HIGH-RESOLUTION NMR SPECTROSCOPIC STUDIES ON THE METABOLISM AND FUTILE DEACETYLATION OF 4-HYDROXYACETANILIDE (PARACETAMOL) IN THE RAT

Paracetamol (4-hydroxyacetanilide, acetaminophen) was synthesized with the acetyl group labelled with (CH3)-H-2 (paracetamol-(CH3)-H-2), and dosed to rats i.p. at 25 mg/kg (N = 5) and 40 mg/kg (N = 3) body weig ht. Paracetamol, with a (CH3)-C-13 in the acetyl group (paracetamol-(C H3)-C-13) was also synthesized and dosed to rats i.p, at 40 mg/kg (N = 3). The metabolism and excretion of the H-2-labelled compound was fol lowed in the rat using 600 MHz H-1 and 92.1 MHz H-2 NMR spectroscopy o f urine collected 0-8, 8-24, 24-32 and 32-48 hr post-dosing. The metab olism of paracetamol-(CH3)-C-13 was also monitored using 600 MHz H-1 N MR spectroscopy of urine collected 0-8, 8-24 and 24-48 hr post-dosing. For paracetamol-(CH3)-H-2 the total recovery of the sulphate, glucuro nide and N-acetyl cysteinyl metabolites via the urine accounted for 61 .2 +/- 14.1% of the 25 mg/kg dose and 61.4 +/- 8.8% of the 40 mg/kg do se. For paracetamol-(CH3)-C-13 the recovery was 102.7 +/- 3.7% indicat ing that the low % urinary recovery with the (CH3)-H-2-labelled drug i s the result of isotope effects on the disposition of paracetamol. In the case of the paracetamol-(CH3)-H-2, quantitative H-1 NMR analysis o f urine showed that 13.3 +/- 0.5 and 10.0 +/- 1.2 mole % (25 and 40 mg /kg, respectively) of the urinary paracetamol sulphate recovered follo wing dosing of the deuterium labelled drug had the (CH3)-H-2 acetyl gr oups replaced by (CH3)-H-1 acetyl groups from endogenous sources. In t he case of the paracetamol-(CH3)-C-13 8.9 +/- 0.7 mole % of the sulpha te conjugate had also been transacetylated to paracetamol-(CH3)-C-12. There was no significant difference between the level of futile deacet ylation observed for the deuterated and C-13-labelled drug. Overall th ese data indicate a high level of deacetylation followed by reacetylat ion (i.e. futile deacetylation) prior to excretion of paracetamol via the nephrotoxic intermediate 4-aminophenol. The level of deacetylation is much higher than has previously been thought which may cast new li ght on the role of 4-aminophenol in the development of paracetamol ind uced nephrotoxicity.