Antipyrine: Radioimmunoassay in plasma and saliva following administration of a high dose and a low dose

A simple and sensitive radioimmunoassay has been developed for the determination of antipyrine levels in plasma and saliva of man. Antiserum to antipyrine was obtained from rabbits immunized with an immunogen prepared by covalently coupling N-(4-antipyrinyl)-succinamic acid to bovine serum albumin (BSA). The radioimmunoassay can detect antipyrine levels as low as 10 nglml of plasma or saliva, using a 0.1-ml sample. This contrasts with the sensitivity of a commonly used spectrophotometric method that can measure about 4,000 nglml using a 2-ml plasma sample. Agreement between the radioimmunoassay and spectrophotometric assay of antipyrine was excellent for plasma (r = 0.98) and saliva (r = 0.97) when samples were analyzed from 6 subjects receiving 18 mg/kg of antipyrine. The correlation between plasma and saliva antipyrine half-lives using the radioimmunoassay and an 18 mg/kg dose of antipyrine was r = 0.90 (p < 0.005). After a dose of 1.8 mg/kg of antipyrine, the drug disappeared monoexponentially from plasma and saliva for at least 51 hr, and the correlation between plasma half-life and saliva half-life was r = 0.97 (p < 0.001) in the 6 subjects. Excellent agreement was also observed between half-lives after the high and low doses of antipyrine (r = 0.99, p < 0.001 for plasma and r = 0.98, p < 0.001 for saliva).