New LHRH analogs for the detection of LHRH positive tumors

Aim: Receptors for luteinizing hormone-releasing hormone (LHRH) can be found in various tumors including breast cancers as shown by previous studies on human specimens. Recently efforts were undertaken to target these receptors with cytotoxic peptide conjugates in experimental animal models (Schally et al.). However, LHRH receptor imaging in humans has not yet been reported. The aim of this study was to develop suitable analogs for this purpose and test the hypothesis that LHRH receptors can be detected and visualized by scintigraphic methods. Methods: Three radiolabeled analogs were prepared and evaluated in-vitro: Analog A is a radioiodinated derivative of the LHRH agonist D-Trp6-LHRH (Triptorelin). Analog B is a MAG-3 conjugate of D-Lys6-LHRH. (D-Lys6-LHRH is known to allow conjugation without substantial loss of receptor binding properties). Analog C is a HYNIC-conjugate of D-Lys6-LHRH. The latter 2 compounds allow for Tc-99m (and also Re-188) labeling. The iodinated compound was labeled using Chloramine-T methods and the Tc-labeled LHRH analogs were prepared by direct SnCl2 mediated labeling of the conjugates of chelator and peptide. (The conjugates were prepared using NHS-esters of the respective chelators, removal of protecting groups and final HPLC-purification.) High radiochemical yields and high specific activity can be obtained with all three compounds. However, first patient studies were carried out with the best characterized I-123-D-Trp6-LHRH. Results: Four patients (3 female, 1 male; mean age 53 y, 42-59) with newly diagnosed breast cancer underwent LHRH receptor scintigraphy after conventional imaging (US, mammography and MRI). After intravenous injection of 2-4 mCi HPLC-purified I-123-labeled D-Trp6-LHRH dynamic imaging was performed for 20 min with subsequent static imaging including whole body scans. The results were matched with conventional imaging. In 3/4 patients the breast cancer lesion was clearly visualized with tumoral tracer accumulation promptly after injection and no significant increase in uptake after 5 min post-injection; in one patient with the smallest and least differentiated tumor (2.1 cm, G3) no definite tracer accumulation could be demonstrated. The known bone metastases of two patients were not detected. No adverse reactions were noted. In conclusion this study shows a first promising proof of concept in LHRH receptor imaging in breast cancer patients