PPAP

Historical-interest compound, not a current pick. PPAP is József Knoll's original catecholaminergic activity enhancer (CAE) — the first molecule he extracted from the deprenyl scaffold to prove that CAE activity is independent of MAO-B inhibition. It worked in animals, never reached human clinical trials, and was superseded by BPAP (~130× more potent in vivo, plus serotonin enhancement) within 7 years. For Dylan: SKIP. The CAE slot in the stack is filled by either low-dose selegiline (Rx, 40+ years of human data) or BPAP (research-chem, the actually-relevant successor) — PPAP offers nothing either of those don't do better.

The CAE concept (Knoll's central claim)

Knoll's late-career work centered on the idea that the brain's catecholaminergic neurons (dopamine and noradrenaline) have an "enhancer regulation" layer — endogenous modulation that increases the amount of transmitter released per impulse without altering the firing rate itself. Trace amines like β-phenylethylamine (PEA) and tyramine were proposed as endogenous enhancers. Synthetic enhancers like PPAP, Knoll argued, mimic this physiological-gated release — fundamentally different from amphetamine (which dumps neurotransmitter chaotically) or reuptake inhibitors (which raise tonic levels).

What PPAP actually does

1. Impulse-coupled DA + NE release enhancement. When a catecholaminergic neuron fires, PPAP increases the magnitude of vesicular release into the synapse. It does NOT trigger release in the absence of an impulse (this is the "physiological gating" claim, the same one that distinguishes selegiline's CAE arm from amphetamine).
2. No MAO inhibition. This was the whole point of synthesizing PPAP — to dissociate CAE activity from MAO-B inhibition and prove the two are mechanistically separate. PPAP has no measurable MAO-A or MAO-B inhibitory activity.
3. No direct release activity. PPAP is taken up into catecholamine axon terminals and vesicles but does not act as a releaser in the amphetamine sense (in fact it interferes with releasers — it can BLOCK amphetamine-induced release at the same terminal).
4. 2025 revision — DAT reuptake inhibition. A 2025 in-vitro screen reported PPAP as a potent dopamine reuptake inhibitor: IC50 57.5 nM at DAT, 571 nM at NET, 19,000 nM at SERT. By dopamine-reuptake potency this puts it 1.3× more potent than amphetamine, 7× more potent than mephedrone, 41× less potent than MDPV. This complicates the original "pure CAE" narrative — PPAP's animal effects may be partly DAT-mediated, not just impulse-coupled CAE.
5. Likely TAAR1 agonism (emerging mechanism, 2020s research). The TAAR1 antagonist EPPTB reverses BPAP's CAE effect — strongly suggesting the entire MAE family (PPAP, BPAP, selegiline at low dose) acts via trace amine-associated receptor 1 agonism. PPAP-specific EPPTB-reversal data is thinner than BPAP's, but the family-level inference applies.

Why BPAP replaced PPAP

Knoll's lab synthesized 65+ analogs. BPAP (1-(benzofuran-2-yl)-2-propylaminopentane, 1999) replaced PPAP's phenyl ring with a benzofuran. Result: ~130× more potent in tetrabenazine-induced depression antagonism (full effect at 0.05 mg/kg in rats vs PPAP's 2.5 mg/kg), plus added serotonin enhancement (PPAP is dopamine + noradrenaline only; BPAP is DA + NE + 5-HT). BPAP became the lab reference compound and PPAP was retired from active investigation.