Paracetamol Metabolite Functions as a Selective Local Anesthetic

Paracetamol, commonly used for the relief of mild to moderate pain for decades, undergoes multiple transformations following oral ingestion. A significant metabolite, AM404, is produced in peripheral nerve cells and appears to exhibit local anesthetic properties.

Although the precise mechanisms behind paracetamol's pain-relieving effects remain somewhat unclear, it is believed that a considerable portion of its analgesic action occurs within the brain and spinal cord. This effect is primarily attributed to AM404, which interacts with cannabinoid receptors and TRPV1 channels in neurons of the central nervous system.

A recent study published in the scientific journal Proceedings of the National Academy of Sciences reveals that paracetamol is metabolized in the liver to 4-aminophenol, which subsequently reaches peripheral tissues. In these tissues, the enzyme fatty acid amide hydrolase (FAAH) facilitates the formation of AM404, as demonstrated through liquid chromatography-mass spectrometry (LC-MS/MS) on isolated trigeminal ganglia.

The study found that AM404 induces a TRPV1-mediated influx of cations at a concentration of 1 µM; however, unlike capsaicin, it does not generate action potentials. Instead, AM404 reduces neuronal excitability in both trigeminal and spinal sensory neurons in a concentration- and time-dependent manner, with an IC₅₀ of approximately 4-8 nM.

Behavioral experiments conducted on rats showed that localized injections of AM404 into their paws effectively diminished responses to mechanical and thermal stimuli. This analgesic effect commenced within 30 minutes and peaked after one hour, remaining localized.

The researchers identified sodium channels NaV1.7 and NaV1.8 as primary targets for AM404. The metabolite quickly and concentration-dependently inhibited sodium currents in both trigeminal and spinal ganglia neurons. This inhibition was also observed in heterologous systems, where AM404 inhibited the human NaV1.8 receptor with an IC₅₀ of around 55 nM, and the NaV1.7 receptor with an IC₅₀ of approximately 22 nM at -80 mV.

AM404 exhibits characteristics typical of local anesthetics. A mutation at the local anesthetic binding site (F1759A in the hNaV1.8 channel) completely abolished AM404's inhibitory effects.

In contrast, paracetamol itself, along with 4-aminophenol and the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI), displayed no effects or even counterproductive effects on the NaV1.7 and NaV1.8 channels at considerably higher concentrations in the millimolar range. Thus, AM404 stands out as the only metabolite that selectively inhibits these two sodium channels in the nanomolar range.

This study presents compelling evidence that the well-known active metabolite of paracetamol, AM404, is produced peripherally and exerts its effects through direct, selective inhibition of pain-specific sodium channels. Its mechanism of action is akin to that of local anesthetics but demonstrates greater selectivity. AM404 holds potential for development as a pain-selective, peripherally acting analgesic, likely with fewer central nervous system side effects.