The relative bioavailability of paracetamol after rectal administration of suppositories containing a mixture of paracetamol, codeine phosphate and buclizine hydrochloride in healthy volunteers

High-dose rectal and oral acetaminophen in postoperative patients--serum and saliva concentrations

BACKGROUND

The primary purpose of the study was to examine the absorption of acetaminophen by measuring serum and saliva concentrations produced by a standard postoperative acetaminophen dosing regimen and secondary to examine the correlation between saliva and serum concentrations of acetaminophen after rectal and oral dosing.

METHODS

Twenty-four women, aged 18-60 years, scheduled for minor gynaecological laparoscopic surgery were studied. Patients received acetaminophen 2000 mg suppositories after surgery and oral doses of 1000 mg at 4 and 8 h postoperatively. Alfentanil was available via patient-controlled analgesia. Saliva and blood samples were collected postoperatively.

RESULTS

At 1, 2, 3, and 4 h after rectal dosing the saliva concentrations (mean+/-SD) were 15.2+/-5.9 micromol/l, 33.7+/-12.5 micromol/l, 45.5+/-19.1 micromol/l, and 55.4+/-23.1 micromol/l, respectively. The serum concentrations at 2 and 4 h were 31.0+/-11.2 micromol/l and 54.8+/-23.8 micromol/l, respectively. Additional oral dosing resulted in saliva concentrations at 5, 8, and 9 h of 99.7+/-49.5 micromol/l, 106.9+/-31.7 micromol/l, and 139.3+/-55.4 micromol/l, respectively, with coincident serum concentrations of 100.1 +/- 50.2 micromol/l, 105.6+/-29.0 micromol/l, and 141.2+/-52.1 micromol/l. After rectal dosing the linear regression resulted in r2=0.96, P<0.001 and saliva/ serum-ratio=0.99. After additional oral dosing the outcome of linear regression was: r2=0.90, P<0.001 and saliva/serum-ratio= 1.00.

CONCLUSION

The slow and ongoing absorption process resulting in no maximum concentration within 4 h after administration of 2000 mg acetaminophen suppositories makes this rectal regimen therapeutically irrational for treatment of postoperative pain. The significant ratio and linear correlation between saliva and serum concentrations of acetaminophen suggests that saliva could be used instead of blood to monitor acetaminophen administration in patients.

Lack of effect of paracetamol on the pharmacokinetics and metabolism of codeine in man

Plasma and urine concentrations of codeine and its measurable metabolites were determined by HPLC in six healthy subjects after a single 30 mg oral dose of codeine either alone or after 7 doses of 1 g paracetamol 8 hourly. After codeine alone, the t1/2 (h), AUC (mumol.l-1.h) and CLR (ml.min-1) for codeine were 2.2, 0.81, and 252 respectively. These were not significantly altered by paracetamol: 2.2, 0.84, and 291 respectively. For codeine-6-glucuronide the values were 2.4, 22.0, and 29.7 respectively. These were not significantly different from those after codeine plus paracetamol: 2.4, 21.9, and 39.6. There were no significant differences between the two treatments in the apparent partial clearances (ml.min-1) of codeine to morphine (88 codeine alone, 70 codeine plus paracetamol), to norcodeine (71 codeine alone, 88 codeine plus paracetamol), and to codeine-6-glucuronide (820 codeine alone, 1022 codeine plus paracetamol). The urinary excretion of codeine-6-glucuronide, morphine, norcodeine, and codeine were not significantly different between the two treatments.