Synthesis and biological evaluation of Ribo 7-N/O/S pyrimidine 9-deaza C-nucleoside analogs as new antiviral agents for inhibiting HCV RNA-dependent RNA polymerases

Hepatitis C infection is caused by the bloodborne pathogen hepatitis C virus (HCV) and can lead to serious liver diseases and, ultimately, death if the treatment is ineffective. This work reports the synthesis and preclinical evaluation of 7 novel 9-O/N/S pyrimidine nucleosides, including compound 12, the triphosphate of known compound 7b. The nucleosides are 9-deaza modifications of adenosine and guanosine with β-2'-C-methyl substituent on the ribose. Within this series of compounds, a 9-deaza furopyrimidine analog of adenosine, compound 7b, showed high anti-HCV activity in vitro, good stability, low toxicity, and low genotoxicity when administrated in low doses, and an adequate pharmacokinetics profile. An improved synthesis of compound 7b compared to a previous study is also reported. Compound 12 was synthesized as a control to verify phosphorylation of 7b occurred in vivo.

Synthesis of novel azasugar-containing 2'β-C-Me 9-deaza nucleosides as potential anti-hepatitis C virus agents

As a part of our ongoing discovery efforts exploring azasugar as agents for treating various unmet medical needs, we prepared analogs of azasugar as potential anti-hepatitis C virus (HCV) agents. Herein we describe the synthesis of novel 2'β-C-Me 9-deazanucleoside azasugar analogs.

Purine nucleoside phosphorylase enables dual metabolic checkpoints that prevent T cell immunodeficiency and TLR7-associated autoimmunity

Purine nucleoside phosphorylase (PNP) enables the breakdown and recycling of guanine nucleosides. PNP insufficiency in humans is paradoxically associated with both immunodeficiency and autoimmunity, but the mechanistic basis for these outcomes is incompletely understood. Here, we identify two immune lineage-dependent consequences of PNP inactivation dictated by distinct gene interactions. During T cell development, PNP inactivation is synthetically lethal with downregulation of the dNTP triphosphohydrolase SAMHD1. This interaction requires deoxycytidine kinase activity and is antagonized by microenvironmental deoxycytidine. In B lymphocytes and macrophages, PNP regulates Toll-like receptor 7 signaling by controlling the levels of its (deoxy)guanosine nucleoside ligands. Overriding this regulatory mechanism promotes germinal center formation in the absence of exogenous antigen and accelerates disease in a mouse model of autoimmunity. This work reveals that one purine metabolism gene protects against immunodeficiency and autoimmunity via independent mechanisms operating in distinct immune lineages and identifies PNP as a potentially novel metabolic immune checkpoint.

Treatment duration with immune-based therapies in Cancer: an enigma

Unlike chemotherapy treatments that target the tumor itself (rather nonspecifically), immune-based therapies attempt to harness the power of an individual patient’s immune system to combat cancer. Similar to chemotherapeutic agents, the dosage and Administration section of labeling for all five currently approved PD-1/PD-L1 inhibitors (immunotherapy) recommends duration of treatment until disease progression or unacceptable toxicity. Overactivation or constitutive activation of the immune system with immune based therapies can lead to T-cell exhaustion and activation induced cell death (AICD) in T- and B-cells. Examples of immune exhaustion and T-cell depletion is noted in preclinical and clinical studies. Overactivation or constitutive activation leading to Immune exhaustion is a real phenomenon and of profound concern as immune cells are the true arsenal for control of the tumor growth. Designing trials rigorously to address the optimum treatment duration with immune based therapies is critical. By addressing this concern now, not only we may improve patient outcomes, but also gather a deeper understanding of the role and mechanisms of the immune system in the control of tumor growth.

Chemotherapy and immune-based therapies provide antitumor effects through completely different mechanisms. Chemotherapeutic agents are cytotoxic in that they directly inhibit basic cellular mechanisms, killing both malignant and nonmalignant cells (hopefully with a preference for malignant cells), while immune based therapies wake-up the host immune system to recognize malignant cells and eliminate them.

While there is a burgeoning excitement surrounding development of immune based therapies for the treatment of cancer, the optimal duration for these therapies need to be explored with equal fervor. Dosing for chemotherapy has been determined over years through large-scale prospective randomized trials to pinpoint the dose which maximizes therapeutic effect while minimizing side effects. Also, due to the mechanism of chemotherapeutic action, the duration of treatment with these agents is generally until disease progression or patient intolerance. However, experience with immune based therapies is limited, with current dosing and duration guidelines based primarily on initial trials required for approval of the agents. Since immune based therapies work by activating the body’s own immune system, there is concern that overactivation or constitutive activation of the immune system may lead to immune exhaustion and depletion of effector T-cells thereby causing decreased anti-tumor effects and possible allowing for tumor progression.

Similar to chemotherapeutic agents, the Dosage and Administration section of labeling for all five currently approved PD-1/PD-L1 inhibitors recommends duration of treatment until disease progression or unacceptable toxicity. However, since immune based therapies work with a completely different mechanism compared to chemotherapy, using the same therapy duration may not be the optimal approach.

In exploring treatment duration with immune based therapies, we need to answer the following: (1) does indefinite treatment with immune based therapies exhaust the immune system counteracting its own mechanism of action leading to tumor progression and (2) how can clinical trials be designed to identify the optimal duration of immune-based therapy that prevents immune cell exhaustion but supports anti-tumor immunity.

Protection against filovirus diseases by a novel broad-spectrum nucleoside analogue BCX4430

A broad-spectrum antiviral small molecule is reported to act as an inhibitor of viral polymerase activity and is shown to be effective in protecting non-human primates from lethal filovirus infection when administered after exposure.

Viruses of the Filoviridae family can cause severe haemorrhagic fever in humans and non-human primates. Mortality rates are extremely high and no vaccines or drugs are currently licensed for the treatment of filovirus diseases. Here Sina Bavari and colleagues report the discovery of a small-molecule viral polymerase inhibitor with in vitro and in vivo antiviral activity against highly pathogenic viruses, including filoviruses such as Ebola virus and Sudan virus. The compound, BCX4430, is an adenosine analogue that acts as a non-obligate chain terminator. Administered either orally or intramuscularly, it can completely protect cynomolgus macaques from Marburg virus, even when administered as late as 48 hours after infection.

Filoviruses are emerging pathogens and causative agents of viral haemorrhagic fever. Case fatality rates of filovirus disease outbreaks are among the highest reported for any human pathogen, exceeding 90% (ref. 1). Licensed therapeutic or vaccine products are not available to treat filovirus diseases. Candidate therapeutics previously shown to be efficacious in non-human primate disease models are based on virus-specific designs and have limited broad-spectrum antiviral potential. Here we show that BCX4430, a novel synthetic adenosine analogue, inhibits infection of distinct filoviruses in human cells. Biochemical, reporter-based and primer-extension assays indicate that BCX4430 inhibits viral RNA polymerase function, acting as a non-obligate RNA chain terminator. Post-exposure intramuscular administration of BCX4430 protects against Ebola virus and Marburg virus disease in rodent models. Most importantly, BCX4430 completely protects cynomolgus macaques from Marburg virus infection when administered as late as 48 hours after infection. In addition, BCX4430 exhibits broad-spectrum antiviral activity against numerous viruses, including bunyaviruses, arenaviruses, paramyxoviruses, coronaviruses and flaviviruses. This is the first report, to our knowledge, of non-human primate protection from filovirus disease by a synthetic drug-like small molecule. We provide additional pharmacological characterizations supporting the potential development of BCX4430 as a countermeasure against human filovirus diseases and other viral diseases representing major public health threats.

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.

A single intramuscular injection of neuraminidase inhibitor peramivir demonstrates antiviral activity against novel pandemic A/California/04/2009 (H1N1) influenza virus infection in mice

New and emerging influenza virus strains, such as the pandemic influenza A (H1N1) virus require constant vigilance for antiviral drug sensitivity and resistance. Efficacy of intramuscularly (IM) administered neuraminidase (NA) inhibitor, peramivir, was evaluated in mice infected with recently isolated pandemic A/California/04/2009 (H1N1, swine origin, mouse adapted) influenza virus. A single IM injection of peramivir (four dose groups), given 1h prior to inoculation, significantly reduced weight loss (p < 0.001) and mortality (p < 0.05) in mice infected with LD90 dose of pandemic A/California/04/2009 (H1N1) influenza virus compared to vehicle group. There was 20% survival in the vehicle-treated group, whereas in the peramivir-treated groups, survival increased in a dose-dependent manner with 60, 60, 90 and 100% survivors for the 1, 3, 10, and 30 mg/kg doses, respectively. Weight loss on day 4 in the vehicle-treated group was 3.4 gm, and in the peramivir-treated groups was 2.1, 1.5, 1.8 and 1.8 g for the 1, 3, 10 and 30 mg/kg dose groups, respectively. In the treatment model, peramivir given 24h after infection as a single IM injection at 50mg/kg dose, showed significant protection against lethality and weight loss. There was 13% survival in the vehicle-treated group while in the peramivir-treated group at 24, 48, and 72 h post infection, survival was 100, 40, and 50%, respectively. Survival in the oseltamivir groups (10 mg/kg/d twice a day, orally for 5 days) was 90, 30 and 20% at 24, 48 and 72 h, respectively. These data demonstrate efficacy of parenterally administered peramivir against the recently isolated pandemic influenza virus in murine infection models.

Combination of peramivir and rimantadine demonstrate synergistic antiviral effects in sub-lethal influenza A (H3N2) virus mouse model

Efficacy of combination of the intramuscularly administered neuraminidase (NA) inhibitor, peramivir, and the orally administered M2 ion channel blocker, rimantadine was evaluated in mouse influenza A/Victoria/3/75 (H3N2) model. Mice were challenged with a sub-lethal virus dose (0-40% mortality in placebo group) and changes in body weights were analyzed by three-dimensional effect analysis to assess mode of drug interactions. Compounds were administered in a 5-day treatment course starting 1h before viral inoculation. The peramivir and rimantadine doses ranged from 0.3-3 mg/kg/d and 5-30 mg/kg/d, respectively. The maximum mean weight loss of 5.19 g was observed in the vehicle-infected group on day 10. In the 1 and 3 mg/kg/d peramivir monotherapy groups, the weight losses were 4.3 and 3.55 g, respectively. In the rimantadine monotherapy group, the weight losses were 3.43, 2.1, and 1.64 g for the 5, 10, and 30 mg/kg/d groups, respectively. Combination of 1mg/kg/d peramivir with 5 and 10 mg/kg/d rimantadine produced weight losses of 1.69 and 0.69 (p<0.05 vs. vehicle and individual agent), respectively, whereas the combination of 3.0 mg/kg/d peramivir with 10 and 30 mg/kg/d rimantadine did not show any weight loss (p<0.05 vs. vehicle and individual agent). The three-dimensional analysis of the weight loss for the majority of the drug combinations of peramivir and rimantadine tested demonstrated synergistic antiviral effects.

Probing the S2 site of factor VIIa to generate potent and selective inhibitors: the structure of BCX-3607 in complex with tissue factor-factor VIIa

Factor VIIa (FVIIa) is a trypsin-like serine protease in the coagulation cascade. Its complex with tissue factor (TF) triggers the extrinsic pathway of the coagulation cascade, generating a blood clot. Research programs at several centers now recognize the important roles played by TF and FVIIa in both the thrombotic and inflammatory processes associated with cardiovascular diseases. Therefore, inhibition of the TF-FVIIa complex is seen as a promising target that is key to the development of clinical candidates for various cardiovascular applications. The crystal structure of the TF-FVIIa enzyme complex has been analyzed in order to design and synthesize small-molecule inhibitors. Using structure-based drug design (SBDD), a new series of inhibitors have been discovered that demonstrate high potency against the TF-FVIIa complex while maintaining substantial selectivity versus other closely related serine proteases such as trypsin, thrombin, factor Xa and plasmin.

Discovery of highly potent small molecule kallikrein inhibitors

Uncontrolled kallikrein activation is involved in diseases such as hereditary angioedema, bacterial septic shock and procedures such as cardiopulmonary bypass. Here we report a series of small molecule compounds that potently inhibit kallikrein activity in vitro. Kinetic studies indicate that some of these compounds are slow binding inhibitors of kallikrein with Ki final less than a nanomolar. The ability of these compounds to inhibit the activity of kallikrein was further confirmed in a plasma model by quantitating the release of bradykinin, an endogenous cleavage product of plasma kallikrein. To understand the inhibitory mechanism of the selected compounds toward kallikrein, the interactions between the selected compounds and kallikrein was explored using molecular modeling based on the information of crystal structures of TF/FVIIa and kallikrein. The information presented in the current study provides an initial approach to develop more selective and therapeutically useful small molecule inhibitors.

An efficient synthesis of acyclic N7- and N9-adenine nucleosides via alkylation with secondary carbon electrophiles to introduce versatile functional groups at the C-1 position of acyclic moiety

The introduction of versatile functional groups, allyl and ester, at the C-1 position of the acyclic chain in acyclic adenine nucleosides was achieved for the first time directly by alkylation of adenine and N6-potected adenine. Thus, the C-1'-substituted N9-adenine acyclic nucleoside, adenine-9-yl-pent-4-enoic acid ethyl ester (11), was prepared by direct alkylation of adenine with 2-bromopent-4-enoic acid ethyl ester (6), while the corresponding N7-regioisomer, 2-[6-(dimethylaminomethyleneamino)-purin-7-yl]-pent-4-enoic acid ethyl ester (10), was obtained in one step by the coupling of N, N-dimethyl-N'- (9H-purin-6-yl)-formamidine (9) with 2-bromopent-4-enoic acid ethyl ester (6). The functional groups, ester and allyl, were converted to the desired hydroxymethyl and hydroxyethyl groups, and subsequently to phosphonomethyl derivatives and corresponding pyrophosphorylphosphonates.

The antithrombotic and anti-inflammatory effects of BCX-3607, a small molecule tissue factor/factor VIIa inhibitor

Tissue factor (TF) is a transmembrane glycoprotein that binds its zymogen cofactor, Factor VIIa (FVIIa) on the cell surface. Together (TF/FVIIa) they activate Factor X (FX) and Factor IX (FIX) and start the extrinsic pathway of blood coagulation. As such, the TF/FVIIa complex plays an important role in normal physiology as well as in thrombotic diseases such as unstable angina (UA), disseminated intravascular coagulation (DIC), and deep vein thrombosis (DVT). In addition to its function as an initiator of coagulation, TF/FVIIa plays an important role in inflammation. Expression of TF on the cell surface and its appearance as a soluble molecule are characteristic features of acute and chronic inflammation in conditions such as sepsis and atherosclerosis. Here we demonstrate that BCX-3607, a small molecule potent inhibitor of TF/FVIIa, reduces thrombus weight in an animal model of DVT. BCX-3607 also decreases the level of interleukin-6 (IL-6) in a LPS-stimulated mouse model of endotoxemia. Additionally, in vitro studies indicate that BCX-3607 blocks the generation of TF/FVIIa-induced IL-8 mRNA in human keratinocytes and reduces the TF/FVIIa-mediated generation of IL-6 and IL-8 in human umbilical vein endothelial cells (HUVEC). Therefore, BCX-3607 might block the TF/FVIIa-mediated coagulation and inflammation associated with pathological conditions.

Synthesis of N6-substituted 9-[3-(phosphonomethoxy)propyl]adenine derivatives as possible antiviral agents

A number of N6-substituted 9-[3-(phosphonomethoxy)propyl]adenine derivatives having hydroxymethyl at C-1' position were prepared from the appropriate 6-chloroadenine derivative. The syntheses of the corresponding prodrugs of these compounds are also reported. These compounds showed poor activity against HCV in replicon assay.

Synthesis of 9-[1-(substituted)-2-(phosphonomethoxy)ethyl]adenine derivatives as possible antiviral agents

Various C-1'-substituted acyclic N9 adenine nucleosides were prepared from 9-[(1-hydroxymethyl)(3-monomethoxytrityloxy)propyl]-N6-monomethoxytrityladenine. The hydroxymethyl was modified to the phosphonomethoxy derivative, and the 3-monomethoxytrityloxy was converted to hydroxyl, methoxy, azido, and amino. Other substituents, such as ethyl and ea-hydroxyethyl were also prepared. The resulting phosphonomethoxy derivatives were converted to prodrugs.

Anti-influenza virus activity of peramivir in mice with single intramuscular injection

In the event of an influenza outbreak, antivirals including the neuraminidase (NA) inhibitors, peramivir, oseltamivir, and zanamivir may provide valuable benefit when vaccine production is delayed, limited, or cannot be used. Here we demonstrate the efficacy of a single intramuscular injection of peramivir in the mouse influenza model. Peramivir potently inhibits the neuraminidase enzyme N9 from H1N9 virus in vitro with a 50% inhibitory concentration (IC(50)) of 1.3+/-0.4 nM. On-site dissociation studies indicate that peramivir remains tightly bound to N9 NA (t(1/2)>24h), whereas, zanamivir and oseltamivir carboxylate dissociated rapidly from the enzyme (t(1/2)=1.25 h). A single intramuscular injection of peramivir (10mg/kg) significantly reduces weight loss and mortality in mice infected with influenza A/H1N1, while oseltamivir demonstrates no efficacy by the same treatment regimen. This may be due to tight binding of peramivir to the N1 NA enzymes similar to that observed for N9 enzyme. Additional efficacy studies indicate that a single injection of peramivir (2-20mg/kg) was comparable to a q.d.x 5 day course of orally administered oseltamivir (2-20mg/kg/day) in preventing lethality in H3N2 and H1N1 influenza models. A single intramuscular injection of peramivir may successfully treat influenza infections and provide an alternate option to oseltamivir during an influenza outbreak.

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 the anti-influenza virus activity of cyclopentane derivatives with oseltamivir and zanamivir in vivo

Cyclopentane derivatives, designated as BCX-1812, BCX-1827, BCX-1898, and BCX-1923, were tested in parallel with oseltamivir carboxylate and zanamivir for the in vivo activity in mice infected with A/Turkey/Mas/76 X A/Beijing/32/92 (H6N2) influenza virus. The compounds were tested orally and intranasally at different dose levels. BCX-1812, BCX-1827, and BCX-1923 showed more than 50% protection at 1mg/kg/day dose level on oral treatment. The intranasal treatment was 100% effective even at 0.01 mg/kg/day for all four compounds. On comparison with oseltamivir carboxylate and zanamivir, these four cyclopentane derivatives have shown equal or better efficacies. The synthesis of two new compounds, BCX-1898 and BCX-1923, is also described.

Syntheses and neuraminidase inhibitory activity of multisubstituted cyclopentane amide derivatives

In further studies aimed toward identifying effective and safe inhibitors of influenza neuraminidases, we synthesized a series of multisubstituted cyclopentane amide derivatives. Amides prepared were 14 examples of N-substituted alkyl or aralkyl types from primary amines, 13 examples of the N,N-disubstituted alkyl, aralkyl, or substituted-alkyl type from secondary amines, and 12 examples from cycloaliphatic or substituted cycloaliphatic secondary amines. These compounds bearing two chiral centers, at position-1 in the ring and position-1' in the side chain attached at position 3, were tested for their ability to inhibit A and B forms of influenza neuraminidase. The 1-ethylpropylamide, diethylamide, dipropylamide, and 4-morpholinylamide showed very good inhibitory activity (IC(50) = 0.015-0.080 microM) vs the neuraminidase A form, but modest activity (IC(50) = 3.0-9.2 microM) vs the neuraminidase B form. Since the parent amides bear two chiral centers (C-1 and C-1'), three of the better inhibitors were tested at higher levels of diastereomeric purity. The diastereomers corresponding to the active forms of the 1-(ethyl)propylamide, the diethylamide, and the dipropylamide (all of the same configuration at the C-1' chiral center), and the diastereomer of the diethylamide representing the active form at both C-1' and C-1 were isolated or synthesized from precursors that were isolated as diastereomers. These diastereomers showed some improvement in neuraminidase inhibition over the parent diastereomeric mixtures. 1-Carboxy-1-hydroxy derivatives of the best active compounds, the diethylamide and the dipropylamide, were also prepared. These compounds were not as active as the compounds without the 1-hydroxy group. In an in vivo study, the C-1' active isomer of the diethylamide from the 1-carboxy series was tested in influenza-infected mice by oral and intranasal administration and found to be very effective only intranasally in preventing weight loss at doses as low as 0.1 (mg/kg)/day.

Synthesis of a potent transition-state inhibitor of 5'-deoxy-5'-methylthioadenosine phosphorylase

Human 5'-deoxy-5'-methylthioadenosine phosphorylase (MTAP) participates in the purine salvage pathway to generate adenine and methylthioribose-1-phosphate, which in turn is converted into adenine nucleotides and methionine. Hence, inhibition of MTA phosphorylase may be an effective target in the design of potential antiproliferative agents. Presented herein is the synthesis of 2-(4-amino-5H-pyrrolo[3,2-d]pyrimidin-7-yl)-5-methylsulfanylmethylpyrrolidin-3,4-diol (1), a potent inhibitor of MTAP.

Purine nucleoside phosphorylase inhibitors in T-cell malignancies

Purine nucleoside phosphorylase (PNP)-deficient children exhibit profound impairment in the T-cell component of their immune systems, but have normal B-cell function. This rare condition provides a model for the development of specific inhibitors of PNP, which should enable selective suppression of T-cell function that may be useful in the treatment of T-cell-mediated diseases. BCX-1777 (BioCryst Pharmaceuticals Inc) is a rationally designed, potent transition-state analog inhibitor of PNP. This review provides a summary of in vitro and in vivo inhibition studies of T-cells by BCX-1777, and the role in this process of plasma deoxyguanosine (dGuo) and intracellular deoxyguanosine triphosphate (dGTP). Preliminary data from a phase I clinical trial of BCX-1777 in patients with T-cell malignancy demonstrated antileukemic activity which can be correlated to an increase in plasma dGuo and intracellular dGTP. This is consistent with results observed in cell cultures, animal studies and PNP-deficient patients. Clinical trials with BCX-1777 have demonstrated that inhibition of PNP leads to T-cell-selective suppression, confirming PNP to be a promising target for the treatment of T-cell-mediated diseases.

Mechanism of inhibition of T-acute lymphoblastic leukemia cells by PNP inhibitor--BCX-1777

Purine nucleoside phosphorylase (PNP) deficiency in humans produces a relatively selective depletion of T cells. BCX-1777 is a potent inhibitor of PNP. BCX-1777 in the presence of deoxyguanosine (dGuo) inhibits the proliferation of CEM-SS [T-acute lymphoblastic leukemia (T-ALL)] cells with an IC(50)=0.015 microM. This inhibition by BCX-1777 and dGuo is accompanied by elevation of dGTP (154-fold) and dATP (8-fold). Deoxycytidine (dCyt) completely and lamivudine (3TC) partially reverse this inhibition caused by BCX-1777 and dGuo. dNTP analysis of these samples indicates that, in the presence of dCyt, where complete reversal of inhibition is observed, dGTP and dATP pools revert back to the control levels. In samples containing 3TC, where partial reversal of inhibition was observed, dGTP decreased from 154-fold to 38-fold and dATP levels further increased from 8-fold to 30-fold compared to the control sample. In CEM-SS cells, inhibition of proliferation by BCX-1777 and dGuo is not due to accumulation of dATP because in the presence of 3TC, where reversal of inhibition is observed, dATP levels are further increased. These studies clearly indicate that inhibition of T cells is due to accumulation of dGTP resulting in cell death with characteristics of apoptosis. The half-life of dGTP in CEM-SS cells is 18 h, which is longer than that observed in human lymphocytes (4 h), suggesting that the nucleotidase level in CEM-SS cells is lower than in human lymphocytes. A 154-fold accumulation of dGTP in CEM-SS cells in the presence of BCX-1777 and dGuo compared to a 15-fold accumulation of dGTP in human lymphocytes suggests that kinase level is higher in CEM-SS cells compared to human lymphocytes. High kinase and low nucleotidase levels make CEM-SS cells more sensitive to inhibition by BCX-1777 and dGuo than human lymphocytes. Currently, BCX-1777 is in phase I/II clinical trial for the treatment of T cell malignancies.

Systematic structure-based design and stereoselective synthesis of novel multisubstituted cyclopentane derivatives with potent antiinfluenza activity

The design and synthesis of novel, orally active, potent, and selective inhibitors of influenza neuraminidase differing structurally from existing neuraminidase inhibitors are described. X-ray crystal structures of complexes of neuraminidase with known five- and six-membered ring inhibitors revealed that potent inhibition of the enzyme is determined by the relative positions of the interacting inhibitor substituents (carboxylate, glycerol, acetamido, hydroxyl) rather than by the absolute position of the central ring. This led us to design potential neuraminidase inhibitors in which the cyclopentane ring served as a scaffold for substituents (carboxylate, guanidino, acetamido, alkyl) that would interact with the four binding pockets of the neuraminidase active site at least as effectively as those of the established six-membered ring inhibitors such as DANA (2), zanamivir (3), and oseltamivir (4). A mixture of the isomers was prepared initially. Protein crystallography of inhibitor-enzyme complexes was used to screen mixtures of isomers in order to identify the most active stereoisomer. A synthetic route to the identified candidate 50 was developed, which featured (3 + 2) cycloaddition of 2-ethylbutyronitrile oxide to methyl (1S,4R)-4[(tert-butoxycarbonyl)amino]cyclopent-2-ene-1-carboxylate (43). Structures of the synthetic compounds were verified by NMR spectroscopy using nuclear Overhauser effect methodology. Two new neuraminidase inhibitors discovered in this work, 50 and 54, have IC(50) values vs neuraminidase from influenza A and B of <1 and <10 nM, respectively. These IC(50) values are comparable or superior to those for zanamivir and oseltamivir, agents recently approved by the FDA for treatment of influenza. The synthetic route used to prepare 50 and 54 was refined so that synthesis of pure active isomer 54, which has five chiral centers, required only seven steps from readily available intermediates. Further manipulation was required to prepare deoxy derivative 50. Because the activities of the two compounds are comparable and 54 [RWJ-270201 (BCX-1812)] is the easier to synthesize, it was selected for further clinical evaluation.

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.

Comparison of the anti-influenza virus activity of RWJ-270201 with those of oseltamivir and zanamivir

We have recently reported an influenza virus neuraminidase inhibitor, RWJ-270201 (BCX-1812), a novel cyclopentane derivative discovered through structure-based drug design. In this paper, we compare the potency of three compounds, RWJ-270201, oseltamivir, and zanamivir, against neuraminidase enzymes from various subtypes of influenza. RWJ-270201 effectively inhibited all tested influenza A and influenza B neuraminidases in vitro, with 50% inhibitory concentrations of 0.09 to 1.4 nM for influenza A neuraminidases and 0.6 to 11 nM for influenza B neuraminidases. These values were comparable to or lower than those for oseltamivir carboxylate (GS4071) and zanamivir (GG167). RWJ-270201 demonstrated excellent selectivity (>10,000-fold) for influenza virus neuraminidase over mammalian, bacterial, or other viral neuraminidases. Oral administration of a dosage of 1 mg/kg of body weight/day of RWJ-270201 for 5 days (beginning 4 h preinfection) showed efficacy in the murine model of influenza virus infection as determined by lethality and weight loss protection. RWJ-270201 administered intranasally at 0.01 mg/kg/day in the murine influenza model demonstrated complete protection against lethality, whereas oseltamivir carboxylate and zanamivir at the same dose demonstrated only partial protection. In the delayed-treatment murine influenza model, oral administration of a 10-mg/kg/day dose of RWJ-270201 or oseltamivir (GS4104, a prodrug of GS4071) at 24 h postinfection showed significant protection against lethality (P < 0.001 versus control). However, when the treatment was delayed for 48 h, no significant protection was observed in either drug group. No drug-related toxicity was observed in mice receiving 100 mg/kg/day of RWJ-270201 for 5 days. These efficacy and safety profiles justify further consideration of RWJ-270201 for the treatment and prevention of human influenza.

Hydrophobic benzoic acids as inhibitors of influenza neuraminidase

Neuraminidase (NA) plays a critical role in the life cycle of influenza virus and is a target for new therapeutic agents. A new benzoic acid inhibitor (11) containing a lipophilic side chain at C-3 and a guanidine at C-5 was synthesized. The X-ray structure of 4-(N-acetylamino)-5-guanidino-3-(3-pentyloxy)benzoic acid in complex with NA revealed that the lipophilic side chain binds in a newly created hydrophobic pocket formed by the movement of Glu 278 to interact with Arg 226, whereas the guanidine of 11 interacts in a negatively charged pocket created by Asp 152, Glu 120 and Glu 229. Compound 11 was highly selective for type A (H2N2) influenza NA (IC50 1 microM) over type B (B/Lee/40) influenza NA (IC50 500 microM).

Design of benzoic acid inhibitors of influenza neuraminidase containing a cyclic substitution for the N-acetyl grouping

A 2-pyrrolidinone ring containing a single hydroxymethyl side chain effectively replaces the N-acetylamino group of 4-(N-acetylamino)-3-guanidinobenzoic acid, a low micromolar inhibitor of influenza neuraminidase. This novel structural template affords new opportunities to evolve more potent benzoic acid inhibitors.

Potent inhibition of influenza sialidase by a benzoic acid containing a 2-pyrrolidinone substituent

On the basis of the lead compound 4-(N-acetylamino)-3-guanidinobenzoic acid (BANA 113), which inhibits influenza A sialidase with a Ki of 2.5 microM, several novel aromatic inhibitors of influenza sialidases were designed. In this study the N-acetyl group of BANA 113 was replaced with a 2-pyrrolidinone ring, which was designed in part to offer opportunities for introduction of spatially directed side chains that could potentially interact with the 4-, 5-, and/or 6-subsites of sialidase. While the parent structure 1-(4-carboxy-2-guanidinophenyl)pyrrolidin-2-one (8) was only a modest inhibitor of sialidase, the introduction of a hydroxymethyl or bis(hydroxymethyl) substituent at the C5' position of the 2-pyrrolidinone ring resulted in inhibitors (9 and 12, respectively) with low micromolar activity. Crystal structures of these inhibitors in complex with sialidase demonstrated that the substituents at the 5'-position of the 2-pyrrolidinone ring interact in the 4- and/or 5-subsites of the enzyme. Replacement of the guanidine in 12 with a hydrophobic 3-pentylamino group resulted in a large enhancement in binding to produce an inhibitor (14) with an IC50 of about 50 nM against influenza A sialidase, although the inhibition of influenza B sialidase was 2000-fold less. This represents the first reported example of a simple, achiral benzoic acid with potent (low nanomolar) activity as an inhibitor of influenza sialidase.

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

The antiproliferative effect of BCX-34 was tested in normal human peripheral blood mononuclear cells (PBMCs) induced to proliferate with OKT3, tetanus toxoid, the mixed lymphocyte reaction, or IL-2. In the case of OKT3, tetanus toxoid, or the MLR the IC50s ranged between 0.7 and 4 microM. With IL-2, the IC50 was 14.6 microM. In T-cells purified by rosetting the IC50 with IL-2 was 0.62 microM. In CD4 or CD8 cells obtained by magnetic activated cell sorting the IC50s with IL-2 were 0.24 and 0.62 microM, respectively. BCX-34 inhibition of proliferation in human PBMCs may not depend entirely upon the accumulation of intracellular dGTP because tetanus toxoid-induced proliferation was inhibited in the absence of deoxyguanosine and was not reversed by deoxycytidine. BCX-34 did not inhibit IL-2 release from PBMCs and did not alter PBMC viability. The results of these studies show that BCX-34 is a potent inhibitor of normal human T-cell proliferation induced by antigenic or IL-2 stimulation. BCX-34 in normal human T-cells has a deoxyguanosine-independent mechanism to suppress in vitro proliferation. BCX-34 appears to have little effect on T-cell viability. The data suggest that BCX-34 may be useful in the treatment of T-cell proliferative disorders.

Generation and characterization of a mutant of influenza A virus selected with the neuraminidase inhibitor BCX-140

Influenza neuraminidase (NA) plays an important role in viral replication, and characterization of viruses resistant to NA inhibitors will help elucidate the role of active-site residues. This information will assist in designing better inhibitors targeted to essential active-site residues that cannot generate drug-resistant mutations. In the present study we used the benzoic acid-based inhibitor BCX-140 to select and characterize resistant viruses. BCX-140 binds to the NA active site in an orientation that is opposite that of a sialic acid-based compound, 4-guanidino-2,4-dideoxy-2,3-dehydro-N-acetylneuraminic acid (GANA). Thus, the guanidino group of BCX-140 binds to Glu-276, whereas in GANA the guanidino group binds to Glu-119. We passaged influenza A/Singapore/1/57 (H2N2) in Madin-Darby canine kidney cells in the presence of BCX-140, and virus resistant to this inhibitor was selected after six passages. The NA of this mutant was still sensitive to inhibition by BCX-140. However, the mutant virus was resistant to BCX-140 in plaque and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. Sequence analysis of hemagglutinin (HA) and NA genes revealed changes in both, although none were in the active site of the NA. Depending on the method of selection of the resistant virus, two types of changes associated with the sialic acid binding site were seen in the HA. One is a change in HA1 of Ala-133 to Thr, a residue close to the binding site, while the other change was Arg-132 of HA1 to Gln, which in HA1 of serotype H3 is a sialic acid contact (Asn-137). Binding studies revealed that both types of resistant viruses had reduced receptor binding affinity compared to that of the wild type. Thus, resistance to BCX-140 was generated by modifying the HA. NA active-site residue 276 may be essential for activity, and thus, it cannot be changed to generate resistance. However, drug-induced changes in the HA can result in a virus that is less dependent on NA activity for growth in cells and, hence, resistant to NA inhibitors.

Design and synthesis of benzoic acid derivatives as influenza neuraminidase inhibitors using structure-based drug design

A series of 94 benzoic acid derivatives was synthesized and tested for its ability to inhibit influenza neuraminidase. The enzyme-inhibitor complex structure was determined by X-ray crystallographic analysis for compounds which inhibited the enzyme. The most potent compound tested in vitro, 5 (4-acetylamino)-3-guanidinobenzoic acid), had an IC50 = 2.5 x 10(-6) M against N9 neuraminidase. Compound 5 was oriented in the active site of the neuraminidase in a manner that was not predicted from the reported active site binding of GANA (4) with neuraminidase. In a mouse model of influenza, 5 did not protect the mice from weight loss due to the influenza virus when dosed intranasally.

Guanidinobenzoic acid inhibitors of influenza virus neuraminidase

The active site of influenza virus neuraminidase (NA) is formed by 11 universally conserved residues. A guanidino group incorporated into two unrelated NA inhibitors was previously reported to occupy different negatively charged sites in the NA active site, A new inhibitor containing two guanidino groups was synthesized in order to utilize both sites in an attempt to acquire a combined increase in affinity. The X-ray crystal structures of the complexes show that the expected increase in affinity could not be achieved even though the added guanidino group binds to the negatively charged site as designed. This suggests that the ligand affinity to the target protein is contributed both from ligand-protein interactions and solvation/conformation energy of the ligand.

In vivo and in vitro pharmacologic activity of the purine nucleoside phosphorylase inhibitor BCX-34: the role of GTP and dGTP

BCX-34 inhibits RBC PNP in vitro from humans, rats, and mice with IC50S ranging from 5 to 36 nM. BCX-34 also, in the presence but not in the absence of deoxyguanosine, inhibits human CCRF-CEM T-cell proliferation with an IC50 of 0.57 microM but not rat or mouse T-cell proliferation up to 30 microM. Inhibition of human T-cell proliferation is accompanied by an accumulation of intracellular dGTP with an associated reduction in GTP. These nucleotide changes do not occur in BC16A mouse T-cells and explain why proliferation is not inhibited by PNP inhibitors in this case. Reduction in intracellular GTP is not essential for the antiproliferative action of BCX-34. Oral bioavailability of BCX-34 in rats is 76%. BCX-34 is orally active in elevating plasma inosine in rats (2-fold at 30 mg/kg), in suppressing ex vivo RBC PNP activity in rats (98% at 3 h. 100 mg/kg), and in suppressing ex vivo skin PNP in mice (39% at 3 h, 100 mg/kg). The results demonstrate that BCX-34 inhibits human PNP and T-cell proliferation, is orally bioavailable in rodents, and pharmacologically active in vivo in rodents after oral dosing with no apparent side effects or toxicity. BCX-34 may, therefore, be useful in treating human T-cell proliferative inflammatory disorders.