Temozolomide in malignant gliomas of childhood: a United Kingdom Children's Cancer Study Group and French Society for Pediatric Oncology Intergroup Study.

PURPOSE: To determine the response rate of the malignant gliomas of childhood to an oral, daily schedule of temozolomide. PATIENTS AND METHODS: A multicenter, phase II evaluation of an oral, daily schedule of temozolomide (200 mg/m(2) on 5 consecutive days) was undertaken in children with relapsed or progressive, biopsy-proven, high-grade glioma (arm A) and progressive, diffuse, intrinsic brainstem glioma (arm B). Evidence of activity was defined by radiologic evidence of a sustained reduction in tumor size on serial magnetic resonance imaging scans. RESULTS: Fifty-five patients were recruited (34 to arm A and 21 to arm B) and received 215 cycles of chemotherapy. Grade 3/4 thrombocytopenia was the most frequent toxic event (7% of cycles). Prolonged myelosuppression resulted in significant treatment delays and dose reductions (17% and 22% of cycles, respectively). Two toxic deaths were documented and were related to myelosuppression and sepsis in one patient and pneumonia in a second. The overall (best) response rate was 12% for arm A (95% confidence interval [CI], 3 to 28 in the study cohort, and 2 to 31 for eligible patients) and 5% and 6%, respectively, for arm B (95% CI, 0 to 26 in the study cohort, and 0 to 27 for eligible patients). Stabilization of disease was also documented and was most noteworthy for brainstem gliomas, where two patients achieved both radiologic static disease and discontinued steroid medication. CONCLUSION: Despite moderate toxicity, objective response rates to temozolomide have been low, indicating that temozolomide has minimal activity in the high-grade gliomas of childhood.

Assays to predict the genotoxicity of the chromosomal mutagen etoposide -- focussing on the best assay.

The topoisomerase II inhibitor etoposide is used routinely to treat a variety of cancers in patients of all ages. As a result of its extensive use in the clinic and its association with secondary malignancies it has become a compound of great interest with regard to its genotoxic activity in vivo. This paper describes a series of assays that were employed to determine the in vivo genotoxicity of etoposide in a murine model system. The alkaline comet assay detected DNA damage in the bone marrow mononuclear compartment over the dose range of 10--100mg/kg and was associated with a large and dose dependent rise in the proportion of cells with severely damaged DNA. In contrast, the bone marrow micronucleus assay was found to be sensitive to genotoxic damage between the doses of 0.1--1mg/kg without any corresponding increases in cytotoxicity. An increase in the mutant frequency was undetectable at the Hprt locus at administered doses of 1 and 10mg/kg of etoposide, however, an increase in the mutant frequency was seen at the Aprt locus at these doses. We conclude that the BMMN assay is a good short-term predictor of the clastogenicity of etoposide at doses that do not result in cytotoxic activity, giving an indication of potential mutagenic effects. Moreover, the detection of mutants at the Aprt locus gives an indication of the potential of etoposide to cause chromosomal mutations that may lead to secondary malignancy.

The effects of dose, route of administration, drug scheduling and MDR-1 gene transfer on the genotoxicity of etoposide in bone marrow.

We have used the bone marrow micronucleus assay (BMMN) as a measure of clastogenicity, in response to etoposide exposure in murine bone marrow. Oral delivery of etoposide resulted in a reduced number of micronucleated polychromatic erythrocytes (MPE) relative to the same dose delivered intraperitoneally (P < 0.001). Daily fractionation of the oral schedule of etoposide led to a more than six-fold increase in cumulative MPE frequency over that observed with the same total, unfractionated dose, with the potency of the response increasing with serial exposure (r = 0.79). Retrovirally-mediated expression of MDR1 in murine bone marrow resulted in partial protection against the clastogenic activity of etoposide relative to mock transduced control mice. The model system developed has indicated a variety of factors able to influence the genotoxicity of etoposide. It should now be possible to further exploit this model in order to define other factors governing haemopoietic sensitivity to etoposide.

Engineering drug resistance in human cells.

Many of the problems with current anti-tumour therapies stem from a lack of specificity for tumour as opposed to normal tissues. To address the problem of collateral toxicity during anti-tumour chemotherapy we have been developing a gene therapy approach to protect normal tissues from the toxic and potentially mutagenic effects of chemotherapeutic agents. As a paradigm for this we have been examining the potential of the DNA repair protein O6-alkylguanine-DNA-alkyltransferase (ATase) to confer genetic chemoprotection to the bone marrow. By transfer and expression of a mutant form of this protein, which is resistant to inactivation by the tumour sensitising agent O6-benzylguanine (O6-beG), we have been able to demonstrate protection of murine bone marrow in vitro from the cytotoxic and clastogenic effects of O6-beG in combination with the anti-tumour agent temozolomide. This protection is seen in multiple lineages, including erythroid and granulocyte/macrophage progenitors, as well as more primitive cells. Importantly, significant protection of the platelet lineage is also seen, with faster recovery of platelets. The multi-lineage protection seen has encouraged us to take this approach forward to clinical trial in the near future.

A radiation-controlled molecular switch for use in gene therapy of cancer.

Ionising radiation induces the expression of a number of radiation-responsive genes and there is current interest in exploiting this to regulate the expression of exogenous therapeutic genes in gene therapy strategies for cancer. However, the radiation-responsive promoters used in these approaches are often associated with low and transient levels of therapeutic gene expression. We describe here a novel radiation-triggered molecular switching device based on promoter elements from the radiation-responsive Egr-1 gene and the cre-LoxP site-specific recombination system of the P1 bacteriophage. Using this system, a single, minimally toxic dose of radiation induced cre-mediated excision of a lox-P flanked stop cassette in a silenced expression vector and this resulted in amplified levels of CMV-promoter-driven expression of the exogenous tumour-sensitising gene, HSV-tk. This strategy could be used in combination with targeted delivery and tumour-specific promoters to elicit the tumour-targeted and prolonged expression of a variety of tumour-sensitising genes and provide an unprecedented level of control and tumour selectivity.

The expression of full length Gp91-phox protein is associated with reduced amphotropic retroviral production.

BACKGROUND AND OBJECTIVE: As a single gene defect in mature bone marrow cells, chronic granulomatous disease (X-CGD) represents a disorder which may be amenable to gene therapy by the transfer of the missing subunit into hemopoietic stem cells. In the majority of cases lack of Gp91-phox causes the disease. So far, studies involving transfer of Gp91-phox cDNA, including a phase I clinical trial, have yielded disappointing results. Most often, low titers of virus have been reported. In the present study we investigated the possible reasons for low titer amphotropic viral production. DESIGN AND METHODS: To investigate the effect of Gp91 cDNA on the efficiency of retroviral production from the packaging cell line, GP+envAm12, we constructed vectors containing either the native cDNA, truncated versions of the cDNA or a mutated form (LATG) in which the natural translational start codon was changed to a stop codon. Following derivation of clonal packaging cell lines, these were assessed for viral titer by RNA slot blot and analyzed by non-parametrical statistical analysis (Whitney-Mann U-test). RESULTS: An improvement in viral titer of just over two-fold was found in packaging cells containing the start-codon mutant of Gp91 and no evidence of truncated viral RNA was seen in these cells. Further analysis revealed the presence of rearranged forms of the provirus in Gp91-expressing cells, and the production of truncated, unpackaged viral RNA. Protein analysis revealed that LATG-transduced cells did not express full-length Gp91-phox, whereas those containing the wild-type cDNA did. However, a truncated protein was seen in ATG-transduced cells which was also present in wild type cells. No evidence for the presence of a negative transcriptional regulatory element was found from studies with the deletion mutants. INTERPRETATION AND CONCLUSIONS: A statistically significant effect of protein production on the production of virus from Gp91-expressing cells was found. Our data point to a need to restrict expression of the Gp91-phox protein and its derivatives in order to enhance retroviral production and suggest that improvements in current vectors for CGD gene therapy may need to include controlled, directed expression only in mature neutrophils.

Enhancing hemopoietic drug resistance: a rationale for reconsidering the clinical use of mitozolomide.

Retroviral gene transfer was used to achieve expression in mouse bone marrow of a mutant form of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (hATPA/GA), which exhibits resistance to inactivation by O6-benzylguanine (O6-beG). After reconstitution of mice with transduced bone marrow, approximately 50% of the bipotent granulocyte-macrophage colony-forming cell (GM-CFC) and multipotent spleen colony-forming unit (CFU-S) hemopoietic populations showed expression of the transgene; this expression was associated with resistance to either mitozolomide or to a combination of O6-beG and mitozolomide, relative to mock-transduced controls. Thus, at a dose of mitozolomide in vivo that allowed only 70% and 62% survival of mock-transduced GM-CFC and CFU-S, respectively, the hATPA/GA CFC were totally resistant to the same dose of mitozolomide (P < .05 and .001, respectively). In the presence of O6-beG, the toxicity of mitozolomide was greatly potentiated. Only 24% and 18%, respectively, of mock-transduced GM-CFC and CFU-S survived combination treatment, whereas 45% (P < .05) and 37% (P < .01) of GM-CFC and CFU-S, respectively, from hATPA/GA mice survived the same combination of doses. Furthermore, as a result of transgene expression, the number of micronucleated polychromatic erythrocytes induced by mitozolomide was significantly reduced (P < .05) by 40% relative to mock-transduced controls, indicating the potential of this approach to reduce the frequency of mutation associated with chemotherapy exposure. The protection against the toxic and clastogenic effects of mitozolomide in both primitive and more mature hemopoietic cells suggests that the severe myelosuppression that halted further clinical investigation of this drug could be substantially ameliorated by the exogenous expression of O6-alkylguanine-DNA alkyltransferase. Therefore, these data raise the prospect for the reinvestigation of mitozolomide and other proscribed drugs in the context of genetically protected hemopoiesis.

Development of synthetic promoters for radiation-mediated gene therapy.

Exposure of cells to ionising radiation results in the activation of specific transcriptional control (CArG) elements within the early growth response 1 (Egr1) gene promoter, leading to increased gene expression. As part of a study investigating the potential use of these elements in radiation-controlled gene therapy vectors, we have incorporated their sequences into a synthetic gene promoter and assayed for the ability to induce expression of a downstream reporter gene following irradiation. In vector-transfected MCF-7 breast adenocarcinoma cells, the synthetic promoter was more effective than the wild-type Egr1 counterpart in up-regulating expression of the reporter gene after exposure to a single 5 Gy dose, and equally effective as the wild-type in U87-MG glioma cells. The level of gene expression achieved using the synthetic promoter was dependent on the inducing radiation dose for both U87-MG and MCF-7 cells, being maximal at 3 Gy and decreasing at 5 and 10 Gy. Furthermore, induction could be repeated by additional radiation treatments. The latter indicates that up-regulation should be additive during fractionated radiotherapy schedules. To demonstrate the potential clinical benefit of such an approach, the synthetic promoters were also shown to drive expression of the herpes simplex virus thymidine kinase gene, leading to enhanced cell killing in the presence of the prodrug ganciclovir (GCV) when compared with cells treated with radiation alone. Our results demonstrate that the synthetic promoter is responsive to low doses of ionising radiation and therefore isolated CArG elements function as radiation-mediated transcriptional enhancers outside their normal sequence context. The continued development and optimisation of such radiation-responsive synthetic promoters is expected to make a valuable contribution to the development of future radiation-responsive vectors for cancer gene therapy.

A novel dual function retrovirus expressing multidrug resistance 1 and O6-alkylguanine-DNA-alkyltransferase for engineering resistance of haemopoietic progenitor cells to multiple chemotherapeutic agents.

Following transduction with a retrovirus (SF1MIH) expressing both the multidrug resistance 1 (MDR1) and O6-alkylguanine-DNA-alkyltransferase (ATase) proteins, human erythroleukaemic progenitor (K562) cells were isolated which were resistant to killing by the MDR1 substrate, colchicine. In colony-forming survival assays, K562-SF1MIH cells exhibited resistance to colchicine and doxorubicin, as well as to the O6-alkylating agents N-Methyl-N-nitrosourea (MNU) and temozolomide. Furthermore, the resistance to doxorubicin was abolished by preincubation with the MDR1 inhibitor verapamil while resistance to MNU was ablated by the specific ATase inactivator, O6-benzylguanine (O6-beG) confirming that resistance to doxorubicin and MNU was conferred by MDR1 and ATase, respectively. When K562-SF1MIH were exposed to combinations of colchicine and MNU or doxorubicin and temozolomide, simultaneous resistance to these agents was observed. Thus, transduction of K562 with SF1MIH conferred dual resistance to these cells. These data offer the prospect of designing vectors that will confer resistance to entire regimens of chemotherapy rather than just to individual components of such drug cocktails, thereby substantially increasing the efficacy of therapy. Furthermore, the use of such dual expression constructs is likely to be highly informative for the design of effective in vivo selection protocols, an issue likely to make a major impact in a clinical context in gene therapy in the near future.

Leptomeningeal melanoma in childhood.

BACKGROUND: Malignant melanoma (MM) is one of the least common types of childhood cancer, accounting for less than 1% of all pediatric malignancies. Neurocutaneous melanosis (NCM) is a rare phakomatosis consisting of congenital abnormal pigmentation of the skin and meninges. The meningeal lesions are particularly prone to malignant change. METHODS: The authors describe 5 patients with NCM and 1 with primary leptomeningeal melanoma (LMM) seen at 2 treatment centers in the north of England over a 13-year period (1984-1997). RESULTS: The clinical features, progress, radiological findings, and treatment of these patients are discussed. All six died within eight months of their diagnosis, illustrating the difficulties faced in treating patients with these conditions. The authors reviewed the published literature on NCM, concentrating on the various therapeutic strategies that have been tried. Very little consistency in approach was found. Malignant skin lesions in NCM may be less responsive than primary malignant melanoma, but the small number of patients with primary LMM or brain metastases of MM make comparisons with NCM difficult. The authors' own series illustrates well the piecemeal nature of therapy for patients with these rare conditions. CONCLUSIONS: The rate of incidence of MM melanoma in the U.K. is increasing, and it will represent an increasing proportion of the pediatric oncologist's workload. A consistent approach to the therapy of patients with metastatic MM and NCM is needed if we are to have any hope of offering more than palliative therapy to these children in the future.

Ifosfamide/etoposide alternating with high-dose methotrexate: evaluation of a chemotherapy regimen for poor-risk osteosarcoma.

Fifteen patients with relapsed osteosarcoma were treated with an intensive combination chemotherapy schedule. Ifosfamide 2.5 g m(-2) daily and etoposide 150 mg m(-2) daily coincidentally for 3 days and high-dose methotrexate 8 g m(-2) (with folinic acid rescue) on days 10-14 in a planned 21 -day cycle. Feasibility, toxicity and response to this alternative combination for the treatment of relapsed osteosarcoma was assessed. There were 98 evaluable cycles for toxicity and tolerability. The majority of cycles were well tolerated. Haematological toxicity of grade 3/4 (common toxicity criteria) was seen in all courses. Renal tubular loss of electrolytes, particularly magnesium, occurred in 71% of cycles. Thirteen per cent of cycles were repeated within 21 days and 61% within 28 days. In the thirteen patients evaluable for response, a partial response rate of 31% was seen after two cycles. However, patients with stable disease continued on therapy, and an overall consequent response rate of 62% was observed. Four patients were alive with no evidence of disease at 8-74 months. Three are alive with disease (at 8-19 months). There were six deaths, all disease related. This regimen exhibits an encouraging response rate in a group of children with poor prognosis disease, with a tolerable toxicity profile.

New perspectives for cancer chemotherapy by genetic protection of haematopoietic cells.

The effectiveness of anti-cancer chemotherapy can be limited by acute suppression of the bone marrow (myelosuppression). There is also a risk of therapy-related secondary haematopoietic malignancy as well as acute and longer term effects in other tissues. Clinical strategies have been established to address some of these problems, particularly toxic effects on the bone marrow (acute myelotoxicity); however, there is still substantial scope for improving the management of chronic toxicity and mutagenicity to haematopoietic cells and collateral damage to non-haematopoietic cells during chemotherapy. In this review, we have discussed a novel strategy that involves the transfer and expression of drug-resistance functions into haematopoietic stem cells and more-mature blood progenitor cells, to overcome both the acute and long-term deleterious effects of anti-tumour treatment in bone marrow. The potential advantages of this approach include: (1) the in vivo selection of protected cell populations, which offers the possibility of intensification or escalation of chemotherapeutic drug doses; (2) a reduction in the frequency of therapy-related leukaemia and (3) tumour sensitisation to chemotherapy at the same time as haematopoietic protection.

Chemoprotective gene transfer I: transduction of human haemopoietic progenitors with O6-benzylguanine-resistant O6-alkylguanine-DNA alkyltransferase attenuates the toxic effects of O6-alkylating agents in vitro.

Retroviral transduction was used to introduce cDNAs encoding two mutants of human O6-alkylguanine-DNA alkyltransferase (hAT), one of which (hATPA) is 16 times more resistant to O6-benzylguanine (O6-beG), and the other (hATPA/GA) which is almost totally refractory to inactivation relative to the wild-type protein, into K562 human erythroleukaemic cells. A colony-forming assay was used to demonstrate significant protection (P < 0.001) against mitozolomide or temozolomide toxicity in K562 clones expressing either hAT mutant, as determined from an in vitro assay of activity. However, protection against these agents was reduced in hATPA expressing cells in the presence of 1 microM O6-beG and was lost in the presence of 20 microM O6-beG while cells expressing hATPA/GA retained protection even in the presence of 20 microM O6-beG (P < 0.001). Using primary human cord blood-derived CD34+ haemopoietic cells in which PCR analysis indicated that up to 70% of progenitors were transduced with retroviral constructs harbouring hATPA/GA, we observed significant protection of the granulocyte-macrophage colony-forming cells against mitozolomide (P < 0.05) and temozolomide (P < 0.001) induced toxicity in the presence of O6-beG. These findings indicate that retrovirus-mediated expression of hATPA/GA in primitive primary human haemopoietic cells is possible and does provide O6-beG-resistant protection for these cells. Using this strategy in patients may simultaneously permit attenuated myelosuppression and increased sensitivity of tumour cells to the effects of O6-alkylating agent chemotherapy. These data, taken together with the study reported by Chinnasamy et al in the accompanying article in this issue showing reduced toxicity and clastogenicity in murine haemopoietic progenitors, make a compelling case to test this strategy clinically.

Chemoprotective gene transfer II: multilineage in vivo protection of haemopoiesis against the effects of an antitumour agent by expression of a mutant human O6-alkylguanine-DNA alkyltransferase.

Murine bone marrow cells were transduced ex vivo with a retrovirus encoding an O6-benzylguanine (O6-beG) insensitive, double mutant form of the human DNA repair protein O6-alkylguanine-DNA alkyltransferase (hATPA/GA). In animals reconstituted with the transduced bone marrow, about 50% of cells in the multipotent spleen colony-forming cells (CFU-S) and lineage restricted granulocyte-macrophage (GM-CFC) haemopoietic progenitor populations were found to be carrying the transgene and this correlated with the frequency of bone marrow cells and spleen colonies which stained positive for hATPA/GA by immunocyto-chemistry. Expression of hATPA/GA was associated with significant in vivo protection of both CFU-S (P = 0.001) and GM-CFC (P < 0.024) against the toxicity of the antitumour methylating agent, temozolomide, given in combination with O6-beG. Expression of hATPA/GA also led to a reduction in the frequency of combined O6-beG/temozolomide-induced micronuclei seen in polychromatic erythrocytes (P < 0.003). This study is the first to demonstrate in vivo protection of multipotent haemopoietic progenitors against the toxic and clastogenic effects of an O6-alkylating agent in the presence of O6-beG. It also represents the first report of reduced clastogenesis as a consequence of expression of an O6-beG-resistant ATase. In the accompanying article we report hATPA/GA-mediated resistance of human CD34+ haemopoietic progenitors to combined O6-beG/O6-alkylating agent toxicity. Together these two reports suggest that a gene therapy strategy whereby protection of normal haemopoietic tissue may be combined with O6-beG-mediated tumour sensitisation may be efficacious in achieving an increase in therapeutic index.

Accelerated telomere shortening in young recipients of allogeneic bone-marrow transplants.

BACKGROUND: The establishment of donor-derived haemopoiesis in the recipients of allogeneic bone-marrow transplants (BMT) involves extensive proliferation of haemopoietic stem cells. The biological consequences of this replicative stress are ill defined, but any "ageing" effect would carry the risk of an increased frequency of clonal disorders during later life. We compared blood-cell mean telomere lengths in donor/recipient pairs. METHODS: Mean telomere length was calculated by in-gel hybridisation to leucocyte DNA from 56 normal individuals aged 0-96 years, and from 14 consecutive BMT recipients (aged 2-14 years) plus their respective donors (aged 2-46 years). Engraftment was confirmed by variable numbers of tandem repeats (VNTR) or gender analysis. FINDINGS: On average, blood-cell telomeres of transplant recipients were 0.4 kb (95% CI -0.2 to -0.6) shorter than those of their respective donors. This degree of telomere loss is equivalent to a median of 15 years' (range 0-40) ageing in the healthy controls. INTERPRETATION: The kinetics of haemopoietic engraftment impose replicative stress on the haemopoietic stem cells, resulting in a pronounced ageing effect, which may be sufficient to accelerate the onset of clonal haemopoietic disorders usually associated with later life. Monitoring of haemopoietic status in BMT recipients as time since BMT increases will be important. Assessment of transplant protocols under development in terms of their effects on telomere shortening is also indicated.

Survival and late effects in medulloblastoma patients treated with craniospinal irradiation under three years old.

Conventional treatment of medulloblastoma has involved surgery to the primary tumour and radiotherapy to the primary site and craniospinal axis. However CNS irradiation in a young child may result in significant side effects. Thus new treatment strategies have emerged which include chemotherapy, given in order to delay radiotherapy, to enable radiation dose reduction to the primary site and craniospinal axis, or even to eliminate radiotherapy completely. Such treatments have not yet been adequately evaluated in terms of survival and late effects. We report a retrospective study of 37 patients under the age of 36 months treated with postoperative craniospinal irradiation, in which the radiation dose to the neuroaxis was below conventional dosage. The overall actuarial 10-year survival rate was 44% and the actuarial 10-year relapse tree survival rate was 54%. Both radiotherapy and chemotherapy contributed to morbidity and mortality. Tour of 16 patients who survived longer than 10 years had no hard neurological signs; all but one patient have required extra support at school. Of nine patients available for work, two have obtained employment but only one has maintained this. No young adults have married. Despite lower doses of radiation, all but 1 survivor has significant spine shortening, and all who reached final height were short. Further work is needed to complete the profile of late effects in this group, which should include the survivors own perceptions of quality of life. It is hoped that multimodality treatment and supportive care can sustain acceptable survival rates but reduce the burden of late effects.

Pediatric oncology and hematology in Manchester, England.

The Manchester pediatric oncology unit is the third largest unit in the United Kingdom, with approximately 120 new referred cases per annum (10% of the U.K. total). Research activities include a gene therapy program, peripheral blood stem cell studies, the genetic epidemiology of childhood cancer, late-effects research (growth, body composition, pulmonary, quality of life), psychosocial studies, and clinical trial organization. Both the clinical oncology service and research activities involve close team coordination and collaboration with scientists both within and outside Manchester. A comprehensive pediatric hematology service is provided. The unit contains the second largest children's hemophilia service in the United Kingdom, serving 200 patients with congenital blood disorders. Twenty-five bone marrow transplants are performed each year (allogeneic, unrelated donor, autologous, and peripheral stem cell) for malignant and nonmalignant disorders. These activities are closely related to local, national, and international research groups.

Possibilities of gene therapies for cancer.

Cancer is an acquired disease in which it is possible to identify a variety of abnormalities at a genetic level. This holds a promise that genetic manipulation of tumour cells will lead to novel therapies. As yet these approaches are constrained by available methods for obtaining gene transfer and subsequent genetic control. However, a number of strategies are already reaching the clinic, including attempts at immunotherapy, prodrug activation and improving host defence against conventional chemotherapy. Further clinical opportunities will occur with improved vector development.

Towards gene therapy of Hurler syndrome.

Allogeneic bone marrow transplantation is the most effective treatment for Hurler's syndrome. However, due to a lack of matched related donors and unacceptable morbidity of matched unrelated transplants, this therapy is not available to all patients. Therefore we have been developing an alternative approach based on transfer and expression of the normal gene in autologous bone marrow. A retroviral vector carrying the full length cDNA for alpha-L-iduronidase has been constructed and used to transduce bone marrow from patients with this disorder. A number of different gene transfer protocols have been assessed including the effect of intensive schedules of exposure of bone marrow to viral supernatant and the influence of growth factors. With these protocols we have demonstrated successful gene transfer into primitive CD34+ cells and subsequent enzyme expression in their maturing progeny. Also, using long-term bone marrow cultures, we have demonstrated high levels of enzyme expression sustained for several months. The efficiency of gene transfer has been assessed by PCR analysis of haemopoietic colonies as around 50%. No advantage has been demonstrated for the addition of growth factors or intensive viral exposure schedules. Indeed a possible disadvantage has been identified for the use of intensive transduction procedures. The enzyme is secreted into the medium and functional localisation has been demonstrated by reversal of the phenotypic effects of lysosomal storage in macrophages. This pre-clinical work forms the basis for a clinical trial of gene therapy for Hurler syndrome.