Thioguanine Combined with Decitabine Can Overcome Resistance to Hypomethylating Agents: Final Results of a Phase I Trial of a Pharmacodynamically-Conceived Thioguanine/Decitabine Combination in Patients with Advanced Myeloid Malignancies

Background: Outcomes are poor for older patients with acute myeloid leukemia (AML), high-risk myelodysplastic syndromes (MDS), or relapsed/refractory disease, and new therapies are needed. Using a chemosensitivity screening assay, we previously demonstrated that combination treatment with thioguanine and decitabine can restore therapeutic efficacy in primary leukemia cells isolated from patients with relapsed/refractory AML. To test the safety and synergistic efficacy of this combination in patients with advanced myeloid malignancies, we performed a Phase I dose-escalation trial of thioguanine given with decitabine.

Patients and Methods: Patients with untreated AML ≥60 years of age and ineligible for standard induction, relapsed/refractory AML, and high-risk or relapsed MDS were eligible. Two thioguanine dose levels were evaluated: 80 and 120 mg/m²/day, given on Days 1-12 of induction and Days 1-7 of maintenance. Decitabine at 20mg/m² was administered on Days 3-12 during induction and on Days 3-7 during maintenance. The primary objective was to determine the maximum tolerated dose (MTD) of thioguanine when given with decitabine. Key secondary objectives were to evaluate the overall response rate (ORR) and progression-free survival (PFS). Patient-specific pharmacodynamic measures to assess the biologic activity of thioguanine-decitabine were also performed. These included an in vitro chemosensitivity assay, BH3 profiling to measure the degree to which the leukemic blasts were primed for apoptosis, and genome-wide analysis of DNA methylation changes.

Results: Twelve patients (median age 67; range 56-83) with de novo AML (n=1), secondary AML (n=6), relapsed/refractory AML (n=4), and chronic myelomonocytic leukemia (CMML) (n=1) were treated. Three patients experienced dose-limiting toxicity (DLT), which were acute renal failure requiring hemodialysis (80 mg/m²), persistent grade 4 leukopenia and thrombocytopenia (120 mg/m²), and grade 4 sepsis preventing continued treatment (120 mg/m²). Thioguanine at 80 mg/m² was determined to be the MTD. Eleven of the 12 patients completed the first induction cycle, and 6 patients completed a second, identical induction cycle. The median number of cycles administered was 3 (range 1-8). One patient experienced a DLT prior to the first response assessment and was removed from study. The ORR in this intent-to-treat study was 67% (8/12). Six patients achieved a CR or CRi, one obtained a morphologic leukemia-free state, and one patient had a PR. Responses were observed in all disease types. Five of the 8 responses, including 4 CR/CRi, were achieved with thioguanine at 80 mg/m², suggesting no loss of efficacy at the MTD compared with the higher dose level. All 11 evaluable patients had ≥50% reduction in bone marrow blast percentages after induction therapy. Six patients had previously received single-agent hypomethylating therapy, and 5 (83%) of these patients responded, demonstrating that thioguanine-decitabine can rescue prior hypomethylating agent failure. Out of the 8 responders, four (50%) proceeded to allogeneic stem cell transplantation (SCT), two relapsed after CR or CRi, one had a CNS-only relapse after achieving a CR, and one patient experienced DLT and was removed from the study. Of the four patients who proceeded to allogeneic SCT, two patients died in CR from transplant-related toxicity, one relapsed, and one patient remains alive and in remission greater than 2 years. Median PFS in responding patients was 42 weeks (range, 10-not reached, weeks). In vitro pharmacodynamic studies currently have been completed on samples from the first 6 patients treated on this trial. The chemosensitivity assay results on pre-treatment mononuclear cells directly correlated with initial response. In addition, significant apoptotic priming of the blasts, as suggested from BH3 profiling, also corresponded to initial clinical response.

Conclusions: Thioguanine-decitabine can be administered safely and induce remission, even among patients who had previously been treated with hypomethylating agents. Intriguingly, preliminary results from the chemosensitivity screening assay and BH3 profiling correlated well with clinical responses. Additional correlative studies including DNA methylation analysis are ongoing to better understand the mechanism of synergy between thioguanine and decitabine. A multi-center Phase II trial is planned.