PPAP

PPAP is phenylethylamine with a propyl group on the nitrogen and another on the carbon next to it. Amphetamine carries a single methyl group in the second position and methamphetamine a methyl in both; stretching those groups to ethyl or propyl is what removes the ability to push dopamine out of resting nerve terminals [3]. Its inventors called amphetamine its parent compound and derived it from selegiline [1][5].

It was made to answer a question about selegiline. Selegiline is a selective MAO-B inhibitor, and it is broken down in the body to methamphetamine and amphetamine [2]. Joseph Knoll's group in Budapest, which had created selegiline in the 1960s, argued that small doses of it also raise the activity of catecholamine neurons in a way that has nothing to do with MAO [6]. To show that directly they looked for selegiline analogues that neither inhibit MAO nor break down to amphetamines, and picked PPAP as the reference compound [1][4].

The 1992 paper that introduced it described a stimulant with an unusual profile. PPAP was taken up into catecholamine nerve endings but did not release their contents, and it inhibited the uptake of noradrenaline, of dopamine and of the amines that act by releasing them. It increased motility at 2 mg/kg and suppressed it only at 50 mg/kg, produced much less stereotyped behaviour than amphetamine or methamphetamine, facilitated learning and retention, reversed the behavioural depression caused by tetrabenazine, and was effective in the forced swimming test. Where amphetamines improved performance in a narrow band of low doses and then impaired it, PPAP improved it over a broad range. The authors suggested depression, Alzheimer's disease and attention-deficit hyperactivity disorder as plausible uses [1].

Later papers reframed it as a "catecholaminergic activity enhancer". The best-documented experiment is simple. Tetrabenazine empties catecholamine stores, and a rat given 1 mg/kg of it an hour before each session barely learns to avoid a foot shock over five days of training. Rats given 1 mg/kg of (−)-PPAP at the same time learned the task and rarely failed to escape, with no rise in the between-trial crossings that track general activity [7]. The group concluded that this effect was not explained by MAO inhibition, by presynaptic receptor blockade, by uptake inhibition or by catecholamine release, and that selegiline and PPAP both act by strengthening the link between a nerve impulse and the release of transmitter [7]. Their wider argument is that phenylethylamine and the amphetamines are enhancers too at low concentrations and releasers at higher ones, and that the releasing action hid the enhancer effect for decades [4].

The repeated-dose data are less tidy than the summaries of them. After 21 daily injections of 0.1 mg/kg, brain regions taken from male rats released one and a half to two times as much dopamine, and nearly twice as much noradrenaline, as those from saline-treated rats. At 0.05 mg/kg noradrenaline release was higher, but dopamine release was significantly lower than control in all three regions measured, and serotonin release from the raphe was lower too [8]. The paper reports the table as showing that PPAP acts like selegiline in very low doses and does not comment on the fall.

PPAP then became the starting point for something stronger. Changing its phenyl ring produced 65 new compounds, from which the benzofuran analogue BPAP was chosen; it was about 100 times more potent than PPAP in the shuttle box [2][9]. The paper that reported this called selegiline and PPAP "relatively weak" enhancers [2]. The group's attention moved to BPAP, and a 2016 review from the same laboratory still named selegiline as the only enhancer available as a drug [6].

PPAP never reached a patient. Knoll wrote in 2001 that it was the first enhancer free of selegiline's unwanted properties, and that its clinical efficacy had never been tested despite all the group's efforts [4]. The literature is also narrow: the work comes from Knoll's group and its collaborators at Fujimoto Pharmaceutical [3][9], apart from one paper on immune cells from another department of the same university [10]. Two of the papers, the 1992 pharmacology paper and a 1994 paper on repeated dosing, are not available online and are cited here from their abstracts [1][5]; the rest were read in full.

It is not a releaser. Amphetamine, methamphetamine and the trace amine phenylethylamine raise dopamine outflow from nerve terminals that are not being stimulated. In superfused rat striatal slices, phenylethylamine and methamphetamine raised resting dopamine release and (−)-PPAP at 10 µM did not [3]. The original paper reached the same conclusion with older methods [1], and the patent reports that the series did not contract the nictitating membrane of an anaesthetised cat, a sign of noradrenaline release, where amphetamine and methamphetamine caused long-lasting contraction [11].

It inhibits uptake. The 1992 paper called PPAP a potent inhibitor of the uptake of releasing amines and of noradrenaline and dopamine themselves [1]. On rabbit pulmonary artery it blocked the noradrenaline-releasing effect of tyramine and phenylethylamine with an ID50 of 1.22 µM, which the authors put down to competition for the neuronal transport system; it was more potent in this test than (−)-methamphetamine, and selegiline was too weak to measure [7][12]. The patent adds an IC50 of 50 nM for noradrenaline uptake in a rat cortex preparation, between desipramine (1–5 nM) and selegiline (7 µM) [11]. That last figure is company data from a filing and has no peer-reviewed counterpart.

The enhancer effect is measured on stimulated release. In isolated rat brain stem loaded with labelled noradrenaline, 2 and 4 µg/mL of (−)-PPAP increased the amount released by each burst of electrical stimulation while leaving the release between bursts unchanged or slightly raised [7]. In a screening table, the lowest concentration that raised stimulated noradrenaline and dopamine release by at least a quarter was 2.5 mg/L (9.6 µM) for both (−)-PPAP and selegiline and 0.05 mg/L for BPAP; (−)-PPAP did not reach that mark for serotonin at 5 mg/L and did at 10 mg/L [9]. The group's other assay removes brain regions from treated rats and measures how much transmitter the tissue gives off [13]. Thirty minutes after a single 0.1 mg/kg injection of (−)-PPAP, striatum, substantia nigra and olfactory tubercle released more dopamine and the locus coeruleus more noradrenaline than tissue from saline-treated rats [14]. In these assays the group rated PPAP about as potent as selegiline [6][14].

Uptake inhibition is not the developers' explanation for that effect. Desipramine, a strong uptake inhibitor, did nothing against tetrabenazine in the shuttle box at 0.5 to 10 mg/kg, and the brain-stem experiments were run with cocaine in the bath, so the 1996 paper ruled uptake inhibition out along with MAO inhibition, presynaptic receptors and release. What remained was stronger coupling between the action potential and transmitter release. The authors assumed that this works through a larger rise in cytosolic calcium, and the supporting observation was indirect: at 4 µg/mL, selegiline and PPAP enhanced the slow inward calcium current in sino-auricular fibres of the frog heart, in three fibres. They noted that calcium would need to be measured in neurons to settle it [7]. A companion paper reported that selegiline and (−)-PPAP reversed tetrabenazine in the shuttle box while the natural trace amines phenylethylamine and tyramine, which are broken down quickly in the body, did not at doses up to 40 mg/kg [12].

A receptor has been proposed but not tested with PPAP. Trace amine-associated receptor 1 (TAAR1) is an intracellular receptor for phenylethylamine and the amphetamines. The group suggested as early as 2002 that the newly discovered trace-amine receptors were the "enhancer receptors" [15]. In rat striatum the TAAR1 antagonist EPPTB blocked the enhancer effect of BPAP and, in a later study, of selegiline [3][16]. The authors propose two binding sites on TAAR1, one for releasers with small side-chain groups and one for enhancers with bulky ethyl or propyl groups, naming PPAP among the latter [3]. PPAP itself was tested only for its lack of releasing action in that paper, not against the antagonist.

It does not behave exactly like BPAP. A weak BPAP analogue, 3-F-BPAP, blocked BPAP's reversal of tetrabenazine in the shuttle box. It did not block selegiline's, and 5 mg/kg of (−)-PPAP still reversed tetrabenazine completely when 5 mg/kg of 3-F-BPAP was injected with it, which the authors took as the first evidence that phenylethylamine-derived and tryptamine-derived enhancers act differently [15]. PPAP acts mainly on catecholamine transmission, whereas BPAP is about equally active on catecholamines and serotonin [3].

What the structure-activity work found. Substituting PPAP's phenyl ring, for example with chlorine or fluorine, gave nothing remarkably more potent, and moving the nitrogen one carbon closer to or further from the ring made the compound weaker. Larger ring systems helped: the naphthyl analogue was about twice as potent in the shuttle box, the indolyl and methylenedioxyphenyl analogues about five times, and the benzofuran, BPAP, about 100 times. In the same test (−)-PPAP was about twice as potent as selegiline [9].

Which enantiomer is the active one is less settled than it looks. Later work uses the (−) form, which has the R configuration and is described as the more active [3], and it is reported as more potent than the (+) form on stimulated transmitter release [2]. The frog-heart paper says the same of the calcium current in its text, but in its own figure, at the single concentration tested, the curve labelled (+)-PPAP shows the larger current, roughly 1.5 µA against 1.1 µA [7]. The abstract of the 1994 repeated-dose study also says the (+) enantiomers of PPAP and p-fluorodeprenyl were the more active [5]; that paper's full text was not available.

Outside the brain. A separate group gave female rats 0.25 mg/kg of (−)-PPAP under the skin and measured serotonin in peritoneal immune cells by flow cytometry. Thirty minutes after a single dose at six weeks of age, serotonin was 19–26% higher in lymphocytes, in mast cells and in the pooled monocytes and granulocytes. Three weeks after three daily doses at weaning it was still 17–24% higher in the last two and unchanged in lymphocytes; the authors noted that the differences were hard to see under the microscope. Selegiline at the same dose had no acute effect and lowered serotonin by 28–40% three weeks later [10].

The only description of PPAP as a material is in the developer's patent, and what follows is the filing's own account [11]. The compound is made by condensing benzyl propyl ketone with n-propylamine and reducing the resulting imine with sodium borohydride. The free base is a fat-soluble oil. The hydrochloride is a crystalline, water-soluble salt melting at 122–124 °C; the later chemistry paper gives 123 °C [9]. The hydrochloride is the form used in the pharmacology papers. Chinoin supplied it for the studies of the 1990s and Fujimoto Pharmaceutical for the most recent one [3][7][17].

The patent's worked examples are hard gelatin capsules, tablets and coated tablets containing 30 mg of the hydrochloride, a 25 mg suppository and a 30 mg/mL solution for injection. These are illustrations of how the compound could be formulated. No dissolution, absorption or bioavailability data accompany them, and no pharmacokinetic study of PPAP in any species was found.

There is no human safety data. PPAP was never tested clinically [4], and no later trial, case report or human pharmacokinetic study was found, so its tolerability, dose ceiling and interactions in humans are unknown.

The measured toxicity is a single table in the patent. The lethal dose for half of the rats was 27 mg/kg intravenously, 50 mg/kg under the skin and 270 mg/kg by mouth [11]. This is company data with no methods beyond the route, and no repeat-dose, genetic or reproductive toxicity study appears in the patent or in the papers read for this page.

Behaviourally it looked milder than amphetamine. It was substantially less effective at inducing stereotyped behaviour, and it depressed motility only at 50 mg/kg [1]. At 1 mg/kg in the shuttle box it did not raise the number of between-trial crossings, the test's measure of general activity [7]. In the patent, 5 mg/kg reduced rats' food intake in the first hour about as much as 0.5 mg/kg of amphetamine, and the series raised metabolic rate less and for a shorter time than amphetamine or selegiline [11].

Its pharmacology still points to the usual stimulant questions. PPAP inhibits noradrenaline uptake at nanomolar concentrations in the patent's assay and raises impulse-evoked catecholamine release [7][11]. None of the papers read here reports heart rate or blood pressure. One interaction runs against expectation: by competing for transport into nerve endings, PPAP prevents tyramine-induced noradrenaline release in isolated artery [12].