Quantitative determination by bioassay of photoactive 8-methoxypsoralen in serum

Photoactive 8-methoxypsoralen (8-MOP) levels in human and animal sera were determined by bioassay with Staphylcoccus aureus serving as the test organism. 8-MOP was extracted from sera with as 9:1 (V/V) ethyl acetate-n-hexane mixture and reconstituted in an aqueous medium following evaporation of the extractant. Bacterial suspensions containing extracted 8-MOP were irradiated for predetermined intervals with longwave ultraviolet light (UVA) at an intensity of 2.1 mw/cm2. Cultures containing known amounts of 8-MOP were used as standards and cytotoxicity (i.e., drug levels) determined by colony counts. The detection limit for 8-MOP was 5 ng/ml with an accuracy of +/- 10% above 10 ng/ml. Concomitant determinations of 8-MOP levels by high pressure liquid chromatography (HPLC) were in excellent agreement with the bioassay results.