Discovery of Indole- and Indazole-acylsulfonamides as Potent and Selective NaV1.7 Inhibitors for the Treatment of Pain

3-Aryl-indole and 3-aryl-indazole derivatives were identified as potent and selective Na_v1.7 inhibitors. Compound 29 was shown to be efficacious in the mouse formalin assay and also reduced complete Freund's adjuvant (CFA)-induced thermal hyperalgesia and chronic constriction injury (CCI) induced cold allodynia and models of inflammatory and neuropathic pain, respectively, following intraperitoneal (IP) doses of 30 mg/kg. The observed efficacy could be correlated with the mouse dorsal root ganglion exposure and Na_V1.7 potency associated with 29.

Discovery of new indole-based acylsulfonamide Nav1.7 inhibitors

Screening of 100 acylsulfonamides from the Bristol-Myers Squibb compound collection identified the C3-cyclohexyl indole 6 as a potent Na_v1.7 inhibitor. Replacement of the C2 furanyl ring of 6 with a heteroaryl moiety or truncation of this group led to the identification of 4 analogs with hNa_v1.7 IC₅₀ values under 50 nM. Fluorine substitution of the truncated compound 12 led to 34 with improved potency and isoform selectivity. The inverted indole 36 also maintained good activity. Both 34 and 36 exhibited favorable CYP inhibition profiles, good membrane permeability and a low efflux ratio and, therefore, represent new leads in the search for potent and selective Na_v1.7 inhibitors to treat pain.

Discovery of morpholine-based aryl sulfonamides as Nav1.7 inhibitors

Replacement of the piperidine ring in the lead benzenesulfonamide Na_v1.7 inhibitor 1 with a weakly basic morpholine core resulted in a significant reduction in Na_v1.7 inhibitory activity, but the activity was restored by shortening the linkage from methyleneoxy to oxygen. These efforts led to a series of morpholine-based aryl sulfonamides as isoform-selective Na_v1.7 inhibitors. This report describes the synthesis and SAR of these analogs.

Development of New Benzenesulfonamides As Potent and Selective Nav1.7 Inhibitors for the Treatment of Pain

By taking advantage of certain features in piperidine 4, we developed a novel series of cyclohexylamine- and piperidine-based benzenesulfonamides as potent and selective Nav1.7 inhibitors. However, compound 24, one of the early analogs, failed to reduce phase 2 flinching in the mouse formalin test even at a dose of 100 mpk PO due to insufficient dorsal root ganglion (DRG) exposure attributed to poor membrane permeability. Two analogs with improved membrane permeability showed much increased DRG concentrations at doses of 30 mpk PO, but, confoundingly, only one of these was effective in the formalin test. More data are needed to understand the disconnect between efficacy and exposure relationships.

BMS-933043, a Selective α7 nAChR Partial Agonist for the Treatment of Cognitive Deficits Associated with Schizophrenia

The therapeutic treatment of negative symptoms and cognitive dysfunction associated with schizophrenia is a significant unmet medical need. Preclinical literature indicates that α7 neuronal nicotinic acetylcholine (nACh) receptor agonists may provide an effective approach to treating cognitive dysfunction in schizophrenia. We report herein the discovery and evaluation of 1c (BMS-933043), a novel and potent α7 nACh receptor partial agonist with high selectivity against other nicotinic acetylcholine receptor subtypes (>100-fold) and the 5-HT_3A receptor (>300-fold). In vivo activity was demonstrated in a preclinical model of cognitive impairment, mouse novel object recognition. BMS-933043 has completed Phase I clinical trials.

B-973, a novel piperazine positive allosteric modulator of the α7 nicotinic acetylcholine receptor

The alpha7 (α7) nicotinic acetylcholine receptor is a therapeutic target for cognitive disorders. Here we describe 3-(3,4-difluorophenyl)-N-(1-(6-(4-(pyridin-2-yl)piperazin-1-yl)pyrazin-2-yl)ethyl)propanamide (B-973), a novel piperazine-containing molecule that acts as a positive allosteric modulator of the α7 receptor. We characterize the action of B-973 on the α7 receptor using electrophysiology and radioligand binding. At 0.1mM acetylcholine, 1μM B-973 potentiated peak acetylcholine-induced currents 6-fold relative to maximal acetylcholine (3mM) and slowed channel desensitization, resulting in a 6900-fold increase in charge transfer. The EC₅₀ of B-973 was approximately 0.3μM at acetylcholine concentrations ranging from 0.03 to 3mM. At a concentration of 1μM, B-973 shifted the acetylcholine EC₅₀ of peak currents from 0.30mM in control to 0.007mM. B-973 slowed channel deactivation upon acetylcholine removal (τ=50s) and increased the affinity of the α7 agonist [³H]A-585539. In the absence of exogenously added acetylcholine, application of B-973 at concentrations >1μM induced large methyllycaconitine-sensitive currents, suggesting B-973 can function as an Ago-PAM at high concentrations. B-973 will be a useful probe for investigating the biological consequences of increasing α7 receptor activity through allosteric modulation.

Development of 4-Heteroarylamino-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octanes] as α7 Nicotinic Receptor Agonists

We describe the synthesis of quinuclidine-containing spiroimidates and their utility as α7 nicotinic acetylcholine receptor (nAChR) partial agonists. A convergent synthetic route allowed for rapid SAR investigation and provided a diverse set of fused 6,5-heteroaryl analogs. Two potent and selective α7 nAChR partial agonists, (1'S,3'R,4'S)-N-(7-bromopyrrolo[2,1-f][1,2,4]triazin-4-yl)-4H-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octan]-2-amine (20) and (1'S,3'R,4'S)-N-(7-chloropyrrolo[2,1-f][1,2,4]triazin-4-yl)-4H-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octan]-2-amine (21), were identified. Both agonists improved cognition in a preclinical rodent model of learning and memory. Additionally, 5-HT_3A receptor SAR suggested the presence of a steric site that when engaged led to significant loss of affinity at that receptor.

Design and Synthesis of a New Series of 4-Heteroarylamino-1'-azaspiro[oxazole-5,3'-bicyclo[2.2.2]octanes as α7 Nicotinic Receptor Agonists. 1. Development of Pharmacophore and Early Structure-Activity Relationship

The design and synthesis of a series of quinuclidine-containing spirooxazolidines ("spiroimidates") and their utility as α7 nicotinic acetylcholine receptor partial agonists are described. Selected members of the series demonstrated excellent selectivity for α7 over the highly homologous 5-HT_3A receptor. Modification of the N-spiroimidate heterocycle substituent led to (1S,2R,4S)-N-isoquinolin-3-yl)-4'H-4-azaspiro[bicyclo[2.2.2]octane-2,5'oxazol]-2'-amine (BMS-902483), a potent α7 partial agonist, which improved cognition in preclinical rodent models.

Discovery of D1 Dopamine Receptor Positive Allosteric Modulators: Characterization of Pharmacology and Identification of Residues that Regulate Species Selectivity

The present studies represent the first published report of a dopamine D1 positive allosteric modulator (PAM). D1 receptors have been proposed as a therapeutic target for the treatment of cognitive deficits associated with schizophrenia. However, the clinical utility of orthosteric agonist compounds is limited by cardiovascular side effects, poor pharmacokinetics, lack of D1 selectivity, and an inverted dose response. A number of these challenges may be overcome by utilization of a selective D1 PAM. The current studies describe two chemically distinct D1 PAMs: Compound A [1-((rel-1S,3R,6R)-6-(benzo[d][1,3]dioxol-5-yl)bicyclo[4.1.0]heptan-3-yl)-4-(2-bromo-5-chlorobenzyl)piperazine] and Compound B [rel-(9R,10R,12S)-N-(2,6-dichloro-3-methylphenyl)-12-methyl-9,10-dihydro-9,10-ethanoanthracene-12-carboxamide]. Compound A shows pure PAM activity, with an EC50 of 230 nM and agonist activity at the D2 receptor in D2-expressing human embryonic kidney cells. Compound B shows superior potency (EC50 of 43 nM) and selectivity for D1 versus D2 dopamine receptors. Unlike Compound A, Compound B is selective for human and nonhuman primate D1 receptors, but lacks activity at the rodent (rat and mouse) D1 receptors. Using molecular biology techniques, a single amino acid was identified at position 130, which mediates the species selectivity of Compound B. These data represent the first described D1-selective PAMs and define critical amino acids that regulate species selectivity.

Discovery of (S,E)-3-(2-fluorophenyl)-N-(1-(3-(pyridin-3-yloxy)phenyl)ethyl)-acrylamide as a potent and efficacious KCNQ2 (Kv7.2) opener for the treatment of neuropathic pain

Acrylamide (S)-6, a potent and efficacious KCNQ2 (Kv7.2) opener, demonstrated significant activity in two models of neuropathic pain and in the formalin test, suggesting that KCNQ2 openers may be useful in the treatment of neuropathic pain including diabetic neuropathy.

Comparative effects of sodium pyrithione evoked intracellular calcium elevation in rodent and primate ventral horn motor neurons

Oral administration of sodium pyrithione (NaP) causes hindlimb weakness in rodents, but not in primates. Previous work using Aplysia neurons has demonstrated that NaP produces a persistent influx of Ca(2+) ions across the plasma membrane. To determine whether this also occurs in mammalian neurons and whether this could underlie the inter-species difference between rodents and primates, we have tested the effects of NaP on intracellular Ca(2+) levels ([Ca(2+)](i)) in rat and monkey motor neurons in vitro. Motor neurons present in spinal cord slices from rhesus monkey embryos (E37 and 56) and from rat E16 were dissected and cultured on glass coverslips. Following 2 weeks (rhesus) or 2-3 days (rat) in culture, neurons were loaded with fura-PE3/AM, and examined for [Ca(2+)](i) changes in response to NaP. Rhesus motor neurons were identified by immunostaining for Islet-1 (MN specific antigen) and neuron specific enolase (NSE). Motor neurons from both species exhibited dose-dependent NaP-evoked increases in [Ca(2+)](i) However, the dose-response curve for the Rhesus motor neurons was significantly shifted to the right of the rat dose-response curve, whereas the overall amplitude of the Ca(2+) rise was similar in both species. As shown previously for the Aplysia neurons, the action of NaP is attenuated by SKF 96365, an inhibitor of store-operated calcium entry. In contrast the action of NaP is unaffected by nifedipine and tetrodotoxin, blockers of voltage-dependent Ca(2+) and Na(+) channels, respectively, or by ouabain, an inhibitor of the plasma membrane Na(+)/K(+) ATPase. Our results indicate that the NaP-induced increase in [Ca(2+)](i) is conserved across species and suggest that the toxicological sensitivity of rodent over primate to pyrithione could be due to the enhanced sensitivity of rodent motor neurons to NaP-evoked intracellular Ca(2+) elevation.

Reduction of mitomycin C is catalysed by human recombinant NRH:quinone oxidoreductase 2 using reduced nicotinamide adenine dinucleotide as an electron donating co-factor

NRH:Quinone Oxidoreductase 2 (NQO2) has been described as having no enzymatic activity with nicotinamide adenine dinucleotide (NADH) or NADPH as electron donating cosubstrates. Mitomycin C (MMC) is both a substrate for and a mechanistic inhibitor of the NQO2 homologue NQO1. NRH:quinone oxidoreductase 2 catalysed the reduction of MMC at pH 5.8 with NADH as a co-factor. This reaction results in species that inhibit the NQO2-mediated metabolism of CB1954. In addition, MMC caused an increase in DNA cross-links in a cell line transfected to overexpress NQO2 to an extent comparable to that observed with an isogenic NQO1-expressing cell line. These data indicate that NQO2 may contribute to the metabolism of MMC to cytotoxic species.

A store-operated Ca(2+) influx pathway in the bag cell neurons of Aplysia

Although store-operated Ca(2+) influx has been well-studied in nonneuronal cells, an understanding of its nature in neurons remains poor. In the bag cell neurons of Aplysia californica, prior work has suggested that a Ca(2+) entry pathway can be activated by Ca(2+) store depletion. Using fura-based imaging of intracellular Ca(2+) in cultured bag cell neurons, we now characterize this pathway as store-operated Ca(2+) influx. In the absence of extracellular Ca(2+), the endoplasmic reticulum Ca(2+)-ATPase inhibitors, cyclopiazonic acid (CPA) or thapsigargin, depleted intracellular stores and elevated intracellular free Ca(2+). With the subsequent addition of extracellular Ca(2+), a prominent Ca(2+) influx was observed. The ryanodine receptor agonist, chloroethylphenol (CEP), also increased intracellular Ca(2+) but did not initiate store-operated Ca(2+) influx, despite overlap between CEP- and CPA-sensitive stores. Bafilomycin A, a vesicular H(+)-ATPase inhibitor, liberated intracellular Ca(2+) from acidic stores and attenuated subsequent Ca(2+) influx, presumably by replenishing CPA-depleted stores. Store-operated Ca(2+) influx was partially blocked by low concentrations of La(3+) or BTP2, and strongly inhibited by either 1-[b-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole (SKF-96365) or a high concentration of Ni(2+). Regarding IP(3) receptor blockers, 2-aminoethyldiphenyl borate, but not xestospongin C, prevented store-operated Ca(2+) influx. However, jasplakinolide, an actin stabilizer reported to inhibit this pathway in smooth muscle cell lines, was ineffective. The bag cell neurons initiate reproductive behavior through a prolonged afterdischarge associated with intracellular Ca(2+) release and neuropeptide secretion. Store-operated Ca(2+) influx may serve to replenish stores depleted during the afterdischarge or participate in the release of peptide that triggers behavior.

(S,E)-N-[1-(3-heteroarylphenyl)ethyl]-3-(2-fluorophenyl)acrylamides: synthesis and KCNQ2 potassium channel opener activity

Replacement of the morpholinyl moiety in (S,E)-N-[1-(3-morpholinophenyl)ethyl]-3-phenylacrylamide (1) with heteroaryl groups led to the identification of (S,E)-N-1-[3-(6-fluoropyridin-3-yl)phenyl]ethyl-3-(2-fluorophenyl)acrylamide (5) as a potent KCNQ2 potassium channel opener. Among this series of heteroaryl substituted acrylamides, (S,E)-N-1-[3-(1H-pyrazol-1-yl)phenyl]ethyl-3-(2-fluorophenyl)acrylamide (9) exhibits balanced potency and efficacy. The syntheses and the KCNQ2 opener activity of this series of acrylamides are described.

Targeting enzymes to cancers - new developments

Two methods of using tumour located enzymes have been described. These are antibody directed enzyme prodrug therapy (ADEPT) and macromolecule directed enzyme prodrug therapy (MDEPT), where the tumour located enzyme converts a non-toxic prodrug into a cytotoxic drug at tumour sites. The alternative use of tumour located enzymes is to inactivate rescue agents that protect cells from antimetabolite action, and is described as 'Antimetabolite with inactivation of rescue agent at cancer sites' (AMIRACS). The leakiness of tumour blood vessels and poor lymphatic drainage allows enzymes to be targeted to many cancers by attachment to polymeric macromolecules (MDEPT), as well as to antibodies and antibody fragments (ADEPT). To avoid systemic toxicity, enzyme activity in blood and normal tissues must be very low before giving a prodrug or rescue agent. Antibodies directed against the enzyme component of macromolecular conjugates have proved to be very efficient at clearing normal tissues. Human enzymes which are over expressed by cancer cells can be exploited particularly if they require co-factors or co-substrates, either in situ or targeted to extracellular sites. Bacterial enzymes have advantages in specificity but require some form of immunological control in view of their immunogenicity. Prodrugs which generate drugs with very short half lives are desirable, and have been developed, including one which has a differential toxicity between prodrug and the active drug of 1000 to 10,000 fold. The range of antimetabolites available for AMIRACS was initially restricted to inhibitors of dihydrofolate reductase but has been greatly extended by the introduction of inhibitors of other enzymes. The limitations of these systems are discussed.

Synthesis and KCNQ2 opener activity of N-(1-benzo[1,3]dioxol-5-yl-ethyl, N-[1-(2,3-dihydro-benzofuran-5-yl)-ethyl, and N-[1-(2,3-dihydro-1H-indol-5-yl)-ethyl acrylamides

Bioisosteric replacement studies led to the identification of N-(1-benzo[1,3]dioxol-5-yl-ethyl)-3-(2-chloro-phenyl)-acrylamide ((S)-3) as a highly potent KCNQ2 opener, and 3-(2,6-difluoro-phenyl)-N-[1-(2,3-dihydro-benzofuran-5-yl)-ethyl]-acrylamide ((S)-4), and N-[1-(2,3-dihydro-1H-indol-5-yl)-ethyl]-3-(2-fluoro-phenyl)-acrylamide ((S)-5) as highly efficacious KCNQ2 openers. In contrast, their respective R enantiomers showed significantly less or no appreciable KCNQ2 opener activity even at the highest concentration tested (10 microM). Because of its high potency and moderate efficacy as well as its convenient synthesis, (+/-)-3 was selected as a reference compound for analyzing efficacies of KCNQ openers in electrophysiology studies. Compounds (S)-4 and (S)-5 demonstrated significant activity in reducing neuronal hyperexcitability in rat hippocampal slices. The synthesis and the KCNQ2 opener activity of these acrylamides are described.

A thallium-sensitive, fluorescence-based assay for detecting and characterizing potassium channel modulators in mammalian cells

Potassium channels have been identified as targets for a large number of therapeutic indications. The ability to use a high-throughput functional assay for the detection and characterization of small-molecule modulators of potassium channels is very desirable. However, present techniques capable of screening very large chemical libraries are limited in terms of data quality, temporal resolution, ease of use, and requirements for specialized instrumentation. To address these issues, the authors have developed a fluorescence-based thallium flux assay. This assay is capable of detecting modulators of both voltage and ligand-gated potassium channels expressed in mammalian cells. The thallium flux assay can use instruments standard to most high-throughput screening laboratories, and using such equipment has been successfully employed to screen large chemical libraries consisting of hundreds of thousands of compounds.

BL-1249 [(5,6,7,8-tetrahydro-naphthalen-1-yl)-[2-(1H-tetrazol-5-yl)-phenyl]-amine]: a putative potassium channel opener with bladder-relaxant properties

BL-1249 [(5,6,7,8-tetrahydro-naphthalen-1-yl)-[2-(1H-tetrazol-5-yl)-phenyl]-amine] produced a concentration-dependent membrane hyperpolarization of cultured human bladder myocytes, assessed as either a reduction in fluorescence of the voltage-sensitive dye bis-(1,2-dibutylbarbituric acid)trimethine oxonol (EC50 = 1.26 +/- 0.6 microM) or by direct electrophysiological measurement (EC50 = 1.49 +/- 0.08 microM). BL-1249 also produced a membrane hyperpolarization of acutely dissociated rat bladder myocytes. Voltage-clamp studies in human bladder cells revealed that BL-1249 activated an instantaneous, noninactivating current that reversed near E(K). The BL-1249-evoked outward K+ current was insensitive to blockade by glyburide, tetraethylammonium, iberiotoxin, 4-aminopyridine, apamin, or Mg2+. However, the current was inhibited by extracellular Ba2+ (10 mM). In in vitro organ bath experiments, BL-1249 produced a concentration-dependent relaxation of 30 mM KCl-induced contractions in rat bladder strips (EC50 = 1.12 +/- 0.37 microM), yet had no effect on aortic strips up to the highest concentration tested (10 microM). The bladder relaxation produced by BL-1249 was partially blocked by Ba2+ (1 and 10 mM) but not by apamin, iberiotoxin, 4-aminopyridine, glyburide, or tetraethylammonium. In an anesthetized rat model, BL-1249 (1 mg/kg i.v.) decreased the number of isovolumic contractions, without significantly affecting blood pressure. Thus, BL-1249 behaves as a potassium channel activator that exhibits bladder versus vascular selectivity both in vitro and in vivo. A survey of potassium channels exhibiting sensitivity to extracellular Ba2+ at millimolar concentration revealed that the expression of the K2P2.1 (TREK-1) channel was relatively high in human bladder cells versus human aortic cells, suggesting this channel as a possible candidate target for BL-1249.

(S)-N-[1-(4-cyclopropylmethyl-3,4-dihydro-2H-benzo[1,4]oxazin-6-yl)-ethyl]-3-(2-fluoro-phenyl)-acrylamide is a potent and efficacious KCNQ2 opener which inhibits induced hyperexcitability of rat hippocampal neurons

(S)-N-[1-(4-Cyclopropylmethyl-3,4-dihydro-2H-benzo[1,4]oxazin-6-yl)-ethyl]-3-(2-fluoro-phenyl)-acrylamide ((S)-2) was identified as a potent and efficacious KCNQ2 opener. This compound demonstrated significant activity in reducing neuronal hyperexcitability in rat hippocampal slices, and the inhibition mediated by (S)-2 was reversed by the KCNQ blocker linopirdine.

Activation of a calcium entry pathway by sodium pyrithione in the bag cell neurons ofAplysia

The ability of sodium pyrithione (NaP), an agent that produces delayed neuropathy in some species, to alter neuronal physiology was accessed using ratiometric imaging of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in fura PE-filled cultured Aplysia bag cell neurons. Bath-application of NaP evoked a [Ca(2+)](i) elevation in both somata and neurites with an EC(50) of approximately 300 nM and a Hill coefficient of approximately 1. The response required the presence of external Ca(2+), had an onset of 3-5 min, and generally reached a maximum within 30 min. 2-Methyl-sulfonylpyridine, a metabolite and close structural analog of NaP, did not elevate [Ca(2+)](i). Under whole-cell current-clamp recording, NaP produced a approximately 14 mV depolarization of resting membrane potential that was dependent on external Ca(2+). These data suggested that NaP stimulates Ca(2+) entry across the plasma membrane. To minimize the possibility that a change in cytosolic pH was the basis for NaP-induced Ca(2+) entry, bag cell neuron intracellular pH was estimated with the dye 2',7'-bis(carboxyethyl-5(6)-carboxy-fluorescein acetoxy methylester. Exposure of the neurons to NaP did not alter intracellular pH. The slow onset and sustained nature of the NaP response suggested that a cation exchange mechanism coupled either directly or indirectly to Ca(2+) entry could underlie the phenomenon. However, neither ouabain, a Na(+)/K(+) ATPase inhibitor, nor removal of extracellular Na(+), which eliminates Na(+)/Ca(2+) exchanger activity, altered the NaP-induced [Ca(2+)](i) elevation. Finally, the possibility that NaP gates a Ca(2+)-permeable ion channel in the plasma membrane was examined. NaP did not appear to activate two major forms of bag cell neuron Ca(2+)-permeable ion channels, as Ca(2+) entry was unaffected by inhibition of voltage-gated Ca(2+) channels using nifedipine or by inhibition of a voltage-dependent, nonselective cation channel using a high concentration of tetrodotoxin. In contrast, two potential store-operated Ca(2+) entry current inhibitors, SKF-96365 and Ni(2+), attenuated NaP-induced Ca(2+) entry. We conclude that NaP activates a slow, persistent Ca(2+) influx in Aplysia bag cell neurons.

Fluorine substitution can block CYP3A4 metabolism-dependent inhibition: identification of (S)-N-[1-(4-fluoro-3- morpholin-4-ylphenyl)ethyl]-3- (4-fluorophenyl)acrylamide as an orally bioavailable KCNQ2 opener devoid of CYP3A4 metabolism-dependent inhibition

The formation of a reactive intermediate was found to be responsible for CYP3A4 metabolism-dependent inhibition (MDI) observed with (S)-N-[1-(3-morpholin-4-ylphenyl)ethyl]-3-phenyl-acrylamide (1). Structure-3A4 MDI relationship studies culminated in the discovery of a difluoro analogue, (S)-N-[1-(4-fluoro-3-morpholin-4-ylphenyl)ethyl]-3-(4-fluoro-phenyl)acrylamide (2), as an orally bioavailable KCNQ2 opener free of CYP3A4 MDI.

(S)-N-[1-(3-morpholin-4-ylphenyl)ethyl]- 3-phenylacrylamide: an orally bioavailable KCNQ2 opener with significant activity in a cortical spreading depression model of migraine

(S)-N-[1-(3-Morpholin-4-ylphenyl)ethyl]-3-phenylacrylamide (2) was synthesized as an orally bioavailable KCNQ2 potassium channel opener. In a rat model of migraine, 2 demonstrated significant oral activity in reducing the total number of cortical spreading depressions induced by potassium chloride.

Telomerase-specific suicide gene therapy vectors expressing bacterial nitroreductase sensitize human cancer cells to the pro-drug CB1954

Telomerase activation is considered to be a critical step in cancer progression due to its role in cellular immortalization. The prevalence of telomerase expression in human cancers makes it an attractive candidate for new mechanism-based targets for cancer therapy. The selective killing of cancer cells can be achieved by gene-directed enzyme pro-drug therapy (GDEPT). In this study we have tested the feasibility of using the transcriptional regulatory sequences from the hTERT and hTR genes to regulate expression of the bacterial nitroreductase enzyme in combination with the pro-drug CB1954 in a suicide gene therapy strategy. hTERT and hTR promoter activity was compared in a panel of 10 cell lines and showed a wide distribution in activity; low activity was observed in normal cells and telomerase-negative immortal ALT cell lines, with up to 300-fold higher activity observed in telomerase positive cancer lines. Placing the nitroreductase gene under the control of the telomerase gene promoters sensitized cancer cells in tissue culture to the pro-drug CB1954 and promoter activity was predictive of sensitization to the pro-drug (2-20-fold sensitization), with cell death restricted to lines exhibiting high levels of promoter activity. The in vivo relevance of these data was tested using two xenograft models (C33a and GLC4 cells). Significant tumour reduction was seen with both telomerase promoters and the promoter-specific patterns of sensitization observed in tissue culture were retained in xenograft models. Thus, telomerase-specific suicide gene therapy vectors expressing bacterial nitroreductase sensitize human cancer cells to the pro-drug CB1954.

Aerobic nitroreduction by flavoproteins: enzyme structure, mechanisms and role in cancer chemotherapy

NQO1 (DT-diaphorase) and its truncated isoenzyme, the metalloenzyme NQO2, can reduce quinone substrates by two-electron transfer. While NQO1 is a known detoxification enzyme, the function of NQO2 is less well understood. Both rat NQO1 and human NQO2 reductively bioactivate the dinitroarene CB 1954 to a cytotoxic product that behaves as a difunctional DNA-crosslinking species with potent anti-tumour activity, although human NQO1 is much less effective. A FMN-dependent nitroreductase from E. coli B also reduces quinones and reductively bioactivates CB 1954. However, this enzyme reduces CB 1954 to the 2- and 4-hydroxylamines in equivalent yield, whereas NQO1 and NQO2 generate only the 4-isomer. The reduction profile is a key factor in the development of anti-tumour prodrugs, where distinct delivery strategies are being evaluated: prodrug therapy, antibody-, macromolecule and gene-directed enzyme prodrug therapy (ADEPT, MDEPT or GDEPT). The flavoprotein enzymes are explored in terms of structure and bioreduction mechanism, particularly for use in the design of novel prodrugs with potential application as chemotherapeutic agents.

Involvement of NADPH: cytochrome P450 reductase in the activation of indoloquinone EO9 to free radical and DNA damaging species

Evidence suggests that DT-diaphorase is involved in the activation and mechanism of cytotoxicity of the investigational indoloquinone anticancer drug EO9 under aerobic conditions. Data also implicate a role for other enzymes including NADPH: cytochrome P450 reductase, especially in low DT-diaphorase tumour cells and under hypoxic conditions. Here, we used purified rat NADPH: cytochrome P450 reductase to provide additional evidence in support of a role for this enzyme in activation of EO9 to generate free radical and DNA-damaging species. Electron spin resonance spectrometry studies showed that NADPH: cytochrome P450 reductase reduced EO9 to a free radical species, including a drug radical (most likely the semiquinone) and reactive oxygen species. Plasmid DNA experiments showed that reduction of EO9 catalysed by NADPH: cytochrome P450 reductase results in single-strand breaks in DNA. The information obtained may contribute to the understanding of the molecular mechanism of DNA damage and cytotoxicity exerted by EO9 and may be useful in the design of future bioreductive drugs.

Recurrent orbital inflammation from metastatic orbital carcinoid tumor

PURPOSE

To report an unusual clinical presentation for carcinoid tumor metastatic to the orbit.

METHODS

Two adult patients with metastatic carcinoid tumor and unilateral orbital masses are described.

RESULTS

Both patients sought treatment for acute unilateral orbital inflammation. Neuroimaging revealed orbital metastases adjacent to the inflamed adnexal tissue. Based on each patient's description of similar, prior, untreated episodes, minimal medical management was initiated. Resolution of the inflammatory signs was spontaneous in one case and assisted by pulsed, systemic corticosteroids in the second case. Compressive neuropathic vision loss occurred 11 months later in the second case.

CONCLUSIONS

Carcinoid tumor metastatic to the orbit may manifest as recurrent orbital and ocular adnexal inflammation. These signs differ from systemic carcinoid syndrome in that they are unilateral, limited to only the orbital and ocular adnexal soft tissues, and resolve over days. Clinicians must carefully differentiate this manifestation from that of tumor necrosis, adnexal infection, or orbital outlet obstruction. This presentation may result from the spontaneous release of local inflammatory mediators intrinsic to the orbital tumor.

Particle electrophoresis for quality assurance and process control

Process control is an increasingly important issue as life science companies world-wide strive for recognition of their manufacturing and product development quality measures according to International Standards Organization (ISO) or good manufacturing practices (GMP) standards. Analytical particle electrophoresis (APE) has the potential for significant contributions, not just to basic research, but also in process development and control in manufacturing environments. An important feature of colloidal (small) particles, which controls their behavior, is their surface charge. Optimization of life science products and process conditions involving small particles (>100 nm) may be approached by a variety of strategies based upon direct measurements of the charge properties of process particles or "reporter" particles. The availability of increasingly powerful instruments and control particle preparations (National Institute of Standards and Technology ((NIST) and others) for validation of instrument operation make the method more attractive than ever. We summarize highly flexible electrophoretic strategies for assessing process consistency both from the perspective of particles being processed as well as the processing environment and describe principles for the use of polymer microspheres both as control particles for validation of instrument operation as well as for probes of the assay medium.

Bioactivation of 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 1954) by human NAD(P)H quinone oxidoreductase 2: a novel co-substrate-mediated antitumor prodrug therapy

A novel prodrug activation system, endogenous in human tumor cells, is described. A latent enzyme-prodrug system is switched on by a simple synthetic, small molecule co-substrate. This ternary system is inactive if any one of the components is absent. CB 1954 [5-(aziridin-1-yl)-2,4-dinitrobenzamide] is an antitumor prodrug that is activated in certain rat tumors via its 4-hydroxylamine derivative to a potent bifunctional alkylating agent. However, human tumor cells are resistant to CB 1954 because they are unable to catalyze this bioactivation efficiently. A human enzyme has been discovered that can activate CB 1954, and it has been shown to be commonly present in human tumor cells. The enzyme is NQO2 [NAD(P)H quinone oxidoreductase 2], but its activity is normally latent, and a nonbiogenic co-substrate such as NRH [nicotinamide riboside (reduced)] is required for enzymatic activity. There is a very large (100-3000-fold) increase in CB 1954 cytotoxicity toward either NQO2-transfected rodent or nontransfected human tumor cell lines in the presence of NRH. Other reduced pyridinium compounds can also act as co-substrates for NQO2. Thus, the simplest quaternary salt of nicotinamide, 1-methyl-3-carboxamidopyridinium iodide, was a co-substrate for NQO2 when reduced to the corresponding 1,4-dihydropyridine derivative. Increased chain length and/or alkyl load at the 1-position of the dihydropyridine ring improved specific activity, and compounds more active than NRH were found. However, little activity was seen with either the 1-benzyl or 1-(2-phenylethyl) derivatives. A negatively charged substituent at the 3-position of the reduced pyridine ring also negated the ability of these compounds to act as cosubstrates for NQO2. In particular, 1-carbamoylmethyl-3-carbamoyl-1,4dihydropyridine was shown to be a co-substrate for NQO2 with greater stability than NRH, with the ability to enter cells and potentiate the cytotoxicity of CB 1954. Furthermore, this agent is synthetically accessible and suitable for further pharmaceutical development. NQO2 activity appears to be related to expression of NQO1 (DT-diaphorase), an enzyme that is known to have a favorable distribution toward certain human cancers. NQO2 is a novel target for prodrug therapy and has a unique activation mechanism that relies on a synthetic co-substrate to activate an apparently latent enzyme. Our findings may reopen the use of CB 1954 for the direct therapy of human malignant disease.

Brain-derived neurotrophic factor induces excitotoxic sensitivity in cultured embryonic rat spinal motor neurons through activation of the phosphatidylinositol 3-kinase pathway

Neurotrophic factors (NTFs) can protect against or sensitize neurons to excitotoxicity. We studied the role played by various NTFs in the excitotoxic death of purified embryonic rat motor neurons. Motor neurons cultured in brain-derived neurotrophic factor, but not neurotrophin 3, glial-derived neurotrophic factor, or cardiotrophin 1, were sensitive to excitotoxic insult. BDNF also induces excitotoxic sensitivity (ES) in motor neurons when BDNF is combined with these other NTFs. The effect of BDNF depends on de novo protein and mRNA synthesis. Reagents that either activate or inhibit the 75-kDa NTF receptor p75NTR do not affect BDNF-induced ES. The low EC50 for BDNF-induced survival and ES suggests that TrkB mediates both of these biological activities. BDNF does not alter glutamate-evoked rises of intracellular Ca2+, suggesting BDNF acts downstream. Both wortmannin and LY294002, which specifically block the phosphatidylinositol 3-kinase (PI3K) intracellular signaling pathway in motor neurons, inhibit BDNF-induced ES. We confirm this finding using a herpes simplex virus (HSV) that expresses the dominant negative p85 subunit of PI3K. Infecting motor neurons with this HSV, but not a control HSV, blocks activation of the PI3K pathway and BDNF-induced ES. Through the activation of TrkB and the PI3K signaling pathway, BDNF renders developing motor neurons susceptible to glutamate receptor-mediated cell death.

Activation of a Ca2+-permeable cation channel produces a prolonged attenuation of intracellular Ca2+ release in Aplysia bag cell neurones

1. Brief synaptic stimulation, or exposure to Conus textile venom (CtVm), triggers an afterdischarge in the bag cell neurones of Aplysia. This is associated with an elevation of intracellular calcium ([Ca2+]i) through Ca2+ release from intracellular stores and Ca2+ entry through voltage-gated Ca2+ channels and a non-selective cation channel. The afterdischarge is followed by a prolonged (approximately 18 h) refractory period during which the ability of both electrical stimulation and CtVm to trigger afterdischarges or elevate [Ca2+]i is severely attenuated. By measuring the response of isolated cells to CtVm, we have now tested the contribution of different sources of Ca2+ elevation to the onset of the prolonged refractory period. CtVm induced an increase in [Ca2+]i in both normal and Ca2+-free saline, in part by liberating Ca2+ from a store sensitive to thapsigargin or cyclopiazonic acid, but not sensitive to heparin. 3. In the presence of extracellular Ca2+, the neurones became refractory to CtVm after a single application but recovered following approximately 24 h, when CtVm could again elevate [Ca2+]i. However, this refractoriness did not develop if CtVm was applied in Ca2+-free saline. Thus, elevation of [Ca2+]i alone does not induce refractoriness to CtVm-induced [Ca2+]i elevation, but Ca2+ influx triggers this refractory-like state. 4. CtVm produces a depolarization of isolated bag cell neurones. To determine if Ca2+ influx through voltage-gated Ca2+ channels, activated during this depolarization, caused refractoriness to CtVm-induced [Ca2+]i elevation, cells were depolarized with high external potassium (60 mM), which produced a large increase in [Ca2+]i. Nevertheless, subsequent exposure of the cells to CtVm produced a normal response, suggesting that Ca2+ influx through voltage-gated Ca2+ channels does not induce refractoriness. 5. As a second test for the role of voltage-gated Ca2+ channels, these channels were blocked with nifedipine. This drug failed to prevent the onset of refractoriness to CtVm-induced [Ca2+]i elevation, providing further evidence that Ca2+ entry through voltage-gated Ca2+ channels does not initiate refractoriness. 6. To examine if Ca2+ entry through the CtVm-activated, non-selective cation channel caused refractoriness, neurones were treated with a high concentration of TTX, which blocks the cation channel. TTX protected the neurones from the refractoriness to [Ca2+]i elevation produced by CtVm in Ca2+-containing medium. 7. Using clusters of bag cell neurones in intact abdominal ganglia, we compared the ability of nifedipine and TTX to protect the cells from refractoriness to electrical stimulation. Normal, long-lasting afterdischarges could be triggered by stimulation of an afferent input after a period of exposure to CtVm in the presence of TTX. In contrast, exposure to CtVm in the presence of nifedipine resulted in refractoriness. 8. Our data indicate that Ca2+ influx through the non-selective cation channel renders cultured bag cell neurones refractory to repeated stimulation with CtVm. Moreover, the refractory period of the afterdischarge itself may also be initiated by Ca2+ entry through this cation channel.

Developments with targeted enzymes in cancer therapy

Cancer therapy based on the delivery of enzymes to tumour sites has advanced in several directions since antibody-directed enzyme/prodrug therapy was first described. It has been shown that methoxypolyethylene glycol (MPEG) can be used to deliver enzyme to a variety of solid tumours. MPEG-enzyme conjugates show reduced immunogenicity and may allow repeated treatment with enzymes of bacterial origin. Enzyme delivery to tumours by polymers can be used to convert a low toxicity prodrug to a potent cytotoxic agent. An example of such a prodrug is CB1954, which can be activated by a human enzyme in the presence of a cosubstrate. Tumour-located enzymes can also be used in conjunction with a combination of antimetabolites and rescue agents. The rescue agent protects normal tissue but is degraded at cancer sites by the enzyme, thus deprotecting the tumour and allowing prolonged antimetabolite action.

Phthalimide analogs of CB 1954: synthesis and bioactivation

Four novel 4-substituted 5-nitrophthalimides (5-substituted-6-nitro-1,3-dihydro-isoindol-1,3-diones), 6, 7, 10 and 11, and the known 5 are prepared as analogs of the dinitrobenzamide prodrug CB 1954, 1, and considered as potential candidates for gene-directed enzyme prodrug therapy. All the phthalimides are poor substrates for Escherichia coli nitroreductase compared to CB 1954. However, 6, 7, 10 and 11 are reduced by both the human and rat forms of DT-diaphorase; 10 is a particularly good substrate but 7 decomposes in phosphate buffer. A cell-line panel consisting of V79 cells that have been engineered to express various levels of either the human or rat forms of DT-diaphorase in an identical cellular background was used to evaluate these compounds as prodrugs activated by this enzyme. The cytotoxic effect of CB 1954 is proportional to the activity of either the rat or human enzyme but cells expressing the rat enzyme were much more sensitive (10000-fold at higher levels of DT-diaphorase activity) than cells expressing comparable levels of the human enzyme. These results demonstrate that the resistance of human tumors to CB 1954 can be accounted for solely by the kinetic properties of the enzyme for this prodrug. The nitrophthalimide analogs overcome this kinetic failing of CB 1954. However, these compounds are not activated to produce cytotoxicity in these DT-diaphorase-expressing cell lines. It is postulated their reduction products fail to undergo an acylation reaction in a manner analogous to CB 1954. Thus, reduction by DT-diaphorase is not predictive of cytotoxicity in this class of prodrugs.

High-performance liquid chromatographic method for sensitive determination of the alkylating agent CB1954 in human plasma

A high-performance liquid chromatography (HPLC) method is described for the measurement of the weak alkylating agent CB1954 in human plasma. CB1954 can be used as an innocuous prodrug designed for activation by bacterial nitroreductases in strategies of gene-directed enzyme-prodrug therapy, and becomes activated to a potent bifunctional alkylating agent. The HPLC method involves precipitation and solvent extraction and uses Mitomycin C (MMC) as an internal standard, with a retention time for MMC of 5.85 +/- 0.015 min, and for CB1954 of 10.72 +/- 0.063 min. The limit of detection for CB1954 is 2.9 ng/ml, and this compares favourably with systems involving direct analysis of plasma (limit of detection 600 ng/ml, approximately). The method is now being used for pharmacokinetic measurements in plasma samples from cancer patients entering phase I clinical trials of CB1954. Results using serial plasma samples from one patient are presented. The patient was treated intravenously with CB1954 (6 mg/m2), and plasma clearance of the drug showed biphasic kinetics with alpha half-life 14.6 min, and beta half-life 170.5 min.

Dexamethasone-mediated inhibition of calcium transients and ACTH release in a pituitary cell line (AtT-20)

In the corticotroph-like murine pituitary tumor cell line, AtT-20, adrenocorticotropic hormone release is triggered by corticotropin-releasing hormone and is attenuated by the synthetic adrenal steroid dexamethasone. The precise mechanisms by which dexamethasone inhibits secretion are under investigation. We examined whether dexamethasone can modulate release via regulation of calcium homeostasis. More specifically, we have evaluated the effects of dexamethasone on calcium current, intracellular calcium concentration, and adrenocorticotropic hormone release. Using perforated patch-clamp and calcium imaging with fura PE3/AM, we found that dexamethasone decreases calcium current and intracellular calcium levels. The inhibition of current by dexamethasone is not, however, altered by the calcium channel antagonists nifedipine (L-type) or omega-agatoxin IVA (P/Q-type), despite the presence of these calcium channel subtypes in AtT-20 cells and the exclusive coupling of adrenocorticotropic hormone release to the L-type channel in these cells. We also evaluated the temporal relationship between dexamethasone-mediated inhibition of secretion and calcium influx. Whereas a prolonged (2 h) incubation with dexamethasone inhibits corticotropin-induced release by approximately 40%, a rapid (10 min) incubation (a time interval sufficient for dexamethasone-mediated inhibition of calcium transients) does not inhibit release. These data suggest, therefore, that dexamethasone does, indeed, modulate calcium homeostasis in AtT-20 cells, but that this effect is not responsible for its inhibition of secretion.

Excitotoxic death of a subset of embryonic rat motor neurons in vitro

We have used cultures of purified embryonic rat spinal cord motor neurons to study the neurotoxic effects of prolonged ionotropic glutamate receptor activation. NMDA and non-NMDA glutamate receptor agonists kill a maximum of 40% of the motor neurons in a concentration- and time-dependent manner, which can be blocked by receptor subtype-specific antagonists. Subunit-specific antibodies stain all of the motor neurons with approximately the same intensity and for the same repertoire of subunits, suggesting that the survival of the nonvulnerable population is unlikely to be due to the lack of glutamate receptor expression. Extracellular Ca2+ is required for excitotoxicity, and the route of entry initiated by activation of non-NMDA, but not NMDA, receptors is L-type Ca2+ channels. Ca2+ imaging of motor neurons after application of specific glutamate receptor agonists reveals a sustained rise in intracellular Ca2+ that is present to a similar degree in most motor neurons, and can be blocked by appropriate receptor/channel antagonists. Although the lethal effects of glutamate receptor agonists are seen in only a subset of cultured motor neurons, the basis of this selectivity is unlikely to be simply the glutamate receptor phenotype or the level/pattern of rise in agonist-evoked intracellular Ca2+.

Reduction of the indoloquinone anticancer drug EO9 by purified DT-diaphorase: a detailed kinetic study and analysis of metabolites

DT-diaphorase has been implicated in the activation and mechanism of cytotoxicity of the investigational indoloquinone anticancer drug EO9. Here, we have used a highly purified DT-diaphorase isolated from rat Walker tumour cells to provide unambiguous evidence for the ability of this enzyme to catalyze reduction of EO9 and to provide a more detailed characterization of the reaction. Under the conditions used hypoxia had no effect on the initial rate of this reduction but did effect the nature and stability of metabolites formed. Electron spin resonance (ESR) spectrometry studies showed that DT-diaphorase reduced EO9 to a highly oxygen-sensitive metabolite that is probably the hydroquinone. In the presence of air, this metabolite is auto-oxidized to generate both drug- and oxygen-based radicals. Comproportionation:disproportionation reactions may also be involved in the generation of these radical species. The identification of these metabolites may contribute to the understanding of the molecular mechanism of DNA damage and cytotoxicity exerted by EO9.

KW-2149 (7-N-[2-[gamma-L-glutamylamino]ethyldithioethyl] mitomycin C): DNA interactions and drug uptake following serum activation

7-N-[2-[gamma-L-glutamylamino]ethyldithio-ethyl] mitomycin C (KW-2149) is a mitomycin-C analogue currently being evaluated in clinical trials. It has been shown that KW-2149 is unusual in that it is activated by serum, resulting in an increase in potency of up to 200-fold. To investigate the mechanism by which KW-2149 is activated, the abilities of mitomycin-C, KW-2149 and its metabolites M-18 (symmetrical disulphide dimer) and M-16 (methyl sulphide form) to interact with DNA were compared, and the influence of serum and glutathione on the sequence-specificity of KW-2149-DNA interactions was determined. Following reduction by glutathione both KW-2149 and M-18 are more efficient crosslinking agents of naked DNA, with the metabolite M-18 showing superior activity. The efficiency of DNA interstrand crosslinking in cells by KW-2149 was also increased by the addition of serum. Using the potassium/SDS precipitation method it was found that KW-2149 and M-18 crosslink protein to DNA whilst mitomycin C and M-16 do not. All four compounds produced almost identical patterns of adducts. Serum and glutathione did not alter the pattern of DNA adducts, but did increase the efficiency of adduct formation. Our earlier studies had indicated that the mechanism of activation of KW-2149 by serum is related to cellular uptake, and we therefore studied the effects of certain metabolic inhibitors, temperature and competitive inhibition on drug uptake. The results suggest that uptake is passive, and this indicates that a component in serum modifies KW-2149 to a form that passively enters cells more rapidly.

Giga-ohm seals on intracellular membranes: a technique for studying intracellular ion channels in intact cells

A method is outlined for obtaining giga-ohm seals on intracellular membranes in intact cells. The technique employs a variant of the patch-clamp technique: a concentric electrode arrangement protects an inner patch pipette during penetration of the plasma membrane, after which a seal can be formed on an internal organelle membrane. Using this technique, successful recordings can be obtained with the same frequency as with conventional patch clamping. To localize the position of the pipette within cells, lipophilic fluorescent dyes are included in the pipette solution. These dyes stain the membrane of internal organelles during seal formation and can then be visualized by video-enhanced or confocal imaging. The method can detect channels activated by inositol trisphosphate, as well as other types of intracellular membrane ion channel activity, and should facilitate studies of internal membranes in intact neurons and other cell types.

Sensitization of colorectal and pancreatic cancer cell lines to the prodrug 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB1954) by retroviral transduction and expression of the E. coli nitroreductase gene

Expression of genes encoding prodrug-activating enzymes can increase the susceptibility of tumor cells to prodrugs, and may ultimately achieve a better therapeutic index than conventional chemotherapy. CB1954 is a weak, monofunctional alkylating agent which can be activated by Escherichia coli nitroreductase to a potent dysfunctional alkylating agent which crosslinks DNA. We have inserted the nitroreductase gene into an LNCX-based retroviral vector, to allow efficient gene transfer and expression in colorectal (LS174T) and pancreatic (SUIT2, BxPC3, and AsPC1) cancer cell lines. A clone of LS174T cells expressing nitroreductase showed > 50-fold increased sensitivity to CB1954, and nitroreductase-expressing clones of pancreatic tumor lines were up to approximately 500-fold (SUIT2) more sensitive than parental cells. Concentrations of CB1954 minimally toxic to nontransduced cells achieved 100% cell death in a 50:50 mix of parental cells with SUIT2 cells expressing nitroreductase; and marked "bystander" cell killing was seen with just 10% of cells expressing nitroreductase. Significant bystander cell killing was dependent on a high cell density. In conjunction with regional delivery of vectors and tumor selectivity of cell entry and/or gene expression, nitroreductase and CB1954 may be an attractive combination for prodrug-activating enzyme gene therapy of colorectal and pancreatic cancer.

The bystander effect of the nitroreductase/CB1954 enzyme/prodrug system is due to a cell-permeable metabolite

The bystander effect is an important part of tumor kill using gene-directed enzyme prodrug therapy (GDEPT). Recently, we have described a novel enzyme prodrug system using bacterial nitroreductase and the prodrug CB1954 (NTR/CB1954). We demonstrate here the presence of a cell-permeable cytotoxic activity in the conditioned growth medium of nitroreductase (NTR)-transduced cells treated with CB1954 and show that its appearance corresponds to the appearance of two metabolites of CB1954 previously identified (Friedlos et al., 1992). The degree of bystander effect and the degree of transferred cytotoxicity correlates with the level of NTR enzyme expression. Two other prodrugs for NTR show little bystander killing and do not produce detectable cell permeable metabolites. The elucidation of the mechanism of the bystander effect may allow the more effective use of NTR/CB1954.

Regulation by insulin of a unique neuronal Ca2+ pool and of neuropeptide secretion

The insulin receptor is a tyrosine kinase receptor that is found in mammalian brain and at high concentrations in the bag cell neurons of Aplysia. We show here that insulin causes an acute rise in intracellular Ca2+ concentration ([Ca2+]i) in these neurons and triggers release of neuropeptide. The insulin-sensitive intracellular Ca2+ pool differs pharmacologically from previously described Ca2+ stores that are sensitive to inositol trisphosphate and from mitochondrial Ca2+ stores. Insulin, but not thapsigargin, stimulates Ca2+ release at the distal tips of neurites, the presumed site of neuropeptide secretion. The effects of insulin on intracellular Ca2+ release and neuropeptide secretion occur without triggering spontaneous action potentials. The insulin-sensitive rise in [Ca2+]i moves into the distal tips of neurites after exposure to a cyclic AMP analogue, a treatment that causes a similar translocation of neuronal vesicles. Our data indicate that Ca2+ release from a distinct intracellular pool associated with secretory vesicles may contribute to secretion of neuropeptide in the absence of neuronal discharge.

Involvement of DT-diaphorase (EC 1.6.99.2) in the DNA cross-linking and sequence selectivity of the bioreductive anti-tumour agent EO9

The chemistry of the mitomycin C-related drug indoloquinone EO9 would suggest that its mechanism of action is likely to involve DNA damage after reductive activation. The ability of this agent to induce DNA damage in intact cells has been examined using alkaline filter elution. After treatment with pharmacologically relevant concentrations of EO9, both DNA strand breaks and interstrand cross-links were detected in rat Walker tumour cells and human HT29 colon carcinoma cells. These cell lines express relatively high levels of DT-diaphorase (NAD(P)H: quinone acceptor oxidoreductase), which is believed to be involved in EO9 activation. The extent of DNA damage was increased by approximately 30-fold under hypoxia in BE colon carcinoma cells that express non-functional DT-diaphorase, but this dramatic hypoxia enhancement was not seen in HT-29 cells. These data are consistent with cytotoxicity studies that indicate that DT-diaphorase appears to be important in EO9 activation under aerobic conditions, but other enzymes may be more relevant under hypoxia. The involvement of DT-diaphorase in DNA damage induction was further investigated using cell-free assays. DNA cross-links were detectable in plasmid DNA co-incubated with EO9, cofactor and DT-diaphorase but not in the absence of this enzyme. In contrast, using a Taq polymerase stop assay, monofunctional alkylation was detected in plasmid DNA without metabolic activation, although the sequence selectivity was altered after reduction catalysed by DT-diaphorase.

Investigation of alternative prodrugs for use with E. coli nitroreductase in 'suicide gene' approaches to cancer therapy

The most commonly employed 'suicide' gene/prodrug system used in cancer gene therapy is the herpes simplex virus thymidine kinase (HSVtk)/ganciclovir system. We have examined the efficacy of an alternative approach utilising the E. coli nitroreductase B enzyme with CB1954 and a variety of other prodrugs. V79 cells transfected with a nitroreductase expression vector were up to 770-fold more sensitive to CB1954 than control non-expressing cells. In general other prodrugs which were found by HPLC to act as substrates for purified E. coli nitroreductase also exhibited increased cytotoxicity against the nitroreductase-expressing cells, although this correlation was not absolute. In particular nitrofurazone (97-fold) and additional aromatic nitro-compounds (nine- to 50-fold) showed a large differential whereas the quinones and the antimetabolite, B-FU, were less effective (< three-fold). The results support the possibility of using nitroreductase and CB1954 for 'suicide gene' therapy and in addition suggest that alternative prodrugs, such as nitrofurazone, warrant further investigation in this novel approach.

Ca2+ influx and activation of a cation current are coupled to intracellular Ca2+ release in peptidergic neurons of Aplysia californica

1. Stimulation of inputs to bag cell neurons in the abdominal ganglion of Aplysia californica causes an increase in their intracellular Ca2+ concentration ([Ca2+]i). We have used thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca2+ pump, to analyse the effects of Ca2+ released from intracellular stores on the electrophysiological responses of bag cell neurons. 2. Using digital imaging of fura-2-loaded isolated bag cell neurons we found that thapsigargin rapidly evoked an increase in [Ca2+]i in somata, with smaller increases in neurites. Thapsigargin-induced elevation of [Ca2+]i peaked at about 1 microM within 5-10 min and then decayed to basal levels by 30 min. 3. Placement of an extracellular vibrating Ca(2+)-selective microelectrode to within 1 micron of somata revealed a relatively large steady-state Ca2+ efflux. Thapsigargin produced a rapid increase in Ca2+ influx. Changes in Ca2+ flux were not detected at neurites. 4. Thapsigargin produced a small depolarization in isolated bag cell neurons in artificial sea water (ASW). Sometimes enhanced depolarizations were observed when extracellular Na+ was replaced by TEA or Tris, but not N-methyl-D-glucamine (NMDG). The depolarization was not blocked by 100 microM tetrodotoxin (TTX), removal of extracellular Ca2+ (0.5 mM EGTA) or addition of 10 mM Co2+ to the bath solution. 5. In voltage-clamp experiments, thapsigargin induced an inward current (ITg) that was recorded in Ca(2+)-free media containing TEA or Tris substituted for Na+. The apparent reversal potential of ITg was -16.8 +/- 1.2 mV in TEA-ASW. Induction of ITg was inhibited in neurons that were microinjected with the Ca2+ chelator BAPTA-Dextran70 or treated with the membrane-permeant analogue BAPTA AM. Activation of ITg was not observed when Na+ was replaced with NMDG. Manipulation of [Na+]o and [K+]o produced shifts in the reversal potential of ITg consistent with the underlying channels being permeable to both Na+ and K+. 6. Thapsigargin did not alter the amplitude or kinetics of voltage-activated Ba2+ currents, but in some experiments it did increase the amplitude of a component of outward K+ current. 7. Thapsigargin neither induced bag cell neurons within the intact ganglion to depolarize and fire spontaneously, nor did it alter the frequency or duration of firing of an electrically stimulated bag cell after-discharge. 8. We conclude that thapsigargin-sensitive Ca2+ pools are present predominantly in the somata of bag cell neurons. Ca2+ that is released from thapsigargin-sensitive Ca2+ stores activates a non-selective cation current that may help sustain depolarization of the somata, but does not by itself trigger an after-discharge.

Insulin receptor in Aplysia neurons: characterization, molecular cloning, and modulation of ion currents

We have isolated the cDNA for a tyrosine kinase receptor that is expressed in the nervous system of Aplysia californica and that is similar to the vertebrate insulin receptor. Binding studies and immunocytochemical staining show that the receptor is abundant in the bag cell neurons. Application of vertebrate insulin to clusters of bag cell neurons stimulates the phosphorylation of the receptor on tyrosine residues, and exposure of isolated bag cell neurons to insulin produces an increase in height and a decrease in duration of the action potentials that can be detected within 15-30 min. These effects were not seen with insulin-like growth factor-1. In voltage-clamped neurons, insulin produces an increase in the amplitude of the voltage-dependent Ca2+ current that can be blocked by preincubation with herbimycin A, an inhibitor of tyrosine kinases. Insulin also enhances a delayed K+ current. We suggest that insulin-like peptides regulate the excitability of the bag cell neurons.

Expression of the bacterial nitroreductase enzyme in mammalian cells renders them selectively sensitive to killing by the prodrug CB1954

A recombinant retrovirus encoding E. coli nitroreductase (NTR) was used to infect mammalian cells. NIH3T3 cells expressing NTR were killed by the prodrug CB1954, which NTR converts to a bifunctional alkylating agent. Admixed, unmodified NIH3T3 cells could also be killed. In contrast to the Herpes simplex virus (HSV) thymidine kinase (TK)/ganciclovir(GCV) enzyme/prodrug system, NTR/CB1954 cell killing was effective in non-cycling cells. Co-operative killing was observed when cells expressing both NTR and TK were treated with a combination of CB1954 and GCV. NTR expression in human melanoma, ovarian carcinoma or mesothelioma cells also rendered them sensitive to CB1954 killing. These data suggest that delivery of the NTR gene to human tumours, followed by treatment with CB1954, may provide a novel tumour gene therapy approach.