Pharmacokinetics of ICG and HPPH-car for Detection of Normal and Tumor Tissue Using Fluorescence, Near-Infrared Continuous Wave Imaging

Abstract

We present in-vivo fluorescent, near-infrared, continuous wave images of indocyanine green (ICG) and carotene-conjugated 2-devinyl-2-(l-hexyloxyethyl) pyropheophorbide (HPPH-car) to discriminate spontaneous canine adenocarcinoma from normal mammary tissue. Following intravenous administration of 1.0 mg/kg ICG or 0.3 mg/kg HPPH-car into the canine, a 20-mW, 780-nm or 100-mW, 660-nm laser diode beam, expanded by a diverging lens to approximately 4 cm in diameter, illuminated the surface of the mammary tissue. Successfully propagating to the tissue surface, ICG or HPPH-car fluorescence generated from within the tissue was collected by an image-intensified, CCD camera fitted with an 830-nm or 710-nrn bandpass interference filter. Upon collecting time-dependent fluorescent images at the tissue surface overlying both normal and diseased tissue volumes and fitting these images to a pharmacokinetic model describing the uptake (wash-in) and release (wash-out) of fluorescent dye, the pharmacokinetics of fluorescent dye was spatially determined. Mapping the pharmacokinetic parameters for ICG indicates that the dye acts as a blood pool or blood persistent agent with enhanced passive uptake into or retention in diseased tissue regions presumably owing to "leaky" angiogenic vessels feeding the tumor. The wash-out of ICG was delayed for up to 72 hours after intravenous injection in tissue volumes associated with disease, for ICG fluorescence was still detected in the diseased tissue 72 hours post-injection. In contrast, HPPH-car pharmacokinetics illustrated active uptake into diseased tissues, perhaps owing to the over-expression of LDL receptors associated with the tumor endothelium. HPPH-car fluorescence was not discernable after 24 hours. This work illustrates the ability to monitor the pharmacokinetic delivery of near-infrared fluorescent dyes within tissue volumes as great as 2 cm from the tissue surface and to differentiate normal from diseased tissue volumes on the basis of parameters obtained from the pharmacokinetic models.