Autologous transplants of P140K-expressing HSCs reduce hematopoietic toxicity of TMZ and benzylguanine therapy.

Scientists report positive data from a first-in-man study evaluating the bone marrow-protective effects of genetically modified autologous hematopoietic stem cell transplants in glioblastoma patients treated using chemotherapy and benzylguanine. The results, published in Science Translational Medicine, showed that of the three patients given the transplants prior to combination therapy, two survived much longer than average, and the third remains alive with no disease progression almost three years later.

The study, led by researchers at the Fred Hutchinson Cancer Research Center’s Clinical Research Division, demonstrates that the engineered stem cells have a chemoprotective effect, maximizing the drug dose that can be administered. The results are described in a paper titled “Extended Survival of Glioblastoma Patients After Chemoprotective HSC Gene Therapy.”

A major barrier to the successful use of cancer chemotherapy is organ toxicity, and primarily bone marrow toxicity (hematopoietic toxicity), which causes blood cell counts to drop and increases susceptibility to infection. Bone marrow toxicity resulting form alkylating agents has been linked with low or absent expression of the methylguanine methyltransferase (MGMT) gene, the researchers explain. However, some tumors, including aggressive brain tumors, paradoxically over-express MGMT, which results in the tumor cells being chemotherapy-resistant but the bone marrow cells being particularly sensitive to alkylating agents. Prognosis is thus particularly poor in patients with glioblastomas that express an unmethylated promoter for the MGMT gene because the increased increased MGMT activity in tumor cells reduces the cytotoxicity of alkylating chemotherapies such as temozolomide (TMZ) by repairing the drug-induced DNA damage.

It’s feasible that overcoming MGMT-related tumor-cell resistance to chemotherapy can be achieved by administering an MGMT inhibitor, O6-benzylguanine (O6BG), in combination with alkylating agent chemotherapy. However, Phase I and II studies have shown that this approach leads to significant hematopoietic toxicity, the researchers continue. In order to try and overcome this, the Fred Hutchinson team carried out a small clinical trial to test a gene therapy approach in which patients received transplants of their own hematopoietic stem cells (HSCs) that had been engineered to carry a mutant MGMT gene (P140K).

The mutant gene engineered into the HSCs displays the same activity as wild-type MGMT, but also confers both the stem cells and their progeny with resistance to O6BG. Prior in vitro and in vivo preclinical work has demonstrated that O6BG-resistant MGMT P140K gene expression can protect the hematopoietic system from toxicity associated with combined O6BG and alkylator chemotherapy.

The reported clinical study involved giving three glioblastoma patients autologous transplants of P140K-modified hematopoietic stem and progenitor cells after nonmyeloablative conditioning with the FDA-approved chemotherapeutic agent BCNU. This step was carried out because data from previous studies indicate that BCNU could facilitate engraftment of transplanted gene-modified cells while maintaining stable disease during transplant recovery.

After BCNU treatment and successful engraftment of modified stem cells, the patients were treated using between three and nine cycles of chemotherapy comprising O6BG and TMZ. Procedural details are reported in the published paper, but the authors stress that all patients demonstrated recovery from BCNU, engraftment of P140K gene-modified cells, and maintenance of stable disease during the transplant period. This, they say, supports “the clinical feasibility of this approach”. Moreover, the dose of BCNU administered prior to HSC transplantation was much lower than doses of the drug used for chemotherapy for glioblastoma, and resulted in only mild myelosuppression with no apparent toxicity.

PCR analyses confirmed that all three stem cell recipients exhibited gene-modified cells among peripheral blood granulocytes and lymphocytes up to 14 months after transplantation. Hematopoietic recovery was observed in each patient after each chemotherapy cycle, and the P140K gene-modified cells were found in multiple hematopoietic lineages after several rounds of combination treatment with O6BG and TMZ.

Importantly, chemotherapy was well tolerated, and all patients demonstrated acceptable hematopoietic toxicity and no significant extramedullary toxicity. “All three patients treated surpassed the median survival for glioblastoma patients with unmethylated MGMT promoter status in tumor cells (12 months), with two of three patients displaying stable disease at 12 months from diagnosis and one patient displaying no evidence for disease progression,” the authors state.

“These data support continued development of P140K-modified hematopoietic progenitor and stem cells as a chemoprotective strategy in cancers, including glioblastoma, where benefit from combined O6BG/DNA alkylating agent chemotherapy has been hindered by hematopoietic toxicity. Lack of extramedullary toxicity in the study patients suggests that this approach will allow for administration of multiple cycles of this chemotherapy, possibly at higher, more effective, doses, potentially leading to better treatment outcomes.”

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