title: Pharmacokinetics of varying doses of nicotinamide and tumour radiosensitisation with carbogen and nicotinamide: clinical considerations.
abstract: Plasma concentrations, after administration of varying doses of nicotinamide, were measured in CBA male mice using a newly-developed high performance liquid chromatography assay. In all dose groups, peak levels were observed within the first 15 min after an i.p. administration of 0.1, 0.2, 0.3 or 0.5 mg g-1 of nicotinamide. There was a clear dose-dependent increase in plasma concentration with increasing dose, with almost a five-fold lower concentration (1.0 vs 4.9 mumol ml-1) achieved with a dose of 0.1 mg g-1 compared with 0.5 mg g-1, respectively. The half-life of nicotinamide increased from 1.4 h to 2.2 h over the dose range (P < 0.01). Comparisons with previous pharmacokinetic data in humans show that clinically-relevant oral doses of 6 and 9 g in humans give plasma levels slightly higher than those achieved at 1 h with doses of 0.1 to 0.2 mg g-1 in mice. Tumour radiosensitisation with carbogen alone, and with carbogen combined with varying doses of nicotinamide (0.05 to 0.5 mg g-1), was investigated using a 10-fraction in 5 days X-ray schedule. Relative to air-breathing mice, a statistically significant increase in sensitisation was observed with both a local tumour control and with an in vivo/in vitro excision assay (P < or = 0.007). With the local control assay, a trend was observed towards lower enhancement ratios (ERs) with decreasing nicotinamide dose (from 1.85 to 1.55); carbogen alone was almost as effective as when combined with 0.1 mg g-1 of nicotinamide. With the excision assay, ERs for carbogen combined with nicotinamide increased with decreased levels of cell survival. At a surviving fraction of 0.02, enhancement ratios of 1.39-1.48 were obtained for carbogen plus 0.1 to 0.3 mg g-1 of nicotinamide. These were lower than those seen with the two higher doses of 0.4 to 0.5 mg g-1 (ERs = 1.63-1.69).
authors.affiliation: CRC Gray Laboratory, Mount Vernon Hospital, Northwood, UK.
authors.forename: A
authors.initials: A
authors.lastname: Rojas
authors.forename: R J
authors.initials: RJ
authors.lastname: Hodgkiss
authors.forename: M R
authors.initials: MR
authors.lastname: Stratford
authors.forename: M F
authors.initials: MF
authors.lastname: Dennis
authors.forename: H
authors.initials: H
authors.lastname: Johns
mesh_terms: D000230:Adenocarcinoma / Q000532:radiotherapy*; D000818:Animals; D002245:Carbon Dioxide / Q000097:blood / Q000493:pharmacokinetics*; D015687:Cell Hypoxia / Q000528:radiation effects; D002470:Cell Survival; D004305:Dose-Response Relationship, Drug; D016018:Least-Squares Analysis; D008297:Male; D008325:Mammary Neoplasms, Experimental / Q000532:radiotherapy*; D051379:Mice; D008808:Mice, Inbred CBA; D009536:Niacinamide / Q000097:blood / Q000493:pharmacokinetics*; D010100:Oxygen / Q000097:blood / Q000493:pharmacokinetics*; D011838:Radiation-Sensitizing Agents / Q000493:pharmacokinetics*
chemical_list: D011838:Radiation-Sensitizing Agents; D002245:Carbon Dioxide; D009536:Niacinamide; C011700:carbogen; D010100:Oxygen