Jacobus P. Petzer

Neuroprotection by caffeine and more specific A2A receptor antagonists in animal models of Parkinson’s disease

A remarkable convergence of epidemiologic and laboratory data has raised the possibility that caffeine reduces the risk of developing Parkinson’s disease (PD) by preventing the degeneration of nigrostriatal dopaminergic neurons. The authors review the evidence that caffeine and more specific antagonists of the adenosine A2A receptor protect dopaminergic neurons in several toxin models of PD. Other studies demonstrating protection by A2A receptor inactivation in animal models of stroke, Huntington’s disease, and Alzheimer’s disease suggest a more global role of A2A receptors in neuronal injury and degeneration. Although the cellular and molecular mechanisms by which A2A receptors contribute to neuronal death are not yet established, several intriguing possibilities have emerged. Now with preliminary clinical data substantiating the antiparkinsonian symptomatic benefit of A2A receptor blockade, the prospects for a complementary neuroprotective benefit have enhanced the therapeutic potential of A2A antagonists in PD.

Inhibition of monoamine oxidase by 8-benzyloxycaffeine analogues.

Based on recent reports that several (E)-8-styrylcaffeinyl analogues are potent reversible inhibitors of monoamine oxidase B (MAO-B), a series of 8-benzyloxycaffeinyl analogues were synthesized and evaluated as inhibitors of baboon liver MAO-B and recombinant human MAO-A and -B. The 8-benzyloxycaffeinyl analogues were found to inhibit reversibly both MAO isoforms with enzyme-inhibitor dissociation constants (K(i) values) ranging from 0.14 to 1.30 microM for the inhibition of human MAO-A, and 0.023-0.59 microM for the inhibition of human MAO-B. The most potent MAO-A inhibitor was 8-(3-methylbenzyloxy)caffeine while 8-(3-bromobenzyloxy)caffeine was the most potent MAO-B inhibitor. The analogues inhibited human and baboon MAO-B with similar potencies. A quantitative structure-activity relationship (QSAR) study indicated that the MAO-B inhibition potencies of the 8-benzyloxycaffeinyl analogues are dependent on the Hansch lipophilicity (pi) and Hammett electronic (sigma) constants of the substituents at C-3 of the benzyloxy ring. Electron-withdrawing substituents with a high degree of lipophilicity enhance inhibition potency. These results are discussed with reference to possible binding orientations of the inhibitors within the active site cavities of MAO-A and -B.

Azure B, a metabolite of methylene blue, is a high-potency, reversible inhibitor of monoamine oxidase

Methylene blue (MB) has been shown to act at multiple cellular and molecular targets and as a result possesses diverse medical applications. Among these is a high potency reversible inhibition of monoamine oxidase A (MAO-A) that may, at least in part, underlie its adverse effects but also its psycho- and neuromodulatory actions. MB is metabolized to yield N-demethylated products of which azure B, the monodemethyl species, is the major metabolite. Similar to MB, azure B also displays a variety of biological activities and may therefore contribute to the pharmacological profile of MB. Based on these observations, the present study examines the interactions of azure B with recombinant human MAO-A and -B. The results show that azure B is a potent MAO-A inhibitor (IC₅₀=11 nM), approximately 6-fold more potent than is MB (IC₅₀=70 nM) under identical conditions. Measurements of the time-dependency of inhibition suggest that the interaction of azure B with MAO-A is reversible. Azure B also reversibly inhibits the MAO-B isozyme with an IC₅₀ value of 968 nM. These results suggest that azure B may be a hitherto under recognized contributor to the pharmacology and toxicology of MB by blocking central and peripheral MAO-A activity and as such needs to be considered during its use in humans and animals.

Role of monoamine oxidase, nitric oxide synthase and regional brain monoamines in the antidepressant-like effects of methylene blue and selected structural analogues

Dual action antidepressants have important therapeutic implications. Methylene blue (MB), a charged compound structurally related to tricyclic antidepressants, acts on both monoamine oxidase (MAO) and the nitric oxide (NO)-cGMP pathway, and has demonstrated antidepressant activity in rodents. We investigated the antidepressant properties of MB and selected structural analogues and whether their actions involve MAO, NO synthase (NOS) and regional brain monoamines. Acute imipramine (IMI, 15 mg/kg), saline, MB, acriflavine (ACR), methylene green (MG), methylene violet (MV), thionine (THI) and tacrine (TAC) (1-60 mg/kg i.p.) were tested for antidepressant activity in the forced swim test (FST), as well as MAO-A/B inhibitory activity. Active antidepressant compounds were subsequently studied at their most effective dose during sub-chronic treatment, followed by behavioural sampling in the FST and assay of cortico-limbic monoamines and hippocampal nitrate (for NOS activity). Only IMI, MB (15, 30, 60 mg/kg) and MG (7.5, 25, 40 mg/kg) reduced immobility in the acute FST. MB, MG and ACR were potent inhibitors of especially MAO-A. Following sub-chronic treatment, IMI (15 mg/kg) increased noradrenergic behaviour in the FST, while MB (15 mg/kg) and MG (15 mg/kg) enhanced serotonergic behaviour. MB and MG bolstered cortico-limbic serotonin (5HT) levels and to a lesser extent l-norepinephrine (l-NE), but did not significantly alter regional dopamine (DA) levels. MB, and to lesser degree MG, reduced hippocampal nitrate levels. MB and MG present with structure-specific antidepressant-like effects following acute and sub-chronic treatment, possibly involving NOS and MAO-A inhibition and cortico-limbic 5HT and l-NE release. A role for MAO-B and DA appears minimal.

Dual-target-directed drugs that block monoamine oxidase B and adenosine A2A receptors for Parkinson’s disease

SummaryInadequacies of the current pharmacotherapies to treat Parkinson’s disease (PD) have prompted efforts to identify novel drug targets. The adenosine A2A receptor is one such target. Antagonists of this receptor (A2A antagonists) are considered promising agents for the symptomatic treatment of PD. Evidence suggests that A2A antagonists may also have neuroprotective properties that may prevent the development of the dyskinesia that often complicates levodopa treatment. Because the therapeutic benefits of A2A antagonists are additive to that of dopamine replacement therapy, it may be possible to reduce the dose of the dopaminergic drugs and therefore the occurrence of side effects. Inhibitors of monoamine oxidase (MAO)-B also are considered useful tools for the treatment of PD. When used in combination with levodopa, inhibitors of MAO-B may enhance the elevation of dopamine levels after levodopa treatment, particularly when used in early stages of the disease when dopamine production may not be so severely compromised. Furthermore, MAO-B inhibitors may also possess neuroprotective properties in part by reducing the damaging effect of dopamine turnover in the brain. These effects of MAO-B inhibitors are especially relevant when considering that the brain shows an age-related increase in MAO-B activity. Based on these observations, dual-target-directed drugs, compounds that inhibit MAO-B and antagonize A2A receptors, may have value in the management of PD. This review summarizes recent efforts to develop such dual-acting drugs using caffeine as the lead compound.

Dual inhibition of monoamine oxidase B and antagonism of the adenosine A2A receptor by (E,E)-8-(4-phenylbutadien-1-yl)caffeine analogues

The adenosine A(2A) receptor has emerged as an attractive target for the treatment of Parkinsons disease (PD). Evidence suggests that antagonists of the A(2A) receptor (A(2A) antagonists) may be neuroprotective and may help to alleviate the symptoms of PD. We have reported recently that several members of the (E)-8-styrylcaffeine class of A(2A) antagonists also are potent inhibitors of monoamine oxidase B (MAO-B). Since MAO-B inhibitors are known to possess anti-parkinsonian properties, dual-target-directed drugs that block both MAO-B and A(2A) receptors may have enhanced value in the management of PD. In an attempt to explore this concept further we have prepared three additional classes of C-8 substituted caffeinyl analogues. The 8-phenyl- and 8-benzylcaffeinyl analogues exhibited relatively weak MAO-B inhibition potencies while selected (E,E)-8-(4-phenylbutadien-1-yl)caffeinyl analogues were found to be exceptionally potent reversible MAO-B inhibitors with enzyme-inhibitor dissociation constants (K(i) values) ranging from 17 to 149 nM. Furthermore, these (E,E)-8-(4-phenylbutadien-1-yl)caffeines acted as potent A(2A) antagonists with K(i) values ranging from 59 to 153 nM. We conclude that the (E,E)-8-(4-phenylbutadien-1-yl)caffeines are a promising candidate class of dual-acting compounds.

Inhibition of monoamine oxidase by selected C5- and C6-substituted isatin analogues

Previous studies have shown that (E)-5-styrylisatin and (E)-6-styrylisatin are reversible inhibitors of human monoamine oxidase (MAO) A and B. Both homologues are reported to exhibit selective binding to the MAO-B isoform with (E)-5-styrylisatin being the most potent inhibitor. To further investigate these structure-activity relationships (SAR), in the present study, additional C5- and C6-substituted isatin analogues were synthesized and evaluated as inhibitors of recombinant human MAO-A and MAO-B. With the exception of 5-phenylisatin, all of the analogues examined were selective MAO-B inhibitors. The C5-substituted isatins exhibited higher binding affinities to MAO-B than the corresponding C6-substituted homologues. The most potent MAO-B inhibitor, 5-(4-phenylbutyl)isatin, exhibited an IC(50) value of 0.66nM, approximately 13-fold more potent than (E)-5-styrylisatin and 18,500-fold more potent than isatin. The most potent MAO-A inhibitor was found to be 5-phenylisatin with an IC(50) value of 562nM. The results document that substitution at C5 with a variety of substituents is a general strategy for enhancing the MAO-B inhibition potency of isatin. Possible binding orientations of selected isatin analogues within the active site cavities of MAO-A and MAO-B are proposed.

Inhibition of monoamine oxidase by indole and benzofuran derivatives.

Monoamine oxidase (MAO) is an important drug target for the treatment of neurological disorders. A series of indole and benzofuran derivatives were synthesised and evaluated as inhibitors of the two MAO isoforms, MAO-A and MAO-B. In general, the derivatives were found to be selective MAO-B inhibitors with K(i) values in the nanoMolar (nM) to microMolar (microM) concentration range. The most potent MAO-B inhibitor, 3,4-dichloro-N-(2-methyl-1H-indol-5-yl)benzamide, exhibited a K(i) value of 0.03 microM and was 99 fold more selective for the B isoform. We conclude that these indole and benzofuran derivatives are promising reversible MAO-B inhibitors with a possible role in the treatment of neurodegenerative diseases such as Parkinsons disease (PD).

Inhibition of monoamine oxidase by selected C6-substituted chromone derivatives.

Chromone has been reported to be a useful scaffold for the design of monoamine oxidase (MAO) inhibitors. In an attempt to discover highly potent MAO inhibitors and to contribute to the known structure-activity relationships (SAR) of MAO inhibition by chromones, in the present study, we have synthesized a series of chromone derivatives substituted at C6 with a variety of alkyloxy substituents, and evaluated the resulting compounds as inhibitors of recombinant human MAO-A and -B. The results document that the C6-substituted chromones are potent reversible MAO-B inhibitors with IC(50) values in the low nM range (2-76 nM). The chromones were also found to bind reversibly to MAO-A, but with lower affinities compared to MAO-B. It may therefore be concluded that C6-substituted chromones are highly potent MAO-B selective inhibitors and promising lead compounds for the development of therapy for neurodegenerative disorders such as Parkinsons disease. The results of this study are discussed with reference to possible binding orientations of a selected C6-substituted chromone in the active site cavities of MAO-A and -B.

Novel monoamine oxidase inhibitors: a patent review (2012 – 2014)

Introduction: Monoamine oxidase (MAO) inhibitors, despite the initial pharmacological interest, are used in clinic for their antidepressant effect and in the management of Parkinson symptoms, due to the established neuroprotective action. Efficacy and tolerability emerged from large-scale and randomized clinical trials. Areas covered: Thirty-six patents range from April 2012 to September 2014. The number of chemotypes with inhibitory effects on MAO is truly high (40 synthetic compounds, 22 natural products and 6 plant extracts reported and licensed), and the present review is comprehensive of all compounds, which have been patented for their relevance to clinical medicine in this period range (27 patents). Moreover, some of the collected patents deal with new formulations of compounds endowed with MAO inhibitory properties (two patents) and new therapeutic options/drug associations for already known MAO inhibitors (seven patents). Expert opinion: The patents reported in this review showed that the interest in this field is constant and mainly devoted to the study of selective MAO-B inhibitors, used as drugs for the treatment of neurological disorders. The development of novel human MAO inhibitors took advantage of the discovery of new therapeutic targets (cancer, hair loss, muscle dystrophies, cocaine addiction and inflammation), the recognized role of MAOs as molecular biomarkers and their activity in other tissues.