Nortadalafil

Nortadalafil (demethyl tadalafil, CAS 171596-36-4) is a synthetic analogue of the clinically approved phosphodiesterase type 5 (PDE5) inhibitor tadalafil, distinguished from the parent compound by the absence of the N-methyl substituent on the piperazinedione ring of the hexahydropyrazinopyridoindole scaffold. The compound retains nanomolar inhibitory potency against PDE5 (IC50 approximately 11 to 12 nM against bovine PDE5, compared to approximately 5 nM for tadalafil) and produces cGMP-mediated smooth muscle relaxation with EC50 values of 300 to 600 nM in rat aortic preparations. Despite this pharmacological activity, nortadalafil has never been submitted for regulatory approval in any jurisdiction and has not been the subject of controlled clinical trials in human subjects. The compound first entered the scientific literature through its detection as an undeclared adulterant in herbal and dietary supplements marketed for erectile dysfunction, a context in which it represents one member of a large and expanding family of synthetic PDE5 inhibitor analogues (including aminotadalafil, N-octylnortadalafil, chloropretadalafil, and others) introduced into the unregulated supplement market to evade analytical screening for the approved parent compounds. Analytical characterization of nortadalafil in adulterated products relies on high-performance liquid chromatography with diode-array and mass spectrometric detection, high-resolution mass spectrometry, and nuclear magnetic resonance spectroscopy. The pharmacokinetic profile of nortadalafil diverges substantially from tadalafil: the reported elimination half-life is approximately 5.9 to 6.2 hours after single oral doses of 40 to 120 mg, compared to the 17.5-hour half-life of tadalafil, a difference attributable to the loss of the N-methyl group and the resulting alteration of hepatic metabolic clearance. No systematic toxicology, no formal pharmacokinetic characterization in healthy volunteers under regulatory oversight, and no controlled efficacy data exist for the compound. Safety concerns are inferred from the PDE5 inhibitor class and include the absolute contraindication of concurrent nitrate administration (risk of severe, potentially fatal hypotension), the interaction with alpha-adrenergic receptor antagonists and potent CYP3A4 inhibitors, and the unknown dose-response and adverse-event profile of a compound consumed without medical supervision at uncharacterized doses in adulterated supplements. This monograph reviews the chemistry, synthesis, and structural relationship of nortadalafil to tadalafil; the molecular pharmacology of PDE5 inhibition and the cGMP-nitric oxide signaling cascade; the limited pharmacokinetic data; the forensic and regulatory context of dietary supplement adulteration; sourcing and quality verification for research applications; handling and reconstitution; stack-interaction considerations; the adverse-event and safety signal inferred from the PDE5 inhibitor class; and a comparative assessment of five PDE5 inhibitor compounds against nortadalafil on five competency standards. The compound is not approved by any regulatory authority. It is available as a research-grade reference standard; investigators should obtain analytical confirmation of identity and purity on every lot.