OPTICAL MEASUREMENT OF PHOTOSENSITIZER CONCENTRATION IN VIVO MARTIN R. AUSTWICK * , JOSEPHINE H. WOODHAMS * ,z , VADZIM CHALAU * , CHARLES A. MOSSE * , CAROLINE ELIOT * , LAURENCE LOVAT * , ALEXANDER J. MACROBERT * , IRVING J. BIGIO y and STEPHEN G. BOWN * * National Medical Laser Centre Charles Bell House, 67-73 Riding House Street University College London London W1W 7EJ, UK y Boston University, BME, 44 Cummington Street Boston, MA 02215, USA z j.woodhams@ucl.ac.uk Most techniques for measuring tissue concentrations of drugs are invasive, time-consuming, and often require the removal of tissue or body °uids. Optical pharmacokinetics (OP) is a minimally invasive alternative giving an immediate result. Pulses of white light are directed at the tissue of interest using a ¯ber optic probe. Scattered light is detected by a second ¯ber immediately adjacent to the ¯rst in the same probe (separation 1.7 mm). Using the photosensitizer dis- ulfonated aluminium phthalocyanine (AlS 2 Pc), OP measurements were made in phantoms and on the mouth, stomach, colon, skin, and liver of normal rats 1 and 24 h after intravenous AlS 2 Pc administration. AlS 2 Pc concentration was determined by calculating the area under the curve (AUC) in the spectral region around the peak drug absorption or measuring the height of the peak. Spectral baseline interpolation removed the need for pre-drug, control optical measure- ments. OP measurements correlated well with values from alkali chemical extraction (CE) of the corresponding tissues, (R 2 0.870.97). OP measurements in the mouth also correlated with CE of less accessible internal organs (R 2 0.770.88). In phantoms, the lowest detectable concentration was 0.1 "g/g. In vivo, results were limited by the lower accuracy in the CE measurements but were almost certainly comparable. An incidental ¯nding was a 1215 nm red shifted component in the spectra observed 1 h after drug administration, suggesting partitioning of the drug in di®erent microenvironment compartments, which could prove to be of considerable interest in future studies. In conclusion, OP shows promise for real-time measurement of concentrations of drugs with suitable absorption peaks. Keywords : Optical pharmacokinetics; chemometrics; photodynamic therapy; noninvasive measurement. 1. Introduction Most current methods for quantifying drug concen- trations in tissue are invasive and usually require tis- sue extraction, making them unsuitable for real-time dosimetry. An important potential application of optical pharmacokinetics (OP) is for the dosimetry of photosensitizers for photodynamic therapy (PDT). This paper addresses the use of OP, a noninvasive Journal of Innovative Optical Health Sciences Vol. 4, No. 2 (2011) 97111 # . c World Scienti¯c Publishing Company DOI: 10.1142/S1793545811001460 97