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 Staphylococcus aureus serving as the test organism. 8-MOP was extracted from sera with a 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 long-wave 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 5ng/ml with an accuracy of ± 10% above 10ng/ml. Concomitant determinations of 8-MOP levels by high pressure liquid chromatography (HPLC) were in excellent agreement with the bioassay results