A phase I study of MGMT-P140K transfected hematopoetic progenitor cells (HPC) combined with TMZ/O6BG dose escalation for newly diagnosed, MGMT unmethylated glioblastoma: Tolerance and evidence of survival benefit.

Abstract

2062 Background: GBM is the most common malignant brain tumor with a median survival of 15 months despite surgery and radio-chemotherapy. The most important mechanism of TMZ resistance is the O6-methylguanine-DNA methyltransferase (MGMT) gene which repairs temozolamide-induced DNA methylation. The MGMT inhibitor O6-benzylguanine (BG) demonstrated efficacy in depleting MGMT and maximizing tumor response in early phase clinical trials. However, MGMT expression is also low in hematopoietic cells, so this approach led to unacceptable bone marrow toxicity and thus has been abandoned. We hypothesized that chemoprotection of hematopoietic HPC with an MGMT mutant (MGMT-P140K) characterized by normal methyltransferase activity, coupled with low affinity for BG would maximize anti-tumor response while enabling patients to tolerate TMZ & BG dose escalation with minimal toxicity. A phase I trial was performed to test this hypothesis. Methods: 10 adults with newly diagnosed MGMT unmethylated, IDH-1 WT, GBM underwent standard surgery and radiation, followed by transplantation with autologous CD34+ HPC engineered to express MGMT-P140K using a lentiviral vector. We tested tolerance and efficacy of three different paradigms for conditioning bone marrow and re-infusion of HPC. To assess chemo-protection, patients’ blood counts and transgene marking were monitored during and after treatment, as was toxicity, response, and progression-free and overall survival. Results: Treatment was moderately toxic with 3/10 patients suffering grade 3-4 hematologic toxicity; no high grade non-hematologic toxicity was observed . Viral transduction rates ranged from 3-75% and were clearly improved in Arm III utilizing BCNU conditioning and intra-patient dose escalation of TMZ/O6GB. In patients tolerating 3 cycles or more, P140K-MGMT gene markings in peripheral blood and bone marrow cells increased 3-26-fold with only mild (Grade 2-3) mylosuppression consistent with chemo-protection as hypothesized. Median PFS and OS was 22 and 31 months respectfully, and three patients in Arm III are healthy and progression free at 36-39 months. OS exceeded RPA predicted survival by 3.3-fold suggesting clinical benefit. Viral insertion site analysis demonstrate lack of clonal dominance. Conclusions: P140K-MGMT transfected HPC enables TMZ/ BG dose escalation with acceptable toxicity and increased survival in a small cohort of selected patients. A phase II study is ongoing. Clinical trial information: NCT01269424.