogs by reintroduced genetically modified autologous bone marrow stromal cells

Canine bone marrow stromal cells were expanded to numbers in excess of 10(9) cells from the initial 10-20 ml of marrow aspirates and transfected to express high levels of human growth hormone (hGH) in vitro. Ex vivo-modified marrow stromal cells were used in a gene therapy model system for the systemic delivery of transgene products in dogs. Adherent bone marrow stromal cell cultures, established and expanded from iliac crest marrow aspirates from each of 8 dogs, were transfected with a hGH gene plasmid expression vector and shown to express from 0.54-3.84 micrograms/10(6) cells per 24 hr hGH in vitro. The transfected plasmid vector does not possess a eukaryotic origin of replication nor does it possess sequences required for efficient integration into the host cell genome. As such, expression was expected to be transient. Transfected cells were autologously reintroduced into each dog by either infusion into a foreleg vein or directly into iliac crest marrow. In two cases, the stromal cells were cryopreserved following transfection, and subsequently thawed and infused. In one case, the expanded stromal cells were first cryopreserved, and then thawed, recultured, transfected, and infused. Reintroduced cell numbers ranged from 2.2 x 10(7) to 2.6 x 10(9), with total hGH expression capacities ranging from 62 to 1,400 micrograms/24 hr. Plasma of each of the dogs contained detectable hGH for a mean of 3.1 days (SD +/- 0.8 day) ranging from 2 to 5 days following reinfusion of cells. Peak plasma levels ranged from 0.10 to 1.76 ng/ml. Similar hGH expression values, based upon total expression capacity of the cells infused and dog body weight, were obtained for all dogs. Vector-modified stromal cells were detectable, by polymerase chain reaction (PCR) analysis, in the peripheral circulation following reinfusion in all 4 dogs analyzed. In 3 of the dogs, modified stromal cells were detected for 8.5-15 weeks. In addition, modified stromal cells were detected in iliac crest marrow of 2 dogs for 9 and 13 weeks, respectively, following reinfusion. In another experiment, cultured bone marrow stromal cells were transfected with a human factor IX (hFIX) plasmid vector. Modified cells (5.57 x 10(8)), with a total hFIX expression capacity of 281 micrograms/24 hr, were reinfused, resulting in detectable hFIX in plasma continuously for 9 days with a peak level of 8 ng/ml on day 1. These results demonstrate that the ex vivo bone marrow stromal cell system is a potentially powerful method by which to deliver secreted transgene product to the systemic circulation of large animals.

Transcriptional Silencing of Retroviral Vectors

Although retroviral vector systems have been found to efficiently transduce a variety of cell types in vitro, the use of vectors based on murine leukemia virus in preclinical models of somatic gene therapy has led to the identification of transcriptional silencing in vivo as an important problem. Extinction of long-term vector expression has been observed after implantation of transduced hematopoietic cells as well as fibroblasts, myoblasts and hepatocytes. Here we review the influence of vector structure, integration site and cell type on transcriptional silencing. While down-regulation of proviral transcription is known from a number of cellular and animal models, major insight has been gained from studies in the germ line and embryonal cells of the mouse. Key elements for the transfer and expression of retroviral vectors, such as the viral transcriptional enhancer and the binding site for the tRNA primer for reverse transcription may have a major influence on transcriptional silencing. Alterations of these elements of the vector backbone as well as the use of internal promoter elements from housekeeping genes may contribute to reduce transcriptional silencing. The use of cell culture and animal models in the testing and improvement of vector design is discussed. Copyright 1996 S. Karger AG, Basel

Systematic evaluation of chimeric marker genes on dicistronic transcription units for regulated expression of transgenes in vitro and in vivo

Plasmid expression vectors combining human cytokine cDNAs and selectable marker genes on dicistronic transcription units were functionally characterized in vitro and in vivo. The internal ribosome entry sequence (IRES) of encephalomyocarditis virus mediated cap-independent translation of the downstream cistron. After cationic lipofection of cells with a dicistronic construct containing the Neor gene downstream of a human interleukin-2 (IL-2) cDNA, all G418-resistant clones secreted high amounts of IL-2. Reversal of the order of the cDNAs was associated with less efficient transgene expression and represented no advantage in comparison to separate expression cassettes. To combine direct in vitro selection of expression with in vivo elimination of cytokine-secreting cells, an improved chimeric cDNA of the Neor and herpes simplex virus (HSV) thymidine kinase (TK) genes was constructed and shown to confer sensitivity to ganciclovir concentrations that can be achieved in human patients. This chimeric marker was coupled on dicistronic constructs with a granulocyte colony-stimulating factor (G-CSF) cDNA as a molecule with easily detectable bioactivity in vivo. Subcutaneous implantation of pCMV.GCSF.ires TK/NEO-transfected CMS-5 cells into syngeneic BALB/c mice resulted in excessive leukocytosis and progressively growing tumors. Treatment with ganciclovir led to normalization of leukocyte counts in all animals, whereas complete regression of tumors was observed in only 3/5 mice. Hypermethylation of the transfected promoter was demonstrated in both ganciclovir-resistant tumors. Thus, transcription units combining selectable markers and genes of interest allow selection of high producer cells in vitro and efficient elimination of transgene-expressing cells in vivo. However, cells that hypermethylate transfected genes to terminate gene expression in vivo may escape conditional ablation.

Granulocyte colony-stimulating factor expression from transduced vascular smooth muscle cells provides sustained neutrophil increases in rats

Granulocyte colony-stimulating factor (G-CSF) regulates granulocyte precursor cell proliferation, neutrophil survival, and activation. Cyclic hematopoiesis, a disease that occurs both in humans and grey collie dogs is characterized by cyclical variations in blood neutrophils. Although the underlying molecular defect is not known, long-term daily administration of recombinant G-CSF eliminates the severe recurrent neutropenia, indicating that expression of G-CSF by gene therapy would be beneficial. As a prelude to preclinical studies in affected collie dogs, we monitored hematopoiesis in rats receiving vascular smooth muscle cells transduced to express G-CSF. Cells transduced with LrGSN, a retrovirus expressing rat G-CSF, were implanted in the carotid artery and control animals received cells transduced with LASN, a retrovirus expressing human adenosine deaminase (ADA). Test animals showed significant increases in neutrophil counts for at least 7 weeks, with mean values of 3,670 +/- 740 cells/microliter in comparison to 1,870 +/- 460 cells/microliter in controls (p < 0.001). Thus, in rats G-CSF gene transfer targeted at vascular smooth muscle cells initiated sustained production of 1,800 neutrophils/microliter, a cell number that would provide clinical benefit to patients. Lymphocytes, red cells and platelets were not different between control and test animals (p > 0.05). These studies indicate that retrovirally transduced vascular smooth muscle cells can provide sustained clinically useful levels of neutrophils in vivo.

Sustained production of human transferrin by transduced fibroblasts implanted into athymic mice: a model for somatic gene therapy

Somatic gene therapy has been proposed as a means of treating inherited diseases involving defective or absent plasma proteins, viral diseases, and cancer. Introduction of the gene of interest into fibroblasts and implantation of these genetically modified fibroblasts using a skin equivalent system may be an attractive model for gene therapy because skin fibroblasts are easily obtained and propagated in culture. This study evaluated expression of the gene for human transferrin (hTf) by genetically modified fibroblasts in vitro and in vivo. NIH 3T3 fibroblasts, which form non-metastasizing tumors in athymic mice, were transduced with a retroviral vector encoding hTf. The transduced cells were cloned by limiting dilution and hTf production by the cloned cells measured. Two clones of cells producing high levels of hTf were used to seed collagen-coated nylon matrices, which were maintained in culture for up to 53 d. The rate of synthesis of hTf by the seeded matrices was constant after 22 d in vitro. Matrices seeded with cloned, transduced cells were implanted subcutaneously into seven athymic mice, and plasma levels of hTf were assessed biweekly. In all animals, the plasma level of hTf was detectable at week 6 after implantation. Levels of hTf remained elevated in the animals until the implants were removed at week 12. At week 10, the level of hTf in the plasma correlated with tumor volume in tumors less than 2000 mm3 in size. The half-life of hTf in the mice was 39.5 h. In this model, gene expression did not decline for the 12-week observation period.

The basic science of gene therapy

The development over the past decade of methods for delivering genes to mammalian cells has stimulated great interest in the possibility of treating human disease by gene-based therapies. However, despite substantial progress, a number of key technical issues need to be resolved before gene therapy can be safely and effectively applied in the clinic. Future technological developments, particularly in the areas of gene delivery and cell transplantation, will be critical for the successful practice of gene therapy.