Pre-clinical Radiopharmaceutical Therapy with CXCR4-targetted compound on a Human CXCR4 Positive Multiple Myeloma Orthotopic Model

Introduction: Multiple myeloma (MM) accounts for approximately 10% of hematological cancers and is considered an incurable disease despite recent treatment advances. The chemokine receptor motif 4 (CXCR4) is overexpressed in many solid tumors and hematological malignancies, including MM, and is an attractive target for radiopharmaceutical therapy (RPT). Data published on peptide and non-peptide CXCR4 radioligands suggest that CXCR4 positive MM patients may benefit from CXCR4 targeted RPT. Last year we developed an orthotopic model of MM applicable to pre-clinical studies involving CXCR4 targeted radiopharmaceuticals. The aim of this study was to investigate the applicability of such model in radiopharmaceutical therapy, including treatment efficacy as well as possible side effects and potential toxicity

Methods: Thirty two male NRG mice were intravenously injected through the tail vein with 10⁶ luciferase transfected L363 cells, a well-established bone marrow homing MM cell line. Animals were monitored clinically once a week for the following 5 weeks, with close attention to weight, gait and behavior. Disease engraftment and spread was also monitored weekly by in vivo bioluminescence imaging in a Perkin Elmer’s IVIS Lumina 5, starting 3 weeks post-injection. On the 6^th week post intravenous tumor inoculation, animals were divided into different groups, a control group and 3 different activity-receiving groups, and were intravenously injected with either 15MBq, 35MBq or 50MBq of ¹⁷⁷Lu-BL34 for therapeutic purpose. Animals were monitored daily following therapy until dying or being humanly euthanized, except for their weight that was measured and recorded once a week. To better observe the in vivo radiopharmaceutical behavior, one subject from the 50MBq group was imaged on a preclinical SPECT-CT scanner 1, 4, 24 and 96 hours post-injection. Four mice presented signs of health decline on the day of treatment with poor grooming and slower movements and were randomly distributed, one per each group, to avoid bias.

Results: Weight gain was observed until the 5th week post-tumor inoculation and the weight was maintained throughout the 6^th week. Weight loss of more than 20% of body weight and hind limb paralysis were the most common symptoms leading to euthanasia in the different groups. Although the three treatment groups survived longer than the control group, statistical significance was achieved in those treated with 35MBq and 50MBq (p values 0.05 and 0.0093 respectively), with animals in these groups outlasting those in the control group a median of 6 and 8 days, respectively (Figure 1). SPECT-CT images showed lower tumor uptake than anticipated based on prior biodistribution studies performed with this radiotracer, and quicker than expected tracer washout, which made images of low quality and unsuitable for interpretation.

Conclusions: With this study we were able to show the applicability of an orthotopic model of Multiple Myeloma in radiopharmaceutical therapy, and that mice treated with a single dose of at least 35MBq of ¹⁷⁷Lu-BL34 responded to treatment. Treatment with the different activities of ¹⁷⁷Lu-BL34 was well tolerated. Further work is needed as many CXCR4-targeted radiopharmaceuticals are highly selective for human CXCR4, which may mask some undesirable effects of RPT in mouse models due to CXCR4 expression in normal hematopoietic stem cells and bone marrow stromal cells.

• Download figure
• Open in new tab
• Download powerpoint