(-)-Deprenyl (D) was developed in the early 1960s by Joseph Knoll, professor and head of the Pharmacological Department of the Semmelweis University in Budapest (Hungary). Knoll, a survivor of Auschwitz and the Dachau death train (Dunn, 1988, pp 209-211), started in the early 1950s his behavioral studies on rats: (i) aiming to understand the mechanism of the manipulability of the behavior of the most developed, domesticable mammals; (ii) to find reasonable explanation why humans possess the most manipulable brain among all living beings on earth; and (iii) to throw light upon the role of the manipulability of human behavior in the birth and development of the human society. He summarized his findings and conclusions in three monographs (Knoll 1969, 2005, 2012).

The early resounding success of his work was the discovery that manipulability of behavior appeared with the development of species capable to fix acquired drives. The rat for example possesses this ability; the mouse is devoid of it.

He realized from the very beginning the extraordinary importance of the catecholaminergic brain machinery (he called it: the engine of the brain) in the fixation of acquired drives. To stimulate the brain engine he used amphetamines as experimental tools. His problem with the amphetamines was that as soon as the dose surpassed the 1-2 mg/kg level they blocked purposeful behavior, because the drug-induced continuous, irresistible release of catecholamines from the intra-neuronal stores in the brain stem neurons resulted in aimless hypermotility. He decided to start a structure-activity-relationship (SAR) study in an attempt to develop an amphetamine-derivative devoid of this unwanted effect. Amphetamine and methamphetamine are long-acting synthetic analogues of b-phenylethylamine (PEA). In order to change substantially their pharmacological profile, Knoll decided to combine in the same molecule the structural features of methamphetamine and pargyline, the newly developed monoamine oxidase inhibitor (MAOI). It was the propargyl group in pargyline which by making a covalent binding with the flavin in the enzyme inhibited monamine oxidase (MAO) activity irreversibly. He designed a series of new structures and asked Mészáros, his close friend, the research director of Chinoin, the Hungarian pharmaceutical company, to contact him with a chemist experienced in the synthesis of phenylethylamines and pargyline. Zoltán Ecsery synthesized about 30 of the compounds designed by Knoll, who selected for the detailed studies E-250, as the one fitting best with his expectations. E-250 was later named deprenyl, to emphasize that the compound was planned for treating depression. The first publication on E-250 appeared in 1964 in Hungarian, followed by a paper in English in 1965. For further pharmaceutical development Knoll chose the (-)-enantiomer. (R)-N-methyl-N-(1-phenylpropan-2-yl)prop-1-yn-3-amine [Selegiline, (-)-Deprenyl, Eldepryl, Jumex, Zelepar, Emsam, Anipryl, and about 100 further trade names]. Selegiline is the presently world-wide available drug, registered in 63 countries to treat Parkinson’s disease (PD), Alzheimer’s disease (AD), major depressive disorder (MDD) and also used as a prophylactic anti-aging compound to slow the age-related-decline of the catecholaminergic brain engine (Knoll 2012).

Knoll designed originally to use D as a new antidepressant and asked his close friend, the psychiatrist Ervin Varga, who worked that time in the Semmelweis University, later in the USA, to perform a clinical study with E-250. Already in 1965 a preliminary note on the promising clinical trial with racemic E-250 in depressed patients was published in German (Varga 1965). The first paper showing that racemic E-250 is an efficient prompt acting antidepressant was published in English in 1967 (Varga and Tringer 1967). The first clinical trial with (-)-E-250 (later named Selegiline) in depressed patients showing its significant antidepressant effect was published in 1971 (Tringer et al. 1971). The finding was later confirmed in a couple of papers; nevertheless, Selegiline was first registered as an antidepressant only in 2006 in the USA. Emsam is the first transdermally applied antidepressant (Bodkin and Amsterdam 2002).

Knoll discovered in 1967 that D is a unique MAO inhibitor which, in contrast to the known ones, does not potentiate the catecholamine releasing effect of tyramine. Thus he realized that his compound must be free of the “cheese effect”. The hypertensive crisis associated with the ingestion of high amounts of tyramine in cheese, the metabolism of which is inhibited by MAO inhibition, restricted in the early 1960s the clinical use of the MAO inhibitors. An exact analysis of this nature of E-250 was published in 1968 (Knoll et al. 1968).  Knoll asked Varga to perform a rapid test investigating the safeness of E-250. Varga found that, as expected, even provocative cheese consumption failed to produce headache or hypertensive crisis. This finding was cited as a personal communication in the Knoll et al. paper (1968), but since Varga left Hungary, the work was not brought to fullness and was never published. The first two publications which exactly proved that D is free of the cheese effect in humans were published in 1978 in England (Elsworth et al. 1978, Sandler et al. 1978).

Knoll realized in 1970 that D is a highly selective inhibitor of B-type MAO and presented his finding at the First International MAO Symposium, held in Cagliari (Sardinia, Italy) in 1971. The first paper which described this novel property (Knoll and Magyar 1972) has become ten years later a citation classic. D became first famous as a key important experimental tool in MAO research.

The finding that D protects the nigrostriatal dopaminergic neurons from the toxic effect of 6-hydroxy-dopamine (6-OHDA) (Knoll 1978) was the first proof of the neuroprotective effect of the drug. The finding that D protects the striatum from the toxic effect of 6-OHDA via the blockade of B-type MAO, the inhibition of the uptake of 6-OHDA into the neuron, the facilitation of scavenger function, and the improvement of the removal of the neurotoxic free radicals (Knoll 1987) catalyzed the discovery that D is significantly enhancing scavenger function in the striatum. Knoll’s discovery that D-treatment significantly enhances in the striatum of both male and female rats the activity of superoxide dismutase (SOD) (Knoll 1988), was soon confirmed (Carillo et al. 1991), and D-induced enhancing of scavenger function was analyzed later in detail in series of papers. It was later described in dozens of papers that D protects neurons against a variety of neurotoxic agents:1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), DSP-4, 5,6-dihyroxyserotonine, AF-64A (Ebadi et al. 2002), and enhances the production of neurotrophins which are natural protective agents of neurons (Shimazu et al. 2003).

The first clinical trial with D, published in Lancet, proved that, in contrast to the known MAO inhibitors, D can be safely combined with levodopa and with this combination the levodopa sparing effect was achieved in patients without signs of significant hypertensive reactions (Birkmayer et al. 1977). This paper and the following Lancet Editorial (1982) initiated the world-wide use of D in PD.

Knoll presented first in his lecture at the Second Strategy in Drug Research IUPAC-IUPHAR Symposium held in Noordwijkerthout (The Netherlands) in 1981 his concept that preventive D medication which facilitates dopaminergic and trace-aminergic activity in the brain is a reasonable strategy to improve the quality of life in the latter decades (Knoll 1982). He presented first in his lecture at the 7^th European Symposium on Basic Research in Gerontology held in Budapest in 1983 experimental evidence proving that this effect of D is unrelated to the inhibition of MAO-B (Knoll 1985). To support his concept, Knoll proposed Birkmayer, that time the only clinician who treated long-lastingly hundreds of patients with D, to analyze in retrospect the survival of his patients treated with D. In an open, uncontrolled study the long term (9 years) effect of treatment with Madopar alone (n=377) or in combination with D (n=564) have been compared in parkinsonian patients. The survival analysis revealed a significant increase of life expectancy in Madopar+D group regardless of the fact whether or not the significant demographic differences between the two groups were taken into account (Birkmayer et. al. 1985). The first longevity study with D on the long living, robust Wistar-Logan rats, starting with two-year old males, was performed between 1985 and 1988. The study furnished unequivocal experimental evidence that prophylactic D-treatment prolongs the life of rats significantly (Knoll 1988, Knoll et al. 1989). The finding was soon confirmed on the short living Fischer F-344 strain of rats (Milgram et al. 1990). D-induced prolongation of lifespan was later further confirmed on rats and demonstrated also on mice, Syrian hamsters, dogs and even on Drosophila melanogaster. Knoll performed with his coworkers a second longevity study with 28-week old Wistar-Logan rats between 1990 and 1994. The aim of this study was to learn how low-dose, lifelong D treatment is influencing the lifespan of low and high performing rats. Out of 1600 sexually experienced male rats the 94 sexually inactive (low performing, LP) and the 99 most sexually active (high performing, HP) rats were selected. The LP rats died significantly earlier than their HP peers and D-treatment eliminated this difference (Knoll et al. 1994).

In the DATATOP multicenter clinical trial (USA, Canada) in 23 University Institutions, the ability of D and a-tocopherol, antioxidant agents that act through complementary mechanisms were studied, expecting to delay the onset of disability necessitating levodopa therapy (the primary end point) in patients with early, untreated PD. Eight hundred subjects were randomly assigned in a two-by-two factorial design to receive D, a-tocopherol, a combination of both drugs, or placebo, and were followed up to determine the frequency of development to the end point. The study proved that the treatment of de novo parkinsonians with D has a unique beneficial influence on the natural history of PD. D-treatment delayed significantly the need for levodopa therapy. The study also revealed that in contrast to the expectation of the authors a-tocopherol was ineffective. The first papers of this study were published in Science and New England Journal of Medicine in 1989 (Tetrud and Langston 1989, Parkinson Study Group 1989). The ineffectiveness of a-tocopherol in this study was explained later. D is enhancing the impulse propagation mediated release of dopamine (catecholaminergic activity enhancer - CAE effect), a-tocopherol is devoid of this property (Miklya et al. 2003).

In the early 1990s Knoll developed (-)-1-phenyl-2-propylaminopentane [(-)-PPAP], the D-analogue equally active with its parent compound but being devoid of the MAO inhibitory property. Zoltán Török performed the chemical work in Chinoin. This study furnished direct evidence that the main effect of D, the specific stimulation of the catecholaminergic brain engine, is unrelated to the inhibition of MAO (Knoll et al. 1992).

It was analyzed in detail between 1994 and 1996 that PEA acts, in a dose-range below the one which is continuously releasing catecholamines from the intra-neuronal stores, as a selective enhancer of the impulse propagation mediated release of catecholamines. Amphetamine and methamphetamine the long acting PEA-derivatives act similarly (Knoll et al. 1996a). Since the catecholamine-releasing property of PEA and the amphetamines concealed their CAE effect (Knoll 2012, pp 13-14), this property remained undetected. D, the only PEA derivative free of the catecholamine releasing property which exerts its CAE effect in concentrations below the dose which inhibits MAO-B activity, enabled the discovery of the enhancer regulation in the catecholaminergic neurons (Knoll and Miklya 1994, Knoll et al. 1996b, Knoll 1998).

The first two papers demonstrating the beneficial effect of D in AD were published in 1987 (Martini et al. 1987; Tariot et al. 1987). Series of clinical studies with small sample sizes confirmed thereafter the usefulness of D in this disease. In 1997 the first controlled trial of D in the treatment of AD was published in New England Journal of Medicine (Sano et al. 1997).

Based on his finding that tryptamine is like PEA a natural enhancer of the impulse propagation mediated release of transmitters from the catecholaminergic and serotonergic neurons Knoll developed R-(-)-1-(benzofuran-2-yl)-2-propylaminopentane [(-)-BPAP], a tryptamine-derived selective enhancer substance which exerts this effect in femto-picomolar concentrations. The chemical part of the SAR study was performed with a group of chemists in the research laboratory of the Fujimoto Pharmaceutical Corporation (Osaka) led by Fumio Yoneda (Knoll et al. 1999). (-)-BPAP is an about 100 times more potent CAE substance than D and acts even more potently on the serotonergic neurons. D is an almost selective CAE substance. With the development of (-)-BPAP the proper experimental tool is now available to search hitherto unknown enhancer-sensitive regulations in the brain.

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Ildiko Miklya

March 13, 2014