Nelarabine: a nucleoside analog with efficacy in T-cell and other leukemias

A phase 1 study to evaluate the safety, pharmacology, and feasibility of continuous infusion nelarabine in patients with relapsed and/or refractory lymphoid malignancies

BACKGROUND

Nelarabine is a purine nucleoside analogue prodrug approved for the treatment of relapsed and refractory T-cell acute lymphoblastic leukemia (R/R T-ALL) and lymphoblastic lymphoma (T-LBL). Although effective in R/R T-ALL, significant neurotoxicity is dose-limiting and such neurotoxicity associated with nucleoside analogues can be related to dosing schedule.

METHODS

The authors conducted a phase 1 study to evaluate the pharmacokinetics and toxicity of nelarabine administered as a continuous infusion (CI) for 5 days (120 hours), rather than the standard, short-infusion approach.

RESULTS

Twenty-nine patients with R/R T-ALL/LBL or T-cell prolymphocytic leukemia (T-PLL) were treated, with escalating doses of nelarabine from 100 to 800 mg/m² /day × 5 days. The median age of the patients was 39 years (range, 14-77 years). The overall response rate was 31%, including 27% complete remission (CR) or CR with incomplete platelet recovery (CRp). Peripheral neuropathy was observed in 34% of patients, including four ≥grade 3 events related to nelarabine. Notably, there was no nelarabine-related central neurotoxicity on study. The maximum tolerated dose was not reached. Pharmacokinetic data suggested no relationship between dose of nelarabine and accumulation of active intracellular ara-GTP metabolite. Higher intracellular ara-GTP concentrations were statistically associated with a favorable clinical response.

CONCLUSION

Preliminary evaluation of continuous infusion schedule of nelarabine suggests that the safety profile is acceptable for this patient population, with clinical activity observed even at low doses and could broaden the use of nelarabine both as single agent and in combinations by potentially mitigating the risk of central nervous system toxicities.

Rational Mutagenesis in the Lid Domain of Ribokinase from E. coli Results in an Order of Magnitude Increase in Activity towards D-arabinose

Development of efficient approaches for the production of medically important nucleosides is a highly relevant challenge for biotechnology. In particular, cascade synthesis of arabinosides would allow relatively easy production of various cytostatic and antiviral drugs. However, the biocatalyst necessary for this approach, ribokinase from Escherichia coli (EcoRK), has a very low activity towards D-arabinose, making the synthesis using the state-of-art native enzyme technologically unfeasible. Here, we report the results of our enzyme design project, dedicated to engineering a mutant form of EcoRK with elevated activity towards arabinose. Analysis of the active site structure has allowed us to hypothesize the reasons behind the low EcoRK activity towards arabinose and select feasible mutations. Enzyme assay and kinetic studies have shown that the A98G mutation has caused a large 15-fold increase in k_cat and 1.5-fold decrease in K_M for arabinose phosphorylation. As a proof of concept, we have performed the cascade synthesis of 2-chloroadenine arabinoside utilizing the A98G mutant with 10-fold lower amount of enzyme compared to the wild type without any loss of synthesis efficiency. Our results are valuable both for the development of new technologies of synthesis of modified nucleosides and providing insight into the structural reasons behind EcoRK substrate specificity.

Repair of DNA damage induced by the novel nucleoside analogue CNDAG through homologous recombination

PURPOSE

We postulate that the deoxyguanosine analogue CNDAG [9-(2-C-cyano-2-deoxy-1-β-D-arabino-pentofuranosyl)guanine] likely causes a single-strand break after incorporation into DNA, similar to the action of its cytosine congener CNDAC, and that subsequent DNA replication across the unrepaired nick would generate a double-strand break. This study aimed at identifying cellular responses and repair mechanisms for CNDAG prodrugs, 2-amino-9-(2-C-cyano-2-deoxy-1-β-D-arabino-pentofuranosyl)-6-methoxy purine (6-OMe) and 9-(2-C-cyano-2-deoxy-1-β-D-arabino-pentofuranosyl)-2,6-diaminopurine (6-NH₂). Each compound is a substrate for adenosine deaminase, the action of which generates CNDAG.

METHODS

Growth inhibition assay, clonogenic survival assay, immunoblotting, and cytogenetic analyses (chromosomal aberrations and sister chromatid exchanges) were used to investigate the impact of CNDAG on cell lines.

RESULTS

The 6-NH₂ derivative was selectively potent in T cell malignant cell lines. Both prodrugs caused increased phosphorylation of ATM and its downstream substrates Chk1, Chk2, SMC1, NBS1, and H2AX, indicating activation of ATM-dependent DNA damage response pathways. In contrast, there was no increase in phosphorylation of DNA-PKcs, which participates in repair of double-strand breaks by non-homologous end-joining. Deficiency in ATM, RAD51D, XRCC3, BRCA2, and XPF, but not DNA-PK or p53, conferred significant clonogenic sensitivity to CNDAG or the prodrugs. Moreover, hamster cells lacking XPF acquired remarkably more chromosomal aberrations after incubation for two cell cycle times with CNDAG 6-NH₂, compared to the wild type. Furthermore, CNDAG 6-NH₂ induced greater levels of sister chromatid exchanges in wild-type cells exposed for two cycles than those for one cycle, consistent with increased double-strand breaks after a second S phase.

CONCLUSION

CNDAG-induced double-strand breaks are repaired mainly through homologous recombination.

Relapsed acute lymphoblastic leukemia: current status and future opportunities

Significant improvements in primary therapy for childhood acute lymphoblastic leukemia (ALL) have led to dramatic increases in cure rates over the past few decades. Relapsed ALL, however, remains more common than new diagnoses of many common pediatric malignancies. Outcomes for patients with relapsed ALL remain poor, especially for patients with early bone marrow relapse. However, most relapse patients do achieve a second complete remission, followed by therapeutic options including further chemotherapy and hematopoietic stem cell transplant. The level of minimal residual disease after achieving second remission or before transplant may predict outcomes. The substantial likelihood of achieving second remission with familiar drug combinations may discourage participation in formal relapse studies. The high likelihood of achieving a third remission may discourage participation in single-agent trials of new drugs, despite the critical need for novel agents with activity against resistant disease that may improve outcomes for recurrent ALL.

Three new drugs for acute lymphoblastic leukemia: nelarabine, clofarabine, and forodesine

The search for more effective and safer anti-leukemia therapies has led to the identification of several new agents that show activity against specific types of acute lymphoblastic leukemia (ALL). Recently, three novel purine nucleoside analogues (nelarabine, clofarabine, and forodesine) have shown promising activity in patients with relapsed or refractory ALL. Of these, nelarabine has shown clinically meaningful benefit in patients with T-cell ALL, with overall response rates ranging from 33% to 60%, the induction of durable complete remissions, and an overall 1-year survival rate of 28% in adults. Clofarabine has also shown promising clinical activity in pediatric patients, with an overall response rate of 30%, and some patients are able to proceed to allogeneic hematopoietic cell transplantation. Forodesine is the most recent novel agent, with a unique mechanism that has shown single-agent activity in relapsed and refractory T- and B-cell leukemias and cutaneous lymphomas. Although clinical experience is limited, treatment-related toxicities appear to be mild. The rationale, pharmacology, and clinical experience to date with these agents in the treatment of patients with refractory acute leukemia are reviewed, with a highlight on ALL.

In vitro cytotoxicity of nelarabine, clofarabine and flavopiridol in paediatric acute lymphoblastic leukaemia

The in vitro efficacies of three new drugs--clofarabine (CLOF), nelarabine (NEL) and flavopiridol (FP) - were assessed in a panel of acute lymphoblastic leukaemia (ALL) cell lines. The 50% inhibitory concentration (IC50) for CLOF across all lines was 188-fold lower than that of NEL. B-lineage, but not T-lineage lines, were >7-fold more sensitive to CLOF than cytosine arabinoside (ARAC). NEL IC50 was 25-fold and 113-fold higher than ARAC in T- and B-lineage, respectively. T-ALL cells were eightfold more sensitive to NEL than B-lineage but there was considerable overlap. FP was more potent in vitro than glucocorticoids and thiopurines and at doses that recent phase I experience predicts will translate into clinical efficacy. Potential cross-resistance of CLOF, NEL and FP was observed with many front-line ALL therapeutics but not methotrexate or thiopurines. Methotrexate sensitivity was inversely related to that of NEL and FP. Whilst NEL was particularly effective in T-ALL, a subset of patients with B-lineage ALL might also be sensitive. CLOF appeared to be marginally more effective in B-lineage than T-ALL and has a distinct resistance profile that may prove useful in combination with other compounds. FP should be widely effective in ALL if sufficient plasma levels can be achieved clinically.

Novel purine nucleoside analogues for T-cell-lineage acute lymphoblastic leukaemia and lymphoma

Purine nucleoside phosphorylase (PNP) deficiency is a rare, inherited immunodeficiency disorder in which the specific molecular defect was identified. Clinically, a lack of PNP manifests as profound T-cell deficiency with minor or variable changes in the humoral system. Biochemically, the absence of PNP results in an increase in plasma deoxyguanosine (dGuo) and a T-cell-specific increase in intracellular deoxyguanosine triphosphate (dGTP). This observation has been the impetus for the search for either inhibitors of the enzyme or PNP-resistant dGuo analogues as potential anti-T-cell-lineage agents over the past 30 years. Forodesine (an inhibitor of PNP) and nelarabine (a PNP-resistant dGuo analogue) proved to be T-cell selective when tested in clinic. This review summarises the preclinical, clinical and pharmacokinetic investigations with these novel agents.

Nelarabine in the treatment of refractory T-cell malignant diseases

T-cell haematological malignancies are uncommon, difficult to treat and, with the exception of T-cell acute lymphoblastic leukaemia, often associated with a poor prognosis. Nelarabine (2-amino-9-beta-D-arabinosyl-6-methoxy-9H-guanine), a synthesised guanosine nucleoside and water-soluble prodrug of ara-G (9-beta-D-arabinofuranosylguanine), has recently been approved by the FDA for the treatment of relapsed/refractory T-cell acute lymphoblastic leukaemia and T-cell lymphoblastic lymphoma in adults and children. Similar to other nucleoside analogues, nelarabine acts by inhibiting DNA synthesis and inducing apoptosis in susceptible cells. Ara-G itself is a water-insoluble molecule, making its clinical use difficult. However, nelarabine is water soluble and rapidly converted to ara-G in vivo. Interestingly, it has been demonstrated that ara-GTP accumulates more readily in T-cells than in B-cells, and this discovery created interest in the development of nelarabine for the treatment of T-cell malignancies. The results of early-phase clinical trials evaluating the use of nelarabine in adults and children with refractory T-cell malignancies have been promising. This article describes the development, pharmacology, toxicity and clinical activity of nelarabine, as well as discusses its potential role in the treatment of T-cell haematological malignancies.