Phase 2 and pharmacodynamic study of oral forodesine in patients with advanced, fludarabine-treated chronic lymphocytic leukemia

Perspectives and challenges in developing small molecules targeting purine nucleoside phosphorylase

As a cytosolic enzyme involved in the purine salvage pathway metabolism, purine nucleoside phosphorylase (PNP) plays an important role in a variety of cellular functions but also in immune system, including cell growth, apoptosis and cancer development and progression. Based on its T-cell targeting profile, PNP is a potential target for the treatment of some malignant T-cell proliferative cancers including lymphoma and leukemia, and some specific immunological diseases. Numerous small-molecule PNP inhibitors have been developed so far. However, only Peldesine, Forodesine and Ulodesine have entered clinical trials and exhibited some potential for the treatment of T-cell leukemia and gout. The most recent direction in PNP inhibitor development has been focused on PNP small-molecule inhibitors with better potency, selectivity, and pharmacokinetic property. In this perspective, considering the structure, biological functions, and disease relevance of PNP, we highlight the recent research progress in PNP small-molecule inhibitor development and discuss prospective strategies for designing additional PNP therapeutic agents.

SAMHD1 Limits the Efficacy of Forodesine in Leukemia by Protecting Cells against the Cytotoxicity of dGTP

The anti-leukemia agent forodesine causes cytotoxic overload of intracellular deoxyguanosine triphosphate (dGTP) but is efficacious only in a subset of patients. We report that SAMHD1, a phosphohydrolase degrading deoxyribonucleoside triphosphate (dNTP), protects cells against the effects of dNTP imbalances. SAMHD1-deficient cells induce intrinsic apoptosis upon provision of deoxyribonucleosides, particularly deoxyguanosine (dG). Moreover, dG and forodesine act synergistically to kill cells lacking SAMHD1. Using mass cytometry, we find that these compounds kill SAMHD1-deficient malignant cells in patients with chronic lymphocytic leukemia (CLL). Normal cells and CLL cells from patients without SAMHD1 mutation are unaffected. We therefore propose to use forodesine as a precision medicine for leukemia, stratifying patients by SAMHD1 genotype or expression.

Release of soluble and vesicular purine nucleoside phosphorylase from rat astrocytes and microglia induced by pro-inflammatory stimulation with extracellular ATP via P2X7 receptors

Purine nucleoside phosphorylase (PNP), a crucial enzyme in purine metabolism which converts ribonucleosides into purine bases, has mainly been found inside glial cells. Since we recently demonstrated that PNP is released from rat C6 glioma cells, we then wondered whether this occurs in normal brain cells. Using rat primary cultures of microglia, astrocytes and cerebellar granule neurons, we found that in basal condition all these cells constitutively released a metabolically active PNP with Km values very similar to those measured in C6 glioma cells. However, the enzyme expression/release was greater in microglia or astrocytes that in neurons. Moreover, we exposed primary brain cell cultures to pro-inflammatory agents such as lipopolysaccharide (LPS) or ATP alone or in combination. LPS alone caused an increased interleukin-1β (IL-1β) secretion mainly from microglia and no modification in the PNP release, even from neurons in which it enhanced cell death. In contrast, ATP administered alone to glial cells at high micromolar concentrations significantly stimulated the release of PNP within 1 h, an effect not modified by LPS presence, whereas IL-1β secretion was stimulated by ATP only in cells primed for 2 h with LPS. In both cases ATP effect was mediated by P2X₇ receptor (P2X₇R), since it was mimicked by cell exposure to Bz-ATP, an agonist of P2X₇R, and blocked by cell pre-treatment with the P2X₇R antagonist A438079. Interestingly, ATP-induced PNP release from glial cells partly occurred through the secretion of lysosomal vesicles in the extracellular medium. Thus, during inflammatory cerebral events PNP secretion promoted by extracellular ATP accumulation might concur to control extracellular purine signals. Further studies could elucidate whether, in these conditions, a consensual activity of enzymes downstream of PNP in the purine metabolic cascade avoids accumulation of extracellular purine bases that might concur to brain injury by unusual formation of reactive oxygen species.

Differential expression pattern of protein markers for predicting chemosensitivity of dexamethasone-based chemotherapy of B cell acute lymphoblastic leukemia

Dexamethasone is considered as a direct chemotherapeutic agent in the treatment of pediatric acute lymphoblastic leukemia (ALL). Beside the advantages of the drug, some problems arising from the dose-related side effects are challenging issues during the treatment. Accordingly, the classification of patients to dexamethasone sensitive and resistance groups can help to select optimizing the therapeutic dose with the lowest adverse effects particularly in sensitive cases. For this purpose, we investigated inhibited proliferation and induced cytotoxicity in NALM-6 cells, as sensitive cells, after dexamethasone treatment. In addition, comparative protein expression analysis using the 2DE-MALDI-TOF MS technique was performed to identify the specific altered proteins. In addition, we evaluated mRNA expression levels of the identified proteins in bone-marrow samples from pediatric ALL patients using the real-time q-PCR method. Eventually, proteomic analysis revealed a combination of biomarkers, including capping proteins (CAPZA1 and CAPZB), chloride channel (CLIC1), purine nucleoside phosphorylase (PNP), and proteasome activator (PSME1), in response to the dexamethasone treatment. In addition, our results indicated low expression of identified proteins at both the mRNA and protein expression levels after drug treatment. Moreover, quantitative real-time PCR data analysis indicated that independent of the molecular subtypes of the leukemia, CAPZA1, CAPZB, CLIC1, and PNP expression levels were lower in ALL samples than normal samples, although PSME1 expression level was higher in ALL samples than normal samples. Furthermore, the expression level of all proteins (except PSME1) was different between high-risk and standard-risk patients that suggesting the prognostic value of them. In conclusion, our study suggests a panel of biomarkers comprising CAPZA1, CAPZB, CLIC1, PNP, and PSME1 as early diagnosis and treatment evaluation markers that may differentiate cancer cells which are presumably to benefit from dexamethasone-based chemotherapy and may facilitate the prediction of clinical outcome.

MicroRNA-1 properties in cancer regulatory networks and tumor biology

Short non-coding microRNAs have been identified to orchestrate crucial mechanisms in cancer progression and treatment resistance. MicroRNAs are involved in posttranscriptional modulation of gene expression and therefore represent promising targets for anticancer therapy. As mircoRNA-1 (miR-1) exerted to be predominantly downregulated in the majority of examined tumors, miR-1 is classified to be a tumor suppressor with high potential to diminish tumor development and therapy resistance. Here we review the complex functionality of miR-1 in tumor biology.

8-Azapurines as isosteric purine fluorescent probes for nucleic acid and enzymatic research

The 8-azapurines, and their 7-deaza and 9-deaza congeners, represent a unique class of isosteric (isomorphic) analogues of the natural purines, frequently capable of substituting for the latter in many biochemical processes. Particularly interesting is their propensity to exhibit pH-dependent room-temperature fluorescence in aqueous medium, and in non-polar media. We herein review the physico-chemical properties of this class of compounds, with particular emphasis on the fluorescence emission properties of their neutral and/or ionic species, which has led to their widespread use as fluorescent probes in enzymology, including enzymes involved in purine metabolism, agonists/antagonists of adenosine receptors, mechanisms of catalytic RNAs, RNA editing, etc. They are also exceptionally useful fluorescent probes for analytical and clinical applications in crude cell homogenates.

Synthesis by microwave-assisted 1,3-dipolar cycloaddition of 1,2,3-triazole 1'-homo-3'-isoazanucleosides and evaluation of their anticancer activity

Racemic 1'-homo-3'-isoazanucleosides have been obtained by microwave-assisted 1,3-dipolar cycloaddition of 3,5-disubstituted proline derivative (±)-2 with different alkynes. The compounds obtained were evaluated for their cytotoxic activities in vitro against human breast carcinoma cell lines (MCF-7), human ovary carcinoma cell lines (A2780) and human lung carcinoma cell lines (NCI-H460).

Apoptosis inducers in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is characterized by a typical defect in apoptosis and is still an incurable disease. Numerous apoptosis inducers have been described. These synthetic compounds and natural products (mainly derived from plants) display antileukemic properties in vitro and in vivo and some have even been tested in the clinic in CLL. They act through several different mechanisms. Most of them involve proteins of the Bcl-2 family, which are the key regulators in triggering the mitochondrial pathway of caspase-dependent apoptosis. Thus, the Mcl-1/Noxa axis appeared as a target. Here I overview natural and synthetic apoptosis inducers and their mechanisms of action in CLL cells. Opportunities for developing novel, apoptosis-based therapeutics are presented.

Gold(I) complexes of 9-deazahypoxanthine as selective antitumor and anti-inflammatory agents

The gold(I) mixed-ligand complexes involving O-substituted derivatives of 9-deazahypoxanthine (HL_n) and triphenylphosphine (PPh₃) with the general formula [Au(L_n)(PPh₃)] (1–5) were prepared and thoroughly characterized by elemental analysis, FT-IR and multinuclear NMR spectroscopy, ESI+ mass spectrometry, single crystal X-ray (HL₅ and complex 2) and TG/DTA analyses. Complexes 1–5 were evaluated for their in vitro antitumor activity against nine human cancer lines, i.e. MCF7 (breast carcinoma), HOS (osteosarcoma), A549 (adenocarcinoma), G361 (melanoma), HeLa (cervical cancer), A2780 (ovarian carcinoma), A2780R (ovarian carcinoma resistant to cisplatin), 22Rv1 (prostate cancer) and THP-1 (monocytic leukaemia), for their in vitro anti-inflammatory activity using a model of LPS-activated macrophages, and for their in vivo antiedematous activity by λ-carrageenan-induced hind paw edema model on rats. The results showed that the complexes 1–5 exhibit selective in vitro cytotoxicity against MCF7, HOS, 22Rv1, A2780 and A2780R, with submicromolar IC₅₀ values for 2 against the MCF7 (0.6 µM) and HOS (0.9 µM). The results of in vitro cytotoxicity screening on primary culture of human hepatocytes (HEP220) revealed up to 30-times lower toxicity of compounds against healthy cells as compared with cancer cells. Additionally, the complexes 1–5 significantly influence the secretion and expression of pro-inflammatory cytokines TNF-α and IL-1β by a similar manner as a commercially used anti-arthritic drug Auranofin. The tested complexes also significantly influence the rate and overall volume of the edema, caused by the intraplantar application of λ-carrageenan polysaccharide to rats. Based on these promising results, the presented compounds could qualify to become feasible candidates for advanced testing as potential antitumor and anti-inflammatory drug-like compounds.

Targeting antiapoptotic and proapoptotic proteins for novel chronic lymphocytic leukemia therapeutics

SUMMARY 

Deficiency in the apoptotic program is one of the hallmarks of chronic lymphocytic leukemia. Defective apoptosis mainly results from the constitutive activation of survival pathways, which leads to the transcription and overexpression of antiapoptotic factors. The latter include proteins of the Bcl-2 family and members of the IAP family. The strategy of inhibiting the expression or activity of these antiapoptotic factors has been extensively investigated. Conversely, upregulation of proapoptotic proteins, notably BH3-only members of the Bcl-2 family (capable of antagonizing their antiapoptotic counterparts) has also been consistently described. Either mechanism can promote apoptosis in chronic lymphocytic leukemia cells ex vivo. The present article recapitulates the mechanistic data and how they contribute to the development of therapeutic agents targeting apoptosis.

Mycoplasma hyorhinis-encoded purine nucleoside phosphorylase: kinetic properties and its effect on the cytostatic potential of purine-based anticancer drugs

A mycoplasma-encoded purine nucleoside phosphorylase (designated PNPHyor) has been cloned and characterized for the first time. Efficient phosphorolysis of natural 6-oxopurine and 6-aminopurine nucleosides was observed, with adenosine the preferred natural substrate (Km = 61 µM). Several cytostatic purine nucleoside analogs proved to be susceptible to PNPHyor-mediated phosphorolysis, and a markedly decreased or increased cytostatic activity was observed in Mycoplasma hyorhinis-infected human breast carcinoma MCF-7 cell cultures (MCF-7.Hyor), depending on the properties of the released purine base. We demonstrated an ∼10-fold loss of cytostatic activity of cladribine in MCF-7.Hyor cells and observed a rapid and complete phosphorolysis of this drug when it was exposed to the supernatant of mycoplasma-infected cells. This conversion (inactivation) could be prevented by a specific PNP inhibitor. These findings correlated well with the high efficiency of PNPHyor-catalyzed phosphorolysis of cladribine to its less toxic base 2-chloroadenine (Km = 80 µM). In contrast, the cytostatic activity of nucleoside analogs carrying a highly toxic purine base and being a substrate for PNPHyor, but not human PNP, was substantially increased in MCF-7.Hyor cells (∼130-fold for fludarabine and ∼45-fold for 6-methylpurine-2'-deoxyriboside). Elimination of the mycoplasma from the tumor cell cultures or selective inhibition of PNPHyor by a PNP inhibitor restored the cytostatic activity of the purine-based nucleoside drugs. Since several studies suggest a high and preferential colonization or association of tumor tissue in cancer patients with different prokaryotes (including mycoplasmas), the data presented here may be of relevance for the optimization of purine nucleoside-based anticancer drug treatment.

Catalytic site conformations in human PNP by 19F-NMR and crystallography

Purine nucleoside phosphorylase (PNP) is a target for leukemia, gout, and autoimmune disorders. Dynamic motion of catalytic site loops has been implicated in catalysis, but experimental evidence was lacking. We replaced catalytic site groups His257 or His64 with 6-fluoro-tryptophan (6FW) as site-specific NMR probes. Conformational adjustments in the 6FW-His257-helical and His64-6FW-loop regions were characterized in PNP phosphate-bound enzyme and in complexes with catalytic site ligands, including transition state analogs. Chemical shift and line-shape changes associated with these complexes revealed dynamic coexistence of several conformational states in these regions in phosphate-bound enzyme and altered or single conformations in other complexes. These conformations were also characterized by X-ray crystallography. Specific (19)F-Trp labels and X-ray crystallography provide multidimensional characterization of conformational states for free, catalytic, and inhibited complexes of human PNP.

Preclinical and clinical evaluation of forodesine in pediatric and adult B-cell acute lymphoblastic leukemia

BACKGROUND

The discovery that purine nucleoside phosphorylase (PNP) deficiency leads to T-cell lymphopenia was the basis for introducing PNP inhibitors for T-cell leukemias. Forodesine is an orally bioavailable PNP inhibitor with picomolar potency. Because T lymphoblasts and indolent chronic lymphocytic leukemia (CLL) B cells inherently elicit favorable pharmacokinetics to accumulate deoxyguanosine triphosphate (dGTP), forodesine demonstrated promising activity in preclinical and clinical settings for patients with T-cell acute lymphoblastic leukemia (T-ALL) and B-cell CLL (B-CLL). However, the use of forodesine in B-cell ALL (B-ALL) is unknown.

PATIENTS AND METHODS

Leukemic blasts obtained from pediatric patients with de novo B-ALL (n = 10) were incubated with forodesine and deoxyguanosine (dGuo), and the biological end points of apoptosis, intracellular dGTP accumulation, and inhibition of RNA and DNA synthesis were measured. Additionally, adult patients with B-ALL (n = 2) were intravenously infused with 80 mg/m(2)/d daily for 5 days. After therapy, clinical response, toxicity, laboratory biomarkers including PNP enzyme inhibition, and plasma forodesine, dGuo, and intracellular dGTP levels were analyzed.

RESULTS

Our in vitro investigations demonstrated that forodesine treatment inhibited proliferation and induced modest apoptosis in de novo B-ALL lymphoblasts. There was time-dependent accumulation of dGTP and inhibition of RNA and DNA synthesis. During therapy, neither patient achieved a complete response (CR), but there was disease stabilization for several weeks in both patients. There was significant maintained inhibition of PNP enzyme in red blood cells, accumulation of forodesine and dGuo in plasma, and intracellular dGTP accumulation in both patients.

CONCLUSION

Our preclinical and clinical investigations suggest that forodesine has activity in B-ALL. However, it needs to be either infused with dGuo or combined with established chemotherapeutic agents based on mechanistic rationale.

Transition States, analogues, and drug development

Enzymes achieve their transition states by dynamic conformational searches on the femtosecond to picosecond time scale. Mimics of reactants at enzymatic transition states bind tightly to enzymes by stabilizing the conformation optimized through evolution for transition state formation. Instead of forming the transient transition state geometry, transition state analogues convert the short-lived transition state to a stable thermodynamic state. Enzymatic transition states are understood by combining kinetic isotope effects and computational chemistry. Analogues of the transition state can bind millions of times more tightly than substrates and show promise for drug development for several targets.

Immucillin-H, a purine nucleoside phosphorylase transition state analog, causes non-lethal attenuation of growth in Staphylococcus aureus

Purine nucleoside phosphorylase (PNP; EC: 2.4.2.1) is a key enzyme involved in the purine salvage pathway. A recent bioinformatic study by Yadav, P. K. et al. (Bioinformation 2012, 8(14), 664–672) reports PNP as an essential enzyme and potential drug target in community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA). We conducted an analysis using the methodology outlined by the authors, but were unable to identify PNP as an essential gene product in CA-MRSA. In addition, the treatment of Staphylococcus aureus cultures with immucillin-H, a powerful inhibitor of PNP, resulted in the non-lethal attenuation of growth, suggesting that PNP activity is not essential for cell viability.

Survival for older patients with acute myeloid leukemia: a population-based study

BACKGROUND

Acute myeloid leukemia is the second most common leukemia among United States adults with a median age of 69 years. We investigated recent clinical practices related to treatments and disease outcomes in older patients with acute myeloid leukemia in the United States.

DESIGN AND METHODS

In this retrospective cohort study, we used Surveillance, Epidemiology, and End Results program data from 2000 through 2007 linked to Medicare enrollment and utilization data in the United States.

RESULTS

Among 5,480 patients with acute myeloid leukemia (median age 78 years, range 65-93), 38.6% received leukemia therapy within three months of diagnosis (treated group). Practice changed with 16.3% of treated patients receiving hypomethylating agents after 2004 when those agents became available. Median survival was two months in the untreated group versus six months in the treated group (P<0.01) with the biggest improvements seen in those aged 65-69 years (10 months vs. 4 months; P<0.01) and 70-74 years (8 months vs. 3 months; P<0.01). In 46 patients receiving allogeneic hematopoietic cell transplantation (0.8%), the median survival from diagnosis was 22 months.

CONCLUSIONS

Therapy for leukemia improves overall survival in older acute myeloid leukemia patients. Based on their comorbidities, most patients up to 80 years of age should be considered for treatment. New therapies including hypomethylating agents and allogeneic hematopoietic cell transplantation are promising and must be compared with other chemotherapy regimens.

Inhibition of pyrimidine and purine nucleoside phosphorylases by a 3,5-dichlorobenzoyl-substituted 2-deoxy-D-ribose-1-phosphate derivative

The 3,5-dichlorobenzoyl-substituted 2-deoxy-D-ribose-1-phosphate derivative, designated Cf2891, was found to inhibit a variety of pyrimidine and purine nucleoside phosphorylases (NPs) with preference for uridine- and inosine-hydrolyzing enzymes [uridine phosphorylase (UP; EC 2.4.2.3), pyrimidine nucleoside phosphorylase (PyNP; EC 2.4.2.2) and purine nucleoside phosphorylase (PNP; EC 2.4.2.1)]. Kinetic analyses revealed that Cf2891 competes with inorganic phosphate (P(i)) for binding to the NPs and, depending on the nature of the enzyme, acts as a competitive or non-competitive inhibitor with regard to the nucleoside binding site. Also, the compound prevents breakdown of pyrimidine analogues used in the treatment of viral infections and cancer. Since NPs are abundantly present in tumor tissue and may be overexpressed due to secondary bacterial infections in immunocompromised patients suffering viral infections, Cf2891 may serve as a lead molecule for the development of inhibitors to be used in nucleoside-based combination therapy.

Plasmodium falciparum parasites are killed by a transition state analogue of purine nucleoside phosphorylase in a primate animal model

Plasmodium falciparum causes most of the one million annual deaths from malaria. Drug resistance is widespread and novel agents against new targets are needed to support combination-therapy approaches promoted by the World Health Organization. Plasmodium species are purine auxotrophs. Blocking purine nucleoside phosphorylase (PNP) kills cultured parasites by purine starvation. DADMe-Immucillin-G (BCX4945) is a transition state analogue of human and Plasmodium PNPs, binding with picomolar affinity. Here, we test BCX4945 in Aotus primates, an animal model for Plasmodium falciparum infections. Oral administration of BCX4945 for seven days results in parasite clearance and recrudescence in otherwise lethal infections of P. falciparum in Aotus monkeys. The molecular action of BCX4945 is demonstrated in crystal structures of human and P. falciparum PNPs. Metabolite analysis demonstrates that PNP blockade inhibits purine salvage and polyamine synthesis in the parasites. The efficacy, oral availability, chemical stability, unique mechanism of action and low toxicity of BCX4945 demonstrate potential for combination therapies with this novel antimalarial agent.

Tumour suppressors miR-1 and miR-133a target the oncogenic function of purine nucleoside phosphorylase (PNP) in prostate cancer

Background:

Our recent analyses of miRNA expression signatures showed that miR-1 and miR-133a were significantly reduced in several types of cancer. Interestingly, miR-1 and miR-133a are located on the same chromosomal locus in the human genome. We examined the functional significance of miR-1 and miR-133a in prostate cancer (PCa) cells and identified the novel molecular targets regulated by both miR-1 and miR-133a.

Methods and Results:

The expression levels of miR-1 and miR-133a were significantly downregulated in PCa compared with non-PCa tissues. Restoration of miR-1 or miR-133a in PC3 and DU145 cells revealed significant inhibition of proliferation, migration, and invasion. Molecular target identification by genome-wide gene expression analysis and luciferase reporter assay showed that purine nucleoside phosphorylase (PNP) was directly regulated by both miRNAs. Silencing of the PNP gene inhibited proliferation, migration, and invasion in both PC3 and DU145 cells. Immunohistochemistry detected positive staining of PNP in PCa specimens.

Conclusions:

Downregulation of miR-1 and miR-133a was a frequent event in PCa and both function as tumour suppressors. The PNP is a novel target gene of both miRNAs and potentially functions as an oncogene. Therefore, identification of novel molecular networks regulated by miRNAs may provide new insights into the underlying causes of PCa oncogenesis.

[Novel conventional therapies in onco-hemathology]

Cytogenetic, molecular and phenotyping features of malignant hematologic diseases succeeded in improving their management by a more accurate stratification of patients according to several groups of risk and by providing a rational for targeted therapy. Three major types of treatment (excluding cellular therapy) are currently available in onco-hematology: conventional chemotherapy, small molecules for targeted therapy and monoclonal antibodies. Conventional chemotherapy with optimization of doses and multidrug-based regimens allowed to substantially improve survival of patients and keeps a place of choice in treatment of these diseases. Targeted treatments came from the cytogenetic and molecular characterization of hemopathies. Thus, the kinase Bcr-Abl, as a result of the translocation t(9;22)(q34;q11), has been successfully targeted by tyrosine kinase inhibitors (TKI) in chronic myeloid leukemia and Ph+ acute lymphoblastic leukemia. Molecular abnormalities like internal-tandem duplication/point activating mutations in FLT3 in some acute myeloblastic leukemia or epigenetic dysregulations in some blood malignancies can also be targeted by small molecules. Hematopoietic malignant cells are phenotypically characterized by expression of cluster of differentiation (CD) on their surface. These CD are detected by flow cytometry using specific antibodies. Monoclonal antibodies targeting different CD have been developed for treatment. Rituximab, an anti-CD20 antibody, was the first monoclonal antibody successfully developed for treatment of malignant hematologic diseases. Since rituximab, many other monoclonal antibodies are being developed. Trends in malignant hematologic diseases presented here will include treatments, which have at least entered phase I/II clinical trials in adult or childhood leukemia. They include some novel drugs of conventional chemotherapy like second-generation nucleoside analogues. We will give an overview of the small molecules targeting the different cellular pathways and we will highlight those appearing as the most promising like novel TKIs. The large field of monoclonal antibodies will be also approached focusing on antibodies developed in leukemias.

Enzymatic transition states, transition-state analogs, dynamics, thermodynamics, and lifetimes

Experimental analysis of enzymatic transition-state structures uses kinetic isotope effects (KIEs) to report on bonding and geometry differences between reactants and the transition state. Computational correlation of experimental values with chemical models permits three-dimensional geometric and electrostatic assignment of transition states formed at enzymatic catalytic sites. The combination of experimental and computational access to transition-state information permits (a) the design of transition-state analogs as powerful enzymatic inhibitors, (b) exploration of protein features linked to transition-state structure, (c) analysis of ensemble atomic motions involved in achieving the transition state, (d) transition-state lifetimes, and (e) separation of ground-state (Michaelis complexes) from transition-state effects. Transition-state analogs with picomolar dissociation constants have been achieved for several enzymatic targets. Transition states of closely related isozymes indicate that the protein's dynamic architecture is linked to transition-state structure. Fast dynamic motions in catalytic sites are linked to transition-state generation. Enzymatic transition states have lifetimes of femtoseconds, the lifetime of bond vibrations. Binding isotope effects (BIEs) reveal relative reactant and transition-state analog binding distortion for comparison with actual transition states.

Evolving insights in the pathogenesis and therapy of cutaneous T-cell lymphoma (mycosis fungoides and Sezary syndrome)

Cutaneous T-cell lymphomas (CTCL) are a heterogeneous group of malignancies derived from skin-homing T cells. The most common forms of CTCL are Mycosis Fungoides (MF) and Sezary Syndrome (SS). Accurate diagnosis remains a challenge due to the heterogeneity of presentation and the lack of highly characteristic immunophenotypical and genetic markers. Over the past decade molecular studies have improved our understanding of the biology of CTCL. The identification of gene expression differences between normal and malignant T-cells has led to promising new diagnostic and prognostic biomarkers that now need validation to be incorporated into clinical practice. These biomarkers may also provide insight into the mechanism of development of CTCL. Additionally, treatment options have expanded with the approval of new agents, such as histone deacetylase inhibitors. A better understanding of the cell biology, immunology and genetics underlying the development and progression of CTCL will allow the design of more rational treatment strategies for these malignancies. This review summarizes the clinical epidemiology, staging and natural history of MF and SS; discusses the immunopathogenesis of MF and the functional role of the malignant T-cells; and reviews the latest advances in MF and SS treatment.

In vitro efficacy of forodesine and nelarabine (ara-G) in pediatric leukemia

Forodesine and nelarabine (the pro-drug of ara-G) are 2 nucleoside analogues with promising anti-leukemic activity. To better understand which pediatric patients might benefit from forodesine or nelarabine (ara-G) therapy, we investigated the in vitro sensitivity to these drugs in 96 diagnostic pediatric leukemia patient samples and the mRNA expression levels of different enzymes involved in nucleoside metabolism. Forodesine and ara-G cytotoxicities were higher in T-cell acute lymphoblastic leukemia (T-ALL) samples than in B-cell precursor (BCP)-ALL and acute myeloid leukemia (AML) samples. Resistance to forodesine did not preclude ara-G sensitivity and vice versa, indicating that both drugs rely on different resistance mechanisms. Differences in sensitivity could be partly explained by significantly higher accumulation of intracellular dGTP in forodesine-sensitive samples compared with resistant samples, and higher mRNA levels of dGK but not dCK. The mRNA levels of the transporters ENT1 and ENT2 were higher in ara-G-sensitive than -resistant samples. We conclude that especially T-ALL, but also BCP-ALL, pediatric patients may benefit from forodesine or nelarabine (ara-G) treatment.

New nucleoside analogs for patients with hematological malignancies

INTRODUCTION

In the last few years, several new purine and pyrimidine nucleoside analogs have been synthesized and made available for both preclinical studies and clinical trials.

AREAS COVERED

This article summarizes recent achievements in the mechanism of action, pharmacological properties and clinical activity and toxicity as well as the emerging role of newer purine and pyrimidine nucleoside analogs potentially active in lymphoid and myeloid malignancies. A literature review was conducted from the MEDLINE database PubMed for articles in English. Publications from 2000 to October 2010 were scrutinized. The search terms used were clofarabine, nelarabine, forodesine, 8-chloroadenosine, LMP-420, azacitidine, decitabine, sapacitabine, troxacitabine, thiarabine and zebularine in conjunction with hematologic malignancies, leukemia and lymphoma. Conference proceedings from the previous 5 years of the American Society of Hematology, European Hematology Association, and American Society of Clinical Oncology were searched manually. Additional relevant publications were obtained by reviewing the references from the chosen articles.

EXPERT OPINION

Several new nucleoside analogs are currently under investigation in preclinical and clinical studies concerning hematological malignancies. Clofarabine, nelarabine, azacitidine and decitabine have been recently approved for the treatment of leukemias and/or myelodysplastic syndromes. Other agents including forodesine, 8-chloroadenosine, LMP-420, sapacitabine, troxacitabine, thiarabine and zebularine seem to be promising for the treatment of lymphoid and myeloid malignancies. However, definitive data from ongoing and future clinical trials will aid in better defining their status in the treatment of hematological disorders.

Influence of bone marrow stromal microenvironment on forodesine-induced responses in CLL primary cells

Forodesine, a purine nucleoside phosphorylase inhibitor, displays in vitro activity in chronic lymphocytic leukemia (CLL) cells in presence of dGuo, which is the basis for an ongoing clinical trial in patients with fludarabine-refractory CLL. Initial clinical data indicate forodesine has significant activity on circulating CLL cells, but less activity in clearing CLL cells from tissues such as marrow. In tissue microenvironments, lymphocytes interact with accessory stromal cells that provide survival and drug-resistance signals, which may account for residual disease. Therefore, we investigated the impact of marrow stromal cells (MSCs) on forodesine-induced response in CLL lymphocytes. We demonstrate that spontaneous and forodesine-induced apoptosis of CLL cells was significantly inhibited by human and murine MSCs. Forodesine-promoted dGuo triphosphate (dGTP) accumulation and GTP and ATP depletion in CLL cells was inhibited by MSCs, providing a mechanism for resistance. Also, MSCs rescued CLL cells from forodesine-induced RNA- and protein-synthesis inhibition and stabilized and increased Mcl-1 transcript and protein levels. Conversely, MSC viability was not affected by forodesine and dGuo. Collectively, MSC-induced biochemical changes antagonized forodesine-induced CLL cell apoptosis. This provides a biochemical mechanism for MSC-derived resistance to forodesine and emphasizes the need to move toward combinations with agents that interfere with the microenvironment's protective role for improving current therapeutic efforts.