Marizomib, a proteasome inhibitor for all seasons: preclinical profile and a framework for clinical trials

The proteasome has emerged as an important clinically relevant target for the treatment of hematologic malignancies. Since the Food and Drug Administration approved the first-in-class proteasome inhibitor bortezomib (Velcade) for the treatment of relapsed/refractory multiple myeloma (MM) and mantle cell lymphoma, it has become clear that new inhibitors are needed that have a better therapeutic ratio, can overcome inherent and acquired bortezomib resistance and exhibit broader anti-cancer activities. Marizomib (NPI-0052; salinosporamide A) is a structurally and pharmacologically unique β-lactone-γ-lactam proteasome inhibitor that may fulfill these unmet needs. The potent and sustained inhibition of all three proteolytic activities of the proteasome by marizomib has inspired extensive preclinical evaluation in a variety of hematologic and solid tumor models, where it is efficacious as a single agent and in combination with biologics, chemotherapeutics and targeted therapeutic agents. Specifically, marizomib has been evaluated in models for multiple myeloma, mantle cell lymphoma, Waldenstrom's macroglobulinemia, chronic and acute lymphocytic leukemia, as well as glioma, colorectal and pancreatic cancer models, and has exhibited synergistic activities in tumor models in combination with bortezomib, the immunomodulatory agent lenalidomide (Revlimid), and various histone deacetylase inhibitors. These and other studies provided the framework for ongoing clinical trials in patients with MM, lymphomas, leukemias and solid tumors, including those who have failed bortezomib treatment, as well as in patients with diagnoses where other proteasome inhibitors have not demonstrated significant efficacy. This review captures the remarkable translational studies and contributions from many collaborators that have advanced marizomib from seabed to bench to bedside.

Phase I trial of NPI-2358 (a novel vascular disrupting agent) plus docetaxel

3571

Background: NPI-2358 is a vascular disrupting agent (VDA) that destabilizes tumor vascular endothelial cell architecture resulting in selective collapse of established tumor vasculature. In murine tumor models NPI-2358 produces tumor regression alone and synergistically with agents such as paclitaxel, docetaxel (TXT) and irinotecan, often with decreased toxicity. Methods: The objective was to determine the Recommended Phase 2 Dose (RP2D) of NPI-2358 in combination with TXT. Patients with previously treated advanced NSCLC or other malignancies where TXT use was appropriate were enrolled. Patients received 75 mg/m2 TXT every 21 days. NPI-2358 was administered IV 2 hours after TXT on Day 1, and alone on Day 8. The dose of NPI-2358 was escalated from the single agent biologic effect dose (BED) of 13.5 mg/m2 to the single agent RP2D of 30 mg/m2 using a 3+3 design. PK was assessed on Days 1 and 8. Results: 13 patients were enrolled of whom 10 had NSCLC. At all dose levels adverse events were consistent with those of both agents given alone. Adverse events commonly associated with NPI-2358 were seen, including nausea, vomiting, fatigue, fever, tumor pain and transient blood pressure elevations. One DLT of nausea, vomiting, dehydration and neutropenia occurred at 30mg/m2. PK analysis did not indicate a drug-drug interaction. Of the patients with NSCLC, 8 had measureable disease of which 2 demonstrated a partial response (PR), with 4 others having lesser regressions. The RP2D was thus 30 mg/m2 of NPI-2358 with 75 mg/m2 TXT. Conclusions: The combination of full dose NPI-2358 and TXT is tolerable. Although a limited data set, activity appears favorable relative to the 5–10% response rate reported with TXT alone in this population. Based on these results, efficacy is now being assessed in Phase 2 (the ADVANCE study), a randomized comparison of TXT ± 30 mg/m2 of NPI-2358 in 2nd- line NSCLC. Of note, entry criteria allow patients with squamous cell carcinoma, as VDAs do not appear to result in unfavorable outcomes associated with some other agents in this sub-population.

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Clinical trial of the novel structure proteasome inhibitor NPI-0052 in patients with relapsed and relapsed/refractory multiple myeloma (r/r MM)

8505

Background: NPI-0052 produces rapid, broad and prolonged inhibition of all 3 catalytic activities of the 20S proteasome. NPI-0052 has a non-peptide based structure that appears to result in a unique proteasome inhibition (PI), signal transduction and safety profile. Preclinical data suggest NPI-0052 may demonstrate an improved therapeutic ratio, with significant activity in hematologic and solid tumor malignancies including MM resistant to bortezomib (BZ) and other agents. Methods: This phase I dose escalation study of NPI-0052 was conducted in patients (pts) with r/r MM, including after BZ and/or lenalidomide. Pts were treated with NPI-0052 IV weekly for 3 weeks in 4-week cycles. The dose of NPI-0052 was escalated utilizing a combination of accelerated titration and 3+3 designs. PI and PK were assayed after the 1st and 3rd doses. Results: 17 patients have been treated at doses ranging from 0.025 mg/m2 to 0.6 mg/m2 without reaching a MTD. One patient experienced reversible creatinine elevation associated with progression of light chain nephropathy at 0.075 mg/m2. Drug- related adverse events have consisted principally of mild-to-moderate fatigue, nausea and diarrhea. In this study at dose levels 0.75 and 0.15 mg/m2, the half-life = 0.5 - 5.0 min; clearance = 4.8 ± 5.2 L/min and Vz = 22.6 ± 44.2 L. PI in whole blood demonstrates dose-dependent chymotrypsin-like inhibition up to 26% at 0.075 mg/m2. Of note, 2 patients remained on study for over 6 months and one year with stable disease and no significant toxicity. Importantly, NPI-0052 does not appear to induce peripheral neuropathy or myelosuppression. Conclusions: NPI-0052 was well tolerated in this study with escalation continuing to a recommended phase II dose (RP2D) for r/r MM. The RP2D (0.7 - 0.8 mg/m2), PI and safety profiles of NPI-0052 have been defined in other diagnoses, and appear unique from BZ in spite of up to 100% proteasome inhibition. In this study, NPI-0052 has effects on PD, PK and may potentially confer clinical benefit at doses which are well below the MTD determined in the other trials. Additional trials in MM are being initiated, including with lenalidomide.

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First-in-human phase I study of the novel structure proteasome inhibitor NPI-0052

3516

Background: NPI-0052 is a second-generation proteasome inhibitor that has a novel structure with preclinical studies suggesting unique proteasome inhibition (PI), signal transduction, efficacy and toxicology profiles, and activities in a broad range of models including bortezomib (BZ) resistant models. Methods: Patients were treated with NPI-0052 IV weekly x 3 in 4-week cycles in a 3+3 design dose escalation to a Recommended Phase 2 Dose (RP2D). PI was assayed in whole blood (PWB) and in PBMCs. Up to 10 patients each (lymphoma and solid tumors) are treated at the RP2D. Results: 39 patients have been treated between 0.0125 mg/m2 to 0.8 mg/m2 for up to 12 cycles. The RP2D was 0.8 mg/m2 and has been well tolerated in this study, with the most notable drug-related adverse events being N/V, fatigue, diarrhea, dizziness, headache, decreases in hemoglobin, and insomnia. Thrombocytopenia or neuropathy did not appear to be induced by NPI-0052. Escalation was halted at 0.8 mg/m2, as DLT of transient “hallucinations” (visual imprints when eyes closed) and dizziness/unsteady gait were reported in another study at 0.9 mg/m2. PK data indicate T1/2 ∼4–10 minutes, with clearance at 3–15 L/min and Vz of 35–85L. PI (chymotrypsin-like activity) in PWB and PBMC were dose dependent (mean D1 and D15 PI in PWB at 0.8 mg/m2 was 75 and 100%) (efficacious doses of BZ have been reported to result in a range of ∼40–70% PI). PI in PBMC returned to ∼baseline within one week of each dose, while inhibition remained in PWB. Stable disease was observed in patients with cervical carcinoma (11 months; maximum response was 24%; PI increased with dose escalation), colorectal, hepatocellular, adenoid cystic, melanoma, granulosis cell and ovarian. Conclusions: NPI-0052 produces dose-dependent pharmacologic effects, with equal to greater PI seen than reported with therapeutic doses of BZ. At the RP2D toxicity is mild-to-moderate, without the toxicity profile associated with BZ. Together with preclinical data this suggests NPI-0052 may have utility in treating patients who are not candidates for BZ as well as in indications and combinations that have not supported BZ use. Additional trials have been initiated including combinations with other targeted agents.

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Clinical trial of NPI-0052 in advanced malignancies including lymphoma and leukemia (advanced malignancies arm)

3582

Background: The novel structure (non-peptide based) of NPI-0052 (NPI) appears to lead to unique proteasome inhibition (PI), toxicology and signal transduction profiles. Preclinical research suggests improvements in therapeutic ratio and activity in hematologic and solid tumor models, leading to clinical trials in patients with myeloma, lymphomas, leukemias, and solid tumors. Methods: Patients with solid tumor, lymphoma or leukemia diagnoses without standard treatment options were treated with IV NPI on Days 1, 8 and 15 of 28-day cycles in a 3+3 design dose escalation to a Recommended Phase 2 Dose (RP2D). Enrollment then began in 10 patient lymphoma and CLL RP2D cohorts. PI (D1 and D15) and PK (D1 and D15) were assayed. Results: 30 patients were treated at doses ranging from 0.1 mg/m2 to 0.9 mg/m2. 0.7 mg/m2 was selected as the RP2D secondary to DLT of transient “hallucinations” (visual imprints when eyes closed) and dizziness/unsteady gait at 0.9 mg/m2. At the RP2D fatigue, parosmia/dysgeusia, transient peri-infusion site pain and lymphopenia were commonly ascribed to NPI. At the RP2D PK data showed: half-life = 31 ± 28 min; AUCtotal = 270 ± 219 ng/mL*min; Cmax = 33.4 ± 34.2 ng/mL; clearance = 7.17 ± 0.40 L/min; volume of distribution (Vz) = 223.3 ± 229.7 L. PI was assayed in blood, indicating a dose:response relationship with mean inhibition of chymotrypsin-like activity up to of 88% Day 1 and 100% Day 15, and inhibition of caspase-like and trypsin-like activity of up to 51% and 72%. PI remained between doses in whole blood (RBC), but recovered between doses in PBMC. Stable disease was induced in 31% of patients, including one each with mantle cell, Hodgkin's lymphoma, follicular lymphoma, sarcoma, prostate carcinoma, and two with melanoma. Conclusions: NPI-0052 produces dose-dependent pharmacologic effects through the predicted efficacious range to an MTD, while producing a toxicity profile that is tolerable and dissimilar to bortezomib in spite of reaching higher PI levels. These data indicate potential for a greater range of uses than other proteasome inhibitors and lead to additional studies being initiated in hematologic malignancies and solid tumors alone and in combination.

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A phase 1 dynamic accelerated titration dose escalation study of the vascular disrupting agent NPI-2358

14097

Background: NPI-2358 is a novel tumor vascular disrupting agent acting on β-tubulin that destabilizes tumor vascular endothelial cells with an additional direct cytotoxic activity. NPI-2358 selectively induces tumor vascular collapse and tumor regression in multiple murine tumor models and also potentiates the effects of chemotherapeutic agents. Preclinical data indicate NPI-2358 may have advantages in terms of safety profile and activity (tumor blood flow remains markedly reduced after 24 hours). Methods: A Phase 1 study of NPI-2358 is being conducted in patients with solid tumors and lymphomas. Patients in this study were treated with NPI-2358 administered as a weekly IV infusion for 3 weeks in 4-week cycles. Eight subjects have been enrolled. The dose of NPI-2358 was escalated in cohorts of 1–3 patients dependent on observed adverse events (any cohort may consist of 1 patient provided no =Grade 2 AE is reported in the prior cohort). A cohort is expanded to 6 patients if a DLT is reported. Escalation was conducted in 100% intervals, and decreased to 50% intervals once a Grade 2 adverse event was reported. In addition to weekly safety monitoring (including ECGs, Troponin I, and blood pressure), echocardiography and pharmacokinetics were performed on Days 1 and 15 (D1 & D15), and a DCE-MRI obtained 4 hours after the first dose was compared to baseline. Results: The dose has been escalated from 2 mg/m2 to 9 mg/m2 (predicted minimum efficacious dose = 7.5 mg/m2) without evidence of dose-limiting toxicity or other significant toxicities including cardiovascular or neurotoxicity. Intrapatient values for Ktrans from DCE-MRIs appear highly reproducible. No responses have been reported, however, one patient with pancreatic adenocarcinoma has had stable disease (SD) for 3 cycles and continues to receive NPI-2358. Average Cmax and AUClast have increased from 16.2 ng/ml to 147 ng/ml, and from 101 to 360 ng/ml*hr respectively. No drug accumulation was observed; T1/2 was 4.2 ±1.9 h, clearance was 45 ±12 L/h and distributive volume was 251±57 L. Conclusions: NPI-2358 is well tolerated at doses above the minimum predicted efficacious dose. PK data show increasing Cmax and AUC with dose escalation and no drug accumulation on a weekly schedule.

No significant financial relationships to disclose.

Synthesis of a novel family of diterpenes and their evaluation as anti-inflammatory agents

The synthesis and biological evaluation of a new family of diterpenes, represented by structures 2 and 3, is presented. These compounds constitute isomeric analogues of acanthoic acid (1) and were examined as potent anti-inflammatory agents. Among them, methyl ester 12 exhibited a low non-specific cytotoxicity, inhibited TNF-alpha synthesis and displayed good specificity in suppressing cytokine expression.

The subtype-selective nicotinic acetylcholine receptor agonist SIB-1553A improves both attention and memory components of a spatial working memory task in chronic low dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys

Monkeys that receive chronic low dose (CLD) 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration develop deficits in spatial delayed-response task performance. The present study examined the extent to which SIB-1553A [(+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride], a novel neuronal nicotinic acetylcholine receptor (nAChR) agonist with selectivity for beta4 subunit-containing nAChRs, could counteract this cognitive deficit produced by CLD MPTP exposure. Prior to MPTP treatment, monkeys displayed a delay-dependent decrement in performance on a variable delayed response task. CLD MPTP treatment caused a shift to a delay-independent pattern of responding on this task, such that short-delay trials were performed as poorly as long-delay trials. At lower doses (e.g., 0.025 mg/kg), SIB-1553A significantly improved performance on short-delay trials but only at 24 h after drug administration. At higher doses (e.g., 0.50 mg/kg), SIB-1553A significantly improved performance on both short- and long-delay trials at both 20 min and 24 h after drug administration. When tested 24 h after drug administration, monkeys performed long-delay trials with greater accuracy than they did under normal (pre-MPTP) conditions. These results suggest that at lower doses, SIB-1553A may be more effective in improving attentional deficits associated with CLD MPTP exposure, whereas at higher doses, SIB-1553A may effectively improve both attentional and memory performance.

SIB-1553A, (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride, a subtype-selective ligand for nicotinic acetylcholine receptors with putative cognitive-enhancing properties: effects on working and reference memory performances in aged rodents and nonhuman primates

Preclinical and clinical data have suggested the potential use of nicotinic acetylcholine receptor (nAChR) ligands for treating cognitive dysfunction associated with neurodegenerative diseases, such as Alzheimer's disease. SIB-1553A, (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride, a novel nAChR ligand with predominant agonist subtype selectivity for beta4 subunit-containing human neuronal nAChRs, was tested in a variety of cognitive paradigms in aged rodents and nonhuman primates after acute and repeated administration. Subcutaneous administration of SIB-1553A improved delayed nonmatching to place performance in aged mice. In aged rhesus monkeys, intramuscular and oral administration of SIB-1553A improved choice accuracy in a delayed matching to sample task. SIB-1553A improved performances in these spatial and nonspatial working memory tasks but was less effective at improving performances in spatial reference memory tasks (i.e., aged rodents exposed to a discrimination task in a T-maze or trained to locate a hidden platform in a water maze). These data suggest that SIB-1553A has a predominant effect on attention/working memory processes. SIB-1553A also induced the release of acetylcholine in the hippocampus of aged rats and was equally effective whether administered acutely or repeatedly (6 weeks of daily subcutaneous administration). Thus, rats repeatedly treated with SIB-1553A exhibit neither tolerance nor sensitization to the effects of the compound. The SIB-1553A-induced cognitive improvement may be in part related to an increase in cholinergic function. The present study provides additional support for the use of subtype-selective nAChR ligands as a potential therapy for the symptomatic treatment of specific cognitive deficits (such as attention/working memory deficits) associated with aging and neurological diseases.

The antinociceptive effect of intrathecal administration of epibatidine with clonidine or neostigmine in the formalin test in rats

The analgesic effect of intrathecal injection of epibatidine, clonidine and neostigmine, compounds that elevate ACh, was examined in the formalin test, a model of post-injury central sensitization in the rat. The compounds were injected alone and in combination. Intrathecal injection of epibatidine alone did not alter pain behaviors, compared to vehicle-treated rats. Intrathecal injection of clonidine dose-dependently reduced tonic pain behaviors (ED(50)+/-95% confidence limits=6.7+/-4.8 microg). The combination of clonidine and epibatidine (C:E), in the ratio of 26:1, dose-dependently reduced tonic pain behaviors; and the ED(50) of C:E was 1.1+/-0.98 microg a significant 6-fold leftward shift of the dose response curve, compared with clonidine alone. The antinociceptive effect of C:E (26:1) was attenuated by pre-treatment with the nAChR antagonist mecamylamine. Neostigmine dose-dependently reduced tonic pain behaviors (ED(50)=1.5+/-1.3 microg). The combination of neostigmine and epibatidine, in a ratio of 8:1, significantly shifted the dose response curve 4-fold to the left (ED(50)=0.4+/-0.3 microg). The effect is mediated in part by the activation of the nAChR and possibly by the enhanced release of ACh. These data demonstrate significant enhancement of the antinociceptive effects of spinally delivered analgesics by a nAChR agonist, suggesting that this class of compounds may have utility as adjuvants when combined with conventional therapeutics.

Differential alterations in nicotinic receptor alpha6 and beta3 subunit messenger RNAs in monkey substantia nigra after nigrostriatal degeneration

Our previous studies showed that alpha4, alpha6, alpha7, beta2, beta3 and beta4 nicotinic acetylcholine receptor messenger RNAs are present in monkey substantia nigra, with a particularly intense and localized labelling of the alpha6 and beta3 subunit messenger RNAs to this brain region. Because loss of nigrostriatal neurons is a central feature of Parkinson's disease and evidence suggests that nicotinic agonists potentiate antiparkinsonian effects of L-dopa, experiments were done to determine whether nicotinic receptor subunit messenger RNAs and binding sites were altered in the basal ganglia after nigrostriatal degeneration. Squirrel monkeys (Saimiri sciureus) were rendered parkinsonian by systemic injection of the selective dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine. Behavioral studies showed that this treatment decreased baseline motor activity to 36+/-11% of control. One month after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration, caudate and putamen dopamine levels were reduced to 51+/-15% and 43+/-6% of control, respectively, while the number of tyrosine hydroxylase-positive neurons in the substantia nigra was 75+/-6% of control. Despite the reduction in nigral cell number after nigrostriatal degeneration, there were no changes in alpha4, alpha7, beta2 and beta4 messenger RNA levels in the substantia nigra. In contrast, alpha6 mRNA levels were significantly increased (143+/-10%) and the beta3 transcript decreased (62+/-6%) in the substantia nigra after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Declines were also observed in [125I]epibatidine binding in both the caudate-putamen and substantia nigra, with no change in alpha7 receptor binding. These results may suggest a dissociation in the regulation of receptor messenger RNA and binding sites, and/or that there are differential alterations in the different receptor subtypes measured using [125I]epibatidine. The changes in the two nicotinic receptor subunit messenger RNAs, alpha6 and beta3, which exhibit a selective localization to the substantia nigra, may indicate that nicotinic receptors containing these subunits are altered after nigrostriatal degeneration.

Antidepressant-like effects of the subtype-selective nicotinic acetylcholine receptor agonist, SIB-1508Y, in the learned helplessness rat model of depression

RATIONALE

Epidemiological studies of smokers suggest that there is a link between nicotine and depression. Nonetheless, few studies have examined the potential use of nicotinic ligands in the treatment of depression.

OBJECTIVES

The goal of this study was to evaluate the effects of SIB-1508Y, a novel subtype-selective ligand for high affinity nicotinic acetylcholine receptors (nAChRs), in the learned helplessness model of depression in rats.

METHODS

In this model, exposure to inescapable foot-shock produces a lasting deficit in escape responses emitted in a subsequent conditioned avoidance procedure (learned helplessness). The effect of SIB-1508Y on learned helplessness was compared to the clinically used antidepressants, imipramine and fluoxetine, and the non-selective nAChR ligand, nicotine.

RESULTS

Similarly to imipramine and fluoxetine, subchronic treatment (5 days) with SIB-1508Y reversed the escape deficit in the learned helplessness model in a dose dependent manner. The effect of SIB-1508Y on learned helplessness was still apparent 1 week following drug administration and was also maintained after 4 weeks of daily administration. In contrast, while nicotine was able to attenuate the learned helplessness deficit, this trend only reached statistical significance after chronic administration. The non-competitive ion channel blocker mecamylamine increased escape failures when administered alone and blocked the effects of SIB-1508Y but not imipramine. SIB- 1508Y also produced an increase in avoidance responding, which suggests an enhancement of learning.

CONCLUSION

These results not only suggest a role for nAChRs in the development of a depressive-like syndrome, but also that subtype-selective nAChR agonists, such as SIB-1508Y, may offer a novel therapeutic approach to the treatment of depression.

Localization of nicotinic receptor subunit mRNAs in monkey brain by in situ hybridization

Nicotinic receptors are implicated in memory, learning, locomotor activity, and addiction. Identification of the specific receptor subtypes that mediate these behaviors is essential for understanding their role in central nervous system (CNS) function. Although expression of nicotinic receptor transcript has been studied in rodent brain, their localization in the monkey CNS, which may be a better model for the human brain, is not yet known. We therefore investigated the distribution of alpha4, alpha6, alpha7, beta2, beta3, and beta4 receptors subunit mRNAs in the monkey brain by using in situ hybridization. alpha4 and alpha7 mRNAs were very widely expressed, with a substantial degree of overlap in their distribution, except for the reticular nucleus of the thalamus in which alpha7 mRNA was much more prominently expressed. beta2 and beta4 mRNA were also widely distributed, although beta4 was more prominently localized in thalamic nuclei than beta2. The distribution of alpha6 and beta3 mRNA was very distinct from that of the other transcripts, being restricted to catecholaminergic nuclei, the cerebellum, and a few other areas. Although there were similarities in distribution of the nicotinic receptor subunit mRNAs in monkey and rodent brain, there were prominent differences in areas such as the caudate, putamen, locus coeruleus, medial habenula, and cerebellum. In fact, the distribution of alpha4 and alpha7 mRNAs in the monkey caudate and putamen was more similar to that reported in the human than rodent brain. These findings have implications for the development of drug therapies for neurological disorders, such as Alzheimer's and Parkinson's disease, in which nicotinic receptors are decreased.

Recombinant human receptors and functional assays in the discovery of altinicline (SIB-1508Y), a novel acetylcholine-gated ion channel (nAChR) agonist

Neuronal nicotinic acetylcholine receptors (nAChRs) are a class of ion channels with significant potential as molecular targets for the design of drugs to treat a variety of CNS disorders. The discovery that neuronal nAChRs are further subdivided into multiple subtypes suggests that drugs which act selectively at specific nAChR subtypes might effectively treat Parkinson's disease (PD), Alzheimer's disease (AD), schizophrenia, ADHD, depression, anxiety or pain without the accompanying adverse side effects associated with non-selective agents such as nicotine (1) and epibatidine. Altinicline (SIB-1508Y) is a novel, small molecule designed to selectively activate neuronal nAChRs and is undergoing clinical evaluation for the treatment of PD. It was selected from a series of compounds primarily on the basis of results from functional assays, including (a) measurement of Ca2+ flux in stable cell lines expressing specific recombinant human neuronal nAChR subtypes; (b) determination of in vitro and in vivo neurotransmitter release; (c) in vivo models of PD. Biological data on both altinicline and the series of compounds from which it was selected are reported.

Nicotinic acetylcholine receptor agonist SIB-1508Y improves cognitive functioning in chronic low-dose MPTP-treated monkeys

Monkeys that receive chronic low-dose 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) administration have difficulty performing numerous cognitive tasks. This study further examines the extent to which chronic low-dose MPTP exposure affects performance of a visual memory task [variable delayed response (VDR)] with both attentional and short-term memory components and assesses the effects of the novel neuronal nicotinic acetylcholine receptor agonist SIB-1508Y and levodopa on cognitive task performance. Before MPTP treatment, these monkeys displayed a delay-dependent decrement in performance on the VDR task and performed well on delayed matching-to-sample and visual pattern discrimination tasks. Chronic low-dose MPTP treatment caused a shift to a delay-independent pattern of responding on the VDR task, such that short-delay trials were performed as poorly as long-delay trials. There were also deficits in performing the delayed matching-to-sample task, whereas visual discrimination performance remained intact. SIB-1508Y normalized the pattern of response on the VDR task by significantly improving performance on short-delay trials and on the delayed matching-to-sample task. These effects lasted up to 24 to 48 h after SIB-1508Y administration. Neither levodopa nor nicotine significantly improved task performance. These results suggest that chronic low-dose MPTP exposure results in a cognitive disturbance that can be corrected by the nicotinic acetylcholine receptor agonist SIB-1508Y but not by levodopa. Thus, SIB-1508Y may be useful in the treatment of the cognitive deficits in Parkinson's disease.

Correlation of nicotinic binding with neurochemical markers in Alzheimer's disease

The loss of neocortical synapses that occurs in Alzheimer's disease (AD) has been shown to correlate with cognitive decline. In addition, marked losses in the cholinergic system in AD, specifically choline acetyltransferase (ChAT) activity and high affinity presynaptic neuronal nicotinic cholinergic receptors (nAChRs), have also been described. We hypothesized that in AD, the loss of [3H]-ligand binding to nAChRs, which are largely presynaptic, would correlate with changes in two other presynaptic markers: synaptophysin (Syn), a measure of synaptic density, and ChAT activity. The midfrontal (MF) cortex of 36 autopsy confirmed (NIA and CERAD criteria) AD patients (mean death age +/- SD 80.1 +/- 8.4 years) who met NINDS-ADRDA criteria for a clinical diagnosis of probable or possible AD, and 11 nondemented controls (mean death age +/- SD 77.9 +/- 8.0) were examined. Synapse counts were quantified by a dotimmunobinding assay for Syn. ChAT activity was assessed by standard biochemical assays. Nicotinic cholinergic receptor binding was assayed using the high affinity nicotinic agonist [3H]-(+/-)-epibatidine ([3H]-EPI). The mean +/- SD Syn in AD (83.4 +/- 31.9 arbitrary units (AU)/mg protein) was significantly lower than controls (126.1 +/- 19.9, p = 0.0003; t-test). The mean ChAT activity in AD (139.0 +/- 75.6 nmol ACh/hr/100 mg protein) was significantly lower than controls (219.6 +/- 70.8, p = 0.004). The mean [3H]-EPI total binding in AD (6.2 +/- 2.8 fmol/mg protein) was significantly lower than controls (14.8 +/- 3.2; p < 0.0001). Syn correlated with [3H]-EPI binding in AD (r = 0.48, p = 0.006; Pearson) but ChAT did not (r = -0.20, p = 0.34). We conclude that loss of high affinity nAChR binding correlates with loss of synapses in AD. The lack of correlation between [3H]-EPI binding and ChAT activity suggests that the targeted receptor populations may not be located exclusively on cholinergic neurons.

Conformationally restricted analogues of nicotine and anabasine

A series of conformationally restricted analogues of nicotine has been synthesized and evaluated as agonists of neuronal acetylcholine receptors. Compound 2 (SIB-1663), which selectively activated human recombinant alpha 2 beta 4 and alpha 4 beta 4 nAChRs, was shown to be active in animal models of Parkinson's disease and pain.

Effects of SIB-1508Y, a novel neuronal nicotinic acetylcholine receptor agonist, on motor behavior in parkinsonian monkeys

The potential antiparkinsonian effects of the centrally acting, subtype-selective neuronal nicotinic acetylcholine receptor agonist (S)-(-)-5-ethynyl-3-(1-methyl-2-pyrrolidinyl)-pyridine (SIB-1508Y) was assessed on motor symptoms and disability scale ratings in three monkeys previously made parkinsonian by chronic exposure to the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Compared with levodopa (L-dopa), SIB-1508Y exerted only mild antiparkinsonian effects when administered alone. Emetic effects of this drug interfered with potential therapeutic effects at higher doses. However, when a low, ineffective dose of SIB-1508Y was combined with low, ineffective doses of L-dopa, a significant clinical effect was observed. These data suggest that subtype-selective nicotinic acetylcholine receptor agonists may hold promise as antiparkinsonian agents, and when administered in combination with L-dopa may allow a reduction in the dose of L-dopa needed to achieve a significant clinical effect.

6-hydroxydopamine lesion of rat nigrostriatal dopaminergic neurons differentially affects nicotinic acetylcholine receptor subunit mRNA expression

Nicotinic acetylcholine receptor (nAChR) subunit mRNA expression in the rat substantia nigra (SN) was assayed by semiquantitative RT-PCR following 6-hydroxydopamine (6-OHDA) lesion of nigrostriatal dopaminergic neurons. Six months after unilateral injection of 6-OHDA or saline into the SN, total RNA was isolated from ipsilateral and contralateral tissue samples. RT-PCR amplifications were performed with template titration using primers specific for sequences encoding 1. nAChR alpha 2-alpha 7 and beta 2-beta 4 subunits 2. Glutamic acid decarboxylase 3. Glyceraldehyde 3-phosphate dehydrogenase for normalization of template mass. PCR products specific for alpha 3, alpha 4, alpha 5, alpha 6, alpha 7, beta 2, beta 3, and glutamic acid decarboxylase were detected in the reactions containing SN RNA. This is the first evidence that alpha 7 may be expressed in the SN. alpha 2 and beta 4 PCR products were not detected in SN reactions, although they were observed in hippocampus and thalamus control reactions. A comparison of ipsilateral and contralateral SN RT-PCR reaction products showed substantial decreases in alpha 5, alpha 6, and beta 3 product yields following 6-OHDA, but not sham treatment. Neither the SN of sham-lesioned rats nor the thalamus of 6-OHDA-lesioned rats yielded similar results, indicating that the effects observed in 6-OHDA-treated SN were not caused by local mechanical damage or a nonspecific response, respectively. Effects of 6-OHDA treatment on alpha 3, alpha 4, alpha 7, beta 2, or glutamic acid decarboxylase product yields from SN samples were small or undetectable. The results suggest that alpha 5, alpha 6, and beta 3 subunit-encoding mRNAs are expressed at substantially higher levels in dopaminergic than in nondopaminergic cell bodies in the SN.

The potential of subtype-selective neuronal nicotinic acetylcholine receptor agonists as therapeutic agents

Neuronal nicotinic acetylcholine receptors (NAChRs) are pentameric ligand-gated ion channel receptors which exist as different functional subunit combinations which apparently subserve different physiological functions as indicated by molecular biological and pharmacological techniques. It is possible to design and synthesize novel compounds that have greater selective affinities and efficacies than nicotine for different NAChRs, which should translate into different behavioral profiles and therapeutic potentials. Examples of NAChR agonists studied are nicotine, SIB-1508Y, SIB-1553A and epibatidine. These compounds have different degrees of selectivity for human recombinant NAChRs, different neurotransmitter release profiles in vitro and in vivo and differential behavioral profiles. Preclinical studies suggest that SIB-1508Y is a candidate for the treatment of the motor and cognitive deficits of Parkinson's disease, whereas SIB-1553A appears to have potential as a candidate for the treatment of Alzheimer's disease. Epibatidine has a strong analgesic profile, however the ratio between pharmacological activity and undesirable effects is so low that it is difficult to envisage the use of this compound therapeutically. Nicotine has a broad profile of pharmacological activity, for instance demonstrating activity in models for cognition and analgesia. As for epibatidine, the adverse effects of nicotine severely limits its therapeutic use in humans. The discovery of subtype-selective NAChR agonists such as SIB-1508Y and SIB-1553A provides a new class of neuropsychopharmacological agents with better therapeutic ratios than nonspecific agents such as nicotine.

Effects of the nicotinic acetylcholine receptor agonist SIB-1508Y on object retrieval performance in MPTP-treated monkeys: comparison with levodopa treatment

This study assessed the relative potencies of levodopa/benserazide and the nicotinic acetylcholine receptor agonist SIB-1508Y on reversal of cognitive and motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys performing an object retrieval task. Monkeys previously taught to perform this task developed significant cognitive deficits after chronic low-dose MPTP exposure. These monkeys then received additional MPTP treatment to superimpose a parkinsonian movement disorder on their preexisting cognitive deficits. Levodopa/benserazide treatment significantly improved motor aspects of object retrieval performance but did not significantly improve cognition. SIB-1508Y (1 mg/kg) alone did not result in a statistically significant improvement in cognition or motor function in symptomatic MPTP-lesioned animals with deficits in both of these areas. However, the combination of SIB-1508Y and levodopa/benserazide caused significant improvements in both cognition and motor aspects of task performance, and did so at one third to one sixth of the levodopa dose necessary to improve only motor function. These results suggest the potential usefulness of SIB-1508Y and levodopa as adjunctive therapies to improve at least some of the cognitive and motor deficits associated with Parkinson's disease.

Pharmacological characterization of SIB-1765F: a novel cholinergic ion channel agonist

Nicotine, the prototypical agonist for neuronal nicotinic acetylcholine receptors (NAChR), nonselectively activates NAChR limiting its use in elucidating the function of NAChR subtypes. SIB-1765F is a subtype selective NAChR agonist that displaces [3H]-nicotine binding with an IC50 of 4.6 nM and [3H]-cytisine binding with an IC50 of 12.2 nM which is 2000- to 6000-fold lower than its displacement of [3H]-QNB or [125I]-alpha-bungarotoxin. SIB-1765F did not inhibit human or rat cholinesterases or the uptake of [3H]-DA in synaptosomal preparations. SIB-1765F mimicked (-)-nicotine in stimulating [3H]-DA release from rat striatal and olfactory tubercle slices, with EC50 values of 99.6 and 39.6 microM, respectively. Such stimulation was sensitive to mecamylamine and DH beta E. SIB-1765F also released endogenous DA in the striatum and the nucleus accumbens as measured by in vivo microdialysis. SIB-1765F was less efficacious than (-)-nicotine at stimulating [3H]-NE release from rat hippocampal slices; in contrast, SIB-1765F increased [3H]-NE release from rat thalamic and cortical slices with efficacies approaching those of (-)-nicotine. Similar to (-)-nicotine and (+/-)-epibatidine, subcutaneous administration of SIB-1765F increased the turnover rate of dopamine ex vivo both in the striatum and olfactory tubercles in a mecamylamine-sensitive manner. Because the release of striatal DA and hippocampal NE appears to be regulated by distinct NAChR, differential effects of SIB-1765F on striatal DA and hippocampal NE release supports the NAChR subtype selectivity of SIB-1765F compared to (-)-nicotine. This is further demonstrated by observations showing that SIB-1765F has a higher affinity for h alpha 4 beta 2 NAChR relative to h alpha 4 beta 4 NAChRs in displacing [3H]-epibatidine binding and increasing cytosolic CA+2 concentration in cell lines stably expressing h alpha 4 beta 2 or h alpha 4 beta 4.

Interactions between a novel cholinergic ion channel agonist, SIB-1765F and L-DOPA in the reserpine model of Parkinson's disease in rats

SIB-1765F, a novel nicotinic acetylcholine receptor agonist, was tested for its efficacy in attenuating reserpine-induced hypolocomotion in rats. SIB-1765F was administered alone or in combination with L-DOPA and its effects were compared to those of nicotine, d-amphetamine and amantadine in the same conditions. Consistent with previous reports, reserpine-induced hypolocomotion was reversed by L-DOPA (plus benserazide), d-amphetamine and amantadine in a dose-dependent manner and the effect of L-DOPA in reserpine-treated rats was potentiated by amantadine. SIB-1765F also increased the locomotor activity of reserpine-treated rats and potentiated the effect of L-DOPA on reserpine-induced hypolocomotion. The onset of potentiation of L-DOPA by SIB-1765F was rapid (< 5 min) compared to the onset of potentiation by amantadine (> 105 min). Interestingly, nicotine did not attenuate reserpine-induced hypolocomotion nor did it affect the action of L-DOPA on reserpine-treated rats. Biochemical analysis of levels of dopamine and its metabolites, dihydroxyphenylacetic and homovanillic acid, indicated that, in contrast to amphetamine, SIB-1765F did not inhibit dopamine reuptake. The effect of SIB-1765F in reserpine-treated rats was attenuated by alpha-methyl-p-tyrosine, implying that SIB-1765F acts by releasing dopamine from both reserpine-insensitive and reserpine-sensitive pools. Our findings demonstrate that nicotinic acetylcholine receptor agonists may offer a new therapeutic approach to the symptomatic treatment of the motor deficits in patients with Parkinson's disease.

Effects of lobeline and dimethylphenylpiperazinium iodide (DMPP) on N-methyl-D-aspartate (NMDA)-evoked acetylcholine release in vitro: evidence for a lack of involvement of classical neuronal nicotinic acetylcholine receptors

Biochemical, behavioral and electrophysiological evidence suggests interactions between pathways containing neuronal nicotinic acetylcholine receptors (NAChRs) and excitatory amino acid receptors. Recently, protective effects of nicotine against N-methyl-D-aspartate (NMDA)-induced toxicity in primary cortical cultures were reported. To address possible interactions between NAChR and NMDA receptor containing pathways, several NAChR agonists were evaluated for their effects on NMDA-evoked [3H]acetylcholine ([3H]ACh) release from slices of rat striatum. Nicotine, cytisine and epibatidine had no effect on NMDA-evoked release or basal release of [3H]ACh over a wide range of concentrations. Lobeline and dimethylphenylpiperazinium iodide (DMPP), however, decreased basal [3H]ACh release and attenuated NMDA-evoked [3H]ACh release with EC50 values of 35 and 155 microM, respectively. The NAChR antagonists, dihydro-beta-erythroidine (DH beta E) and d-tubocurarine had no effect on NMDA-evoked [3H]ACh release, whereas mecamylamine attenuated the NMDA-evoked [3H]ACh evoked release with an EC50 value of 144 microM. Methyllycaconitine (MLA), a highly selective and potent antagonist of the alpha-bungarotoxin-sensitive alpha 7 NAChR subtype, also had no effect on NMDA-evoked [3H]ACh release at concentrations upto 10 microM. The inhibitory effects of DMPP and lobeline on NMDA-evoked [3H[ACh release were relatively insensitive to mecamylamine, d-tubocurarine, MLA and DH beta E. In addition, DMPP or lobeline-induced attenuation of basal [3H]ACh release was insensitive to blockade by sulpiride, a dopamine (D2) receptor antagonist. In contrast to their effects on NMDA-evoked striatal [3H]ACh release, both DMPP and lobeline increased basal release of striatal [3H]DA and hippocampal [3H]norepinephrine ([3H]NE) and did not attenuate NMDA-evoked release of these two transmitters. Instead, DMPP and lobeline appeared to have an additive effect on both NMDA-evoked hippocampal [3H]NE release and striatal [3H]DA release. These pharmacological results suggest that the inhibitory effects on lobeline and DMPP on striatal [3H]ACh release are independent of their interactions with classical NAChRs or the NMDA receptor complex itself.

Effects of a novel cholinergic ion channel agonist SIB-1765F on locomotor activity in rats

SIB-1765F ([+/-]-5-ethynyl-3-(1-methyl-2-pyrrolidinyl)pyridine fumarate) is a novel nicotinic acetylcholine receptor (NAChR) agonist displaying a different in vitro pharmacological profile than nicotine and epibatidine, suggestive of NAChR subtype selectivity. Our study describes the effects of SIB-1765F on locomotor activity in rats, which were compared to those observed for nicotine and epibatidine. The three NAChR agonists decreased or increased locomotor activity in rats naive or habituated to the test apparatus, respectively. The transient reduction in locomotor activity induced by SIB-1765F was quantitatively similar to those induced by nicotine and epibatidine but, unlike the effects of nicotine and epibatidine, was not blocked by the NAChR antagonists mecamylamine and dihydro-beta-erythroidine, suggesting different mechanisms of action. Furthermore, SIB-1765F produced a larger and longer-lasting increase in locomotor activity when administered to rats familiar with the test apparatus. Mecamylamine and dihydro-beta-erythroidine but not hexamethonium blocked the increase in locomotor activity induced by SIB-1765F, suggesting that SIB-1765F elicits this effect predominantly through the activation of central NAChR. The SIB-1765F-induced increase in locomotor activity was also attenuated by selective D1 and D2 dopamine receptor antagonists, implying that this increase in locomotor activity is mediated through the activation of dopamine receptors subsequent to the release of dopamine. Based on these results, SIB-1765F appears to have a different locomotor activity profile than nicotine and epibatidine.

Evaluation of anti-nociceptive effects of neuronal nicotinic acetylcholine receptor (NAChR) ligands in the rat tail-flick assay

In the present investigation, anti-nociceptive effects of neuronal nicotinic acetylcholine receptor (NAChR) ligands, (+)- and (-)-nicotine, cytisine, methylcarbamylcholine (MCC), dimethylphenylpiperazinium iodide (DMPP), and (+/-)-epibatidine were evaluated in the rat tail-flick assay both after subcutaneous (s.c.) and intracerebroventricular (i.c.v.) administration. The pharmacology of the tail-flick response to NAChR ligands after s.c. and i.c.v. routes was similar. Epibatidine was the most potent ligand examined with a longer duration of action than any other agonist. (-)-Nicotine was more active than (+)-nicotine indicating stereospecificity. ICV administration studies indicated an apparent partial agonist activity for (+)-nicotine in the tail-flick response. Tail-flick responses to NAChR agonists are independent of opioid and muscarinic pathways and appear to be mediated both by central and peripheral NAChR recognition sites. Central administration of MCC activates both NAChR and muscarinic anti-nociceptive mechanisms. Studies employing the alpha-adrenergic receptor alkylating agent, phenoxybenzamine or the noradrenergic neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), suggested that the NAChR-noradrenergic and NAChR-serotoninergic interactions play an important role in the tail-flick response. Studies employing a selective alpha-bungarotoxin-sensitive NAChR receptor antagonist, methyllycaconitine (MLA), suggested a minimal role for these receptors in the tail-flick response. The biochemical studies also indicated that a sub-population of NAChR receptors are located pre-synaptically on noradrenergic and/or serotoninergic pathways in the hippocampus.

Epibatidine: a nicotinic acetylcholine receptor agonist releases monoaminergic neurotransmitters: in vitro and in vivo evidence in rats

The effect of the neuronal acetylcholine-gated ion channel receptor agonist (+/-)-epibatidine was studied on neurotransmitter release in vitro and motor behavior in vivo. (+/-)-Epibatidine (3-300 nM) caused a concentration- and calcium-dependent release of [3H]-dopamine from striatal slices and [3H]-norepinephrine release from hippocampal and thalamic slices. (+/-)-Epibatidine-induced neurotransmitter release was inhibited in all three regions by mecamylamine (3 microM). In contrast, D-tubocurarine (10-100 microM) inhibited only (+/-)-epibatidine-induced [3H]-norepinephrine release from the hippocampus and the thalamus. Conversely, dihydro beta-erythroidine (3-100 microM) inhibited (+/-)-epibatidine-induced [3H]-dopamine release in the striatum without significantly altering [3H]-norepinephrine release from either the hippocampus or the thalamus. This is consistent with the observation that different nAChRs modulate dopamine release as compared with norepinephrine release. The effect of (+/-)-epibatidine on both [3H]-dopamine and [3H]-norepinephrine release was tetrodotoxin-sensitive, suggesting the involvement of sodium channels. (+/-)-Epibatidine (1-3 micrograms/kg s.c.) produced ipsilateral turning in the unilaterally [6(OH)-DA]-lesioned rat. This effect was mimicked by (-)-nicotine (0.35 mg/kg s.c.). Both (+/-)-epibatidine- and (-)-nicotine-induced turning were significantly inhibited by mecamylamine (3 mg/kg s.c.), indicating that the turning response was mediated by nAChRs. (+/-)-Epibatidine also increased locomotor activity in a dose-dependent manner. (+/-)-Epibatidine-induced hyperactivity was blocked by D1 and D2 receptor antagonists, SCH 23390 and eticlopride, respectively, suggesting that both dopamine receptor subtypes might be required for the locomotor effect of (+/-)-epibatidine. These results demonstrate that (+/-)-epibatidine displays nAChR agonist activity in the rat CNS and that certain effects are mediated via nAChR-stimulated catecholamine release and subsequent activation of corresponding receptors.

Pharmacological characterization of neuronal acetylcholine gated ion channel receptor-mediated hippocampal norepinephrine and striatal dopamine release from rat brain slices

Neuronal acetylcholine-gated ion channel receptor-mediated [3H]-norepinephrine ([3H]-NE) and [3H]-dopamine ([3H]-DA) release from rat hippocampal and striatal slices, respectively, were compared. The nicotinic receptor agonists (-)-nicotine, (-)-cytisine and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) increased both [3H]-NE and [3H]-DA release in a concentration-dependent manner. The rank order of potency for the three agonists was DMPP > (-)-cytisine > (-)-nicotine for evoking [3H]-NE release and (-)-cytisine > DMPP = (-)-nicotine for releasing [3H]-DA. (-)-Cytisine acted as a partial agonist in stimulating DA release as it displayed lower efficacy and inhibited the agonistic effect of (-)-nicotine. (-)-Cytisine and (-)-nicotine were equally effective in stimulating NE release. The responses to a maximally effective concentration of (-)-nicotine, (-)-cytisine or DMPP on [3H]-NE release were blocked by 1 microM tetrodotoxin (TTX). In contrast, the effects of the various agonists on [3H]-DA release were not blocked by tetrodotoxin. The nicotinic receptor antagonists, d-tubocurarine (3-100 microM) and mecamylamine (1.0-10 microM) blocked the 3H-NE release induced by (-)-nicotine and DMPP in the rat hippocampal slice, whereas dihydro beta-erythroidine (3-300 microM) was without effect. In the striatum, mecamylamine (0.3-10 microM) and dihydro beta-erythroidine (3-100 microM) blocked the responses mediated by both agonists whereas d-tubocurarine (3-100 microM) was ineffective.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAA receptor complex function in frontal cortex membranes from control and neurological patients

The functional integrity of the GABAA receptor-benzodiazepine (BZ) recognition site-Cl- ionophore complex was assessed by means of [35S]TBPS (t-butylbicyclophosphorothionate) binding to frontal cortex membranes prepared from frozen postmortem brain tissue taken from control (n = 4), Alzheimer (n = 7), Parkinson (n = 3) and Huntington's chorea (n = 2) patients. Specific [35S]TBPS binding was similar in control, Parkinson's disease and Huntington's chorea brains, but was significantly reduced (78% control, P less than 0.01) in frontal cortex membranes from Alzheimer's patients. The linkage between the BZ recognition sites and the GABAA receptor-linked Cl- ionophore was functionally intact in these membranes as BZ site agonists (zolpidem, alpidem, flunitrazepam and clonazepam) enhanced [35S]TBPS binding under the conditions used (well-washed membranes in the presence of 1.0 M NaCl). Zolpidem (BZ1 selective) exhibited a biphasic enhancement in control membranes whereas the other compounds induced a bell-shaped concentration-response curve. The enhancement of [35S]TBPS binding by alpidem, flunitrazepam and clonazepam was greater in frontal cortex membranes from Alzheimer's patients than in controls whereas it tended to be reduced in membranes from the brains of Huntington's chorea patients. These studies demonstrate the functional integrity of the GABAA receptor macromolecular complex and also the usefulness of [35S]TBPS binding in the study of human postmortem tissue.

The activity of zolpidem and other hypnotics within the gamma-aminobutyric acid (GABAA) receptor supramolecular complex, as determined by 35S-t-butylbicyclophosphorothionate (35S-TBPS) binding to rat cerebral cortex membranes

The present study compares the effects of different hypnotics acting at omega 1/omega 2 sites (zolpidem, zopiclone, flunitrazepam and triazolam) on 35S-t-butylbicyclophosphorothionate (35S-TBPS) binding to well-washed rat cerebral membranes, in the presence of 1M NaCl. Under these conditions, all compounds enhanced 35S-TBPS binding in the 0.05 to 10 microM range with EC50 values and maximal enhancement of: zolpidem, 84 nM and 36%; flunitrazepam, 8 nM and 41%; zopiclone, 171 nM and 51%; triazolam, 2 nM and 42%. Under these conditions, gamma-aminobutyric acid enhanced 35S-TBPS binding with an EC50 of 240 nM and a 38% maximal increase. The EC50 values for the stimulation of 35S-TBPS binding are well correlated, with (r = 0.97) the affinity of these compounds at omega 1/omega 2 sites, and are in the same concentration range. This enhanced binding was due to an altered apparent affinity for the 35S-TBPS recognition site without any change in the number of sites (Scatchard analysis). The effect of zolpidem and other hypnotics was antagonized by flumazenil. This was an apparently competitive antagonism in the case of zolpidem or flunitrazepam, whereas for zopiclone, increasing the concentration of the hypnotic did not overcome the antagonism. Bicuculline only partially antagonized the hypnotic-induced enhancement of 35S-TBPS binding. This antagonism was more effective for zopiclone (-57%) than for either zolpidem (-33%) or flunitrazepam (-30%). Zolpidem and the other hypnotics studied induced a fast component of dissociation which was not observed in the control membranes. These findings are consistent with the hypothesis that omega 1/omega 2 agonists increase the frequency of openings of the chloride ionophore, with both gamma-aminobutyric acid-A receptor-dependent and -independent mechanisms.

The toxicity of gossypol to the male rat

When (+/-) gossypol acetic acid was administered to male Sprague-Dawley rats for 26 weeks, the most significant toxicological finding was marked suppression of body weight gain in rats receiving 25 mg/kg per day. Minor biochemical changes were noted at this dosage level. Terminal studies showed 6 out of 20 rats receiving 25 mg/kg per day to have varying degrees of testicular pathology. Five mg/kg per day was shown to be a "no effect" level.