The DNA Crosslinking Agent SJG-136 (SG2000, BN2629) Is Highly Potent in Acute Myeloid Leukaemia and Is Synergistic with Cytarabine

Standard treatment of acute myeloid leukemia (AML) results in a median survival of approximately 1 year. However, increased understanding of the complex biology of AML has led to the introduction of more targeted therapies, including gemtuzumab ozogamycin and a number of FLT3 inhibitors, in the expectation that they will improve outcomes in this condition. The future is likely to see combinations of new targeted drugs together with more standard therapies. The challenge will be to identify the optimal combinations for individual patients based on their specific disease markers. SJG-136 (SG2000, BN2629) is a pyrrolo[2,1-c][1,4]benzodiazepine dimer which binds into the minor grove of DNA with a preference for 5′Œ-GATC-3′Œ sequences. The crosslinks that SJG-136 forms are more persistent than those formed by nitrogen mustard agents like melphalan due to their slow rate of repair. We have previously shown that SJG-136 is very potent against primary chronic lymphocytic leukaemia (CLL) cells so we set out to investigate its effect on AML cell lines and primary AML. The AML cell lines, NB4 and HL60, had LD₅₀ values of 5.75nM and 12.10nM respectively following exposure to SJG-136 for 48 hours. Apoptosis induction was dose-dependent, as evidenced by Annexin V/propidium iodide assay and caspase-3 activation assay. Subsequently, primary AML cells from 37 diagnostic samples were isolated and treated with SJG-136 and cytarabine yielding an average LD₅₀ of 19.4nM (±22.8) and 5.7mM (±5.6) respectively. SJG-136 was >100 times more potent than cytarabine in samples derived from the same patients (P< 0.0001). We next investigated the potential for synergy between cytarabine and SJG-136 in the AML cell lines (cytarabine:SJG-136, 100:1). NB4 cells demonstrated a combination index (CI) for the two agents of 0.37 and HL60 cells showed a CI of 0.69 indicating that these two agents were synergistic in the two cells lines under these conditions. The primary AML samples were also tested in vitro for synergy between SJG-136 and cytarabine at the same ratio. In these samples the mean CI was 0.56 (± 0.49) with synergy being demonstrated in 29/37 patient samples. Closer examination of these results showed that the 8 patient samples with a CI greater than 1 were all particularly sensitive to cytarabine as a single agent. Therefore, the increased sensitivity of these patient samples to cytarabine may account for their apparent lack of synergy as calculated by the median effect method. These results indicate that SJG-136 has significant potential as a therapy for AML both as a single agent and in combination with cytarabine.