35P - Exploring plerixafor as a vector molecule in nuclear medicine for targeting CXCR4 receptor overexpression in vivo

Background

The use of targeted radionuclide-tagged vector molecules forms the basis of the foundation of the emerging science of nuclear medicine. These vector molecules play a pivotal role in screening the extent of disease spread in cancer patients. Chemokine receptor 4 (CXCR4) overexpression is noted in > 75% of cancers such as breast cancer, lymphoma, lung cancer, etc. Plerixafor is a CXCR4 antagonist used to mobilize hematopoietic stem cells. It binds to CXCR4 by three acidic residues in binding pockets Asp171, Asp262, and Glu288. Modified plerixafor can be radiolabelled for diagnostic use and targeted radionuclide therapy.

Methods

Optimization of various parameters (type of bifunctional chelator, temp., pH, time, and reaction volume) for conjugation of plerixafor. Further, radiolabelling with ⁶⁸Ga and ¹⁷⁷Lu radionuclides was standardized. Other quality control checks such as radionuclide purity; radiochemical purity; sterility; pyrogenicity; serum stability and immunoreactivity to CXCR4 receptor were performed. In-vivo physiological biodistribution studies were conducted in normal rats. After seeking Institutional Ethical clearance, ⁶⁸Ga-Plerixafor PET/CT imaging was performed in lymphoma patients. Results were correlated with ¹⁸F-FDG uptake for proof of concept.

Results

Modified plerixafor was confirmed by MALDI-TOF with changes in molecular weight in mass spectra 1078-1087 Da (Plerixafor-DTPA) and 1014-1038 Da (Plerixafor-NOTA). Radionuclide and radiochemical purity of ⁶⁸Ga and ¹⁷⁷Lu Plerixafor was ≥ 99%. Synthesized radiotracers were stable, sterile, and pyrogen-free. Radioligand binding assay confirmed immunoreactivity with high target efficacy (Kd 57.16 nM) of ¹⁷⁷Lu-Plerixafor towards CXCR4 expressing cancer cells. Furthermore, the log absolute IC50 concentration of ¹⁷⁷Lu-Plerixafor cytotoxicity studies was 2.628 nM. Nuclear receptor expression was confirmed with immunocytochemistry. In-vivo physiological biodistribution of ⁶⁸Ga-Plerixafor was in the liver, spleen, and lung. In addition, ⁶⁸Ga-Plerixafor targeted similar lesions seen in ¹⁸F-FDG PET/CT.

Conclusions

Radiolabelled Plerixafor elicits high in-vivo target efficacy towards CXCR4 overexpressing cancer cells.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

The authors.

Funding

DST-INSPIRE Fellowship.

Disclosure

All authors have declared no conflicts of interest.