Effects of a novel purine nucleoside phosphorylase inhibitor, BCX-34, on activation and proliferation of normal human lymphoid cells

Enzymatic Transition States and Drug Design

Transition state theory teaches that chemically stable mimics of enzymatic transition states will bind tightly to their cognate enzymes. Kinetic isotope effects combined with computational quantum chemistry provides enzymatic transition state information with sufficient fidelity to design transition state analogues. Examples are selected from various stages of drug development to demonstrate the application of transition state theory, inhibitor design, physicochemical characterization of transition state analogues, and their progress in drug development.

Forodesine in the treatment of relapsed/refractory peripheral T-cell lymphoma: an evidence-based review

T-cell lymphoma is a rare hematologic malignancy with an incidence rate between 10% and 20% of that of non-Hodgkin lymphomas. Patients with peripheral T-cell lymphoma (PTCL) generally have a poor prognosis when treated with cyclophosphamide, doxorubicin, vincristine, and prednisolone (CHOP)/CHOP-like chemotherapy; once relapse occurs, it is mostly regarded as an incurable disease. To overcome the chemorefractoriness of PTCL, several novel agents have been developed. Since the first approval of pralatrexate, a dihydrofolate reductase inhibitor, for relapsed/refractory PTCL by the US Food and Drug Administration, several new agents, such as romidepsin (histone deacetylase inhibitor), brentuximab vedotin (antibody–drug conjugate targeting CD30), chidamide (histone deacetylase inhibitor), and mogamulizumab (anti-CC chemokine receptor 4 monoclonal antibody), have been approved as a therapeutic option for relapsed/refractory PTCL in several countries, including the US, Europe, China, and Japan. Forodesine is a novel, potent purine nucleoside phosphorylase inhibitor that is effective against T-cell malignancies. Although the clinical development of forodesine was discontinued in the US and Europe, a multicenter Phase I/II study of oral forodesine for relapsed PTCL was recently completed in Japan. The overall response rate was 24% (10 of 41 patients), which included four patients with complete response. In general, the toxicity of forodesine is manageable. As the study met the primary end point, forodesine was approved for the treatment of relapsed/refractory PTCL in Japan in March 2017, which was the first approval of forodesine in the world. As forodesine is an oral formulation, it is more convenient than other novel intravenous agents approved for PTCL. However, it is necessary to appropriately manage opportunistic infections and secondary lymphomas possibly associated with long-lasting lymphocytopenia caused by forodesine. In this manuscript, we have summarized the currently available evidence for forodesine and discussed the clinical implications for PTCL treatment.

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.

Metabolites of purine nucleoside phosphorylase (NP) in serum have the potential to delineate pancreatic adenocarcinoma

Pancreatic Adenocarcinoma (PDAC), the fourth highest cause of cancer related deaths in the United States, has the most aggressive presentation resulting in a very short median survival time for the affected patients. Early detection of PDAC is confounded by lack of specific markers that has motivated the use of high throughput molecular approaches to delineate potential biomarkers. To pursue identification of a distinct marker, this study profiled the secretory proteome in 16 PDAC, 2 carcinoma in situ (CIS) and 7 benign patients using label-free mass spectrometry coupled to 1D-SDS-PAGE and Strong Cation-Exchange Chromatography (SCX). A total of 431 proteins were detected of which 56 were found to be significantly elevated in PDAC. Included in this differential set were Parkinson disease autosomal recessive, early onset 7 (PARK 7) and Alpha Synuclein (aSyn), both of which are known to be pathognomonic to Parkinson's disease as well as metabolic enzymes like Purine Nucleoside Phosphorylase (NP) which has been exploited as therapeutic target in cancers. Tissue Microarray analysis confirmed higher expression of aSyn and NP in ductal epithelia of pancreatic tumors compared to benign ducts. Furthermore, extent of both aSyn and NP staining positively correlated with tumor stage and perineural invasion while their intensity of staining correlated with the existence of metastatic lesions in the PDAC tissues. From the biomarker perspective, NP protein levels were higher in PDAC sera and furthermore serum levels of its downstream metabolites guanosine and adenosine were able to distinguish PDAC from benign in an unsupervised hierarchical classification model. Overall, this study for the first time describes elevated levels of aSyn in PDAC as well as highlights the potential of evaluating NP protein expression and levels of its downstream metabolites to develop a multiplex panel for non-invasive detection of PDAC.

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

Forodesine is a new and potent purine nucleoside phosphorylase (PNP) inhibitor. Patients with chronic lymphocytic leukemia (CLL) with primary resistance to fludarabine-based therapy or with progressive disease were eligible for oral forodesine (200 mg/d) for up to 24 weeks. Eight patients with median lymphocyte count of 35.9 x 10(9)/L and median serum beta2 microglobulin level of 6.45 mg/L were treated. Six had Rai stage III to IV and were previously heavily treated (median prior therapy = 5). Two had transient decrease in lymphocyte count to normal, whereas in 5, disease progressed. Adverse events were mild. Steady-state level of forodesine ranged from 200 to 1300 nM and did not reach desired 2 microM level. PNP inhibition ranged from 57% to 89% and steady-state 2'-deoxyguanosine (dGuo) concentration median was 1.8 microM. Intracellular deoxyguanosine triphosphate (dGTP) increase was very modest, from median of 6 microM to 10 microM. Compared with in vivo, in vitro incubations of CLL lymphocytes with 10 or 20 microM dGuo and forodesine (2 microM) resulted in accumulation of higher levels of dGTP (40-250 microM) which resulted in increase in apoptosis. Forodesine has biologic activity in CLL; pharmacodynamic parameters suggest that an alternate dosing schedule and/or higher doses to achieve greater intracellular dGTP may be beneficial in this patient population.

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.

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.

A proof-of-principle pharmacokinetic, pharmacodynamic, and clinical study with purine nucleoside phosphorylase inhibitor immucillin-H (BCX-1777, forodesine)

The discovery of purine nucleoside phosphorylase (PNP) deficiency and T lymphocytopenia suggested that inhibition of this enzyme could serve as a therapeutic target. Inhibitors of PNP failed until structure-based synthesis of immucillin-H (BCX-1777, forodesine), a transition-state analog of PNP. The picomolar potency for PNP, T cell-selective cytotoxicity, and animal studies provided the rationale for use of forodesine in T-cell malignancies. Five patients were treated with an intravenous infusion of forodesine (40 mg/m2) on day 1; treatment continued on day 2; forodesine was administered every 12 hours for an additional 8 doses. Plasma and cellular pharmacokinetics and pharmaco-dynamics were investigated. Median peak level of forodesine (5.4 microM) was achieved at the end of infusion. This level was sufficient to increase plasma 2'-deoxyguanosine (dGuo) concentrations in all patients. Intracellular deoxyguanosine triphosphate (dGTP) increased by 2- to 40-fold in 4 of 5 patients (8 of 9 courses) and correlated with antileukemia activity in 4 patients. However, objective responses were not observed. This was the first clinical study in humans to demonstrate the plasma pharmacokinetics and the pharmacodynamic effectiveness of the PNP inhibitor, forodesine; however, regrowth of leukemia cells in the blood and marrow after course 1 suggested that a different therapeutic schedule should be considered for future studies.

Comparison of in vivo efficacy of BCX-1777 and cyclosporin in xenogeneic graft-vs.-host disease: the role of dGTP in antiproliferative action of BCX-1777

Purine nucleoside phosphorylase (PNP) deficiency in humans produces a relatively selective depletion of T-cells. Inhibitors of PNP are therefore of interest as potential T-cell selective immunosuppressive agents. BCX-1777 is a potent inhibitor of PNP and in vitro T-cell proliferation. Inhibition of human T-cells by BCX-1777 and deoxyguanosine (dGuo) is accompanied by deoxyguanosine triphosphate (dGTP) accumulation. Unlike human T-cells, mouse, rat, dog and monkey T-cells are neither inhibited (IC50>100 microM) nor accumulate dGTP in the presence of BCX-1777 and dGuo. Cells pretreated with BCX-1777 and dGuo for 24 h (to elevate dGTP levels) prior to stimulation demonstrated 80% inhibition similar to the inhibition observed with cells treated with BCX-1777 and dGuo during the stimulation and proliferation process. This further confirms that inhibition of T-cells is due to the accumulation of dGTP in these cells. Deoxynucleotide (dNTP) analysis of the cells treated with BCX-1777 and dGuo for 48 h showed no significant change in deoxycytidine triphosphate (dCTP) and deoxyadenosine triphosphate (dATP) pools. However, a decrease (2-fold) in thymidine triphosphate (dTTP) pools, and a large increase in dGTP pools (15-fold) were observed. Results from various groups have shown that alteration in the dNTP supply results in DNA fragmentation and cell death with characteristics of apoptosis. Indeed, apoptosis is observed in human T-lymphocytes treated with BCX-1777 and dGuo. To compare the in vivo efficacy of BCX-1777 with another potent T-cell inhibitor, cyclosporin, these drugs were tested in a xenogeneic graft-vs.-host disease model (XGVHD). In this model, human lymphocytes are engrafted into severe combined immunodeficient mice (SCID) mice inducing severe XGVHD. The efficacy of BCX-1777 in the XGVHD model was comparable to cyclosporin and a combination of BCX-1777 and cyclosporin treatment showed a trend towards increased efficacy compared to cyclosporin alone. These results suggest that BCX-1777 may be useful for the treatment of disease characterized by activated T-cell responses.

Purine nucleoside phosphorylase inhibitor BCX-1777 (Immucillin-H)--a novel potent and orally active immunosuppressive agent

Patients with purine nucleoside phosphorylase (PNP) deficiency present a selective T-cell immunodeficiency. Inhibitors of PNP are, therefore, of interest as potential T-cell selective immunosuppressive agents. BCX-1777 is a potent inhibitor of PNP from various species including human, mouse, rat, monkey and dog, with IC50 values ranging from 0.48 to 1.57 nM. BCX-1777, in the presence of 2'-deoxyguanosine (dGuo, 3-10 microM), inhibits human lymphocyte proliferation activated by various agents such as interleukin-2 (IL-2), mixed lymphocyte reaction (MLR) and phytohemagglutinin (PHA) (IC50 values < 0.1-0.38 microM). BCX-1777 is a 10-100-fold more potent inhibitor of human lymphocyte proliferation than other known PNP inhibitors like PD141955 and BCX-34. Nucleotide analysis of human lymphocytes indicate that inhibition of proliferation by BCX-1777 correlates with dGTP levels in the cells. BCX-1777 has excellent oral bioavailability (63%) in mice. At a single dose of 10 mg/kg in mice, BCX-1777 elevates dGuo to approximately 5 microM. BCX-1777 was not effective in mouse T-cell models such as delayed type hypersensitivity (DTH) and splenomegaly because mouse T-cells do not accumulate dGTP as do human T-cells. However, in the human peripheral blood lymphocyte severe combined immunodeficiency (hu-PBL-SCID) mouse model, BCX-1777 was effective in prolonging the life span 2-fold or more. This is the first known example of a PNP inhibitor that elevates dGuo in mice similar to the levels observed in PNP-deficient patients. Furthermore, these dGuo levels are also required for in vitro T-cell inhibition by BCX-1777. Thus, BCX-1777 represents a novel class of selective immunosuppressive agents that could have therapeutic utility in various T-cell disorders.

Purine nucleoside phosphorylases: properties, functions, and clinical aspects

The ubiquitous purine nucleoside phosphorylases (PNPs) play a key role in the purine salvage pathway, and PNP deficiency in humans leads to an impairment of T-cell function, usually with no apparent effects on B-cell function. This review updates the properties of the enzymes from eukaryotes and a wide range of prokaryotes, including a tentative classification of the enzymes from various sources, based on three-dimensional structures in the solid state, subunit composition, amino acid sequences, and substrate specificities. Attention is drawn to the compelling need of quantitative experimental data on subunit composition in solution, binding constants, and stoichiometry of binding; order of ligand binding and release; and its possible relevance to the complex kinetics exhibited with some substrates. Mutations responsible for PNP deficiency are described, as well as clinical methods, including gene therapy, for corrections of this usually fatal disease. Substrate discrimination between enzymes from different sources is also being profited from for development of tumour-directed gene therapy. Detailed accounts are presented of design of potent inhibitors, largely nucleosides and acyclonucleosides, their phosphates and phosphonates, particularly of the human erythrocyte enzyme, some with Ki values in nanomolar and picomolar range, intended for induction of the immunodeficient state for clinical applications, such as prevention of host-versus-graft response in organ transplantations. Methods of assay of PNP activity are reviewed. Also described are applications of PNP from various sources as tools for the enzymatic synthesis of otherwise inaccessible therapeutic nucleoside analogues, as coupling enzymes for assays of orthophosphate in biological systems in the micromolar and submicromolar ranges, and for coupled assays of other enzyme systems.

Treatment of cutaneous T-cell lymphoma from a dermatologist's perspective

Mycosis fungoides, the most common form of cutaneous T-cell lymphoma, is a helper/memory epidermotrophic T-cell lymphoma presenting as skin lesions. At the current time, curative therapy does not exist, and many patients have chronic skin lesions for many years, with successful treatment limited to the skin. Mycosis fungoides appears to start as a human lymphocyte antigen-restricted immune response, which may be antigen or superantigen driven, in early stages. From a dermatologist's perspective, removing the stimulating antigen(s), treating infections, preserving the skin barrier, targeting the abnormal clone, preserving the cytotoxic response, and using skin-directed therapy early in the disease are sensible strategies. As the disease progresses to involve more of the skin surface, systemic therapies, especially biological response modifiers (interferon and retinoids), phototherapy, or photopheresis help to preserve the patient's innate immunity and are widely used. New agents including bexarotene (a rexinoid) and DAB(389)IL-2 (interleukin-2 diphtheria fusion protein) offer new therapeutic options that are advantageous for treatment of mycosis fungoides in later stages.

Pharmacokinetics and pharmacodynamics of peldesine (BCX-34), a purine nucleoside phosphorylase inhibitor, following single and multiple oral doses in healthy volunteers

The pharmacokinetic parameters of peldesine (BCX-34) were investigated after single and multiple oral doses in two groups of healthy adult volunteers. The pharmacodynamic elevation of endogenous inosine and 2'-deoxyguanosine was simultaneously monitored. The first group of 8 subjects received an intravenous dose (18 mg/m2) and five oral doses (30, 63, 108, 144, and 192 mg/m2) of drug. A second group of 12 subjects received 160 mg/m2 in four and in six divided doses orally. Serial blood samples and total urine outputs were collected during dosing and for at least 24 hours after the last dose was administered. One set of samples was analyzed using high-pressure liquid chromatography/ultraviolet (LC/UV) methods, validated for intact drug in human plasma and urine samples. Another set of samples was analyzed for the biomarkers, inosine and 2'-deoxyguanosine, using high-pressure LC with either mass spectrometry (MS) or electrochemical detection (EC) methods. The pharmacokinetic parameters of inosine and 2'-deoxyguanosine were calculated using noncompartmental methods and correlated against the pharmacokinetic parameters of BCX-34. For the single-dose study, the results exhibited linear pharmacokinetics over the dose range from 30 to 144 mg/m2. The calculated terminal half-life was 3.5 +/- 1.0 h, and the absolute bioavailability of the oral formulation was approximately 51%. Analysis of urine in the first 24 hours of collection accounted for approximately 82% of the absorbed intact drug. Evaluation of the multiple-dose pharmacokinetics indicated that steady-state blood concentrations were achieved by 24 hours when the drug was administered four or six times a day. A drug dose-related elevation of plasma 2'-deoxyguanosine was observed. This phenomenon was not seen with plasma inosine levels. However, analysis of urine samples showed an increase in inosine output with an increase in the drug dose. The calculated terminal half-life of inosine and 2'-deoxyguanosine was 15.3 +/- 1.8 h and 1.3 +/- 0.1 h, respectively.