- Email: [email protected]
Tacrine: In vivo veritas
Accepted Manuscript Title: Tacrine: In vivo veritas Author: Bevyn Jarrott PII: DOI: Reference:
S1043-6618(16)31397-4 http://dx.doi.org/doi:10.1016/j.phrs.2016.12.033 YPHRS 3456
To appear in:
Pharmacological Research
Received date: Accepted date:
12-12-2016 21-12-2016
Please cite this article as: Jarrott Bevyn.Tacrine: In vivo veritas.Pharmacological Research http://dx.doi.org/10.1016/j.phrs.2016.12.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tacrine : In vivo veritas
Corresponding author: Bevyn Jarrott Florey Institute of Neuroscience & Mental Health University of Melbourne Parkville 3010, Australia Email: [email protected] Abstract Tacrine was initially synthesised in 1945 as part of a project seeking antibacterial drugs to treat infected wounds in soldiers. However, it was inactive in vitro against common strains of bacteria. Serendipitously, it was injected in vivo into dogs anaesthetised with chloroform and morphine and noted to immediately counter the respiratory rate depression caused by morphine but not block analgesia. Subsequent studies showed that tacrine was an acetylcholinesterase inhibitor. When combined with morphine in ampoules it was possible to inject larger doses of morphine without causing respiratory depression and it was marketed for10 years in Australia. Tacrine was also used alone for treating acute anticholinergic syndrome in the 1980s. Shortly after this, it was hypothesised by William Summers that it could be of benefit in treating the early stages of Alzheimer’s dementia and an IND was granted by the US Food and Drug Administration and a use patent awarded to Summers. It was the first of four anticholinesterases to be approved for treating this condition although its variable pharmacokinetics was a disadvantage.
Key words: Tacrine; cholinesterase inhibitor; block of opiate-induced respiratory depression; antagonist of acute anticholinergic syndrome; oral treatment of Alzheimer’s dementia. Chemical compound reported in this article: Tacrine (PubChem CID:1935; 1,2,3,4-tetrahydroacridin-9-amine) Aminoacridine (PubChem CID:7019; acridin-9-amine)
1.
Discovery of Tacrine The discipline of Pharmacology in Australia commenced with the establishment of
a Department of Pharmacology at the University of Sydney led by Professor Roland Thorp in 1949 and at the University of Melbourne led by Professor Frank Shaw in 1953. Prior to that, drug research and discovery occurred mainly in Departments of Physiology and Chemistry. The following is an account of the synthesis and pharmacological evaluation of tacrine from 1945 to 1980 in Australia and then its subsequent development in the USA as the first drug approved by the US Food & Drug Administration (FDA) for the treatment of Alzheimer’s dementia.
During the Second World War (1939-45) both these universities actively collaborated in a project to develop potential antibacterial drugs for treating infected wounds in soldiers since this was a major source of fatalities in the First World War. At the University of Sydney, Adrien Albert was a leading authority on the chemical synthesis of heterocyclic compounds as well as a leader in relating pharmacological properties of compounds to physical chemical properties such as basicity, degree of ionisation at different pH and lipid solubility which he documented in detail in his textbook Selectivity Toxicity. During this period he and his staff synthesised at least 80 compounds of the acridine class to expand upon the topical antiseptic, 9-aminoacridine (pharmacoepia name of Aminoacridine, Trade name of Monacrin). Albert collaborated with an experienced bacteriologist at the University of Melbourne, Sydney Rubbo who was trained in the classical in vitro assays of antibacterial agents pioneered by Paul Ehrlich and Gerhard Domagk in Germany in the early 1900s.
Earlier, acridine dyes such as Acriflavine and Aminoacridine had been found to be
effective antiseptics when applied to the skin but were too toxic and ineffective in treating blood septicemia so the goal was to find a non-toxic compound that was antibacterial in vivo. While that goal was not achieved by Albert and Rubbo, fortunately it was achieved in the early 1940s by the efforts of an Australian pathologist at the University of Oxford, Professor Howard Florey, who succeeded in the large scale production and purification of water soluble penicillin that had been discovered serendipitously by Alexander Fleming of St.Mary’s Hospital, London in 1928.
What did emerge from Albert & Rubbo’s structure-activity studies of 77 new acridines was the finding that the synthesis of an 9-amino-acridine analog with a saturated C ring (1,2,3,4-tetrahydro-9-amino-acridine) was completely inactive as an antibacterial compound in vitro and so no further compounds were made with this saturated ring (Albert et al, 1945). While Rubbo carried out in vitro assays of these compounds against a range of gram-positive and gram-negative bacteria, no in vivo studies were included. Such studies were then done in the Dept of Physiology at the University of Melbourne by a senior lecturer, Frank Shaw (who became the foundation Professor of Pharmacology in 1953) and a postgraduate student, Geoffrey Bentley (who later became the foundation Associate Professor of Pharmacology in the Dept of Physiology at Monash University in 1967). As they had a plentiful supply of 9-amino-acridine, they injected this into a dog anesthetised with chloroform and a high dose of morphine in order to estimate its pharmacokinetic profile. Serendipitously, they noted that this compound immediately increased the depressed respiratory rate of the dog induced by morphine and the dog quickly regained consciousness. This was unexpected but clearly of interest and they had
access to many of the acridines synthesised by Albert for testing in this model. Again, unexpectedly, 1,2,3,4-tetrahydro-9-amino-acridine was more potent than 9amino-acridine in stimulating morphine-induced respiratory depression and arousing the dog from its unconscious state and appeared less toxic than 9-amino-acridine. They published an extensive paper on this action of 7 acridines and other heterocyclic compounds (Shaw & Bentley, 1949). In another paper (Shaw & Bentley 1952) they concluded that 1,2,3,4-tetrahydro-9-amino-acridine would be the preferred compound for clinical trials in acute morphine poisoning as it was of clinical significance to have a drug that could reverse the respiratory depression from high doses of morphine but not block the analgesic action. This was done in collaboration with Dr Wally Moon at the Austin Hospital and showed that 1,2,3,4-tetrahydro-9amino-acridine could be combined in the same ampoule as morphine in ~ equal concentrations which allowed larger doses of morphine to be given to a group of 60 patients with intractable pain of carcinoma without producing respiratory depression or narcosis (Stone et al, 1961). A use patent for this combination in an ampoule was filed in 1961 by H.W. Woods Pty Ltd of Melbourne and an Australian patent (No.263669) was granted in 1965. In 1963, 1,2,3,4-tetrahydro-acridin-9-amine HCl was given the pharmacoepia name of Tacrine.
1.
Mechanism of action of tacrine With regard to the mechanism of action of tacrine, Bentley found by classical
pharmacological experiments that tacrine was a potent cholinesterase inhibitor and was likely to be potentiating cholinergic transmission in the brain and periphery which could explain the respiratory stimulation in morphine-treated animals. This was
confirmed by a definitive biochemical assay by Dr Shirley Freeman who was a Senior Lecturer in the new Dept of Pharmacology in 1965. It was found that tacrine was ~ 100 times more potent in inhibiting pseudocholinesterase than acetylcholinesterase. Tacrine appears to bind to a hydrophobic site near the active site of these cholinesterases. Dr Freeman then became a Laboratory Head at the Australian Government Defence Science & Technology Organisation where her group carried out extensive research into the molecular actions of tacrine, both using a variety of esterases and also on muscarinic receptors and potassium channels in brain to assess if it had a role in treatment of nerve poisoning by organophosphorus compounds in chemical warfare. They compiled a definitive review of the complex pharmacological profile of tacrine with 162 references in 1991 (Freeman & Dawson, 1991). It is worth summarising their conclusion: “ Briefly, tacrine is a potent and reversible inhibitor of cholinesterases. Its action has both competitive and noncompetitive components. It is an antagonist of muscarinic M1 and M2 receptors, but interacts only weakly with nicotinic receptors. Binding to other neurotransmitter receptors is either weak or virtually non-existent.”
2.
Tacrine for treating Alzheimer’s dementia The above studies may suggest that nothing further needed to be discovered of
tacrine. Not so. Clinical studies with it continued and through the research of Dr William Summers in the USA, he found that tacrine was very effective in treating acute anticholinergic syndrome from overdoses of tricyclic antidepressant drugs as it was relatively safe in reversing the induced coma (Summers et al., 1980). Interestingly, he gained experience in the clinical use of tacrine and was encouraged by Dr Sam Gershon nearby at the Missouri Institute of Psychiatry. Sam Gershon had
been a Senior Lecturer in Frank Shaw’s Dept of Pharmacology from 1954-62 and had given tacrine to patients with glycolate-induced psychosis. Given that a selective loss of central cholinergic neurons were then found to be a feature in Alzheimer’s dementia, Summers reasoned that the remaining cholinergic transmission at the early stages of the disease could be enhanced by the administration of an inhibitor of acetylcholinesterase which passed through the blood-brain barrier. He applied to the US Patent Office and received in 1989 a use patent for tacrine for the treatment of Alzheimer’s disease (US 4,816,456) and then to the FDA for an Investigational Number for a Drug (IND) application which was sponsored by Dr Sam Gershon. The IND was granted and clinical trials in dementia patients commenced. In September 1993, tacrine became the first FDA approved treatment for Alzheimer’s disease. Summers licensed its use for the treatment of Alzheimer’s disease to Parke Davis who marketed it orally as Cognex capsules. Its advantage was that it could be given orally as well as by injection and it also crossed the blood-brain barrier. As more laboratories studied tacrine in Alzheimer’s patients, considerable debate over its effectiveness ensued due to conflicting clinical trial results because of the difficulty of studying this disease and to large individual pharmacokinetic variation which influenced both efficacy and the incidence of adverse effects in patients. In summary, tacrine had a palliative effect in treating patients with mild to moderate dementia but did not alter the course of the underlying neurodegeneration. Its clinical pharmacokinetics has been reviewed by Madden et al (1995) and Summers has reviewed and assessed the conflicting clinical studies with tacrine (Summers, 2006).
3.
Future research of tacrine derivatives Finally, in recent years, papers have been published in medicinal chemistry
journals of several classes of new chemical entities formed by combining tacrine with other pharmacophores such as an antioxidant (Trolox) or a flavonoid scaffold or a muscarinic M1 agonist with the strategy of maintaining cholinesterase inhibition plus adding beneficial actions such as activity against beta-amyloid formation and aggregation or inhibition of beta-secretase activity (reviewed by Romero et al., 2013). Clearly it will be many years before it can be established if these multifunctional hybrid drugs are effective against Alzheimer’s disease.
Thus from a compound that was reported as inactive as an antibacterial agent in 1945 it was discovered serendipitously in a dog in vivo preparation to have an interesting pharmacological action. It is a salutary lesson of not solely using an in vitro screening assay for a new chemical entity. Currently, Chemical Abstracts lists 3954 references to papers on or using tacrine in their SciFinder database.
References: Albert A, Rubbo SD, Goldacre RJ, Davey ME & Stone JD (1945) The influence of chemical constitution on antibacterial activity. Part II: A general survey of the acridine series. Br J Exp Pathol 26:160-192. Freeman SE & Dawson RM (1991) Tacrine: a pharmacological review. Prog Neurobiol 36: 257-277. Madden S, Spaldin V & Park BK (1995) Clinical pharmacokinetics of tacrine. Clin Pharmacokinet 28: 449-457. Romero A, Cacabelos R, Oset-Gasque MJ, Samadi A & Marco-Contelles J (2013) Novel tacrine-related drugs as potential candidates for the treatment of Alzheimer”s disease. Bioorg & Med Chem Lett 23: 1916-1922. Shaw FH & Bentley G (1949) Some aspects of the pharmacology of morphine, with special reference to its antagonism by 5-amino-acridine and other chemically related compounds. Med J Aust ii: 868-874. Shaw FH & Bentley G (1952) Morphine antagonism. Nature 169: 712-713. Stone V, Moon W & Shaw FH (1961) Treatment of intractable pain with morphine and tetrahydroaminacrine. Brit Med J 1: 471-473. Summers WK, Kaufman KR, Altman F & Fischer JM (1980) THA- A review of the literature and its use in treatment of five overdose patients. Clin Toxicol 16: 269-281. Summers WK (2006) Tacrine and Alzheimer’s treatments. J Alzheimer’s Dis 9: 439445.
S1043-6618(16)31397-4 http://dx.doi.org/doi:10.1016/j.phrs.2016.12.033 YPHRS 3456
To appear in:
Pharmacological Research
Received date: Accepted date:
12-12-2016 21-12-2016
Please cite this article as: Jarrott Bevyn.Tacrine: In vivo veritas.Pharmacological Research http://dx.doi.org/10.1016/j.phrs.2016.12.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tacrine : In vivo veritas
Corresponding author: Bevyn Jarrott Florey Institute of Neuroscience & Mental Health University of Melbourne Parkville 3010, Australia Email: [email protected] Abstract Tacrine was initially synthesised in 1945 as part of a project seeking antibacterial drugs to treat infected wounds in soldiers. However, it was inactive in vitro against common strains of bacteria. Serendipitously, it was injected in vivo into dogs anaesthetised with chloroform and morphine and noted to immediately counter the respiratory rate depression caused by morphine but not block analgesia. Subsequent studies showed that tacrine was an acetylcholinesterase inhibitor. When combined with morphine in ampoules it was possible to inject larger doses of morphine without causing respiratory depression and it was marketed for10 years in Australia. Tacrine was also used alone for treating acute anticholinergic syndrome in the 1980s. Shortly after this, it was hypothesised by William Summers that it could be of benefit in treating the early stages of Alzheimer’s dementia and an IND was granted by the US Food and Drug Administration and a use patent awarded to Summers. It was the first of four anticholinesterases to be approved for treating this condition although its variable pharmacokinetics was a disadvantage.
Key words: Tacrine; cholinesterase inhibitor; block of opiate-induced respiratory depression; antagonist of acute anticholinergic syndrome; oral treatment of Alzheimer’s dementia. Chemical compound reported in this article: Tacrine (PubChem CID:1935; 1,2,3,4-tetrahydroacridin-9-amine) Aminoacridine (PubChem CID:7019; acridin-9-amine)
1.
Discovery of Tacrine The discipline of Pharmacology in Australia commenced with the establishment of
a Department of Pharmacology at the University of Sydney led by Professor Roland Thorp in 1949 and at the University of Melbourne led by Professor Frank Shaw in 1953. Prior to that, drug research and discovery occurred mainly in Departments of Physiology and Chemistry. The following is an account of the synthesis and pharmacological evaluation of tacrine from 1945 to 1980 in Australia and then its subsequent development in the USA as the first drug approved by the US Food & Drug Administration (FDA) for the treatment of Alzheimer’s dementia.
During the Second World War (1939-45) both these universities actively collaborated in a project to develop potential antibacterial drugs for treating infected wounds in soldiers since this was a major source of fatalities in the First World War. At the University of Sydney, Adrien Albert was a leading authority on the chemical synthesis of heterocyclic compounds as well as a leader in relating pharmacological properties of compounds to physical chemical properties such as basicity, degree of ionisation at different pH and lipid solubility which he documented in detail in his textbook Selectivity Toxicity. During this period he and his staff synthesised at least 80 compounds of the acridine class to expand upon the topical antiseptic, 9-aminoacridine (pharmacoepia name of Aminoacridine, Trade name of Monacrin). Albert collaborated with an experienced bacteriologist at the University of Melbourne, Sydney Rubbo who was trained in the classical in vitro assays of antibacterial agents pioneered by Paul Ehrlich and Gerhard Domagk in Germany in the early 1900s.
Earlier, acridine dyes such as Acriflavine and Aminoacridine had been found to be
effective antiseptics when applied to the skin but were too toxic and ineffective in treating blood septicemia so the goal was to find a non-toxic compound that was antibacterial in vivo. While that goal was not achieved by Albert and Rubbo, fortunately it was achieved in the early 1940s by the efforts of an Australian pathologist at the University of Oxford, Professor Howard Florey, who succeeded in the large scale production and purification of water soluble penicillin that had been discovered serendipitously by Alexander Fleming of St.Mary’s Hospital, London in 1928.
What did emerge from Albert & Rubbo’s structure-activity studies of 77 new acridines was the finding that the synthesis of an 9-amino-acridine analog with a saturated C ring (1,2,3,4-tetrahydro-9-amino-acridine) was completely inactive as an antibacterial compound in vitro and so no further compounds were made with this saturated ring (Albert et al, 1945). While Rubbo carried out in vitro assays of these compounds against a range of gram-positive and gram-negative bacteria, no in vivo studies were included. Such studies were then done in the Dept of Physiology at the University of Melbourne by a senior lecturer, Frank Shaw (who became the foundation Professor of Pharmacology in 1953) and a postgraduate student, Geoffrey Bentley (who later became the foundation Associate Professor of Pharmacology in the Dept of Physiology at Monash University in 1967). As they had a plentiful supply of 9-amino-acridine, they injected this into a dog anesthetised with chloroform and a high dose of morphine in order to estimate its pharmacokinetic profile. Serendipitously, they noted that this compound immediately increased the depressed respiratory rate of the dog induced by morphine and the dog quickly regained consciousness. This was unexpected but clearly of interest and they had
access to many of the acridines synthesised by Albert for testing in this model. Again, unexpectedly, 1,2,3,4-tetrahydro-9-amino-acridine was more potent than 9amino-acridine in stimulating morphine-induced respiratory depression and arousing the dog from its unconscious state and appeared less toxic than 9-amino-acridine. They published an extensive paper on this action of 7 acridines and other heterocyclic compounds (Shaw & Bentley, 1949). In another paper (Shaw & Bentley 1952) they concluded that 1,2,3,4-tetrahydro-9-amino-acridine would be the preferred compound for clinical trials in acute morphine poisoning as it was of clinical significance to have a drug that could reverse the respiratory depression from high doses of morphine but not block the analgesic action. This was done in collaboration with Dr Wally Moon at the Austin Hospital and showed that 1,2,3,4-tetrahydro-9amino-acridine could be combined in the same ampoule as morphine in ~ equal concentrations which allowed larger doses of morphine to be given to a group of 60 patients with intractable pain of carcinoma without producing respiratory depression or narcosis (Stone et al, 1961). A use patent for this combination in an ampoule was filed in 1961 by H.W. Woods Pty Ltd of Melbourne and an Australian patent (No.263669) was granted in 1965. In 1963, 1,2,3,4-tetrahydro-acridin-9-amine HCl was given the pharmacoepia name of Tacrine.
1.
Mechanism of action of tacrine With regard to the mechanism of action of tacrine, Bentley found by classical
pharmacological experiments that tacrine was a potent cholinesterase inhibitor and was likely to be potentiating cholinergic transmission in the brain and periphery which could explain the respiratory stimulation in morphine-treated animals. This was
confirmed by a definitive biochemical assay by Dr Shirley Freeman who was a Senior Lecturer in the new Dept of Pharmacology in 1965. It was found that tacrine was ~ 100 times more potent in inhibiting pseudocholinesterase than acetylcholinesterase. Tacrine appears to bind to a hydrophobic site near the active site of these cholinesterases. Dr Freeman then became a Laboratory Head at the Australian Government Defence Science & Technology Organisation where her group carried out extensive research into the molecular actions of tacrine, both using a variety of esterases and also on muscarinic receptors and potassium channels in brain to assess if it had a role in treatment of nerve poisoning by organophosphorus compounds in chemical warfare. They compiled a definitive review of the complex pharmacological profile of tacrine with 162 references in 1991 (Freeman & Dawson, 1991). It is worth summarising their conclusion: “ Briefly, tacrine is a potent and reversible inhibitor of cholinesterases. Its action has both competitive and noncompetitive components. It is an antagonist of muscarinic M1 and M2 receptors, but interacts only weakly with nicotinic receptors. Binding to other neurotransmitter receptors is either weak or virtually non-existent.”
2.
Tacrine for treating Alzheimer’s dementia The above studies may suggest that nothing further needed to be discovered of
tacrine. Not so. Clinical studies with it continued and through the research of Dr William Summers in the USA, he found that tacrine was very effective in treating acute anticholinergic syndrome from overdoses of tricyclic antidepressant drugs as it was relatively safe in reversing the induced coma (Summers et al., 1980). Interestingly, he gained experience in the clinical use of tacrine and was encouraged by Dr Sam Gershon nearby at the Missouri Institute of Psychiatry. Sam Gershon had
been a Senior Lecturer in Frank Shaw’s Dept of Pharmacology from 1954-62 and had given tacrine to patients with glycolate-induced psychosis. Given that a selective loss of central cholinergic neurons were then found to be a feature in Alzheimer’s dementia, Summers reasoned that the remaining cholinergic transmission at the early stages of the disease could be enhanced by the administration of an inhibitor of acetylcholinesterase which passed through the blood-brain barrier. He applied to the US Patent Office and received in 1989 a use patent for tacrine for the treatment of Alzheimer’s disease (US 4,816,456) and then to the FDA for an Investigational Number for a Drug (IND) application which was sponsored by Dr Sam Gershon. The IND was granted and clinical trials in dementia patients commenced. In September 1993, tacrine became the first FDA approved treatment for Alzheimer’s disease. Summers licensed its use for the treatment of Alzheimer’s disease to Parke Davis who marketed it orally as Cognex capsules. Its advantage was that it could be given orally as well as by injection and it also crossed the blood-brain barrier. As more laboratories studied tacrine in Alzheimer’s patients, considerable debate over its effectiveness ensued due to conflicting clinical trial results because of the difficulty of studying this disease and to large individual pharmacokinetic variation which influenced both efficacy and the incidence of adverse effects in patients. In summary, tacrine had a palliative effect in treating patients with mild to moderate dementia but did not alter the course of the underlying neurodegeneration. Its clinical pharmacokinetics has been reviewed by Madden et al (1995) and Summers has reviewed and assessed the conflicting clinical studies with tacrine (Summers, 2006).
3.
Future research of tacrine derivatives Finally, in recent years, papers have been published in medicinal chemistry
journals of several classes of new chemical entities formed by combining tacrine with other pharmacophores such as an antioxidant (Trolox) or a flavonoid scaffold or a muscarinic M1 agonist with the strategy of maintaining cholinesterase inhibition plus adding beneficial actions such as activity against beta-amyloid formation and aggregation or inhibition of beta-secretase activity (reviewed by Romero et al., 2013). Clearly it will be many years before it can be established if these multifunctional hybrid drugs are effective against Alzheimer’s disease.
Thus from a compound that was reported as inactive as an antibacterial agent in 1945 it was discovered serendipitously in a dog in vivo preparation to have an interesting pharmacological action. It is a salutary lesson of not solely using an in vitro screening assay for a new chemical entity. Currently, Chemical Abstracts lists 3954 references to papers on or using tacrine in their SciFinder database.
References: Albert A, Rubbo SD, Goldacre RJ, Davey ME & Stone JD (1945) The influence of chemical constitution on antibacterial activity. Part II: A general survey of the acridine series. Br J Exp Pathol 26:160-192. Freeman SE & Dawson RM (1991) Tacrine: a pharmacological review. Prog Neurobiol 36: 257-277. Madden S, Spaldin V & Park BK (1995) Clinical pharmacokinetics of tacrine. Clin Pharmacokinet 28: 449-457. Romero A, Cacabelos R, Oset-Gasque MJ, Samadi A & Marco-Contelles J (2013) Novel tacrine-related drugs as potential candidates for the treatment of Alzheimer”s disease. Bioorg & Med Chem Lett 23: 1916-1922. Shaw FH & Bentley G (1949) Some aspects of the pharmacology of morphine, with special reference to its antagonism by 5-amino-acridine and other chemically related compounds. Med J Aust ii: 868-874. Shaw FH & Bentley G (1952) Morphine antagonism. Nature 169: 712-713. Stone V, Moon W & Shaw FH (1961) Treatment of intractable pain with morphine and tetrahydroaminacrine. Brit Med J 1: 471-473. Summers WK, Kaufman KR, Altman F & Fischer JM (1980) THA- A review of the literature and its use in treatment of five overdose patients. Clin Toxicol 16: 269-281. Summers WK (2006) Tacrine and Alzheimer’s treatments. J Alzheimer’s Dis 9: 439445.