Bryostatin-1 - Wiley Online Library

CNS Drug Reviews Vol. 12, No. 1, pp. 1–8 © 2006 The Authors Journal compilation © 2006 Blackwell Publishing Inc.

Bryostatin-1: Pharmacology and Therapeutic Potential as a CNS Drug Miao-Kun Sun and Daniel L. Alkon Blanchette Rockefeller Neurosciences Institute, Rockville, MD, USA Keywords: Alzheimer’s disease — Antidepressants — Bryostatin-1 — Cognition activation — Depression — Learning and memory — Mood — Protein kinase C.

ABSTRACT Bryostatin-1 is a powerful protein kinase C (PKC) agonist, activating PKC isozymes at nanomolar concentrations. Pharmacological studies of bryostatin-1 have mainly been focused on its action in preventing tumor growth. Emerging evidence suggests, however, that bryostatin-1 exhibits additional important pharmacological activities. In preclinical studies bryostatin-1 has been shown at appropriate doses to have cognitive restorative and antidepressant effects. The underlying pharmacological mechanisms may involve an activation of PKC isozymes, induction of synthesis of proteins required for long-term memory, restoration of stress-evoked inhibition of PKC activity, and reduction of neurotoxic amyloid accumulation and tau protein hyperphosphorylation. The therapeutic potential of bryostatin-1 as a CNS drug should be further explored.

INTRODUCTION The bryostatins, a family of at least 20 marine natural products with a chemical structure of macrocyclic polyketides, were first isolated from the bryozoan Bugula neritina by Pettit et al. in 1968 and characterized chemically in 1982 (33). Their main pharmacological mechanism of action is modulation of protein kinase C (PKC) activity. The prototypic member, bryostatin-1 (Fig. 1) possesses a unique pharmacological profile as a cancer chemotherapeutic. It is currently in phases I and II trials against a variety of cancers (8,13,19,27) and produces behavioral and cognitive effects when appropriate drug levels are achieved in the brain (14,39). It inhibits tumor invasion, tumor growth in vitro and in vivo, and angiogenesis. It also synergizes with other anticancer agents (2), reverses multidrug resistance (3,37), stimulates the immune system (11), enhances cognition, and has antidepressant effects. The focus of this short review is on pharmacology of bryostatin-1 and evaluation of its therapeutic potential in the treatment of dementia and depression. Address correspondence and reprint requests to: Miao-Kun Sun, Ph.D., Blanchette Rockefeller Neurosciences Institute, 9601 Medical Center Drive, Academic and Research Bldg., Room 319, Rockville, MD 20850, USA. Phone: +1 (301) 294-7181, Fax: +1 (301) 294-7007, E-mail: [email protected]

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M-K. SUN AND D. L. ALKON OH

O

OAc

11

O

B

O

O

A

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3

OH 19

O

H

OH

C

O

O

26

O

OH O

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FIG. 1. Chemical structure of bryostatin-1.

PHARMACODYNAMICS Bryostatin-1 is a partial agonist of several members of the PKC family (see below). Its pharmacological effects appear to depend on its action on the PKC isozymes. Bryostatin-1 binds PKC isozymes with IC50 subnanomolar concentrations and a dissociation half-time of several hours [12]. The binding of bryostatin-1 to PKC results in PKC activation, autophosphorylation, and translocation to the cell membrane. Bryostatin-1-bound PKC is then downregulated by ubiquitination and degradation in proteasomes. Downregulation is most significant when the PKC isozymes are exposed to high and/or prolonged high concentrations of bryostatin-1. It should be, however, mentioned here that modulation of PKC activity may not be the only biological action of bryostatins (25). For example, in B16/F10 melanoma cells (26) epi-bryostatin-1, a bryostatin-1 stereoisomer, with much lower affinity for PKC, exhibits the same potency in inhibiting cell growth as bryostatin-1 (41), suggesting the involvement of aPKC activity-independent action. Nevertheless, modulation of PKC activity is the only well established biological mechanism by which bryostatin-1 produces a variety of biological effects. Bryostatin-1 can modulate the activity of two of the three subgroups of PKC isozymes. PKC activity, with the active site of isozymes usually located at the C-terminus, is involved in the phosphorylation of serine and threonine residues. PKC isozymes are divided into three subgroups, which differ in their molecular structures and co-factor requirements: classical PKC (cPKC; á, âI, âII, and ã isoforms), novel PKC (nPKC; ä, å, å¢, ç, è, and ì isoforms), and atypical PKC (aPKC; æ and ë/i). The cPKC and nPKC contain regulatory C1 domains (C1A and C1B) and are activated by diacylglycerol, phorbol esters, and bryostatins. The difference between cPKC and nPKC is that cPKC has a C2 region corresponding to the Ca2+ binding site and requires Ca2+ as a co-factor for activation, whereas the nPKC does not contain the C2 region and, therefore, does not require Ca2+ as a cofactor for activation. The third group, aPKC, does not bind phorbol esters or bryostatins

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and contains no C2 region and one of the repeated cysteine-rich zinc finger binding motifs within the C1 domain. Bryostatin-1 is, therefore, expected to modulate nPKC activity, independent of a Ca2+ signal. It activates cPKC only when associated with Ca2+ signaling. aPKC activity is not sensitive to bryostatin-1 administration. Interestingly, Ca2+ signals play an important role in synaptic transmission and information processing. Bryostatin-1 thus possesses a unique action profile: it will not affect cPKC activity in neurons that are not functioning as an active part of the signaling processing circuit with significant Ca2+ influx and intracellular Ca2+ release. Studies of structure-activity relations of bryostatin-1 (Fig. 1) for PKC isoforms (44–46) indicate that the intact 20-membered macrolactone ring is needed for good PKC binding activity while the A-ring and the B-ring exocyclic olefin can be deleted from the 20-membered structure without inducing much change in PKC-binding affinity. The C[26] free hydroxyl is essential for a good interaction with the PKC isozymes, while the C[1] carbonyl group is also important for a high affinity (44). The C[19] hydroxyl group may be involved in the interaction with the lipid bilayer during binding, while the C[3] hydroxyl group plays an important role in the conformation of the molecule. The A-ring region can be extensively modified without affecting binding affinity. Thus the C[9] region could be modified as needed to tune ADME (absorption, distribution, metabolism, and excretion) and pharmacokinetic characteristics (46). The C[20] region is a non-pharmacophoric site and can also be modified to tune analogs for function and physical properties without significantly affecting their binding affinity for the PKC isozymes (44). These positions can thus be used to change for improvement of physical properties and selectivity, and for incorporation of additional functionality, such as a fluorophore. Bryostatin-1 can mimic certain effects of phorbol esters in some biological systems. However, several other properties of bryostatin-1 are distinct from those of the phorbol esters (22). Of critical importance and unlike the phorbol esters, this compound lacks tumor-promoting capabilities and actually counteracts tumor promotion induced by phorbol esters (17), an important pharmacological property when clinical potential is considered.

Role in Alzheimer’s Disease Numerous reports (10,16,24) imply a critical role of deficient functions of PKC isozymes in the pathogenesis of Alzheimer’s disease (AD). Intensive efforts have, therefore, been focused on the development of bryostatin-1 and its analogs to treat dementia of the Alzheimer’s type. AD, a devastating and progressive decline of memory and other cognitive functions (e.g., a reduced ability to learn, loss of memory, decreased attention, judgment, and decision-making), robs the affected individuals of quality of life. The main histopathological hallmarks of AD brain are extracellular senile plaques and intracellular neurofibrillary tangles at late stages of the disorder, particularly in the hippocampus and related neural structures that play an essential role in memory formation. Three pharmacological profiles favor potential use of bryostatin-1 and its analogs to restore cognitive dysfunction especially that associated with AD. 1) PKC isozymes are activated by synaptic inputs and intracellular signals that are involved in information processing and play an important role in various types of learning and memory (4,6). Reduced PKC activity is associated with AD (10,16), at least partially due to a direct inhibition of PKC activity by neurotoxic Aâ (24). Activation of PKC with bryostatin-1 induces the de novo synthesis of proteins necessary and sufficient for subse-


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quent long-term memory consolidation in Hermissenda (5). PKC inhibition, on the other hand, impairs learning and memory. Mice deficient in PKCb have been found to show normal brain anatomy and normal hippocampal synaptic transmission, paired facilitation, and long-term potentiation of the glutamatergic synaptic responses but a loss of learning in both cued and contextual fear conditioning (42). Bryostatin-1 selectively activates cPKC isozymes in those neurons that are active, as part of the information processing network with Ca2+ signaling. Activation of PKC with bryostatin-1 promotes stabilization of growth-associated protein 43 (GAP-43) mRNA, resulting in an increased GAP-43 protein level in human neuroblastoma cells (34). In addition, evidence has been provided that bryostatin-1, at appropriate doses (bilateral intracerebroventricular 0.64 or 2 pmoles/site), directly enhances learning and memory in Wistar rats (39). 2) Activation of certain PKC isozymes has therapeutic values in reducing the formation of extracellular senile plaques, one of the AD’s main histopathological hallmarks. Bryostatin-1 activates á-secretase, probably through PKC á, ä, or å (14,21,47,50). At subnanomolar concentrations, it enhances the secretion of á-secretase product soluble amyloid precursor protein (sAPP)á in fibroblasts from AD patients and reduces brain Aâ40 and Aâ42 accumulation in AD double-transgenic mice (14). In APP[V7171] transgenic mice, PKC activation has also been found to reduce Aâ40 accumulation in the brain (14). 3) PKC activation inhibits glycogen synthase 3 kinase (15,23) and thereby reduces ô protein hyperphosphorylation (7) and intracellular neurofibrillary tangles, another main histopathological hallmark of AD. The combination of memory-enhancing action and reduction in brain amyloid burden and ô protein hyperphosphorylation may represent a promising multi-target strategy with one agent and an effective therapeutic approach against AD.

Role in Depression Mood disorders are among the most prevalent forms of mental illness. They are recurrent, life threatening (due to the risk of suicide), and a major cause of morbidity worldwide. Several forms of depression affect 2–5% of the U.S. population, and up to 20% of this population suffers from milder forms of the illness. Efforts to enhance monoamine activity in the brain pharmacologically have led to the development of several clinically effective antidepressants. Unfortunately, progress in the development of new and improved antidepressants has been limited. The new drugs developed so far have essentially the same mechanism of action as the older ones and, as a result, the efficacy of the newer agents and the ranges of depressed patients they treat are no better than those of the older drugs. Today’s treatment thus remains suboptimal, with only ~50% of all patients demonstrating complete remission, although many more (up to 80%) show partial response. In addition, the current pharmacological treatments, including the use of the second generation of antidepressants, selective serotonin reuptake inhibitors, are associated in some cases with a disturbing “impulsive” tendency for suicide. The adverse effects as well as limited antidepressant efficacy of the available drugs call for new antidepressants. It is of interest that dementia and depression appear to share common structural and signal pathway injuries (38). An involvement of PKC activity in depression and its treatment has been implicated in several studies. PKC may be involved in 5-hydroxytryptamine (5-HT2A) receptor desensitization (35). Activation of the hypothalamic-pituitaryadrenal axis causes downregulation of PKC isozymes in rat brain (30). Reduced PKC activity occurs in the prefrontal cortex and the hippocampus in suicide victims (28,31,32)


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and in fibroblasts cultured from skin biopsies from patients with melancholic depression (1). The human plasma membrane serotonin transporter is a substrate for PKC. PKC activation or phosphatase inhibition downregulate uptake of 5-HT. This effect is probably mediated by reduction of the expression of 5-HT transporter (36). Imipramine, a tricyclic antidepressant, but not citalopram, has been shown to increase PKC levels in primary rat neuronal cultures (29). Along this line of an important role of PKC activity in mood regulation is the observation that bryostatin-1, at appropriate doses, has antidepressant activity in rats (39). In an open space swim model of induced depressive behavior, bryostatin-1, at 100 nmoles/kg i.v., significantly reduced non-searching immobility in rats. This antidepressant effect of bryostatin-1 is largely abolished by co-administration of 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), a PKC inhibitor, suggesting the involvement of PKC activation in its antidepressant action. It is, therefore, likely that bryostatin-1 and/or its analogs could be developed as new antidepressants.

PHARMACOKINETICS In clinical trials as an antitumor agent, bryostatin-1 is given either as a bolus intravenous injection or a continuous infusion. It is difficult to perform pharmacokinetic analysis of bryostatin-1 in humans as its in vivo blood levels are not easily detectable by mass spectrometry or high-performance liquid chromatography. Trial design has been hampered by lack of human pharmacokinetic data. Efforts have been made to develop more specific and sensitive methods for detection of bryostatin-1 in human plasma (49). These efforts may lead to an improvement in the design of future trials. Pharmacokinetics of bryostatin-1 has been, however, studied in animals. In mice, studies with [C26-3H]bryostatin-1, i.v. or i.p. reveal a wide distribution of the drug with high levels in lungs, liver, gastrointestinal tract, and fat (48), whereas its oral bioavailability, to our knowledge, has not been clearly established. Bryostatin-1 is relatively stable to metabolic breakdown in vivo. High-performance liquid chromatography analysis indicates that radioactivity is mainly associated with the intact molecule, suggesting that the compound remained largely intact. Following intravenous administration to mice, the plasma disappearance curve of bryostatin-1 fits a two compartment model, with half-lives of 1.05 and 22.97 h, respectively. In contrast, the plasma disappearance curve for bryostatin-1 administered i.p. is better described by a first order absorption one-compartment model, with an absorption half-life of 0.81 h and an elimination half-life of 28.76 h, respectively. The majority of radioactivity in plasma was associated with the intact drug for up to 24 h after dosing. During the first 12 h after i.v. administration of bryostatin-1, urinary excretion represented the major pathway of drug elimination, with 23.0 ± 1.9% (mean ± S.D.) of the administered dose excreted. Within 72 h after i.v. administration, approximately equal amounts of radioactivity (40%) were excreted in feces and in urine. After i.v. administration of bryostatin-1 to mice, a brief increase in its brain levels was observed, reaching several nanomoles after an i.v. dose of 40 mg/kg (48). These brain levels were sufficient to activate PKC isozymes that were sensitive to bryostatin-1, but the brain levels rapidly declined after administration of the drug. After i.p. injection of the same dose to mice, on the other hand, the peak brain levels were much lower, but the increases were longer-lasting than those observed after an i.v. dose. These data indicate that bryostatin-1 can pass through the blood-brain barrier, although the brain levels of the drug were much lower than its plasma levels (48).


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ADVERSE EFFECTS AND HUMAN TOXICITY The maximum tolerated dose of bryostatin-1 in humans has been found to be about 25 ìg/m2/week, given intravenously over up to 8 weeks (9,20,26). Toxic and adverse reactions are rare and generally mild. Myalgia is the dose-limiting toxicity in humans. Myalgia occurs at one to two days after infusion and tends to get worse with repeated administration. The symptoms are eased by exercise but return on resting. The calves, thighs and extraocular muscles are affected first but myalgia becomes more generalized as therapy continues. Myalgia affecting the muscles of hypopharynx may result in frontal headache and odynophasia. Lasting impairment of oxidative metabolism in muscle mitochondria may be responsible for myalgia (18). Other reported adverse reactions in humans include fatigue and lethargy; they are common but generally mild. Less common adverse effects include low-grade pyrexia, nausea and anorexia. In a toxicological study in rats, lethargy, unsteadiness, and hematuria have also been observed following bryostatin-1 treatment. Adverse hematological toxicities in humans are not common, although thrombocytopenia and leucopenia have been reported. Mild abnormalities of liver function have also been reported. Bryostatin-1 toxicity appears to be age-dependent. Children can tolerate higher doses of the drug. The maximal tolerated dose of bryotaxin-1 in children has been reported to be 44 ìg/m2 (43). Myalgia and photophobia are the dose limiting toxic effects of bryostatin-1 in children. It remains to be determined whether the age-dependent toxicity of bryostatin-1 would limit its utility in aged AD patients.

CONCLUSIONS Several studies have suggested that at appropriate doses bryostatin-1 can produce beneficial effects on cognition and mood, through modulation of PKC activity. Activation of PKC isozymes represents a potential therapeutic strategy in improving memory and mood, although we do not know precisely which PKC isozyme(s) may mediate the observed effects. PKC isozymes are involved in a variety of vital functions and neurological disorders so that a long-term PKC activation may cause wide and severe reactions. For instance, all forms of PKC isozymes are sensitive to oxidative stress. It remains to be studied whether a sustained increase in the plasma levels of bryostatin-1 would further sensitize PKC isozymes to oxidants. However, intermittent PKC activation with lower doses over an extended period may avoid such reactions. Multiple but intermittent doses of bryostatin-1 or its analogs, administered chronically, are expected to result in brief but multiple “peaks” of PKC activation with smaller doses, maximizing effects of PKC cascade activation but minimizing potential “tolerance” or PKC downregulation that is associated with the exposure to high and/or long-lasting high concentrations of bryostatin-1. Based on the data available in clinical trials, bryostatin-1 is well tolerated as an antitumor agent. This means that its adverse side effects are rare, generally mild, and reversible. It possesses several pharmacological actions, including functional restoration of a reduced PKC signal cascade that occurs in dementia and depression, a reduction in the accumulation of neurotoxic amyloid, and an inhibition of tau protein hyperphosphorylation. Structural modifications of bryostatin-1 may lead to compounds that possess bryostatin-1 activity but can pass through the blood-brain barrier much easier, so that they may achieve the desired brain effects at smaller doses. Bryostatin-1 and its analogs (44,46) may be developed as therapeutic drugs for the treatment of dementia, depression, and/or disorders such as attention-deficit/hyper-


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activity disorder in which cognitive enhancement is desirable and co-morbid depression is prominent.

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