Pharmacokinetics and biochemistry studies on nicotinamide in the mouse

Nicotinamide sensitizes murine tumours to the effect of radiation, but the pharmacokinetics are not well characterized at doses that are achievable in humans. In the mouse, nicotinamide given i.p. at doses of 100-500 mg/kg showed biphasic elimination with dose-dependent changes in half-life. The initial half-life increased significantly (P < 0.05) from 0.8 to 2 h and the terminal half-life increased from 3.4 to 5.6 h over the dose range studied. Clearance, however, decreased significantly from 0.3 to 0.24 l kg-1 h-1 only at the highest dose. Peak concentrations increased in a dose-dependent manner from 1,000 to 4,800 nmol/ml. The main plasma metabolite in the mouse is nicotinamide N-oxide, the peak concentration of which increased only from 80 to 160 nmol/ml. The N-oxide, which is also a weak radiosensitizer, is subject to reduction to the parent nicotinamide following administration at a dose of 276 mg/kg; peak concentrations of the N-oxide of 1900 nmol/ml were reached in 10 min, whereas concentrations of nicotinamide produced by reduction reached a maximum of 144 nmol/ml at 1 h. Elimination of the N-oxide was also biphasic, with initial and terminal half-lives being 0.39 and 1.8 h, respectively. The bioavailability of both drugs given via the i.p. as compared with the i.v. route was close to 100%. Tumour concentrations of nicotinamide paralleled those in the plasma after a short lag. Tumour nicotinamide adenine dinucleotide (NAD) concentrations were elevated by factors of 1.5 and 1.8 following doses of 100 and 500 mg/kg nicotinamide, respectively. Maximal concentrations were seen after 3-6 h, but levels remained elevated for 16 h. No change in tumour energy charge or in plasma 5-hydroxytryptamine was detected following a dose of 500 mg/kg nicotinamide.