Safety of MGMT Gene Transfer into Hematopoietic Progenitors: A Phase I Clinical Trial Using Retroviral Gene Transfer of G156A MGMT To Protect Hematopoiesis during BG Plus Alkylating Agent Treatment of Advanced Malignancies.

The G156A mutation of the DNA repair gene O⁶-methylguanine DNA-methyltransferase (MGMT) confers hematopoietic cell resistance to O⁶-benzylguanine (BG) and the DNA-alkylating agents BCNU and temozolomide (TMZ). We conducted a Phase I clinical trial utilizing MGMT-mediated chemoprotection to improve treatment of advanced solid tumors. Here we report the safety and feasibility of this approach. Autologous CD34⁺ cells were transduced ex vivo with a retroviral vector MFG- G156A MGMT (K. Cornetta, NGVL at Indiana U.), then reinfused into non-myeloablated patients. Eight patients were accrued to the study and 6 patients were infused with a mean of 7.3 × 10⁵ cells/kg obtained post transduction. Of the 6 patients who received cells, all received one cycle of chemotherapy (BG + BCNU or TMZ) 3 days prior to cell infusion and 3 patients received a second cycle of either BG + BCNU or BG + TMZ 4–6 weeks post infusion. No patient experienced grade 2 or greater infusion related toxicity, and one patient went off study prior to receiving cells due to neurological toxicity during cytokine administration. All patients who received G156A MGMT+ cells went off study due to progressive disease, and survival ranged from 2 weeks to 9 months post infusion. No patient showed evidence of replication competent retrovirus (RCR) when assessed for the presence of the viral envelope by quantitative PCR for GALV sequences (performed by the IU Vector Production Facility). The mean vector copy number in post transduction cells (n= 7 patients) was 0.29 copies per genome by quantitative PCR (Q-PCR), with a mean CFU transduction efficiency of 24%, and a mean of 9% CD34 cells expressing the G156A MGMT transgene by flow cytometry. Retroviral insertion sites in pre-infusion CFUs from 2 patients analyzed by linear amplification-mediated (LAM)-PCR were polyclonal, with an average of 1.4 retroviral insertions per positive CFU. Transgene positive cells were detected in vivo in 3 patients. In the first patient, after BG and BCNU at 6 weeks, the transgene was detected in 1 of 100 BM CFU by Q-PCR and one insertion site was detected in this clone by LAM-PCR. The patient went off study at 8 weeks, then received TMZ alone (300 mg/m²/day for 5 days). At 16 weeks, the transgene was detected at low frequency in BM MNCs and granulocytes by Q-PCR and in BM granulocytes as an internal band in LAM-PCR, although no other insertions sites were detectable. In a second patient, the transgene was detected in 2 of 30 BM CFU by PCR and in BM granulocytes and MNCs by LAM PCR at 5 weeks. In a third patient treated with BG + TMZ 4 weeks post cell infusion, 3 of 50 PB CFU were transgene positive by Q-PCR at week 28. These data demonstrate that infusion of retroviral MGMT transduced hematopoietic cells is safe and are the first to show emergence and low level persistence of transduced mutant G156A MGMT cells after nonmyeloablative conditioning in humans. Our future studies include using a lentiviral vector to achieve better levels of gene transfer and reduced risk of insertional mutagenesis.