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

Protection of committed murine haemopoietic progenitors against BCNU toxicity does not predict protection of primitive, multipotent spleen colony-forming cells – implications for chemoprotective gene therapy

“…The size of the protective effect in committed progenitors (1.4-to 3.5-fold) was similar to that previously reported by others following transduction of bone marrow with wild-type or single mutant forms of hAT. 40,42,43,46,59 Given this effect and from our experience with temozolomide and mitozolomide, 49,50 we expected to see a similar protective effect in the more primitive day 12 CFU-S compartment. However, whilst a significant effect on survival was seen in the absence of O 6 -beG there was no protective effect in the presence of O 6 -beG.…”

“…ATase-mediated protection of lineage-specific or bipotent bone marrow progenitors, both in vitro and in vivo, against the toxicity of a range of O 6 -alkylating agents has been demonstrated. [40][41][42][43][44][45]59 Some of these studies have made use of mutant ATases, which exhibit various degrees of resistance to the tumour sensitiser O 6 -beG, [46][47][48][49][50] and have shown that haemopoietic cell protection is maintained under conditions where an otherwise resistant tumour might be expected to be sensitised to treatment. Despite the fact that this strategy is often referred to as stem cell protection, little direct qualitative evaluation of any protective effects that may occur in primitive haemopoietic cells has been undertaken.…”

“…38,39 An approach to circumventing the dose limiting toxicity of O 6 -alkylating agents, whilst simultaneously sensitising tumours via O 6 -beG, would be to enhance the repair capacity of bone marrow for O 6 -alkG via expression of a mutant version of human ATase (hAT) that is both refractory to O 6 -beG inactivation and can efficiently repair O 6 -alkG in DNA. Increasing the expression of hAT [40][41][42][43][44][45] or O 6 -beG-resistant mutants of hAT, [46][47][48][49][50] either in murine bone marrow or in human primary haemopoetic cells, has been shown to afford some degree of protection against O 6 -alkylating agentinduced toxicity in the bipotent granulocyte-macrophage progenitor population. These observations have stimulated interest in a clinical trial to protect bone marrow against BCNUinduced myelosuppression, even though evidence of enhanced resistance to BCNU in multipotent haemopoietic progenitors has not yet been reported.…”

Therapeutic Modulation of DNA Damage and Repair Mechanisms in Blood Cells

“…While this is still an area of active investigation, there is no evidence to date that overexpression of mutant forms of MGMT and exposure to alkylating agents can lead to leukemia. To this end, several groups have demonstrated that hematopoietic cells transduced with retroviral vectors that express MGMTmutant proteins do not show an increased in the frequency of mutations or chromosomal aberrations upon challenge with O 6 alkylating agents (Allay et al, 1997;Chinnasamy et al, 1998a;Chinnasamy et al, 1998b;Dumenco et al, 1993;Fairbairn et al, 2000;Liu et al, 1994;Liu et al, 1999;Reese et al, 2001). Additionally, there have been no signs of hematopoietic clonal expansion in a canine large animal model in which animals transplanted animals MGMT P140K -transduced cells received alkylator therapy over a prolonged time period that also included some escalation in alkylator dose.…”

Live and let die: In vivo selection of gene-modified hematopoietic stem cells via MGMT-mediated chemoprotection

“…In practice, there is little evidence that such a chemoprotective strategy would promote leukemia in cells which overexpress MGMT. In fact, gene modified hematopoietic cells demonstrate a significant reduction in mutation frequency and chromosomal aberrations upon challenge with O 6 alkylating agents [ 31,[82][83][84] and mice transgenic for human MGMT show a decrease in cellular transformation frequency upon drug treatment [85][86][87][88][89]. Perhaps most compelling in this regard is the observation that there was no evidence of hematopoietic malignancy following an extended, dose escalating chemotherapeutic regimen in a canine large animal model which enabled selection of gene modified cells [ 62 ].…”