Fluorescence detection of occult non-palpable metastases in lymph nodes following injection of low doses of Photofrin II®

Extraction procedures to quantitate Photofrin® II concentration in tissues correlated with fluorescence measurements from instrumentation developed for in vivo fluorimetry were applied for the detection of low drug levels in occult metastases of the lymph nodes. A sensitive fluorimeter was developed to overcome the limitation of natural background autofluorescence signals. The new device circumvents this limitation by reliably subtracting the normal tissue background signal from the combined fluorescence of DHE and natural background at 630. These techniques have been initially applied to detect low levels of drug in DBA mice bearing the SMT-F tumor, which has been extensively studied in our laboratory. The data show the ability of the techniques to detect very low levels of porphyrin in the tumors after low, non-therapeutic doses of injected photosensitizer. The Pollard rat prostatic adenocarcinoma (PA-III) model was chosen for this study because of its characteristic pattern of metastases involving only ipsilateral lymph nodes. Early studies on this lymph node model have shown the ability of the detection device to measure low levels of drug in non-palpable occult metastases in the nodes. The findings show that the detection of small numbers of metastatic cells is possible (<100 cells) with injected DHE doses of 0.25 mg/kg.